H41B-0494
Field and Laboratory Investigations of Coastal Dune Morphodynamics
Coastal dunes are important features along many coastlines, owing to their role in sediment budgets, their use as ecologically unique habitat, and their ability to protect onshore resources from wave attack. Skillful predictions of the erosion and overtopping rates of these features are needed to quantify coastal vulnerability during major storm events. Knowledge of post-storm recovery and subsequent dune growth rates is critical to developing quantitative sediment budgets and ultimately for predicting future shoreline positions. We have been conducting both long-term field and large-scale laboratory studies to improve our understanding of dune morphodynamics and will present results of dune behavior, including various feedback mechanisms, at scales ranging from individual storm events to decadal trends. A large-scale physical model study of dune erosion was recently performed at Oregon State University's O.H. Hinsdale Wave Research Laboratory producing a comprehensive, near prototype-scale data set of hydrodynamics, sediment transport, and morphological evolution during extreme dune erosion events. The laboratory moveable bed beach/dune system was brought to equilibrium with pre-storm random wave conditions. It was subsequently subjected to attack from steadily increasing water levels and offshore wave heights simulating a natural storm surge hydrograph. Observations made include inner surf zone and swash free surface and velocities as well as wave-by-wave estimates of topographical change at high spatial resolution through the use of stereo video imagery. Initial results suggest strong feedbacks between the evolution of the foreshore profile during the storm and episodic dune slumping events. Beach topographic data have been collected quarterly along southwest Washington and northwest Oregon since 1997 resolving the seasonal to interannual morphological variability of a nearly 160-km long high-energy dissipative coastline. Major climate events (such as El Ninos) cause region-wide dune erosion/scarping due to high water levels and increased storminess. However, subsequent dune recovery rates have been variable and appear linked to variations in short-term shoreline change rates and sediment budgets. At interannual scale regions of high shoreline progradation rates experience relatively high dune growth rates. At longer time scales, overall dune morphology is again linked to shoreline change rates but with the highest foredune ridges occurring in areas of relative stable shorelines at decadal scale.
H41B-0495
Spectral Analysis of LIDAR Data to Examine Beach Cusp Activity Along the Atlantic Coast of Florida
Beach cusp activity in the high-tide region of the beach face responds to a variety of parameters that have been studied and modeled over the past 30 years. During this time the typical evaluation method required field investigations to make observations regarding the size, length and overall predominance of the cusps. The advent of LIght Detection And Ranging (LIDAR) allows us to examine beach cusp data on long stretches of coastline. Interpreting LIDAR horizontal coordinates in conjunction with a specified elevation consistent with the berm crest, we generate time series with oscillations corresponding to cusp horns and embayments. We examine LIDAR data collected along portions of the Atlantic coast of Florida and evaluate beach cusp signals using wavelet analysis and s-transform analysis as a means to identify beach cusp activity. This new method of analysis seeks to provide insight into the spatial and temporal persistence of beach cusps along long stretches of coast. The large data set now available allows us to investigate the effects of individual parameters on beach cusp activity at both temporal and spatial scales.
H41B-0496
Elwha River Delta: Geomorphology of a Mixed-Sediment Beach
The Elwha River drains the Olympic Peninsula of Washington and forms a mixed grain-size delta in the Strait of Juan de Fuca. The Elwha River has been dammed for almost a century, and a pending dam removal project is expected to reconnect upstream sediment sources to the river mouth. Topographic and grain-size mapping of the delta during 1939-2007 is synthesized and the geomorphology and shoreline changes of this system are described. Data sources include historical aerial photographs, airborne LIDAR, semiannual RTK DGPS topographic surveys and grain-size analyses from digital photographs. The delta is divided into three geomorphic regions: west delta, river mouth and east delta. The river mouth is the most complex region due to the river channel movement, side-channels, and bars immediately offshore of the mouth. The east and west delta differ in beach profile and shoreline change rates. The west delta is steep, cuspate and lacks a low-tide terrace. Further, the west delta has exhibited little semi-annual or inter-annual shoreline change. In contrast, the east delta has a steep foreshore, flat low tide terrace that is dominated by cobble, and a consistent trend of erosion during the surveys. These observations can be used to track coastal changes following dam removal on the Elwha River..
