OS35E-01
Seasonal Changes in Sand Level and Wave Energy on Southern California Beaches
During the last four years, nine airborne LIDAR flights mapped changes in sand level between the low tide waterline and the backbeach for an 80~km stretch of coastline between Point La Jolla and Dana Point. The spatially dense (multiple data points per m$^2$) but temporally infrequent LIDAR surveys (collected by R. Gutierrez, U Texas) are supplemented with more frequent surveys at two focus sites within the overflown region. Monthly surveys, spanning about 7~km at Torrey Pines Beach and 3~km at San Onofre Beach, are obtained with GPS-equipped all terrain vehicles and personal watercraft. Twice yearly, surveys are extended to about 8~m depth. EOF analysis was used to estimate the alongcoast variation of seasonal fluctuations in sand volume (above mean sea level). At most locations, erosion occurs during winter and accretion during summer. However, seasonal sand level fluctuations are about three times larger at Torrey Pines than at San Onofre. Wave conditions in Southern California are alongshore variable due to the changing coastline orientation and shadowing by the offshore Channel Islands. Observations with directional wave buoys are combined with a numerical model to estimate the wave field with high alongshore and temporal resolution, over the alongshore span and time period of the overflights. At all locations, waves are most energetic in winter when northly swells are dominant. However, owing to alongshore variable exposure to both southern (dominant in summer) and northern (dominant in winter) swells, the EOF-estimated seasonal change in wave energy at Torrey Pines is about twice as large as at San Onofre. Furthermore, within the 7~km long Torrey Pines region, there is a good correlation between small-scale (0(1km)) variations in the magnitude of seasonal wave and sand volume changes. In these cases, the magnitudes of seasonal fluctuations in wave energy and sand volume appear correlated. However, some other locations do not follow this pattern, and sand level fluctuations are often more alongshore variable than wave energy fluctuations. Ongoing work explores the importance of additional factors (e.g. wave obliquity, sediment size, and beach width) on seasonal sand level changes.
OS35E-02
Assessing Spring Nearshore Currents and Sediment Transport in the Saco River Estuary, Saco Bay, ME, USA.
The Saco River is a major source of sediment for the Saco Bay beach system, providing an estimated 10,000-16,000m$^{3}$ of sand per year, primarily during the spring freshet. The 1867 construction of a jetty on the north side of the inlet has altered the Saco River's ability to deliver sediment to the bay and changed the rate and patterns of sand movement throughout the bay. The jetty system now channels the Saco River and extends the estuary 2032 m out to sea and the river mouth must be dredged every 10 years. Beaches adjacent to the northern jetty have eroded severely in the past century, with neighboring Camp Ellis suffering the loss of 33 properties since 1968. Previous estimates of river sediment output were based on limited measurements of upstream sediment discharge and the shoaling rate of the Camp Ellis anchorage at the river mouth. To better constrain the rate of sand introduction to the Saco Bay from the Saco River in April 2005 a series of instruments were deployed on three moorings inside and outside the Saco River jetty system. Instruments deployed included an acoustic Doppler current profiler (ADCP), a conductivity temperature sensor, an optical back scatter sensor (OBS), two single-beam acoustic current meters equipped with two pressure sensors. Temperature/salinity data taken near the bed indicate the extension of the estuary's salt wedge at least 1500 m seaward of the actual river mouth on the ebb tide when river discharge exceeds 280 m$^{3}$s$^{-1}$. Current meter data show a strong shear in the estuary during the latter flood and early ebb tides. During spring tides and high-discharge events, flow reversals are evident in the near surface. During neap tides under mean-discharge conditions, tidal flow reversals are less apparent throughout the water column. Both tidal and residual currents will be discussed in terms of potential sediment transport.
OS35E-03
Modeling the Large-Scale Morphodynamics of Barrier Island Coasts Under Conditions of Rising Sea Level
As sea level continues rising to an estimated 48 cm above current MSL by 2100 (IPCC 2001), hurricanes of greater intensity will subject the coast to larger waves and higher storm surges making low-lying barrier islands increasingly vulnerable. A barrier island will ultimately respond to rising sea level in one of two ways - either it will transgress across underlying strata or it will become submerged. Recent studies suggest that some East Coast barriers will break up and become submerged within decades. Other studies show that barriers in Louisiana have already submerged while others are in the process of narrowing in place and submerging. Several factors determine whether a barrier island will respond to sea-level rise by transgression or by submergence. These factors include the initial topography and morphology of the barrier, the geologic framework underlying the barrier, the availability and supply of sediment, the rate of sea-level rise, the frequency and intensity of coastal storms, as well as anthropogenic modifications to the coast. The relative importance of these factors in determining the large-scale morphodynamic response of barrier islands in the future is not yet well understood. A morphological-behavior model, GEOMBEST, which simulates the evolution of coastal morphology and stratigraphy resulting from changes in sea level and sediment supply over a variety of time scales, provides insight into how barriers might respond to rising sea level in the future. The model is based upon behavior rules originating from "Bruun rule" concepts, with additional parameters to allow simulation of more complex real-world scenarios. GEOMBEST as described in Stolper et al. (2005) differs from some other coastal evolution models by allowing specification of substrate characteristics in the coastal tract. Additional parameters allow open sediment budgets in the shoreface and backbarrier. Morphological evolution is driven by disequilibrium between the shoreface and a user-specified theoretical equilibrium profile that maintains its vertical position relative to sea level. Prior to running forward simulations, GEOMBEST is used to hindcast the evolution of the modern morphology and stratigraphy for locations along the Outer Banks of North Carolina. Forward simulations in North Carolina predict future rates of shoreline translation for a range of sea-level rise scenarios and suggest that at a minimum, the rate of landward migration of barriers will increase in response to rising sea level. Additional simulations provide insight into the relative importance of various factors such as sediment supply, the rate of sea-level rise and the underlying geologic framework, in determining the large-scale morphodynamic evolution of the barriers. Following successful application of the model in North Carolina, the model will be applied to other barrier coasts such as the Isles Dernieres or Chandeleur Islands on the Louisiana Coast.
