HR: 0800h
AN: OS21B-1212    [Abstracts]
TI: Observations of Sediment Resuspension and Advection in Southern Lake Michigan
AU: * Hawley, N
EM: nathan.hawley@noaa.gov
AF: Great Lakes Environmental Research Laboratory, 2205 Commonwealth Blvd., Ann Arbor, MI 48105 United States
AB: An instrumented tripod with three transmissometers located at 0.2 m, 0.5 m, and 0.8 m above the bottom was deployed for approximately one month during the fall of 2003. The mooring was located in 12 m of water in southeastern Lake Michigan. In addition to the transmissometers, a pressure sensor was used to record surface wave activity, and an acoustic current profiler was moored nearby to measure the current velocities. Sediment traps were mounted on the mooring at 0.9 and 1.5 meters above the bottom. Several storms during the deployment resuspended bottom material; bottom stresses due to combined wave and current action exceeded 1 Pascal during these events. Bottom sediment at the site is over \99% sand, but the sediment traps contained a large percentage of finer-grained material (\50% in the lower trap and \63% in the upper one). During the storms the uppermost transmissometer frequently recorded the highest beam attenuation coefficient, while the bottom transmissometer recorded the lowest attenuations. This result is counter-intuitive since the attenuation coefficient is linearly related to the concentration of suspended sediment and should decrease - rather than increase - as the distance from the bottom increases. These observations may be due to a decrease in particle size with increasing elevation above the bottom. The transmissometer records also show several intervals when the attenuation coefficient was high even though the bottom stresses were small. These episodes are presumably due to the advection of fine-grained material resuspended elsewhere and then transported to the site. However, the source of this fine- grained material is unknown.
DE: 4239 Limnology
DE: 4536 Hydrography
DE: 4558 Sediment transport
SC: Ocean Sciences [OS]
MN: 2004 AGU Fall Meeting