HR: 0800h
AN: H51I-0894    [Abstracts]
TI: Spatial variation of bed material grain size over a large dune in the Jade Bay tidal channel, German Wadden Sea (preliminary results)
AU: * Thompson, J A
EM: jessicathompson@umail.ucsb.edu
AF: Department of Earth Science, University of California Santa Barbara, Webb Hall BLDG 526, Santa Barbara, CA 93106, United States
AU: Ernstsen, V B
EM: ernstsen@uni-bremen.de
AF: Marum - Center for Marine Environmental Sciences, University of Bremen, Leobener Strasse, Bremen, 28359, Germany
AB: High-resolution bathymetry of a large dune with a height of ~4 m, a length of ~70 m and a cross- channel width of ~400 m was measured in the Jade Bay tidal channel, German Wadden Sea, using a multibeam echosounder (MBES) system. A total of 34 bed material samples were collected along 5 transects (trough-crest-trough) across the dune using a Shipek^TM grab sampler. High-resolution flow velocity measurements, by means of an acoustic Doppler current profiler (ADCP), were conducted along 3 of these transects during a tidal cycle. Along the channel, mean grain sizes increase from trough to crest along all 5 transects. This increase is most pronounced in the center of the channel (trough ~340 μm, crest ~610 μm) compared to near the channel sides (trough ~425 μm, crest ~500 μm). Across the channel, mean grain sizes decrease along the crest (center ~610 μm, sides ~500 μm), but increase along the trough (center ~340 μm, sides ~425 μm) from the channel center toward the channel sides. The along-channel increase in mean grain size from trough to crest is coherent with an increase in the maximum near-bed flow velocity (calculated as the average flow velocity in the interval from 1 to 2 m above the bed). Moreover, this increase in near-bed flow velocity from trough to crest is more pronounced in the center of the channel (trough ~0.7 m/s, crest ~1.1 m/s) than near the channel sides (trough ~0.9 m/s, crest ~1.1 m/s), which is in line with the larger gradients in mean grain size in the center than along the sides. The higher flow velocities recorded at the crest relative to the trough is due to flow continuity. The across-track increase in mean grain size in the trough from the channel center toward the channel sides is consistent with an increase in the maximum near-bed flow velocity (center ~0.7 m/s, sides ~0.9 m/s). The decrease in mean grain size on the crest from the channel center toward the channel sides cannot simply be explained by lower near-bed flow velocities near the sides of the channel. Alternatively, this small decrease in mean grain size may be explained by secondary currents oblique to the main current direction generated either by the strong ebb-flood gyre in the channel as an effect of the Coriolis Force or by topographically steering of the large dunes. More detailed measurements of the near-bed hydrodynamics are required to fully understand the complexity of this dune field.
DE: 1862 Sediment transport (4558)
SC: Hydrology [H]
MN: 2007 Fall Meeting