Hurricane Isaac storm surge deposition in a coastal wetland along ...

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Hurricane Isaac storm surge deposition in a coastal wetland along Lake Pontchartrain

Hurricane Isaac storm surge deposition in a coastal wetland along Lake Pontchartrain, southern Louisiana Kam-biu Liu†, Terrence A. McCloskey‡, Thomas A. Bianchette+, Gregory Keller∞, Nina S.N. Lam#, Jaye E. Cable@, Jill Arriola§ †Dept of Oceanography and Coastal Sciences Louisiana State University Baton Rouge, LA, USA, 70803 [email protected]

‡ Dept of Oceanography and Coastal Sciences Louisiana State University Baton Rouge, LA, USA, 70803 [email protected]

+ Dept of Oceanography and Coastal Sciences Louisiana State University Baton Rouge, LA, USA, 70803 [email protected]

∞Dept of Geology and Geophysics

# Dept of Environmental Sciences Louisiana State University Baton Rouge, LA, 70803 [email protected]

@Dept of Marine Sciences University of North Carolina Chapel Hill, NC, USA, 27599 [email protected]

Louisiana State University Baton Rouge, LA, USA, 70803 [email protected]

www.cerf-jcr.org

§Dept of Marine Sciences University of North Carolina Chapel Hill, NC, USA, 27599 [email protected]

ABSTRACT Liu, K.., McCloskey, T.A., Bianchette, T.A., Keller, G., Lam, N.S.N., Cable, J.E., Arriola, J. 2014. Hurricane Isaac Storm Surge Deposition in a Coastal Wetland along Lake Pontchartrain, Southern Louisiana. In: Green, A.N. and Cooper, J.A.G. (eds.), Proceedings 13th International Coastal Symposium (Durban, South Africa), Journal of Coastal Research, Special Issue No. 70, pp. 266-271, ISSN 0749-0208. www.JCRonline.org

Hurricanes play an important role in shaping the coast of Louisiana. Although the sedimentary signatures of hurricane deposits have been documented in several different coastal environments along the northern Gulf coast, no studies have as yet documented the signatures in wetlands adjacent to large, inland brackish water bodies. In this paper we present results of a case study documenting the distribution and characteristics of storm surge deposits related to Hurricane Isaac (2012) in a wetland on the western shore of Lake Pontchartrain, Louisiana. Hurricane Isaac, a category1 storm, made landfall near the mouth of the Mississippi River on August 28, 2012. Due to its large size and slow movement, Isaac generated strong easterly winds across Lake Pontchartrain, producing a large storm surge along the west shore of the lake and unprecedented flooding in the surrounding lowlands. Loss-on-ignition, XRF, radioisotopic, and grain-size analyses conducted on sediment cores and surface samples from the area identify two distinct sedimentary signatures for the Hurricane Isaac deposits. Near the lake shore the signature is characterized by a laminated silty sand with a geochemical profile closely resembling that of lake bed material. Storm deposits located in a brackish swamp ~ 1km inland consist of a dark, low-organic mud with low concentrations of terrestrial metals and elevated concentrations of Br, S, and Cl. Differences in the storm signal are explained by the differing effect of topographical features on the depositional and transportation processes occurring at the two sites. Utilizing the geochemical/compositional signatures as a hurricane-generated storm surge proxy indicates the possible occurrence of a similar event predating the historical record. ADDITIONAL INDEX WORDS: Paleotempestology, Hurricane Isaac, storm surge, storm deposit, wetland sedimentation, Lake Pontchartrain, Louisiana.