OS35E-04
Testing SWAN under Hurricane Conditions in the Gulf of Mexico: Hurricanes Katrina and Rita (2005)
There have been 21 named storms in the Atlantic Basin this year even it is still 1.5 months before the end of the 2005 Hurricane season. Among them, Hurricane Katrina is one of the deadliest and costliest natural disasters in U.S. history. The 30 ft storm surge on the Mississippi Gulf coast generated by Katrina's 140 mph wind is the highest storm surge ever recorded in the United States. The high sea generated by Hurricanes Katrina and Rita damaged offshore oil rigs in the Gulf of Mexico. The combination of high surge and waves flooded and destroyed coastal bridges, roadways, buildings, houses, etc on the north Gulf Coast. Because of the lack of nearshore wave measurements under hurricane conditions, the SWAN (Simulation of WAves in Nearshore areas) wave model has often been used to provide storm wave information in coastal areas by coupling with storm surge models. The surge and wave information is needed for the assessment of hurricane's damage and rebuilding the devastated coasts. Evaluations of SWAN have been limited to nearshore wave simulations with less severe wind storms. The purpose of this study is to test SWAN under the hurricane conditions of Katrina and Rita in the Gulf of Mexico using a fairly large number of offshore buoy measurements. We seek answers to the question on whether SWAN is able to serve as a basin-scale wave model as well as a nearshore wave model under hurricane conditions. The SWAN model in curvilinear coordinates is employed. High grid resolution is placed along the hurricane path and in the coastal region of interest. The spatial varying grid spacing allows for resolving the structure of the wind field near the eye of the hurricane while keeping the computational time under control. The wind fields that drive the wave model are taken from different sources and parametric hurricane wind models, including the Hurricane Research Division's surface wind analysis data. We examine the sensitivity of the wave model to the choice of the wind fields. The modeled directional spectra are compared with the field observations from 10 buoys for Katrina and 8 buoys for Rita. Detailed results will be presented at the conference.
http://www.southalabama.edu/usacterec/research.html
OS35E-05
Nearshore wave climate variability during Hurricane Ivan
Hurricane Ivan entered the Gulf of Mexico through the Yucatan Strait and advanced toward the northern Gulf coast for 52 hours with maximum sustained winds near 60 m/s until the eye made landfall at 0700 UTC on September 16, 2004 near Gulf Shores, AL. The duration and intensity of the storm caused large waves and surge resulting in extreme beach overwash and multiple island breaches over a 200 km length of coastline. Observations of significant spatial variability in beach response to the storm may have been forced by a longshore-variable wave climate. To investigate this the Simulating WAves Nearshore (SWAN) spectral wave model has been used to hindcast the nearshore wave climate of Hurricane Ivan. NOAA /AOML/Hurricane Research Division surface wind analysis grids and a parametric hurricane wind profile model as input for SWAN. Modeled two-dimensional wave spectra were compared to spectra measured at seven deep-water NOAA/NDBC operational buoys in the eastern Gulf of Mexico in order to verify the model results. While maximum wave heights occurred in close proximity to the maximum wind forcing in deep water, model results suggest that local wind forcing may not have been the controlling parameter in wave heights as the hurricane moved over the shelf. The relationships between wind forcing gradients, shelf bathymetry, and wave parameters in the immediate nearshore region were investigated to determine how the wave climate varied with distance from the storm.
OS35E-06
Large-Scale Coastal Change Forced by Hurricane Ivan
Category-3 Hurricane Ivan came ashore between the mouth of Mobile Bay, Alabama and Pensacola, Florida with 195-km/hour sustained winds on September 16, 2004. Hurricane-force winds extended out from the eye 168 km, forcing extensive changes to the coast. In a cooperative effort between the U.S. Geological Survey, NASA, and Corps of Engineers, the hurricane's impact zone was surveyed with airborne lidar before and after landfall. The surveys were conducted using NASA's EAARL (Experimental Advanced Airborne Research Lidar) and Corps of Engineers' CHARTS (Compact Hydrographic Airborne Rapid Total Survey), and included all of the Gulf-front sandy beaches of Alabama and the Florida Panhandle. The before and after surveys were compared and show extensive beach and dune changes as well as the opening of new inlets through barrier islands. The kinds of impacts varied along the coast dependent on the coast's elevation. For example, the coast is low, generally less than 2 m NAVD, along the barrier islands near Gulf Shores, AL close to where the right eyewall, and the hurricane's strongest winds, made landfall. Here, the barrier islands were completely inundated by storm surge. The sea-level gradient between Gulf and back bays drove a strong landward current that transported sand across the island and into the back bays and opened a new inlet. In contrast, ten kilometers to the east in Orange Beach, AL, the topography was higher and the response of the system was dune erosion of over 20 m. In places, the vertical scour approached 3 m and undermined structures including several five-story condominium towers that had been built on top of dunes. These are the largest buildings to ever be completely destroyed in the United States by a hurricane. Using wave simulations (presented in a companion paper by Thompson et al.), model estimates of storm surge, and airborne lidar surveys of coastal elevations, we will test whether the areas that experienced overwash, inundation, and dune erosion can be differentiated with a simple model based on the elevation of extreme wave runup relative to dune elevation.
OS35E-07
Sediment Transport Across the Continental Shelf in Onslow Bay, North Carolina During Hurricane Ophelia
Hurricane Ophelia made landfall along the southeastern North Carolina coast on September 14th, 2005. Prolonged effects were felt along the southeastern US coast as the hurricane lingered offshore for approximately one week before eventually making landfall. An extensive instrumentation array maintained by the Coastal Ocean Research and Monitoring Program was present on the shelf as the storm approached the area and passed over Onslow Bay, NC. Acoustic Doppler Current Profilers, meteorological stations, and conductivity-temperature recorders were located across the continental shelf from the nearshore to the mid-shelf. Simultaneous measurements of wind velocities, current velocities, and wave properties were obtained as the storm approached the area and passed directly over the instrumentation array. Sustained winds on the inner-shelf reached 30 m/s while bottom current velocities ranged from 80 to 90 cm/s towards the southeast at the inner- and mid-shelf sites. Wave heights reached approximately 6 meters at the inner- and mid-shelf sites and up to 3 meters in the nearshore. Calculated shear velocities based on bottom wave and current velocities indicate that sediment transport was occurring at all depths across the shelf where instrumentation was located. Temporal and spatial variations in sediment transport magnitude and direction were determined at each location as the storm traversed the shelf.
http://www.cormp.org
OS35E-08
The Influence of Storms on Circulation and Sediment Transport in Long Bay, South Carolina, U.S.A.
