Time-averaged near-bed suspended sediment concentrations under waves and currents: Comparison of measured and model estimates

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Abstract

Profiles of suspended sediment concentration and velocity were measured over a 15-day period at a near-shore site off Santa Cruz, CA in Monterey Bay. The concentration and velocity data were collected with an Acoustic Backscattering System (ABS) and Acoustic Current Profiler (ACP) that were mounted on a bottom tripod. High-resolution bottom scanning sonar was also attached to the tripod to provide images of bed features during the experiment. Hourly time-averaged near-bed concentrations of suspended sediment were calculated from three models and compared with the measurements. Surface waves and currents that were generated by a storm of moderate intensity caused bed stresses that exceeded threshold stress for D50=0.02 cm, the median size of the moderately well-sorted bottom sediment, over a period of about 7 days. Estimates of the concentration at 1 cm above the bottom, Ca1, were obtained using the ABS measurements. These observations have been compared with predictions for the concentration at 1 cm above the bottom, C1. Nielsen's models for reference concentration Co [Nielsen, P., 1986. Suspended sediment concentrations under waves. Coastal Engineering 10, 32-31; Nielsen, P., 1992. Coastal Bottom Boundary Layers and Sediment Transport, Advanced Series on Ocean Engineering. World Scientific, Hackensack, NJ.] are purely wave-based and do not include effects of bottom currents on bed stress and bedform scales. C1 calculated from this model compared well with measured Ca1 when currents were weak and small oscillatory ripples were observed in the sonar images. However, during the 3-day period of highest bottom stresses modeled C1 did not compare well to Ca1. The other two models for C1, Glenn and Grant [Glenn, S.M., Grant, W.D., 1987. A suspended sediment stratification correction for combined wave and current flows. Journal of Geophysical Research 92(C8), 8244-8264.] and van Rijn and Walstra [Van Rijn, L.C., Walstra, D.J.R., 2004. Description of TRANSPOR2004 and implementation in Delft3D-ONLINE. Interim Report prepared for DG Rijkswaterstaat, Rijksinstituut voor Kust en Zee. Delft Hydraulics Institute, The Netherlands.], accounted for combined wave-current stresses and included different formulations for predicting bedform scales. C1 calculated from these models compared well with measurements throughout the early and middle period of the storm, but underpredicted measured values during the latter part of the experiment. An alternative method based on dimensional analysis provided a straightforward way to relate C1 to bed stress and sediment parameters. This new relationship based on dimensional analysis explained about 77% of the variability in Ca1 derived from the ABS data.

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Publication type Article
Publication Subtype Journal Article
Title Time-averaged near-bed suspended sediment concentrations under waves and currents: Comparison of measured and model estimates
DOI 10.1016/j.csr.2007.10.006
Volume 28
Issue 3
Year Published 2008
Language English
Publisher Elsevier
Description 15 p.
First page 470
Last page 484
Country United States
State California
City Santa Cruz
Other Geospatial Monterey Bay
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