wave motions. Small scale shear, γ

wave motions. Small scale shear, γ , was calculated as γ = (ε/ν)1/2[sec−1], where
ν is the kinematic viscosity.
Time series current velocity profiles were collected for the duration of each
seasonal cruise using a 1,200 kHz Broadband Acoustic Doppler Current Profiler
(ADCP) mounted in the hull of a surface buoy in a downward looking configuration.
The ADCP buoy was deployed at 39◦ 22.098 N, 76◦ 07.523 W in approximately
11 m of water. The ADCP collected data at 5-min intervals, sampling 0.5-m bins
through the water column starting at 1.56 m below the surface and extending to
approximately 1 m above the seabed. For each sample, 120 water column pings
were averaged, which gives an estimated standard deviation of the measured velocity
of about 0.5 cm sec−1. ADCP data was post-processed using MATLAB software,
where all bad data points (including all data below 0.75 m above the seabed) were
removed. North and east velocities were translated into along and cross-channel
components, using a projected angle for the along channel direction of 230◦ T
(ebb positive).
A microcosm erosion testing system was used to test the erodibility of bottom
sediments collected using the Cape Henlopen Ocean Instruments multicorer. The
erosion system consisted of two 10 cm Gust Microcosms,58 which use a spinning
disk with central suction to generate a controllable, nearly uniform shear stress. A
Campbell Datalogger controlled the system and stored data. During erosion experiments,
a sequence of increasing levels of shear stress was applied to undisturbed
cores. The effluent from each Microcosm was passed through a turbidimeter and time
series of turbidity were measured. The effluent was collected, filtered, and weighed
to determine the actual mass eroded during each step, which was used to calibrate
the turbidimeter. Erosion rate was subsequently calculated as the product of pumping
rate and suspended sediment concentration, and the data were analyzed according to
the formulation of Sanford and Maa.59
10.4 RESULTS
Disaggregated particle sizes of suspended sediment in upper Chesapeake Bay were
very fine, consisting of either clays or fine silts (Table 10.1). The four pumped samples
listed in Table 10.1 were all collected from just above the bottom at anchor during
two of the seasonal cruises in 2002. No data are available from the October cruise
because insufficient material was collected to run a standard gravimetric analysis.
Slightly more coarse material (d75 = 16 µm) appears to have been resuspended
under strong tidal forcing, but over the weakest flood tide sampled, only clay particles
smaller than 1 µm were in suspension. There were few apparent seasonal differences
in disaggregated suspended particle size, with greater differences between sequential
flood and ebb tides than between May and July.
In contrast to these very fine disaggregated particle size distributions, much of
the material in suspension was packaged in large flocs. These particles were often
visible to the naked eye in the settling tubes and erosion microcosm, and were even
more apparent in the magnified video images from the VISTA (Figure 10.4). The
flocs visible in Figure 10.4b, collected approximately 1.5 m above bottom during