evaluation of coastal erosion hazard zones along ...

State of Oregon Department of Geology and Mineral Industries Vicki S. McConnell, State Geologist

Open-File Report OFR O-04-09

EVALUATION OF COASTAL EROSION HAZARD ZONES ALONG DUNE AND BLUFF BACKED SHORELINES IN LINCOLN COUNTY, OREGON: CASCADE HEAD TO SEAL ROCK TECHNICAL REPORT TO LINCOLN COUNTY By

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GEOLOGY A ND

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George R. Priest1 and Jonathan C. Allan1

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INDUSTR

O N D E PA

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1937

2004

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Oregon Department of Geology and Mineral Industries, Coastal Field Office, 313 SW 2nd St., Suite D, Newport, OR 97365

Photo is a from historical photos provided by Paul Komar and shows deformation in the early 1940’s at the headwall of the Jumpoff Joe Landslide, Nye Beach, Newport.

NOTICE The Oregon Department of Geology and Mineral Industries is publishing this paper because the information furthers the mission of the Department. To facilitate timely distribution of the information, this report is published as received from the authors and has not been edited to our usual standards.

Oregon Department of Geology and Mineral Industries Open-File Report Published in conformance with ORS 516.030

For copies of this publication or other information about Oregon’s geology and natural resources, contact: Nature of the Northwest Information Center 800 NE Oregon Street #5 Portland, Oregon 97232 (503) 872-2750 http://www.naturenw.org Oregon Department of Geology and Mineral Industries OFR O-04-09

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TABLE OF CONTENTS 1.0 2.0 3.0

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5.0 6.0 7.0 8.0

EXECUTIVE SUMMARY ............................................................................................................ 1 INTRODUCTION ........................................................................................................................4 METHODS .................................................................................................................................. 6 3.1 Active Erosion Hazard Zone ............................................................................................... 6 3.2 Dune-backed Shorelines ..................................................................................................... 8 3.2.1 The Geometric Model ............................................................................................ 8 3.2.1.1 Wave Runup ............................................................................................ 8 3.2.1.2 Tides ...................................................................................................... 11 3.2.1.3 Beach Morphology ................................................................................. 12 3.2.1.4 Scenarios of Coastal Change in Lincoln County.................................... 13 3.3 BLUFF-BACKED SHORELINES ......................................................................................16 3.3.1 Introduction ..........................................................................................................16 3.3.2 The Bluff Retreat Model .......................................................................................16 3.3.3 Data Used for Drawing Bluff Erosion Hazard Zones ............................................ 19 3.3.3.1 Angle of Repose.....................................................................................19 3.3.3.2 Erosion Rate Data ..................................................................................29 3.3.3.2.1 Buff Top Retreat .................................................................... 29 3.3.3.2.2 Bluff Toe Retreat ................................................................... 29 3.3.3.3 Gradual Subaerial Erosion ..................................................................... 33 3.3.3.4 Block Failure Data .......................................................................................................39 3.4 Landslide Mapping (Mass Movements) ............................................................................ 42 3.4.1 Introduction ............................................................................................................42 3.4.2 Prehistoric Mass Movements (PHls, PHb, PHf) ..................................................... 43 3.4.3 Potentially Active Mass Movements (PAls, PAb, PAf) ............................................ 43 3.4.4 Active Mass Movements (Als, Ab, Af) .................................................................... 43 3.4.5 Quaternary Landslides (Qls) .................................................................................. 43 3.4.6 Landslide Terrain (Qls?) .........................................................................................43 3.4.7 Extent and Quality of Landslide Mapping .............................................................. 44 3.5 Explanation of Geologic Data ........................................................................................... 45 3.6 Mapping Technique for Bluff Erosion Hazard Zones ......................................................... 50 3.6.1 Description of the zones ........................................................................................50 3.6.2 Uncertainty in Spatial Location of the Zones ......................................................... 51 3.6.3 General Procedure for Drawing Bluff Hazard Zones ............................................. 51 RESULTS AND DISCUSSION ..................................................................................................54 4.1 Landslide Hazards ............................................................................................................54 4.2 Active Hazard Zone...........................................................................................................59 4.3 Beach-dune Erosion Hazard Zones .................................................................................. 62 4.4 Bluff Erosion Risk Zones ...................................................................................................68 SUMMARY AND CONCLUSIONS ............................................................................................70 RECOMMEDATIONS ...............................................................................................................74 ACKNOWLEDGMENTS ........................................................................................................... 76 REFERENCES CITED..............................................................................................................77

Oregon Department of Geology and Mineral Industries OFR O-04-09

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APPENDIX A Erosion Hazard Polygons ....................................................................................................................81 Salmon River Spit ................................................................................................................................ 82 North Roads End, Lincoln City ............................................................................................................83 South Roads End, Lincoln City ............................................................................................................84 North Lincoln City ................................................................................................................................ 85 D River, Lincoln City ............................................................................................................................86 South Lincoln City ............................................................................................................................... 87 North Taft, Lincoln City ........................................................................................................................88 South Taft, Lincoln City ........................................................................................................................89 Central Salishan Spit ...........................................................................................................................90 South Salishan Spit .............................................................................................................................91 Gleneden Beach .................................................................................................................................. 92 Lincoln Beach ...................................................................................................................................... 93 Fogarty Creek - Fishing Rock ..............................................................................................................94 Boiler Bay - Pirate Cove ......................................................................................................................95 Depoe Bay ........................................................................................................................................... 96 Whale Cove ......................................................................................................................................... 97 Cape Foulweather ............................................................................................................................... 98 Otter Crest ........................................................................................................................................... 99 Otter Rock-beverly Beach .................................................................................................................100 Beverly Beach (Spencer Creek - Wade Creek ..................................................................................101 Moolack Beach (Carmel Knoll – Moolack Creek) .............................................................................102 Moolack Beach (Schooner Point – Schooner Creek) ........................................................................103 North Newport (Yaquina Head - Agate Beach) ..................................................................................104 North Newport (Agate Beach Wayside – NW 17th) ............................................................................105 Newport (Nw 11th – SW 2nd) ...............................................................................................................106 Newport (SW 9th Street – Yaquina Bay) ............................................................................................107 Newport (South Beach) .....................................................................................................................108 Henderson Creek – Grant Creek (SW 66th – SW 82nd)......................................................................109 Holiday Beach (Moore Creek – Thiel Creek; SW 82nd – 100th) .......................................................... 110 North Lost Creek Wayside (SE 116th – Se 127th) ............................................................................... 111 Lost Creek – North Ona Beach ......................................................................................................... 112 Ona Beach ........................................................................................................................................ 113 Seal Rock .......................................................................................................................................... 114 APPENDIX B Shoreline Geology And Landslide Maps ........................................................................................... 115 Salmon River Spit .............................................................................................................................. 118 North Roads End, Lincoln City .......................................................................................................... 119 South Roads End, Lincoln City ..........................................................................................................120 North Lincoln City ..............................................................................................................................121 D River, Lincoln City ..........................................................................................................................122 South Lincoln City .............................................................................................................................123 North Taft, Lincoln City ......................................................................................................................124 Oregon Department of Geology and Mineral Industries OFR O-04-09

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South Taft, Lincoln City ......................................................................................................................125 Central Salishan Spit .........................................................................................................................126 South Salishan Spit ...........................................................................................................................127 Gleneden Beach ................................................................................................................................ 128 Lincoln Beach .................................................................................................................................... 129 Fogarty Creek - Fishing Rock ...........................................................................................................130 Fishing Rock (Close Up) ...................................................................................................................131 Fogarty Creek (Close Up) .................................................................................................................132 Boiler Bay - Pirate Cove ....................................................................................................................133 Depoe Bay ......................................................................................................................................... 134 North Depoe Bay –Pirate Cove (Close Up) ....................................................................................... 135 South Depoe Bay (Close Up) ............................................................................................................136 Whale Cove ....................................................................................................................................... 137 Whale Cove (Close Up) ....................................................................................................................138 Cape Foulweather .............................................................................................................................139 Otter Crest ......................................................................................................................................... 140 North Otter Crest (Close Up) .............................................................................................................141 Otter Rock-beverly Beach .................................................................................................................142 Otter Crest Landslde-devils Punchbowl (Close Up) ..........................................................................143 Johnson Creek Landslide (Close Up) ................................................................................................144 Beverly Beach (Spencer Creek - Wade Creek ..................................................................................145 Spencer Creek (Close Up) ................................................................................................................146 South Spencer Creek-beverly Drive (Close Up) ................................................................................147 Moolack Beach (Carmel Knoll – Moolack Creek) .............................................................................148 Moolack Beach (Schooner Point – Schooner Creek) ........................................................................149 Schooner Point (Close Up) ................................................................................................................150 North Newport (Yaquina Head - Agate Beach) ..................................................................................151 Schooner Creek Landslide (Close Up Of Active Slide Block Outlined In White) ............................... 152 South Yaquina Head At Agate Beach (Close Up) ..............................................................................153 North Newport (Agate Beach Wayside – NW 17th) ............................................................................154 Newport (NW 11th – SW 2nd) .............................................................................................................155 Jumpoff Joe Landslide (Close Up) ....................................................................................................156 South Nye Beach (Close Up) ............................................................................................................157 Newport (Sw 9th Street – Yaquina Bay) ............................................................................................158 Mark Street Landslide (Close Up) .....................................................................................................159 Newport (South Beach) .....................................................................................................................160 Henderson Creek – Grant Creek (SW 66th – SW 82nd)......................................................................161 Henderson Creek (Close Up) ............................................................................................................162 Grant Creek (Close Up) .....................................................................................................................163 Holiday Beach (Moore Creek – Thiel Creek; SW 82nd – 100th) ..........................................................164 North Lost Creek Wayside (SE 116th – SE 127th) ..............................................................................165 Lost Creek – North Ona Beach .........................................................................................................166 Ona Beach ........................................................................................................................................ 167 Seal Rock .......................................................................................................................................... 168 Oregon Department of Geology and Mineral Industries OFR O-04-09

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APPENDIX C Digital Files ........................................................................................................................................ 169 APPENDIX D Erosion Rate Data from Mathiot (1973) .............................................................................................171 APPENDIX E 1868 and 1900 to 1993 Erosion Rate Data ....................................................................................... 172 APPENDIX F House-to-bluf Erosion Rate Data from Priest and Others (1994) ...................................................... 179 APPENDIX G Erosion Measurements by Rubbersheeting ......................................................................................189 FIGURES 1

Map of western Lincoln County showing location of the study area. Box shows maximum extent of area covered by digital geographic information files containing data on geology and landslides. Erosion information covers only open coastal (versus estuarine or river) areas. .......................................................................................................................................... 5

2

Schematic diagram showing possible dune erosion hazard zones. ........................................... 7

3

The foredune erosion model (Komar and others, 1999). NGVD = national geodetic vertical datum. .............................................................................................................. 9

4

The geometric model used to assess the maximum potential beach erosion in response to an extreme storm (After Komar and others, 1999). ............................................ 9

5

Elevation changes along the Oregon coast, measured by geodetic surveys (Vincent, 1989). The elevation changes are relative to the global increase in sea level, with positive values representing a rise in the land at a higher rate than the increase in sea level, while negative values represent the progressive submergence of the land. [from Komar, 1997]. .................................................... 12

6

Schematic illustration of block failure on a bluff, angle of repose, and erosion rate in relation to possible hazard zones. These factors can be combined in a variety of different ways to produce hazard zones. ...................................... 17

7

Gradual versus episodic bluff erosion. Note how the landslide toe position remains stable as the headwall retreats; hence erosion rate for bluffs with landslides is better measured at the headwall. Also note that the dangerous part of the bluff is much wider for the


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landslide-prone bluff, since another maximum block failure could occur at any point in the erosion cycle. .................................................................................... 19 8

Near vertical upper slope of otherwise vegetated sea cliff composed of Pleistocene marine terrace deposits and grass-covered talus in the exposure. Note the scar of a recent cantilevered block failure (see text explanation of this process). Rip rap was installed at the bluff toe to protect it from wave erosions. ...................................................................................................21

9

Close up of scar of a cantilevered block failure from photograph of Figure 8. ......................... 22

10

Nye Beach view of stable ~1.5:1 (horizontal : vertical) slopes (measured from LIDAR elevation data) of vegetated talus developed on Pleistocene paleosols (lower half of bluff) overlain by Pleistocene marine terrace sands (upper half of bluff). Upper picture was taken sometime before ~1917-1920; left picture is a 2001 close up of the north end of the bluff in the right hand picture. Note the historic Sylvia Beach Hotel in the upper picture appears on the left side of both lower pictures. It is clear that this hotel has been about the same distance back from the bluff edge for ~100 years, so bluff top retreat in front of the hotel has been near zero. Hence, once talus cover prevents further erosion by cantilevered block fall, gradual erosion at the angle of repose is near zero. An extreme rainfall event caused a small debris flow that stripped talus from the bluff where the small group of people are standing in lower right hand photo. Rejuvenated stress-release fracturing and block falls are continuing to erode the top of the bluff from this point south. ..................................... 23

11

Near vertical sea cliff of Pleistocene marine terrace deposits in the Lincoln City area (photo courtesy of Paul Komar). Cliff erosion is from undercutting by waves and cantilevered block fall driven by stress release fractures. Wave attack is vigorous enough to remove all talus from this bluff, resulting in a near-vertical slope continuously exposed to further erosion. .......................................................................................................24

12

Slump block about 30 feet wide in Pleistocene paleosol, laminated carbonaceous clayey silit and sand deposits at Nye Beach. Note that cantilevered block falls are keeping the upper part of the bluff near vertical. Note also that this slump and the cantilevered block falls occurred on a well-vegetated slope. While the vegetation does not prove that wave undercutting did not trigger this slump, the vegetation is suggestive that waves were probably not the main factor for either type of failure. Groundwater saturation probably played a more significant role in this particular case. .......................................................................................24


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13

Cape Foulweather area illustrating steep lower sea cliffs but overall 1:1 slope in the zone of rapid soil creep that directly responds to wave erosion (active hazard zone). Note the debris flow chute in the center of the slide. This is where Highway 101 fill collapsed into a rapidly moving debris flow during heavy rains in December 1999. Older slide scars are also present. Photograph is courtesy of the Oregon Department of Transportation. ..................................................................................................25

14

Talus-free slopes in mudstone of the Astoria Formation at Beverly Beach on the south side of Spencer Creek (at bridge in background). Slopes average about 1:1, horizontal to vertical. The orange unit in the middle background is Pleistocene to Holocene colluvium that mantles the valley walls north and south of Spencer Creek. Paleosols (ancient soils) of this unit also underlie the beach around the mouth of Spencer Creek. These deposits have ancient tree stumps that are exposed when sand is removed by winter wave erosion. A Carbon-14 age of outermost wood on a stump on the beach at Spencer Creek (just off the upper left edge of the photo) is 3,480 ± 60 years before present (B.P.) and records the last time that relative sea level was low enough to allow trees to grow as low in elevation as present sea level. The sample was collected in this investigation after winter waves had removed the beach sand covering the ancient forest soils that mark the seaward extension of the ancestral Spencer Creek valley. ...................................................... 25

15

Wave erosion of a fine-grained sandstone below harder, blocky jointed tuff of the Astoria Formation at Nye Beach produces a vertical to overhanging slope. Tuff is hardened by zeolite cement (Alan Niem, 2003, written communication). Note basalt rip rap in background provides protection to the public walkway. .........................................................................................................................26

16

Vegetated, talus-covered bluff of Tertiary sedimentary rock overlain by Quaternary marine terrace deposits at the mouth of Yaquina Bay. Most of the slope is on the order of 1.4 to 1.5:1, horizontal to vertical and has not been subjected to significant wave erosion since construction of the jetties in the late 1800’s. .................................................................................................26

17

Nye Beach view of gently dipping (strike N9ºW; dip 13ºW) Astoria Formation siltstone and sandstone overlain by Pleistocene paleosols and marine terrace deposits. The Astoria Formation is an aquitard. When groundwater percolating downward through the permeable Pleistocene units encounters the Astoria Formation, it flows laterally along the contact, producing springs. The softer Pleistocene deposits are eroding faster than the Tertiary rocks in response to a combination of groundwater-driven soil creep, wind erosion, and attack by the largest storm waves. ............................................. 27


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18

Projection of angle of repose for landslides in Tertiary sedimentary rocks. .............................. 28

19

Location of major landslides discussed in the text. All of these slides are translationsl block slides in fine-grained Tertiary sedimentary rocks (siltstone and mudstone). See Appendix B for detailed location and geology of each slide. ...............................................................................................................30

20

Wade Creek area of Beverly Beach where bluff toe erosion rate was estimated from 1939 and 1993 air photos. Low rampart of shallow marine Astoria Formation sandstone is overlain by highly disaggregated landslide debris. Erosion of the bluff toe by failure of large blocks is unlikely in this geologic setting. Bluff toe erosion is most likely by gradual wearing away of small pieces of the sandtone. This is a winter beach condition; this area is covered in beach sand in the summer. ................................................................................. 34

21

Blue line is 1994 shoreline from the USGS base map; red line is a 1927 shoreline from National Ocean Service T-sheets; the dashed line is the 1998 shoreline derived from 1998 LIDAR elevation data. There is much less difference in position between these shorelines than the same shorelines plotted for Tillamook County littoral cells, where the 1927 shoreline is well east of the 1994 shoreline. .......................................................................................................35

22

Same age shorelines as in Figure 21 but at Gleneden Beach in the Siletz littoral cell. .......................................................................................................................36

23

Topographic map of the Johnson Creek Landslide cutting Highway 101 ~3 miles north of Newport, Oregon (see Appendix B, Johnson Creek Landslide geologic map for an overview of the geology). Highway 101 is the linear feature passing north-south through the center of the map. Landslide block boundaries are shown in blue. This landslide is the subject of a five-year joint research investigation by Oregon Department of Geology and Mineral Industries and Oregon Department of Transportation. The investigation commenced in the fall of 2002 with geotechnical drilling along Cross Section A-A’ and topographic mapping. Blue dots are 3 piezometer holes; red dots are 3 adjacent inclinometer holes; light brown numbers are elevation contours labeled at 0.5 m intervals. .......................................................................... 40

24

Vertical cross section through the Johnson Creek Landslide. Inclinometer holes LT-1, LT-2, and LT-3 were drilled in the fall of 2002. Cross section shows thick siltstone (dashed pattern) with interbedded thin sandstone beds (red, yellow, blue) of the Astoria Formation dipping about 17º west. Capping reddish pink unit is Pleistocene sand deposits. Unpublished cross section from A.R. Niem and W.A. Niem (2003). ....................................................................... 41


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25

LB-10 sample locality at Lincoln Beach, Lincoln County, Oregon. Don Christensen (former Lincoln Beach resident and member of the DOGAMI Governing Board) is pointing a chain saw at the Pleistocene stump that was sampled. Note that the stump is at the bottom of a section of organic-rich paleosol but lies on top of some less organic-rich Pleistocene deposits that have been eroded during the strong wave activity that affected this beach in 1997 and 1998. A stump from paleosol deposits at beach level but stratigraphically equivalent to the upper peat horizon indicated in the photo yielded an age of 1920 ± 60 radiocarbon years B.P. The picture was taken by Roger Hart. ................................................................................................................49

26

Quaternary faulting at Fogarty Creek and Fishing Rock drops youngest Pleistocene marine terrace sand (unit Qtc) to beach level north of Fishing Rock. Qc = deposits of ancient colluvium and forest soils; Tdb = Depoe Bay Basalt; Ta = Astoria Formation; Qal = Quaternary alluvium............................................... 55

27

Small Syncline (big arrow = plunge direction of axis; little arrows = direction of dip of limbs) in southern part of Nye Beach at Newport drops the Pleistocene marine terrace deposits to within ~6-7 feet of beach level. Syncline plunges only about 1º to the east. Drop in elevation of Pleistocene marine terrace from top of map to synclinal axis is ~14 feet. Large active slide blocks occur in the southern part of the map where Tertiary sedimentary rocks again become a significant part of the cliff face. Ab = active landslide block; Qtc = Quaternary marine terrace deposits; Ta – Tertiary Astoria Formation. ............................ 55

