Coastal Studies Institute, Louisiana State University, Baton Rouge, LA 70803. Abstract ... nier plain coast in southwestern Louisiana has not been quantified.
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GULF COAST ASSOCIATION OF GEOLOGICAL SOCIETIES TRANSACTIONS
VOL. XLV, 1995
Historical Shoreline Dynamics Along the Chenier Plain of Southwestern Louisiana Mark R. Byrnes, Randolph A. McBride, Qiang Tao, and Lisa Duvic Coastal Studies Institute, Louisiana State University, Baton Rouge, LA 70803
Abstract A computer-based shoreline mapping methodology, within a framework of a geographic information system, was used to compile and analyze changes in historical shoreline position between Sabine and Southwest Passes in southwestern Louisiana for the period 1883 to 1994. Regional patterns of change depict systematic shifts between shore retreat and advance in relation to sediment supply from the westwarddirected Atchafalaya River mudstream and erosion of marsh/chenier deposits, as well as shoreline orientation relative to storm and normal wave conditions. Net shoreline position change for the period 1883 to 1994 illustrates seven cells of accretion and erosion and one cell showing no net change. The western 17 km of coast (5 km east of Sabine Pass to west Ocean View Beach) illustrates net advance of 3.5 m/yr. Moving east, the next 15 km of coast (Ocean View Beach to the eastern end of the revetment along Highway 82) shows net retreat at 1.2 m/yr. A 2-km segment of coast bracketing Holly Beach shows no net change, but an 8.5-km length of coast east of this area has been retreating at a rate of 1.4 m/yr since 1883. Shoreline change from 0.7 km west of the Calcasieu Pass jetties east to the old Mermentau River mouth has averaged +2.7 m/yr; however, the 63-km segment of coast east of this area shows net retreat at a rate of 8.7 m/yr. From western Mulberry Island to Cheniere au Tigre, shoreline advance has been dominant at 2.8 m/yr, whereas east of this zone to Southwest Pass average shore erosion at 2.9 m/yr is illustrated. Although shoreline retreat is dominant over much of the chenier plain (average change rate for the period of record is -2.6 m/yr), long-term trends indicate net shoreline advance between Sabine and Calcasieu Passes (0.7 m/yr) - and average shoreline retreat between Calcasieu and Southwest Passes (3.8 m/yr) since 1883.
Introduction Regional trends in shoreline position change provide fundamental information for establishing planning and policy decisions with respect to coastal zone management strategies. In recent years, significant effort has been placed on establishing a data base for quantifying long- and short-term trends in wetland loss (Britsch and Dunbar, 1993), shoreline position change (McBride et al., 1991, 1992), and seafloor change (List et al., 1991, 1994) for the Mississippi River Delta Plain. These data play a significant role in the development of coastal restoration and management activities in Louisiana. However, shoreline and seafloor change data for the chenier plain coast in southwestern Louisiana has not been quantified to the same degree of accuracy as for the delta plain. Morgan and Larimore (1957) performed the first regional analysis of historical shoreline change for the chenier plain to establish the 1812 (Louisiana admitted to the Union) shoreline boundary for ascertaining the extent of Louisiana's jurisdiction over resource ownership in the offshore coastal zone. Morgan and Morgan (1983) updated results of the original study using aerial photography taken in 1969. Both studies indicated six zones of retreat and advance; however, the magnitude and, in some cases, direction of change varied between the original analysis time period (1883/86-1954) and the update time period (1954-1969). The only other study of historical shoreline change for the chenier plain coast was completed by Mitchell (1987) for the area between Ocean View Beach and Holly Beach. Data analyses for the period 1883 to 1985 indicated net shoreline retreat for the entire study area (about 2 m/yr). The pur-
pose of this paper is to quantify changes in high-water shoreline position for the period 1883 to 1994 within a framework of processresponse variables affecting short- and long-term changes, and to identify the relative importance of natural and human-induced influences on coastal evolution along the chenier plain. This is the first time regional shoreline position change has been re-evaluated for the chenier plain since 1969; data compilation and analysis procedures have changed significantly, allowing more detailed and accurate results.
