Hydrological connectivity in vegetated river deltas : the effects of spatial variability and patchiness on channel-island exchange
River deltas are threatened regions of great societal and environmental importance, and their continued survival depends upon a greater understanding of their formation and evolution. Hydrological connectivity in river deltas is important for delivering flow and sediment to the island interior and is responsible for a large portion of the ecosystem benefits that deltas provide, which could be leveraged for restoration projects using nature-based engineering. However, the process is still poorly understood. The roughness of island vegetation is known to significantly limit channel-island connectivity, but the importance of the spatial distribution of vegetation is, as-of-yet, unknown. Using a 2D hydrodynamic model, we investigate the influence of vegetation percent cover, patch size, and stem density on the fraction of discharge allocated to the islands of an idealized delta complex, modeled after the Wax Lake Delta in coastal Louisiana. We find that spatial heterogeneity can substantially alter connectivity when vegetation is dense and covers less than a “disconnectivity” threshold near 50% of the island domain, near the theoretical percolation limit. Above this threshold, models can accurately approximate vegetation as uniform. Below this threshold, however, preferential flow-paths develop in the islands, which greatly alter the hydraulics, transport capabilities, and residence time distribution of the delta complex, with respect to what is seen in uniform vegetation cases. Our results suggest that patchiness has substantial hydrogeomorphic and biogeochemical implications which should be considered when modeling deltaic systems.