H41B-0497
Sandbar Formation Under Surface Waves
We report a combined theoretical and experimental study of sandbar formation under simple-harmonic surface waves. For coarse grains and weak waves, an established empirical rule of bedload transport is used with an asymptotic theory for the fluid flow. The surface waves are governed by potential theory and a depth-linear eddy viscosity is employed in the turbulent boundary layer at the seabed. The derived bed stress is used to predict the sand-bed evolution. Laboratory experiments and corresponding numerical simulations for both high and low beach reflection are discussed. For weak reflection, the shear stress associated with the return current is found to be important. Partial simulation of a field record in Cape Cod Bay is also described. The dependence of bar morphology on sediment grain size is examined experimentally. In particular, new quantitative data on sandbar formation and sediment sorting on a bed of mixed colored sands of significantly different grain sizes under standing waves is presented to motivate future study.
H41B-0498
Influence of Beach Scraping on Beach Profile Morphology: Fire Island, New York
Fire Island is part of a barrier island system located just south of Long Island, New York. The island is 50 km long, oriented southwest-northeast, and varies in width from 150 meters to 1 kilometer. Established communities on Fire Island are part of Fire Island National Seashore (FIIS) which is managed by the National Park Service. The island is densely populated, and thus mitigating coastal erosion caused by large-scale storm waves has become an important issue. Severe nor'easter storms in 1991, 1992, and 1993 caused substantial erosion and property damage. This prompted communities within FIIS to conduct a pilot study in which the preventative, non-structural practice of beach scraping was employed as a method of erosion control. Beach scraping is the anthropogenic movement of sand from the berm to the back beach creating an artificial foredune. Currently, there is no published research that explores the morphologic influence of beach scraping on Fire Island, although the practice is still in place today for a number of communities. This study assesses changes caused by beach scraping using a temporally robust beach profile dataset of over 150 profiles, spanning thirteen years. Three study areas were chosen based on location (western, central, and eastern parts of Fire Island) and data availability in scraped and adjacent control areas. Analyzed characteristics include beach width, beach volume, slope (dune, beachface, global), berm crest elevation, and dune crest elevation. Initial results indicate a detectable difference in the behavior of the beach between scraped and control areas. Seasonal signals show beach width decreasing substantially westward from the scraped profile location, which is in the direction of net littoral transport. Anthropogenic relocation of berm material to the foredune zone during scraping places sediment in the back beach area that might otherwise be mobilized by storm waves, therefore depriving downcoast beaches of sediment. Longer-term comparisons (decadal) indicate that the beach is both widening and increasing in volume in the western study area (by 30-50%), which corresponds spatially to a persistent accretional cell that has been identified in current studies along this section of Fire Island.
H41B-0499
Modeling Evolution of the Chandeleur Barrier Islands, Southeastern Louisiana: Initial Exploration of a Possible Threshold Crossing
Airborne photographic and lidar observations of the 72 km-long Chandeleur Island arc in southeastern Louisiana since August 2005 indicate that large volumes of sediment were removed from the islands during and following Hurricane Katrina and suggest that a return to pre-storm island configuration may be unlikely. Others have suggested, based on recent field observations, that the southern portion of the Chandeleur Islands may be showing signs of becoming an inner shelf shoal. In contrast to these observations, plentiful sand has been observed in the nearshore farther to the north; based on this finding it has been suggested that at least the northern portion of the Chandeleur Islands may be poised for recovery. Given the range of observations, it is unclear if Hurricane Katrina initiated a threshold crossing in the Chandeleurs causing the subaerial, landward- migrating barrier islands to begin evolving as submerged sand shoals. If a threshold crossing has not yet occurred and the Chandeleurs do recover from the impact of Hurricane Katrina, it remains uncertain how imminent a threshold crossing may be. To better understand the potential future evolution of the Chandeleur Islands and to assess the combination of factors that are likely to cause a threshold crossing in this environment, a series of initial model experiments are being conducted using the morphological-behavior model GEOMBEST. This model simulates the evolution of coastal morphology and stratigraphy resulting from changes in relative sea level and sediment supply, and provides insight into how barriers evolve over time scales ranging from decades to millennia. Vibracore logs, geophysical records, bathymetric surveys, and lidar surveys provide data necessary to design the model domain, while sediment budget studies, estimates of sea-level rise rates, and measurements of shoreline change rates provide input and calibration parameters. Late Holocene model runs simulate the evolution of 42 km-long North Chandeleur Island as it migrated from the distal end of the St. Bernard Delta to its modern position. Building on the late Holocene simulation, we present a series of initial, multi-decadal forward model experiments that assess the combination of factors, including relative sea-level rise rates, sediment supply rates, and geologic framework, that are likely to initiate a threshold crossing in the Chandeleur Islands.