28

Looking southeast at the base of sea cliff near the axis of the syncline in Figure 27. Basal lag gravel of the youngest Pliestocene marine terrace are only about 3 feet above beach level and dip about 1º east at this locality. Erosion prior to deposition of the Pleistocene marine terrace gravel and underlying paleosol deposits has cut into the Astoria Formation lowering it 5.3 feet more than would be the case from the syncline alone. Paleosol deposits underlying the basal marine terrace gravel were probably deposited during a low sea stand (glaciation). Blue bill-cap hat (10 inches long) is for scale. ................. 56

29

Close up of basal marine terrace gravel in Figure 28. Note cobbles above blue hat. ............. 56

30

Close up of base of paleosol in Figure 28. Note fossil woody debris in probable forest soils at the base of the deposit (at blue hat). Astoria siltstone and sandstone is the reddish jointed material below. Paleosol is soft, sandy silty clay and clayey silt. ...........................................................................................................57

31

Tertiary sedimentary rock, probably Nye Mudstone (base) overlain by Pleistocene paleosols, colluvial deposits, and debris flows which are cut by basal gravel of the youngest Pleistocene marine terrace. Picture is ~1000 feet north of Nye Creek in a slide


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block at the south end of the Jumpoff Joe Landslide. Outcrop records erosion of the Astoria Formation by an ancestral Nye Creek, deposition of subaerial deposits and debris flows during a Pleistocene low stand (glaciation), erosion of a wave-cut platform across these deposits during the last high-stand (interglacial period), and deposition of beach gravel and overlying sand during marine regression. Green hat in upper part of picture is about 10 inches long. ................................................................. 58 32

Close up of basal marine terrace gravel (orange) and underlying low-stand paleosol, colluvial, and debris flow deposits from Figure 31. Green hat is about 10 inches long. ........................................................................................... 59

33

Jumpoff Joe landslide and small intact headland of west-dipping Astoria Formation and underlying Nye Mudstone; note that wave-cut platform is about 38 feet above the beach at the headland where no Pleistocene low-stand deposits lie between the basal gravel of the Pleistocene marine terrace and the underlying Tertiary rocks. Figure 34 below is shows the rest of the landslide to the right where pictures in Figures 31 and 32 were taken. .................................................................................................60

34

South end of Jumpoff Joe landslide block at Nye Beach showing location of pictures in Figures 31 and 32. Note how the Pleistocene paleosol thickens beneath the marine terrace sands at the expense of the underlying Tertiary sedimentary rocks. By the time the Tertiary rocks essentially disappear out of the section at the south end of the slide block (right at stairway), the slide ends. There are no large translational landslides immediately south because there is no significant Tertiary sedimentary rock in the cliff face. .............................................................................................61

35

Probable Quaternary fault at Grant Creek lowers bluff height and amount of Tertiary sedimentary rock in the cliff face south of the creek. ............................................... 61

36

Example of a sawed log located in situ in the foredune at Salishan Spit. Picture was taken within a few weeks after the log was exposed by wave erosion; the weathered surface on the saw cut indicates that it was not cut in this brief time interval but when the log was transported by waves probably decades earlier. The log was subsequently covered by dune sand before being exhumed by renewed erosion. Photo is courtesy of Paul Komar. ...........................................................................................................61

37

Salishan Spit historical shorelines and erosion hazard zones (red = high, orange = moderate, and yellow low risk). For simplicity, the active hazard zone is not shown. Note how little difference there is between the moderate- and low-risk zones. Historic shorelines show that fluctuations of the Siletz River channel


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have moved the end of the spit laterally as much as 600 feet, but a rip rap revetment installed along the open coastal shore in the developed portion of the spit prevents shoreline change. The river channel has eroded the east side of the spit, prompting installation of rip rap (Komar and Rea, 1976). If the rip rap fails, the central part of the spit could be breached under the worst cases of open coastal shoreline retreat. .................................... 64 38

Beach accretion caused by construction of jetties at Yaquina Bay in the late 19th Century. The effect is most pronounced at South Beach and disappears north and south of the jetties. The base map is a 1993 orthophoto. 1928 and 1952 shorelines are from NOS T sheets (National Ocean Service topographic maps); the 1998 shoreline is derived from LIDAR data flown by USGS and processed by NOAA. All shorelines approximate mean higher high water (~4 feet above geodetic mean sea level). Most areas affected by accretionary shorelines in this area are Oregon State Park lands. Note the numerous linear, vegetated dune ridges that mark accretion at South Beach since jetty construction. The pre-jetty shoreline can be seen as the west edge of the heavily vegetated zone east of these linear features. ........................................................................... 67

TABLES 1

Peak storm wave statistics for the Newport wave buoy for the major 1997-98 El Niño and 1998-99 La Niña (Allan and Komar, 2002). ............................................ 10

2

Average extreme-wave projections based on data from four NDBC wave buoys located offshore the Pacific Northwest coast. ................................................................ 11

3

Extreme annual tides (Shih and others, 1994). Note all elevations are relative to the NGVD’29 datum. ................................................................................................13

4

Summary of bluff erosion data that can be used to calculate bluff hazard zones. Only maximum observed (empirical) block failure width is listed (versus most probable or average width), because this is generally the only empirical data that can be easily obtained in most areas. Quantitative slope modeling or regional empirical analyses would be required to establish a mean or most probable block failure width. Angle of repose refers to the ideal slope angle for unconsolidated talus of the bluff material. ............................................... 17

5

Slopes of repose by material type. ...........................................................................................28

6

Bluff toe erosion data for Lincoln County based on comparison of 1993 digital orthophotos to 1939 air photos along continuous lengths of shoreline, so rates are derived from east-west change of the bluff toe over 54 years. See Appendix G for details.


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Std. Dev = Standard deviation calculated by weighting the individual deviations from the weighted mean rate by shoreline length applicable to each measurement. Error is measurement error estimated from lateral differences in location between geographic features common to the two photos; Nye ms = Nye Mudstone; Astoria = Astoria Formation sandstone and siltstone. ...........................................................................................................32 7

Open coastal bluff toe erosion rates assumed for calculations of coastal erosion hazard zones in Lincoln County. The mean plus measurement error plus one standard deviation from the mean is used for a maximum erosion rate in each case except the Beverly Beach littoral cell. Discussion of the mean and conservative rates used there are given in the text. ...................................... 34

8

Recommended maximum block failure widths for coastal bluffs of Lincoln County. All data are from empirical observations of local landslides. Sedimentary rocks are considered to be seaward dipping if the angle between bluff trend and bedding strike is £18º; this is the angle below which block failure width appears to increase abruptly. Bluff height means topographic relief from the bluff toe to the top seaward edge. Data are from empirical data of Allan and Priest (2001) from Tillamook and Lincoln Counties with supplemental data in Lincoln County to account for Pleistocene marine terrace sand bluffs, bluffs cutting at high angles (>18º) to the bedding strike, and Tertiary sedimentary rock bluffs somewhat lower in height than those addressed by Allan and Priest. Note that some bluff types listed here do not occur in Lincoln County (e.g. coastal bluffs of Tertiary sedimentary rock in excess of 200 feet high), but are included in order to show the entire empirical data set for both Tillamook and Lincoln Counties. .................................................................................................................................. 39

9

Landslide map units; “Present?” refers to presence on the Lincoln County coast. .................. 45

10

Carbon-14 isotopic age data obtained by the Oregon Department of Geology and Mineral Industries in coastal Lincoln County. Native map projection for these data is Oregon North State Plane, US Survey feet, here converted to longitude-latitude coordinates. BP = before present; Convent_Age = Conventional Age; Meas Age = measured age. ............................................. 47

11

Site and sample descriptions of carbon-14 samples. ............................................................... 47

12

Maximum potential erosion distances determined for the Salmon River spit. ..............................................................................................................62

13

Maximum potential erosion distances determined for the Lincoln City littoral cell at Salishan Spit. ...................................................................................63


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14

Maximum potential erosion distances determined for the Agate Beach area. ..............................................................................................................65

15

Maximum potential erosion distances determined for the accretionary beach north of the North Jetty at Yaquina Bay. .............................................. 65

16

Maximum potential erosion distances determined for the South Beach area. ....................................................................................................................65

17

Maximum potential erosion distances determined for the north Ona Beach area. .............................................................................................................65

18

Maximum potential erosion distances determined for the central Ona Beach area. ...........................................................................................................66

19

Minimum, mean, and maximum lateral distances of bluff top retreat should erosion continue for 60-100 years. These distances define the landward boundaries of the high-, moderate-, and low-risk hazard zones, respectively, when added to the lateral distance of the projected angle of repose for talus of each bluff. Table illustrates the uncertainty of predicting future bluff retreat from erosion rate and maximum block failure width. Values in parentheses are actual mapped widths, taking into account the limitations of the digital base maps, topographic data, and drawing accuracy. “Fine-grained interbeds” in the table refers to interbeds of siltstone, mudstone, or silty fine-grained sandstone with low resistance to shearing forces and consequent slope failure. .................................................... 69

A1

Digital files used in this report. See main report for detailed descriptions. ............................ 169


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1.0 EXECUTIVE SUMMARY This report describes and documents a range of coastal geologic hazard zones distinguished for the Lincoln County coastline. In particular, the report focuses on identifying minimum and maximum potential erosion distances for bluffs and for dune backed shorelines using two quite different but complementary approaches. In both types of shorelines four zones were defined, an active hazard zone characterized by existing, active erosion processes, and three zones of potential future erosion, high-, moderate-, and low-risk zones that respectively depict decreasing risks of becoming active in the future. Of most interest to planners are the landward boundaries of the high- and low-risk zones. The landward boundary of the high-risk zone defines a conservative but reasonable limit of expansion of the active hazard zone in the next 60-100 years. The landward boundary of the low-risk zone defines the outermost limit of expansion of the active hazard zone in a worst-case scenario. This scenario could be a catastrophic event such as a great earthquake on the Cascadia subduction zone, coupled with severe storms. For example, a Cascadia earthquake would directly cause widespread landsides on steep slopes, while remobilizing existing landslides. Near instantaneous subsidence of the coast by up to ~3 feet during a Cascadia event would lead to extensive retreat of dune and bluff backed beaches in northern Lincoln County. These erosion hazard zones were defined by detailed analysis of coastal erosion processes affecting the County. The most important conclusions reached from this analysis are: 1)

Analyses of historical shoreline changes along the Lincoln County coastline indicate that the dune-backed shorelines respond episodically to such processes as the El Niño/La Niña Southern Oscillation, and as a result of rip current embayments that cause “hotspot erosion” of the coast. Previous work in this area and in Tillamook County suggests that such processes can cause up to 125 ft of beach erosion in one or a few large storm cycles. Thus, the coastline undergoes periods of both localized and widespread erosion, with subsequent intervening periods in which the beaches and dunes rebuild. Nevertheless, because the record of such occurrences is relatively short, limited to 30 years at best, the effects of extremely large storms, or storms-in-series remain largely unknown, except for qualitative observations (e.g. sawed logs in dunes).

2)

Coastal change adjacent to the unmodified bay mouths and spit ends has been dramatic in the past. These features are capable of migrating over large distances (up to 600 feet) over a several years in response to changes in both the sediment supply and the predominant wave conditions. The prime example is Salishan Spit, which undergoes large lateral changes in the position of northern (unarmored) portion.

3)

Hazard zones on dune-backed beaches for the Lincoln City littoral cell were determined from a geometric model, whereby property erosion occurs when the total water level produced by the combined effect of extreme wave runup (R) plus the tidal elevation (ET), exceeds some critical elevation of the fronting beach, typically the elevation of the beach-dune junction (EJ). Three scenarios were used to model erosion hazard zones on dune-backed beaches: o

Scenario 1 (HIGH risk). This scenario is based on a large storm wave event (wave heights ~47.6 ft high) occurring over the cycle of an above average high tide, coincident with a 3.3


1

ft storm surge. Under this scenario, the mapped width of the high-risk hazard zone was found to range from 138 to 510 ft. The following two scenarios (MODERATE and LOW-risk events) are one of two “worst case” events identified for the Lincoln City littoral cell. Both scenarios have low probabilities of occurrence.

5)

o

Scenario 2 (MODERATE-risk). This scenario is based on an extremely severe storm event (waves ~52.5 ft high) coupled with a long-term rise in sea level of 1.31 ft. Maximum potential erosion distances (MPED) mapped under this particular scenario range from 279 to 772 ft.

o

Scenario 3 (LOW-risk). This scenario is similar to scenario 2 above but incorporates a 3.3 ft vertical lowering of the coast as a result of a Cascadia subduction zone earthquake. MPED mapped for scenario 3 ranged from 316 to 928 ft.

Hazard zones on bluff-backed shorelines were mapped based on an understanding of several geological parameters including bluff erosion rates, potential for block failures, and empirically determined angles of repose for the bluff materials. o

Scenario 1 (HIGH-risk) portrays the zone of bluff retreat that would occur if only gradual erosion at a relatively low mean rate were to occur after the slope reaches and maintains its ideal angle of repose (for talus of the bluff material). The time interval of erosion was assumed to be 60 years. The width of the high-risk hazard zone generally ranged from 20 to 30 ft wide, depending on the type of geology. Where slopes were steeper than the angle of repose for talus of the bluff material, the zone width was increased by the lateral distance necessary to accommodate retreat to the angle of repose.

o

Scenario 2 (MODERATE risk) portrays an average amount of bluff retreat that would occur from the combined processes of block failures, retreat to an angle of repose, and erosion for ~60-100 years. The moderate-risk hazard zone boundary was placed halfway between the high- and low-risk boundaries, and resulted in bluff retreat that generally ranged from 40 to 225 ft, depending on the type of geology.

o

Scenario 3 (LOW risk) illustrates a “worst case” for bluff retreat in ~60-100 years. This zone accommodates a maximum bluff slope failure, subsequent erosion back to its ideal angle of repose, and gradual bluff retreat for ~100 years. For bluffs composed of Pleistocene marine terrace deposits and paleosols, an additional retreat of the bluff top in response to subaerial erosion is achieved by making sure that the projected bluff top retreat corresponds to at least a 50 percent factor safety for the ideal slope of repose of 1.5 horizontal to 1 vertical (i.e. a 2:1 slope). Low-risk hazard zone widths ranged from 60 to 420 ft wide, depending on the type of geology.

o

In all cases, the minimum hazard zone width that could be mapped at the scale of the base maps is 20 feet, so even hard rock basalt and sandstone bluffs with negligible erosion rates were assigned zones with this minimum width. Many of these bluffs have high-, moderate-,


2

and low-risk zones of 20 feet each (total of 60 feet). o

Width of all erosion hazard zones for bluffs was locally increased to accommodate for the presence of inland landslides that could be intercepted by coastal retreat.

6)

An analysis of maximum single slide block failure width in both Tillamook and Lincoln County revealed that maximum width increases with bluff height but that above a threshold height width increases to hundreds of feet. This height is ~70-80 feet for seaward-dipping fine-grained Tertiary sedimentary rocks and ~150-160 feet for bluffs with clay-rich Pleistocene deposits at the base. Bluffs with Pleistocene sedimentary deposits at the base in the study area were lower than 150 feet of height, so large slide blocks did not form.

7)

An active erosion hazard zone (AHZ) has also been identified for the Lincoln City and Beverly Beach littoral cells. For dune-backed shorelines, the AHZ encompasses the active beach to the top of the first vegetated foredune, and includes those areas subject to large morphological changes adjacent to the mouths of bays due to inlet migration. On bluff-backed shorelines the AHZ includes actively eroding coastal bluff escarpments and active or potentially active coastal landslides.

8)

The report and GIS files identify landslides, slide blocks, and earth flow deposits located within about 1 mile of the coast, and classifies these features as active, potentially active, or prehistoric. Landslides, landslide topography, and pertinent geologic units mapped in previous studies were compiled for a strip of the coast about 13 miles wide (width of two standard USGS 7.5’ quadrangles). Large landslides with single block failures of hundreds of feet are limited to bluffs dominated by weak siltstones and mudstones of Tertiary age. Bluffs composed of mostly Quaternary sedimentary deposits did not form large landslides but failed in small slumps up to ~31 feet wide. Downward tectonic displacement of the youngest Quaternary marine terrace north Fishing Rock-Fogarty Creek, in a small area of Nye Beach, and south of Thiel Creek produced bluffs composed almost entirely of Quaternary deposits and devoid of large landslides. Significant earth flows were very rare and all appear to be moderately consolidated, probably indicating relatively ancient (Pleistocene or early Holocene?) deposition.

9)

New carbon-14 isotopic age determinations for some Pleistocene and Holocene deposits along the coastline give insight into the long-term history of coastal erosion and deposition. Systematic dating and mapping of the uppermost Holocene forest soils may in the future put constraints on long-term rates of coastal retreat.


3

2.0 INTRODUCTION The Oregon Department of Geology and Mineral Industries (DOGAMI) has been commissioned by Lincoln County to carry out an assessment of existing and potential coastal erosion hazards along the northern Lincoln County shoreline. This study focuses on the area from Cascade Head to Seal Rock (Figure 1). The purpose of this investigation is to assist County planners in effective decision-making adjacent to the shoreline. A secondary purpose is to provide a digital replacement for previously published chronic geologic hazard maps for this same segment of coastline (Priest and others, 1994; Priest, 1994 and 1997). For comparison to the current mapping and data, geographic information system (GIS) files of line work for the 1994 and 1997 erosion hazard maps are included on this CD as files Coast3-_arcs, Coast3-_ellipses, Coast3-_lines, Coast3-_shapes, Coast3-_text, Coast3-_textgrp (see Appendix C for complete list of digital files). It should be stressed that this is a regional investigation and is not intended for use as a site-specific analysis tool. However, the investigation can be used to identify areas in need of more detailed site-specific geotechnical studies. The response of coastal shorelines in the form of erosion or accretion is exceedingly sensitive to a multitude of complex factors that include the beach sediment budget, wave energy, water-level fluctuation, near shore morphology, shoreline orientation, and the geology. Because many shorelines are composed of unconsolidated sediments, including significant stretches of the Oregon coast, they are able to respond rapidly and are among the most dynamic and changeable of all landforms. It is this dynamism at the coast that makes beaches such an integral and important landform as they moderate the effects of wave energy. Beaches and dunes therefore provide an essential buffering mechanism, protecting properties and infrastructure from wave Oregon Department of Geology and Mineral Industries OFR O-04-09

attack. Notwithstanding this, because bluffs are also characteristic of much of the Oregon coast, erosion of these features is often accelerated by large storms during the winter months due to removal of beach sediment from the base of the bluffs. This process enables waves to directly attack the bluff toe, causing it to be undercut. Eventually, the bluff begins to retreat, either gradually or in the form of major landslides that can cause catastrophic loss of property. These problems may be exacerbated by extreme rainfall events and associated high groundwater levels which can trigger large landslides, regardless of wave erosion. Increasingly, the natural response of coastal shorelines to erode has come into conflict with the “built” environment due to the rapid growth in population and increased urbanization of coastal margins. Such development is characteristic of much of the Oregon coast, including significant sections of the Lincoln County shoreline (e.g. Lincoln City, Salishan Spit, Gleneden Beach, and Lincoln Beach), and is the product of escalating property values and the desire to establish infrastructure as close as possible to the ocean’s edge (Schlicker and others, 1973; Komar, 1997; Priest, 1999). Once the properties are established, the expectation is that the coast will remain where it is. Clearly, for sensible shoreline management to occur, sufficient technically sound information on the likelihood and magnitude of shoreline change must be provided to decision makers so they can make informed choices regarding shoreline management practices. That is the ultimate objective of this investigation.