Background The chenier plain of southwestern Louisiana is a marginal deltaic environment that has developed over the past 3,000 to 4,000 yrs B.P. through a series of transgressions and regressions (Gould and McFarlan, 1959). Although the timing of deposition and source of sediment to cheniers is not completely understood, the general mechanism of deposition for the chenier plain is related to shifts in position of the Mississippi River mouth; west-oriented flow added significant quantities of sediment to the area whereas east-oriented flow resulted in sediment reduction to the chenier plain and net transgression. As such, mud deposits along the coast were reworked by coastal processes, resulting in shoreline recession, coastal erosion, and the formation of narrow perched beaches (Hoyt, 1969). These beaches are composed of shell and sand, and are relatively thin (1 to 3 m) due to a general lack of sand-sized material. When the beaches become stranded by regressive mud deposits (due to Mississippi River mouth reorientation to the west), they become cheniers (see Hoyt [1969] and Otvos and Price [1979] for a complete description of chenier genesis) which may or may not be excavated during subsequent transgression after the Mississippi River switches course. The outer coast of the Chenier Plain between Sabine Pass and Southwest Pass (Figure 1) is in a microtidal setting with diurnal tides ranging from 0.6 to 0.8 m (mean to spring). Locally-generated wind waves from the south and southeast are dominant 18 and 22 percent of the time, respectively (Nakashima et al., 1987). Breaking wave heights average 0.5 m with a 5-sec period, and net longshore sand transport is to the west at 47,000 to 76,000 m3/yr (US Army Corps of Engineers, 1971). These rates are not calculated using wave statistics, but instead were determined from historical shoreline erosion trends. Relative sea-level change determined from tide gage records indicates a rate of about 0.54 cm/yr (Penland and Ramsey, 1990). The primary processes adversely impacting shoreline change are hurricane waves and storm surge. The maximum recorded storm surge in this area was 3.97 m during Hurricane Audrey in 1957, and average storm surge elevation between 1939 and 1986 was 1.85 m relative to mean sea level (MSL). A number of coastal engineering projects constructed during the period of record have affected long- and short-term shoreline response in the study area. The jetties at Sabine and Calcasieu Passes were constructed in the late 1800s under authority of the River and Harbor Act of 1872 for navigation purposes (US Army Corps of Engineers, 1961). Because the structures extend seaward of the coast, littoral sediment transport is blocked at these boundaries, interrupting natural longshore movement of sediment in the littoral zone and effecting downdrift beaches. Construction of the Sabine Pass jetties consisted of stone on a foundation mat of brush that began in 1883 and 1885 (Morton, 1975). The east and west jetties were completed to a length of 7.7 and 6.7 km, respectively. A similar method of construction was used to build jetties at the entrance of Calcasieu Pass starting in 1893 (east) and 1896 (west) (US Army Corps of Engineers, 1961). Modifications were made to the struc-
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GULF COAST ASSOCIATION OF GEOLOGICAL SOCIETIES TRANSACTIONS
94°00'
VOL. XLV, 1995
92°00'
Kilometers Universal Transverse Mercator Zone 15 NAD 1983
29°20'
Figure 1. Map of study area in southwestern Louisiana. tures over the next 45 years, and by 1942, jetty lengths were 3.2 km (east) and 2.5 km (west). Chronic erosion along Highway 82 between Holly and Peveto Beaches resulted in construction of a 5km long revetment in 1970 made of Gobi Blocks underlain by filter cloth (Dement, 1977). Although the project has exceeded expectations, significant maintenance efforts have persisted through the years in response to storms. Consequently, the Louisiana Department of Transportation and Development constructed a series of six segmented breakwaters along the western end of the revetment in 1985 to trap longshore sediment at areas of accelerated erosion and revetment damage (Nakashima et al., 1987). The positive performance of these structures led to the construction of an 11.6 km seg. mented breakwater system (85 breakwaters) between Ocean View Beach and Holly Beach (see Bymes and McBride, 1995). Finally, the jetties at Lower Mud Lake Outlet were constructed in 1970s for navigation purposes, but a byproduct of construction has been sand trapping from the littoral system on the east side of the entrance, downdrift erosion, and closure of the old Mermentau River mouth in the early 1980s.