H41B-0500
New Perturbation Solution for Tidal Water Table Fluctuations in Unconfined Aquifers with Sloping Beaches
Tidal water table fluctuations are important characteristics of coastal unconfined aquifers and have been linked to beach profile changes. Mathematical models of such fluctuations based on the Boussinesq equation are subjected to a moving boundary condition induced by the beach slope. Approximate analytical solutions of these models have been previously derived using the perturbation method based on the perturbation variable ε=Acot(φ)\sqrt{neω/(2KD) (A and ω are tidal amplitude and frequency, respectively; φ is the beach angle; K, ne and D are the hydraulic conductivity, effective porosity and mean thickness of the aquifer, respectively). The applicability of these solutions is limited by the condition of ε < 1, which may not hold at coasts with relatively flat beaches. Here we present a different perturbation approach using a new perturbation variable β=ε/(1+ε), which by definition is less than unity. Comparison of the new solution with previous analytical and a numerical ("exact") solution shows significant improvement of the new perturbation approach.
H41B-0501
High-resolution (spatial and temporal) Hydrodynamic Modeling in the Lower Mississippi River Delta
The lower Mississippi River is a highly engineered system existing in one of the world's largest deltas. This system is subject to a variety of spatial and temporal forcings due to its large watershed (drains about 41% of the continental U.S.) and from the Gulf of Mexico. Future perturbations on this system are anticipated due to the impacts of global climate change (e.g., rising eustatic sea level, changes in weather patterns) and from proposed modifications to the system such as diversion structures aimed at providing freshwater nutrients and sediments to the rapidly degrading coastal wetlands. Numerical modeling will play a large role in improving our understanding and management of the system and the ability to properly design future structural features. These models will need to have the necessary spatial and temporal resolution to account for the many important processes in the river, the Gulf of Mexico, and in the wetland areas where small distributary channels will form and wetting/drying must be accounted for. This paper will investigate the ability of a 2D shallow water and sediment model to reproduce the complex distributary development associated with flow diversions into quiescent bays. A reach of the Lower Mississippi River from Point a la Hache to the Gulf of Mexico was used as a test domain to evaluate the performance and capabilities of the U.S. Army Corps of Engineers ADaptive Hydraulics (ADH) model. ADH is an unstructured finite element modeling system that includes unsaturated Richards' equations for groundwater, Navier Stokes for nonhydrostatic flow calculations, and Shallow Water equations. ADH conducts automated refinement and coarsening of the mesh based upon flow characteristics. In this case the 2D shallow water model is being used. It includes coupled flow and sedimentation. An unstructured mesh was developed for the study area which includes detailed bathymetry and topography from available survey data. The mesh is fine enough to capture the changes in bathymetry and relies upon the automated refinement in ADH to capture flow details. Numerical experiments conducted include modeling 1) existing flow conditions in the river, 2) the effects of sea level rise, 3) a hypothetical diversion opening connecting the river to the adjacent wetlands. Model capabilities and limitations, such as how much variability it can simulate, were evaluated by sequentially including more complete flow and sedimentation descriptions. Investigating the sediment depositional patterns provides information useful for design and tests the capabilities of ADH in modeling variability in sediment transport and capturing some of the fine details that will have a large impact in the wetland areas. Results from these tests are being used to define further research questions and possible scenarios.