Long Bay is a sediment-starved, arcuate embayment located along the eastern coast of South Carolina. The rates and pathways of sediment transport are important to coastal communities in this region because they determine the availability of sediments for beach renourishment. Sediment dynamics in this region are strongly influenced by winds generated from local storms that create varying circulation patterns and transport sediments. Instruments at eight sites offshore of Myrtle Beach, SC measured tides, surface waves, currents, salinity, temperature, suspended sediment concentrations, and bed forms from October 2003 to April 2004. These measurements, in conjunction with meteorological data, were used to identify patterns of circulation and sediment transport in relation to three predominant storm patterns for this region: 1) the passage of a cold front, 2) the passage of a warm front and 3) the passage of a tropical storm. The passage of a cold front is accompanied by a rapid change in wind direction from predominantly northeastward to southwestward. The passage of a warm front is accompanied by a change in wind direction from predominantly southwestward to northeastward. Tropical storms passing offshore are accompanied by a change in wind direction from southwestward to southeastward as the storm moves from south to north. Wind-generated waves and currents are mostly aligned with wind direction during each storm. Sediments are mobilized by bottom wave orbital motions and remain in suspension. During the passage of fronts, the wind speed decreases as the wind direction rotates causing a temporary decrease in wave height, and reducing the amount of sediment in suspension. The rotation in wind direction causes a change in wave and current direction. During the passage of tropical storms strong winds, waves, and currents to the south are sustained. Sediment mobilized during the tropical storm remains in suspension after its passage. Interaction of all three types of storms creates complex pathways of sediment transport. We compare the circulation and sediment transport patterns caused by each type of storm.
OS35E-09
Bottom Scour Under Hurricane Ivan in the Gulf of Mexico
Bottom scour typically occurs under storms when wave action resuspends sediment and background currents transport the resuspended sediment away. Here we present observations of extensive bottom scour along the outer continental shelf during Hurricane Ivan's passage. The scour resulted in the displacement of more than 100 million cubic meters of sediment form a 35 x 15 km region directly under the storm's path. Sediment resuspension was accomplished by the extreme waves generated by Ivan and transported by the strong near-bottom wind-driven currents. The sediment was transported primarily to the southwest, suggesting sediments may have accumulated near the shelf break and on the upper continental slope. The near-bottom wave orbital velocities generated by Ivan show that major hurricanes can induce significant stress on bottom sediments. When this occurs near the shelf break where growth faults are known to exist, it suggests major hurricanes may be a possible mechanism for initiating major mass wasting (slump) events through growth fault failure that could generate a tsunami within 160 km of the Gulf Coast. Recent studies showing that hurricanes produce extremely large waves over the outer shelf, that we are in a period of increased hurricane activity, and that hurricanes are becoming more destructive (i.e., higher intensity for longer periods), suggest the potential risk for a hurricane initiated slump and associated tsunami has increased for the foreseeable future.
OS35E-10
The simulation of a severe storm event at Lake Athabasca, Canada
Ground-penetrating radar stratigraphy of the Williams river delta on the southern shore of Lake Athabasca, Canada, suggests that the delta experienced several pronounced erosion events (Smith et al, 2004). These events are characterized by the erosion of between 3x10$6$ m$^3$ and 6x10$^6$ m$^3$ of sand from the shelf of the delta eastward during a storm event. In this study, our objective is to assess the magnitude and duration of a potential storm that could result in the projected erosion. We are simulating possible storm events and are concentrating on the predominant northwesterly storm approach. We are utilizing a state-of-the-art wave generation/propagation model to assess wave growth and shoaling/refraction over the lake. Subsequently, a circulation model is utilized to assess the strength of the resulting longshore currents due to both wave forcing as well as wind-induced pressure gradients. Finally, we assess the transport potential of these currents using a simple transport formulation that assumes that the waves suspend the sediment and currents transport it. We report on required storm magnitude and duration. This project has been carried out as part of the Research Experience for Undergratuates (REU) program at the Oregon State University on "Interdisciplinary Approaches to Coastal Processes and Hazards Mitigation". Andrew Collier by was supported as an REU student by the National Science Foundataion (EEC-0244205).
OS35E-11
Predictive Capabilities of a Simple Model for Estimating the Longshore-Variable Coastal Response to Hurricanes
A simple model that defines four storm impact regimes (Sallenger, 2000) is used to hindcast the potential coastal response along a 30-km stretch of the North Carolina coast to the landfall of Hurricane Bonnie in late August 1998. Storm-induced water levels were calculated as the sum of modeled storm surge, astronomical tide and wave runup, estimated from offshore wave conditions and local beach slope using an empirical parameterization. Water levels were compared to lidar-derived measures of dune and berm elevation measured one year prior to hurricane landfall to predict the spatially-varying coastal response. Locations where water levels reached the base of the dune were expected to experience dune erosion. Areas where water levels exceeded the elevation of the dune or berm crest were likely to overwash. Predictions were compared to the observed response quantified using a lidar topographic survey collected following hurricane landfall. The overall accuracy of the model in predicting one of three impact regimes was 52%. Accuracy within the overwash regime increased to 76%. The scaling model not only allows for prediction of the general coastal response to storms but also provides a framework for examining the longshore-variable magnitudes of observed coastal change. Beach volume change within locations that experienced overwash or dune erosion was two times greater than locations where wave runup was confined to the active beach face. Comparisons of pre-storm surveys to a calm weather survey collected two years after the storm's landfall show continued beach volume loss at overwash locations. Here, the volume of sand eroded from the beach during Bonnie was balanced by the volume of overwash deposits, indicating that the majority of the sand removed from the beach was transported landward across the island rather than being transported directly offshore. In overwash locations, sand was removed from the nearshore system and is unavailable for later beach recovery, resulting in a more permanent response than observed within the other regimes. Results from a similar study of the impacts of Hurricane Floyd on the North Carolina coast further support the predictive capabilities of the storm scaling model and illustrate that the impact regimes provide a framework for scaling the processes and magnitudes of hurricane-induced coastal change.