4

Salmon River Spit

ROADS END LINCOLN CITY Salishan Spit

$EVILS,AKE $EVILS,AKE Devils Lake

3ISiletz 3I LETZ ""Bay LETZ AY AY

LINCOLN BEACH DEPOE BAY OTTER ROCK

Yaquina Head

NEWPORT

Yaquina Bay UIN A"AY 9AQ 99AQ AQUI UIN NA"AY A"AY

SOUTH BEACH SEAL ROCK

STUDY AREA AY "" AY EA Bay LSEA !!LS Alsea

WALDPORT

0

5

miles

10

YACHATS

Figure 1. Map of western Lincoln County showing location of the study area. Box shows maximum extent of area covered by digital geographic information files containing data on geology and landslides. Erosion information covers Oregon Department of Geology and Mineral Industries OFR O-04-09

5

3.0 METHODS The erosion hazard mapping methods used here are substantially the same as those used by Allan and Priest (2001) for Tillamook County. A variety of approaches have been used to define coastal erosion hazard zones along the Lincoln County shoreline. In particular, significant time was spent during the summers of 1991, 1992, 1996, 1999 and 2000 “walking” the coast, classifying the many landslide features adjacent to the beach into one of three categories; active, potentially active, and prehistoric. Inland landslides within about 1 mile of the coastline were generally identified by analysis of aerial photographs. In some cases inland landslides are listed simply as Quaternary landslides, without any interpretation about activity, especially if no field examination or other information could be found. In other cases areas identified in previous investigations as “landslide terrain” were shown. All map data have subsequently been incorporated into MAPINFO, Geographic Information System (GIS) software. Digital files of all vector and point data was also translated into ArcView (shape file) format in two map projections, State Plane, 1983 feet and Oregon Lambert Conformal, 1997 feet (see Appendix C for summary of digital data files included on this disk). The following sections present in more detail the approaches that have been used to establish erosion hazard zones on dune and bluff-backed shorelines. 3.1 Active Erosion Hazard Zone

An active erosion hazard zone (AHZ) (Figure 2) was mapped for dune- and bluff-backed shorelines throughout the study area based on a combination of purely geomorphic observations, and from an analysis of historical shoreline positions. The AHZ is the least speculative of the designated coastal hazard zones since it depends on easily identifiable coastal features that may be seen on modern aerial photos supplemented with current field data. On dune-backed beaches, the Oregon Department of Geology and Mineral Industries OFR O-04-09

AHZ distinguishes the zone of beach variability, a region in which beaches undergo considerable change (e.g. changes in the position of the shoreline (height and width) relative to some known datum point). Thus, it represents the portion of beach that is known to have changed in recent times due to large wave events and changes in sediment supply. It is therefore the zone that can be expected to change in the immediate future. As a result, there can be no doubt that building within the active hazard zone represents considerable risk. The landward boundary of the AHZ was drawn on 1993 orthophotos or 1994 digital orthophoto quadrangles (DOQ) at the top of the first continuously vegetated foredune. The higher resolution 1993 orthophotos were used where available; the lower resolution 1994 DOQ’s were used elsewhere, mainly in areas with sparse population. Furthermore, the landward boundary was adjusted to include 1998 Light Detection and Ranging (LIDAR) topographic data, indicative of shoreline recession that has occurred since 1993-1994. It is important to note that the AHZ as defined here should not be confused with the “active dune” or “active foredune” used by State regulators (e.g. OCZMA, 1979; DLCD, 1995). For example, OCZMA (1979) defines the Active Foredune as those dunes that possess insufficient vegetative cover to retard wind erosion, while Goal 18 (Beaches and Dunes) of Oregon’s Statewide Planning Goals and Guidelines prohibits the residential and commercial development of beaches and active foredunes (DLCD, 1995).On bluff-backed beaches the AHZ was mapped from the shoreline to the top edge of bluffs, sea cliffs, and the headwall of active, or potentially active shoreline landslides. Consistent with the view held for dune-backed shorelines, building within the active hazard zone along bluff-backed shorelines also reflects considerable risk from direct wave 6

$5.%%2/3)/./.(!:!2$:/.%3 #URRENT(AZARD:ONE ACTIVEEROSIONAREA

&UTURE(AZARD:ONE ,OW (AZARD :ONE

-ODERATE (AZARD :ONE

(IGH (AZARD :ONE

)MMINENT (AZARD :ONE

!. ', % /& 2% 0/ 3%

YEAREVENT COSEISMIC

YEAREVENT

YEAREVENT

YR EVENT

Figure 2. Schematic diagram showing possible dune erosion hazard zones.

attack at the bluff toe or from slope instability. The seaward boundary of the AHZ was established as the MHWL derived from an analysis of the NOS T-sheet shoreline positions and from the LIDAR data. This approach is discussed further below. These data were also used to identify the AHZ along the spit ends and around the mouths of the estuaries. Where those geomorphic features are uncontrolled (e.g. by the construction of jetties), the results clearly highlight the highly dynamic nature of both the spit ends and the mouths of the estuaries. Supplementary mapping of the AHZ was carried out through stereoscopic viewing of 1993 aerial photos and from field reconnaissance. All map data were then transferred by inspection to standard USGS DOQ’s using MapInfo software. Oregon Department of Geology and Mineral Industries OFR O-04-09

Some interpretation was needed when mapping the AHZ around the mouths of bays. In particular, where considerable accretion has occurred beside the jetties, we drew the landward boundary at the top of the first major foredune, even if only sparsely vegetated. An exception is the area immediately adjacent to a jetty where strong rip currents cause larger potential erosion. In those areas the zone was drawn at the point of first continuous grassy vegetation. The lateral extent of this rip current effect was judged by the extent of the associated embayment. For areas of highly active inlet migration, such as the end of Salishan Spit, even the grassy dunes were considered highly vulnerable to changing spit position. As a result, the landward boundary there was drawn at the first densely vegetated dune with trees and/or shrubs. 7

The maximum extent of shoreline variability on dune-backed beaches can also be estimated from oceanographic factors using empirical modeling techniques rather than direct geomorphic observations. The advantage of these techniques is that they can depict erosion events that may be difficult or impossible to define by geomorphic field observations of the effects of past erosion events. An example is sea level rise, which to some extent makes all past storm events and even coseismic subsidence events, somewhat less erosive than equivalent events in the future. The geometric model of Komar and others (1999) will be used in this investigation. 3.2 Dune-Backed Shorelines 3.2.1 The Geometric Model

For property erosion to occur on sandy beaches, the total water level produced by the combined effect of wave runup (R) plus the tidal elevation (ET), must exceed some critical elevation of the fronting beach, typically the elevation of the beach-dune junction (EJ). This basic concept is depicted in Figure 3, and in an expanded form as the geometric model in Figure 4. Clearly, the more extreme the total water level elevation, the greater the resulting erosion that occurs along both dunes and bluffs (Komar and others, 1999). As can be seen from Figure 4, estimating the maximum amount of dune erosion (DEmax) is dependant on identifying the total water level elevation, WL, which includes the combined effects of extreme high tides plus storm surge plus wave runup, relative to the elevation of the beach-dune junction (EJ). Therefore, when the WL > EJ the beach retreats landward by some distance, until a new beach-dune junction is established, whose elevation approximately equals the extreme water level. Since beaches along the high-energy Oregon coast are typically wide and Oregon Department of Geology and Mineral Industries OFR O-04-09

have a nearly uniform slope (tan ß), the model assumes that this slope angle is maintained, and the dunes are eroded landward until the dune face reaches point B in Figure 4. As a result, the model is geometric in that it assumes an upward and landward shift of a triangle, one side of which corresponds to the elevated water levels, and then the upward and landward translation of that triangle and beach profile to account for the total possible retreat of the dune (Komar and others, 1999). An additional feature of the geometric model is its ability to accommodate further lowering of the beach face due to the presence of a rip current. This feature of the model is represented by the beach-level change ΔBL shown in Figure 4, which causes the dune to retreat some additional distance landward until it reaches point C. As can be seen from Figure 4, the distance from point A to point C depicts the total retreat, DEmax, expected during a particularly severe event that includes the localized effect of a rip current. Critical then in applying the model to evaluate the susceptibility of coastal properties to erosion, is an evaluation of the occurrence of extreme tides (ET), the runup of waves (R), and the joint probabilities of these processes along the coast (Ruggiero and others, 2001). 3.2.1.1 Wave Runup

Detailed studies of wave runup along the Oregon Coast, under a range of wave conditions and beach slopes (Ruggiero and others, 1996; 2001), have yielded the following relationship R2% = 0.27(SHSOLO)½

(1)

for estimating the 2% exceedence runup (R) elevation, where S is the beach slope (tan ß), HSO is the deep-water significant wave height, LO is the deep-water wave length given by where T is the wave period, and g is acceleration due to gravity (9.81 ms-2). Therefore, estimates of the 8

Figure 3. The foredune erosion model (Komar and others, 1999). NGVD = national geodetic vertical

Figure 4. The geometric model used to assess the maximum potential beach erosion in response to an extreme storm (After Komar and others, 1999). Oregon Department of Geology and Mineral Industries OFR O-04-09

9

wave runup elevation depend on knowledge of the wave heights and periods. Since a major objective of this investigation is to estimate the maximum potential erosion (DEmax) that may occur in response to sustained periods of wave attack during extreme storm events (Figure 4), it is important to examine the probabilities of extreme wave occurrence offshore from the Pacific Northwest (PNW) coast. Wave data (wave heights and periods) have been measured in the North Pacific using wave buoys and sensor arrays for almost 30 years. These data have been collected by NOAA, which operates the National Data Buoy Center (NDBC), and by the Coastal Data Information Program (CDIP) of Scripps Institution of Oceanography. Previous analyses of these data through 1996 by Ruggiero and others (1996; 2001) indicated that the projected 100-year extreme storm would generate a deep-water significant wave height on the order of 33 ft. However, during the 1997-98 El Niño that height was exceeded by one storm, and by four 100-year storms during the 1998-99 La Niña winter, with the March 2-3, 1999 storm having generated deepwater significant wave heights of

46 ft (Table 1). Finally, a sixth 100-year storm occurred during the winter of January 2000. In response to the large wave events that occurred during the latter half of the 1990s, the wave climate of the eastern North Pacific has been re-examined to determine the probabilities of extreme wave occurrence offshore from the PNW coast (Komar and Allan, 2000; Allan and Komar, 2001). Using standard techniques of extreme value analysis, the 10- through 100year extreme values for the deep-water significant wave heights were determined for several wave buoys located along the West Coast of the U.S. These analyses yield 100-year storm wave heights that ranged from 46 to 55.1 ft, for four wave buoys offshore from the PNW coast. Apart from highlighting the extreme nature of the wave climate in the eastern North Pacific, these results also emphasize the variability of the wave climate along the coasts of Washington and Oregon due to deviations in the predominant storm tracks. To accommodate this type of variation in our analyses and for input into Equation 1, the extreme wave height estimates were averaged, so that mean 10- through 100-year extreme value

TABLE 1. PEAK STORM WAVE STATISTICS FOR THE NEWPORT WAVE BUOY FOR THE MAJOR 1997-98 EL NIÑO AND 1998-99 LA NIÑA (ALLAN AND KOMAR, 2002) Buoy #46050

Date

Significant wave height (feet)

Wave Period (s)

Wave Breaker height (feet)

El Niño (1997-98)

19-20 Nov.

34.5

14.3

38.4

La Niña (1998-99)

25-26 Nov. 6-7 Feb. 16-17 Feb. 2-3 Mar.

35.4 33.1 32.8 46.3

12.5 12.5 20.0 16.7

37.1 35.4 42.3 51.8

La Niña (1999-00)

16-17 Jan.

39.7

14.2

43.0


10

significant wave heights could be determined for the Oregon coast. These values are presented in Table 2. TABLE 2. AVERAGE EXTREME-WAVE PROJECTIONS BASED ON DATA FROM FOUR NDBC WAVE BUOYS LOCATED OFFSHORE THE PACIFIC NORTHWEST COAST Projection

Mean water elevation (feet)

5 10

39.7 44.3 47.6 49.2 52.5

25 50 100

Analyses have also been undertaken of the range of wave periods that are experienced in the eastern North Pacific (Komar and Allan, 2000). These data have been examined using joint-frequency graphs of the significant wave heights versus the spectral-peak periods, the latter being the region where most of the wave energy occurs. The analyses have revealed that the largest wave heights tend to correspond to spectral-peak periods that range from 15 to 17 seconds, with some storm events producing periods up to 20 seconds. Since Equation 1 is particularly sensitive to the magnitude of the wave period, we have focused on the longer period wave events in our modeling of wave runup elevations. 3.2.1.2 Tides

The elevation of the sea, in part controlled by the astronomical tide, is extremely important for the occurrence of beach and property erosion along the Oregon coast (Komar, 1986). This process is particularly enhanced when large waves are superimposed on top of elevated water levels, so that wave processes are able to reach much higher elevations on the shore. It is the comOregon Department of Geology and Mineral Industries OFR O-04-09

bined effect of these processes that invariably leads to toe erosion on coastal dunes and bluffs, and eventually shoreline recession. The actual level of the measured tide can be considerably higher than the predicted level provided in most standard tide tables, and is a function of a variety of atmospheric and oceanographic forces, which ultimately combine to raise the mean elevation of the sea. These latter processes also vary over a wide range of time-scales, and may have quite different effects on the coastal environment. For example, strong onshore winds coupled with the extreme low atmospheric pressures associated with a major storm, can cause the water surface to be raised along the shore as a storm surge. Along the PNW coast, the role of storm surges in coastal hazard applications has for the most part been ignored, largely because the storm surge elevations were thought to be quite small. For example, analyses of daily mean water levels up through 1996 at Newport, Oregon, revealed that the surges are typically of the order of 0.3 to 0.5 ft (Ruggiero and others, 1996). However, recent analyses of storm surges that occurred during the 1997-98 El Niño and 1998-99 La Niña winters revealed surges that were on the order of 1.3 to 2.0 ft, which suggest that much larger storm surge heights can be experienced along the PNW coast (Allan and Komar, 2002). As a result, any analysis of future coastal change should include a storm surge component. Much longer-term processes that depend on offshore water temperatures and ocean currents can also influence the monthly-averaged water levels observed along the coast (Komar and Allan, 2000). In particular, analyses of the South Beach, Yaquina Bay tide gauge located in Newport, reveal a seasonal increase in mean water levels along the Oregon coast that occurs between summer and winter. This seasonal rise in mean water levels is on the order of 0.7 to 1.3 ft, and is 11

a function of changes in the water temperature and effects from ocean currents (Komar and others, 2000). As noted earlier, major climate events such as El Niños can also have a dramatic impact on water level elevations along the U.S. West Coast. For example, during the 1982-83 El Niño, water levels along the Oregon coast were raised by about 1.6 ft, and remained elevated for several months (Huyer and others, 1983). These findings were reinforced in a subsequent investigation of water levels during the 1997-98 El Niño by Komar and others (2000).

next 100 years, which is likely to influence the long-term stability of shorelines in the PNW.

Having described the various process elements that are required as input into the geometric

Astoria

Lincoln City

Newport

Florence

3.2.1.3 Beach Morphology

Tillamook

1

Bandon

2

Coos Bay

3

Cresent City

Land Movement Relative to Sea Level (mm/year)

4

Gold Beach

Long-term trends in the level of the sea can also be identified along the Oregon coast, which relate to the global (eustatic) rise in mean sea level that has been occurring over the past several thousand years. However, these changes in mean sea level are complicated due to on-going changes in the level of the land that are also occurring along the Oregon coast. For example, Vincent (1989) demonstrated that the southern Oregon coast is rising at a faster rate than the global rise in mean sea level, whereas the northern Oregon coast, including Lincoln County, is being slowly submerged by the rise in mean sea level (Figure 5). Recent analyses of the Newport tide gauge data by Flick and others (1999), indicate that mean sea level is rising at a rate of 3.7 mm per year. Assuming this rate of sea level rise continues, we can expect a further increase in mean sea level of about 1.3 ft along the Lincoln coastline over the

It is therefore apparent that the Oregon coast experiences highly variable mean-water levels, with the occurrence of extreme high tides being a contributing factor to the development of erosion problems (Komar and others, 1999). To accommodate the huge variability in tidal elevations experienced along the Oregon coast, an extreme value analysis (similar to that used to estimate the probabilities of the extreme wave heights) has been used to analyze the tidal elevations for the South Beach, Yaquina Bay tide gauge (Shih and others, 1994; Ruggiero and others, 1996; 2001). Table 3 presents the 5- through 100-year expected extreme tide levels (ET) determined for the South Beach, Yaquina Bay tide gauge. These data are referenced to the National Geodetic Vertical Datum of 1929 (NGVD’29) datum. As can be seen from Table 3, the expected 50- and 100-year tide is on the order of 8.2 ft, and likely includes the effects of an El Niño. Furthermore, it is apparent from Table 3 that there is in effect little difference in the extreme tidal elevations estimated for the 5- through 100-year expected tides, with the difference amounting to only about 1.0 ft.

0 -1 -2 -3 -4

Land is rising faster than eustatic sea level rise 42

43

Land is being submerged by rising sea level 44 Latitude

45


Figure 5. Elevation changes along the Oregon coast, measured by geodetic surveys (Vincent, 1989). The elevation changes are relative to the global increase in sea level, with positive values representing a rise in the land at a higher rate than the increase in sea level, while negative values represent the progressive submergence of the land. [from Komar, 1997].

46

12

TABLE 3. EXTREME ANNUAL TIDES (SHIH AND OTHERS, 1994). NOTE ALL ELEVATIONS ARE RELATIVE TO THE NGVD’29 DATUM

Projection

Mean water elevation (feet)

5 10

7.2 7.5 7.9 8.2 8.2

25 50 100

model, it remains for the morphological variables of the beach to be determined. These last variables include determinations of the beach slope (tan ß) and the beach-dune toe elevation (EJ). In the absence of surveyed beach morphology data along the Lincoln County coast, a remote sensing technology, LIDAR, was used to assess the morphology of beaches at the end of the 1998 El Niño winter. These data were obtained from the National Oceanic and Atmospheric Administration (NOAA) Coastal Services Center website , operated in tandem with the United States Geological Survey (USGS) and NASA. The LIDAR data consists of x, y, and z values of land topography that are derived using a laser ranging system mounted on board a De Havilland Twin Otter aircraft. To measure the coastal topography, the aircraft flies at an altitude of approximately 700 meters at a rate of about 60 m.s-1, and surveys a several hundred meter wide swath of the shoreline, acquiring a value of the surface elevation every few square meters (USGS, 2000). Subsequent analyses of the LIDAR data by NOAA staff have revealed that the data have a vertical accuracy within ±0.5 ft, while the horizontal accuracy of these measurements are within ± 2.6 ft. As noted by the USGS, use of LIDAR enables hundreds of kilometers Oregon Department of Geology and Mineral Industries OFR O-04-09

of coastline to be mapped in a single day, with data densities that are unsurpassed using traditional survey technologies. Furthermore, subsequent survey runs using the same system can provide unprecedented data, which may be used to investigate the magnitude, spatial variability, and causes of coastal changes that occur during severe storms. All LIDAR data obtained from the NOAA/USGS/NASA website were in the 1983 Oregon State Plane Coordinate system, while the elevations were relative to the North American Vertical Datum of 1988 (NAVD’ 88). Once the LIDAR data were obtained from NOAA, the data were subsequently pruned (e.g. data points located in the surf zone were removed), and then analyzed using a triangulation approach to generate a grid data set. This process was accomplished using VERTICAL MAPPER (contour modeling and display software), which operates seamlessly within MAPINFO’s GIS software. Having generated the grid data, detailed contour maps and cross-sections of the beach morphology could then be constructed along the northern and central Lincoln County coastline. Identification of the beach-dune junction (EJ) was accomplished by inspection of the topographic maps contoured at intervals of two feet. Features used to distinguish the beach-dune junction included erosion scarps, major breaks in slope, or some combination. Average beach slopes west of the EJ were determined from representative cross sections across the detailed contour maps. Segments of shoreline with similar slope were identified and a these slopes were used in calculations of erosion from the geometric model. 3.2.1.4 Scenarios of Coastal Change in Lincoln County