H41B-0502
Medium-term sediment accumulation in the urbanized lower Hudson River estuary
Estuaries are dynamic systems that naturally act as sediment traps at the land-ocean interface. Sediment accumulation in urbanized systems like the lower Hudson River estuary is also affected by human activities such as channelization, dredging, harbor construction, and ship traffic. Sediment accumulation in this urban estuary was examined using a unique research approach that links Geographic Information Systems (GIS) with radioisotopic (Cesium-137) dating over the medium-term (decadal to centennial) time scales. Raster analyses were performed to determine sediment accumulation/erosion patterns based on bathymetric data over the last 100 years. These results indicate that sediment accumulation was negligible in the natural river channel, where sediment surface is in equilibrium with its physical regime. Protected marginal areas (such as abandoned harbor slips) and previously dredged areas (such as around the southern tip of Manhattan Island and in the Bay Ridge Flat area) are currently sites of rapid sediment accumulation. Vertical profiles of Cesium-137 measured in sediment cores collected in the lower Hudson River estuary provided time-marker horizons that could be used to estimate sediment accumulation rates over a 40-year time span (1960-70 to 2007). Results from Cs-137 profiles confirmed the raster analyses and indicated that sediment accumulation rates ranged from 0 to 1 cm/yr in the natural river channel and subtidal bank areas, to as high as 15-20cm/yr in areas affected by human activities (Pier 32, Battery and Bay Ridge Flat). Overall, GIS and radioisotopic dating are complementary to each other and combined they provide a more accurate estimate for medium-term sediment accumulation in the lower Hudson River estuary.
H41B-0503
Simulating the effects of hyperpycnal events on the stratigraphy of Poverty Shelf, New Zealand
The hydrologic-transport model, HydroTrend indicates that suspended sediment discharge of the Waipaoa River, New Zealand increased from 2.3 to 15 Mt/y over the last 3000 years. Prior to the arrival of European colonists in the nineteenth century A.D., volcanic eruptions, natural fires and severe storms controlled erosion rates within the basin. Since then, clearing of much of the indigenous forest for sheep farming has caused suspended sediment discharge of the Waipaoa to increase by 850%. HydroTrend simulations indicate the Waipaoa was not able to generate hyperpycnal discharges before the arrival of European colonists. However, because of deforestation in the headwaters, suspended sediment concentrations of the Waipaoa are now able to exceed 40 kg/m3 during large flood events. The river density of these events is great enough to cause the manner by which sediment is transported from the river to change from a surface plume to a hyperpycnal plume. Although these hyperpycnal events are rare (recurrence intervals greater than 2 years), simulations suggest these events carry approximately one fifth of the total sediment load. Observational data of hyperpycnal flows are scarce as they often only occur during extreme weather events. Given the proper boundary conditions, these events have the potential to transport large amounts of sediment over the sheltered Poverty Bay shelf, and into the deep ocean. For this study, we have used HydroTrend results as input to the basin filling model, sedflux (coupled with the hyperpycnal plume model, sakura), to investigate the impact of these hyperpycnal events on the stratigraphy of the Poverty Bay shelf. We note that while some flood events generate hyperpycnal flows that are able to bypass the shelf, others are unable to ignite and deposit the bulk of their sediment on the shelf.
H41B-0504
Morphology and forcing agents of the Monterey Canyon System: a quantitative geomorphic analysis
Landforms are shaped by tectonics and climate, which control deformation, deposition, and erosion at the fluid/solid interface. The recent re-emergence of geomorphology was spurred by the realization that the shape of the land is not only the result of many processes, but it also affects them. Tectonics builds mountains and geomorphic agents and climate conditions modify them. Thus geomorphologic processes and tectonics are major agents of change and landform is the variable that links them, the expression of a steady-state equilibrium controlled by multiple feed-backs. Measuring and parameterizing the shape of the landscape is the first step in understanding many natural processes critical to environment, such as terrain instability and fluid circulation. Furthermore, a multidimensional parameterization of the land surface (e.g., geomorphic indices) is useful in comparing different landforms and in isolating the effect of specific factors, such as the level of tectonic activity. So far geomorphology has dealt primarily with land areas, but an expansion to the sub-aqueous environment is already in the making due to high-resolution bathymetry increase. This research compares submarine and land morphology as they manifest tectonics in a rapidly transform continental margin. To this end the research has analyzed high-resolution multibeam bathymetry to interpret modern submarine processes from a study of geomorphic change. This research is applied on the Monterey Bay (California) due to the complex geomorphology of both the Coast Ranges and the seafloor. Tectonic has controlled the evolution and the present geometry of the branches and meanders of the Monterey Canyon system, one of the largest submarine canyon systems in the world. In particular the Monterey Canyon is an erosional, presently active feature, which has resulted from both canyon cutting and mass wasting. The deeper parts of the canyon have been progressively offset from the headward part by strike-slip faulting, and are now located in northern Monterey Bay. In this context a clear understanding of the physical processes that trigger submarine slumps has been the basis of the research. The high-resolution multibeam record of the seafloor of the MBR collected by MBARI will be analyzed to measure the parameters describing the geometry of the submarine canyons of the Monterey Canyon System. Our quantitative geomorphologic analysis will include slope angle, sinuosity, and width of the canyon axis and of the thalweg. The interpretation of these geomorphologic parameters will help to define active processes along the submarine canyons of the MBR and to unveil the relationships between continental deformation and canyon formation in a transform continental margin. Moreover, to broaden constraints on geophysical properties and evolution of the area, we had used landform surface high resolution DEM.
