OS23A-1231
A Beach Monitoring Using Satellite Data, Airborne- and Ground-based LIDAR in the East Coast of Korea
Studies on changes in coastlines and erosion/accumulation in the beach environments are actively being conducted. Various satellites and airborne remote sensing system are being used as a method of monitoring recent changes in the coasts. In this study, 2-D and 3-D morphologic changes have been investigated by IKONOS and Kompsat-2 MSC and measured by airborne and ground-based LIDAR system. These data and in situ measurements were conducted at the same time in the Ulgin coast area of the East Sea (Japan Sea) and compared to perform an accuracy analysis. Sedimentary facies results with ocean current data were also used for understanding of sedimentary processes in the beach area according to dyke construction.
OS23A-1232
Comparison of Laser Scanning Methods for Beach Morphology Investigations
Surveying the active beach face is fundamental to understanding the inherent relationships between hydrodynamic conditions and the ever changing geomorphology of the shoreline. New laser scanning technology allows for rapid, high-resolution surveys of beach face elevation. For example, approximately 125,000 square meters of beach can be scanned in 10-45 minutes, depending on the instrument used and the level of resolution intended for the survey. Elevations can be resolved to less than 3 mm. The two common laser scanning methods, one based on phase-shift in the laser signal, the other in time of flight of a laser pulse, are evaluated for their use in studying the active shoreline. A phase-shift laser scanner was employed to survey a 500m stretch along the active beach face at successive low tides along the Atlantic Coast of Florida at Melbourne Beach in February of 2008. A time-of-flight laser scanner was employed to measure a similarly-sized stretch of the active shoreline during successive low tides at Tairua Beach on the Pacific Coast of North Island, New Zealand during July 2008. The results of both surveys are presented, outlining the advantages and disadvantages of each. We describe pre-survey set-up required for a scan, preferred methods for data collection and post-processing of the data for each survey collected.
OS23A-1233
Beach Nourishment Dynamics in a Coupled Large-Scale Coastal Change and Economic Optimization Model
Global climate change is predicted to have significant consequences for shoreline evolution from both sea level rise and changing wave climates. Because many coastal communities actively defend against erosion, changing environmental conditions will influence rates of nourishment. Over large coastal regions, including many towns, the anticipated future rate of nourishment is assumed to be proportional to the expected evolution of the shoreline in the region. This view neglects the possibility of strong coupling between the spatial patterns of nourishment and the distribution of property values within the region. To explore the impact of this coupling, we present a numerical model that incorporates the physical forces of alongshore sediment transport and erosion due to sea level rise as well as the economic forces that drive beach replenishment including the economic benefits of enhanced or maintained beach width and the costs of replenishing. Results are presented for a Carolina-like coastline and show how natural shoreline change rates are altered as the wave climate changes (because of changing storm behaviors). Results also show that the nourishment rate is conserved for varying property value distributions when the nourishment cost is unrelated to past nourishment and, in contrast, increasing nourishment cost as available sand for nourishment is depleted causes strong coupling between the property value distribution and erosion patterns. This strong coupling significantly alters the rate of nourishment and hence the depletion of available sand for nourishing.
OS23A-1234
Understanding Impacts of Coastal Storms From 2007-08 in National Parks
The United States National Park Service (NPS) is assessing the storm vulnerability of resources in coastal parks. After the active 2004-2005 hurricane seasons a project was initiated to better understand how storms affect the morphology of these areas. Landforms such as barrier islands and spits, open ocean and bayside beaches as well as backbarrier ecosystems such as tidal flats and salt marshes are being examined. Impacts from tropical and extratropical cyclones from 2007-08 will be looked at in detail. An extratropical cyclone in the northeastern United States in April 2007 caused significant changes to several parks. A new inlet formed through a barrier spit within the boundaries of Cape Cod National Seashore in Massachusetts. The new inlet has captured a significant portion of the tidal prism and will likely become the primary inlet. Seasonal homes and park structures were lost due to the subsequent inlet widening and erosion. Along the southern shore of Long Island, New York coastal dune erosion >12m was documented during the same storm at Fire Island National Seashore. This storm, though not extraordinarily powerful, illustrates the role extratropical cyclones play as primary drivers of coastal change along the northeast coast. Tropical cyclones play a similar role in the coastal morphodynamics along the southeastern Atlantic coast, Caribbean and Gulf of Mexico. Hurricane Gustav was a Category 2 storm at landfall in September 2008 and produced significant change to coastal areas in Louisiana and Mississippi. The recovery seen at some barrier islands since Hurricane Katrina was lost during this event, demonstrating how barrier islands recovering from previous storms are more vulnerable to less intense events. At Gulf Islands National Seashore, West Ship Island, which lost considerable subaerial volume during the 2004-2005 hurricane seasons, was completely inundated during Hurricane Gustav and Jean Lafitte National Historical Park and Preserve in Louisiana experienced storm surges >3.5m. The NPS is conducting park-specific assessments with partners to determine impacts from anthropogenic alterations to sediment transport, natural variations in morphology and coastal landform evolution. These data will provide a better understanding of how and why coastal areas respond and recover to storm events. These data will allow the NPS to develop park-specific, resource-based management documents that help park staff better understand and manage their resources in a post-storm setting and inform and educate the public regarding these natural phenomena.
OS23A-1235
Quantifying Barrier Island Overwash Through Sedimentological, Geophysical, and Geospatial Analyses: Onslow Beach, NC
A combination of sedimentological, geophysical, and geospatial datasets were utilized to quantify the amount of barrier island sediment that can be attributed to overwash events. A variety of processes including inlet dynamics, longshore and aeolian transport, and oceanic overwash are capable of driving barrier island migration. As sea level continues to rise and concerns of potential increased storm activities are peaked, it is important that we parameterize these primary mechanisms of sediment exchange and work towards physics- based models of barrier island migration. High-resolution chirp sub-bottom profiles were collected in Onslow Bay (depths <10 m) and along the back side of the island in the Atlantic Intracoastal Waterway. These data were subsequently coupled with ground- penetrating radar (GPR) collected over the top of the barrier island and used to estimate the depth of the shallowest cohesive substrate. The distance between this cohesive substrate and topographic elevations of the island (determined from lidar) enable an estimate of the total volume of sediment on Onslow Beach. Information from the GPR, in combination with numerous sediment vibracores, provide an estimate of what percentage of the active sand prism was deposited in overwash events. A time series of aerial photographs and satellite imagery reveals how the surficial expression of overwash fans has changed in both area and distribution since the 1950s and is interpreted in the context of published historical shoreline erosion rates for the region.
