Sediment Transport Time Scales and Trapping Efficiency in a Tidal River

Abstract

Observations and a numerical model are used to characterize sediment transport in the tidal Hudson River. A sediment budget over 11 years 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 (mm s−1), so transport speeds vary by orders of magnitude from clay (0.01 mm s−1) to coarse silt (1 mm s−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 0.1 mm s−1 and 1 mm s−1 range from several months to several years for transport through the tidal river and several years to several decades through the estuary.

Response to Channel Deepening of the Salinity Intrusion, Estuarine Circulation, and Stratification in an Urbanized Estuary

“…Tidal amplitude has more than doubled due to dredging in the tidal freshwater region of the upper ~60 km of the Hudson, but the increases in tidal amplitude in the harbor and lower estuary have been smaller (Ralston et al, ), so the corresponding effects on sediment transport are expected to be small. The increase in salinity intrusion is likely to increase the landward extent of sediment trapping, change the location of estuarine turbidity maxima, and increase the time scales for transport through the estuary (Ralston & Geyer, ). However, to this point the Hudson has not undergone a dramatic shift in sediment concentration like that observed in several European estuaries.…”

Bigger Tides, Less Flooding: Effects of Dredging on Barotropic Dynamics in a Highly Modified Estuary

“…Tidal influence extends landward of the salinity intrusion, a region Hoitink and Jay () define as the tidal freshwater zone. Under high discharge conditions, the river input can modify the tide in the upper ~40 km of the Hudson such that the lowest water levels occur during neap rather than spring tides (Ralston & Geyer, ), which is the tidal river definition in Hoitink and Jay ().…”

“…The model grid and forcing configuration is the same for both cases, and only the bathymetry is changed. Idealized, constant discharge scenarios were run with simplified tidal forcing (M 2 , S 2 , and N 2 components) to represent spring‐neap variability for a range of discharge conditions: 150, 300, 600, 2,000, and 5,000 m 3 /s (Ralston & Geyer, ). Constant discharge simulations were run until reaching periodic equilibrium with the tidal variability, up to 6 months for the low discharge cases.…”

Effects of flushing flows on the transport of mercury-polluted particulate matter from the Flix Reservoir to the Ebro Estuary

“…The more erodible part of these deposits can leak from the reservoir along with the lighter fine suspended sediment and be transported downstream. In addition, environmental management may induce changes in physicochemical conditions, leading to the release of pollutants (Frémion et al, 2016) Many pollutants are linked to fine particulate matter (<63 µm) (Feng et al, 1998;Israelsson et al 2014) that can bypass the dams and flow downstream to be trapped in estuaries due to the convergence of flows associated with their circulation (Yellen et al, 2017;Ralston and Geyer, 2017;Burchard et al, 2018). There, fine particles and pollutants are recirculated and can be periodically deposited and resuspended (Garnier et al, 1991;Grabemann et al, 1997;Turner and Millward, 2002;Geyer and Ralston, 2018).…”