H41B-0505
Stratigraphic Evidence of Salt Marsh Erosion Along North Shore of Delaware Bay Associated With Sever Storms.
Detailed stratigraphy documented from over 50 hand-driven cores obtained from the Back Creek marsh near Sea Breeze, south New Jersey shows that this marsh along the north shore of Delaware Bay experienced significant erosion several times during the past few thousand years. Eroded marsh deposits were replaced by a regressive sequence of tidal mud, low marsh peat and high marsh peat. Tidal mud units that filled eroded accommodation space vary in thickness from 1 m to 3 m. Given the overall horizontal facies boundaries within the regressive sequences, which suggest rapid post-erosion infilling and marsh recovery, we hypothesize that the observed erosion of marsh deposits was related to the passage of hurricanes. We recognize four erosion surfaces, the deepest three of which are overlain by complete regressive sequences. The fourth regressive sequence is incomplete, suggesting that the system is still recovering from recent storm erosion. Pending radiometric dates, we estimate, on the basis of depth and a published relative sea-level curve for Delaware Bay, that the deepest two erosion surfaces date to approximately 800-700 cal BP and 600-400 cal BP. The third storm-erosion event is estimated to have occurred less than a century ago. Similar depths of erosion surfaces and distinct shape of eroded spaces indicate that the erosion could have happened under storm-surge conditions.
H41B-0506
Exploring Morphological Process Indicators in Regional Patterns of Shoreline Change
Beach morphology changes over a continuum of time and spatial scales, from ephemeral patterns (100 – 102 m alongshore) such as cusps and scarps that develop and dissipate over days to weeks, to persistent plan-view bumps in the shoreline (103 - 105 m), such as promontories, embayments, and capes, that tend to erode and accrete over years to millennia. Localized sediment cycling at smaller scales is strongly associated with cross-shore transport, while theoretical and data-analysis findings attribute large-scale shoreline evolution to alongshore sediment-transport processes; deep-water incident waves interacting with a sediment- covered shoreface are predicted to set up gradients in alongshore sediment transport that, depending on their approach angle, can exaggerate or smooth away shoreline perturbations. Patterns of alongshore sediment flux thus produce a correlation between shoreline-position change and shoreline curvature that depends on whether high- or low-angle waves dominated the wave climate over a given time span. Recent work on the northern North Carolina Outer Banks using lidar surveys to measure shoreline position shows a significant negative correlation, strongest at long time (decadal) and spatial (km) scales, between position change and shoreline curvature, suggestive of diffusion driven by a low-angle-dominated regional wave climate over the last decade. Correlations between position change and shoreline curvature, however, can arise from processes other than gradients in alongshore transport strictly related to shoreline curvature. Near the towns of Kitty Hawk, Kill Devil Hills, and Nags Head, a series of oblique sandy bars and gravel-floored troughs ~1 km wide define the nearshore bathymetry for approximately 5 - 8 km alongshore. Wave propagation over these features likely causes alongshore variations in breaking wave height and angle that affect alongshore sediment transport and patterns of shoreline change. From annual lidar surveys and vehicle-based shoreline measurements taken sub-annually for nine years, we find km-scale undulations in the shoreline that might arise from such bar-controlled modifications of nearshore processes. We identify patterns of shoreline change that in some areas suggest alongshore translation of these undulations, and less-regular changes in location and amplitude in others. Alongshore translation of features with a well-defined wavelength should give negative correlations between curvature and shoreline change, with a correlation magnitude that increases as the translation approaches half a wavelength (180° phase shift). Such behavior would also create a maximum correlation at zero-lag in a cross-correlation analysis, as we observe. We present data analyses and numerical modeling designed to diagnose the relative influences of the processes driving (over various time and spatial scales) the observed shoreline change in this enigmatic region. We also compare observations from the northern Outer Banks with two other internally continuous beach segments north and south of Cape Hatteras, introducing higher-angle wave climates and a greater range of known shoreface bathymetry to the investigation.