OS23A-1236
Coastal Dune Building Capabilities of Native and Exotic Beach Grasses: Initial Results From a Moveable Bed Wind Tunnel Experiment
Coastal dune morphology results from the interplay between wind, waves, sediment supply, and vegetation. Over the last 100 years in the U.S. Pacific Northwest, two exotic beach grasses (Ammophila arenaria and A. breviligulata) have replaced much of the native beach grass (Elymus mollis), and formed large continuous, and relatively stable, foredunes where open low-lying dynamic dune systems had previously dominated. Field measurements demonstrate a correlation between dominant beach grass species and foredune height, suggesting the potential for an ecological control on coastal vulnerability. Over a 20 year period, and across a broad range of sediment supply along the coast, A. breviligulata is associated with lower foredune heights than A. arenaria. Based on these correlations, we hypothesize that the A. arenaria is more effective at entrapping and stabilizing sand which results in steep and tall foredune morphology along reaches where this species dominates. A moveable bed wind tunnel experiment is being performed at Oregon State University's O.H. Hinsdale Wave Research Laboratory to test this hypothesis. Our experiments will quantify the relative sand trapping capabilities among the three beach grass species across different planting densities and wind speeds. Results from the experiment will help parameterize a computational fluid dynamics model describing dune building capabilities of different species, species' densities, wind speeds, and sediment supplies. Wind tunnel experiments and modeling results will be combined with field data, both biological and physical observations, to inform coastal protection measures and dune ecosystem management.
OS23A-1237
The Relative Importance of Cross- and Along-shore Sediment Transport in Planform and Profile Adjustments of a Gravel Barrier Beach
Slapton is a macrotidal gravel barrier beach in Devon, UK. Its stability is threatened by accelerated sea-level rise and long-term (decadal-centurial scale) redistributions of sediment alongshore between adjacent beaches within Start Bay. Fortnightly surveys have been conducted regularly at spring low tide since October 2006. An ARGUS camera system has continuously monitored the beach since the autumn of 2005. Historical observations and measured profiles taken by the Field Studies Council Slapton Ley, have been made since 1972. In addition, several field campaigns have been carried out, in autumn 2005 and springs of 2007 and 2008, measuring profile response over tidal- and sub-tidal cycle timescales. The relative importance of sediment transport gradients operating in the cross- and along-shore directions, and resulting net sedimentation patterns and changes in the beach profile and planform, has scale- dependency. Despite being 'swash-aligned', planform adjustments induced by alongshore gradients in sediment transport are dominant. Modelled net alongshore sediment transport rates are at least an order of magnitude greater than those derived from measurements, and thus these models must be treated as transport potentials, and require improvement. At time scales at or less than a tidal cycle, beachface change is dominated by cross-shore sedimentation, and although lack of sediment continuity over these scales suggests some net alongshore transport, antecedent cross-shore sedimentation dictates where and for how long. During storms, whilst the immediate manifestation of change is cross-shore, the antecedent along- shore redistribution of sediment has a crucial role to play in determining the general integrity of the barrier, as well as the location and magnitude of overwash and significant shoreline recession. The complicated interplay between cross- and along-shore sedimentation processes, at overlapping scales, highlights the importance of continuous monitoring for sound management decisions at this and similar sites.
OS23A-1238
Controls on alongshore transport in the Santa Barbara Littoral Cell; a process-based model analysis
A process-based model (Delft3D) is used to investigate the patterns of littoral drift along the Santa Barbara
Littoral Cell that stretches from Pt. Conception to the Mugu Submarine Canyon. Identification of the sediment
transport patterns, pathways and potential is essential for sustainable coastal zone management along this
heavily anthropogenically-altered coastline. An advanced optimalization procedure is applied to schematize
the wave climate in 10 wave height, period and direction classes. Calibration of the littoral drift rates is based
on comparison with dredging volumes from the entrance channels to Santa Barbara and Ventura Harbors.
Preferential erosion or sedimentation hotspots are identified by analysis of the divergence or convergence of
sediment transport. Waves are shown to play a key role in distributing river-supplied sediments along the
coast. The transport capacity is strongly dependant on the alongshore variation in wave height due to wave
sheltering, diffraction and focusing by the Northern Channel Islands, and on the local orientation of the
structurally-controlled coastline.
http://walrus.wr.usgs.gov/coastal_processes/sbventura/
OS23A-1239
Characterizing Nearshore Sediment Transport in the Santa Barbara Littoral Cell Using Repeated Shallow Bathymetric Surveys
The Santa Barbara Littoral Cell (SBLC) extends 150 km from Point Conception to the Point Mugu Submarine Canyon. Alongshore sediment transport rates in the SBLC can be very high but vary by at least three-fold throughout this complex region. Natural factors contributing to transport variability include wave sheltering and focusing by the Channel Islands, proximity to river inputs, and framework geological controls on coastal orientation. Sediment in the SBLC generally moves from west to east due to the predominant northwest swell, but localized transport gradients can have significant impacts on beach accretion and erosion. Coastal development in the area, particularly the construction of jetties, breakwaters, groins and seawalls, has modified nearshore sediment transport in the SBLC, but the degree to which development-related changes in sediment transport are impacting coastal resources remains unclear. This study uses three years of nearshore bathymetry data (collected biennially from October 2005 to October 2008) to examine nearshore sediment transport along the 90 km of SBLC coastline between Goleta and the Point Mugu Submarine Canyon. Data for each survey were collected along up to 248 cross- and along-shore survey lines using the third-generation USGS Coastal Profiling System (personal watercraft equipped with hydrographic surveying equipment), which allowed for collection of high-quality bathymetric data in water depths as shallow as 1 meter. Spatial and temporal changes in bathymetry are used to characterize alongshore variations in sediment transport, seasonal bar migration, and statistical depth-of- closure in support of a publicly-funded regional sediment transport study.