INTRODUCTION Coastal erosion has been a very serious problem in Louisiana for decades (Barras et al., 2008). Although many of the proximate causes such as subsidence, sea level rise, and landward transgression of salinity are gradual processes, erosion often occurs in pulses, driven by the passage of tropical cyclones (TCs), particularly hurricanes (http://coastal.er.usgs.gov/hurricanes/ isaac/coastal-change/). Because of the importance of these TCgenerated geomorphic changes, including a debate over the relative importance of hurricane-delivered sediments and their effect on marsh elevation (Turner et al., 2006; Törnqvist et al., 2007), several studies have investigated the depositional signatures of TCs along the northern Gulf coast. Studied ____________________ DOI: 10.2112/SI70-045.1 received 1 December 2013; accepted 21 February 2014. © Coastal Education & Research Foundation 2014

environments include coastal wetlands (Turner et al., 2006), a deltaic marsh (Reese et al., 2008), a backbarrier lagoon (Liu et al., 2011), and beach-ridge plains (Williams, 2009, 2011). However, as the severely indented coastline of Louisiana is immensely varied, consisting of a large number of bays, lakes, marshes and swamps that vary greatly in terms of size, salinity, vegetational communities and degree of fluvial influence, it can be expected that geomorphic response to TCs will also vary by location and environment type. Therefore, expanding the spectrum of environments for which depositional signatures of TCs are known is an important objective, in order not only to improve our ability to accurately assess the geomorphologic impact of TCs, but also increase our understanding of the sedimentary processes involved. In this study we advance this objective by examining the geochemical and stratigraphic characteristics of material deposited

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Figure 1. Location of study site. Google Earth imagery displays a), the northern Gulf of Mexico, b), southeastern Louisiana, c), the western margin of Lake Pontchartrain, d) the community of Frenier, showing the locations of cores LAP-1, FRE-4 and FRE- 5, and the elevated rail line (marked in yellow). The white line in b) represents the track of Isaac during the 36 hour period from 5 PM, August 28 (lower white dot) through 5 AM, August 30 (upper white dot). The red box in d) is the area displayed in Figure 2.

by Hurricane Isaac (2012) in a wetland on the margins of Lake Pontchartrain in southern Louisiana.

STUDY SITE Lake Pontchartrain is a large (1,600 km2), shallow (~ 4 m) ovalshaped brackish lagoon lying east of the Mississippi River just north of New Orleans (Li et al., 2008). As the lake receives freshwater input along the northern and western edges and is connected to the Gulf of Mexico by a few small openings along the eastern edge, salinity decreases from east to west. Immediately to the west of Lake Pontchartrain and connected to it by two narrow passes lies Lake Maurepas, a smaller (240 km2), shallower (~3 m), and fresher lake (Keddy et al., 2007). An extensive wetland, occupied by cypress swamps and bottomland hardwood forests, surrounds Lake Maurepas, extending from the western edge of Lake Pontchartrain to the city of LaPlace, located on the eastern bank of the Mississippi River (Figure 1c). During Hurricane Isaac the entire area suffered unprecedented flooding, forcing the closure of the interstate highway in LaPlace, and requiring the evacuation of trapped residents in several neighborhoods (Boquet, 2012; Thibodeaux, 2012). Frenier is a small community located in a bottomland hardwood forest along the western edge of Lake Pontchartrain. A raised (~2 m) railroad line runs parallel to the lake’s shoreline ~250-400 m inland behind the community (Figures 1, 2). Although Frenier regularly floods during tropical cyclones, local residents relate that the inundation associated with Isaac was significantly deeper and longer-lasting than for other recent events (Hurricanes Katrina, 2005; Rita, 2005; Gustav, 2008) (L. Lipps, personal communication).

We extracted cores from two sites in the area. Core LAP1 was extracted ~ 1 km west of Lake Pontchartrain in a low area consisting of a mix of cypress swamp, innumerable black-water ponds, and hardwood forest, with a water table generally at or above ground level (Figure 1d). Cores FRE4 and FRE5 were extracted from the area immediately around the community of Frenier, which is much dryer, with no open water. Our coring sites in this area were selected to capture the area of maximum storm surge and wave energy as determined by the spatial distribution of the storm debris, with cores FRE4 and FRE5 taken from the area most heavily covered by hurricane debris (Figures 1d, 2).