H41B-0507
PX and PXT: New Methods for Calculating Shoreline Change Rates
It is imperative that coastal erosion studies produce valid erosion rates and erosion hazard predictions to aid in the development of public policy and protect coastal resources. Currently, the Single-Transect method is the most common shoreline change model, which calculates a rate at each shore-normal transect without regard to influences of data from adjacent transects along a beach. Improving on Single-Transect, the University of Hawaii Coastal Geology Group has developed the PX (Polynomial in distance X) and PXT (Polynomial in distance X and Time) shoreline change rate calculation methods, which model all the shoreline positions within a beach simultaneously using polynomial techniques. PX is a special case of PXT that models shoreline change rates spatially along a beach. PXT not only models the shoreline change spatially, but it lets the rate change with time (acceleration). This is an important advance, as beaches may not erode or accrete at a constant (linear) rate. A linear sum of basis functions characterizes the shoreline change rate for both PX and PXT. These methods are an improvement on previous methods as they produce more meaningful, i.e., statistically significant rates and erosion hazard predictions. To date, PX and PXT improve the significance in the rate by 25% on Maui. We use an information criterion (gMDL) to (1) identify the number of coefficients of the basis functions that are needed to describe shoreline change in PX and PXT, and (2) compare different methods to determine which method best describes shoreline change. We present an overview of the PX and PXT methods and results from a shoreline change study of the beaches of southeast Oahu, Hawaii, utilizing these rate calculation methods.
H41B-0508
Results from the Southeast Oahu, Hawaii, Shoreline Erosion Study Utilizing the PX and PXT Shoreline Erosion Rate Methods
It is imperative that coastal erosion studies produce valid erosion rates and erosion hazard predictions to aid in the development of public policy and protect coastal resources. Currently, the Single-Transect method is the most common shoreline change model, which calculates a rate at each shore-normal transect without regard to influences of data from adjacent transects along a beach. Improving on Single-Transect, the University of Hawaii Coastal Geology Group has developed the PX (Polynomial in distance X) and PXT (Polynomial in distance X and Time) shoreline change rate calculation methods, which model all the shoreline positions within a beach simultaneously using polynomial techniques. PX is a special case of PXT that models shoreline change rates spatially along a beach. PXT not only models the shoreline change spatially, but it lets the rate change with time (acceleration). This is an important advance, as beaches may not erode or accrete at a constant (linear) rate. A linear sum of basis functions characterizes the shoreline change rate for both PX and PXT. These methods are an improvement on previous methods as they produce more meaningful, i.e., statistically significant rates and erosion hazard predictions. We use an information criterion (gMDL) to (1) identify the number of coefficients of the basis functions that are needed to describe shoreline change in PX and PXT, and (2) compare different methods to determine which method best describes shoreline change. The southeast coastline of Oahu, Hawaii, features a range of beach morphologies and littoral dynamics well suited for further testing of the PX and PXT shoreline change rate calculation methods. The PX and PXT methods find significant rates for 70% of the study area versus 28% significant rates with the Single-Transect method. In companion with the work presented by Ayesha Genz on the PX and PXT rate methods, we present results from the Southeast Oahu Shoreline Study as a demonstration of the utility of the new rate calculation methods and for comparison with the previously used rate methods. http://www.soest.hawaii.edu/coasts/
H41B-0509
Tracking Changes in Coastal and Nearshore Morphology in the Southern Beaufort Sea Using Synthetic Aperture Radar