OS23A-1240
Interannual cross-shore sandbar migration
Nearshore sandbars are ubiquitous features along micro- to mesotidal, wave-dominated sandy coasts. The most conspicuous characteristic of nearshore multiple sandbar systems along sea-dominated coasts is interannual net offshore migration (NOM), while that of similar systems along swell-dominated coasts appears to be the on/offshore migration in response to individual wave events. As originally described for the Dutch coast, a sandbar that exhibits interannual NOM is generated close to the shoreline and then migrates net offshore for several years to finally decay in the outer nearshore. It is not known why sandbars in swell- dominated settings behave differently. Here we investigate an approximately 7.5-year data set of daily 10-minute time exposure video images of the swell-dominated double-barred northern Gold Coast, Australia. We show that this system does exhibit NOM behaviour albeit with a highly episodic character. This hitherto unreported NOM type, which we term episodic NOM, is triggered by events with wave heights exceeding three to four times the annual average. The outer sandbar then migrates some 75 to 100 m further offshore than usual and subsequently decays in the outer nearshore by slow onshore migration during the low-energy weeks following the major storm. The next, moderately high wave event causes the rapid (≈ 20-30 m/day) offshore migration of the inner bar to become the new outer bar and the onshore birth of a new bar. Between the large wave events both the outer and the inner bar respond primarily to the annual signal in wave height, whereby both bars are located further offshore during the higher (lower) energy winter (summer) months. Episodic NOM is thus strongly steered by the chronology in wave events, while interannual NOM appears to be controlled by cumulative wave energy in which the timing of individual storms is unimportant. Because the sediment volume involved in sandbar migration strongly controls the relaxation time and the morphological inertia within the sandbar system, we speculate that small sandbars (maximum height < 0.5- 1 m), such as along the Gold Coast, exhibit episodic NOM, while large and hence more inert sandbars (maximum height of 1 m or more), such as found along the Dutch coast, develop interannual NOM. As small sandbars tend to be observed more frequently along swell-dominated rather than along sea-dominated coasts, wave period might be a critical parameter controlling sandbar size.
OS23A-1241
On the O'Brien - Jarrett - Marchi law: origins, limits, extensions.
The relationship between the total water volume entering a lagoon during a characteristic tidal cycle (i.e. the prism) and the size of its inlet is well established empirically since the classic work of O'Brien and Jarrett widely cited in the geomorphic and hydrodynamic literature. Less known is a rather deep theoretical explanation proposed by Marchi. This paper reviews the empirical and theoretical evidence on which the relation is based, and deepens its implications to a wider class of morphodynamic processes. The importance of the extended relation is noteworthy, because it allows quantitative predictions of the long-term morphological evolution of tidal landforms in response to physical and ecological forcings, per se a subject of great theoretical and practical importance. In particular, we show, by collecting evidence from disparate sources, that a relation analogous to those investigate by O'Brien, Jarrett and Marchi holds asymptotically for estuaries and for whole tidal networks. The latter requires that the peak flows and local tidal prisms driven by landforming spring fluctuations are defined by a time-invariant equivalent of the lagoonal surface proper to any channel cross-section. The role of the lagoonal surface is taken by a mean watershed defined by an energy landscape analogous to topography where gravity is the leading force, here obtained from averaged free-surface gradients. Empirical evidence, largely gathered within the lagoon of Venice, supports our claims. We conclude that the depth of the empirical and theoretical validations, and the breadth and the importance of its implications, suggest to refer thereinafter to the O'Brien-Jarrett-Marchi law relating tidal prisms and watershed surfaces to cross-section sizes anywhere within a tidal landscape.
OS23A-1242
Modeling the effect of wind-waves on the evolution of marshplains and mudflats
A one-dimensional sediment and bed model was constructed to assess the long-term evolution of shallow intertidal zones. Observations indicate that shallow intertidal basins tend to be characterized by a bimodal distribution of water depths into higher marshplain and lower mudflats. The model is used to investigate the mechanisms and factors affecting the transition of a shallow coastal region into either a marshplain accreting to mean high-high water level, or eroding into mudflat. Wind waves and tidal currents both affect the deposition and erosion of sediments. For two weeks, field data was collected at a tidal wetlands restoration site in San Francisco Bay, California to provide a comparison with model results. Two locations within the field site with different wind climates are compared to show the impact of wind on the accretion rate. Model sensitivity is tested for parameters, such as the fetch length, initial marsh elevation, and mass flux of sediment, to study their relative importance to accretion. The model provides a tool in engineering and design applications in developing strategies for restoring or protecting existing tidal marshes.
OS23A-1243
Advanced Scour Monitoring at Indian River Inlet, Delaware
The scour holes threatening the bridge over Indian River Inlet, Delaware are unique in their size and location, and thus demand an innovative approach to their observation. Typical bridge scour occurs at the base of support structures. However, scour at Indian River Inlet has developed two large holes over 25m deep that flank the bridge. The deepest part of one hole sits just 45m away from the bridge foundation, and has been migrating toward the bridge at a rate that has alarmed Delaware Department of Transportation (DelDOT) engineers. Recently a real-time bridge and bathymetric monitoring system was installed, giving engineers the ability to make operational decisions regarding the safety of the bridge, and providing scientists a unique opportunity to investigate the coupled inlet morphologic evolution and hydrodynamics over multiple time scales. The monitoring system consists of two identical sets of instruments permanently mounted to subaqueous bridge piers, with one three dimensional imaging sonar and one horizontal ADCP comprising each pair. This new system represents significant progress in scour monitoring at Indian River Inlet. Previous efforts only provided snapshots of the storied evolution of the bathymetry or tidal forcing - and thus offered diminished potential for understanding the complicated inlet dynamics. Data from the instruments are automatically uploaded to University of Delaware servers and processed twice per day. Preliminary analysis of observed diurnal evolution in bathymetry along with associated forcing will be presented.
