The Role of a Flood-Tidal Delta in the Evolution of a Tidal Inlet: Chatham Harbor, Cape Cod, Massachusetts

An inlet formed through Nauset Beach, a barrier spit within Cape Cod National Seashore during an extratropical storm on 2 January 1987. The inlet, still unarmored and unmanaged, continues to evolve naturally. This study documents the relationship between the tidal inlet and the primary flood-tidal delta from 1987 to 2005. Fieldwork included intertidal bedform surveys and a tidal current velocity survey on the flood-tidal delta and a sidescan sonar survey in the harbor. Intertidal and subtidal sedimentary features were delineated from a series of georeferenced vertical aerial photographs and their temporal and spatial relationships to one another were documented.

Field surveys documented bimodal, topographically-controlled ebb-tidal flow over the flood-tidal delta. Bedforms were oriented between 60-75° oblique to the direction of unobstructed ebb-tidal flow. The ebb-tidal prism could not be accommodated in the main tidal channel thus increasing the hydraulic slope and allowing water to flow over the flood-tidal delta.

The links between the two-dimensional morphology of the flood-tidal delta and the inlet are clear. A feedback mechanism linking the flood-tidal delta with the migration of the main ebb channel was documented in the study area. The main ebb channel of the inlet migrates downdrift as material entrained in the longshore sediment transport system becomes incorporated into the swash platform. The size of the flood-tidal delta increases as growth of the swash platform allows more sediment to enter the harbor. As the surface area, and particularly the width, of the flood-tidal delta increases the ebb-tidal flow is restricted which results in narrower and deeper ebb channels. This channelization of ebb-tidal flow increases the shallower areas in the harbor increasing the sediment transport during flood-tidal flow thus continuing the cycle.