H41C-0304 0800h
Sedimentary archives of extreme flooding events in the Northeastern Caribbean
Flooding associated with both hurricanes and tsunamis can be extremely hazardous to coastal communities. Accurately reconstructing the past history and intensity of these extreme events is valuable when assessing future risks to specific coastal regions. Overwash deposits preserved within backbarrier lagoons and salt ponds can provide a means for documenting previous flooding activity; however, the details of how these individual flooding events are recorded within these environments is still not completely understood. An analysis of sediment cores collected from a lagoon system in the Northeastern Caribbean (Laguna Playa Grande, Vieques, Puerto Rico) reveal coarse-grained deposits interlaminated within fine-grain lagoon sediments. These coarse-grained laminae consist of beach and nearshore sediments and can be mapped across the lagoon. These coarse-grained strata are likely overwash deposits that may provide a high-resolution record of extreme flooding events in the region over the last 5000 years. Understanding the sedimentary record of historical events of known intensity and duration is necessary, however, in order to make inferences about the cause of prehistoric coarse-grained layers. Vieques has experienced relatively little coastal development and contains a well documented history of storm and tsunami activity since its colonization in the 16th century. The island thus provides an excellent opportunity to study how recent extreme flooding events have been preserved in a natural lagoon system. Overwash deposits preserved at Laguna Playa Grande over the last 200 years are analyzed in order to identify which historically documented flooding events appear to have left a sedimentological record. These deposits are initially identified based on high resolution grain-size analysis and x-radiographic imaging. Activity levels for Pb-210 and C-137 are measured to date recent sediments (over the past 200 yrs) and to link individual coarse-grained strata with documented flooding events. Radiocarbon methods are used to date the age of sediments on the 100-1000 year time scale. Diatomic assemblage distributions and chemical compositions within deposits (percent calcium carbonate, siliciclastic, and organics) are analyzed to identify source areas for specific sedimentary units. The grain-size distributions within flood deposits are also analyzed in order to develop methods for quantifying the wave and currents conditions present during times of mobilization and deposition. Results from this study show that documented hurricane and tsunami events in the Northeast Caribbean are recorded within the sediment stratigraphy at Laguna Playa Grande. These findings emphasize the accuracy to which lagoon systems can record extreme flooding events and helps to validate using sediment cores from these environments to construct a high-resolution, prehistoric record of tropical cyclone and tsunami activity for the region.
H41C-0305 0800h
Sediment Transport and Deposition in Tidal Riparian Wetlands During Drought Conditions
Rates of sediment deposition in coastal riparian wetland are determined, in part, by the amount of sediment available in the flooding waters. In the Lower Cape Fear River (LCFR) estuary in southeastern North Carolina total suspended solids (TSS) are received from three major tributaries: Cape Fear mainstem (CFR), Northeast Cape Fear (NECFR), and Black Rivers (BR). The sediment delivered to the estuary depends on the tributary characteristics and climatological conditions in the watersheds. In coastal North Carolina, two primary types of streams, brown-water and black-water, drain vastly different headwater areas and therefore contrast greatly in the amount of sediment they carry and can potentially deposit in adjacent swamps. TSS concentrations in the brown-water river (Cape Fear River) range from 5-35 mg L$^{-1}$, but TSS are typically lower (<10 mg L$^{-1}) in the black-water streams. From spring 2000 to winter 2002 severe drought conditions were present, particularly in the piedmont region. Although streamflow was reduced during drought conditions, mean TSS concentrations were significantly higher during the drought than during normal conditions for both the CFR and NECFR. At a swamp adjacent to CFR total and inorganic suspended sediment concentrations decreased slightly, but not significantly over time during drought conditions, suggesting a loss of suspended sediment to the wetland surface. During normal conditions TSS and ISS concentrations increased slightly, but not significantly, suggesting inhibited settling or export. Sediment deposition data during the drought was consistent with the flux data indicating deposition did not differ significantly between normal and drought conditions. However, total and inorganic deposition was higher during the drought period. At a swamp adjacent to the NECFR suspended sediment concentrations did not vary relative to high tide for either normal or drought conditions. Sediment trap data indicated little difference in deposition between drought and normal conditions. These results indicate short-term changes in climatological conditions may affect deposition rates in tidal riparian swamps. Additional data are needed to examine the extent of drought effects on deposition.
