Observations and Modeling of Morphological Change in a Tidal Channel at Seasonal to Annual Time Scales

Tidal channels can be highly dynamic systems with strong currents, mobile bedforms, and relatively rapid morphological change. We use a combination of shipboard bathymetric sonar measurements and structure-from-motion analysis of airborne drone imagery to map in detail the bathymetry of the tidal channel at the entrance to Nauset Estuary (MA, USA). Nauset is a lagoon with extensive salt marsh located on Cape Cod and lies behind a barrier spit, and the channel connecting it to the Atlantic Ocean has been evolving as the location of the inlet migrates along-coast due to wave-driven processes. Digital elevation models (DEMs) of the channel are constructed based on individual surveys separated by several months to a year. Differences between DEMs are used to characterize the spatial patterns of change, including estimates of magnitude of sand transport. A hydrodynamic and sediment transport model of the system is developed using COAWST, and the model is calibrated based on observations from velocity and water level sensors during two different deployment periods. Model bathymetries based on the DEMs are used to quantify the effects of morphological change on tidal dynamics (frictional losses, overtide generation). The model is used to quantify sediment transport in the channel due to tidal currents at spring-neap to seasonal time scales, and the results are compared with observed velocity time series and changes in bathymetry. The results are evaluated for sensitivity to the model formulations for bottom friction and bedload transport.