The Bed Stress Minimum in Tidal Rivers

Bed stress patterns control erosion and deposition in tidal rivers and thereby govern changes in geomorphology. Management of river discharge and shipping channel geometry perturbs rivers from their natural state, leading to hotspots of sand deposition and erosion. Here, we investigate the along-channel variability in bed stress for a tidal river of constant depth with semidiurnal tides and convergent geometry using a Fourier decomposition of the quadratic bed stress and analytical approximations of tidal and river velocity. Under some river discharge and tidal conditions, bed stress profiles exhibit a local bed stress minimum, xmin, within a region marked by strong gradients in cross-sectionally averaged velocity. These gradients can lead to convergent sediment fluxes and shoaling near xmin. Factors decreasing river velocity (flow management, channel deepening, and weak channel convergence) move xmin and depositional areas upstream. Analytical estimates of xmin were validated using fifty-two two-dimensional Adaptive Hydraulics (AdH) numerical model simulations and agree well with the sediment transport behavior of three prototype systems (Columbia River, Hudson River, and Delaware Estuary). Climate changes in seasonal flow cycles and mean river discharge, and the reservoir management response to these changes, may significantly alter the dynamics of xmin, affecting ecosystem dynamics and the stability of wetlands and coastal beaches as sea level rises. The analytical formulation of xmin developed herein will make it easier to understand how climate and human-induced changes to a river can impact long-term erosion/accretion patterns and can help guide future investments for managing sediment.