H41C-0306 0800h
Morphological controls on marsh creek network flow patterns
An experimental study to investigate flow characteristics and salt marsh morphology was carried out in the North Inlet/Winyah Bay Estuary, South Carolina. Acoustic Doppler current meters (ADVs) were placed at the terminus of two abutting creek networks that were separated by a topographic divide. An additional acoustic profiler was placed near the mouth of one of the creeks. Flow patterns observed during spring tide indicated an abrupt switch in current direction during flood before vanishing at slack high water. This reversal in flow direction was mirrored during the ebb phase, in which initial ebbing did not drain the creek but rather flowed further onto the marsh surface. This pattern was also seen in the profile measurements at all depths. In contrast, the current moved up both creeks on flood and out on ebb during the neap phase. The marsh surrounding the study site is located within a meander of the main tidal channel. Although the cause of the spring tide flow pattern is not completely understood, it may be related to inundation of the marsh island producing a net fluid transport down the pressure gradient formed by the propagating tidal wave. Therefore, topography exerts the dominant control on the flow over the marsh surface during neap tide, yet topography and hydrodynamics exert control during spring tide.
H41C-0307 0800h
Wetland Platform Erosion by Wave Action and its Implications for Future Mitigation Projects in South Louisiana
The transition of marshland to open water in south Louisiana continues at an alarming rate, and freshwater diversions and marsh restoration projects have been implemented or are planned to address this loss. This project examines the magnitude and impact of wave-induced subaqueous platform erosion that occurs during and following subsidence of the subaerial marsh. Study and control sites have been chosen in Breton Sound, where an existing freshwater diversion project (Caernarvon) has been operational since 1991; Barataria Bay, where the Davis Pond diversion has been active since 2003; and the Deltas National Wildlife Refuge (DNWR), where marshes are still receiving significant freshwater and sediment from the Mississippi River. Study grids are established along marsh fringes in bay-fronting, gulf-fronting, and interior ponds that have a variety of orientations and open water fetch to predominant wave attack and in recent years (since the 1930s) have shown significant wetland loss. Subaqueous platform elevation and stratigraphy are examined with vibracores and transit elevation transects and detailed bathymetric maps of the 1 km grids are also made with an Odom Hydrotrak HT100 fathometer. Preliminary results suggest that the overall magnitude of wave-induced erosion is extreme (deflation of 1-1.5 m) in Barataria Bay sites, but may not be as great a magnitude in Breton Sound or the DNWR sites. Shoreline orientation to wave attack, the composition and resistance to wave re-working of the underlying sediment, or the presence of submerged aquatic vegetation (SAV) that serves to stabilize the substrata may help explain the resultant site to site variability. Predominant in many sites, once the aerial marsh has submerged, a portion of the peat deposits is preserved below more recent onlapping bay bottom sediments.
H41C-0308 0800h
Beach Profiles of Kamchatka, Russia: an Analysis of how Process Controls Beach Geomorphology.
The geomorphology of the eastern coastlines of Kamchatka, Russia are governed by processes that range from instantaneous to geologic in duration. This study focuses specifically on the processes that are involved in beach-ridge genesis by investigating three bays influenced by very different processes. This study provides results from the summer, 2002 field season.
H41C-0309 0800h
Topography of a Salt Marsh Island
Subtle variations in salt marsh topography affect over-marsh tidal currents and likely influence marsh sediment accumulation and resulting landforms. A DEM with vertical accuracy of ±0.02 m was developed using real-time kinematic (RTK) GPS over a relatively pristine, uninhabited, 1.05 km2 salt marsh island. The island has 22 discrete tidal creek networks that typically become dry at low tide. The highest elevations are associated with discontinuous levees along the island perimeter, giving the island an overall "bowl" shape. The broad salt marsh platform consists of subtle ridges and valleys with up to 0.30 m of relief. The ridges separating discrete tidal creek networks. These ridges, however, are discontinuous and create hydraulic connections between discrete creek networks before the maximum tide height occurs. Topographic index analyses reveal two types of creek networks, one set that is relatively stable and a second set that may be undergoing headward extension.
