Contribution of Hurricane Rita Storm Surge Deposition to Long-Term Sedimentation in Louisiana Coastal Woodlands and Marshes

Transcription

1 Journal of Coastal Research SI ICS2009 (Proceedings) Portugal ISSN Contribution of Hurricane Rita Storm Surge Deposition to Long-Term Sedimentation in Louisiana Coastal Woodlands and Marshes H.F.L. Williams and W.M. Flanagan Department of Geography, University of North Texas, Denton 76203, USA ABSTRACT WILLIAMS, H.F.L. and FLANAGAN, W.M., Contribution of Hurricane Rita Storm Surge Deposition to Long-Term Sedimentation in Louisiana Coastal Woodlands and Marshes. Journal of Coastal Research, SI 56 (Proceedings of the 10th International Coastal Symposium), Lisbon, Portugal, ISSN A field survey of Hurricane Rita storm surge deposits in the Chenier Plain of Southwest Louisiana was conducted in November 2005, and repeated in April The results show that the storm surge deposits are well-preserved in coastal woodland and marsh environments. Along a transect of pits, oriented approximately parallel to the direction of storm surge inundation, the preserved deposits form a wedge of sandy to muddy sediments that fines and thins inland. Maximum thickness of the sediment wedge is about 25 cm immediately landward of a sandy beach, tapering to a few cm of sandy mud about 420 m inland. Long-term sedimentation rates in soils underlying the storm surge deposits, determined by Cesium-137 dating, vary from 0.24 to 0.71 cm per year. A comparison of storm surge sedimentation to these long-term sedimentation rates, suggests that a single hurricane deposit may be the equivalent of over a century of non-storm-surge sedimentation. Based on the estimated recurrence interval of large hurricanes in the study area, the results suggest that storm surge deposits may account for between one third and two thirds of long-term sedimentation. It is concluded that hurricanederived deposition is a significant contribution to long-term sedimentation in these near-shore coastal environments. ADDITIONAL INDEX WORDS: cesium-137 dating, hurricane washover, wetlands, washover terrace, chenier hurricanes over a short time period can be superimposed, resulting in stacked deposits and increased vertical aggradation (HAYNE and CHAPPELL, 2001; WANG and HORWITZ, 2006). WILLIAMS (in press) documented storm surge sedimentation in southwest Louisiana s Chenier Plain resulting from Hurricane Rita (September 24, 2005). The storm surge deposits formed a wedge of washover sediments, up to 50 cm thick and extending about m inland. Deposition occurred in coastal woodlands and palustrine marshes immediately landward of sandy beaches. This paper presents the results of a study made to assess preservation of the storm surge deposits and their contribution to long-term sedimentation in these nearshore environments. INTRODUCTION In recent decades considerable attention has focused on the role of hurricane storm surge sedimentation as a mechanism of aggradation in salt and brackish marshes bordering the Gulf of Mexico (BAUMANN, DAY and MILLER, 1984; REJMANEK, SASSER and PETERSON, 1988; CONNER, et al., 1989; REED 1989; DAY, et al., 1995; NYMAN, CROZIER and DELAUNE, 1995; STONE et al., 1997; PARSONS, 1998; CAHOON, 2006; TURNER, et al., 2006, 2007). These studies are part of a larger effort aimed at improving understanding of marsh aggradation and guiding more effective management strategies to combat rapid conversion of marshes to open water due a combination of regional subsidence and eustatic sea-level rise (DELAUNE et al., 1989; MILAN, et al., 1995; CALLAWAY, DELAUNE and PATRICK, 1997; REED, DE LUCA and FOOTE, 1997; TURNER, MILAN and SWENSON, 2006). Hurricane storm surges also deposit large amounts of sediment in terrestrial environments immediately landward of sandy beaches along many km of coastline, forming washover terraces (GUIDROZ, STONE AND DARTEZ, 2006; SALLENGER, et al., 2006). For example, OTVOS (1999) reported up to 90 cm of sand aggradation immediately landward of East Belle Fontaine beach near Biloxi Mississippi, resulting from a storm surge generated by Hurricane Georges (1998). WANG, et al., (2006) described a washover terrace on the northwest coast of Florida resulting from Hurricane Ivan in The terrace extended about 100 km along the shoreline, was over 200 m wide and up to 1.2 m thick. There is also evidence that storm surge deposits from two or more STUDY AREA The study area is undeveloped land near the small beachfront community of Constance Beach in Cameron Parish, Louisiana, about 35 km east of the Texas-Louisiana border. The coastline is fronted by wide sandy beaches and backed by alternating shoreparallel linear tracts of woodland and palustrine marsh (Figure 1). The area has low relief, on the order of about 1 m. The woodlands occupy subdued ridges elevated about 0.5 to 1 m above adjacent lower-lying marshlands. The area is part of the Chenier Plain of southwest Louisiana. Multiple shore parallel, sand-rich beach ridges separated by low-lying, muddy, fresh to brackish marshes characterize this coastal plain. 1671