The previous sections have described the ranges of variables required for input into the geometric model. This section discusses the three scenarios 13

used for modeling maximum potential erosion distances (MPED) on dune-backed beaches in Lincoln County. Figures 4 and 5 reveal that the measured tides (ET) and the wave runup levels (R) calculated from Equation 1 are combined to yield a total water level (WL) elevation, which is then input into the geometric model. When WL exceeds the elevation of the beach-dune toe, erosion occurs and the dune retreats landward until a new beach-dune toe is established, which approximately equals the total water elevation caused by the storm. However, the addition of the measured tides and wave runup components together, e.g. the 50-year runup level combined with the 50-year tide, is not as straightforward as it seems, due to the fact that these processes have been found to operate independently from each other (Komar and others, 1999; Ruggiero and others, 2001). In other words, the occurrence of an extreme storm does not necessarily mean that it will occur concurrently with an extreme tide. As a result, because both variables are occurring independently, it is necessary to consider their joint probabilities of occurrence, which is the product of the two individual probabilities. Thus, a 50year runup level combined with a 50-year tide would yield a joint return period of about 2,500 years (50 x 50 = 2500 years). To some degree, one can get around this problem by applying various combinations of the extreme tides plus the wave runup elevations. For example, a 50-year storm runup event may be combined with a 2-year extreme tide to yield a 100-year total water level. A better approach might be to evaluate the total water levels associated with particular storms, the combined mean-water level (tides + surge + El Niño effects) and the wave runup, and then analyze the probabilities of these levels (Komar and others, 1999). Analyses of this type however, have yielded values that closely approximate those derived using the approach that sums the Oregon Department of Geology and Mineral Industries OFR O-04-09

individual values, suggesting that either technique is useful. Finally, it should be noted that the analyses of extreme water levels undertaken previously (Shih and others, 1994; Ruggiero and others, 1996; 2001), excludes the most recent high water levels generated during the 1997-98 El Niño and 1998-99 La Niña winter. As a result, future efforts are planned to better establish the total water levels that may be experienced along the Oregon coast. In developing the three scenarios below, we have attempted to steer clear of such terminology as the 100-year extreme event, which can often be misconstrued. Instead, we have defined our scenarios according to high-, moderate-, and low-risk hazard zones, which respectively indicate decreasing probability levels of occurrence, with the high-risk scenario having the greatest chance of occurrence during the next 60 - 100 years. These time intervals are typical planning horizons of interest to coastal planners. Because of the difficulties of identifying the most appropriate combination of extreme high waves and tides, the following scenarios assume that a major storm occurs over the course of an above average high tide. This is consistent with the approach taken by Komar and Allan (2000) in developing their scenarios of high waves and water levels. Along the central Oregon coast, the Mean Higher High Tide averages about 8.38 ft (2.55 m) relative to Mean Lower Low Water. When converted to the NAVD’88 datum, this amounts to an elevation of 7.55 ft (2.3 m). Thus, when other variables are added to this, all of the elevations will be relative to the NAVD’88 datum. Scenario 1 describes a HIGH-risk hazard zone. The variables included in this scenario are: • 47.6 ft (14.5 m) significant wave height, • 17 second peak spectral wave period, • 7.55 ft (2.3 m) Mean Higher High Tide, 14

• 1.31 ft (0.4 m) monthly mean water level, • 3.28 ft (1.0 m) storm surge. This particular scenario is similar to the 2-3 March 1999 La Niña storm, which caused widespread damage along the Oregon coast. The scenario assumes that a major storm occurs over the course of an above average high tide. To accommodate the monthly rise in mean water levels between summer and winter, an additional 1.31 ft has been added to the high tide. Furthermore, because the extreme storms that occurred during the 1997-98 El Nino and 1998-99 La Nina winter produced significant storm surges, we have included a 3.28 ft storm surge component as part of this scenario. When combined, these data yield a water elevation of 12.14 ft relative to the NAVD’88 datum. Scenario 2 describes a MODERATE-risk hazard zone, and includes the following variables: • • • • • •

52.5 ft (16.0 m) significant wave height, 20 second peak wave period, 7.55 ft (2.3 m) Mean Higher High Tide, 1.31 ft (0.4 m) monthly mean water level, 5.58 ft (1.7 m) storm surge, 1.31 ft (0.4 m) sea level rise.

The MODERATE-risk hazard zone is one of two “worst case” scenarios. This particular scenario assumes that the rise in wave heights identified offshore from the PNW coast by Allan and Komar (2000a; 2000b; 2002) continues over the course of the next century. In effect, the 52.5 ft significant wave height used in this scenario is similar to the predicted 100-year storm wave shown in Table 2. The variables used to generate the water levels are the same as those shown in scenario 1, except that we have incorporated a larger storm surge component (5.58 ft). Furthermore, this scenario includes a 1.31 ft rise in mean sea level expected to occur over the next 100 years. This Oregon Department of Geology and Mineral Industries OFR O-04-09

rise in mean sea level is an estimate based on existing trends determined for the South Beach, Yaquina Bay tide gauge (Flick and others, 1999). This combination of events has an extremely low probability of occurrence. However, the results are still useful in that they provide a landward limit of potential erosion (assuming no long-term trends in the coast) due to a particularly severe storm. Scenario 3 describes a LOW-risk hazard zone, and includes the following variables: • • • • • • •

52.5 ft (16.0 m) significant wave height, 20 second peak wave period, 7.55 ft (2.3 m) Mean Higher High Tide, 1.31 ft (0.4 m) monthly mean water level, 5.58 ft (1.7 m) storm surge, 1.31 ft (0.4 m) sea level rise. 3.28 ft (1.0 m) lowering of the coast due to a Cascadia subduction zone earthquake.

The LOW-risk hazard zone is the second “worst case” scenario, and incorporates all of the variables used in scenario 2, but with the added feature of a Cascadia subduction zone event. These events have been shown to occur in response to large earthquakes in the Cascadia margin, and have a recurrence interval of approximately 500 years (Darienzo and Peterson, 1995; Geomatrix, 1995; Atwater and Hemphill-Haley, 1996). These types of events can cause some parts of the PNW coast to be abruptly lowered by 0 – 6.6 ft (Peterson and others, 2000). Because of the lower coastal elevations, wave processes will therefore be able to reach much further up the beach. As a result, it can be expected that wave erosion would be widespread under this scenario with extensive coastal retreat. Furthermore, the process of erosion is likely to persist for several decades until the coastal environment has achieved a new state of dynamic equilibrium, and as interseismic strain builds up on the locked Cascadia subduc15

tion zone interface. Under this scenario, we have adopted a value of 3.28 ft coseismic subsidence event for the Lincoln County coast, which is “typical” for this part of the northern Oregon coast based on paleoseismic analyses of previous subduction events (Peterson and others, 1997; 2000). 3.3 Bluff-Backed Shorelines 3.3.1 Introduction

This section describes a methodology whereby four coastal erosion hazard zones can be drawn for bluffs of Lincoln County. The basic techniques used here are modified from Gless and others (1998), Komar and others (1999), and Allan and Priest (2001). The zones are as follows: 1) Active hazard zone: The zone of currently active mass movement, slope wash, and wave erosion. 2) The other three zones define high-, moderate-, and low-risk scenarios for expansion of the active hazard zone by bluff top retreat. Similar to the dune-backed shorelines, the three hazard zones depict decreasing levels of risk that they will become active in the future, but an increase in the magnitude of erosion. These hazard zone boundaries are mapped as follows: a.

High-risk hazard zone: The boundary of the high-risk hazard zone will represent a best case for erosion. It will be assumed that erosion proceeds gradually at a mean erosion rate for 60 years, maintaining a slope at the angle of repose for talus of the bluff materials.

b. Moderate-risk hazard zone: The boundary of the moderate-risk hazard zone will be drawn at the mean distance between the high- and low-risk hazard zone boundaries. Oregon Department of Geology and Mineral Industries OFR O-04-09

c.

Low-risk hazard zone: The low-risk hazard zone boundary represents a “worst case” for bluff erosion. The worst case is for a bluff to erode gradually at a maximum erosion rate for 100 years, maintaining its slope at the angle of repose for talus of the bluff materials. The bluff will then be assumed to suffer a maximum slope failure (slough or landslide). For bluffs composed of poorly consolidated or unconsolidated sand, another worst case scenario will be mapped that assumes that the bluff face will reach a 2:1 slope as rain washes over it and sand creeps downward under the forces of gravity. For these sand bluffs, whichever method produces the most retreat will be adopted.

In order to understand how these zones are defined, it is useful to examine what variables are generally used for erosion hazard zone calculations and how they relate to the way bluffs actually erode. 3.3.2 The Bluff Retreat Model

Table 4 summarizes those variables that are generally used for calculating bluff erosion hazard zones (see Komar, 1997, Gless and others, 1998, and Komar and others, 1999 for further discussions), while Figure 6 illustrates those parameters, and one approach (of many) that may be used to map bluff hazard zones. Note that the major policy decisions used for delineating the hazard zones are: 1) Which hazard zones will be useful for planning, and; 2) What planning horizons (projected number of years in the future) should be used for erosion rate calculations. To understand how to apply these factors, it is 16

useful to discuss first how bluffs actually erode. Bluff erosion generally occurs in the following steps:

forcing event for failure may be a function of: •

1. Erosion of the bluff toe occurs in response to waves, and subaerial processes (weathering, slope wash, mass wasting, and wind erosion).

• •

2. Slope failure occurs and blocks of various sizes may slide, fall, or topple. The final

•

The critical slope stability angle is exceeded; Exposure of weak rock layers in the bluff face; Unusually high ground water pressure (i.e. pore pressure); Stress-release fracturing (see Hampton,

TABLE 4. SUMMARY OF BLUFF EROSION DATA THAT CAN BE USED TO CALCULATE BLUFF HAZARD ZONES. ONLY MAXIMUM OBSERVED (EMPIRICAL) BLOCK FAILURE WIDTH IS LISTED (VERSUS MOST PROBABLE OR AVERAGE WIDTH), BECAUSE THIS IS GENERALLY THE ONLY EMPIRICAL DATA THAT CAN BE EASILY OBTAINED IN MOST AREAS. QUANTITATIVE SLOPE MODELING OR REGIONAL EMPIRICAL ANALYSES WOULD BE REQUIRED TO ESTABLISH A MEAN OR MOST PROBABLE BLOCK FAILURE WIDTH. ANGLE OF REPOSE REFERS TO THE IDEAL SLOPE ANGLE FOR UNCONSOLIDATED TALUS OF THE BLUFF MATERIAL Erosion Data

Planning Horizon

Added to Account for Uncertainities

Average Erosion Rate (ft/year) Stable Slope Angle or Angle of Repose (Projected from the bluff toe to top) Maximum Block Failure Width (ft)

x 60-100 years

+ error (1-2σ or some %) + error (generally 10-50 %) + error (generally 10-50 %)

Not applicable x number of blocks per 60-100 years

&UTURE%ROSION(AZARD:ONE ,OW (AZARD :ONE

(IGH (AZARD :ONE

-ODERATE (AZARD :ONE

!CTIVE%ROSION(AZARD:ONE E OP SL ED LL EN WA EP AD STE HE OS E ER LIDE P /V DS 2E N OF LA GLE !N

"LOCKFAILURE E GL !N

WIDTH

2 OF OS EP E

LOCK 3LIDEB IDTH FAILUREW

7AVE%ROSION YEARS

YEARS

%ROSION

Figure 6. Schematic illustration of block failure on a bluff, angle of repose, and erosion rate in relation to possible hazard zones. These factors can be combined in a variety of different ways to produce hazard zones. Oregon Department of Geology and Mineral Industries OFR O-04-09

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• • •

2002, for explanation); Severe wave erosion event; Seismic shaking from an earthquake, or; Combination of any or all of the above factors.

3. The size of blocks that fall or slide is a function of the strength and the type of material, degree of weathering, its structure (bedding, jointing, faulting, and fracturing), and bluff height. If only small sloughs, topples, and falls of material occur, then the bluff will erode back gradually, maintaining a more or less constant slope angle until wave erosion is no longer effect in causing bluff retreat. Wave erosion generally keeps the slope steep enough so the forces tending to cause the bluff to suffer a slope failure are just balanced by the forces opposing failure; in other words, the ratio of these forces, or factor of safety, is equal to ~1.0. On the other hand, some bluffs subject to deep bedrock landslides retreat in a highly episodic fashion, resisting erosion for long periods and then failing in large slide blocks, once the factor safety decreases below 1.01. 4. Subaerial erosion (sheet wash, soil creep, etc.) becomes the main process of bluff retreat once waves cannot reach the bluff effectively, either because of slide debris or sand in front of the bluff or because the bluff has retreated so far that waves cannot effectively erode the base. Where landslide debris continues moving seaward in front of the bluff escarpment or headwall, additional block failures at an unstable headwall can also occur. These failures may occur virtually in lock step with slide 1

movement on highly unstable headwalls. If the toe of the bluff does get protected for an extended period of time and it is relatively resistant to large landslide failures, subaerial weathering, slope wash, and mass movement will erode the slope to progressively lower angles as talus accumulates in front of the bluff. This process will continue until the talus cone reaches the top of the bluff. At this point retreat of the bluff becomes extremely slow, and the slope will maintain an angle approximately equal to the angle of repose of the bluff material. Bluffs in this condition are generally vegetated from top to bottom. See the discussion of below on angle of repose for examples of this process. In drawing erosion hazard zones, we will endeavor to emulate these various modes of bluff retreat. Predicting whether a particular part of a bluff will erode away over the life of a proposed development depends on understanding the influence of several parameters. These include: • • • • • • • •

•

Bluff slope; Bluff height; Bluff material properties Vulnerability to stress-release fracturing; Groundwater level and resulting pore pressures; Surface water runoff; Wave climate; How much and what type of material is present at the toe of the bluff (e.g. slide debris, dune sand, logs, gravel, etc.) that can buttress the slope, dissipate wave energy, or, in some cases, act as ballistics that will erode the bluff; Whether any buttressing material is mov-

The factor of safety is the ratio of forces resisting slope failure to forces promoting failure.


18

•

ing seaward (active slide blocks), and; Vegetative cover.

It is critical to understand that each bluff must be judged based on the local geology, likely future climate (rainfall and storms), and its current state of instability in the erosion cycle. Owing to limitations of this regional county investigation, we will only be able to take into account parameters such as the bluff slope, height, material properties (rock or soil composition), and the historical response of broad classes of bluff to coastal erosion. As a result, detailed, site-specific investigations are necessary to provide projections of the erosion hazard for a particular development on coastal bluffs. This report is no substitute for site-specific investigations.

3.3.3 Data Used for Drawing Bluff Erosion Hazard Zones 3.3.3.1 Angle of Repose

Overall slope angles in the study area are ~34° ± 2º (1.5 horizontal: 1 vertical) or ~45° ± 2º (1:1). The former slope is characteristic of talus slopes in Quaternary or Tertiary sedimentary material, while the latter is for talus-laden slopes of basalt. The angle of repose for loose clean sand is 33° 41’, while 45° is the angle for hard weathered rock (Merriman and Wiggin, 1947), so these values make sense for slopes composed of talus of these bluff materials. Vegetated, talus-laden slopes of Pleistocene

Figure 7. Gradual versus episodic bluff erosion. Note how the landslide toe position remains stable as the headwall retreats; hence erosion rate for bluffs with landslides is better measured at the headwall. Also note that the dangerous part of the bluff is much wider for the landslide-prone bluff, since another maximum block failure could occur at any point in the erosion cycle.


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paleosol and colluvium overlain by Pleistocene marine terrace deposits at Nye Beach are at a mean angle of 34° (1.5:1), while the same material in the same area reaches an overall slope angle of 40-42° where wave exposure prevents vegetation growth. Sea cliffs of Pleistocene marine terrace sand at Lincoln City for the most part lack vegetation or significant talus and lie at an average angle of 54° but can be near vertical in many areas. In the Taft area of Lincoln City dunes protect the bluff from erosion, allowing the entire bluff to be vegetated. Overall slope angle is ~3439°(1.5:1 to 1.2:1) at Taft, and bluff erosion rate is negligible (0-0.1 ft/yr, according to Priest and others (1994). Steep slopes like the non-vegetated slopes at Lincoln City are maintained in Quaternary sedimentary deposits by both wave action and stressrelease fracturing. In all types of Pleistocene soil and sand deposits, there are local exposures near the top of the coastal bluff that tend to reach near-vertical slope owing to stress-release fracturing and cantilevered block falls2 (Figures 8 and 9; see Hampton, 2002, for explanation). These failures are facilitated by reduction of cohesion from groundwater saturation (Hampton, 2002). Talus accumulation on the bluff face eventually shuts off this process (Figure 10); thereafter erosion is by gradual soil creep or episodic events like debris flows, big storm waves, or slide block failures. Where wave energy is sufficient to clean off talus and vegetation, the entire sea cliff can become near vertical (Figure 11). If continuously attacked by waves, such slopes will remain steep and never reach the angle of repose of the talus material. The stress-release block fall process does not mean that larger block failures, either rotational slumps or translational slides, do not occur in these Pleistocene deposits. In fact slumps are common in the same areas where gradual erosion

by cantilevered block fall is occurring (Figure 12). That is why hazard zones mapped in this investigation take into account block failures in addition to slope angle and erosion rate. Many hard sandstone and jointed basalt cliffs are near-vertical and appear to have persisted in this condition with only minor erosion for many decades. Clearly these slopes are maintained by vigorous wave erosion that undercuts the bluff and removes talus. In most cases the near-vertical cliff gives way up slope to progressively lower slope angles, forming a curving, convexup profile (Figure 13). At Cape Foulweather the basalt bluff has slopes of 67-86° from sea level to ~60-90 feet elevation but an overall slope of 3457° (mean of 45° for 11 spot measurements) from the toe to the east boundary of the zone of rapid soil creep (active hazard zone) at ~100-440 feet elevation. Heavily vegetated slopes with deeply weathered basalt and colluvial deposits above the active hazard zone boundary are on the order of 30° and lower. In estimating a stable slope for these rocks, a 1:1 (45°) slope is thus appropriately conservative. Bluffs composed of Tertiary sedimentary rocks (mudstone, siltstone, tuff, and sandstone of the Miocene Astoria and Nye Formations) are mostly at~36-60° (mean of ~45°) where bare of talus and continuously eroding (Figure 14); nevertheless, undercutting by waves can also produce nearvertical slopes (Figure 15). Hard sandstone bluffs of upper Astoria Formation at the Inn at Otter Crest and Devils Punch Bowl have near vertical slopes because they are headlands fully exposed to wave attack. In measurements near the North Jetty at Yaquina Bay (Figure 16), a vegetated, talus-covered bluff of Astoria Formation sandstone, siltstone, mudstone, and tuff has slopes of 31-41° averaging about 36° (1.4:1). The simplifying assumption of a 1.5:1 stable talus slope for

2

According to Hampton (2002) cantilevered block falls are protrusions on the cliff face or thin tabular blocks that fall off and leave behind a near vertical cliff surface. The initial failures commonly leave an arch-shaped overhang. They occur in weakly lithified sea cliffs owing to release of horizontal confining stress as increasing groundwater saturation decreases sediment cohesion. Individual failures are generally less than 1 cubic meter and only the outer meter or so of sediment is removed in any one failure episode. Oregon Department of Geology and Mineral Industries OFR O-04-09

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4 feet

Figure 8. Near vertical upper slope of otherwise vegetated sea cliff composed of Pleistocene marine terrace deposits and grass-covered talus in the exposure. Note the scar of a recent cantilevered block failure (see text explanation of this process). Rip rap was installed at the bluff toe to protect it from wave erosions.


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Cantilevered block failure

4 feet

Figure 9. Close up of scar of a cantilevered block failure from photograph of Figure 8. Oregon Department of Geology and Mineral Industries OFR O-04-09

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Figure 10. Nye Beach view of stable ~1.5:1 (horizontal : vertical) slopes (measured from LIDAR elevation data) of vegetated talus developed on Pleistocene paleosols (lower half of bluff) overlain by Pleistocene marine terrace sands (upper half of bluff). Upper picture was taken sometime before ~1917-1920; left picture is a 2001 close up of the north end of the bluff in the right hand picture. Note the historic Sylvia Beach Hotel in the upper picture appears on the left side of both lower pictures. It is clear that this hotel has been about the same distance back from the bluff edge for ~100 years, so bluff top retreat in front of the hotel has been near zero. Hence, once talus cover prevents further erosion by cantilevered block fall, gradual erosion at the angle of repose is near zero. An extreme rainfall event caused a small debris flow that stripped talus from the bluff where the small group of people are standing in lower right hand photo. Rejuvenated stress-release fracturing and block falls are continuing to erode the top of the bluff from this point south.


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Figure 11. Near vertical sea cliff of Pleistocene marine terrace deposits in the Lincoln City area (photo courtesy of Paul Komar). Cliff erosion is from undercutting by waves and cantilevered block fall driven by stress release fractures. Wave attack is vigorous enough to remove all talus from this bluff, resulting in a near-vertical slope continuously exposed to further erosion.

