Coupled Evolution of Waves and Fluid Mud Layers
S.
Jaramillo and A. Sheremet,
Civil & Coastal Engineering, University of Florida, Gainesville,
M. A. Allison,
Earth and Environmental Sciences, Tulane University,
S. Dartez
Coastal Studies Institute, Louisiana State University,
Civil & Coastal Engineering, University of Florida, Gainesville,
M. A. Allison,
Earth and Environmental Sciences, Tulane University,
S. Dartez
Coastal Studies Institute, Louisiana State University,
Summary of a poster presented at the AGU Ocean Sciences Meeting help in Honolulu Hawaii, 2006. A pdf version of the poster can be downloaded (see Links).
Abstract
In an ongoing field experiment two instrument clusters are deployed at various locations on the muddy subaqueous delta of the Atchafalaya River, Louisiana, to conduct high resolution observations of water and sediment motion. The experiment studies fluid mud formation and its effects on wave propagation under wave conditions associated with winter cold fronts.
Background
Limited observations show that waves propagating above muddy seafloors can be strongly dissipated. Wells and Coleman (1981) recorded more that 90% wave energy dissipation across the 20-km wide shallow shelf of Surinam; Mathew et al. (1995) observed 95% energy loss as waves crossed 1.1 km-wide mudbanks off the coast of India.
Various models have been proposed to describe mud-induced wave dissipation, e.g. soft bottom damping Shemdin et al., 1980; viscous fluid-mud layers Dalrymple and Liu, 1978; formation of a high-density near-bottom mud layer Sheremet et al., 2005; surface-internal wave interaction waves Hill and Foda, 1999. However, the physical mechanisms for wave dissipation over muddy seabeds remain poorly understood.
The state of near-bottom sediment changes significantly during a storm. Waves and currents are sensitive to the state of the sea bed (Neill and Allison, 2005; Sheremet et al., 2005). In the absence of detailed coherent sediment and hydrodynamic measurements, characteristc sediment parameters are inferred based on postulated dissipation mechanisms and rheological models (e.g viscous, visco-elastic, or poro-elastic medium).
Field experiment
The goal of this project is to conduct high resolution observation of sediment and wave motion. Two instrumented platforms were designed to monitor fluid motion throughout the entire water column, with increased resolution (2 cm bins) in the first 50 cmab. Suspended sediment concentration is monitored using acoustic backscatter intensity and optical sensors. Acoustic backscatter sensors (ABS) are used to monitor sea bed motions.
UF tripod before the first deployment on January 16, 2006.
The experiment site is
the shallow (~10 m depth) inner shelf off
Atchafalaya Bay, Louisiana. There are 4 deployments planned, with two
already executed. The
instruments will be deployed in total for a period of about 3 months,
and will record continuously. About once every two weeks the pods are
retrieved, cleaned, and redeployed. During onsite service, memory cards
are also replaced with empty ones, allowing for data retrieval without
interrupting the deployment. During the first two runs, the UF tripod
was in about 5.4 m depth of water; the Tulane tripod was first in 2.5-m
depth, then moved near the 4.5-m isobath.
Deployment configurations of the UF (blue) and Tulane (red) instrumented pods.
Lighter color symbols mark future deployments.
Extensive and
high-resolution field
observations are needed. Data should provide detailed, coherent
measurements of sediment and water dynamics. This experiment focuses on
high resolution (but local) observations of hydro- and sediment
dynamics. Future work includes extending the spatial coverage of the
experiment, improved monitoring of sediment dynamics, as well as data
analysis and modeling of wave propagation in muddy environments.
References
Dalrymple, R.A., and P.L.-F Liu, JPO 8, 1121-1131, 1978.
Hill, D. F. and Foda, M. A.. JGR 104(C5), 10951-10957, 1999.
Jiang, F., and A.J. Mehta, JCR 11(3), 918-926, 1995.
Mathew J., M. Baba, and N. Kurian, JCR 11, 168-178, 1995.
Neill, C.F. and M.A. Allison, Marine Geology 214, 411-430, 2005.
Shemdin O.H., S.V. Hsiao, H.E. Carlson, K. Hasselmann, and K. Schulze, JGR 85, 5012-5018, 1980
Sheremet, A. and G. W. Stone, JGR 108, C11, 24, 2003.
Sheremet, A., A. J. Mehta, and J. M. Kaihatu, 5th Sym. Ocean Waves, Madrid, Spain, 2005.
Wells, J. T. and Kemp, G. P., Lecture Notes on Coast. Est. Studies 14, p. 44-65, 1984.