2 Contribution of Hurricane Rita Storm Surge Deposition Figure 1. Study area near Constance Beach, Louisiana, showing location of pits 1-8, visible sandy washover deposits (white arrows) and approximate track of Hurricane Rita (inset). NOAA image acquired September 25th, METHODS A survey of storm surge sedimentation in the study area was made in November 2005, approximately six weeks after Hurricane Rita made landfall, and again in April 2007, to assess long-term preservation of the deposit. The storm surge sediments were examined in a series of eight pits excavated by spade along a transect beginning near the seaward edge of coastal woodlands and ending approximately 500 m inland. Pits 1, 2, 3, 6, 7 and 8 were located in coastal woodland tracts, whereas pits 4 and 5 were located in an intervening palustrine marsh (Figure 1). The transect was oriented approximately perpendicular to the coast and aligned with the hurricane wind direction as determined from numerous scour marks and the leaning of displaced piers. The location of each pit was recorded by a sub-meter accuracy global positioning system and marked with a flagged stake to facilitate site identification at a later date. In each pit, the soil underlying the storm surge deposit formed a clear abrupt boundary with the overlying sediments. The thickness of storm surge sedimentation at each location was estimated by averaging the thickness at each side of the pit. In the vicinity of pit 8 the storm surge deposit consisted of only a thin and discontinuous veneer (1-2 cm) of mud (this site was not included in the remainder of the study). A level transit was used to measure the elevation of each pit relative to a nearby road intersection which has an estimated elevation of 2.1 m (National Geodetic Vertical Datum 1929) based on spot heights shown on a 1998 United States Geological Survey topographic map (USGS, 1998). Pit 1 was located approximately 10 m inland from, and 0.3 m below, the crest of the adjacent beach berm. In November 2005, samples of the soils underlying the storm surge deposit in pits 1-7 were collected for Cesium-137 analysis. Samples of uniform volume were collected at 2-cm intervals from the side of each pit down to 50 cm depth below the soil surface. At pits 4 and 5, the soils were waterlogged at a shallow depth; here a spade was used to excavate 5-cm-thick soil monoliths from the side of each pit below the water table and soil samples were collected from the monoliths. These sampling methods caused no compaction of the soil samples. All pits were filled in when sampling was completed. Cesium-137 concentrations were counted on a gamma ray spectrometer having a solid state Ge(Li) detector. Gamma rays corresponding to the branched decay of cesium-137 to barium- 137, which yields a gamma of MeV, were measured for hours per sample. Cesium-137 concentrations are reported as Bq kg -1 of dried sediment. Average background radiation was calculated and subtracted from the counts. Counting errors are reported as 2-sigma error bars. In April 2007, new pits were excavated within 1 m of the original sites, to assess preservation of the deposit over the 1.5- year period. Thickness of the preserved deposit at each location was estimated by averaging the thickness at each side of the pit. RESULTS Preservation of Storm Surge Deposit Considerable changes in the thickness of the storm surge deposit occurred between November 2005 and April The greatest changes occurred at pit 1 where sediment thickness decreased by 50% (from 50 cm to 25 cm), and at pits 4 and 5 where sediment thickness approximately doubled (from 13 cm to 24.5 cm and from 11 cm to 20 cm, respectively). Small decreases in sediment thickness were recorded at pits 2 and 7 and small increases were recorded at pits 3 and 6 (Figure 2). In November 2005, the surface of the storm surge deposit was unvegetated, consisting of rippled sand, covered by a thin veneer of mud in low spots. Localized scouring around trees and bushes had formed undulations with an estimated relief of cm. By April 2007, vegetation was well-established in the study area (Figure 3). Given the stabilizing effect of the relatively dense vegetation cover, it is assumed that the storm surge sediment thicknesses recorded in 2007 will be preserved in the long-term. Although roots had penetrated the entire thickness of the storm surge deposit at all seven pit sites, most roots and resulting bioturbation appeared concentrated in about the upper 2-3 cm of the sediments. Bioturbation had not obscured the sharp contact Figure 2. Nearshore environments and thickness of storm surge deposit in November 2005 and April Numbers refer to pits shown in Figure

3 Williams and Flanagan between the storm surge deposit and underlying soils. Cesium-137 Dating Soils at all seven pit sites contained Cesium-137 in their upper parts, attesting to recent (post-1950) sedimentation. However, most profiles of Cesium-137 concentration did not conform to the ideal shape with a clear 1963 peak (Figure 4). It may be that this has resulted from relatively low sedimentation rates, a relatively coarse sampling interval (2 cm) and vertical mixing caused by bioturbation. It is also possible that reworked sediments containing Cesium-137 have washed into these sites, increasing near-surface Cesium-137 concentrations or that the upper parts of some soils have been eroded, truncating the profiles (although WILLIAMS (in press) observed no evidence of erosion of soil surfaces by Hurricane Rita s storm