Figure 12. Slump block about 30 feet wide in Pleistocene paleosol, laminated carbonaceous clayey silit and sand deposits at Nye Beach. Note that cantilevered block falls are keeping the upper part of the bluff near vertical. Note also that this slump and the cantilevered block falls occurred on a well-vegetated slope. While the vegetation does not prove that wave undercutting did not trigger this slump, the vegetation is suggestive that waves were probably not the main factor for either type of failure. Groundwater saturation probably played a more significant role in this particular case. Oregon Department of Geology and Mineral Industries OFR O-04-09

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.EAR VERTICALCLIFF

Figure 13. Cape Foulweather area illustrating steep lower sea cliffs but overall 1:1 slope in the zone of rapid soil creep that directly responds to wave erosion (active hazard zone). Note the debris flow chute in the center of the slide. This is where Highway 101 fill collapsed into a rapidly moving debris flow during heavy rains in December 1999. Older slide scars are also present. Photograph is courtesy of the Oregon Department of Transportation.

Figure 14. Talus-free slopes in mudstone of the Astoria Formation at Beverly Beach on the south side of Spencer Creek (at bridge in background). Slopes average about 1:1, horizontal to vertical. The orange unit in the middle background is Pleistocene to Holocene colluvium that mantles the valley walls north and south of Spencer Creek. Paleosols (ancient soils) of this unit also underlie the beach around the mouth of Spencer Creek. These deposits have ancient tree stumps that are exposed when sand is removed by winter wave erosion. A Carbon-14 age of outermost wood on a stump on the beach at Spencer Creek (just off the upper left edge of the photo) is 3,480 ± 60 years before present (B.P.) and records the last time that relative sea level was low enough to allow trees to grow as low in elevation as present sea level. The sample was collected in this investigation after winter waves had removed the beach sand covering the ancient forest soils that mark the seaward extension of the ancestral Spencer Creek valley. Oregon Department of Geology and Mineral Industries OFR O-04-09

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Figure 15. Wave erosion of a fine-grained sandstone below harder, blocky jointed tuff of the Astoria Formation at Nye Beach produces a vertical to overhanging slope. Tuff is hardened by zeolite cement (Alan Niem, 2003, written communication). Note basalt rip rap in background provides protection to the public walkway.

Figure 16. Vegetated, talus-covered bluff of Tertiary sedimentary rock overlain by Quaternary marine terrace deposits at the mouth of Yaquina Bay. Most of the slope is on the order of 1.4 to 1.5:1, horizontal to vertical and has not been subjected to significant wave erosion since construction of the jetties in the late 1800’s.


26

these sedimentary rock bluffs is thus appropriately conservative. Bluffs in Lincoln County fronted by significant sandy beaches from Fogarty Creek south are composed of west-dipping (15-20º) Tertiary sedimentary rocks at the base and unconformably overlying, near horizontal Pleistocene marine terrace or paleosol deposits in the upper part. Erosion occurs by both wave attack and subaerial erosion. The softer, more permeable Pleistocene deposits generally have significant groundwater, while the underlying Tertiary rocks are less permeable, leading to springs and seeps at the contact between the two units (Figure 17). The resulting accelerated subaerial erosion at these contacts, coupled with wind erosion of marine terrace sands, and attack by the highest storm waves often causes upper part of the bluff to

retreat somewhat faster than the hard rock toe (Figure 17). A stable slope angle for these situations is best constructed by projecting the slope from each geologic contact or break in slope. The same is true for any bluff composed of multiple rock units. Slope angle must therefore be used in conjunction with a thorough understanding of the geology of the bluffs. Rock type (lithology) and any tendency toward slide block failures must be taken into account. For example, the bluff edge could retreat much farther than might be predicted from the simple angle of repose approach, should the bluff be subject to large slide block failures. Nevertheless, in spite of some uncertainty about how large a block might fail, and the slight over estimation of the slope of repose for hard rock bluffs, the slope angle approach is

Figure 17. Nye Beach view of gently dipping (strike N9ºW; dip 13ºW) Astoria Formation siltstone and sandstone overlain by Pleistocene paleosols and marine terrace deposits. The Astoria Formation is an aquitard. When groundwater percolating downward through the permeable Pleistocene units encounters the Astoria Formation, it flows laterally along the contact, producing springs. The softer Pleistocene deposits are eroding faster than the Tertiary rocks in response to a combination of groundwater-driven soil creep, wind erosion, and attack by the largest storm waves. Oregon Department of Geology and Mineral Industries OFR O-04-09

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TABLE 5. SLOPES OF REPOSE BY MATERIAL TYPE Bluff Material

Slope of Repose (horizontal:vertical)

Basalt Tertiary sedimentary rocks Pleistocene marine terrace deposits, paleosols, colluvium, and alluvium

a relatively straight forward way to get a quick assessment of the likelihood of slope failure and where the bluff edge might eventually be located. Table 5 summarizes slopes of repose that are used to estimate bluff retreat in this study. On bluffs with an existing landslide, the slope

1:1 1.5:1 1.5:1

of repose was projected from the inferred base of the headwall landward to the bluff top using the techniques of Allan and Priest (1991, their Appendix D). For bluffs composed of seaward dipping Tertiary sedimentary rock, the following formula was used where an existing large translational slide was present (Figure 18):

Equation 1: S = R [D+ H – (tan β (D - L tan µ)/(tan β – tan α))] S = horizontal distance from the slide contact at the foot of the headwall to the projected intersection of the slope of repose behind the headwall α = shear plane dip below the headwall = 87° β = dip of the main slide plane beneath the slide block = 3° or 18:1 R = slope of repose (cotangent of angle of repose) for the headwall material = 1.5 H = Vertical height of the exposed headwall D = (Elevation of slide top at foot of headwall) – (elevation at the slide toe) L = Horizontal width of the slide mass from toe to its top at the foot of the headwall

3 (

A ²

$

,ANDSLIDE

,

Figure 18. Projection of angle of repose for landslides in Tertiary sedimentary rocks. Oregon Department of Geology and Mineral Industries OFR O-04-09

28

The 1.5:1 slope of repose is the value assumed for Tertiary sedimentary rocks and marine terrace sands that are typically found in the headwall of most large landslides in the area. The headwall shear plane dip of 87° dip is based on the inclination of a fresh scarp exposed in Quaternary marine terrace sand at the headwall of the Johnson Creek Landslide near Otter Rock (Figure 19). Although it is likely that this shear angle becomes shallower (smaller) in the subsurface, we will, to be conservative, assume that the 87° dip persists at depth when projecting location of headwall-slide plane intersections for landslides with similar geology (Quaternary marine terrace sand capping seaward dipping, fine grained Tertiary sedimentary rock). The 3° slide plane dip is taken from observations of the minimum dip of the slide plane for the Schooner Creek landslide (Allan and Priest, 2001). This dip is similar to the 3.5° dip of the slide plane inferred from geotechnical data at the Mark Street landslide in Newport (interpretations of unpublished geotechnical data from the City of Newport, 1999-2000), the 4° dip inferred in a cross section by Swanson (1974) through the Moolack Beach Landslide, and the mean dip of ~6° that can be fit through the curving slide plane at the Johnson Creek Landslide (see further discussion and cross section of the Johnson Creek Landslide in the section on block failures). Assuming the lowest possible dip is a conservative assumption, because it yields the deepest possible projected depth to the headwallslide plane intersection and therefore the largest S value. 3.3.3.2 Erosion Rate Data 3.3.3.2.1 Buff Top Retreat

Time and funding for this project were insufficient to carry out a detailed analysis of mean rates of bluff top retreat from combinations of gradual toe erosion and episodic block failures. This type of analysis requires precise measurement of local bluff retreat on historical photos through detailed field measurements tied to Oregon Department of Geology and Mineral Industries OFR O-04-09

geographic markers, and by rectification of historical photography. Owing to the infrequency of block failures, especially very large ones, large observation times are necessary to establish an overall rate of retreat. Whereas there is some spot bluff top retreat data for the County from a previous investigation (Appendices E and F), this information is sparse and subject to large measurement errors (Priest and others, 1994). The only place with an observation period long enough to begin to provide a sense of this longterm bluff top retreat was at Newport where bluff top or landslide headwall position on an 1868 topographic map could in some places be compared to modern photos and maps (see digital file 1868or1900_to_1993rates; Appendix E). Even these measurements were subject to large error owing to significant error in the 1868 survey and difficulty in matching geographic features to modern maps (Priest and others, 1994). Another problem with the 1868 data are that erosion rates in the Nye Beach area of Newport have probably changed significantly since 1868, because jetties constructed since that time have caused beach and dune sand accretion. 3.3.3.2.2 Bluff Toe Retreat

Bluff toe retreat is a better estimate of gradual wave erosion not influenced by large block failures. In this study we are treating block failure events as a separate variable, so it is important to obtain some estimate of gradual erosion of bluff toes not influenced by large (≥ ~40 feet-wide) blocks. We tried to establish some conservative estimates of these rates by rubbersheeting historical photography to modern orthophotos in areas that are fully exposed to wave action for large portions of the year (Appendix G). We also had access to observations of toe retreat by Mathiot (1973; Appendix D) who monitored a series of survey stakes over the 1972-1973 winter. In some cases there are features on historical photos that may be scaled to the same features on 1993 orthophotos. In a few areas there were 29

/TTER#REST,ANDSLIDE

/ 4 4 % 22/#+ Johnson Creek Landslide

Beverly Beach Landslide

Carmel Knoll Landslide

Moolack Beach Landslide

Schooner Creek Landslide

Yaquina Head Landslide

Jumpoff Joe Landslide

NEWPORT

Mark Street Landslide

0

1

2

Yaquina Bay

miles Figure 19. Location of major landslides discussed in the text. All of these slides are translationsl block slides in finegrained Tertiary sedimentary rocks (siltstone and mudstone). See Appendix B for detailed location and geology of each slide. Oregon Department of Geology and Mineral Industries OFR O-04-09

30

enough such features on both 1939 and the 1993 aerial photos to allow crude rubber sheeting of the 1939 photos to the 1993 orthophotos, using MapInfo geographic information software. In these areas continuous 1939 and 1993 bluff toe positions could be compared for hundreds of feet north-south. A mean erosion rate for the entire shoreline segment could then be calculated by weighting the east-west change in bluff toe position with the length of affected shoreline north-south. This weighted mean then gives a good overall measure of bluff toe retreat for some typical beach and bluff conditions. Table 6 summarizes the data generated by this technique. The rates for all of these areas compare favorably with earlier spot erosion rate estimates by Priest and others (1994) and Mathiot (1973) with one exception. The 935-foot segment of Gleneden Beach analyzed here has a rate about half of that calculated from the mean of 15 house-to-bluff edge rates of Priest and others (1994). Examination of historical photos for the 935-foot segment reveals no evidence of rip cell embayments known to accelerate erosion at Gleneden Beach; hence the segment is not representative of the worst-case for erosion. The spot rate data show that rip embayments produced “hot spot” erosion of up to ~60 feet between 1967 and 1993, much of this occurring in a few severe storm events. When two spot rates of -2.33 ft/yr from one rip embayment are subtracted from 1994 data set, the mean bluff top retreat is -0.4 ft/yr, close to the -03.ft/yr obtained here from a continuous segment of shoreline. Mathiot (1973) monitored survey stakes at Gleneden Beach for a 250-day period and found 3.75 (-0.45 ft/yr), 4.0 (0.49 ft/yr), and 14 inches (-1.7 ft/yr) of bluff erosion at three widely separated sites, illustrating the high variability of erosion in this setting. On the 1993 photos rip cell embayments comprise about 20 percent of the Lincoln Beach-Gleneden Beach shoreline, so it is not surprising that the segment chosen here for analysis lacked such embayments. The embayments are also known to migrate along Gleneden Beach, so any area Oregon Department of Geology and Mineral Industries OFR O-04-09

could experience this accelerated erosion at some point over a 100-year period, although it would be necessary for an embayment to coincide with one or more particularly intense storm seasons to produce the highest erosion (~60 feet). The rate of rip cell migration is unknown so the probability of any area experiencing the worst-case erosion is unknown. Komar (2002) summarized the evidence that erosion should be greatest at Gleneden Beach relative to the rest of the Siletz littoral cell, owing to the coarseness of the sand there. The following quote summarizes his analysis: “The longshore variations in beach sand grain sizes in turn produce systematic longshore changes in the beach slope, in the character of the surf-zone processes, and to the morphodynamic responses of the beaches to winter storms, all documented by the collection of beach profiles at intervals along the length of the cell (Shih and Komar, 1994). Again the results conform with the morphodynamics model of Wright and Short (1983), the beach being most dynamic and sea-cliff erosion impacts greatest in Gleneden Beach toward the center of the cell, and progressively decreasing to the north and south as the beaches become finer grained and more dissipative.”

The data here suggest that where there is no lowering of the beach profile by a rip embayment, the overall bluff erosion rate is similar to that of the fine-sand dissipative beaches with either Tertiary sedimentary rocks or Pleistocene Marine Terrace deposits at the bluff toe. The essentially straight coastline at the Siletz littoral cell suggests that the overall erosion rate at Gleneden Beach cannot be greatly different than in areas north and south; nevertheless, Komar (2002) correctly points out that Gleneden Beach bluffs can experience severe “hot spot” erosion events much larger than at dissipative beaches. Gradual shoreline retreat for all sedimentary rock and soil bluffs is thus assumed to be~-0.3 feet per year where there is a significant sandy beach. Where there is little sand, as at the sand-starved Beverly Beach 31

TABLE 6. BLUFF TOE EROSION DATA FOR LINCOLN COUNTY BASED ON COMPARISON OF 1993 DIGITAL ORTHOPHOTOS TO 1939 AIR PHOTOS ALONG CONTINUOUS LENGTHS OF SHORELINE, SO RATES ARE DERIVED FROM EAST-WEST CHANGE OF THE BLUFF TOE OVER 54 YEARS. SEE APPENDIX G FOR DETAILS. STD. DEV = STANDARD DEVIATION CALCULATED BY WEIGHTING THE INDIVIDUAL DEVIATIONS FROM THE WEIGHTED MEAN RATE BY SHORELINE LENGTH APPLICABLE TO EACH MEASUREMENT. ERROR IS MEASUREMENT ERROR ESTIMATED FROM LATERAL DIFFERENCES IN LOCATION BETWEEN GEOGRAPHIC FEATURES COMMON TO THE TWO PHOTOS; NYE MS = NYE MUDSTONE; ASTORIA = ASTORIA FORMATION SANDSTONE AND SILTSTONE. Location

Beach type

Bluff toe composition

Erosion Rate (ft/yr)

Error (ft/yr)

Std. Dev. (ft/yr)

N-S length (ft)

Wade Creek (Beverly Beach)

Dissipative3; sandstarved

-0.81

0.40

0.08

1727

Holiday Beach to Lost Creek State Wayside Lincoln City (Wecoma Beach) Gleneden Beach

Dissipative, fine sand

Tertiary Sedimentary Rock (Astoria) Tertary Sedimentary Rock (Nyems) Marine Terrace Sand Marine Terrace Sand

-0.30

0.13

0.02

3423

-0.31

0.19

0.01

1182

-0.30

0.37

0.01

935

Dissipative, fine sand Reflective, coarse sand

3

According to Komar (1998, p. 47), a dissipative beach is “the type having a low-sloping profile, such that waves first break well offshore and continuously lose energy when they travel as breaking bores across the wide surf zone.”

littoral cell, overall bluff toe retreat is assumed to be~-0.8 feet per year. The large measurement error at both Gleneden Beach and at Beverly Beach (Table 6) casts doubt on the significance of either measurement, but until better data can be obtained by rectification of the 1939 aerial photography, these values are all that are available. In the case of Beverly Beach, Mathiot (1973) did a high-precision measurement of erosion at the toe of the bluff where the –0.8 ft/yr measurement in Table 6 was generated. He monitored the bluff toe change for a 250-day (September 1972 to May 1973) observation using a semi-permanent survey marker. The short observation interval and small number of data points in the Mathiot study make his data of questionable accuracy. Nevertheless, his rate works out to –0.82 ft/yr in this same locality. While this lends some confidence to the accuracy of the rubber sheeting technique for eroOregon Department of Geology and Mineral Industries OFR O-04-09

sion rate measurement at this locality, it does not prove that it is representative of the erosion rate in the Astoria Formation throughout the entire cell. For example, six spot rates by Mathiot in Astoria Formation from other parts of the Beverly Beach littoral cell have a mean of –0.6 ft/yr, because four of them are about –0.5 ft/yr; two are north and two south of the mid-cell locality chosen for the rubber sheeting measurement. Using –0.8 ft/yr for the entire cell is thus considered a conservative (high) rate; -0.5 ft/yr is probably more representative of a mean rate for the cell as a whole. While none of these data are particularly compelling in terms of accuracy, they are all that is available until the 1939 aerial photography can be ortho-rectified. Based on the modest amounts of erosion for the 54-year observation interval (i.e. ~17 feet or less for all but Beverly Beach) and observations of 32

bluff geology, all rates in Table 6 represent gradual bluff retreat rather than episodic failure of large (≥ ~40 feet) blocks. Even in the case of Beverly Beach, where the mean erosion was ~43 feet over the 54 year observation period, large block failures are probably not possible in the particular segment chosen for rate measurement. Retreat of the bluff toe at that locality is by gradual wearing away of a rampart of seaward-dippingTertiary sedimentary rock several feet high. The bluff above this rampart is composed of shallow landslide debris and colluvium that is so disaggregated that large block movements are unlikely and would not affect the toe erosion rate, even if they occurred (Figure 20). According to Benumof and Griggs (1999) the most important control on the rate of bluff erosion is material properties, if the bluff is not protected from open wave attack by a dune field or some other barrier. For hard rock (basalt and hard sandstone) headlands, this condition always applies, since they lack significant protection from beach sand. Basalt and hard sandstone bluffs in the study area have erosion rates that are smaller than the measurement error. Allan and Priest (2001) found that basalt and hard sandstone bluffs in Tillamook County eroded at about -0.1 ± 0.1 ft/yr. This rate will be assumed here. Table 7 summarizes the bluff toe erosion rate data that will be used in this investigation. All of the rates are similar to those used by Allan and Priest (2001) for Tillamook County, except for the rates for Quaternary deposits. Rates for Quaternary deposits in the Allan and Priest (2001) study were adjusted to a 100 percent safety factor by multiplying the values by 2. This factor was added, because (1) Quaternary deposits, owing to the weakness of the material, are the most responsive to variations in wave attack from short-term changes in the beach; and (2) Allan and Priest (2001) found that there was widespread accretion on many beaches in Tillamook County during at least part of the time of observation for rate meaOregon Department of Geology and Mineral Industries OFR O-04-09

surements (1939 to 1994 and 1967 to 1994), biasing measured rates toward lower values. Most of the Quaternary deposits at beach level in Lincoln County are in the Lincoln City-Gleneden Beach area (Siletz littoral cell). There seems to be no obvious evidence in the Siletz cell of accretion noted in the Tillamook study (Figures 21 and 22); hence no additional safety factor is added here. The factor was also more appropriate for the Tillamook County bluffs of Quaternary deposits, since the vast majority of them were large Holocene dunes that could respond very rapidly to even short term changes in shoreline conditions. In Lincoln County Quaternary deposits in coastal bluffs are mostly Pleistocene marine terrace sands that are much more consolidated than the Holocene dunes of Tillamook County. These consolidated sands will probably respond much less quickly to short-term shoreline changes than the Holocene dunes. 3.3.3.3 Gradual Subaerial Erosion

While the above data are useful for wave erosion estimates, rates of gradual subaerial erosion from wind, slope wash, and soil creep are also important. For example, at the Jumpoff Joe landslide, a large translational block slide in Newport, about 160 feet of slide debris was removed by waves between 1939 and 1993, giving an erosion rate of about -3 feet per year. Until this slide debris is removed by waves, retreat of the headwall behind the landslide will be by subaerial processes such as wind, sheet wash, and soil creep. In other areas dunes effectively prevent wave erosion, so subaerial erosion is the dominant factor. Again, since we are treating episodic block failures as a separate variable, we need to estimate gradual subaerial erosion in areas where there has not been large block failures during the observation interval. We attacked this problem by looking at high precision house-to-bluff data and by rubbersheeting historical air photos to modern orthophotos in places where we could see from 33

Figure 20. Wade Creek area of Beverly Beach where bluff toe erosion rate was estimated from 1939 and 1993 air photos. Low rampart of shallow marine Astoria Formation sandstone is overlain by highly disaggregated landslide debris. Erosion of the bluff toe by failure of large blocks is unlikely in this geologic setting. Bluff toe erosion is most likely by gradual wearing away of small pieces of the sandtone. This is a winter beach condition; this area is covered in beach sand in the summer.