Data Sources and Analysis Methods Coastal planning and management strategies often rely on various types of scientific data for decision support. Of these types, historical shoreline data sets and associated change analyses represent essential information needs. As such, data organization, compilation, and accuracy requirements become critical for performing reliable analyses and implementing appropriate management options. Prior to widespread use of computer-based routines for capturing and analyzing changes in shoreline position, manual and semi-automated techniques were applied for assessing shoreline evolution in coastal areas. Currently, accurate quantitative documentation of coastal change depends on electronic digitizers and computer processing that integrate computer-aided design (CAD), computer cartography, and geographic information system (GIS) software. Byrnes et al. (1991) address the issues of mapping and analysis standards, and database development, for shoreline position information. These procedures were developed for science and coastal zone management application using state-of-science technology. McBride et al. (1991, 1992) applied these procedures for analyzing shoreline change along the Mississippi River deltaic plain, and McBride and Byrnes (1995) have used shoreline change data for the coastal zone of Louisiana and Mississippi to classify regional trends in shoreline response for management application. An historical shoreline change analysis was completed using maps and a global positioning system (GPS) survey for evaluating
long-term trends relative to short-term shoreline response. A computer-based shoreline mapping methodology, within a framework of a GIS (Byrnes and Hiland, 1994a, 1994b, 1995), was used to compile and analyze changes in historical shoreline position between Sabine and Southwest Passes in southwest Louisiana. The data base consists of three cartographic-based shorelines (1883/86, 1923/34, and 1947/57) and a global positioning system (GPS) field survey completed in May 1994 (Table 1). Shoreline position data were captured using a large-format, highprecision digitizer according to original projection, ellipsoid, and datum using Intergraph computer mapping hardware and software (McBride et al. 1991). Metadata associated with shoreline compilation were recorded using Oracle as the relational database management system under Intergraph's Modular GIS Environment. Once compiled, shoreline data were converted to a common datum (North American Datum of 1983 [NAD83]), projection (Universal Transverse Mercator [UTM]), and ellipsoid (Geodetic Reference System 1980 [GRS80]) before creating spatial overlays or performing change analysis. The Automated Shoreline Analysis Program (ASAP) was used for quantifying shoreline change at 50-m longshore intervals. Data quality assurance was monitored continually, and an analysis of inherent and operational errors was performed to gauge the significance of measured change. When considering all potential inherent errors in data compilation and analysis, these estimates apply to each individual data set. In making comparisons of shoreline position, error is additive. Assuming individual errors represent standard deviations, a root-mean-square (rms) approach can be applied to provide a realistic assessment of combined potential errors (Merchant, 1987; Crowell et al., 1991). Table 2 summarizes estimates of potential positional error for the primary data sources used in this study. The rms errors for 1883/86 and 1923/34 T-sheets are about ±15.2 m, whereas the 1947/57 data contain about ±16.7 m of potential error. The GPS survey provided the most accurate measurement of shoreline position with an estimated maximum rms error of ±5.8 m. Table 3 is a summary of maximum rms error associated with shoreline change estimates for all time periods. A component of this paper addresses short-term shoreline response relative to long-term trends using beach profile data for an 11.6-km segment of coast between Ocean View Beach and Holly Beach. The method by which shoreline data often are extracted from beach profiles is inconsistent with methods used for collecting shoreline position data from field surveys and aerial photography. The position of the high-water shoreline determined in the field or from aerial photography is an interpretation of the upper limit of average wave activity at high tide; relative to geomorphology, this
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BYRNES, McBRIDE, TAO, DUVIC
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Table 1. Summary of Shoreline Source Data Characteristics for the Study Area Date