surge). Given the lack of a clear 1963 peak in most profiles, dating was based on the assumption that the depth at which the Cesium-137 concentration falls to zero corresponds to the year 1950 (MILAN, et al., 1995). This provides a conservative estimate of maximum sedimentation rates because it is unlikely that bioturbation or other processes could raise all traces of Cesium-137 from lower to higher in the soil profile, thus raising the apparent position of the 1950 marker horizon. Long-term average sedimentation rates were calculated by dividing the depth to the 1950 marker horizon by 55 years (Table 1). Figure 3a.View inland from between pit 2 and pit 3, November The unvegetated surface of the storm surge deposit consists of rippled sand (foreground) or a thin veneer of mud (center right). Surface undulations have resulted from scouring around trees and bushes. b. Similar viewpoint in April 2007 vegetation has become well established in the study area. Figure 4. Profiles of Cesium-137 in soils at each pit site. Error bars are +2-sigma. Subtraction of background radiation resulted in small negative counts for some samples. CB: Constance Beach. DISCUSSION The results show considerable redistribution of the storm surge sediments occurred between November 2005 and April Erosion of the unvegetated deposit surface after November 2005 probably resulted from wind deflation or rain washing sediment from higher to lower elevations. Exposure to winds blowing inland from the Gulf of Mexico likely contributed to the loss of the upper 25 cm of the deposit at pit 1. The addition of relatively large amounts of sediment at pits 4 and 5 may reflect the ability of the marshland surface to act as a sediment trap (due to wetter conditions, dense vegetation cover and generally lower elevations). The overall effect of the redistribution of sediment was to even out the thickness of the storm surge deposit between pits 1 and 5, where the difference between minimum and maximum thickness changed from 39 cm in 2005 to 5 cm in 2007 (Figure 2). This presumably resulted from movement of sediment by wind or rain from high points to low points (e.g. scour hollows) on the deposit surface. There is little indication of either net loss or net gain of sediment between 2005 and 2007; the average thickness of the deposit, based on measurements from pit sites, changed very little between the two surveys (18.4 cm in 2005, and 18 cm in 2007). Long-term average sedimentation rates derived from Cesium- 137 dating vary from 0.24 to 0.71 cm/year (Table 1). The preserved thicknesses of storm surge sediments at the pit sites are equivalent to decades to over a century of non-storm-surge sedimentation. Site 4, on the margin of the palustrine marsh, had the lowest long-term sedimentation rate (0.24 cm/year). The preserved thickness of storm surge sedimentation (24.5 cm) at this site is equivalent to 102 years of non-storm-surge sedimentation. Even at the most landward sites (pits 6 and 7) the relatively small thicknesses of preserved storm surge sediments (6.2 cm and 8 cm, respectively) are equivalent to 20 years of non-storm-surge sedimentation (Table 1). The significance of hurricanes to long-term sedimentation in the study area depends on the frequency and amount of storm surge sedimentation. There are many factors that determine whether or not a hurricane generates a storm surge of sufficient height and 1673

4 Contribution of Hurricane Rita Storm Surge Deposition Table 1. Long-term sedimentation rates in soils underlying the storm surge deposit derived from Cesium-137 dating; preserved thickness of storm surge sedimentation, and years of non-storm-surge sedimentation represented by the storm surge deposit. Pit Distance Depth to 1950 Average sedimentation Preserved thickness of storm Number of years Inland (m) marker horizon (cm) rate (cm/year) surge sedimentation (cm) equivalent energy to transport sediment into a particular site on the coast. These factors include distance from the eye of the storm, position in the left or right quadrant at landfall, magnitude of the hurricane, areal extent of the hurricane, forward speed of the storm, local tide levels, wave set up, wave run up, an available source of sediment, the height of topographic barriers to inundation and local topographic and/or bathymetric factors that either enhance or diminish storm surge height. There is not a simple correlation between hurricane magnitude and storm surge sedimentation because a small hurricane making landfall a short distance west of a site may result in a larger storm surge and more sedimentation than a larger hurricane making landfall farther west of the site or even a similar distance to the east of the site. In the absence of historical field survey data, the frequency of storm surge sedimentation must be estimated based on records of the magnitude and location of nearby landfalling hurricanes. Hurricane Rita made landfall approximately 35 km west of the study area as a Category 3 hurricane with maximum sustained winds of 185 km/h, generating a storm surge of 4-5 m in southwest Louisiana (KNABB et al., 2006; FEDERAL EMERGENCY MANAGEMENT AGENCY (FEMA), 2006). In the historical record maintained by the National Oceanic and Atmospheric Administration (NOAA, 2008) there are two other hurricanes