HURRICANE ISAAC Hurricane Isaac made landfall near the mouth of the Mississippi River on August 28, 2012 as a category 1 hurricane, with maximum sustained wind speed of 70 kts (Berg, 2013) (Figure 1b). However, due to an unusual combination of features, both the societal and biophysical effects of this storm greatly exceeded the norm for hurricanes of this magnitude along the northern Gulf coast. Isaac was a very large storm; tropical-storm-force winds extended >320 nautical miles at landfall. It was also a very slow moving storm, at times remaining either stationary and drifting backwards on its path. This resulted in an extremely long duration for sustained tropical force winds (up to 45 hours) in coastal areas, heavy rainfall (>50 cm in New Orleans), and a vast amount of water pushed westward against the southeastern shoreline of Louisiana (Berg, 2013, US Army Corps of Engineers, 2013). These conditions were powerful enough to force the Mississippi River to flow backwards for 24 hours, with the storm surge


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Figure 2. Frenier coring site. Red lines mark the approximate boundaries of a fairly narrow debris field resulting from Hurricane Isaac. As the debris originated in the lakeside buildings, the location and orientation of the field indicate the primary direction (from the northeast) of wind and waves at the time of maximum impact. Cores FRE4 and FRE5 were sited within the debris field. advancing >300 river miles upstream (US Army Corps of Engineers, 2013). In Lake Pontchartrain the sustained easterly wind piled water against the western shore, and storm surge occurred during a period of already elevated water levels (Berg, 2013, US Army Corps of Engineers, 2013), resulting in unprecedented inundations for the area. In the area immediately surrounding Frenier the elevated rail line acted as a barrier to the free movement of water, resulting in differing depths and durations of inundation for the areas east and west of the line. Although both areas were connected hydrologically during the time of maximum water level, drainage was very different for the two areas. Flood waters in the area east of the rail line were able to drain directly back into Lake Pontchartrain, while the area to the west remained flooded much longer as drainage in that area required a slow traverse across the large wetlands and Lake Maurepas.

METHODS Cores FRE4 and FRE5 were extracted by means of a 7.62 cm diameter aluminum push tube. FRE4 reached a depth of 46 cm, the 36 cm of sediments contained in the core indicates that 10 cm of compaction occurred. FRE5 captured the top 30 cm of sediments without compaction. LAP1, extracted with a Russian peat borer, consists of three overlapping sections pulled from within a small (~25 cm diameter) area with a total core length of 119 cm. A sample of surface material, ~5 cm in depth, was obtained at the same location by means of a putty knife. Peat borer sections and surface samples were photographed in the field. All coring and sampling locations were determined by a handheld GPS unit. Cores were sealed in the field and transported to The Global Change and Coastal Paleoecology Laboratory at Louisiana State University, where they were kept in a cold room (4°C) until opened, at which time the cores were photographed and the composition, structure, color and stratigraphy described. Loss-on-ignition (LOI) analysis was conducted continuously at 1 cm resolution (Liu and Fearn, 2000) to determine water, organic, carbonate, and residual percentages. Grain size analysis was

performed on 38 clastic samples for FRE4 with a Beckman Coulter ls 13 320 laser diffraction particle size analyzer. Sampling resolution was 0.5 cm until a depth of 3 cm, after which material was sampled every cm, except for cm 33. Both the top of LAP1 and the surface material collected from the immediate area were analyzed for the short-lived radioisotopes, 7Be (t1/2 = 53 days; 477 keV gamma emission) and 234Th (t1/2 = 24 days; 63 keV gamma emission) by gamma spectrometry using an intrinsic germanium well detector (Canberra®) at the University of North Carolina. All sediment material was collected on 9/26/2012, 29 days after Hurricane Isaac made landfall. Sediment samples were shipped to UNC where they were homogenized and packed into 10-mm diameter gamma vials. Samples were counted within 1 to 3 months of collection for 1 to 2 days each. Total 234Th and 7Be activities were determined on all samples and corrected using the detector efficiency for each gamma peak as determined from counting standard reference material calibrated by the International Atomic Energy Agency (IAEA-300 marine sediment). Supported 234Th was determined after greater than six half-lives of 234Th had passed and the excess 234Th activity had decayed to less than 1%. Sediments were recounted and the remaining 234Th was taken as the supported activity. Supported 234 Th was deducted from the total 234Th (first count), and the excess 234Th and 7Be activities are reported as decay-corrected to the time of collection (± 1σ analytical error). We evaluate the new or event deposition based on the presence and magnitude of these decay-corrected activities. Elemental concentrations were determined for four sediment samples taken from the bed of Lake Pontchartrain in front of the study site and for all three cores at 2 cm resolution by a Delta Premium DP-4000 X-ray fluorescence (XRF) analyzer. A small quantity of gray clay from a depth of 101 cm in LAP1 was sent to Beta Analytic, Inc. in Miami, FL, USA for bulk sediment radiocarbon dating. Dates were calibrated to calendar years with the Calib 6.0 program (http://calib.qub.ac.uk/calib/calib.html), using the Reimer et al. (2009) dataset.