TABLE 7. OPEN COASTAL BLUFF TOE EROSION RATES ASSUMED FOR CALCULATIONS OF COASTAL EROSION HAZARD ZONES IN LINCOLN COUNTY. THE MEAN PLUS MEASUREMENT ERROR PLUS ONE STANDARD DEVIATION FROM THE MEAN IS USED FOR A MAXIMUM EROSION RATE IN EACH CASE EXCEPT THE BEVERLY BEACH LITTORAL CELL. Estimated Mean Bluff Toe Erosion Rate (ft/yr)

Conservative Rate (ft/yr)

Basalt Resistant sandstone Fine-grained Tertiary sedimentary rocks at the Beverly Beach littoral cell

-0.1 -0.1 -0.5

-0.2 -0.2 -0.8

Fine-grained Tertiary sedimentary rocks at all other areas

-0.3

-0.45

Quaternary deposits at Gleneden-Lincoln Beach

-0.3

-0.68

Quateranary deposits at all other localities

-0.3

-0.51

Rock Type


34

Figure 21. Blue line is 1994 shoreline from the USGS base map; red line is a 1927 shoreline from National Ocean Service T-sheets; the dashed line is the 1998 shoreline derived from 1998 LIDAR elevation data. There is much less difference in position between these shorelines than the same shorelines plotted for Tillamook County littoral cells, where the 1927 shoreline is well east of the 1994 shoreline.


35

Figure 22. Same age shorelines as in Figure 21 but at Gleneden Beach in the Siletz littoral cell.


36

the photography that deep-seated slide blocks had not formed during the observation period.

digital file House_bluff_eros_rates2). These rates average –0.08 ± 0.08 ft/year or ~-0.1 ft/yr.

In coastal Lincoln County subaerial erosion is probably at a maximum in freshly formed, near-vertical escarpments of Quaternary marine terrace sand. Sloughing, sheet wash and wind erosion should all be highly effective in removing the friable sand from such cliffs. A landslide headwall like that behind the large Jumpoff Joe landslide is an ideal case study of maximum subaerial erosion in isolation from wave erosion, since the headwall was formed only a few decades ago, is tens of feet high, near vertical, composed of terrace sand, and protected from wave erosion by the landslide. We were able to use rubbersheeting to obtain two estimates of subaerial erosion rate. One was obtained from analysis of 506 feet of bluff (north-south) on a 1967 ODOT photo; the other from 637 feet of bluff on a 1939 USACE photo. The rate from the 1939 photo is -0.5 ± 0.6 ft/yr; for the 1967 photo the rate is -0.4 ± 0.4 ft/yr; and the mean erosion rate is -0.4 ± 0.5 ft/yr, weighted for shoreline length measured and measurement error. Two high precision spot rates measured by scaling house-to-bluff distances from field measurements to the 1939 air photo yielded rates of –0.45 ± 0.29 ft/yr and -0.51 ± 0.29 ft/yr (Priest and others, 1994; digital file House_ bluff_eros_rates2). We conclude that subaerial erosion rate of Quaternary marine terrace sand escarpments can be as high as -0.5 ft/yr.

What about the maximum subaerial erosion rates in headscarps composed of hard rocks like sandstone and basalt? No large active or potentially active landslides have been mapped in coastal Lincoln County in resistant sandstone or basalt, so this problem does not come up for the current exercise. One would expect that subaerial erosion of headscarps in these rocks would be exceedingly slow, probably far less than the wave erosion rate of –0.1 ft/yr (Table 7). We conclude that gradual subaerial erosion rate is at most ~-0.5 ft/yr for the most erosion-prone coastal bluffs, ones that are near-vertical, tens of feet high, and entirely or mostly composed of Pleistocene sediment. Vegetated bluffs with seaward slope near the angle of repose are generally guarded from erosion by wide beaches, dunes, or slide debris and have gradual subaerial erosion rates no larger than ~-0.1 ft/yr.

What about coastal bluffs that are protected from most wave erosion but are at a later stage of erosional development with vegetated slopes close to the angle of repose? As illustrated in Figure 10, in areas without vigorous wave erosion, vegetated slopes at the angle of repose, even in soft Pleistocene sediment have subaerial erosion rates near zero. High precision house-to-bluff erosion rate measurements at a vegetated, dune-guarded bluff of Quaternary marine terrace sand at or near the angle of repose in southern Lincoln City also had a near-zero erosion rate (see data points b292, b293 and b294 of Priest and others, 1994; Oregon Department of Geology and Mineral Industries OFR O-04-09

For mapping erosion hazard zones we will assume that an area could become vulnerable to bluff top retreat from subaerial processes if it is on a sedimentary rock or Pleistocene sediment bluff guarded by slide debris or dunes that could protect the bluff from wave erosion during some part of the planning horizon (e.g. 60-100 years). If the bluff is at the angle of repose, the assumed rate of bluff top retreat will be –0.1 ft/yr for that portion of the planning horizon. Since the mapping scheme used here always starts with finding the point behind the bluff top where the angle of repose projects, only the –0.1 ft/yr rate will be used where subaerial erosion applies. 3.3.3.4 Block Failure Data

Erosion rate is only one part of the puzzle in predicting how a bluff will respond to erosion. The size of episodic block failures must also be taken into account, if the objective is to understand not only how much erosion will happen over hundreds of years but also what could happen 37

in a single or many block failure events. Block failures could be translational slides, slumps, or even large topples in the case of some high hard rock bluffs. The rate (and thus the probability) of block failures of various sizes, especially large ones, is unknown, since hundreds of years of detailed observations are not available. On the other hand, some maximum block failure widths can be derived from field measurements and analysis of aerial photographs. The location and degree of historic activity of the existing slides and large rock topples is an essential starting point for establishing the likelihood, extent, and rate of propagation of bluff slope failures. The basic techniques are discussed by Allan and Priest (2001; their Appendix C) in their study of the Tillamook County coastline. Table 8 summarizes the maximum block widths that will be used for the Lincoln County coastline.

rived maximum block failure widths from Alan and Priest (2001, their Appendix C and Table 7) were utilized. We also used new observations of maximum block failures in Pleistocene marine terrace sands and Tertiary sedimentary rocks (Table 8).

One difference between Tillamook and Lincoln County geology is the more common occurrence of Pleistocene marine terrace sand in Lincoln County bluffs. At Tillamook County Pleistocene deposits in bluffs are clay-rich paleosols and debris-flow deposits prone to large slumps and translational block slides. Such deposits do occur in coastal bluffs of Lincoln County but are not as common as marine terrace sands and are in most places interbedded with abundant fluvial and marine terrace sand or gravel. Another difference from Tillamook County is the lack of Holocene dune sheets forming high (200-foot) bluffs.

While block width does increase with bluff height, it does so in complex ways, apparently increasing to hundreds of feet at some threshold bluff height that varies from about 150-160 feet for bluffs with Pleistocene soil at the base (e.g. The Capes landslide in Tillamook County), to ~70-80 feet for bluffs with seaward-dipping Tertiary sedimentary rocks (e.g. the Jumpoff Joe Landslide in Lincoln County). In both cases the increase in block failure width was caused by translational block sliding. In some cases the slide planes have very low inclinations, lower than the bedding dip. In other cases the slide planes roughly follow bedding in the Tertiary sedimentary rocks, curving upward near the toe of the slide once the slide plane reached below the elevation of the wave cut platform. The Johnson Creek Landslide immediately south of Otter Rock is an example of the latter type. The Jumpoff Joe landslide in Newport appears to be an example of the former. The main difference between the two seems to be that the Jumpoff Joe slide is in weak poorly bedded Nye Mudstone, whereas the Johnson Creek slide is in siltstone with numerous interbeds of relatively strong sandstone and zeolitized tuff.

In order to establish empirical block failure widths, data were gathered from Tillamook and Lincoln Counties in a study by Allan and Priest (2001). They concluded that some landslide blocks might actually represent fragments of earlier larger blocks, whereas other large intact slide blocks may have undergone unknown amounts of wave erosion. Both factors tended to bias the data to smaller block failures. For the purpose of this report, the largest identified block failure width was used in prediction of “worst case” bluff retreat. Empirical equations and locally de-

Data from drilling and geologic mapping at the Johnson Creek Landslide in Lincoln County (Figure 23) are consistent with control of block failure width in seaward dipping Tertiary sedimentary rock weak siltstone and mudstone units. The main slide plane is translational and deflected down the dip of strong sandstone and tuff units until it reaches below sea level about 200 feet east of the sea cliff. The slide plane then apparently flattens out within about 100 feet of the sea cliff, forming a wedge or rotational failure surface (Figure 24). The complex geometry of this


38

TABLE 8. RECOMMENDED MAXIMUM BLOCK FAILURE WIDTHS FOR COASTAL BLUFFS OF LINCOLN COUNTY. ALL DATA ARE FROM EMPIRICAL OBSERVATIONS OF LOCAL LANDSLIDES. SEDIMENTARY ROCKS ARE CONSIDERED TO BE SEAWARD DIPPING IF THE ANGLE BETWEEN BLUFF TREND AND BEDDING STRIKE IS ~18º; THIS IS THE ANGLE BELOW WHICH BLOCK FAILURE WIDTH APPEARS TO INCREASE ABRUPTLY. BLUFF HEIGHT MEANS TOPOGRAPHIC RELIEF FROM THE BLUFF TOE TO THE TOP SEAWARD EDGE. DATA ARE FROM EMPIRICAL DATA OF ALLAN AND PRIEST (2001) FROM TILLAMOOK AND LINCOLN COUNTIES WITH SUPPLEMENTAL DATA IN LINCOLN COUNTY TO ACCOUNT FOR PLEISTOCENE MARINE TERRACE SAND BLUFFS, BLUFFS CUTTING AT HIGH ANGLES (>18º) TO THE BEDDING STRIKE, AND TERTIARY SEDIMENTARY ROCK BLUFFS SOMEWHAT LOWER IN HEIGHT THAN THOSE ADDRESSED BY ALLAN AND PRIEST. NOTE THAT SOME BLUFF TYPES LISTED HERE DO NOT OCCUR IN LINCOLN COUNTY (E.G. COASTAL BLUFFS OF TERTIARY SEDIMENTARY ROCK IN EXCESS OF 200 FEET HIGH), BUT ARE INCLUDED IN ORDER TO SHOW THE ENTIRE EMPIRICAL DATA SET FOR BOTH TILLAMOOK AND LINCOLN COUNTIES. Bluff Height and Material Causing Block Failure

Maximum Block Failure Width (ft)

Basalt subject mostly to rock fails and topples in bluffs 150 feet high. Pleistocene marine terrace sand bluffs ≥40 feet high. Pleistocene marine terrace sand bluffs -0.34 FT/YR); ASSIGNED MEAN = 0.5 X (-0.34 FT/YR). Als LESS THAN 0.05" CHANGE AT 1:10,000, 1868-1993(>-0.34 FT/YR); ASSIGNED MEAN = 0.5 X (-0.34 FT/YR). Qtc/Ta LANDSLIDE HEADWALL CHANGE: 1868 T-MAP TO 1993 PHOTO Als LANDSLIDE HEADWALL CHANGE: 1868 T-MAP TO 1993 PHOTO PAb BLUFF TOP OR HEADWALL, 1868 T-MAP TO 1993 PHOTOS Als WEIGHTED MEAN, 1868 T-MAP TO 1993 JUMPOFF JOE AREA PHOTOS Als WEIGHTED MEAN, 1868 T-MAP TO 1993 JUMPOFF JOE AREA PHOTOS Als WEIGHTED MEAN, 1868 T-MAP TO 1993 JUMPOFF JOE AREA PHOTOS Als WEIGHTED MEAN, 1868 T-MAP TO 1993 JUMPOFF JOE AREA PHOTOS Ab WEIGHTED MEAN, 1868 T-MAP TO 1993 JUMPOFF JOE AREA PHOTOS Ab WEIGHTED MEAN, 1868 T-MAP TO 1993 JUMPOFF JOE AREA PHOTOS Ab WEIGHTED MEAN, 1868 T-MAP TO 1993 JUMPOFF JOE AREA PHOTOS Ab WEIGHTED MEAN, 1868 T-MAP TO 1993 JUMPOFF JOE AREA PHOTOS Ab

Geomorphic Setting

QLS

ACTIVE DEEP LANDSLIDE

QLS

ACTIVE SHALLOW LANDSLIDE

SS

BLUFFED BEACH

QLS

ACTIVE DEEP LANDSLIDE

SS QLS QLS QLS QLS MS MS MS MS MS

POTENTIALLY ACTIVE DEEP SLIDE BLOCK ACTIVE DEEP LANDSLIDE ACTIVE DEEP LANDSLIDE ACTIVE DEEP LANDSLIDE ACTIVE DEEP LANDSLIDE ACTIVE DEEP SLIDE BLOCK ACTIVE DEEP SLIDE BLOCK ACTIVE DEEP SLIDE BLOCK ACTIVE DEEP SLIDE BLOCK ACTIVE DEEP SLIDE BLOCK

174

11

Eros. Best12 Northing Rate Error Rate Error (ft) Transect (ft/yr) (ft/yr) (ft/yr) (ft/yr)

Geographic Feature

ID

Easting (ft)

29

7274708.8 378640.8 #d239

0.61 0.96 -1.35 0.63 NEWPORT

N

30

7274661.4 378499.9 #d240

0.33 0.33

0 0.34 NEWPORT

N

31

7274594.4 378365.7 #d241

0.33 0.33

0 0.34 NEWPORT

N

32

7274527.5 378231.4 #d242

0.61 0.33 -1.35 0.63 NEWPORT

N

33

7274460.5 378097.2 #d243

0.61 0.33 -1.35 0.63 JUMPOFF JOE

N

34

7274409.9 377956.4 #d244

0.61 0.33 -1.35 0.63 JUMPOFF JOE

N

35

7274364.9 377813.3 #d245

0.61 0.33 -1.35 0.63 JUMPOFF JOE

N

36

7274319.9 377670.2 #d246

0.61 0.33 -1.35 0.63 JUMPOFF JOE

N

37

7274274.9 377527.1 #d247

0.61 0.33 -1.35 0.63 JUMPOFF JOE

N

38

7274229.9

377384 #d248

0.61 0.33 -1.35 0.63 JUMPOFF JOE

N

39

7274184.9 377240.9 #d249

0.61 0.33 -1.35 0.63 JUMPOFF JOE

N

40

7274139.9 377097.8 #d250

0.61 0.33 -1.35 0.63 JUMPOFF JOE

N

41

7274094.9 376954.7 #d251

0.61 0.33 -1.35 0.63 NYE BEACH

N

42

7274083.2 376805.4 #d252

0.61 0.33 -1.35 0.63 NYE BEACH

N

43

7274073.7 376655.7 #d253

0.61 0.33 -1.35 0.63 NYE BEACH

N

44

7274064.3

376506 #d254

0.61 0.33 -1.35 0.63 NYE BEACH

N

45

7274054.9 376356.3 #d255

0.61 0.33 -1.35 0.63 NYE BEACH

N

46

7274045.5 376206.6 #d256

0.61 0.33 -1.35 0.63 NYE BEACH

N

47

7274034.4

0.61 0.33 -1.35 0.63 NYE BEACH

N

376057 #d257


SPS?13

Base15 of Fms14. Bluff

Comment on Best Erosion Rate WEIGHTED MEAN, 1868 T-MAP TO 1993 JUMPOFF JOE AREA PHOTOS Ab LESS THAN 42 FEET OF EROSION SINCE 1868. PAb LESS THAN 42 FEET OF EROSION SINCE 1868. PAb WEIGHTED MEAN, 1868 T-MAP TO 1993 JUMPOFF JOE AREA PHOTOS Als WEIGHTED MEAN, 1868 T-MAP TO 1993 JUMPOFF JOE AREA PHOTOS Als WEIGHTED MEAN, 1868 T-MAP TO 1993 JUMPOFF JOE AREA PHOTOS Als WEIGHTED MEAN, 1868 T-MAP TO 1993 JUMPOFF JOE AREA PHOTOS Als WEIGHTED MEAN, 1868 T-MAP TO 1993 JUMPOFF JOE AREA PHOTOS Als WEIGHTED MEAN, 1868 T-MAP TO 1993 JUMPOFF JOE AREA PHOTOS Als WEIGHTED MEAN, 1868 T-MAP TO 1993 JUMPOFF JOE AREA PHOTOS Ab WEIGHTED MEAN, 1868 T-MAP TO 1993 JUMPOFF JOE AREA PHOTOS Qtc/Ta WEIGHTED MEAN, 1868 T-MAP TO 1993 JUMPOFF JOE AREA PHOTOS Ab WEIGHTED MEAN, 1868 T-MAP TO 1993 JUMPOFF JOE AREA PHOTOS Ab WEIGHTED MEAN, 1868 T-MAP TO 1993 JUMPOFF JOE AREA PHOTOS Ab WEIGHTED MEAN, 1868 T-MAP TO 1993 JUMPOFF JOE AREA PHOTOS Ab WEIGHTED MEAN, 1868 T-MAP TO 1993 JUMPOFF JOE AREA PHOTOS Ab WEIGHTED MEAN, 1868 T-MAP TO 1993 JUMPOFF JOE AREA PHOTOS Ab WEIGHTED MEAN, 1868 T-MAP TO 1993 JUMPOFF JOE AREA PHOTOS Ab WEIGHTED MEAN, 1868 T-MAP TO 1993 JUMPOFF JOE AREA PHOTOS Ab

MS MS MS QLS QLS QLS QLS QLS QLS SS SS MS MS MS MS MS MS MS MS

Geomorphic Setting ACTIVE DEEP SLIDE BLOCK POTENTIALLY ACTIVE DEEP SLIDE BLOCK POTENTIALLY ACTIVE DEEP SLIDE BLOCK ACTIVE DEEP LANDSLIDE ACTIVE DEEP LANDSLIDE ACTIVE DEEP LANDSLIDE ACTIVE DEEP LANDSLIDE ACTIVE DEEP LANDSLIDE ACTIVE DEEP LANDSLIDE ACTIVE DEEP SLIDE BLOCK BLUFFED BEACH (small headland) ACTIVE DEEP SLIDE BLOCK ACTIVE DEEP SLIDE BLOCK ACTIVE DEEP SLIDE BLOCK ACTIVE DEEP SLIDE BLOCK ACTIVE DEEP SLIDE BLOCK ACTIVE DEEP SLIDE BLOCK ACTIVE DEEP SLIDE BLOCK ACTIVE DEEP SLIDE BLOCK

175


ID

Easting11 (ft)