http://www.geol.sc.edu/torres
H41C-0310 0800h
Response of an Ecomorphodynamic Model of Tidal Marsh Development to Changes in Sea Level Rise Rates and Sediment Supply
We have developed a three dimensional model of tidal marsh accretion and channel network development that couples physical sediment transport processes with biomass productivity. A new, simplified, water routing technique facilitates application of the model over the large spatial and temporal scales necessary to address morphologic and biologic responses to changing environmental conditions. The model algorithm, representing a tidal cycle, begins with a basin flooded to high tide water depth. Water parcels in each model cell are routed out of the basin according to a parametrically represented water surface. Outside the boundaries of the channel network, the surface features a constant curvature and flow directions determined by the surface's gradient (e.g. Rinaldo et al., 1999). Inside the channel network, water is routed to cells with progressively decreasing along-channel distances from the outlet. The cumulative volume of water flowing through each cell is divided by a characteristic time of strong flow during a tidal cycle to determine local discharge (Fagherazzi and Furbish, 2001). A local rate of erosion, related to discharge, and deposition, proportional to high tide water depth, is calculated in each cell after every drainage. Biological productivity is additionally calculated in each cell as a function of bed surface depth below high tide and influences the rate of deposition and slope driven transport processes such as creek bank slumping. Changes in bed elevation after each drainage iteration are extrapolated to one month of tidal cycles. With a steady, moderate rise in sea level, the model builds a marsh platform and channel network with accretion rates everywhere equal to the rate of sea level rise, meaning water depths and biological productivity remain temporally constant. Establishment of vegetation plays a critical role in developing a horizontal platform incised by a well defined channel network. In early stages of basin filling, or if vegetation influences are not included, channels are broad and grade gradually into inter-channel surfaces. An increase in the rate of sea level rise, or a reduction in sediment supply, causes equilibrium depths in unvegetated regions to increase and channels to deepen and widen. If the new equilibrium depths in vegetated regions of the marsh exceed the optimal depth of biological productivity, reduced biological productivity will cause lower deposition rates on the platform. In this situation, the marsh system is in an unstable condition where channels expand laterally and platform vegetation is rapidly converted to open water. If the new equilibrium water depths only approach the optimal depth of biological productivity, the marsh platform will respond with increased productivity and greater deposition rates. Vegetation on the marsh platform can promote a meta-stable equilibrium where the platform maintains elevation relative to the rising sea level, although disturbance to vegetation could cause irreversible drowning of the marsh platform. Dynamics between biological productivity and predicted equilibrium depths thus have a strong control on the fate of salt marshes in the context of sea level rise and changes in land use/sediment supply and these dynamics may explain patterns of marsh loss.
H41C-0311 0800h
Mega-tsunami deposits or evidence of uplift within the Hawaiian Islands
For several years there has been a controversy over the origin of coral-bearing deposits on the island of Lanai (Hawaii). Studies underway have expanded the study of marine deposits from Lanai to adjacent islands. Coral-bearing deposits are present at elevations up to 190 m on Lanai, 90 m on Maui, 90 m on Molokai, 30 m Oahu, 30 m on Niihau, roughly 75 m on Kauai (as well as a few m above sea level on the Kohala Volcano on the island of Hawaii). The deposits show a persistent pattern of increased weathering, color change, increasing age and increase in the number of fossils now extinct in Hawaiian waters, with elevation above modern sea level. Changes in slope are also observed reflecting changing relative sea level. A review of radiometric ages suggests in-situ corals as well as marine conglomerates were deposited near sea level and were contemporaneous. The distribution, stratigraphy and age of marine sediments around the islands are consistent with a history of uplift combined with changing sea level. We document the age, rock and fossil characteristics and distribution of sub-aerially exposed marine sediments, in the Hawaiian Island chain. We suggest that the Hawaiian Islands have experienced lithospheric adjustments during the last 500,000 years that have left marine deposits exposed above sea level.
H41C-0312 0800h
Internal structures and developing mechanisms of tidal sand ridges in the East China Sea
*Abstract:* The internal structures of the tidal sand ridges in the East China Sea are characterized by the southwest-dipping beddings and several erosion surfaces, which resulted in young sand ridges overlying old ones and forming a unique multi-layer complex structure. These characteristics reveal the multi-phase development of the tidal sand ridges, and their multi-repetitive forming history of accumulation, erosion and accumulation. There are two mechanisms interpreting the migration and evolution of these sand ridges. One mechanism concerns the evolution tidal sand ridge itself. The tidal current difference along two flanks of a ridge makes the sediments migrate toward the side with lower current speed. This is so-called auto-cyclic process of sand ridge developing. For the tidal sand ridges in the East China Sea, one side is dominated by flood currents in a NW direction, the other side is dominated by ebb current in a SE direction. Due to the influences of topography gradient of the continental shelf and the runoff from Chinese main land, the velocity of the ebb currents is larger than that of the flood currents, so the sediment is transported to southwest continuously, and it is a certainty for the sand ridges migrate toward SW and form the beddings inclining toward SW. The other mechanism is allo-cyclic process, which relates to global or regional hydrodynamic changes, such as sea-level fluctuations or storm waves. The post-glacial sea level rose rapidly in a stepwise pattern caused by the four postglacial melting water pulses (MWP-1A, 1B, 1C, 1D) (Liu et al., 2004) and it should play an important role in this mechanism. Both mechanisms exist in the development of the tidal sand ridges in the East China Sea, so there is some difficulties to distinguish them clearly. It is obvious that the tidal sand ridges of the East China Sea have evolved continuously since the postglacial period; four stages of sand ridges have formed in sequence: younger ridges overlaid the old ones and finally formed the multi-layer complex structure. *Key words:* East China Sea, tidal sand ridge, internal structure, auto-cyclic processes, allo-cyclic process
H41C-0313 0800h
Application of Sedimentologic-Geophysical Analysis for Coastal Zone Management in Albania.
