OS11F-01 INVITED
Mechanisms of wave dissipation on the Louisiana shelf: observations of short wavelength lutocline internal mode waves
Field observations were performed on the Louisiana shelf, west of Marsh Island, in the spring of 2007 and 2008 to elucidate the mechanisms of wave energy dissipation over a muddy seafloor. During the 2008 observations the Atchafalaya River discharge was approximately double the 1930-2007 average, which produced large quantities of new mud on the inner shelf. Acoustic backscatter profiler (ABS) measurements showed deposition of 20 cm in 5 to 7 m water depths after each of three wave events with wave heights of near 2 m on the 9 m isobath. While total wave energy dissipation was greatest during the high energy periods, the normalized dissipation (Dissipation/Energy Flux, with dimensions of an inverse length scale) was largest after the wave events, as the recently deposited mud layer consolidated from 20 cm to 10 cm thickness. The ABS data also showed waves on the lutocline with heights ranging from 10 cm during periods of high total wave energy dissipation to 2 cm during periods of maximum normalized dissipation. The amplitude of these waves is much larger than predicted by models for the surface mode (with the wavenumber of the interface mode matching the surface wavenumber) of a two layer, water over viscous higher density mud system. Analysis based on the continuity equation and measurements of orbital velocity decay away from the interface show that these waves have short wavelength (1.5 to 3 m) relative to the surface waves (~ 60 m), but oscillate at the same frequency. These observed waves are consistent with the internal mode solutions to the two layer dispersion equation. The dispersion equation also shows that in the high viscosity, or thin normalized mud layer (mud layer thickness/viscous boundary thickness) regime the dissipation due to the internal waves with the observed amplitudes can be significantly larger than the dissipation due to the surface mode.
OS11F-02
Sediment Classification in Tidal Flats via Heat Flux Observations
Thermal signals in tidal flat environments have strong spatial and temporal gradients as a result of variable
forcing by solar radiation during low tides and estuarine advection during high tides. Here, we present
infrared and in situ observations of surface heat fluxes in two distinct tidal flat environments on the coast of
Washington State, USA. The Skagit Flats are river-dominated and sandy, compared with the Willapa Flats
which are tide-dominated and muddy. At both sites, we show that the uptake of heat by sediments during
low-tide exposure is directly related to the water content, and thus the composition, of the surface layer.
Generally, the sandy sediments of the Skagit Flats have a lower water content and a faster uptake of heat,
compared with the muddy sediments of the Willapa Flats. In addition, small-scale variations in the heat flux at
each site are related to local morphology (eg, channels, bedforms). The uptake of heat can be quantified
via airborne infrared imagery, and this is suggested as a new method for geotechnical remote sensing.
http://www.tidalflats.org/
OS11F-03
Thermal remote sensing of tidal flat bathymetry and circulation
Tidal flats exhibit strong thermal gradients due to mixing water masses, solar heating and latent and sensible
heat flux. During a pilot experiment in August 2008, the thermal signal of the Skagit Bay tidal flats in
northwest Washington State was measured using airborne and tower-based long-wave infrared imaging. A
small network of in situ temperature loggers recorded water column and sediment temperatures. The
strong temperature difference observed between the water and the exposed flat sediments revealed the tidal
flat bathymetry as the tide flooded and ebbed. Waterlines identified in the thermal imagery and in situ
tide measurements will be used to generate large-scale (kilometer) and small-scale (meter) scale bathymetry
maps over the braided network of channels and shoals of the flats. In addition, the surface extent and
evolution of the relatively warm freshwater plume from the Skagit River is traceable in the thermal images as
it extends into the colder Puget Sound seawater. The circulation of the river plume over the changing tide
and on complex bathymetry will be explored via airborne thermal mapping and combined with in situ
thermometry of the water column. The larger goal of this work is to eventually produce gross circulation and
bathymetry maps over an annual cycle to provide a broad framework for understanding the geomorphology
and hydrodynamics in this dynamic system.
http://www.tidalflats.org
OS11F-04
Mud Rheology and Wave Dissipation on a Shallow Muddy Shelf
Wave dissipation characteristics are studied based on field measurements collected on Atchafalaya Shelf, Louisiana, USA, in Spring 2008. During energetic storms, large swells liquefy the bed, resuspend the mobilized sediment, and produce wave-supported fluid-mud layers which last for the duration of the storm. As the bed sediment is reworked, wave dissipation rate increases rapidly, reducing swell energy by an average of 30-40% over about 4 km. Surprisingly, the largest net dissipation rates (up to 60% energy decrease over 4 km) are observed in the wake of the storm, when the water column is nearly clear of sediment and no fluid- mud layers are detected. The analysis of the vertical structure of wave phase suggests an increased role of bottom sediment rheology; direct observations (bed sampling) indicate that at this stage the bed is typically in an under-consolidated state, better described by non-Newtonian (e.g., visco-elastic) approximations. We employ numerical inversion techniques based on nonlinear three-wave interaction models to study the rheological properties of the bed sediment, and understand the importance of non-Newtonial rheology for wave dissipation processes.
