Morphodynamic Response of Tidal Mudflats to Marine Cohesive Sediment Influx

Abstract

Estuaries adjust their plan form, low water cross-section and intertidal profile to achieve a stable configuration when external changes in sediment fluxes and sediment types occur (Townend, 2010). Tidal mudflats are part of the intertidal profile; tidal mudflats accrete when suspended cohesive sediment influx increases and retreats when sediment influx diminishes (Pritchard & Hogg, 2003). Therefore, morphological changes to tidal mudflat induces changes in the estuary morphology. The stable estuary morphology depends on morphodynamic equilibrium conditions and the relevant processes that induce morphological changes.

Recent studies had still used a pre-determined tidal flat geometry in order to determine stable estuarine morphodynamic equilibrium conditions (Mariotti & Fagherazzi, 2013; Lanzoni & D’Alpaos, 2014). Nonetheless, these studies have shown which relevant processes are needed to form tidal flat and channel morphodynamics. The aim of this study is to illustrate morphological effects of tidal mudflat on the overall estuary morphology by including sand-mud interactions.

This study focuses on tidal mudflat morphology within a funnel shape estuary. A process-based numerical approach is used to simulate tidal mudflat morphodynamics with the following processes: 1) tide-generated currents, 2) wave generated currents, 3) fluvial discharge, 4) fluvial non-cohesive sediment influx, and 5) marine cohesive sediment influx. Waves were not simulated properly in the model configuration used in this study.

Model results suggest that tidal mudflats are wider in the estuary mouth and become narrower further upstream. The tidal mudflat widths upstream can become wider when the mud suspended load increases upstream. Lateral migrating channel bends can induce a local narrowing of tidal mudflat by erosion. Furthermore the length scales of estuary width are lower when cohesive sediment influx is included and higher when cohesive sediment influx is absent. Length scale of estuary depth decreases for simulations with cohesive sediment influx while the length scale of depth increases for the simulations without cohesive sediment influx.