The areas under investigation encompassing a large expense of land of south- western part of Albanian coast zone. The paper examines the Quaternary history of dual (sedimentologic models and the evaluation impact of the geological environment in expansion of ancient civilization in this region). The work presented here is result of continued study about four last years and goes on in our days. The neotectonic structure of Butrint region is consisted off horst- graben structure, E-W trending. The Quaternary formations fill the lowest part of the region and have different origins and consisted of alluvial and lagoon deposits, about 80-m thickness. The lagoonal deposits are common around Butrint lake wile ancient town (Hellenistic- Roman- Byzantine) was extended mainly on the soft Holocene sediments. The soundings data, particularly resistivity variation are the base of sedimentologic and lithological studies due to the lack of boreholes. Two cycles of sedimentation can be observed within the thickness of 40- 50m: the first, gravel and sands and second mainly of clays in upper part of the cross section. In addition to, V.E.S. data and resistivity maps point out the features of sedimentologic environment distinguishing Pleistocene and Holocene deposits, delineated of water-bearing coarse-grained sands and gravels and land-sea interaction separating salty waters areas. Based on the sedimentologic and structural factors studied and geophysical maps and cross-sections, plenty of geomorphic problems are obvious now. This studies show the evidence to the advancement of the coastline is still occurring and the shoreline is still on the move.
H41C-0314 0800h
Tidal Influence on Behaviour of a Coastal Aquifer Adjacent to a Low-Relief Estuary
Tide-induced groundwater flow will influence the degree of salt-water intrusion and contaminant transport in an unconfined coastal aquifer. However, the magnitude of such influence is uncertain, especially for a mildly sloping beach. Here a costal aquifer adjacent to a low-relief estuary at Ardeer (Scotland) is investigated both with field monitoring and numerical modelling, in order to ascertain the tidal influence on the groundwater dynamics, salt water intrusion, and chemical migration to the estuary. A field survey and aquifer monitoring were carried out to acquire data on the aquifer geology, water table fluctuations and salinity distributions. The observed groundwater fluctuations were asymmetric and skewed in time. Analysis revealed that the semi-diurnal fluctuations were not efficiently filtered out in the area near the beach, although their magnitude decreased greatly further inland. The measured salinity distributions indicated that salt-water intrusion occurred both in the bottom part of the estuary and near the beach surface. A numerical model (SEAWAT) was used to simulate flow processes in the cross-section perpendicular to the estuary. Results show that the simulated groundwater table fluctuation was in good agreement with the collected data. The case with a vertical beach (an extreme condition opposite to a low-relief beach) was also simulated for the purpose of comparison. It was found that tidal fluctuations affected significantly the hydrodynamics in the aquifer. Especially for the mildly sloping beach, a circulation cell was formed below the beach when the groundwater was retreating to the estuary, a feature that did not appear in the vertical beach case. Within the aquifer, the asymmetry in water table fluctuations was captured by the simulation with the mildly sloping beach, but was less obvious in the vertical beach simulation. Comparison of the predicted salinity distributions in the aquifers shows that the mild beach slope enhanced salt water intrusion. This occurred in two ways (at the mildly sloping beach): first, the salt water wedge in the lower part of the aquifer advanced further into the inland area and secondly, the infiltrated salt water induced a large mixing zone just under the sloping beach. Contaminant transport from the aquifer to the estuary was also affected. The mildly sloping beach tended to reduce the horizontal advection of the plume towards the estuary but accelerated the upward contaminant migration towards the beach surface, which potentially puts estuarine ecology at risk with beach pollution.