OS23A-1244
Importance of wind, fetch and water levels on waves and wave-generated shear stresses in a shallow coastal lagoon
Wind waves and the bed shear stresses they produce are critical for the morphological and ecological equilibrium of shallow tidal basins. Wave-generated shear stresses are the main mechanism responsible for sediment erosion on tidal flats, and regulate both sediment concentrations in the water column and, together with tidal currents, sediment export to salt marshes and to the ocean. We analyze the response of a system of shallow tidal basins along the Eastern Shore of Virginia, U.S., to wind-wave events, with a specific focus on the interplay of basin morphology, tidal elevation and wind direction on depth, fetch and the resulting wave- generated shear stresses. Our analysis indicates that the potential for erosion is the highest when the salt marshes are submerged. Under these conditions the direction of the wind is critical, with maximum wave heights and erosion potential occurring for winds blowing along the barrier islands of the basin (NNE-SSW). We identify four bottom shear stress regimes produced by wind waves in the Virginia Coastal Reserve as a function of water elevation. For elevations between MLLW and MSL the increase in water depth dominates the increase in wave height thus reducing the bottom shear stresses. For elevations between MSL and MHHW the flooding of the salt marshes increases fetch, wave height and bottom shear stresses, producing the largest resuspesion events in the bay. Finally, for elevations above MHHW, the increase in depth reduces the average bottom shear stresses, thus reducing possible erosion in the tidal flats.
OS23A-1245
Study Of Estuary Entrances Through One Dimensional Modelling
This paper investigates the use of a one dimensional hydrodynamic model, Dynlet in analyzing rivers that are highly mobile and prone to closure. Wairoa river, located in Hawkes Bay, New Zealand has a gravel barrier beach seperating the estuary from the sea and is often closed or near closed during low flow events. The river has to be dredged regularly to prevent flooding and as such, it is of interest to the research to study the behavior of the entrance. A one dimensional model has been created using data provided by Hawkes Bay regional council and from the model, a method is proposed allowing for the prediction of entrance dimensions. In addition, the equilibrium flow area of the river is used as a comparison with other researchers in this field.
OS23A-1246
Modelling An Estuary Over Long Time Periods
Due to the special geographic features and high rate of biological productivity of the estuary; analyzing the evolution of an estuary over long time periods is important in understanding and predicting environmental changes in the natural ecosystem. To predict the behavior the varied flow conditions in the natural phenomena, a simpler mathematics model is required to analyze it. Averaging methods are extensively used for fluid studies. Based on the changing flow conditions, the time span could be divided into three periods, long time, medium and short period. In this paper, a long-term averaging model for the estuary is introduced to predict the flow variation, sediment transport and bed morphology change. In addition, various variables which may influence the river system are discussed and some reasonable values of these variables are estimated based on the experience. By introducing the time scales and instantaneous variables into the one- dimensional hydraulic equations of momentum, sediment transport and bed morphology change, three simplified equations are obtained. A case study on the Snowy River is discussed in this paper.
OS23A-1247
EXPLORING THE RELATIONSHIP BETWEEN LARGE-SCALE SHORELINE CHANGE AND WAVE CLIMATE
In sandy coastal locales such the North Carolina Outer Banks, an alongshore survey (extending tens of kms) of cross-shore shoreline change can sometimes be described by a power law, for which the scaling exponent is a measure of the surveyed area's fractal roughness. Such self-affine signals, recently identified in repeated shoreline measurements, could be related to shoreline behaviors characterized in extant work as the emergent results of feedback mechanisms within a complex, wave-driven system. Theoretical insights into sandy shoreline evolution suggest that the gradients in alongshore sediment transport that drive large-scale shoreline change are strongly dependent on the angle of incidence between shoreline orientation and deep- water waves. In this framework, representation of shoreline behavior simplifies to an effective diffusivity that is largely a function of incidence angle. As wave angle increases, the effective diffusivity of the shoreline decreases. For large incidence angles (~ 45°), effective diffusivity becomes negative and alongshore shoreline perturbations tend to grow; even a coast dominated by diffusive, low-angle waves experiences a seasonally integrated mix of waves that approach from high angles. Regardless of whether small-scale shoreline undulations arise from the high-angle wave instability or from more localized processes that tend to roughen the alongshore coastal profile (such as sandbar development and transformation), how changes in small-scale roughness (< km) integrate into the regional-scale shoreline signal remains poorly understood. We hypothesize that the fractal roughness of the signal of sandy shoreline change is a function of wave climate, such that fractal roughness is negatively correlated with effective diffusivity. To test this idea, we dovetail shoreline change measurements (taken from North Carolina Outer Banks lidar surveys recorded 1996 - 2006) with a one-line numerical model of shoreline behavior forced by different prevailing wave climates. Stretches of beach to the north and south of Cape Hatteras should have different effective diffusivities based on their relative orientations to the prevailing wave climate. Hindcast data for the past two decades shows the wave climate for the Carolina Capes as slightly weighted by waves approaching from the northeast. Is the North Carolina coast getting more or less rough, and if so, where? and over what temporal and spatial scales? We employ shoreline position change and shoreline curvature change as metrics of roughness and compare field surveys over annual and decadal time scales; we then match our results against similar parameters extracted from the model output.
OS23A-1248
Large-scale seabed evolution: the geomorphic history of the outer Thames estuary
The seabed of the outer Thames estuary (southern North Sea) comprises a complex array of ridges, banks and channels. Under the classification of Dyer and Huntley (1999), open shelf ridges exist across the shoreface to the east, wide estuary mouth banks occupy the central region and headland-associated banner banks are located to the north. Although some studies of hydrodynamics, sediment transport and morphology within the region exist, there has been little consideration of historical seabed behaviour. The work described here synthesises chart and sea atlas archives from the 1600s through to present to reconstruct the geomorphic history of the outer Thames seabed across a region >5,000km2. A comprehensive picture of seabed evolution is achieved through a) time-series examination of feature depths over the last 400 years and b) spatial analysis of bathymetric surfaces (interpolated from soundings) over the last 200 years. Results show that although the gross framework and arrangement of features has changed little historically, seabed dynamics are spatially and temporally variable. There is evidence of progressive long-term vertical accretion and degradation, in addition to step-functional or cyclic variability. Changes are often associated with shifts in bank and channel configuration. This is particularly true of estuary-mouth banks within the central Thames estuary. Here, features in close proximity exhibit opposing trends where channels and banks migrate, causing processes of constriction, attachment and detachment. Headland- associated banks in the northern part of the region do not display a clear trend, but there is evidence to suggest a more significant change in their lateral extent and orientation with respect to the shoreline. There is also no evidence that the shelf ridges have changed over their documented history: however, interpretation of change in these narrow and linear features is more susceptible to errors associated with survey resolution. Significantly, where features show progressive vertical change, there does not seem to be a close association with sea-level forcing, despite a regional 20th century relative sea-level rise of about 2.4mm/yr. At this regional scale, greater importance is credited to tidal-sculpting as a mechanism forcing changes in planform resulting in localised stepped and cyclic changes.