Nearshore morphology in the Mackenzie Delta region of the Beaufort Sea is poorly known because much of the region is very shallow (< 2 m) and the water is highly turbid. Synthetic Aperture Radar (SAR) has been used to map nearshore morphology of lakes in Alaska by taking advantage of the ability of radar waves to penetrate freshwater ice. This technique has been extended to the Mackenzie Delta nearshore region where winter ice forms from river water that is sufficiently fresh so as to be transparent at SAR frequencies. SAR allows the delineation of sea ice that freezes to the seabed (bottom-fast ice or BFI). A time series of imagery throughout a winter depicts the progressive growth of areas where BFI occurs and if sea ice thickness is known at the time of imaging, the delineation of BFI zones represents a proxy for bathymetry. Progressive development of BFI was mapped through the winters of 2003-07 and isolated images from other years are available. The nearshore morphology of the region as revealed by BFI mapping is characterized by extensive nearshore shoals that form at the mouths of active distributaries and are separated by wide, slightly deeper embayments. Narrow channels can be seen to transect the shoals both aligned with and orthogonal to the river outflow. Detailed images from thick ice years depict channels fanning out to feed distributary mouth bars. Comparison of images acquired over more than 10 years suggest that shoal migration can exceed 100 m per year and channel incision of the shoals to depths of >5 m has occurred. The BFI imagery suggests that there is sufficient room beneath the sea ice cover to permit river discharge to reach the shelf without requiring extensive networks of sub- ice channels. The distribution of bottomfast ice also constrains discharge during winter and spring. High inflows occurring during winter surges may lift the ice canopy or over flow onto the surface of the ice disrupting transportation networks. Negative surges in winter can seal off channels. Rising spring discharge is routed over the surface of the nearshore ice when the capacity of the constrained sub-ice drainage network is exceeded causing extensive flooding of the ice surface and strudel drainage to occur.
H41B-0510
Coastal Morphodynamic Response of a Shallow Sediment Mound Forced by Strong Tidal Currents and Large Waves
Shoreline erosion along Ocean Beach, located south of the Golden Gate inlet, is an imminent problem threatening viable infrastructure. In an effort to reduce the erosion, dredged material has been placed within the nearshore region immediately offshore of the erosion ‘hot-spot' during three annual operations. As part of an effort to understand the physical processes causing morphologic evolution in the inner shelf and nearshore regions, a numerical model simulating tidal currents and transformation of waves from offshore to the nearshore region has been hydrodynamically calibrated and validated with field data. The numerical model is used to investigate nearshore morphodynamics including the development of bars, displacement of material locally introduced to the system, and movement of the inferred mean shoreline position on seasonal and inter-annual time-scales. The feedback between longer term (decadal) shoreline changes in response to regularly scheduled placement of sediment and the response to varying offshore wave conditions are also investigated.
H41B-0511
Correlation of Sub-Aerial Beach Change with Numerical Model Derived Nearshore Wave Conditions
Wave-induced sediment transport on and off of beaches is difficult to understand and predict without thorough knowledge of the nearshore wave conditions. Wave data is commonly provided by a buoy located offshore in deep water that measures waves prior to shoaling and refraction. Irregular bathymetry causes dissimilar refraction and shoaling and can lead to variable wave conditions in the nearshore environment. To account for wave propagation over varying bathymetry, numerical wave models are good tools for estimating the nearshore wave climate from offshore wave data. Ocean Beach in San Francisco, CA is an energetic, intermediately sloping beach that was the subject of frequent sub-aerial topographic surveys in 2005 and 2006, with some surveys being as close as two days apart. Sediment volume change derived from these surveys was correlated to nearshore wave heights estimated from offshore buoy measurements and the application of the numerical wave model SWAN (Simulating WAves Nearshore). The SWAN model was used to create a "look-up" table of nearshore wave heights from over 4500 combinations of offshore wave heights, periods, and directions. The model was run using a nested grid scheme using three separate spatial resolutions, with the finest being closest to shore. Correlations between the sub-aerial beach volume data at five morphologically different reaches of Ocean Beach and the SWAN derived wave heights from just outside of the surf zone (in 5, 7.5, or 10 m of water depending on wave height) are generally low, with R2 values less than 0.5, with the highest being 0.61. Although the coefficients of determination are low in most instances the significance exceeds 90%. The reason for the low coefficients of determination is not known but is currently being investigated; some possible reasons are improper characterization of the lengthy time series of wave data between surveys (up to 28 days), or the ignored effect of strong along-shore directed tidal currents (O(~1m/s)) on breaking waves in the surf zone.