RESULTS LAP1 and surface samples LAP1 is dominated by a very uniform grayish-brown, loworganic clay. Except for a minor peak in organics at 90 cm, from 13-101 cm there is no significant stratigraphic change, with very little variation in either the wet weight percentages of water (3040%), or the dry weight percentages of organics (seldom above 7%), carbonate (always 90%). However, both the top 12 cm and the bottom 17 cm do display clear stratigraphic changes. The top 4 cm are a slightly organic (13-22%) mud, with water content from 64-75%. The next four cm (5-8) are much wetter (water percentages >80%) and more organic (32-65%). From 8-12 cm, water and organic values decrease to levels only slightly above that associated with the clay, which begins at 13 cm (Figure 3). A similar pattern occurs at the bottom of the core, as below 101 cm organic and water percentages first rise, reaching values of 28% and 65% respectively at 107 cm, and then drop, until by 111 cm water and organic levels are only slightly higher than the clay. Another very small organic rise occurs at 118-119 cm at the very bottom of the core. Shifts in elemental concentrations mirror these compositional changes, as the core top and bottom share an elemental signature, which is distinctly different from the clay signature. The clay interval occupying the middle of the core is characterized by high



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Figure 3. Core LAP1. LOI curves (left) display water, organic and carbonate contents of the core at 1-cm resolution, resolution of concentrations of selected elements (center) is 2 cm. Photo (right) shows the sharp contact between the ubiquitous mud layer found around LAP1 and the underlying organic material. 7Be analysis shows the mud layer to have been deposited shortly before the time of collection, which occurred 29 days after the passage of Hurricane Isaac. The 14C age and the 2σ calibrated date range are listed next to a red bar indicating the depth of the sample (101 cm) on the LOI curves (left). concentrations of such terrestrial metals as Ti, V, Cr, Fe, and Zn, and low concentrations of Br, Sr, and Ca, while the more organic sections at the top and bottom of the core display the reverse pattern (Figure 3). The bulk sediment sample from 101 cm in LAP was dated at 1280±30 14C yr BP, which calibrates to a single calendar date range of 1177-1286 BP (Table 1). Significant amounts of both 7 Be and 234Th were found to be present in all four samples that were analyzed, with 7Be values ranging from 2.47 to 8.37 dpm/g and 234Th values ranging from 1.30 to 34.1 dpm/g (Table 2).

four cm is the largest in the core (Figure 4). Elemental concentrations in the uppermost unit in both FRE4, and FRE5 are markedly different from downcore material, although achieving a good match with sediment samples taken from the lake bed immediately off shore, having much higher concentrations of Ca, Table 2. Shortlived radioisotopic results for samples from the LAP1 site. Sample

FRE4, FRE5 FRE4 and FRE5 have very similar chemical elemental profiles (Figure 4). A thin section of laminated brown sandy material at the top of each core (3 cm thick in FRE4, 2 cm in FRE5) is characterized by extremely low water and organic percentages, (in fact, in both cores both water and organics values reach core minima in this interval). These sediments occur immediately above rooty silt intervals (from 4-12 cm in FRE4, from 3-6 cm in FRE5) with either the highest water and organic values (FRE4) or

Depth

7

Be

234

Th excess

(cm)