OS23A-1249
Linking Sediment Management Practices, Ebb-Tidal Delta Evolution, and Shoreline Change in the San Francisco Bay Coastal System
San Francisco Bay, California, is among the most anthropogenically-altered estuaries in the United States, but the effect of human activities on sediment transport to the coastal ocean has not been quantified. Since the discovery of gold in 1848, the bay floor has been regularly altered by a range of activities, including infill by hydraulic mining debris, mining of sand for bay development, dredging of harbors and waterways, and mining of sand and gravel for use as construction aggregate. Results from the compilation of historical records indicate that over 60 million m3 of sand-sized material has been removed for development and aggregate use alone. Long-term effects on the amount of sediment delivered to the coastal ocean are assessed using hydrographic survey data of the San Francisco Bar, a 120 km2 ebb tidal delta at the mouth of the bay. Data from 1855, 1873, 1900, 1955, and 2005 are compiled to create bathymetric models and compared to quantify net sedimentation volumes and rates of sediment deposition and erosion. Results from the last 50 years demonstrate that the ebb tidal delta has lost approximately 92 million m3 of material. Historical shoreline positions of ~20 km of adjacent beaches are also analyzed to determine if there is a statistical link between bay sediment removal, ebb tidal delta change, and beach evolution. This system-wide approach is unique to the region and emphasizes the often overlooked connection between San Francisco Bay and the coastal ocean.
OS23A-1250
Numerical Modeling of Large-Scale Rocky Coastline Evolution
Seventy-five percent of the world's ocean coastline is rocky. On large scales (i.e. greater than a kilometer), many intertwined processes drive rocky coastline evolution, including coastal erosion and sediment transport, tectonics, antecedent topography, and variations in sea cliff lithology. In areas such as California, an additional aspect of rocky coastline evolution involves submarine canyons that cut across the continental shelf and extend into the nearshore zone. These types of canyons intercept alongshore sediment transport and flush sand to abyssal depths during periodic turbidity currents, thereby delineating coastal sediment transport pathways and affecting shoreline evolution over large spatial and time scales. How tectonic, sediment transport, and canyon processes interact with inherited topographic and lithologic settings to shape rocky coastlines remains an unanswered, and largely unexplored, question. We will present numerical model results of rocky coastline evolution that starts with an immature fractal coastline. The initial shape is modified by headland erosion, wave-driven alongshore sediment transport, and submarine canyon placement. Our previous model results have shown that, as expected, an initial sediment-free irregularly shaped rocky coastline with homogeneous lithology will undergo smoothing in response to wave attack; headlands erode and mobile sediment is swept into bays, forming isolated pocket beaches. As this diffusive process continues, pocket beaches coalesce, and a continuous sediment transport pathway results. However, when a randomly placed submarine canyon is introduced to the system as a sediment sink, the end results are wholly different: sediment cover is reduced, which in turn increases weathering and erosion rates and causes the entire shoreline to move landward more rapidly. The canyon's alongshore position also affects coastline morphology. When placed offshore of a headland, the submarine canyon captures local sediment, increases weathering and erosion around the headland, and eventually changes the headland into an embayment! Improvements to our modeling approach include refining the initial conditions. To create a fractal, immature rocky coastline, self-similar river networks with random side branches were drawn on the shoreline domain. River networks and side branches were scaled according to Horton's law and Tokunaga statistics, respectively, and each river pathway was assigned a simple exponential longitudinal profile. Topography was generated around the river networks to create drainage basins and, on a larger scale, represent a mountainous, fluvially-sculpted landscape. The resultant morphology was then flooded to a given elevation, leaving a fractal rocky coastline. In addition to the simulated terrain, actual digital elevation models will also be used to derive the initial conditions. Elevation data from different mountainous geomorphic settings such as the decaying Appalachian Mountains or actively uplifting Sierra Nevada can be effectively flooded to a given sea level, resulting in a fractal and immature coastline that can be input to the numerical model. This approach will offer insight into how rocky coastlines in different geomorphic settings evolve, and provide a useful complement to results using the simulated terrain.
OS23A-1251
Autodrowning and Phased Evolution of Fluviodeltaic System During Steady Rise of Relative Sea Level
A series of flume-tank experiments in which fluvial deltas were built with steady rise of relative sea level (rate rslr > 0), constant sediment discharge (qS) and constant upstream water discharge (qW) substantiates a set of inbuilt morphodynamic processes that are predicted from the theory of shoreline autoretreat. With the steady external forcing of the basin, the deltaic shoreline migrates basinwards during an early stage but inevitably begins to retreat landwards after a while (autoretreat). Where hinterland slope is steeper than the delta's foreset slope, the retreating shoreline sooner or later becomes attached to the hinterland basement. This critical moment is referred to as 'autodrowning', after which the entire depositional system is drowned to develop as a subaqueous cone. An extended understanding of autoretreat and autodrowning suggests an additional autogenic process. If the initial downstream length of feeding alluvial river exceeds a critical downstream distance (Lcrt) that is specified primarily with qS/rslr, the shoreline takes a modified autoretreat-autodrowning trajectory. Result of the experiments indicates that there are three discrete phases in evolution of the fluviodeltaic system whose shoreline is retreating landwards with constant and continuous rise of sea level. The first phase is characterized by nondeltaic shoreline, alluvial length exceeding Lcrt, and sediment-starvation of the subaqueous surface. During the second phase, alluvial length is less than Lcrt, and deltaic sedimentation is reactivated whereby the sediment- starved surface becomes overlain by delta foreset deposits. The third phase, beginning with the attainment of autodrowning is a phase of nondeltaic sedimentation by a subaqueous cone. In first two phases, depositional system proceeds in a non-equilibrium state that accounts large-scale autogenic changes. These autogenic, sequential morphodynamic processes reflect large-scale, autogenic response of the depositional system to steady rise of sea level.
