Influence of vegetation and flood regimes on deltaic floodplain deposition

In deltaic environments, sedimentation occurring during high freshwater discharges is essential to prevent floodplains from drowning with rising sea levels. Vegetation has long been considered as one of the main drivers of overbank deposition because it reduces flow velocity on the floodplains, and thus the percentage of sediment volume trapped on the floodplains with respect to the volume of sediments entering from the adjacent channel (trapping effect, FT). However, it has recently been shown that dense vegetation patterns can behave as a barrier for sediments and water fluxes, thus increasing the percentage of sediments exported seaward through the channel (buffering effect: FB) and reducing the percentage volume of sediments flowing from the channel to the floodplains with respect to the volume of sediments that enters the system (1-FB). FB and FT determine the trapping efficiency of the system, TE = (1-FB)⸱FT, i.e., the percentage of sediments trapped on the floodplains with respect to the volume entering the system. The buffering effect has been shown to prevail over the trapping effect only in the Wax Lake Delta and only for a few specific hydrographs. Therefore, there is a need to systematically investigate the impact of floodplain vegetation on sediment trapping and buffering. For this purpose, we conduct numerical simulations with the Deltf3D model to analyze sediment deposition over floodplains for several different channels and floodplains geometries, vegetation characteristics, and flood conditions (i.e., peak magnitude, duration, and hydrograph skewness). The model domain consists of a simplified riverine environment, constituted of a rectilinear channel surrounded by rectangular floodplains. Our results indicate that the trapping and buffering effects are modulated by a third important effect, which we name the piling-up effect. It consists of a general increase in water level along the channel for higher vegetation densities and heights, due to higher friction. Overall, vegetated scenarios display a decrease in longitudinal water velocity on the floodplains with respect to the non-vegetated ones. Thus, the residence time of sediments suspended over the floodplains increases, enhancing deposition. Piling-up also increases the settling time of sediments, due to the higher water depth on the floodplain. Our results show that the increase in resident time is larger than the increase in settling time, overall favoring deposition. In the transversal direction, piling-up increases water and sediment fluxes, both advective and diffusive, from the channel into the floodplains, thus decreasing FB. Our simulations indicate that the overall effect of piling-up is to increase the trapping efficiency TE of the system. Finally, we identify the parameter space for which trapping effects are larger than buffering effects, thus leading to more deposition in the vegetated case than in the non-vegetated one. We also identify the vegetation characteristics that maximize sediment deposition given the channel and floodplain geometries. This information can be used for targeted floodplain restoration strategies.