Effects of low-tide rainfall on channel morphodynamics of tidal mudflats

Abstract

Tidal channels provide a pathway for exchange of water, sediment and nutrients in coastal wetlands, significantly affecting their eco-functions. Morphological evolution of tidal channels is influenced by both periodic tidal and episodic rainfall conditions. The latter, being a short-term hydrodynamic process, have received little consideration. Here, we investigated the impact of rainfall on tidal channel evolution, based on field investigation of geomorphic change and a numerical model (MIKE21 FM). The results indicate that intense low-tide rainfall promotes formation of tidal channels. Rainfall induces rill erosion, facilitating rapid formation of a rill network. Initial tidal currents converging through the rill network enhance sediment transport, leading to rapid development of a tidal channel network. The tidal channel network inherits features from the rainfall-induced rill network, retaining up to 34% of these characteristics after 30 tidal cycles. Furthermore, long durations, steep slope, micro-tidal conditions, and peaked rainfall pattern enhance the effect of rainfall on tidal channel network morphodynamics. There exists a critical slope threshold for a specific high-intensity rainfall. Rainfall dominates channel length when slope is above this threshold, whereas tides become dominant as slope is below it. These findings provide an explanation for the rapid growth of tidal channels during rainy seasons in earlier field observations, indicating that rainfall condition should be incorporated into research of coastal wetland evolution.