OS23A-1252
non-Bruun shoreline response to sea-level rise: simple models of the dynamic behavior of overwashing barriers and soft rock shores
The prospect of continued acceleration of rising sea levels presents a unique challenge to our ability to use historical measurements to predict future coastal changes. As a result, we often must rely on models to predict the future of the coast. The so-called 'Bruun rule' underlies most of our long-term understanding of shoreline response to sea-level rise. Concisely, this approach is based upon the concepts of mass conservation and preservation of a shoreface shape. Although models based upon these 'Bruun rule' concepts can shed much understanding, they are in essence geometric, or 'morphokinematic', relying on these base assumptions and necessarily casting a fuzzy eye towards underlying processes. Accordingly, an understanding of process can be lost as these predictions are independent of the rate of sea-level rise, depending only on the total change. For example, geometric models make the same shoreline change predictions regardless of whether a sea-level rise of one meter took decades or millennia. Here, we theoretically investigate two separate (but related) cases where the shoreline can respond dynamically to both the rate and the rate of change of sea-level rise: overwashing barriers and soft rock coasts. Overwashing barriers present an apparent paradox. Although higher sea levels should direct sediment offshore due to shoreface oversteepening, non-equilibrium behavior must occur somehow to provide the net onshore sediment flux that would be necessary for a barrier to survive long-term sea-level rise. A simple model suggests that understeepening of the shoreface by overwash causes a long-term net shoreward sediment flux; shoreface response to sea-level rise becomes 'slaved' to the rate and timing of barrier overwash. Simulations suggest that the dynamically coupled shoreface-overwash system can demonstrate surprising long-term behavior. A dynamic response to sea-level rise also can occur when the coast consists of lithified sediment that can be broken down into fine sediment by wave attack. Recent results using the detailed SCAPE model suggest that If an eroding 'soft rock' cliff retains a low volume beach-- either because the cliff supplies little coarse-grained sediment or if the supplied sediment is quickly removed by alongshore sediment transport gradients-- a dynamic equilibrium shape arises. We investigate the behavior of such a detachment-limited system by providing a simplistic theoretical framework to understand the relationship between the rate of sea-level rise and the rate of erosion.
OS23A-1253
Complexities in Barrier Island Response to Sea-Level Rise: Insights from Model Experiments
As sea level rises, a barrier island will respond either by migrating landward across the underlying substrate to higher elevations or by disintegrating if there is no longer sufficient sand volume and relief above sea level to prevent inundation during storms. Using the morphological-behavior model GEOMBEST, we investigate the sea-level rise response of a complex coastal environment to changes in variety of factors, thus yielding insights into barrier island evolution. Our base case is a simplified Holocene run which simulates a possible scenario for the evolution of a 25-km stretch of the North Carolina Outer Banks over the last 8500 years. Varying one parameter at a time, we explore the degree to which changes in sea-level rise rate, sediment supply/loss rate, offshore limits to sediment transport, substrate erodibility, substrate composition and depth- dependent response rate produce changes in average landward barrier island migration rate, average depth of substrate erosion, and final barrier island volume. As expected, sensitivity analyses reveal that within the range of possible values for the North Carolina coast, sea-level rise rate, followed by sediment supply rate, is the most important factor in determining barrier island response to sea-level rise. More surprisingly, the analyses in aggregate indicate that barrier island evolution is highly sensitive to the range of substrate slopes encountered throughout landward migration (i.e., the slope history); as a barrier encounters a continually changing substrate slope, island volume constantly changes, moving toward equilibrium with the current average slope. Through this process, steeper average slope histories produce smaller barrier islands while shallower average slope histories produce larger barrier islands. In both cases, secondary effects on substrate erosion depth and migration rate also result. Notably, similar geometric effects explain an insensitivity of simulation results to changes in offshore sediment transport limits, or shoreface depth. Results indicate that although simple analytical calculations may predict coastal response to sea-level rise in simplified coastal environments (e.g., constant slope, constant sea-level rise, etc.), morphological-behavior modeling is necessary to provide critical insights regarding changes that may occur in environments having complex geometries, especially when multiple parameters are likely to change at the same time.
OS23A-1254
Late Pleistocene Estuarine Channel-Fill Sediments on the New Jersey Shelf: Response to Glacio-eustatic Sea Level Rise
The shallow New Jersey continental shelf has been an area of study for many decades. Evidence for the shelf's response to the Late Pleistocene glacio-eustatic sea level rise, confined principally to interpreted estuarine fill strata within fluvial channel systems, has been imaged extensively with high resolution chirp seismic data. However, little work has been done on sediment samples of these units. This study aids in the ground truthing of previous seismic stratigraphic work done on the New Jersey shelf channel-fill sediments by analyzing cores collected during the summer of 2007. The infilled channels from the study area are characterized as fossil riverine systems that developed on the exposed shelf during the Last Glacial Maximum lowstand ~18 ka. Subsequent sea level rise ~15-10 ka flooded and modified the valleys, forming estuaries. Allen and Posamentier's (1993) stratigraphic model of transgressive estuarine deposition posits the following ordered sequence: (1) a fluvial lag deposit, (2) estuarine mixed sands and muds (3) basin fill mud, and finally (4) estuarine mouth complex sediments. Seismic data over the buried channels on the outer New Jersey shelf (~80 m water depth) exhibit clearly identifiable seismic facies bounded by mostly horizontal boundaries that can be correlated to this interpretation (Nordfjord et al. 2006) We collected one core into these sediments, which likely sampled the estuarine mouth complex and basin fill muds. Previously collected cores supplement our analysis of these sediments. Seismic data over mid-shelf (~30 m water depth) valley channel fill stratigraphy is strikingly different, with finely-laminated U-shaped layering throughout the section, and often with cut-and-fill sequences. We collected 7 cores into these sediments. The cores ranged from 25cm to 5.8m in length. Thus far we have conducted grain size analysis via gross division of the percent of sand, silt, and clay. Initial results indicate that the laminated mid-shelf channel fill units are dominated by fine sands, suggesting a much higher-energy estuarine environment than when the outer-shelf channels were filled. Further grain size analysis will be conducted by sedigraph and settling tube. Radiocarbon analysis of the stratigraphy is being provided by the shell fragments and organic mud within the samples. We obtained one reliable shell fragment 14C date of 9389 ±79 years and eight bulk carbon 14C dates ranging from 10975 to 11445 years, with a strong terrestrial signature. The foraminiferal assemblages found within the sediment samples should eventually aid in determining the depositional environment.
