The Influence of River Plume Discharge and Winds on Sediment Transport into a Coastal Mangrove Environment

Abstract

We investigate how the physical forcing factors of river discharge and winds affect sediment delivery to, and retention within, mangrove-lined coastal regions. We use an idealized numerical model, broadly similar to the Firth of Thames deltaic system in New Zealand, to isolate and explore the underlying processes without some of the complexities of the real system. Total sediment transport and the relative contributions of riverine and bed-sourced sediment into the forest are assessed using a transect along the edge of the forest region. The model results demonstrate that both river discharge and winds alter the distribution of sediment transport, and that the spatial patterns relate to different regions of the river plume. At the river mouth (the near-field region), irrespective of the discharge employed, sediment fluxes are directed into the mangrove forest, indicating an accretionary environment consistent with satellite observations. Here, contributions from the riverine and bed-sourced sediments are similar. For small to medium discharge scenarios (up to \(\sim\) 280 m\(^{3}\) s\(^{-1}\), flow speeds \(\sim\) 0.6 m s\(^{-1}\)), mass loads increase with river discharge. However, in the case of large discharge events, the high momentum in the near-field region allows the river plume to effectively transport sediment through the full width of forested region and out of the forest front. In the mid- and far-field regions of the plume, tidal influences also play a stronger role. Suspended sediment is primarily composed of bed-sourced material and transported out of the forest. Weaker winds are found to affect the far- and mid-field regions of the river plume. Stronger winds are able to reshape the entire plume structure, also including the near-field, such that sediment deposition is enhanced when winds are directed towards the forest.