OS35E-12
Nowcasting of Coastal Evolution Through Assimilation of Remote Observations and Morphological Model
Information on the actual state of the nearshore zone is crucial in many coastal management and naval applications. Obtaining sufficiently detailed information from in situ measurements or model predictions alone is not feasible. Sophisticated use of high-resolution video observations in combination with a 2DH morphological model opens the door towards the nowcasting of nearshore hydrodynamics and bathymetric evolution. Our approach involves the application of a morphological area model to compute nearshore wave propagation and flow circulation patterns. Key-element of the model is the updating of bathymetry on the basis of high-resolution Argus video observations of the surf zone. This is achieved through assimilation of video-observed and model-predicted patterns of wave dissipation. An artificial morphological process is added to the processes already in place. This additional process extracts or adds sediment to the sea bed based on the mismatch between remotely sensed estimates of wave dissipation and its modeled equivalent. Given the high resolution of video observations in time, this approach allows for virtually continuous monitoring of the evolution of surf zone bathymetry. Application at Duck (NC, USA) has demonstrated the model's potential to map surf zone bathymetry. After calibration of the breaker parameter gamma on the basis of wave height, the model was run for the period 2-25 October 1997, starting from a 1994 measured bathymetry. Results show that model performance is best around the bar crests. At deeper water, model deviations increase owing lack of wave dissipation in that area. The rms error over the entire model domain decreases from 0.5 to 0.35 m throughout the simulation period. Present work aims at the application of the model at Egmond (NL) and the inclusion of additional data sources for assimilation purposes. The latter particularly involves bathymetrical estimates derived from remote observations (radar and/or video) of wave celerity. Celerity-based techniques are known to show best performance outside the region of actual wave breaking, which makes them complementary to dissipation-based estimates of bathymetry. Activities include the collection and analysis of 2Hz timestack arrays at Egmond, in combination with regular time exposure images. Confidence intervals are determined for each bathymetry estimate, to enable a weighed assimilation of the different data sources and the model. This work is funded by the Office of Naval Research ONR (BeachWizard project) and co-sponsored by the Dutch Ministry of Public Works (Rijkswaterstaat).
OS35E-13
High-Precision Time-series Measurements of pH in Kaneohe Bay, Oahu
Spatial and temporal variability in pH was examined in Kaneohe Bay from May 2004 through December 2005. These parameters were examined over a series of transects through the main ship channel and out into the open ocean. In addition to routine CTD profiles, we obtained continuous profiles of pH using a profiling electrode (SBE-18 pH). The electrode yielded very precise pH data within each of the time-series cruises. However, electrode drift between cruises was very large. In order to maintain accurate pH measurements over the 18-month sampling period, we calibrated the electrode response on each of the cruises with high-precision spectrophotometric pH measurements on discrete water samples collected along the transect. Calibration samples for pH were either analyzed immediately after sample collection or were preserved with Mercuric Chloride for later analysis. Preserved samples yielded pH values that were consistent with those run immediately following sample collection, indicating that calibration samples can be preserved. Our pH time-series data set shows considerable temporal and spatial variability in the Kaneohe Bay. Temporal variability is stochastic and is probably related to variable input of terrestrial runoff. However, biological and physical conditions in the bay appear to combine to yield a pattern of spatial variability that is fairly consistent throughout the year.
OS35E-14
Freshwater Discharge and Tidal Range Impacts on Sediment Transport in the Passaic R., NJ, USA
Contaminated sediments in the Passaic River and Newark Bay are the result of long periods of industrialization and manufacturing. While many aspects of the hydrodynamics in this system have been characterized by state funded projects (NJDEP, NJDOT) our knowledge of the sediment transport in the system has been lacking but is critical to remediation efforts. Three month long deployments of an array of 5 moorings that included ADCP, CTD, pressure and OBS sensors reveal the sediment dynamics in the river across a range of tidal and freshwater discharge forcings. The direction of net sediment transport in the river is governed by river discharge and its magnitude is mediated by tidal range. Net sediment flux is out of the estuary during high discharge and into the estuary during low discharge. Spring tides amplify the net transport in both directions. During periods of high freshwater discharge secondary currents create a region of strong shear high in the water column. The evolution of this rotating cell during ebb and its deepening is a persitant feature of the cross channel velocity. The rotating cell strengthens and reaches the bottom slightly after peak along channel ebb currents. This is accompanied by the maximum near bottom stresses throughout the tidal cycle, maximum sediment resuspension, and net flux down the estuary. During lower discharge periods the sediment transport dynamics more closely resemble that of a typical partially mixed estuary. Net sediment flux is into the estuary and a well developed turbidity maximum migrates up and down the length of the estuary.
OS35E-15
The Hydrodynamics of Zihuatanejo Bay, Mexico
Zihuatanejo is a small bay sited at the Pacific Coast of South Mexico. It has 4 kilometers long in the main axes, and an average depth of 15 meters. Wind and tide are the main forcing of the circulation. A three dimensional model is used to simulate the hydrodynamics in Zihuatanejo Bay and the results are compared with measurements taken during the winter.
OS35E-16
Seasonal Characterization of Currents in Northern California's Humboldt Bay
Currents at the entrance of Humboldt Bay are highly variable owing to the migration of shoals produced from shifting sand bars and seasonal alongshore currents. Tidal current studies of Humboldt Bay were conducted by NOAA's National Ocean Service Center for Operational Oceanographic Products and Services during the winter of 2002/2003 and during the summer of 2004 using acoustic Doppler current profiler (ADCP) arrays. Tidal flow and strength from two stations occupied in 2002 and two stations occupied in 2004 are evaluated. Maximum observed current outside the jetties near the harbor entrance was northerly during the winter and reversed during the summer, as expected, owing to the movement of seasonal alongshore currents. Currents within the Entrance Channel differed seasonally from the expected along channel flow in strength and direction.