OS23A-1255
Earliest Marine beds in the Jurassic sedimentary record near the Huajuapan-Petlalcingo region, southern Mexico and their paleogeographic implications
A paleogeographic model of Jurassic-Cretaceous is presented, the study area is the region near Huajuapan de Leon in the Mixteca Terrane, Mexico where a sedimentary successions constituted by interlayered terrestrial and marine beds, provides evidence of transgression and regression episodes. The sediments in the study zone were deposited over a Paleozoic metamorphic basement, the Acatlan Complex. The stratigraphic features in the Middle Jurassic of the terrestrial beds indicate a depositional elements varying from alluvial fans to floodplains and channel deposits, represented by conglomerates, sandy conglomerates and sandstones. After, in the same epoch (Bajocian and Bathonian age) a transgression coming from the Pacific Ocean covered the region. A transitional zone between continental and marine sediments is situated between Tezoatlan and Petlalcingo, the actual cross section consists in the earliest marine beds: limestones interlayered with terrestrial beds. Fossil contents in this beds indicate an age between the Oxfordian and the Tithonian. During this period of transgression the paleogeography was dominated by a small bay with shallow waters connected in the south with the Pacific Ocean, represented principally by limestone and dolomite units. At the end of the transgression, volcanic episodes occurred and the land emerged again. The sedimentary beds were later affected by tectonic activity that produced a normal fault near Zapotitlan, putting the metamorphic basement in contact with the sedimentary sequence.
OS23A-1256
Coupled Wave-Hydrodynamic Modeling Over Sorted Grain Size Features (Rippled Scour Depressions) on the Inner Shelf
Sorted grain-size features, also known as rippled scour depressions, are persistent cross-shore structures found in many nearshore environments, characterized by sharp gradients in grain size and gentle relief in the alongshore direction. The formation of these features is not completely understood, but self-organization and feedback have been proposed as explanations for their persistence. Sorted grain-size features near the Martha's Vineyard Coastal Observatory (MVCO), Massachusetts, are characterized by bathymetric lows with coarse (0.5 mm) sand and large ripples (heights of 0.10-0.15 cm, wavelengths of 0.6 to 0.8 m) and bathymetric highs with fine sand (0.125 mm) and small ripples (heights of 0.01 m, wavelengths of 0.1 m). The features extend from the shallowest region surveyed (6 m) to depths of 17 m, 3 km offshore, with a maximum alongshore width of less than 1 km. Wave-current interaction is an important component of the inner shelf and circulation at MVCO. However, subtle changes in topography and hydrodynamic roughness associated with these features complicate evaluation of numerical models, especially because of the spatial and temporal variability in alongshore flows and wave forcing near MVCO. We have investigated these features and the inner-shelf circulation using nested and coupled wave-hydrodynamic models. The models were calibrated using water-level, current, and wave data from MVCO and several wave buoys. The suite of nested models applied here began with basin- scale (5-km grid for the eastern seaboard) wave models that provided spectral wave forcing to a regional wave model (1-km grid for the southern New England shelf). The regional wave model and a regional hydrodynamic model at the same scale were then run uncoupled, to provide boundary conditions for a cascade of nested, coupled, wave-hydrodynamic models. The final nested model, at an 8-m grid resolution, resolved the sorted grain-size features. At this level, a sediment-transport model with time-dependent ripple geometry and various roughness formulations was applied to investigate the maintenance of these features and their influence on circulation under realistic conditions. Given an initial bed sediment distribution, the roughness introduced by these features significantly alters the quasi-steady-state distribution within the bed, and highlights the concept of feedback and self-organization.
OS23A-1257
A Preliminary Study of Sedimentation in the Mekong Sub-aqueous Delta, Southern Vietnam
The Mekong River is the third largest river systems of Asia in terms of sediment load, but, until now, no high- resolution seismic data have been obtained to document the nature of the sedimentation deposit in its coastal area. Two high resolution seismic surveys were conducted in the coastal area of Mekong River Delta (MRD) in 2006 and 2007 separately, unveiling a low gradient, up to 20 m thick subaqueous delta system. Combination of inland borehole cores and seismic profile successively documents the subaqueous part of this fast prograding delta system. The coastal area of the MRD is dividend into 4 zones according to their clinoform characters. Pb-210 profile in most of the gravity cores shows highly fluctuating activities. Calculated accumulation rate in the cape and northwest side of Camau Peninsula is between 0.23 and 0.47 cm/yr. Late Quaternary sediment budget is updated based on the area and thickness of deltaic sediments. Unlike other large river systems in Asia, approximately 80% of Mekong delivered sediments has been trapped within the MRD participating its fast progradation in the last 3000 yrs. Further explanation of this unique sedimentation process is expected with the combination of hydrodynamic data, which is still very limited, in situ continuous observation, and so on.
OS23A-1258
A Study on Marine Geology and Sedimentary Environment in the Coastal Zone around East Sea Research Institute in the South Korea
Bathymetry and sedimentary environment were investigated in the western margin coastal zone of the East Sea, the surrounding sea of East Sea Research Institute (Korea Ocean Research and Development Institute), Uljin, Korea. Precise bathymetry and seabed images were obtained using multi-beam and thicknesses of sedimentary layer were found through seismic survey. Submarine topography deepens parallel to the coastline to -60 meters and rock mass distributed in the southeast of study area. Change of bathymetry was accumulated with multi-beam data and a topographical map on land and sea was framed by unified processing of submarine and land (LIDAR data) topography data. A seabed classification map was made through comparison of analysis on seabed image data and sediment sampling results. By finding the thickness of sedimentary layer through seismic survey, a sedimentary thickness map on the investigated sea area was established. Futhermore, monitoring data of bathymetry, substructure and sedimentary environment will be secured through successive geological investigation.
