Hydrological Connectivity in Vegetated River Deltas: The Importance of Patchiness Below a Threshold

Hydrological connectivity in coastal deltas is important for delivering flow, sediment, and nutrients to the island interiors. The roughness of island vegetation limits connectivity, but how important is the spatial distribution of vegetation? Using hydrodynamic modeling, we test the influence of vegetated percent cover, patch size, and stem density on the discharge-fraction allocated to the islands of an idealized delta complex, modeled after Wax Lake Delta. We find that heterogeneity has negligible effects when vegetation is relatively sparse but is important when vegetation is relatively dense and covers less than a disconnectivity threshold of 40-50% of the islands, near the theoretical percolation limit. Below this threshold, preferential flow paths develop in the islands, which alter the hydraulics and residence time distribution of the delta complex and enhance potential sediment transport with respect to model runs with uniform vegetation. Patchiness has hydrogeomorphic and biogeochemical implications, which should be considered when modeling deltaic systems. Plain Language Summary Coastal river deltas are vulnerable regions of tremendous economic and ecological importance. The long-term sustainability of deltaic systems depends in part on the delivery of water, sediment, and nutrients to the interior of deltaic islands. The vegetated wetlands within these islands are typically hot spots of land growth and biological nutrient removal, which can counteract land loss and prevent the expansion of coastal oxygen-depleted zones. Increasing the amount of island vegetation is known to decrease the fraction of flow that enters the islands, but it has yet to be shown how the spatial arrangement of vegetation patches affects this channel-island hydrological connectivity. In the present study, we numerically model flows in a simplified river delta to determine the influence of three vegetation characteristics on connectivity: the percentage of the island covered by patches, patch size, and patch density (how tightly packed the vegetation is within each patch). We find that all three characteristics can increase connectivity when compared to similar model runs with spatially uniform vegetation. More specifically, the spatial configuration becomes important when patches are dense and cover less than 40-50% of the deltaic islands. The results of this study have implications for our ability to model the success of restoration projects.