Data Source
Comments and Map Numbers
1883/86
USC&GS TopographicMaps (1:20,000)
First surveyed shoreline using standard engineering techniques; 1883 Sabine Pass to Calcasieu Pass (T-sheets 1642, 1643, 1644); 1884 Calcasieu Pass to Beach Prong Bayou (T-sheets 1654, 1655); 1886 Beach Prong Bayou to Southwest Pass (T-sheets 1684, 1686,1688)
1923/34
USC&GS Topographic Maps (1:20,000)
All maps produced from field surveys; 1923 - Sabine Pass to 3 km west of Calcasieu Pass (T-sheets 4057,4061); 1924 - 3 km west of Calcasieu Pass to 5 km west of the old Mermentau River entrance (T-sheet 4060); 1933 - 5 km west of the old Mermentau River entrance to Rollover Bayou (T-sheets 4791,4795, 4922); 1934 - Rollover Bayou to Southwest Pass (T-sheets 4922,4923,4924)
1947/57
USC&GS Topographic Maps (1:20,000)
All maps produced from interpreted aerial photography; December 1947 - 5.6 km west of DeWitt Canal to Southwest Pass (T-sheets 9110, 9111, 9112, 9113); April 1957 - Sabine Pass to 5.6 km west of DeWitt Canal (T-sheets 10629 through 10640)
GPS Survey (1:1); one stationary unit (base station) and one roving unit
Differential corrections were applied for accurate estimates of highwater shoreline position
May 1994
Table 2. Estimates of Potential Error Associated with Shoreline Position Surveys Traditional Engineering Field Surveys (1883/86 and 1923/34 shorelines) Location of rodded points Location of plane table Interpretation of high-water shoreline position at rodded points Error due to sketching between rodded points
±1 m ±2 to 3 m ±3 to 4 m up to ±5 m Map Scale 1:20,000
Cartographic Errors (all maps for this study) Inaccurate location of control points on map relative to true field location Placement of shoreline on map Line width for representing shoreline Digitizer error Operator error
up to ±6 m ±10 m ±6m ±2m ±2m Map Scale 1:20,000
Aerial Surveys (1947/57 shoreline) Delineating high-water shoreline position
±10 m
GPS Survey (1994 shoreline) Delineating high-water shoreline Position of measured points
±1 to 3 m ±2 to 5 m (specified); ±1 to 3 m (field tests)
Sources: Shalowitz, 1964; Ellis 1978; Kruczynski and Lange, 1990; Anders and Byrnes, 1991; Crowell et al., 1991 position generally is recognized as the berm crest or an active scarp at the toe of a dune (Byrnes and Hiland, 1995). Therefore, no vertical datum is referenced explicitly when dealing with historical shoreline position data (Shalowitz, 1964; Anders and Byrnes, 1991). Typically, beach profile data are referenced to a vertical datum, so shoreline position was interpreted from the profile in a manner consistent with that used to determined shoreline position in the field. As such, reliable comparisons between profile shoreline position and topographic shoreline surveys can be made for evaluating shortterm trends.
described qualitatively to provide a regional perspective of change in the study area, and quantitative data are tabulated for gaging trends in the magnitude and rate of shore response. Cumulative and incremental change rates are summarized for evaluating temporal trends, and spatial variability is assessed by averaging rates of change for coastal segments having similar response characteristics (retreat versus advance) for each time interval. Beach profile data for the period.1990 to 1994 are used to evaluate short-term adjustments in shoreline position and profile sand volume relative to engineering activities and long-term trends.
Historical Shoreline Change
Long-Term trends
The magnitude and direction of shoreline position change were evaluated for the Chenier Plain to assess long- and short-term response to natural coastal processes and engineering activities for the period 1883 to 1994. Patterns of shoreline movement are
Regional patterns of shoreline response for two primary geomorphic zones (determined by major boundaries to littoral transport) illustrate similar temporal trends but different spatial response. Table 4 is a quantitative summary of trends since the initial 1883/86
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GULF COAST ASSOCIATION OF GEOLOGICAL SOCIETIES TRANSACTIONS
Table 3. Maximum Root-Mean-Square (rms) Potential Error for Shoreline Change Data Date
1923/34
1947/57
±21.5'
±22.6
1883/86
(±0.4 to ±0.5)2 (±0.3 to ±0.4) 1924/34
1994 ±16.3 (±0.1 to ±0.2)
±22.6
±16.3
(±0.7 to ±1.0)
(±0.2 to ±0.3 ) ±17.7
1947/57
(±0.4 to ±0.5) 1
Magnitude of potential error associated with high-water shoreline position change (m); Rate of potential error associated with high-water shoreline position change (m/yr).