OS23A-1259
Evaluation of Acoustic Doppler Velocimeters (ADVs) and Pulse Coherent Acoustic Doppler Profiler (PCADP) in Estimating Suspended Sediment Concentration
Laboratory experiments were conducted to estimate the suspended sediment concentration (SSC) or its profile using acoustic backscatter strengths. Three acoustic Doppler velocimeters (ADVs) with different frequencies (5, 10 and 16 MHz) and a pulse coherent acoustic Doppler profiler (PCADP) with 1.5 MHz were used with selected sediments: two different commercial clays and Clay Bank sediment in the York River. Each ADV showed different backscatter responses depending on the sediment type and SSC. Not all devices had a good linear relationship between backscatter strength and SSC. Within a limited range of SSC, however, the backscatter strength can be well correlated with the SSC. Compared with optical backscatter sensor (OBS), ADV's backscatter signals were too noisy to be directly converted to the fluctuation of SSC, due to high amplification ratio and small sampling volume. There is another unexpected response for the backscatter strength: high signals from small particles but low signals from large particles. This might be caused by the internal gain setting built in ADVs, which should be further clarified. For profiling the SSC, a PCADP was tested in a controlled water tank. The sound attenuation by sediments was included in the signal inversion algorithm since most target regions measured by PCADP are located at near-bed high concentration layer. Clay Bank sediment showed a higher correlation coefficient (r2=0.92) between range- corrected volume scattering and PCADP signal when SSC<10 g/L. On the other hand, kaolinite clay exhibited a much smaller range of SSC for linear correlation. This different response might be attributed to the fact that, other than SSC, the acoustic response is controlled by the particle size in suspension at a given frequency. The profiling test for Clay Bank sediments showed that the PCADP-derived SSC profile has a good agreement with sample- and OBS-derived outcomes. This study suggests that both ADV and PCADP are useful instruments to estimate the SSC if the aforementioned questions are clarified and the particle (floc) size is sufficiently large enough to be sensed.
OS23A-1260
Mud deposition on the meso-tidal beach due to Typhoon Durian on 4-5th December 2006 in BaDong beach, Vietnam
BaDnog beach is located in the southeastern margin of the Mekong River Delta, Tra Vinh province, southern
VietnamCIt is a typical meso-tidal beach system. Since November 2005 we have repeatedly studied surface
sedimentation and morphological change controlled by Asian monsoon effects. On March 2007, we
described strange mud layers and mud clasts around the high-tide shoreline on Lines A and BT. The
diameters of the mud clasts are 1-30 cm and the roundness varies from angular block shape to ellipse
spherical shape. Furthermore, we dug among sand bars near the coast and high-tide shoreline. Then we
found some mud layers under the beach and sand bars. According to comparison with field data of March
2007 and November 2006, we summarized five important characteristics: (1) Mud layers were found only in
the south area such as Lines A and BT because this area had a depositional tendency during March 2007
and November 2006. (2) Mud layers were distributed between the surfaces of March 2007 and November
2006, and mostly spread over the surface of November 2006. (3) The thickness of the mud layers is 1-30 cm.
The maximum number of mud layers is three. These are associated with thin sand layers, which consist of
very fine sand, derived from beach and associated traction structures such as plane bed and wave ripple. (4)
The mud includes much organic materials and its color suggests a subtidal mud origin. The southern
Vietnam region has rarely been damaged by strong typhoon since 1997. But, Typhoon Durian hit Vietnam's
southern regions on 4-5th December, 2006. We guess the mud layers were brought from the subtidal area to
the high-tide shoreline due to the strong waves of this typhoon. Also we guess the mud clasts were washed
out from buried mud layers under the beach and sand bars due to the strong waves during the winter
monsoon after the typhoon.
http://staff.aist.go.jp/nanayama-f/
OS23A-1261
Relating Forage-Fish use With Spatial and Temporal Variability of Particle-Size Distributions of Mixed Grain-Size Beaches in the Elwha and Dungeness Drift Cells , Central Strait of Juan de Fuca, Washington State.
The removal of two dams on the Elwha River of Washington State is expected to release 1.38 x107 m3 of gravel, sand and silt into the Elwha River and nearshore marine environment of the Strait of Juan de Fuca (Randle et al., 1996). Waves and tides are expected to distribute these sediments widely upon beaches within the Elwha littoral cell with implications for many biotic organisms including forage fish such as surf smelt (Hypomesus pretiosus) and sand lance (Ammodytes hexapterus)(Shaffer et al., 2008). Forage fish spawning surveys (Moulton and Penttila, 2000), consisting of beach foreshore surface photographs and bulk sediment samples allow correlation between sediment particle-size distributions and forage-fish egg frequencies. Seasonal and spatial variability of mean grain-sizes of mixed sand-gravel beaches is characterized with traditional sieve (Church et al. 1987) and photographic methods (Adams, 1979). Sample sites are categorized by geomorphic setting (e.g. embayments, bluffs, and spits) and sampling frequency reflects expected annual seasonal variation in sediment transport processes . Mean sediment grain-sizes measured from photographs are correlated with mean grain-sizes from sieved bulk sediment samples. Comparison of mean grain-sizes from surface and 10-centimeter depth photographs where the surface lag has been removed, suggest an improved relationship between bulk sieve sample data and data obtained from photographic analysis. This finding demonstrates that photographic analysis of grain-sizes on mixed grain-size beaches from surface samples only, may skew mean grain-size due to the presence of a surface lag layer. Observed particle-size distributions are strongly polymodal. Geomorphic setting of beaches is an important factor in the observed range of mean grainsizes based on proximity to sediment sources such as feeder bluffs. These results will be used to compare forage fish habitat form and function response to future sediment inputs from dam removals.