(dpm/g)

(dpm/g)

Surface sample

0-1 1-2

3.55 ± 0.17 8.37 ± 0.17

1.30 ± 0.19 3.56 ± 0.19

LAP1

0-4

6.96 ± 0.42

11.57 ± 0.85

4-7

2.47 ± 0.44

34.05 ± 0.77

Sr, Zr, and Mn and lower concentrations of Fe, Co, and Br than other core material (Figure 5). Table 1. Radiocarbon results for the bulk sediment sample from core LAP1. Lab # 360464

Core LAP1

Depth (cm) 101

Age C yr BP 1280±30

14

Age (2σ) Cal yr BP 1177-1286

DISCUSSION

%

Sedimentary signatures 1

the highest water and second highest organic values (FRE5) in their respective cores. Farther downcore the material is gray silt that transitions into clay near the core bottoms. Occasional intervals with slightly elevated organic values occur. Grain size analyses conducted on FRE4 indicate that the material in the top

234

Th and 7Be are short-lived, particle reactive radionuclides. 7Be is derived from cosmic ray spallation in the upper atmosphere and is deposited on the earth’s surface during precipitation events. 234 Th is a daughter of the ubiquitous primordial earth element, 238 U, and occurs in excess of its 238U parent when sediment is washed into an area from another location. Because both radionuclides appear in sediments from an external source, we can use them to evaluate the initiation of an event and/or ascertain how rapidly sediments were deposited. The presence of these isotopes within the mud layer at the top of LAP1 constrains the


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Topographic/geographic controls

Figure 4. Core FRE4. LOI curves for FRE4 (middle) show a distinctive dip in the top four cm (shaded box), which presents as a laminated clastic material (top left), with appreciably larger grain size.

date of the initiation of deposition of this layer to a point in time shortly before our collection, which occurred 29 days after Hurricane Isaac made landfall. Additionally, the high activities of both excess 234Th and 7Be indicate that the sediments were deposited over an extremely short time span. The contact between this material and the underlying higher-organic sediments is both visually striking and ubiquitous among samples; random sampling in the area surrounding LAP1 produced nearly identical results at multiple sites. Often a layer of well-preserved intact leaves and forest debris could be observed immediately below the mud. The dramatic stratigraphic mismatch between the mud cap and the underlying material is paralleled by equally clear differences in elemental concentrations, which are marked by a downcore increase in terrestrial metals and a decrease in Br, S, and Cl (Figure 3). The most likely interpretation of this sequence is that the surficial mud was deposited by Hurricane Isaac and that the more organic underlying layer represents the forest floor at the time of the storm. The sedimentary signature at LAP1 seems clear; namely a dark, low-organic mud, with low concentrations of terrestrial metals and high concentrations of Br, S, and Cl. As at the LAP1 site, the surface units in the Frenier cores display major differences with the underlying material in terms of grain size, color, texture, and structure, as seen in FRE4 (Figure 4). FRE5 exhibits a nearly identical stratigraphy, capped by the same unique top unit. It seems reasonable to identify these anomalous surface units as Hurricane Isaac storm deposits. Their elemental profiles suggest that a significant portion of this material was derived from lake bed material. The signature of this deposit is characterized by a light-colored, low-organic, laminated silty sand with high concentrations of Ca, Sr, Zr, and Mn, and low concentrations of Fe, Co, and Br. This does not match the signature from LAP1.