shoreline. Between Sabine and Calcasieu Passes (geomorphic zone 1; about 48 km long), average shoreline advance is shown for all time intervals except 1947/57 to 1994. Although it appears that net shoreline advance has decreased significantly since 1957, the lack of survey information for the western 4.96 km of this zone (an area of consistent accretion) in 1994 creates an artificially lower average rate of change. For example, if it is assumed that shoreline change rates along the western 4.96 km of zone 1 (1883 to 1994) are consistent with change rates for the period 1883 to 1957, average change would be 1.4 m/yr, twice that indicated in Table 4. Change trends at points east of this area (adjacent 1 km) suggest that this estimate may be slightly high but reasonable for qualifying average change rates. Thus, a trend of increasing shoreline retreat with time for geomorphic zone 1 is apparent, but the magnitude of change likely is lower than that shown in Table 4. Geomorphic zone 2, Calcasieu Pass to Southwest Pass (about 126 km long), illustrates little variation in average change rates for all time periods. However, unlike geomorphic zone 1, shoreline retreat dominates at an average rate of -3.8 m/yr (1883-1994). An increase in the magnitude of retreat is illustrated for the period 1947/57 to 1994, suggesting that erosive processes are becoming more dominant with time. This increase is consistent with trends for zone 1 and results of Morgan and Morgan (1983). Consequently, average change rates for the entire study area (about 174 km long) illustrate the same trend, that is, slightly increasing retreat rates with
VOL. XLV, 1995
time. Average change for the period of record is about -2.6 m/yr; however, the magnitude would be decreased to about -2.4 m/yr if the reasoning presented above for the western 4.96 km of shoreline in zone 1 were applied. Overall, average shoreline change rates are relatively consistent, but a subtle trend of increasing retreat suggests that sediment availability to the subaerial beach is being reduced slowly with time for the entire study area. Although average shoreline movements over the period of record show little variation in trend, eight cells of shoreline advance and retreat were identified for the period 1883 to 1994. Figure 2 shows net shoreline change within geomorphic zone 1; the plot of shoreline change at 50-m increments is spatially referenced with the map, providing a quantitative summary of spatial variability between Sabine and Calcasieu Passes. Three primary cells of change can be identified. First, the region west of Ocean View Beach to Sabine Pass shows net shoreline advance at a rate of 3.5 m/yr. Second, the area between Ocean View Beach and just west of Holly Beach is net erosional. The average shoreline change rate for this cell is -1.2 m/yr, but maximum retreat rates reach 2.5 m/yr. The high-water shoreline for the Holly Beach area has remained stable for the 111 -yr time period; although significant storm damage does occur over short periods of time, shoreline position readjusts and has been unchanged since 1883. Third, an 8.45 km erosion zone updrift of Holly Beach and 0.7 km downdrift of the Calcasieu River entrance jetties illustrates net retreat at a rate of 1.4 m/yr (Figure 2). Maximum retreat rates in this cell reach 2.8 m/yr (1883-1994) and provide sand to downdrift beaches. The two remaining cells include an area of no change gulfward and slightly to the east of Holly Beach (approximately 1.6 km long) and an area of net shoreline advance next to the west Calcasieu jetty (about 0.7 km long). Table 5 is a summary of spatial and temporal trends in shoreline position change between Sabine and Calcasieu Passes. Three trends emerge that support changes in average change rates for the entire zone. One, the length of shoreline in cell 1 is decreasing with time and the magnitude of shoreline advance is declining. Two, the length of shoreline encompassing cell 2 (erosional zone) is increasing with time, and the magnitude of change remains relatively consistent. Three, the length of shoreline in cell 4 (net retreat) shows significant change, and the magnitude of retreat is increasing steadily. All three observations indicate a reduction in sediment availability to the subaerial beach. To the east, between Calcasieu Pass and Southwest Pass (geomorphic zone 2), alternating cells of erosion and accretion generally are associated with greater magnitudes of change than those to the
Table 4. Spatial and Temporal Trends in Shoreline Position Change for the Chenier Plain, Southwestern Louisiana 1923/34 to
1883/86 to 1923/34 1.71 3.82 47.773 -3.5 5.4 124.39
1
1947/57
1994
1947/57
1947/57 to 1994
1994
Geomorphic Zone 1 - Sabine Pass to Calcasieu Pass 0.7 2.1 2.7 2.6 3.8 4.7 47.77 42.78" 47.51
0.4 2.3 42.914
-0.6 2.4 42.914
Geomorphic Zone 2 - Calcasieu Pass to Southwest Pass -3.8 -3.3 -2.7 5.7 6.1 8.8 125.74 124.965 124.55
-4.0 6.4 123.635
-4.5 5.8 124.905
Entire Study Area - Sabine Pass to Southwest Pass -2.1 -2.6 -1.8 -1.2 -2.9 -3.5 5.5 6.1 5.5 8.2 5.41 6.0 167.74 172.16 166.54 173.51 172.06 167.81 average shoreline position change rate (m/year); 2 sample standard deviation (± m/year); 3 length of shoreline analyzed (km); 4 the westernmost 4.96 km of shoreline was not surveyed in 1994; the westernmost 0.73 km of shoreline was not surveyed in 1994
