Backwater Hydrodynamics in Complex Channel Networks

Abstract:

A fundamental characteristic of many river deltas is that they branch into distributary networks as they approach their receiving basin. Recent studies have shown that the pattern of sediment transport in the lower reaches of the trunk channel is controlled by interaction with the downstream water level, and that channel bed friction plays a critical role in modulating the relationship. However these studies have typically concentrated on regional scales, and not investigated the terminal distributary channels which have an additional upstream water level boundary constraint set by the trunk channel.

Intuition gained from studies of trunk channels would suggest that friction is a dominant control in terminal distributary channels too. Contrary to this intuition, we present observations suggesting that in many cases large and small distributary channels emanating from the same crevasse have similar water surface slopes and velocities. We consider the hypothesis that big and small terminal distributary channels act similarly because the down-channel pressure gradient overwhelms inter-channel variations in bed friction over the observed distributary length. We then adapt existing models of backwater flow to account for the fixed upstream boundary condition in terminal distributary channels. Our results have implications for the management of coastal river systems and for our understanding of channel bifurcation stability.