The coalescence of two mangrove-lined river plumes and consequences for sediment transport and deposition in a coastal environment

Abstract

We employed idealized numerical experiments using Delft3D to investigate how the interactions of coalescing buoyant river plumes with mangrove vegetation affect sediment transport and deposition in the coastal zone. Our model system, based on the Firth of Thames in the North Island of New Zealand, is designed to represent two mangrove-lined rivers debouching into a coastal bay. Sediment transport patterns into the three adjacent mangrove forest regions (central, western, and eastern) were assessed by quantifying the sediment fluxes over a tidal cycle. The interactions between the buoyant river plumes resulted in a coalesced river plume that flowed along the central mangrove forest between the rivers. The dominant contributors to the total sediment transport into the mangrove forests varied with both the total and the relative flows through the two rivers. When both rivers were forced with their respective flows at the 50th percentile of observed flow, the analysis of sediment transport fluxes along the central forest revealed sediment deposition inside the mangrove forests lining the river mouth nearly four times greater than in the intermediate regions along the central forest; consistent with satellite imagery of the Firth of Thames. Winds of 5 m s⁻¹ modify the overall sediment transport patterns by deflecting the river plume in the direction of the wind. The magnitude of sediment flux is the highest (nearly 80% compared to the no-wind scenario) in the case of an easterly wind scenario as the alongshore transport is enhanced by Coriolis deflection and wind direction aligned with the alongshore flows of the larger plume.