Bathymetric controls on sediment transport in the Hudson River estuary: Lateral asymmetry and frontal trapping

TLDR

Lateral depth gradients generate baroclinic pressure gradients and stratification that drive sediment transport, with sediment convergences at bathymetric fronts creating secondary turbidity maxima, and lateral transfer occurring from higher to lower bed stress depending on pycnocline elevation relative to the bed. Field observations and numerical models show that sediment transport is laterally segregated between channel and shoals, with landward flux in the channel and seaward on shoals, frontal trapping at bathymetric transitions, and pronounced spring‑neap cycle variations, resulting in a heterogeneous distribution of sediment transport and trapping along the estuary.

Abstract

Analyses of field observations and numerical model results have identified that sediment transport in the Hudson River estuary is laterally segregated between channel and shoals, features frontal trapping at multiple locations along the estuary, and varies significantly over the spring‐neap tidal cycle. Lateral gradients in depth, and therefore baroclinic pressure gradient and stratification, control the lateral distribution of sediment transport. Within the saline estuary, sediment fluxes are strongly landward in the channel and seaward on the shoals. At multiple locations, bottom salinity fronts form at bathymetric transitions in width or depth. Sediment convergences near the fronts create local maxima in suspended‐sediment concentration and deposition, providing a general mechanism for creation of secondary estuarine turbidity maxima at bathymetric transitions. The lateral bathymetry also affects the spring‐neap cycle of sediment suspension and deposition. In regions with broad, shallow shoals, the shoals are erosional and the channel is depositional during neap tides, with the opposite pattern during spring tides. Narrower, deeper shoals are depositional during neaps and erosional during springs. In each case, the lateral transfer is from regions of higher to lower bed stress, and depends on the elevation of the pycnocline relative to the bed. Collectively, the results indicate that lateral and along‐channel gradients in bathymetry and thus stratification, bed stress, and sediment flux lead to an unsteady, heterogeneous distribution of sediment transport and trapping along the estuary rather than trapping solely at a turbidity maximum at the limit of the salinity intrusion.