OS35E-17
Sediment Dynamics of a Fluvially Influenced Inner-Shelf Environment, Long Bay, North Carolina, USA
Long Bay, a coastal embayment extending from Cape Fear to Cape Romain, occupies the southernmost section of the Carolina Cape complex that is part of the South Atlantic Bight. The offshore geology in this region is composed of limestone and siltstone units overlain by a thin veneer of fine to medium sands, shell hash, and organic muds. In the most northern portion of Long Bay, discharge from the Cape Fear River (CFR) influences the composition of and distribution of bottom sediments. This study seeks to describe the shelf sediments in the vicinity of the Cape Fear River plume and to identify the mechanisms that result in mobilization of these sediments. The sediment load of the CFR is low and the average annual discharge is approximately 280 m3/s. The CFR receives a considerable amount of dissolved organic material from its upstream tributaries causing discoloration near the mouth and increasing organic content of suspended and bottom sediments. Surface grabs of bottom sediments are collected bimonthly at seven sites in the vicinity of river plume. The samples are analyzed for percent sand/mud, percent organics, and mean grain size. Concurrently, water samples are collected at 3 depths using a Niskin CTD rosette to determine suspended sediment concentrations (SSC). SSC is relatively low and ranges from 8-45 mg/L, but peaks over 100 mg/L are periodically observed. High SSC concentrations are usually coincident with periods of high discharge and/or wave energy. Wave energy is moderated by the east-west orientation of the coastline and the presence of Frying Pan Shoals in the eastern section of the bay. The majority of waves are less than 1m with a dominant direction from the south. Wind tends to blow from two directions south-southwest in spring and summer and northeasterly in fall and winter with an average of approximately 4.8m/s. The primary forcing mechanism in the area is wind, which dictates significant wave height and wind-driven currents. Quantifying the effects of the physical parameters (discharge, wind, waves, currents, and tides) allows a better understanding of the range of physical mechanisms necessary for sediment mobilization and transport in Long Bay which have implications for sand sources for beach renourishment projects and benthic ecology.
OS35E-18
Numerical modeling of fine sediment deposition in storm event using ECOMSED
Fine sediment deposition poses great risk and damage to Chinese harbors. A single storm event of Oct. 10~13, 2003 caused more than 2.5m deposition inside the access channel of Huanghua Port, Hebei, China. Based on the information gained from field and lab measurements, a numerical model that integrates SWAN (Booij, et al. 2004) and ECOMSED (HydroQual, 2002) is utilized to study the fine sediment transport processes of Huanghua Port. Our integrate model showed satisfactory accuracy of wave, tide and sediment concentration when comparing with field data. However, modeling of sediment deposition appears to be the most difficult part of the research and continuing study is being carried out.
OS35E-19
Observations of Wave-Sediment Interaction, Louisiana, USA
Water and cohesive sediment dynamics are monitored in the first 1 meter above the bottom, as part of an ONR-funded study of the characteristics of lutocline and fluid mud formation and their effects on wave propagation in muddy coastal environments. The experiment site selected for this study is the muddy inner shelf (less than 20 m water depth) fronting Atchafalaya Bay, Louisiana. The observation system is composed of two tripod-based instrument clusters including fiber-optic spectrometer (FOS), acoustic backscatter sensor (ABS), upward-looking ADCP, pressure and optical backscatterance sensors (OBS), and downward-looking PC-ADP. The tripods have been deployed at various locations on the topset and foreset region of the muddy subaqueous delta of the Atchafalaya River, to examine cross- and/or along-shelf wave evolution and sediment state variability under a variety of hydrodynamic forcing conditions. Deployment timing and instrument data collection is focused upon examining the primary wave events that impact this section of the coast--winter cold fronts and tropical cyclones.
OS35E-20
Modeling boundary layer and gravity-driven fluid mud transport
A model is developed to predict field observed resuspension processes of fluid mud under tidal forcing and wave-supported gravity-driven transport. The model is based on a simplified two-phase modeling framework for particle-laden flow, which concurrently captures boundary-layer-driven and gravity-driven sediment transport, turbulence-sediment interactions and mud rheology. It is also computationally efficient to enable physical-based simulation of fine sediment processes over several tidal cycles. Given floc size and density, the model predicts time-dependent flow velocity and concentration profiles that are qualitatively similar to the observed tidal driven fluid mud at Amazon shelf. Driven by wave and current, the model also predicts the wave-supported turbidity fluid mud flow that are similar to field data measured at Eel river and Po river shelves. Motivated by field observations, numerical experiments further suggest that the behavior of wave-supported turbidity flow is rather complex and depends on the intensity of wave-current forcing, sediment property, slope and the supply of erodible sediment. Funding provided by ONR.
OS35E-21
Observations of Particle Dynamics During OASIS 2004 and 2005
Processes controlling the vertical distribution of particles in the ocean are not fully understood, partly because particles in aquatic systems are predominantly packaged as flocs, which have markedly different transport properties than single-grain particles. Despite the importance of flocs, understanding of the controls on their size and packaging within the water column is poor, in part due to the difficulty of sampling and characterizing the often delicate particles in situ. Less-intrusive in-water methods such as optical and acoustical backscatter, laser particle sizing, and microphotography combined with image processing techniques are able provide estimates of single grain and floc size spectra and abundance, as well as proxies for their composition. Here we present coincident measurements of particle size spectra (using laser particle sizing and microphotography), bulk optical and acoustical properties, waves, currents, and estimates of shear stress, that were measured in September 2004 and 2005, at $\sim$1.25 meters above bottom in shallow waters (12m) south of Martha's Vineyard, Massachusetts during the ONR Optics, Acoustics, and Stress In Situ (OASIS) project. During resuspension events, decoupling of particle concentration and size distribution is illustrative of processes such as particle flocculation and de-flocculation within the water column. Time series of particle size distributions, combined with in situ estimates of stress, are used to assess flocculation rates and mechanisms near the ocean bottom.
