Impacts of tidal-channel connectivity on transport asymmetry and sediment exchange with mangrove forests

Mangrove forests provide many valuable services, including wave- and tidal-energy dissipation, sediment accumulation, and substrate stabilization. But, their global extent is rapidly shrinking. Sedimentary research that is needed to inform restoration projects has so far focused on characterizing flow patterns through forests associated with single rivers, nearshore environments, or terminating tidal channels. However, flows within the most extensive mangrove forests tend to be driven through channels that connect to multiple estuaries or other channels with temporally lagged tides. For this study, in-situ observations of water and sediment flux were obtained in two tidal channels near the Amazon River to characterize how connectivity impacts sediment transport in coastal mangrove forests. The low-connectivity channel connects to one estuary whereas the high-connectivity channel connects two estuaries. We conclude that channel connectivity controls the amount of sediment stored in channels that is available to supply mangrove forests. Although phase-duration asymmetry is flood dominant in both channels, velocity asymmetry becomes ebb dominant during tidal cycles when the mangrove flats are inundated. Additionally, tidal elevation and water velocity are not in quadrature for the high-connectivity channel, as they are for the low-connectivity channel. As a result, the high-connectivity channel acts as a conduit for sediment from one estuary to another rather than simply importing and retaining sediment like the low-connectivity channel. High-connectivity channels distribute point-source sediment along coastlines and provide sediment to estuaries and channels that may not have their own direct source. As restoration efforts of mangrove forests continue to grow in response to coastal threats such as sea-level rise and diminished fluvial-sediment delivery, the location of these efforts can benefit from the conclusion that channel connectivity influences spatial dispersal of sediment supply to mangrove forests.