OS11F-05
A numerical modeling framework for fluid mud transport in estuary and continental shelf
It is important to understand the fate of terrestrial sediment in the coastal ocean, because it determines for example the seabed properties and coastal geomorphology. Very often, fine sediment delivered by the river becomes cohesive in estuary and continental shelf. When significant amount of cohesive sediments are available, they transport as fluid mud, a concentrated aggregate suspension of more than 10 g/l. Fluid mud transport essentially occurs within the thin wave boundary layer near the seabed. Therefore, typical large- scale coastal models cannot resolve fluid mud transport and require further bottom module parameterizations. A numerical modeling framework for fine sediment transport developed here is based on Equilibrium Eulerian Approximation to the multiphase flow equations and incorporates closures on turbulence-sediment interaction, mud rheology, bed erodibility and the dynamics of floc break-up and aggregation. In this talk, the implementation of these critical cohesive sediment transport mechanisms is briefly introduced and an overview of several applications of this modeling framework is presented. In the 1DV formulation of this framework, the dynamics of wave-supported gravity-driven mudflows, which has recently been identified as major offshore fine sediment transport mechanism on the continental shelf, is studied. In the 2DV formulation, wave-mud interaction and sedimentation at salinity stratified river mouth are simulated. To resolve detailed turbulence-sediment interactions and to improve the closures in the Reynolds- averaged approach, Direct Numerical Simulation (DNS) of fluid mud transport under oscillatory flow is carried out in a full 3D formulation based on pseudo-spectrum scheme. It is our goal to understand processes at different scales using this numerical modeling framework in order to provide more reliable parameterizations for large-scale coastal models. Funding of this study is supported by ONR, NSF and NOPP.
OS11F-06
An Airborne Scanning LiDAR System for Ocean and Coastal Applications
We have developed an airborne scanning LiDAR (Light Detection And Ranging) system
and demonstrated its functionality for terrestrial and oceanographic measurements.
Differential GPS (DGPS) and an Inertial Navigation System (INS)
are synchronized with the LiDAR,
providing end result vertical rms errors of approximately 6~cm.
Flying 170~m above the surface,
we achieve a point density of ~ 0.7 m-2
and a swath width of 90 to 120~m over ocean and 200~m over land.
Georeferencing algorithms
were developed in-house and earth-referenced data are
available several hours after acquisition.
Surveys from the system are compared with
ground DGPS surveys and existing airborne surveys of fixed targets.
Twelve research flights in a Piper Twin Comanche from August 2007 to July 2008
have provided topography of the
Southern California coastline
and sea surface wave fields
in the nearshore ocean environment.
Two of the flights also documented the results of the
October 2007 landslide
on Mt.~Soledad in La Jolla, California.
Eight research flights aboard a Cessna Caravan surveyed the topography, lagoon, reef,
and surrounding seas of Lady Elliot Island (LEI)
in Australia's Great Barrier Reef in April 2008.
We describe applications for the system, including
coastal topographic surveys, wave measurements, reef research, and ship wake studies.
http://airsea.ucsd.edu/
OS11F-07
Assimilation of 2D Currents in the Surf-zone: Bathymetric Sensitivity and Inversion
The accuracy of surf-zone circulation predictions depends on two factors: model validity (e.g. that of physical parameterizations), and the quality of observational data (i.e. model inputs). The latter includes bathymetric survey data, which is often collected with spatial and temporal resolution much larger than the relevant physical scales. Thus, significant bathymetric uncertainty is often present, and the corresponding error in the model prediction can be difficult to quantify and/or correct. Here, we examine the problem of bathymetric sensitivity in the context of a hindcast of PUV measurements taken at Duck, NC, during the fall of 1997. We show, using inverse methods (the Ensemble Kalman Filter), that two-dimensional bathymetric change taking place on hourly time-scales had a significant impact on hindcast accuracy for surf-zone currents. Likewise we also show, with cross-validation, that PUV measurements can be used to compute an accurate 2D bathymetric inversion, thus minimizing the observationally-derived error and improving the overall prediction of the flow field.
OS11F-08
Optical Polarization in the Nearshore
A recent addition to the suite of optical remote sensing methods that have been used to study nearshore processes is the use of imaging polarimetric cameras. Both the degree of polarization and the azimuth of polarized light contain information about the imaged surfaces from which light has been reflected or scattered. In 2007, a polarimetric Argus camera was installed atop the tower at Duck, NC. This talk will examine the various polarization signatures that can be exploited, including the potential for measuring the sea surface slope spectrum of nearshore surf zone waves, the slope of the foreshore beach, water content of foreshore sediments and bubble signatures of dissipating waves.