OS35E-22
Sediment Convection due to Flow Reversal Over Flat Bed.
The turbulent convection process of sediment suspension was investigated in relation to the development of turbulent structures in boundary layer flows over a flat bed. The variation of turbulent vortex structures was numerically generated using Large Eddy Simulation, and a high accuracy sediment particle trajectory model was coupled into the LES to describe the sediment motions. It was found that near-bed vortex cores were well aligned horizontally in the steady turbulent boundary layer flows. In the pulsating flow, however, the distribution of vortices varied in time according to flow phases. During maximum flow rate, the horizontal vortices retarded settling of sediment particles, maintaining them in downstream motions. These horizontal structures disappeared at the decelerating flow phase. Instead, it started to develop vertically organized vortices. The sediment particles were captured and actively dispersed by these vertical turbulent structures, showing a strong evidence of sediment convective flux at this time. This sediment convection became stongest at the time of flow reversal.
OS35E-23
Morphodynamic and Volumetric Response of the Beach and Nearshore to Storm Forcing
Geophysical observations from the nearshore of the northeastern coast of North Carolina have shown that the presence of shore-oblique sand bars and exposures of underlying strata may influence the spatial variability of shoreline response over decadal timescales. It has also been shown that decadal shoreline change and the volume of nearshore sediment are highly correlated, such that low nearshore sediment volumes correspond to regions of increased shoreline erosion. Despite these connections, little is known about the simultaneous response of shore-oblique sand bars and subsurface exposures, nearshore sediment volume, and shoreline behavior to storm forcing. Geophysical data were collected in the nearshore (defined here as the region from the shoreline to the 15-m isobath) of four 1-km2 sites in northeastern North Carolina in March 2003 following a nor'easter; in May 2003 following an extended period of fair weather; in November 2003 following Hurricane Isabel; and finally, in June 2004 after a period of fair weather. RTK-GPS was used to map the sub-aerial beach from the toe of the dune to the water line for the May 2003, November 2003, and June 2004 sampling periods. Our results contradict the current understanding of how sediment is distributed between the beach and the nearshore after storm events. First, many sites show a decrease in nearshore sediment volume after a nor'easter despite the fact that our surveys appeared to extend seaward of depth of closure. Second, sand bar configuration (morphology) appears to influence the exchange of sediment between the beach and the nearshore. The initial nearshore sediment volume of an area with a shore-parallel bar was comparable to the final volume. However, at a site with a large shore-oblique bar, the final volume was greater than the initial volume, suggesting the sediment never returned to the beach and remained in the nearshore. Finally, though it has been estimated that 1-2 million cubic meters of sediment is transported alongshore annually in the study area, the underlying, non-sandy stratum remained exposed, even after energetic events. Given that an older, outcropping stratum is not completely covered by sediment after a storm event in light of the immense amount of alongshore drift, our results raise questions about the mechanisms responsible for alongshore transport in similar wave-dominated settings. Understanding the redistribution of sediment in the context of volumetric change of the beach and nearshore may provide insight to the feedback mechanisms responsible for variable short-term shoreline responses to large-scale forcing.
OS35E-24
Modeling cross-shore sandbar behavior on the time scale of weeks
Waves, currents, and sediment transport in the nearshore depend strongly on bathymetry. When gradients in the sediment fluxes result in the modification of this bathymetry, for instance, as on/offshore sandbar migration, subsequent waves and current patterns are altered as well. This, in turn, may lead to further modifications of the depth profile. Coupled waves-currents-bathymetric evolution models, however, still struggle to reproduce natural bar behavior on time scales of a few days to weeks. Most of the existing models are intended primarily to predict the amount of beach erosion during a storm, failing to reproduce onshore bar migration during relatively quiescent wave conditions. The development and objective testing of a more realistic waves-currents-bathymetric evolution model thus remains a challenging research goal. We test the ability of a coupled bed elevation model to hindcast cross-shore sandbar behavior observed daily during a 44-day period in 1987 along the pier of the Hazaki Oceanographic Research Station, Japan. The bar crest moved 75~m offshore, which occurred primarily during two storm events with offshore root-mean-square wave heights in excess of 2~m. Offshore migration rates peaked at 20~m/day during these storms. During the intermediate low-energy conditions the bar moved onshore at rates of 0--6~m/day. The applied model can be considered as a successor to the Roelvink and Stive (1989, {\it J. Geophys. Res.}) model, differing in the flow module and the sediment transport description. The cross-shore sediment transport in the model is due to the skewness of nonbreaking waves, bound infragravity waves, undertow, and boundary layer streaming. The model is forced with an initial depth profile and time series of offshore wave height, period, direction and water level. The model's free parameters were calibrated by minimizing the squared difference between predicted and observed depth profiles using an automated global stochastic-deterministic search algorithm. The model was found to be able to accurately hindcast the observed bed evolution, both during the storms and the intermediate low-energy conditions. After the 44-day simulation period, the model skill, defined as 1 minus the ratio of the squared prediction to the persistence error, amounted to about 0.8. Consistent with earlier field observations, offshore bar migration in the model is caused primarily by undertow-induced suspended load, although at high tide onshore transport by nonbreaking skewed waves is also quite appreciable. Onshore bar migration during low-energy conditions is predicted to result mainly from bedload transport induced by boundary layer streaming. The generality of our findings is presently being tested by hindcasting sandbar behavior at Duck, N.C. and at Egmond, The Netherlands during the Duck94 and 1998 Coast3D experiments, respectively. Funded by the Netherlands Organization for Scientific Research NWO under grant VIDI-864.04.007.
OS35E-25
Video Observations of the Evolution of Nearshore Bars - an Application of Complex 2-D Empirical Orthogonal Functions
Video images of beaches provide long-term measurements of coastal morphodynamics, such as beach state change, rip current formation and rip current evolution. Techniques for extracting quantitative information on beach morphology from the images remain limited, however. In order to address this problem, existing complex empirical orthogonal function analysis techniques are extended to the 2-dimensional spatial domain and applied to investigate temporal and spatial variation in video measurements of beach morphology. The technique is applied to quantify the evolution of bars and rip currents during a storm event at a single barred beach on the East Coast of the Coromandel Peninsula, New Zealand. The analysis was successful at measuring the migration rate and direction of dominant morphological features during a storm. The change in beach state between a transverse bar and rip state and a long-shore bar and trough state was found to be associated with a distinct temporal shift in the relative size and phases of the empirical orthogonal functions.
