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JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 110, C06014, doi:10.1029/2003JC001814, 2005

Estimating hydrodynamic roughness in a wave-dominated environment with a high-resolution acoustic Doppler profiler

Jessica R. Lacy

U.S. Geological Survey, Santa Cruz, California, USA


Christopher R. Sherwood

U.S. Geological Survey, Woods Hole, Massachusetts, USA


Douglas J. Wilson

Imagenex Technology Corporation, Port Coquitlam, British Columbia, Canada


Thomas A. Chisholm

Department of Environmental Science and Engineering, Oregon Graduate Institute, Beaverton, Oregon, USA


Guy R. Gelfenbaum

U.S. Geological Survey, Menlo Park, California, USA


Abstract

Hydrodynamic roughness is a critical parameter for characterizing bottom drag in boundary layers, and it varies both spatially and temporally due to variation in grain size, bedforms, and saltating sediment. In this paper we investigate temporal variability in hydrodynamic roughness using velocity profiles in the bottom boundary layer measured with a high-resolution acoustic Doppler profiler (PCADP). The data were collected on the ebb-tidal delta off Grays Harbor, Washington, in a mean water depth of 9 m. Significant wave height ranged from 0.5 to 3 m. Bottom roughness has rarely been determined from hydrodynamic measurements under conditions such as these, where energetic waves and medium-to-fine sand produce small bedforms. Friction velocity due to current u *c and apparent bottom roughness z 0a were determined from the PCADP burst mean velocity profiles using the law of the wall. Bottom roughness k B was estimated by applying the Grant-Madsen model for wave-current interaction iteratively until the model u *c converged with values determined from the data. The resulting k B values ranged over 3 orders of magnitude (10−1 to 10−4 m) and varied inversely with wave orbital diameter. This range of k B influences predicted bottom shear stress considerably, suggesting that the use of time-varying bottom roughness could significantly improve the accuracy of sediment transport models. Bedform height was estimated from k B and is consistent with both ripple heights predicted by empirical models and bedforms in sonar images collected during the experiment.

Received 7 February 2003; accepted 24 March 2005; published 30 June 2005.

Keywords: bottom boundary layer; hydrodynamic roughness; sediment transport.

Index Terms: 3022 Marine Geology and Geophysics: Marine sediments: processes and transport; 4546 Oceanography: Physical: Nearshore processes; 4558 Oceanography: Physical: Sediment transport (1862); 4211 Oceanography: General: Benthic boundary layers; 4217 Oceanography: General: Coastal processes.

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Citation: Lacy, J. R., C. R. Sherwood, D. J. Wilson, T. A. Chisholm, and G. R. Gelfenbaum (2005), Estimating hydrodynamic roughness in a wave-dominated environment with a high-resolution acoustic Doppler profiler, J. Geophys. Res., 110, C06014, doi:10.1029/2003JC001814.