H41C-0315 0800h
Active-Margin Coastal Morphology as a Reflection of Holocene Seismotectonic Evolution: An Example From Eastern Kamchatka, Southwestern Bering Sea
We studied the Holocene seismotectonic evolution of Soldatskaya Bay, Kamchatskiy Peninsula, Kamchatka, via coastal geomorphology and morphodynamics. In this bay we examined morphology, geological structure and ages of beach ridges, which compose a marine terrace. Each relic beach ridge marks the position and altitude of the shoreline at the time when it completes its formation. In areas of active Holocene volcanism it is possible to estimate the age of beach ridges using tephrochronology, because sandy soils overlying the beach ridges contain multiple ash layers, most of which have been mapped and dated. So the beach ridges (morphology of marine terraces) can reflect the changes of relative sea level in time. In general these changes may be caused by eustatic or tectonic movements. For the last 5000 years in the eastern Kamchatka region, the difference in elevation of the beach ridges commonly marks the difference in sea level caused by strong seismotectonic movements. If the hydrodynamic conditions during the formation of the beach ridges are similar, and other factors can be eliminated, the difference in their elevations is the net amplitude of tectonic deformation during the time interval between their formation. For Soldatskaya Bay we measured the elevations and estimated the ages of beach ridges spanning nearly 5000 years from the present. Based on the obtained data a paleogeomorphological reconstruction of Soldatskaya Bay was carried out. From this reconstruction, we calculated average rates of relative sea level change and progradation in time intervals bracketed by marker tephra. Estimated relative sea level change differs between the southern and northern parts of the Bay, pointing to seismotectonic control. Rates also vary in time. Higher rates were estimated for shorter time intervals, with slower rates characterizing longer time intervals. This happens because long time periods are represented by average seismotectonic rates which mask short-term, high-amplitude coseismic deformation related to individual earthquakes.
H41C-0316 0800h
Estimating Coastal Slopes Vulnerability in Dakar Peninsula, West Senegal
Dakar Peninsula, where most of the economically active places in Senegal are concentrated, has been repeatedly subjected to rockfalls and coastal erosion on the stretch of coastline between Pointe des Abattoirs and Pointe de Dakar. Coastal scientists, engineers and political leaders are increasingly being called upon to assess the physical and socio-economic impacts of such slope degradation, and hence explore appropriate response strategies. The objective of this terrain hazard mapping study was to collect information to enable the prevention and control of slope failure and slope degradation to be made on a rational basis. The area to be investigated was defined and sub-areas were further subdivided into 12 individual sections and assigned hazard and risk categories or levels derived from the field data.The present study evaluated the risk of slope failure within a conventional probabilistic framework based on slope outcrop inventory and structural discontinuities measurements, geomorphological mapping combined with air photograph interpretation, review of pre-existing geotechnical data as well as the sampling of rock specimen for laboratory testing and analysis. Detailed sea level rise analysis reported elsewhere leads to the conclusion that the background geology of the area should be looked as the cause to the several coastal hazards, among which slope instability poses the most important urban management problems in Dakar. From the results of the study it was possible to infer that local rockfalls and degradation of rock slopes are closely linked to the geological setting of the area, notably local rock type and structure, local hydrogeological conditions, local topography and geomorphological processes, as well as a poor city planning and overpopulation. The results also showed that geomorphological mapping can provide an effective tool to the engineering geologist in anticipating the slope failure processes which may otherwise go unnoticed. Eventually the probabilistic method has been found to work consistently for this case study and therefore suggests that the orientation and distribution of structural discontinuities play a key role for assessing the risk of slope failure with acceptable degree of accuracy.
H41C-0317 0800h
Alongshore coastline instability: wave climate analysis and comparisons to nature
Deep-water, not breaking, wave angles best predict how gradients in wave-driven alongshore sediment transport shape a coastline. As relative wave angles (between wave crests and the shoreline trend) increase, the smoothing influence (diffusivity) of alongshore sediment transport decreases, with the diffusivity reaching zero for relative deep-water angles around 45 degrees (although breaking angles may be much less). For even greater relative wave angles, coastline evolution becomes anti-diffusive, where bumps grow rather than shrink. Although their details differ, different formulations for alongshore sediment transport all predict this instability for large deep-water wave angles. Numerical modeling suggests that wave climates dominated by anti-diffusive, `high-angle' waves can cause a coastline to self-organize into large-scale rhythmic or quasi-rhythmic configurations, resembling natural features such as cuspate forelands, cuspate spits, and alongshore sandwaves. Along a real coast, wave angles and heights typically change more rapidly than significant long-term evolution takes place. Some days the waves exert a smoothing influence, on other days waves tend to roughen the coastline. Summing over all of a wave climate's diffusivity contributions for a given shore orientation yields the long-term coastal stability. At natural locations with interesting features such as capes and sandwaves, measured and hindcast wave values suggest that high-angle waves dominate for the regional shoreline trend. However, along the capes of North Carolina, U.S.A., measured climates are locally low-angle dominant despite regional suggested instability. Numerical simulations also exhibit this phenomenon, where shadowing by neighboring protuberances and local coastline self-adjustment combine to evolve a coast predominantly low-angle despite regionally high-angle wave climates. These comparisons support the hypothesis the instability affects natural shoreline shapes, and further emphasize how long-range interactions and emergent behavior (such as shadowing by emergent coastline features) can affect shoreline evolution.