H41B-0512
Estimating sand grain sizes from digital imagery via chord-distribution functions
Variations in beach sediment size, supply, and composition are important parameters in understanding patterns of sediment transport and erosion. Traditional methods of characterizing sediments (including mechanical sieving, water column settlement, and laser diffraction) involve recovery of a sample and its removal to the lab for measurement, a time and labor intensive process. Consequently, despite the evidence suggesting the importance of grain size and variability in sediment transport and morphodynamic processes and the inherent spatial and temporal variability of sediments at many locales, most studies are based on relatively few samples. In-situ, rapid characterization of sediments would allow greater spatial data coverage and, consequently, tighter constraints in sediment transport and erosion models. Rubin (J. Sediment. Res., 74, 160-165, 2004) suggested one approach, analyzing the autocorrelation of digital images of sediments to determine grain size distribution rapidly. The technique has the advantages of rapid data collection (via a digital camera) and a relatively straight forward data processing algorithm. However, the autocorrelation technique is not always able to discriminate robustly between different grain morphologies. Using the same tools (digital imagery and computational resources), I examine an alternative approach to characterization of beach sediments: using chord-distribution functions to discriminate grain size distribution and variability. Chord-distribution functions have been shown to be extremely sensitive to small changes in particle morphology in biphasic media (Levitz and Tchoubar, J. Phyisque I, 2, 771-790, 1992). Initial results indicate that with proper pre-processing of the digital image, the technique robustly characterizes the distribution of grains in beach sediment samples from Oahu's north shore.
H41B-0513
Field observations and morphodynamic modeling of spontaneous tidal network formation within a constructed salt marsh
We have monitored and analyzed, through remote sensing and ancillary field surveys, the rapid (O(1) year) development of a tidal network within a newly established artificial salt marsh in the Venice Lagoon. After the construction of the salt marsh, a network of volunteer creeks established themselves away from an artificially constructed main channel (with mean and maximum annual headward-growth rates of 11 m/yr and 18 m/yr, respectively). The rapid formation of this system of tidal creeks provides a unique opportunity to test the reliability of a model of tidal network initiation and development, previously proposed by the authors. The restored marsh presents the characteristics of a controlled environment analogous to a large-scale field laboratory, as it allows comparison of the morphologic features of real and simulated network structures under the reasonable assumption of neglecting accretion and deposition processes over the timescales of observation. Our results compare favorably with observational evidence, showing that the model proves reasonably capable of reproducing the main features of the actual channel-network patterns. The model reproduces statistical network characteristics of eco-morphodynamic and hydrodynamic relevance and captures the dominant modes of the network-incision process.
H41B-0514
Daily and Decadal Coastal Changes Along the Yellow River Delta
We use the dynamic Yellow River Delta in China as one of the world's best examples to demonstrate rapid changes along coastal regions. Several factors contribute to the changes: 1) the river carries a heavy sediment load, leading to coastal deposition and frequent river course changes; 2) the lower reaches of the river support a high population density, leading to extensive development of the delta region and water over-subscription. On very short time scales, the delta area also changes dramatically because the low-lying coastal region is easily flooded or exposed by tides in conjunction with wind-driven sea level fluctuations. While several researchers have correlated the amount of delta growth with annual river discharge, we use satellite imagery from various sources to provide a regional assessment of change that occurs on very small time scales (days to weeks). Observations are correlated to local climate data, when available. With these data sets, we provide a new summary of the long- term rates of change of the Yellow River delta, giving consideration to the substantial short-term coastline differences. While the 20-year interval over a defined region shows the delta area changing by up to 400 sq km (and an overall growth of more than 200 sq km), differences in delta area (over the same region) from day to day or within a few weeks can be as high as 100 sq km. Coastal changes of this magnitude and at such high frequency present challenges for defining the "normal" coastline configuration and highlight the effect of the temporal scale in characterizing changes along deltas.