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BYRNES, McBRIDE, TAO, DUVIC 93°51'
0
40'
5
10
15
30'
20
25
30
93°20'
35
40
45
50
Distance Alongshore (km) Figure 2. Net shoreline change between Sabine Pass and Calcasieu Pass, 1883 to 1994. Change rates shown on the plot below the map correspond with graphic representations of advance and retreat. Values along the base of the plot indicate average rates of change for designated sections of coast (see Table 5). Table 5. Spatial and Temporal Trends in Shoreline Position Change for Geomorphic Zone 1 (Sabine Pass to Calcasieu Pass) 1923/24
1883 to Cell Number 1
2
3
4
5
1
1923/24
1957
1994
4
1957
1957 1994
4
1994 4
4.6»
4.9
3.5
5.3
2.8
1.9
3.32
2.9 23.23
1.8
3.9 24.03
1.6
0.9
16.31 -1.2
12.91
22.733 -1.4
-1.1
16.90 -1.2
-1.0
-1.6
0.8
0.6
0.7
0.4
0.7
0.9
14.26
11.71 0.2
7.23 0.6
14.70
0.3 0.3
15.11 0.1
19.65 0.4
0.1
0.1
0.3
0.1
0.2
4.58
3.47 -1.7 0.8 7.87 5.3 3.9
1.62 -1.4
5.90 -2.4
0.81
4.02
0.9 8.45 2.2 1.4
1.0 8.26 9.2
-3.9 3.2 5.79
t.49
0.71
-1.3 0.8 10.07 1.9 1.3 0.74
-2.0 0.7 5.49 2.6 1.7 0.71
7.0 2.09
0.1
average shoreline position change rate (m/year); sample standard deviation (± m/year); 3 length of shoreline analyzed (km); 4 the westernmost 4.96 km of shoreline was not surveyed in 1994
10
20
30
40
60
70
80
Distance Alongshore (km)
50
90
100
110
Figure 3. Net shoreline change between Calcasieu Pass and Southwest Pass, 1884/86 to 1994. Change rates shown below the map correspond with graphic repr retreat. Values along the base of the plot indicate average rates of change for designated sections of coast (see Table 6).
20'
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29*30'
93*25'
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BYRNES, McBRIDE, TAO, DUVIC
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Table 6. Spatial and Temporal Trends in Shoreline Position Change for Geomorphic Zone 2 (Calcasieu Pass to Southwest Pass) Cell Number
6
7
8
9
10
11
12
1947/57
1994 4
1947/57 1994 4
1947/57
1994 4
5.71
3.9
2.7
5.6
5.02 2.03 3
1.8
2.9
3.1 1.2
0.7
21.49
1.0 24.74
20.98
18.88
18.55
-0.4
-1.2
-8.7
-4.9
-0.5
-1.1
0.3
0.5
2.6
2.5
0.2
0.6
5.98
0.51
63.29
3.5 1.7 13.84
2.4 2.1
2.8 1.6
1.13 2.2
0.88 1.4
3.7
21.78
2.8 5.54
2.1
5.58
-1.1 0.6
-8.5 2.6
-2.9 1.0
2.8
3.6 2.08
-9.5 2.9
-9.2 64.60
3.3
10.3
63.23 5.3
2.6
1 7
4.6
2.7
2.2
5.49
19 01
19.92 -2.2
11.62 -1.2
11.50 0.8
-3 0
63.08
1.47
63.47 4
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1883-1994 1957-1994 1990-1994
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eaa Highway 82 551 +00 Beach Profile Station # -10-
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7 8 9 10 11 12 Distance Alongshore (km)
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Figure 5. Long- and short-term beach response for an 18.5-km segment of coast between a position 4.5 km west of Ocean View Beach and about 1 km west of Holly Beach. zone 1 averaged 0.7 m/yr; however, for the period 1957 to 1994, net shoreline retreat was indicated at an average rate of 0.6 m/yr. Although these data do not include the westernmost 4.96 km of prograding shoreline, the relative trend suggests that future shoreline response may be controlled by erosive processes. Geomorphic zone 2 (Calcasieu Pass to Southwest Pass) also contains cells of advance and retreat; however, average shoreline response indicates net retreat for all time intervals. Long-term shoreline change for this 125-km segment of coast is -3.8 m/yr, and incremental changes show increasing retreat rates. Spatial variability in shoreline change is much greater than that in geomorphic zone 1, and the average magnitude of change for the 63-km long cell of erosion (-8.7 m/yr) is 7 times the rate calculated for the Constance Beach area. Overall, temporal and spatial trends in shoreline response indicate increasing shoreline retreat with time. Besides being a function of incident wave energy, shoreline change results indicate that factors such as shoreline orientation to dominant wave processes, sediment supply, and engineering structures have a profound influence on coastal response. Short-term beach profile data for the period 1990 to 1994 can be summarized by segments of coast. The eastern segment, an area fronting the revetment along Highway 82, illustrates slight shoreline advance and major aggradation on the beach profile. The central segment, from just west of the revetment terminus to Ocean View Beach, shows significant shoreline retreat in most areas and slight aggradation on the subaqueous beach profile. The segment west of Ocean View Beach is experiencing shoreline advance and profile aggradation as the downdrift segment of the study area. These data show that beaches behind updrift breakwaters (east zone) fill faster as sediment is transported from east to west. Also, there appears to be a relationship between distance offshore to the breakwater and coastal response. In the Constance Beach area, breakwaters were constructed about 45 m farther offshore than
those to the east. Segments of shore with the greatest retreat rates and a sand deficit on the beach profile exist in this area. A compounding problem in the central segment is the downdrift influence of the revetment (ie., accelerated shoreline retreat rates) on coastal change. Long-term shoreline change data suggest that this is a significant problem. Sand replenishment could be used to mitigate this problem.