OS35E-26
Modeling Bed Morphology Under Waves and Currents
Bedforms, such as sand ripples, play an important role in the dynamics of the coastal bottom boundary layer and thus affect circulation patterns and the dissipation of wave energy. The dynamics and behavior of bedforms, however, are not well understood, especially those that form under a progressive wave field or combined wave and current boundary layers. While laboratory and field experiments have been conducted to analyze the complex flow fields around bedforms and to apply that knowledge to predictive morphological models, they have been limited in their scope. Furthermore, the hydrodynamic forcing conditions of such experiments are not sufficiently well known to drive a physics based modeling system with skill. This has made it difficult to evaluate deficiencies in the sediment transport sub-models for bedload and suspended load, separate from deficiencies in the coupled wave, current, and two-phase flow system. It is beneficial, therefore, to investigate the importance of different transport formalizations on the dynamics of bedforms over a range of wave and current conditions and compare the results to field and laboratory data. Utilizing a three-dimensional, volume of fluid Navier-Stokes model that resolves time-dependent surface gravity waves, we simulate the dynamics of bedforms such as sand ripples under various wave and current conditions. The bed evolution is modeled by testing a variety of bedload transport formulae under the constraints of sediment continuity, where gradients in the bedload transport flux provide information about the time-dependent bed geometry. The modeling domain is resolved on a variable mesh to capture sea-bed and hydrodynamic boundary layer features at the millimeter scale and larger flow properties at the centimeter scale. Additionally, such accurate representation of the flow field near the bed resolves the turbulent structures that contribute to localized variability in the bed shear stress. Recent laboratory experiments conducted on the formation of sand ripples exhibit properties conducive to performing numerical simulations with our nonlinear, finite-difference model and also provide a unique opportunity for model-data comparison and testing of sediment transport sub-models.
OS35E-27
Testing Energetics-Based Models for Onshore Sediment Transport
While a number of models for cross-shore sediment transport exist, the energetics-based models have received the most attention. Comparisons of these models against field data demonstrate that the models typically predict offshore transport better than onshore transport. This shortcoming has led to the development of several different hypotheses for onshore sediment transport. One problem in the validation of these hypotheses is the lack of available datasets involving onshore bar migration, which is typically a lengthy process. A recently collected laboratory data set has substantially augmented the onshore migration data available to researchers for validating sediment transport models. Three bar migration datasets (one offshore and two onshore) were collected during the CROSSTEX experiment in the Large Wave Flume at Oregon State University. Waves were generated for 15 minute durations followed by 2-D bathymetric surveys of the beach profile. A cross-shore array of near-bottom Acoustic Doppler Velocimeters collected the near-bottom current observations necessary to drive energetics-type sediment transport models. A time series representing storm waves (large wave heights, short wave periods) generated a substantial sandbar that moved offshore. Subsequently, a time series representing milder wave conditions (small wave heights, long wave periods) caused the sandbar to migrate shoreward and decay in amplitude. A secondary, inner sandbar formed as a result of these milder wave conditions. Next, the inner bar was forced shoreward under controlled wave conditions until the beach was nearly planar, providing the second onshore migration case. Wave conditions were tuned during this event to ensure continued onshore sandbar migration. The three sandbar migration datasets will be used to drive the energetics-type model of Bailard (1981) similar to the studies of Thornton et al. (1996) and Gallagher et al. (1998). Furthermore, inclusion of the normalized form of acceleration skewness, "aspike", as suggested by Drake and Calantoni (2001) and positively demonstrated by Hoefel and Elgar (2003), will also be tested for the different migration cases. The accuracy of predicting onshore sediment transport with these models will be quantified. Finally, the contribution of various physical processes to the net onshore sediment transport will be discussed.
OS35E-28
Model Simulations of Bar Evolution on a Large-Scale Laboratory Beach
The dominant role of the undertow during erosional events in energetic surf zones is reasonably well understood, and models based on quasi-steady hydrodynamics provide successful predictions of profile erosion and offshore bar migration. Conversely, outside the surfzone or in low energy conditions, the wave signal and therefore the skewness of velocity and acceleration become relatively more important. In these accretional regimes, wave-averaged transport based on steady state models typically fail to predict observed onshore transport or bar migration when used with coefficients similar to those employed in successful offshore transport simulations. Conversely, existing simulations based on boundary layer model predictions of bed shear stress have been successful in predicting onshore bar migration, but have not often been tested in erosional conditions due to inaccuracies in model representations of undertow. This presentation will report a test of an extension of the coupled Boussinesq/boundary layer model described by Long et al, that incorporates a detailed description for wave roller flux to avoid undertow underprediction. The vertical boundary layer calculation is based on the fully nonlinear Boussinesq model predicted velocities above the bed that result from a shoaling wave train. After obtaining the instantaneous bed shear stress, sediment transport is estimated using the Meyer-Peter Muller formula. Instantaneous transport is accumulated over several wave periods, after which the equation for morphology change is integrated using an Euler-WENO scheme. The model calculations will be conducted in predictive mode under both erosional and accretional wave conditions in order to assess model skill in performing short term assessments of beach profile evolution for single storm events. The data set on cross-shore profile evolution was obtained in the large wave flume facility at Oregon State University during summer 2005 as part of the CROSSTEX experiment.
