Tidal impacts on downstream hydraulic geometry of a tide-influenced delta

Abstract

Variations in river hydraulic geometry (HG) are of fundamental importance to fluvial geomorphological research and catchment management. In tide-influenced river deltas, channel geometry displays a mixed scaling behavior since the channel forming discharge is of both tidal and river origins. This study aims to map the tidal signature on delta morphology in the Pearl River Delta (PRD) channel network. The model results of a 2D numerical model are used to analyze the spatial variations of river and tidal discharges and their relations with representative channel geometry throughout the delta. Downstream HG of the distributary channels in the PRD features distinct characteristics in three zones. The transition points in morphology, which splits the delta into river- and tide-dominated parts, coincide with those in the mean flow velocity, the ratio of maximum tidal discharge amplitude (MTDA) to fluvial discharge. The ratio of MTDA to river discharge scales with bifurcation order. The water level setup created by non-linear tidal and fluvial interactions affects the discharge division at tidal bifurcations. In general, the net tidal impact is to attenuate the inequality in discharge division in the PRD. The tide is non-negligible in the cross-sectional morphology and plays a decisive role in the formation of the river channel in the seaward portion of the delta.