Acknowledgements Support for this project was provided by the U.S. Geological Survey, Coastal Geology Program and the Louisiana Department of Natural Resources (DNR), Coastal Restoration Division. The authors would like to thank Mr. Steven Underwood and Dr. Bill Good (DNR), the DNR field support group, Mr. Mark Bradshaw (ConTerra Systems, San Francisco, CA) for GPS support, Mr. Matt Hiland (Intergraph Corporation) for GIS and GPS support, Mr. Guthrie Perry (Rockefeller Refuge) for logistical support, Mr. James Manning (Louisiana Department of Wildlife and Fisheries), and Dr. Nicholas C. Kraus (Texas A&M University - Corpus Christi, Blucher Institute) and Mr. Matthew Taylor (CSI) for critically reviewing the document.
References Anders, F.J. and M.R. Byrnes, 1991, Accuracy of shoreline change rates as determined from maps and aerial photographs: Shore and Beach, v. 59(1), p. 17-26. Britsch, L.D. and J.B. Dunbar, 1993, Land loss rates: Louisiana coastal plain: Journal of Coastal Research, v. 9(2), p. 324-338. Byrnes, MR., R.A. McBride, and M.W. Hiland, 1991, Accuracy standards and development of a national shoreline change database, in: N.C. Kraus, K.J. Gingerich, and D.L. Kriebel, eds., Coastal Sediments '91: American Society of Civil Engineers, New York, NY, p. 1027-1042.
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122 GULF COAST ASSOCIATION OF GEOLOGICAL SOCIETIES TRANSACTIONS Byrnes, M.R. and M.W. Hiland, 1994a, Shoreline position and nearshore bathymetric change (Chapter 3), in: N.C. Kraus, L.T. Gorman, and J. Pope, eds., Kings Bay Coastal and Estuarine Monitoring and Evaluation Program: Coastal Studies: Technical Report CERC-94-09, Coastal Engineering Research Center, Vicksburg, MS, p. 61-143. Byrnes, M.R. and M.W. Hiland, 1994b, Compilation and analysis of shoreline and bathymetry data (Appendix B), in: N.C. Kraus, L.T. Gorman, and J. Pope eds., Kings Bay Coastal and Estuarine Monitoring and Evaluation Program: Coastal Studies: Technical Report CERC-94-09, Coastal Engineering Research Center, Vicksburg, MS, p. B1-B89. Byrnes, M.R. and M.W. Hiland, 1995, Large-scale sediment transport patterns on the continental shelf and influence on shoreline response: St. Andrew Sound, Georgia to Nassau Sound, Florida, U.S.A., in: J.H. List and J.H.J. Terwindt eds., Large-Scale Coastal Behavior: Special Issue in Marine Geology. Byrnes, M.R. and R.A. McBride, 1995, Preliminary Assessment of Beach Response to a Segmented Breakwater System: Constance Beach and Vicinity, 1990-1994: Final Report to Louisiana Department of Natural Resources, Coastal Restoration Division, Baton Rouge, LA, 102 p. Crowell, M., S.P. Leatherman, and M.K. Buckley, 1991, Historical shoreline change: error analysis and mapping accuracy: Journal of Coastal Research, v. 7(3), p. 839-852. Dement, L., 1977, Two new methods for erosion protection in Louisiana: Shore and Beach, v. 45, p. 31-38. Gould, H.R. and E. McFarlan, 1959, Geologic history of the Chenier Plain, southwestern Louisiana: Transactions Gulf Coast Association Geological Societies, v. 9, p. 261-272. Hoyt, J.H., 1969, Chenier versus barrier, genetic and stratigraphic distinction: American Association Petroleum Geologists Bulletin, v. 53, p. 299-396. Huh, O.K., H.H. Roberts, L.J. Rouse, and DA. Rickman, 1991, Fine grain sediment transport and deposition in the Atchafalaya and Chenier Plain sedimentary systems, in: N.C. Kraus, K.J. Gingerich, and D.L. Kriebel, eds., Coastal Sediments '91: American Society of Civil Engineers, New York, NY, p. 817-830. List, J.H.,B.E. Jaffe, and A.H. Sallenger, 1991, Large-scale coastal evolution of Louisiana's barrier islands, in: N.C. Kraus, K.J. Gingerich, and D.L. Kriebel , eds., Coastal Sediments '91: American Society of Civil Engineers, New York, p. 1532-1546. List, J., BE. Jaffe, AJ. Sallenger, Jr., S.J. Williams, R.A. McBride, and S.P. Penland, 1994, Louisiana Barrier Island Erosion Study - Atlas of Seafloor Changes from 1878 to 1989: Miscellaneous Investigations Series 1-2150B, U.S. Geological Survey, Reston VA, 82 p. McBride, R.A., Hiland, M.H., Penland, S., Williams, S.J., Byrnes, M.R., Westphal, K.A., Jaffe, B., and Sallenger, A.H., Jr., 1991, Mapping barrier island changes in Louisiana: techniques, accuracy, and results: in, N.C. Kraus, K.J. Gingerich, and D.L. Kriebel, eds., Coastal Sediments '91: American Society of. Civil Engineers, New York, p. 1011-1026.
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McBride, R.A., Penland, P.S., Hiland, M., Williams, S.J., Westphal, K.A., Jaffe, B., and Sallenger, A.H., Jr., 1992., Analysis of barrier shoreline change in Louisiana from 1853 to 1989,.in, S.J. Williams et al., eds., Atlas of Barrier Island Changes in Louisiana from 1853 to 1989: Miscellaneous Investigations Series 1-2150-A, US Geological Survey, p. 36-97. McBride, R.A. and M.R. Byrnes, 1995, A megascale systems approach for shoreline change analysis and coastal management along the northern Gulf of Mexico: Gulf Coast Association of Geological Societies Transactions, v. 45, p. Merchant, D.C., 1987, Spatial accuracy specification for large scale topographic maps: Photogrammetric Engineering and Remote Sensing, v. 53, p. 958961. . Mitchell, J., 1987, A Remote Sensing Analysis of Erosional and Depositional Trends Along Louisiana Chenier Plain Beaches: M.S. Thesis, Department of Geology and Geophysics, Louisiana State University, Baton Rouge, LA, 140 p. Morgan, J.P. and P.B. Larimore, 1957, Changes in the Louisiana shoreline: Gulf Coast Association of Geological Societies Transactions, v. 7, p. 303310. Morgan, J.P. and D.J. Morgan, 1983, Accelerating Retreat Rates Along Louisiana's Coast: Louisiana Sea Grant Program, Center for Wetland Studies Louisiana State University, Baton Rouge, LA, 41 p. Morton, R.A., 1975, Shoreline Changes Between Sabine Pass and Bolivar Roads: Bureau of Economic Geology Circular 75-6, University of Texas, Austin, TX, 43 p. Nakashima, L.D., J. Pope, and J.L. Dean, 1987, Initial response of a segmented breakwater system, Holly Beach, Louisiana, in, N.C. Kraus, K.J. Gingerich, and D.L. Kriebel, eds., Coastal Sediments '91: American Society of Civil Engineers, New York, p. 1399-1414. Otvos, E.G. and W.A. Price, 1979, Problems of chenier genesis and termihologyan overview: Marine Geology, v. 31, p. 251-263. Penland, S. and K.E. Ramsey, 1990, Relative sea-level rise in Louisiana and the Gulf of Mexico: 1908-1988: Journal of Coastal Research, v. 6, p. 323342. Shalowitz, A.L., 1964, Shore and Sea Boundaries: U.S. Department of Commerce, Coast and Geodetic Survey, U.S. Government Printing Office, Washington, D.C., 420 p. US Army Corps of Engineers, 1961, Calcasieu River and Pass, Louisiana, Design Memorandum No. 2, Jetty Rehabilitation: New Orleans District, New Orleans, LA, 8 p. US Army Corps of Engineers, 1971, Survey of Holly Beach and Vicinity, Louisiana, Series Number 95: New Orleans District, New Orleans, LA, 19 p.