OS35E-29
Design of Large Scale Experiments on Onshore Bar Migration
The Cross-shore Sediment Transport Experiment (CROSSTEX) is a multi-year scientific research program designed to improve process-based models of nearshore sediment and wave dynamics. The experimental phase of the program was a coordinated series of near-prototype scale experiments in controlled laboratory conditions, and was conducted in the Long Wave Flume at Oregon State University's O.H. Hinsdale Wave Research Laboratory during the summer of 2005 (see Maddux et al. abstract in this session for a summary). One portion of these experiments (An Experimental Study of Onshore Bar Movement, Ozkan-Haller et al., {\em NSF}) was focused on onshore-directed sediment transport, i.e. beach recovery processes. In this poster we will describe a Research Experience for Undergraduates project funded by the {\em National Science Foundation} that involved the design of a large scale, mobile bed experiment for studying cross-shore sediment transport. The requirements of the experimental design were fairly unique in the sense that the process of interest, i.e. onshore bar migration, is inherently slow and in addition the experiments were performed at near prototype scale. The design process involved collecting a data set of previous onshore bar migration observations (field and lab) and performing a scaled comparison between these observations and the design conditions we expected for our experiments in the Large Wave Flume. In addition, existing beach profile parameterizations intended to predict the occurrence of offshore vs. onshore transport conditions, such as the Dean number, were tested against these data and used in the lab design process. The utility of equilibrium beach profile techniques for this problem was also assessed. Further results to be discussed include a summary and analysis of the profile change observations and these observations will be related to observations of sediment flux and the underlying distribution of sediments across the profile.
OS35E-30
Sea-Bed Response to Non-Breaking Waves
The sea-bed response to non-breaking waves was investigated in a large scale laboratory setting, conducted as a part of the 2005, multi-institutional collaborative CROss Shore Sediment Transport Experiment (CROSSTEX). Research was carried out at the large scale wave flume at the O.H. Hinsdale Wave Research Laboratory located at Oregon State University. The focus of this investigation is on the bed state characteristics in response to non-breaking waves. Observations of bedform geometry were measured with a submersible laser-video Particle Image Velocimetry (PIV) system and a two-axis variable frequency sonar system. Additionally, the free stream velocity was measured with an Acoustic Doppler Velocimeter. The observations were obtained at two water depths offshore of the wave breaking region. Bed states and ripple types were examined for a range of random wave conditions (20 < Hs < 60 cm; 4 < T < 8 s). Geometries ranged from flat beds to anorbital ripples, consistent with observations of {\it Clifton} [1976]. The anorbital ripples showed a sensitivity of ripple heights to wave conditions, but no variability in ripple wave length. In a manner consistent with {\it Hay and Mudge} [2005] we examined the relative importance of second-order (wave energy, u$_{rms}$) and third-order (Sk and As) wave statistics. These observations showed that wave energy, inferred by u$_{rms}$, is a critical parameter in determining bed state geometry. However, the results showed little sensitivity to the third-order wave statistics, wave skewness or asymmetry.
OS35E-31
Sediment suspension and cross-shore transport observed in a large wave flume
This investigation is part of the Cross-shore Sediment Transport Experiment (CROSSTEX) project, multi-investigator project conducted in a large wave flume to improve the fundamental understanding of nearshore processes. The objective of this investigation is to study wave transformation, sediment suspension, and bed transformation in the surf zone under strongly plunging and weakly spilling waves over short time scales (several seconds). This project focuses on the role of wave breaking turbulence in the process of sediment suspension and transport. The experiment was conducted in the large wave flume at Oregon State University's O.H. Hinsdale Wave Research Laboratory. The flume is 104 meters long, 3.7 meters wide, and 4.6 meters deep with a programmable flap-type wavemaker. A moveable seabed representative of a natural barred beach was created using 612 m3 (800 y3) of Oregon beach sand. Test were run for a range of irregular wave conditions to provide erosive and accretive conditions. Fluid velocity and sediment concentration were measured at various cross-shore and vertical locations using a dense instrument array. This array consisted of nine acoustic-Doppler velocimeters (Nortek Vectrinos) sampling at 50 Hz to measure the fluid velocity near the bed and in the upper water column. Three electro magnetic current meters were deployed to measure the fluid velocity above trough level. The sediment concentration was measured at different elevations above the bed using two new fiber optic backscatter sensors (FOBS). Eight optical backscatter sensors (OBS) were deployed in the cross-shore direction to measure sediment concentration and co-located with ADVs to provide sediment flux. Details will be presented on the turbulence generated by breaking waves and the effects it has on the process of sediment suspension and transport. Specifically, the sediment concentration will be analyzed in the cross-shore and vertical directions to determine the affects of the downward transport of turbulence from wave breaking to distinguish it from the flux of sediment due to undertow and the wave-induced flow.
OS35E-32
Wave breaking turbulence and sediment suspension observed over a moveable bed in a large-scale wave flume.
CROSSTEX (Cross-Shore Sediment Transport Experiment) project is a collaboration among seven institutions to study wave transformation, sediment suspension, and bed transformation across the nearshore zone. This experiment was conducted at the O.H. Hinsdale Wave Research Laboratory, at Oregon State University, in the large wave flume (104 m x 3.7 m x 4.6 m). To model a natural barred beach 600 cubic meters of sand, with a nominal grain size of 0.2 mm, were filled into the tank. A large-scale laboratory setup was near prototype scale and provided control, repeatability, and eliminated alongshore variable, wind, and tides. This research project studied the turbulence generated from breaking waves in the surf zone. This turbulence can be transferred from the top of the water surface to the bottom of the water column and put sediments into suspension. Previous experiments with a fixed bed have shown that this energy is able to reach close to the bottom boundary layer before completely dissipating. This experiment used a moveable bed to measure both the sediment concentration as well as the velocity to determine the correlation between wave breaking turbulence and sediment suspension. Data were collected in the large wave flume over a period of two weeks. The instantaneous fluid velocity and the concentration of sediment was measured in vertical profiles at several cross-shore locations in both onshore and offshore bar migration conditions. An Acoustic Doppler Velocimeters (ADVs), optical backscatter sensors (OBSs), fiber optical backscatter sensors (FOBS), a void fraction probe, a wave gauge, and an underwater camera were deployed. The goal of this project is to find the correlation between turbulence and sediment suspension, and use it to provide a more realistic pick-up function for sediment transportation models. Time-averaged quantities might not accurately describe this correlation because waves that cause large sediment suspension events can account for a large percentage of the total energy, even though they only occur a small percent of the time, as small scale experiments have shown. To create accurate beach transformation models, this phenomenon needs to be fully understood.