Sediment Transport Time Scales and Trapping Efficiency in a Tidal River

Observations and a numerical model are used to characterize sediment transport in the tidal Hudson River. A sediment budget over 11years including major discharge events indicates the tidal fresh region traps about 40% of the sediment input from the watershed. Sediment input scales with the river discharge cubed, while seaward transport in the tidal river scales linearly, so the tidal river accumulates sediment during the highest discharge events. Sediment pulses associated with discharge events dissipate moving seaward and lag the advection speed of the river by a factor of 1.5 to 3. Idealized model simulations with a range of discharge and settling velocity were used to evaluate the trapping efficiency, transport rate, and mean age of sediment input from the watershed. The seaward transport of suspended sediment scales linearly with discharge but lags the river velocity by a factor that is linear with settling velocity. The lag factor is 30-40 times the settling velocity (mms(-1)), so transport speeds vary by orders of magnitude from clay (0.01mms(-1)) to coarse silt (1mms(-1)). Deposition along the tidal river depends strongly on settling velocity, and a simple advection-reaction equation represents the loss due to settling on depositional shoals. The long-term discharge record is used to represent statistically the distribution of transport times, and time scales for settling velocities of 0.1mms(-1) and 1mms(-1) range from several months to several years for transport through the tidal river and several years to several decades through the estuary.