Insights into Sediment Transport in the DeGrey River Delta: Cyclonic Influences and Bedform Persistence

This study characterizes the sediment transport dynamics of the fluvial portion of the DeGrey River delta, a dryland tide/wave-dominated delta along the Pilbara coast of northwestern Australia. The primary focus lies in the discernment of discrete deposits resulting from annual flood events in this ephemeral river, primarily driven by cyclones and tropical depressions during the austral summer.

Methodology combines water discharge data, digital elevation models (DEM), and Sentinel-2 change detection to model flow depth and flooding extent during storm events, linking it to riverbed shear stress and the formation of discrete flood deposits. A time series of DEM datasets, consisting of a 1 m aerial survey (2021) and drone photogrammetry surveys (2022 and 2023) were used to generate differential DEMs to accurately detect yearly morphological changes within the river channel. Field surveys of selected sites indicating presence of flood deposits enabled characterization of grain size, porosity, water flow, and structural elements.

Cyclone floods in the region cause propagation of preexisting dunes, unit bars, and compound bars. These events predominantly shape lower-flow-regime structures within medium-grained sand. Unit bars exhibit down-climbing cross-stratified sets, with variations in thickness contingent on their location within the channel ranging from 0.4–1.6 meters. Lower-flow-regime bar and bedform morphology persists and propagates between flows, despite the occurrence of intense flash floods, often generating discharges in excess of 100,000 ML/day. This challenges conventional expectations of channel excavation and the preservation of upper-flow-regime bedforms in the wake of such extreme events.