48

7274023.3 375907.4 #d258

0.61 0.33 -1.35 0.63 NYE BEACH

N

49

7274012.2 375757.8 #d259

0.61 0.33 -1.35 0.63 NYE BEACH

N

50

7274001.1 375608.2 #d260

0.66 0.66 -0.68 0.68 NYE BEACH

N

51

7273990 375458.6 #d261

0.66 0.66 -0.42 0.68 NYE BEACH

N

52

7273962.4 375311.6 #d262

0.66 0.66 -0.42 0.68 NYE BEACH

N

53

7273927.3 375165.8 #d263

0

0

0

0 NYE BEACH

Y

54

7273892.2

0

0

0

0 NYE BEACH

Y

55

7273857.2 374874.2 #d265

0.66 0.66 -0.25 0.68 NYE BEACH

N

56

7273822.1 374728.4 #d266

0.66 0.66 -0.25 0.68 NYE BEACH

N

57

7273787 374582.6 #d267

0.66 0.66 -0.25 0.68 NYE BEACH

N

58

7273751

374437 #d268

0.66 0.66

0 0.68 NYE BEACH

Y

59

7273714.5 374291.5 #d269

0.66 0.66

0 0.68 NYE BEACH

Y

60

7273678.1

0.66 0.66

0 0.68 NYE BEACH

Y

61

7273641.6 374000.5 #d271

0.66 0.66 -0.87 0.68 NEWPORT

N

62

7273605.1

373855 #d272

0.66 0.66 -0.87 0.68 NEWPORT

N

63

7273568.7 373709.5 #d273

0.66 0.66 -0.87 0.68 NEWPORT

N

64

7273532.2

373564 #d274

0.66 0.66 -0.87 0.68 NEWPORT

N

65

7273495.8 373418.5 #d275

0.66 0.66 -0.87 0.68 NEWPORT

Y

375020 #d264

374146 #d270

Geographic Feature


SPS?13


Comment on Best Erosion Rate WEIGHTED MEAN, 1868 T-MAP TO 1993 JUMPOFF JOE AREA PHOTOS Ab WEIGHTED MEAN, 1868 T-MAP TO 1993 JUMPOFF JOE AREA PHOTOS Qtc BLUFF TOP OR HEADWALL, 1868 T-MAP TO 1993 PHOTOS Qtc BLUFF TOP OR HEADWALL, 1868 T-MAP TO 1993 PHOTOS Qtc BLUFF TOP OR HEADWALL, 1868 T-MAP TO 1993 PHOTOS Qtc NO CHANGE SINCE 1868; VISUAL ARTS CENTER SEA WALL PRESENT SINCE 1920'S Qtc NO CHANGE SINCE 1868; VISUAL ARTS CENTER SEA WALL PRESENT SINCE 1920'S Qtc BLUFF TOP OR HEADWALL, 1868 T-MAP TO 1993 PHOTOS Qtc BLUFF TOP OR HEADWALL, 1868 T-MAP TO 1993 PHOTOS Qtc BLUFF TOP OR HEADWALL, 1868 T-MAP TO 1993 PHOTOS Qtc BLUFF TOP OR HEADWALL, 1868 T-MAP TO 1993 PHOTOS Qtc BLUFF TOP OR HEADWALL, 1868 T-MAP TO 1993 PHOTOS Qtc BLUFF TOP OR HEADWALL, 1868 T-MAP TO 1993 PHOTOS Qtc/Ta BLUFF TOP OR HEADWALL, 1868 T-MAP TO 1993 PHOTOS Qtc/Ta BLUFF TOP OR HEADWALL, 1868 T-MAP TO 1993 PHOTOS Qtc/Ta BLUFF TOP OR HEADWALL, 1868 T-MAP TO 1993 PHOTOS Qtc/Ta BLUFF TOP OR HEADWALL, 1868 T-MAP TO 1993 PHOTOS Qtc/Ta BLUFF TOP OR HEADWALL, 1868 T-MAP TO 1993 PHOTOS Qtc/Ta

MS

Geomorphic Setting ACTIVE DEEP SLIDE BLOCK

SS

BLUFFED BEACH

Qtc

BLUFFED BEACH

Qtc

BLUFFED BEACH

Qtc

BLUFFED BEACH

Qtc

BLUFFED BEACH

Qtc

BLUFFED BEACH

Qtc

BLUFFED BEACH

Qtc

BLUFFED BEACH

Qtc

BLUFFED BEACH

Qtc

BLUFFED BEACH

Qtc

BLUFFED BEACH

SS

BLUFFED BEACH

SS

BLUFFED BEACH

SS

BLUFFED BEACH

SS

BLUFFED BEACH

SS

BLUFFED BEACH

SS

BLUFFED BEACH

176

11


Geographic Feature

ID

Easting (ft)

66

7273459.3

373273 #d276

0.66 0.66 -0.87 0.68 NEWPORT

N

67

7273422.4 373127.6 #d277

0.66 0.66 -0.87 0.68 NEWPORT

Y

68

7273377.1 372984.6 #d278

0.66 0.66 -0.87 0.68 NEWPORT

Y

69

7273331.8 372841.6 #d279

0.66 0.66 -0.87 0.68 NEWPORT

N

70

7273286.5 372698.6 #d280

0.66 0.66 -0.87 0.68 NEWPORT

N

71

7273241.2 372555.6 #d281

0.66 0.66 -0.87 0.68 NEWPORT

N

72

7273195.9 372412.6 #d282

0.66 0.66 -0.87 0.68 NEWPORT

N

73

7273150.6 372269.6 #d283

0.66 0.66 -0.87 0.68 NEWPORT

N

74

7273105.3 372126.6 #d284

0.66 0.66 -0.87 0.68 NEWPORT

N

75

7273060 371983.6 #d285

0.66 0.66 -0.87 0.68 NEWPORT

N

76

7273022.9 371838.3 #d286

0.66 0.66 -0.87 0.68 NEWPORT

N

77

7272988.2 371692.4 #d287

0.66 0.66 -0.87 0.68 NEWPORT

N

78

7272953.6 371546.5 #d288

0.66 0.66 -0.87 0.68 NEWPORT

N

79

7272918.9 371400.6 #d289

0.66 0.66 -0.87 0.68 NEWPORT

N

80

7272896.7 371252.5 #d290

0.66 0.66 -0.87 0.68 NEWPORT

N

81

7272879.4 371103.5 #d291

0.66 0.66 -0.87 0.68 NEWPORT

N

82

7272862.1 370954.5 #d292

0.66 0.66 -0.87 0.68 NEWPORT

N

83 84

7272844.8 370805.5 #d293 7272827.5 370656.5 #d294

0.66 0.66 -0.87 0.68 NEWPORT 0.66 0.66 -0.87 0.68 NEWPORT

N N


SPS?13

Comment on Best Erosion Rate BLUFF TOP OR HEADWALL, 1868 T-MAP TO 1993 PHOTOS BLUFF TOP OR HEADWALL, 1868 T-MAP TO 1993 PHOTOS BLUFF TOP OR HEADWALL, 1868 T-MAP TO 1993 PHOTOS BLUFF TOP OR HEADWALL, 1868 T-MAP TO 1993 PHOTOS BLUFF TOP OR HEADWALL, 1868 T-MAP TO 1993 PHOTOS BLUFF TOP OR HEADWALL, 1868 T-MAP TO 1993 PHOTOS BLUFF TOP OR HEADWALL, 1868 T-MAP TO 1993 PHOTOS BLUFF TOP OR HEADWALL, 1868 T-MAP TO 1993 PHOTOS BLUFF TOP OR HEADWALL, 1868 T-MAP TO 1993 PHOTOS BLUFF TOP OR HEADWALL, 1868 T-MAP TO 1993 PHOTOS BLUFF TOP OR HEADWALL, 1868 T-MAP TO 1993 PHOTOS BLUFF TOP OR HEADWALL, 1868 T-MAP TO 1993 PHOTOS BLUFF TOP OR HEADWALL, 1868 T-MAP TO 1993 PHOTOS BLUFF TOP OR HEADWALL, 1868 T-MAP TO 1993 PHOTOS BLUFF TOP OR HEADWALL, 1868 T-MAP TO 1993 PHOTOS BLUFF TOP OR HEADWALL, 1868 T-MAP TO 1993 PHOTOS BLUFF TOP OR HEADWALL, 1868 T-MAP TO 1993 PHOTOS BLUFF TOP OR HEADWALL, 1868 T-MAP TO 1993 PHOTOS BLUFF TOP OR HEADWALL, 1868


Geomorphic Setting

Qtc/Ta SS

BLUFFED BEACH

Qtc

SS

BLUFFED BEACH

Qtc

SS

Ab

QLS

Ab

QLS

Ab

QLS

Ab

QLS

Ab

QLS

Ab

QLS

Ab

QLS

Ab

QLS

Ab

QLS

Ab

QLS

Ab

QLS

Ab

SS

Ab

SS

Ab

SS

Ab Ab

SS SS

BLUFFED BEACH ACTIVE DEEP LANDSLIDE BLOCK ACTIVE DEEP LANDSLIDE BLOCK ACTIVE DEEP LANDSLIDE BLOCK ACTIVE DEEP LANDSLIDE BLOCK ACTIVE DEEP LANDSLIDE BLOCK ACTIVE DEEP LANDSLIDE BLOCK ACTIVE DEEP LANDSLIDE BLOCK ACTIVE DEEP LANDSLIDE BLOCK ACTIVE DEEP LANDSLIDE BLOCKACTIVE DEEP ACTIVE DEEP LANDSLIDE BLOCK ACTIVE DEEP LANDSLIDE BLOCK ACTIVE DEEP LANDSLIDE BLOCK ACTIVE DEEP LANDSLIDE BLOCK ACTIVE DEEP LANDSLIDE BLOCK ACTIVE DEEP LANDSLIDE BLOCK ACTIVE DEEP

177

11

ID

Easting (ft)


Geographic Feature

SPS?13

85 7272810.1 370507.5 #d295

0.66 0.66 -0.87 0.68 NEWPORT

N

86 7272792.8 370358.5 #d296

0.66 0.66 -0.87 0.68 NEWPORT

N

87 7272800.6 370208.8 #d297

0.66 0.66 -0.87 0.68 NEWPORT YAQUINA BAY 0 0.05 0 0.05 STATE PARK YAQUINA BAY 0 0.05 0 0.05 STATE PARK YAQUINA BAY 0 0.05 0 0.05 STATE PARK YAQUINA BAY 0 0.05 0 0.05 STATE PARK YAQUINA BAY 0 0.05 0 0.05 STATE PARK YAQUINA BAY 0 0.05 0 0.05 STATE PARK YAQUINA BAY 0 0.05 0 0.05 STATE PARK YAQUINA BAY 0 0.05 0 0.05 STATE PARK YAQUINA BAY 0 0.05 0 0.05 STATE PARK

N

88 7272809.5 370059.1 #d298 89 7272837.6 369914.6 #d299 90 7272905.7

369781 #d300

91 7272973.8 369647.4 #d301 92

7273042 369513.8 #d302

93 7273100.9 369375.9 #d303 94 7273151.4 369235.5 #d304 95

7273166 369086.2 #d305

96 7273158.8 368937.8 #d306


N N N N N N N N N


Comment on Best Erosion Rate T-MAP TO 1993 PHOTOS BLUFF TOP OR HEADWALL, 1868 T-MAP TO 1993 PHOTOS Ab BLUFF TOP OR HEADWALL, 1868 T-MAP TO 1993 PHOTOS BLUFF TOP OR HEADWALL, 1868 T-MAP TO 1993 PHOTOS < 5' OF BLUFF RETREAT SINCE TURN OF THE CENTURY < 5' OF BLUFF RETREAT SINCE TURN OF THE CENTURY < 5' OF BLUFF RETREAT SINCE TURN OF THE CENTURY < 5' OF BLUFF RETREAT SINCE TURN OF THE CENTURY < 5' OF BLUFF RETREAT SINCE TURN OF THE CENTURY < 5' OF BLUFF RETREAT SINCE TURN OF THE CENTURY < 5' OF BLUFF RETREAT SINCE TURN OF THE CENTURY < 5' OF BLUFF RETREAT SINCE TURN OF THE CENTURY < 5' OF BLUFF RETREAT SINCE TURN OF THE CENTURY

SS

Ab

SS

Ab

SS

Ab

SS

Geomorphic Setting LANDSLIDE BLOCK ACTIVE DEEP LANDSLIDE BLOCK ACTIVE DEEP LANDSLIDE BLOCKACTIVE DEEP PREHISTORIC DEEP SLIDE BLOCK ACTIVE DEEP LANDSLIDE BLOCK

Qtc/Ta SS

BLUFFED BEACH

Qtc/Ta SS

BLUFFED BEACH

Qtc/Ta SS

BLUFFED BEACH

Qtc/Ta SS

BLUFFED BEACH

Qtc/Ta SS

DUNED BLUFF

Qtc/Ta SS

DUNED BLUFF

Qtc/Ta SS

DUNED BLUFF

Qtc/Ta SS

DUNED BLUFF

178

APPENDIX F House-To-bluf Erosion Rate Data From Priest And Others (1994) Field Number, Easting16 Northing Rate Error Photo Years17, Geographic ID (ft) (ft) Transect (ft/yr) (ft/yr) Worker Location NO. 1715043002, 671 7294166 512519.6 #b80 -0.24 0.11 92, GOOD ROADS END NO. 1715043002, 672 7294159 512369.7 #b81 -0.24 0.11 92, GOOD ROADS END NO. 171601800, 3 7294100 511020.6 #b90 -0.08 0.11 67-92, GOOD ROADS END NO. 171601800, 4 7294094 510870.7 #b91 -0.08 0.11 67-92, GOOD ROADS END NO.31-1, 67-93, 5 7294001 509976.1 #b97 -0.1 0.1 DIEBENOW ROADS END NO. 171701700, 6 7293982 509827.3 #b98 -0.08 0.11 GOOD ROADS END NO. 32-1, 677 7293963 509678.5 #b99 -0.08 0.11 93, DIEBENOW ROADS END NO. 34-1, 678 7293944 509529.7 #b100 -0.2 0.07 93, DIEBENOW ROADS END NO. 171701800, 9 7293726 508349.6 #b108 -0.24 0.08 67-92, GOOD ROADS END NO. 171701800, 10 7293698 508202.2 #b109 -0.24 0.08 67-92, GOOD ROADS END NO. 36-1, 6711 7293670 508054.8 #b110 -0.05 0.07 93, DEIBENOW ROADS END NO. 172000501, 12 7293605 507762.1 #b112 -0.28 0.11 67-92, GOOD ROADS END NO. 36-4, 6713 7293504 507323.4 #b115 -0.2 0.08 93, INGMAR ROADS END NO. 36-4, 6714 7293471 507177.1 #b116 -0.2 0.08 93, INGMAR ROADS END NO.36-6, 67-93, 15 7293404 506884.6 #b118 -0.22 0.21 DIEBENOW ROADS END NO.36-6, 67-93, 16 7293371 506738.4 #b119 -0.22 0.21 DIEBENOW ROADS END NO.37-1, 67-91, 17 7293338 506592.2 #b120 -0.2 0.09 INGMAR ROADS END NO. 38-1, 67- LINCOLN 18 7292986 504980.2 #b131 -0.67 0.07 93, INGMAR CITY NO. 40-1, 67- LINCOLN CITY 19 7292805 504098.3 #b135 -0.29 0.07 93, INGMAR NO. 40-1, 67- LINCOLN 20 7292775 503951.3 #b136 -0.29 0.07 93, INGMAR CITY NO. 172305100, LINCOLN 21 7292428 502032.3 #b151 -0.43 0.02 67-92, GOOD CITY 22 7292105 500109 #b164

-0.24

0.07 NO. 63-1, 67-

LINCOLN

SPS?18 Fms.19

Fm.20 Bluff Base

Geomorphic Setting

Y

Qc

QC

BLUFFED BEACH

Y

Qc

QC

BLUFFED BEACH

Y

Qc

QC

BLUFFED BEACH

Y

Qc

QC

BLUFFED BEACH

N

Qtc

Qtc

BLUFFED BEACH

N

Qtc

Qtc

BLUFFED BEACH

N

Qtc

Qtc

BLUFFED BEACH

N

Qtc

Qtc

BLUFFED BEACH

Y

Qtc

Qtc

BLUFFED BEACH

N

Qtc

Qtc

BLUFFED BEACH

N

Qtc

Qtc

BLUFFED BEACH

N

Qtc

Qtc

BLUFFED BEACH

N

Qtc

Qtc

BLUFFED BEACH

N

Qtc

Qtc

BLUFFED BEACH

N

Qtc

Qtc

BLUFFED BEACH

N

Qtc

Qtc

BLUFFED BEACH

N

Qtc

Qtc

BLUFFED BEACH

Y

Qtc

Qtc

BLUFFED BEACH

N

Qtc

Qtc

BLUFFED BEACH

N

Qtc

Qtc

BLUFFED BEACH

N

Qtc

Qtc

BLUFFED BEACH

N

Qtc

Qtc

BLUFFED BEACH

Note: Data from GIS file House_bluff_eros_rates2. 16 Oregon State Plane 1983 US Survey feet. 17 67-92 = comparison of distances on 1967 to 1992 air photo; 67-93 = 1967 to 1993 air photo, etc. 18 SPS = shoreline protection structure, usually basalt quarry rock (rip rap); Y = yes; N = no. 198 Fms = rock formations in the bluff face; see main text and Appendix B for explanation of geologic symbols. 20 Rock formation in the base of bluff; see main text and Appendix B for explanation of geologic symbols. Oregon Department of Geology and Mineral Industries OFR O-04-09

179

Fm.20 Field Number, Bluff Rate Error Photo Years17, Geographic Easting16 Northing ID (ft) (ft) Transect (ft/yr) (ft/yr) Worker Location SPS?18 Fms.19 Base 93, INGMAR CITY 23 7292009 499516.6 #b168

-0.19

24 7291985 499368.5 #b169

-0.24

25 7291961 499220.4 #b170

-0.09

26 7291937 499072.3 #b171

-0.12

27 7291841 498480.1 #b175

-0.09

28 7291589 497306.5 #b183

-0.25

29 7291557 497159.8 #b184

-0.25

30 7290613 493072.7 #b212

-0.14

31 7290575 492927.6 #b213

-0.12

32 7290537 492782.6 #b214

-0.12

33 7290498 492637.5 #b215

-0.34

34 7290460 492492.5 #b216

-0.42

35 7290422 492347.4 #b217

-0.42

36 7290231 491622.2 #b222

-0.14

37 7289741 489737.8 #b235

-0.34

38 7289735 489587.9 #b236

-0.34

39 7289121 487645.1 #b250

-0.16

40 7289103 487496.2 #b251

-0.2

41 7289085 487347.3 #b252

-0.21

42 7289066 487198.4 #b253

-0.21

43 7288665 483922.6 #b275

-0.16

44 7288647 483773.7 #b276

-0.18

45 7288355 481391.3 #b292

-0.09

46 7288337 481242.4 #b293

-0.09

47 7288327 481092.9 #b294

-0.07

48 7285033 465158.7 #c98

-0.12

NO. 172501500, LINCOLN 0.06 67-92, GOOD CITY NO. 65-1, 67- LINCOLN 0.12 93, DIEBENOW CITY NO. 65-2, 67- LINCOLN 0.12 93, DIEBENOW CITY NO. 65-3, 67- LINCOLN CITY 0.08 93, INGMAR NO. 66-0, 67- LINCOLN CITY 0.08 93, INGMAR NO. 67-1, 67- LINCOLN 0.08 93, DIEBENOW CITY NO. 67-1, 67- LINCOLN 0.08 93, DIEBENOW CITY NO. 75BR-2, 67-93, LINCOLN 0.09 INGMAR CITY NO. 75BR-3, 67-93, LINCOLN 0.08 INGMAR CITY NO. 75BR-3, 67-93, LINCOLN 0.08 INGMAR CITY NO. 75BR-4, 67-93, LINCOLN 0.08 INGMAR CITY NO. 75BR-5, 67-93, LINCOLN 0.1 DIEBENOW CITY NO. 75BR-5, 67-93, LINCOLN 0.1 DIEBENOW CITY NO. 76-3, 67- LINCOLN 0.07 93, DIEBENOW CITY NO. 173401701, LINCOLN 0.2 67-92, GOOD CITY NO. 173401701, LINCOLN 0.2 67-92, GOOD CITY NO. 173601300, LINCOLN 0.08 67-92, GOOD CITY NO. 173601400, LINCOLN 0.08 67-92, GOOD CITY NO. 173602000, LINCOLN 0.08 67-92, GOOD CITY NO. 173602000, LINCOLN 0.08 67-92, GOOD CITY NO. 173901500, LINCOLN 0.08 67-92, GOOD CITY NO. 90-1, 67- LINCOLN 0.07 93, DIEBENOW CITY NO. 96-1, 670.08 93, INGMAR TAFT NO. 96-1, 670.08 93, INGMAR TAFT NO. 96-2, 670.08 93, INGMAR TAFT NO. 175404000, GLENEDEN 0.03 67-92, GOOD BEACH