H41C-0318 0800h
Geologic Framework Versus Surficial Processes: Call it a Draw? (in a Model Under Some Circumstances)
Models often involve the implicit assumption that changes to the shape of a sedimentary coastline, such as the Southeast Coast of the United States, are due to wave-driven alongshore sediment transport. This assumption is most valid along coastlines where the shoreface, or the nearshore area strongly influenced by waves, remains covered with a veneer of sediment, as weathering of the shoreface produces sand at a rate at least commensurate with local erosion rates. However, if the rocks or semi-lithified deposits composing the shoreface weather slowly, so that the sediment supply is locally or regionally insufficient to keep up with the combined demands of a divergence of alongshore transport and cross-shore losses, the geologic framework can exert a significant influence on coastline evolution. A more complete analysis requires incorporating the effects of underlying geology into models treating nearshore sediment transport. We start with a model recently developed to investigate large-scale interactions over long time periods. This preliminary model is similar to "one-line" models used in many coastal engineering applications although it can treat arbitrarily complex shoreline shapes and wave climates. We incorporate weathering of the underlying geology along a lithologically heterogeneous coast into the preliminary model. We do not explicitly simulate the complex set of physical, chemical, and biological processes involved in shoreface weathering. Instead, we include the effects of weathering in a simple modeling framework that relates weathering rate to the thickness of sediment covering the underlying material. We treat thickness as a proxy for how often the underlying material is exposed as a possibly patchy sand cover shifts around. Since fine-grained sediment is lost offshore and is unavailable to the nearshore system for transport, the amount of material delivered to the nearshore system from shoreface weathering is also modulated by the fraction of coarse material composing the rock. Analyses of coastline evolution based solely on lithological heterogeneities can implicitly lead to an apparent paradox. It has been suggested that where a shoreface is locally composed of material that is more mobile or contains a larger fraction of fine material than surrounding shoreline segments, long-term shoreline retreat rates are higher. Subtle divots in the coastline corresponding to such locations seem to support that idea. However, in the long term, such spatially heterogeneous shoreline retreat rates would tend to produce a progressively more crenulated coastline, in conflict with the observation that sandy coastlines tend to be fairly smooth on the regional scale. Our groundbreaking model combining lithologic influences and sediment-transport processes offers a resolution to this apparent paradox. Shoreline divots do tend to form in the model where the shoreface weathers more rapidly or is composed of finer material. However, alongshore transport tends to partially fill in such divots, slowing the weathering rates there while increasing them in adjacent areas where mobile sediment is preferentially removed. The long-term result is an alongshore-uniform erosion rate. Subtle shoreline undulations reflecting the heterogeneous geologic framework can exist in the model, but they their amplitude reaches a steady state rather than increasing with time. In this model, when the regional erosion rate is greater than the rate that the slowest-weathering rocks along a shoreline can transform into mobile material, bays flanked by rocky headlands develop.
H41C-0319 0800h
Surprising Long Range Effects of Local Shoreline Stabilization in a Large-Scale Coastline Model
As coastlines continue to retreat and threaten communities, roads, and other infrastructure, humans increasingly employ shoreline stabilization techniques to maintain the shoreline in its current position. Examples of shoreline stabilization techniques include beach nourishment and seawall construction. During beach nourishment, sand is typically dredged from locations offshore and placed on the beach. Seawalls or revetments, on the other hand, are hardened concrete structures which prevent the shoreline from retreating further yet do not add sand to the nearshore system. Coastal engineers and scientists have only addressed the local and relatively short-term effects of shoreline stabilization. Can beach nourishment or seawalls affect coastline behavior tens or hundreds of kilometers away in the longer term? We adapted a recently developed model of large-scale, long-term shoreline change to address such questions. On predominately sandy shorelines, waves breaking at oblique angles to the shoreline orientation drives the alongshore transport of sediment. Though traditionally believed to smooth out shoreline features, Ashton, et. al. (2001) have shown that alongshore-driven sediment transport can cause more complex shoreline evolution. Their model showed the spontaneous formation of large-scale features such as capes and cuspate forelands (e.g. the shape of the coastline of the Carolinas) using simple sediment transport relationships. This model accounts for non-local shoreline interactions, such as wave "shadowing." In this work, we have further developed the large-scale shoreline model to include the effects that shoreline stabilization techniques have on shoreline position and sediment supply. In one set of experiments, we chose an initial shoreline with cape-like features separated by approximately 100 kilometers, roughly similar to that of the coast off the Carolinas. In each individual experiment, we nourished a different 10 kilometer section of coastline. In each case, we compared the coastline that evolved after 50, 100, and 200 years with the coastline in a control run which received no nourishment. We found several surprising results. The difference in shoreline position attributable to the nourishment project is as much as 1 kilometer regionally (up to 10's of kilometers away in the alongshore direction after 200 years). Changes on the order of 10's of meters were seen as far away as hundreds of kilometers. Nourishment directly altered the geometry of the adjacent cape. Shadowing propagated the changes over long distances, changing the local wave climate felt by distant capes, and thereby altering their position. The distant changes in shoreline position attributable to the nourishment were concentrated entirely near the capes rather than in the cuspate bays. These experiments suggest that the long-term effects of a single, local beach nourishment project can be much more widespread than might be expected.