that had similar characteristics to Hurricane Rita, made landfall within 100 km west of the study area and may have caused similar storm surge sedimentation. Hurricane Audrey (1957) made landfall about 17 km west of the study area as a Category 4 hurricane with maximum sustained winds of 231 km/h and an estimated storm surge of up to 3.6 m. An unnamed Category 3 hurricane in 1886 made landfall approximately 5 km west of the study area. The hurricane had maximum sustained winds of 194 km/h. No data is available on storm surge height. A frequency of three hurricanes in the historical period ( ) yields a recurrence interval of 52 years. Assuming all three hurricanes caused the same amount of storm surge deposition as Hurricane Rita allows estimation of the contribution of hurricanes to long-term sedimentation in the study area (Table 2). Table 2. Estimated contribution of hurricane storm surges to long-term sedimentation based on a recurrence interval of 52 years. Pit Non-storm-surge sedimentation (cm) Storm surge sedimentation (cm) Percentage contribution of hurricanes The results in Table 2 suggest that hurricane-related deposition varies from about one third to two thirds of long-term sedimentation in the study area. It is possible that much of Louisiana s Chenier Plain coastline experienced similar levels of sedimentation. TURNER, et al., (2006), surveyed storm surge deposits a few weeks after Hurricane Rita s landfall. They report thicker, sandier, low-organic-content sediments, probably derived from immediately offshore, occur within a few km of the shoreline along 150 km of coastline, from Sabine Pass to eastern Vermilion Parish. The deposits were generally thicker nearer landfall and also thicker on the margins of inlets such as Sabine Pass, probably because of reduced resistance to the passage of the storm surge. The deposits generally became thinner and discontinuous farther from shore, although freshly-deposited muddy organic-rich sediments were found up to 18 km inland. These sediments, although deposited by the storm surge, may have resulted from local reworking of marsh deposits and therefore may not constitute a net gain of sediment to the marsh (PARSONS, 1998). The data from TURNER et al., (2006) and WILLIAMS, (in press) suggest that Hurricane Rita s washover deposits thin rapidly inland and extend a few hundred meters to perhaps 2 km from shore, probably depending on local conditions (e.g. dense stands of coastal woodlands may have limited inland penetration of washover sediments to about 500 m in the area studied by WILLIAMS, in press) (Figure 5). Based on the findings of the present study, this sediment probably was subject to some redistribution by wind and rain, but was presumably preserved once vegetation was re-established. The preserved washover deposits constitute a net gain of sediments for these nearshore environments; textural and microfossil analyses performed by WILLIAMS (in press) showed that the beach and immediate offshore zone were the likely sources of washover sediments and no evidence was found of erosion and re-deposition of sediment from woodland and marsh surfaces. Figure 5. Thickness of Hurricane Rita storm surge deposit in nearshore environments of Louisiana s Chenier Plain. Data from WILLIAMS (in press) and TURNER et al., (2006; data from the shoreline to 2000 m inland only). 1674

5 Williams and Flanagan CONCLUSIONS Hurricane Rita s storm surge deposit represents a significant contribution to long-term sedimentation in coastal woodlands and marshes of the study area. Preserved thicknesses of the deposit are the equivalent of decades to over a century of non-storm-surge sedimentation. Based on an estimated recurrence interval of hurricane storm surges in the study area of 52 years, hurricane derived sedimentation represents a major - if not predominant - source of sediment input for these nearshore environments, supplying perhaps one to two thirds of long-term sedimentation. There is evidence that similar washover sedimentation occurred along 150 km of southwest louisiana s coastline during Hurricane Rita s storm surge, extending hundreds of meters to perhaps 2 km inland. Hurricane-derived sedimentation may therefore be an important sediment source for the entire Chenier Plain coastline. The findings of this study add to a growing body of research that indicates hurricane storm surges may be a significant source of sediment input for a variety of nearshore environments along the northern Gulf of Mexico. Coastal management strategies aimed at combating the effects of sea-level rise by encouraging surface accretion should design management plans that recognize hurricane contributions. LITERATURE CITED BAUMANN, R.H., Day, J.W., and MILLER, C.A., Mississippi deltaic wetland survival: sedimentation versus coastal submergence. Science 224, CAHOON, D.R., A review of major storm impacts on coastal wetland elevations. Estuaries and Coasts, 29, CALLAWAY, J.C., DELAUNE, R.D., and PATRICK, W.H., Sediment accretion rates from four coastal wetlands along the Gulf of Mexico. Journal of Coastal Research, 13, CONNER, W.H., DAY, J.W., BAUMANN, R.H., and RANDALL, J.M., Influence of hurricanes on coastal ecosystems along the northern Gulf of Mexico. 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