The fact that we have clear, but differing, sedimentological/ geochemical storm signals for the two locations raises some interesting avenues of investigation. Because both signatures originated with the same event, the differences must be due to variations in the effects of the transportation and/or depositional processes. Investigating these differences should therefore help us better understand the working of these processes during tropical cyclones. Geographical and topographic features can exert important control over the hydrodynamics of storm waves and the movement of water during periods of sustained flooding (Bianchette et al., 2009). Of particular importance in this case is the elevated rail line just to the west of Frenier. Roughly ~2 m in height, the road bed acted as an effective barrier to water movement and the transport of materials during Isaac, as evidenced by the stranding of a small fishing boat and large cargo container on the lake side of the embankment by the storm. This suggests that large-grained traction load material would not have been transported inland of the rail line. The presence of this barrier also likely complicated wave and sedimentation patterns east of the rail line, with sediment-laden bottom flow rebounding lakeward after colliding with the embankment. Conceivably, the resulting temporal variability in wave energy and direction and velocity of flow could have contributed to the laminated nature of the storm deposit by affecting the parameters determining the settling of large-grained sediments. Therefore, very different patterns of storm activity may have occurred at the two coring sites. Lakeward of the embankment (FRE4, FRE5), we can hypothesize a turbulent storm surge-driven mixture of nearshore/beach lake bed sediments (light-colored, larger-grained) and loose/dislodged surface material from the bottomland forest, with the energy reflected from the constraining embankment causing erratic movement of both bottom sediments and surface water. After passage of the storm, the relatively quick drainage of the flood water would have carried much of the suspended load back to the lake. However, a very different flood event could have occurred at LAP1, west of the embankment. Due both to distance and the presence of the rail line, only small-grained lake material, carried as suspended load in the upper part of the water column, would have reached the site.

Figure 5. Core FRE4, FRE5 and lake material. The surface units are distinctly different from downcore material, both compositionally and in elemental concentration for both FRE4 (top) and FRE5 (bottom).The elemental concentrations of this material is very similar to that from lake bed samples (bottom).



This lake material would have formed a smaller portion of the total sediment load than at FRE4, FRE5, as the flood waters, though less turbulent, entrained large amounts of the loose, finegrained dark swamp material forming the forest floor or floating in or resting on the bottom of the small pothole ponds. Much of this material would have subsequently been deposited at the site, after settling out of the water column during the long, slow drainage process. This variability in the sedimentary response to tropical cyclones highlight the difficulty in creating viable proxies for correctly identifying hurricane events in the sedimentary record. The field of paleotempestology uses such signatures derived from historical storms as sedimentary proxies for establishing multi-millennial landfall records for coastal locations. The ultimate purpose is to correlate these records with known paleoclimatic conditions in order to identify the climatic/atmospheric mechanisms forcing changes in tropical cyclone activity (Liu and Fearn 2000; Donnelly and Woodruff, 2007; McCloskey and Liu, 2012). As nearly all previously described modern analogs have been derived from sea-edge locations, the identification of sedimentary signatures from this inland site may prove useful. The FRE4, FRE5 signature is similar to the common coastal proxy, which uses large-grained, light-colored clastics as a proxy for hurricanegenerated overwash lobes. The LAP1 proxy is a bit more subtle. Because the storm deposit consists mainly of autocthonous material (resuspended pond/forest sediments) with a very small marine/lake component, the event layer itself may not be recognizable in the sedimentary record. However, in this case the mud deposit may have created a recognizable, though indirect, proxy by covering, and thereby preserving, the underlying forest surface. This higher-organic sediment, which by a combination of weathering and bioturbation would normally degrade into the featureless, low-organic silty clay found lower in the core, will perhaps, by its burial, be preserved as a distinct layer. Recognizing a buried forest surface layer as marking a hurricane provides another tool for researchers looking to extend the landfall record, particularly, given the necessity of thick draping mud layer at large distances inland, for the identification of extremely large events. A potential candidate layer occurs near the bottom of LAP1, dated to ~1200 yr BP (years before present) (Figure 3), although further investigation is needed.

CONCLUSIONS Anomalous surface layers, 2-4 cm in thickness occur over a wide area of forested wetlands on the western edge of Lake Pontchartrain, Louisiana, USA. Short-lived radioisotopic analysis strongly suggests that this material was deposited rapidly, as a unit, at the approximate time of the passage of Hurricane Isaac, a category 1 hurricane which caused unprecedented flooding in the area in August, 2012. The surface layer covering a swamp forest ~1 km inland consists of a dark, low-organic mud, with low concentrations of terrestrial metals and high concentrations of Br, S, and Cl. However, the distinguishing characteristics of the posited storm deposit in a bottomland hardwood forest