H41C-0320 0800h
The Role of Intense Storms on Backbarrier Morphodynamics: Examples From the New York/New Jersey Bight
Intense storms can significantly modify coastal landforms. Understanding the influence of these relatively rare, but potentially important, events on the evolution of coastal systems is important if we are to reliably forecast future changes. In the New York/New Jersey Bight the most intense storms are landfalling tropical cyclones that approach the region from the south. Since European settlement, four severe tropical cyclones, occurring in 1693, 1788, 1821, and 1893, have made landfall in the New York/New Jersey Bight. Each of these storms resulted in a rise in water level of over 2.5 meters above mean sea level (MSL) in New York City. Storm surges of this magnitude can overtop and breach barrier beaches creating inlets and depositing overwash deposits across the surface of backbarrier marshes. Severe winter storms, near miss, and minimal hurricanes impacting the region in the 20th century caused water levels to rise approximately 1.5-2 meters above MSL. Events of this magnitude likely caused erosion of the beach face, and limited overtopping and breaching restricted to areas with little or no dune development. Backbarrier sediments can preserve an archive of environmental changes. We collected a series of vibracores from four backbarrier marshes in the New York/New Jersey Bight. High-resolution grain-size and loss-on-ignition analyses were used to characterize the sediments and yield evidence of multiple storm-induced deposits. Heavy metal pollution horizons, pollen stratigraphic data, and C-14 ages were used to provide chronological control. In order to link the dynamics of the barriers with the sedimentary framework of the backbarrier estuary, we used ground-penetrating radar (GPR) to map the subsurface character of the barrier sediments. Our results indicate that intense tropical cyclones are very important in shaping the barrier and backbarrier environments in the New York/New Jersey Bight. Backbarrier and barrier sediments reveal records of overwash deposition and inlet formation associated with intense tropical cyclone landfalls. Evidence of three historic hurricane strikes in 1893, 1821, and 1693 has been recovered from the western Long Island sites. Only the 1821 hurricane is preserved in the recent sediments of Cheesequake, NJ. The age of many of the prehistoric storm-induced deposits among the four sites in the New York Bight overlap, suggesting that some of the overwash layers may have been deposited by the same prehistoric storms.
H41C-0321 0800h
Regional Long-term Coastal Change in Southern California
The USGS National Assessment of Coastal Change project addresses issues that include the impact of severe storms, identifying coastal vulnerability, and the effects of potential accelerated sea-level rise. One of the principal tasks is to calculate long-term shoreline change rates using a method that is consistent and repeatable at a national scale. We have undertaken the analysis of the open-coast shorelines for California, Oregon and Washington. Methodologies that have already been developed for the Gulf of Mexico are applied to the West Coast and modified where necessary to adjust to the differences in coastal geomorphology between East and Gulf Coast-type shorelines and the variable geomorphology of the West Coast. In addition to measuring coastal change along linear and sometimes dune-backed beaches, the methods must also incorporate the means to measure long-term change along narrow beaches backed by cliffs, pocket beaches and headlands, and high-relief stretches of coast. For the completed analysis of Southern California, the dataset includes nearly 150 historical maps dating back as far as the mid-1800s. In addition, lidar data, collected for the entire U.S. West Coast in 1998, is used in the analysis. Three coastal change reference features are used for the Southern California analysis: high water line and mean high water shorelines for the sandy beaches, and the cliff edge. The long-term shoreline change rates for Southern California, generated using a linear regression method on four shorelines, are surprisingly low for a region that is generally thought to be experiencing widespread erosion. Our analysis for an approximately 120-year period shows that only 6% of the 277 km of coastline analyzed is undergoing long-term erosion, at an average rate of 0.4±0.2 m/yr. This pattern changes somewhat in the last 25 years, where the percent of eroding coastline increases to 21% and the average erosion rate is 1.1±0.2 m/yr. The highest erosion rates are in the northern portion of the region near Santa Barbara, and to the south near Oceanside. The completed cliff erosion rates for San Diego County indicate that of the 53 km of the coastline that are backed by cliffs, 34% are experiencing erosion at an average rate of 0.4±0.16 m/yr. Both the beach and cliff erosion analyses provide a regional perspective of coastal change trends for Southern California that had not previously been available and initial results suggest that the general trend is not long-term shoreline erosion.
