Longitudinal scour-bar pattern in large convergent estuaries on low-lying coastal plains

Abstract

A seaward decrease–increase–decrease trend in the longitudinal bed level, defined as scour-bar pattern, is observed in several large estuaries on low-lying coastal plains such as the Amazon, Yangtze and Fly. Why this pattern forms has not yet been well explained. The mechanism for the formation of this pattern is explored here using a one-dimensional morphodynamic model, with varied geometry, fluvial water discharge, sediment flux and tide. Results show that for those estuaries with bed slopes in the order of 10−5 and sediment concentration in the order of 0.1 kg m−3, bars are likely to occur for channels with relatively weak convergence and wide mouth, while scours are nearly ubiquitous. Scours are classified as fluvial and tidal based on the relative strength of river and tide, which can be distinguished by bed slope and water depth. A balance between river and tide can lead to no scours or bars. Sediment reduction and sea level rise enhances the relative strength of tide, and transition of a fluvial scour to a tidal scour may occur.