Geomorphic Setting

N

Qtc

Qtc

BLUFFED BEACH

N

Qtc

Qtc

BLUFFED BEACH

N

Qtc

Qtc

BLUFFED BEACH

N

Qtc

Qtc

BLUFFED BEACH

N

Qtc

Qtc

BLUFFED BEACH

N

Qtc

Qtc

BLUFFED BEACH

N

Qtc

Qtc

BLUFFED BEACH

N

Qtc

Qtc

BLUFFED BEACH

N

Qtc

Qtc

BLUFFED BEACH

N

Qtc

Qtc

BLUFFED BEACH

N

Qtc

Qtc

BLUFFED BEACH

N

Qtc

Qtc

BLUFFED BEACH

N

Qtc

Qtc

BLUFFED BEACH

N

Qtc

Qtc

BLUFFED BEACH

N

Qtc

Qtc

BLUFFED BEACH

N

Qmt

N

Qtc

Qtc BLUFFED BEACH BASA LT BLUFFED BEACH

N

Qtc

Qtc

BLUFFED BEACH

N

Qtc

Qtc

BLUFFED BEACH

Y

Qtc

Qtc

BLUFFED BEACH

N

Qtc

Qtc

BLUFFED BEACH

N

Qtc

Qtc

BLUFFED BEACH

N

Qtc

Qtc

DUNED BLUFF

N

Qtc

Qtc

DUNED BLUFF

N

Qtc

Qtc

DUNED BLUFF

N

Qtc

Qtc

BLUFFED BEACH 180

Field Number, Rate Error Photo Years17, Geographic Easting16 Northing ID (ft) (ft) Transect (ft/yr) (ft/yr) Worker Location NO. 111-1, 67- GLENEDEN 49 7284884 464577.5 #c102 -0.82 0.08 93, DIEBENOW BEACH NO. 111-1, 67- GLENEDEN -0.82 0.08 93, DIEBENOW BEACH 50 7284847 464432.2 #c103 NO. 115-1, 67- GLENEDEN 51 7284549 463269.8 #c111 -0.05 0.07 93, DIEBENOW BEACH NO. 115-1, 67- GLENEDEN 52 7284511 463124.5 #c112 -0.05 0.07 93, DIEBENOW BEACH NO. 115-2, 67- GLENEDEN -0.14 0.07 93, DIEBENOW BEACH 53 7284474 462979.2 #c113 NO. 175703100, GLENEDEN 54 7283878 460654.4 #c129 -0.14 0.07 67-92, GOOD BEACH NO. 175708800, GLENEDEN 55 7283803 460363.8 #c131 -0.12 0.07 67-92, GOOD BEACH NO. 175708800, GLENEDEN -0.12 0.07 67-92, GOOD BEACH 56 7283766 460218.5 #c132 NO. 175807900, GLENEDEN 57 7283505 459201.4 #c139 -2.33 0.11 67-92, GOOD BEACH NO. 175807800, GLENEDEN 58 7283468 459056.1 #c140 -2.33 0.11 67-92, GOOD BEACH NO. 176206900, LINCOLN -0.44 0.04 67-92, GOOD BEACH 59 7282127 454293 #c173 NO. 176207100, LINCOLN 60 7282082 454149.8 #c174 -0.36 0.04 67-92, GOOD BEACH NO. 176402200, LINCOLN 61 7281412 452001.8 #c189 -1.07 0.1 67-92, GOOD BEACH NO. 176402400, LINCOLN 62 7281367 451858.6 #c190 -1.07 0.1 67-92, GOOD BEACH NO. 159-1, 67-93, 63 7280050 448418.7 #c219 -0.03 0.07 DIEBENOW

SPS?18 Fms.19

Geomorphic Setting

N

Qtc

Qtc

BLUFFED BEACH

N

Qtc

Qtc

BLUFFED BEACH

N

Qtc

Qtc

BLUFFED BEACH

N

Qtc

Qtc

BLUFFED BEACH

N

Qtc

Qtc

BLUFFED BEACH

N

Qtc

Qtc

BLUFFED BEACH

N

Qtc

Qtc

BLUFFED BEACH

N

Qtc

Qtc

BLUFFED BEACH

Y

Qtc

Qtc

BLUFFED BEACH

Y

Qtc Qc/Qt c Qc/Qt c Qc/Qt c Qc/Qt c

Qtc

BLUFFED BEACH

QC

BLUFFED BEACH

QC

BLUFFED BEACH

QC

BLUFFED BEACH

QC

BLUFFED BEACH

N N Y N N

64 7276982 445187 #c256

-0.16

0.14 NO. 165-2, 67-93, INGMAR

N

65 7275969 444928.8 #c264

-0.13

0.11 NO. 165-4, 67-93, INGMAR

N

66 7275844 444497.6 #c267

-0.22

0.11 NO. 165-5, 67-93, INGMAR

N

67 7275802 444353.4 #c268

-0.22

0.11 NO. 165-5, 67-93, INGMAR

N

68 7275441 442451.8 #c281

-0.2

0.14 NO. 165-6, 67-93, INGMAR

N

69 7275436 442301.9 #c282

-0.06

0.12 NO. 165-7, 67-93, INGMAR

N

70 7275432 442152 #c283

-0.08

0.08 NO. 165-8, 67-93, INGMAR

N

71 7275427 442002.1 #c284

-0.13

0.11 NO. 165-9, 67-93, INGMAR

N

72 7275423 441852.2 #c285

-0.13

N

73 7276377 438502.6 #c313

-0.07

74 7276512 438455.9 #c314

-0.07

75 7276469 436844.9 #c325

-0.03

76 7276456 436695.5 #c326

-0.03

77 7273328 430320.9 #c380

-0.17

0.11 NO. 165-9, 67-93, INGMAR NO. 180216600, 0.1 67-92, GOOD DEPOE BAY NO. 180216600, 0.1 67-92, GOOD DEPOE BAY NO. 165-16, 670.06 93, DIEBENOW DEPOE BAY NO. 165-16, 670.06 93, DIEBENOW DEPOE BAY NO. 167A-1, 67- WHALE 0.09 93, DIEBENOW COVE

78 7273207 428825.9 #c390

-0.02

0.08 N0. 167A-5, 67-93, INGMAR N


Fm.20 Bluff Base

N N N N N

Qtc/Ta SS BASA Qc/Tcf LT BASA Qc/Tcf LT BASA Qc/Tcf LT BASA Qc/Tcf LT BASA Qc/Tcf LT BASA Qc/Tcf LT BASA Qc/Tcf LT BASA Qc/Tcf LT BASA Qc/Tcf LT Qtc/T wc SS Qtc/Td BASA b LT Qtc/Td BASA b LT Qtc/Td BASA b LT Qtc/T wc SS Tcf

BLUFFED BEACH HEADLAND HEADLAND HEADLAND HEADLAND HEADLAND HEADLAND HEADLAND HEADLAND HEADLAND POCKET BEACH POCKET BEACH POCKET BEACH POCKET BEACH POCKET BEACH

BASA HEADLAND 181

Field Number, Easting16 Northing Rate Error Photo Years17, ID (ft) (ft) Transect (ft/yr) (ft/yr) Worker

79

7273194 428676.4 #c391

80

7275063 413690.2 #c500

81

7274314 396128.2 #d76

82

7274274 395983.6 #d77

83

7273836 394393 #d88

84

7272902 390613.8 #d114

85

7272752 390353.8 #d116

86

7270141 389280.1 #d137

87

7275449 385435.3 #d193

88

7275466 385286.2 #d194

89

7275499 384988 #d196

90

7275516 384838.9 #d197

91

7275240 380826.6 #d224

92

7275206 380680.5 #d225

93

7275172 380534.4 #d226

94

7275139 380388.3 #d227

95

7275103 380242.7 #d228

96

7274528 378231.4 #d242

97

7274461 378097.2 #d243

98

7274365 377813.3 #d245

99

7274320 377670.2 #d246

100

7274275 377527.1 #d247

101

7274230 377384 #d248

102

7274185 377240.9 #d249

103

7274083 376805.4 #d252

-0.56

Fm.20 Bluff Geographic Location SPS?18 Fms.19 Base LT

0.14 NO. 167A-6, 67-93, INGMAR NO. 182200601, 67-92, GOOD; ERROR NOT DETERMINED, BUT PROBABLY ABOUT 0.1 FT/YR BASED ON CITY OF ANALOGOUS OTTER -0.09 N.D. DATA ROCK NO. 193-1, 67- MOOLACK -1 0.13 93, DIEBENOW BEACH NO. 193-1, 67- MOOLACK -1 0.13 93, DIEBENOW BEACH NO. 198-1, 67- MOOLACK -0.38 0.14 93, DIEBENOW BEACH MOOLACK -1.06 0.08 BEACH MOOLACK BEACH -0.58 0.07 NO. 215-1, 67- YAQUINA -0.17 -0.13 93, INGMAR HEAD NO. 221-2, 67- AGATE -0.19 0.13 93, DIEBENOW BEACH NO. 221-2, 67- AGATE -0.19 0.13 93, DIEBENOW BEACH NO. 222-1A, 67- AGATE -0.67 0.3 93, DIEBENOW BEACH NO. 183803400, AGATE -0.88 0.15 67-92, GOOD BEACH NO. 233-1A, 67-0.23 0.08 93, DIEBENOW NEWPORT NO. 233-1B, 67-0.21 0.08 93, DIEBENOW NEWPORT NO. 233-4, 67-0.15 0.08 93, DIEBENOW NEWPORT NO. 184102400, -0.44 0.08 67-92, GOOD NEWPORT NO. 184102400, -0.44 0.08 67-92, GOOD NEWPORT NO.239-0, 39-0.45 0.29 93, INGMAR NEWPORT NO.239-1, 39- JUMPOFF -0.51 0.29 93, INGMAR JOE NO.241-1, 67- JUMPOFF -2.26 0.07 93, DIEBENOW JOE NO.241-1, 67- JUMPOFF -2.26 0.07 93, DIEBENOW JOE NO.243B-1, 67- JUMPOFF -5.09 0.07 93, DIEBENOW JOE NO.243B-1, 67- JUMPOFF -5.09 0.07 93, DIEBENOW JOE NO.243B-1, 67- JUMPOFF -5.09 0.07 93, DIEBENOW JOE -4.2

0.3 NO.244-1, 39-


Geomorphic Setting

N

Tcf

BASA LT HEADLAND

N

PHb

SS

N

Als

QLS

N

Als

QLS

N

Als

QLS

N

Als

QLS

N

Als

N

Tcf

QLS BASA LT HEADLAND

N

Qtc/Tn MS

BLUFFED BEACH

N

Qtc/Tn MS

BLUFFED BEACH

N

Qtc/Tn MS

BLUFFED BEACH

N

Qtc/Tn MS

N

Ab

MS

N

Ab

MS

N

Ab

MS

N

Ab

MS

N

Ab

MS

N

Als

QLS

N

Als

QLS

N

Als

QLS

N

Als

QLS

N

Als

QLS

N

Ab

SS

PREHISTORIC DEEP SLIDE BLOCK ACTIVE DEEP LANDSLIDE ACTIVE DEEP LANDSLIDE ACTIVE DEEP LANDSLIDE ACTIVE DEEP LANDSLIDE ACTIVE DEEP LANDSLIDE

N

Ab

SS

BLUFFED BEACH ACTIVE SHALLOW SLIDE BLOCK ACTIVE SHALLOW SLIDE BLOCK ACTIVE SHALLOW SLIDE BLOCK ACTIVE SHALLOW SLIDE BLOCK ACTIVE DEEP SLIDE BLOCK ACTIVE DEEP LANDSLIDE ACTIVE DEEP LANDSLIDE ACTIVE DEEP LANDSLIDE ACTIVE DEEP LANDSLIDE ACTIVE DEEP LANDSLIDE ACTIVE DEEP SLIDE BLOCK ACTIVE DEEP SLIDE BLOCK

NYE BEACH N

Ab

MS

ACTIVE DEEP SLIDE 182

Field Number, Rate Error Photo Years17, Easting16 Northing Worker ID (ft) (ft) Transect (ft/yr) (ft/yr) 93, INGMAR 104

7274074 376655.7 #d253

-0.75

105

7274064

376506 #d254

-0.75

106

7274055 376356.3 #d255

-6.6

107

7274046 376206.6 #d256

-0.57

108

7274034

376057 #d257

-1.36

109

7274023 375907.4 #d258

-0.09

110

7274012 375757.8 #d259

-0.76

111

7274001 375608.2 #d260

-0.76

112

7273605

373855 #d272

-0.9

113

7273569 373709.5 #d273

-0.1

114

7273377 372984.6 #d278

-0.07

115

7273332 372841.6 #d279

-0.07

116

7272879 371103.5 #d291

-0.51

117

7272862 370954.5 #d292

-0.51

118

7270289

347262 #e126

-0.93

119

7270267 347113.5 #e127

-0.93

120

7270225 346816.5 #e129

-1.67

121

7270203

346668 #e130

-1.67

122

7268835 336711.1 #e197

-0.45

123

7266946 326974.3 #e265

-1.53

124

7266648 325051.6 #e278

-0.11

125

7266633 324902.4 #e279

-0.11

126

7293922 509381.4 #b101

0

127

7293894

509234 #b102

0

128

7289949 490610.7 #b229

0

129

7289908 490466.4 #b230

0

130

7288785

479588 #b305

0

131

7288903 479495.4 #b306

0

132

7284362 462543.3 #c116

0

Fm.20 Bluff Geographic Location SPS?18 Fms.19 Base

NO.245-1, 390.08 93, DIEBENOW NYE BEACH NO.245-1, 390.08 93, DIEBENOW NYE BEACH NO.245-1B, 670.3 93, INGMAR NYE BEACH NO.245-2, 670.09 93, DIEBENOW NYE BEACH NO.245-3, 670.09 93, DIEBENOW NYE BEACH NO.245-4, 670.09 93, DIEBENOW NYE BEACH NO.245-5, 670.09 93, DIEBENOW NYE BEACH NO. 245-5, 0.09 DIEBENOW NYE BEACH NO. 256-1, 670.13 93, INGMAR NEWPORT NO. 256-2, 670.13 93, INGMAR NEWPORT NO.258-1, 670.13 93, INGMAR NEWPORT NO.258-1, 670.13 93, INGMAR NEWPORT NO.262-2, 670.13 93, DIEBENOW NEWPORT NO.262-2, 670.13 93, DIEBENOW NEWPORT NO. 290-1, 67-93, 0.09 DIEBENOW NO. 290-1, 67-93, 0.09 DIEBENOW NO. 292-1, 67-93, 0.09 DIEBENOW NO. 292-1, 67-93, 0.09 DIEBENOW NO. 295A-1, 67-93, 0.07 DIEBENOW NO. 306-1, 67-93, 0.07 DIEBENOW NO. 311-1, 67- CITY OF 0.08 93, DIEBENOW SEAL ROCK NO. 311-1, 67- CITY OF 0.08 93, DIEBENOW SEAL ROCK NO. 34-2, 670.07 93, INGMAR ROADS END NO. 34-2, 670.07 93, INGMAR ROADS END NO. 173400101, LINCOLN 0.03 67-92, GOOD CITY NO. 173400101, LINCOLN 0.03 67-92, GOOD CITY 98A-1, 67-93, 0.08 INGMAR TAFT 98A-1, 67-93, 0.08 INGMAR TAFT NO. GLENEDEN 0.07 1725601500, 67- BEACH


Geomorphic Setting BLOCK

N

Ab

MS

N

Ab

MS

N

Ab

MS

N

Ab

MS

N

Ab

MS

N

N

Ab MS Phb(Q tc/Ta) SS Phb(Q tc/Ta) SS

ACTIVE DEEP SLIDE BLOCK ACTIVE DEEP SLIDE BLOCK ACTIVE DEEP SLIDE BLOCK ACTIVE DEEP SLIDE BLOCK ACTIVE DEEP SLIDE BLOCK ACTIVE DEEP SLIDE BLOCK PREHISTORIC DEEP SLIDE BLOCK PREHISTORIC DEEP SLIDE BLOCK

N

Qtc/Ta SS

BLUFFED BEACH

N

Qtc/Ta SS

BLUFFED BEACH

Y

Qtc/Ta SS

N

N

Als QLS Phb(Q tc/Ta) SS Phb(Q tc/Ta) SS

BLUFFED BEACH ACTIVE DEEP LANDSLIDE PREHISTORIC DEEP SLIDE BLOCK PREHISTORIC DEEP SLIDE BLOCK

N

Qtc/Tn MS

BLUFFED BEACH

N

Qtc/Tn MS

BLUFFED BEACH

N

Qtc/Tn MS

BLUFFED BEACH

N

Qtc/Tn MS

BLUFFED BEACH

N

BLUFFED BEACH

N

Qtc/Tn MS Qtc/[T yq] MS Qtc/Ty q SS Qtc/Ty q SS Qal/Qt c Qtc Qal/Qt c Qtc

N

Qtc

Qtc

BLUFFED BEACH

N

Qtc

Qtc

BLUFFED BEACH

N

Qtc

Qtc

DUNED BLUFF

N

Qtc

Qtc

DUNED BLUFF

N

Qtc

Qtc

BLUFFED BEACH

N

N

N N N N

BLUFFED BEACH BASALT-GUARDED BLUFFED BEACH BASALT-GUARDED BLUFFED BEACH BLUFFED BEACH BLUFFED BEACH

183

16

ID

Easting (ft)

Field Number, Northing Rate Error Photo Years17, (ft) Transect (ft/yr) (ft/yr) Worker 92, GOOD

Geographic Location SPS?18 Fms.19

133 7275364 439903.5 #c298

0

134 7275360 439753.6 #c299

0

135 7275355 439603.7 #c300

0

NO. 165-11, 67-93, 0.12 DIEBENOW NO. 165-11, 67-93, 0.12 DIEBENOW NO. 165-12, 67-93, 0.14 DIEBENOW

136 7273219 428975.4 #c389

0

0.08 NO. 167A-3, 67-93, INGMAR N

137 7274845 387268.7 #d180

0

138 7275532 384689.8 #d198

0

139 7273287 372698.6 #d280

0

140 7273241 372555.6 #d281

0

NO. 217-2, 67- AGATE 0.08 93, INGMAR BEACH NO. 222-2, 67- AGATE 0.14 93, DIEBENOW BEACH NO.258-2, 670.07 93, DIEBENOW NEWPORT NO.258-2, 670.07 93, INGMAR NEWPORT


Fm.20 Bluff Base

N

Qtc/Tc BASA f LT Qtc/Tc BASA f LT Qtc/Tc BASA f LT BASA Tmcf LT PHb (Qtc/T a) SS

N

Qtc/Tn MS

N

Als

QLS

N

Als

QLS

N N N

Geomorphic Setting

HEADLAND HEADLAND HEADLAND HEADLAND PREHISTORIC DEEP SLIDE BLOCK BLUFFED BEACH ACTIVE DEEP LANDSLIDE ACTIVE DEEP LANDSLIDE

184

APPENDIX G Erosion Measurements By rubberSheeting NORTHERN LINCOLN CITY (WECOMA BEACH): Green area at bluff edge is estimated bluff toe retreat between 1939 and 1993. Green lines on streets and buildings show match of 1939 rubbersheeted photo to the 1993 orthophoto (red lines and the base map). Bluff is composed of Pleistocene marine terrace sand.


185

GLENEDEN BEACH: Green area at bluff edge is estimated bluff toe retreat between 1939 and 1993. Green lines on streets and buildings show match of 1939 rubbersheeted photo to the 1993 orthophoto (red lines and the base map). Bluff is composed of Pleistocene marine terrace sand. Note that the east-west registration is much better than the north-south registration of the two photos.

0

100

200

feet

Gleneden Beach

N ep t u

ne Av e

Laurel St


186

BEVERLY BEACH (WADE CREEK AREA): Green area at bluff edge is estimated bluff toe retreat between 1939 and 1993. Green lines on roads show match of 1939 rubbersheeted photo to the 1993 orthophoto base map. Some roads have disappeared since 1939. Highway 101 in this area approximately follows a road that was on the 1939 photos. There is significant error in the eastern part of the photo where the 1993 location of the old coast highway differs from the 1939 rubbersheeted photo. Bluff is composed of siltstone and sandstone of the Astoria Formation overlain by landslide debris.


187

HOLIDAY BEACH: Green area at bluff edge is estimated bluff toe retreat between 1939 and 1993. Small areas of red show error (bluff growth) from the rubbersheeting technique unable to account for radial displacement of the near-vertical bluff. Green line shows match of west edge the coast highway in 1939 rubbersheeted photo to the same feature on the 1993 orthophoto. Bluff is composed of several feet of Tertiary Nye Mudstone at the base overlain by Pleistocene marine terrace sand.


188