H41C-0322 0800h
A Method of Discriminating Littoral Cells Using Stacked Erosion Transect Data
Historical aerial photogrammetry and erosion-based statistics offer a new approach to identify littoral cells within alongshore contiguous beach systems, which are usually determined by visual assessment that becomes difficult to analyze when sediment input occurs from multiple sources. Historical aerial photogrammetry, making use of topographic surveys (NOAA T-Sheets) and vertical aerial photographs, has been successfully utilized in calculating shoreline change rates on beaches. We employ the Maui historical shoreline database to identify littoral cells on a decadal time scale. Shoreline change rates are calculated at shore normal transects spaced 20 m alongshore. These datasets are typically limited and characterized by low signal-to-noise ratio. To reduce noise and non-random errors, we stack data from transects belonging to the entire length of a beach onto one plot and calculate change rates using the Weighted Least Squares (WLS) method. We compare stacked rates with the individual transect rates to determine disagreements between them by using the F-test statistic at a 95% confidence interval. We divide the beach into littoral cells where disagreements occur until a majority of the individual transects agree with the stacked rate. This procedure shows that various types of beaches on Maui breakdown into different numbers of littoral cells.
H41C-0323 0800h
Identification and Spatial Distribution of Remotely Sensed Sand on Fringing Reefs of Oahu, Hawaii
What surficial spatial patterns predominate among sand accumulations on fringing reefs? Do these patterns provide clues to transport mechanisms and factors governing deposition? Understanding of these spatial characteristics is integral for studying sand as a resource, a benthic habitat, and a mobile geologic unit on the sea floor. Using Quickbird imagery of fringing reefs on Oahu, Hawaii, we tested multiple data processing techniques with a linear classifier that utilized a fixed set of training groups and a truth image. Results show the Lyzenga method provided the best quantitative results, and analyst review finds this method to be the best qualitative spatial display as well. Highly accurate spatial data produced from the classified image, combined with bathymetry data from Shoals LIDAR were then utilized to begin identifying spatial distribution of the sandy substrate across the sea floor. Using basic geomorphologic reef descriptions and general spatial characteristics of the sandy substrate is the first step in identifying patterns of distribution within and juxtaposition to the reef. These results, when compiled from several sites, may begin to provide better understanding of sand's role in the reef geomorphology, though current results are not yet sufficient for predicting sandy substrate characteristics from remotely sensed data.
H41C-0324 0800h
Quantifying Shoreline Change Using Mean High Water and High Water Line Shorelines: Should Proxy-Datum Offsets be Incorporated?
Studies of large-scale geomorphic change in coastal environments often rely on shoreline change analyses to determine rates and identify alongshore patterns of change. Although the high water line has traditionally been the shoreline indicator of choice in such endeavors, the use of datum-based shorelines derived from lidar is becoming more common. For this reason, it is important to understand the effects of combining traditional shoreline indicators and datum-based shorelines in shoreline change analyses. We compare a high water (proxy-based) shoreline, interpreted from aerial photography, with a mean high water (datum-based) shoreline derived from a lidar survey. The aerial photography and lidar surveys were collected simultaneously along 45 km of Assateague Island, which has a relatively steep reflective beach (1V:14H), and experiences a moderately energetic wave climate (annual average significant wave height = 1.2 m). Results show an average horizontal offset of approximately 20 m between the two types of shoreline indicators, with the mean high water shoreline seaward of the high water line. Vertical offsets are also substantial and are correlated with foreshore beach slope and corresponding variations in wave run-up. Incorporating the average horizontal offset into both an end-point and a linear regression shoreline change analysis produces average shoreline change rate shifts of -0.5 m/yr and -0.1 m/yr, respectively. The rate shift increases with increasing horizontal offset and decreasing measurement intervals. However, results suggest that in most cases the error due to a rate shift will be small relative to shoreline change rates. Investigation of pre-survey hydrodynamic conditions using a total water level model suggests that the high water line was generated by a combination of large waves and a high tide several days prior to the aerial survey. This result illustrates the complexity of the high water line as a shoreline indicator and calls into question traditional definitions of this feature, which consider the high water line a wetted bound or "marks left by the previous high tide." Especially for this reason, datum-based shorelines provide a more reliable measure of coastal change than traditional high water line shorelines.