The Woods Hole Sea Grant (WHSG) and Cape Cod Cooperative Extension (CCCE) recently released a Longshore Sediment Transport report for Cape Cod. This is a useful report for the public , coastal managers, conservation commissions, and landowners in that it explains the dynamic processes that shape our beaches, how those processes are altered by human development, and how they can change over time. It also provides an excellent qualitative summary of the net movements of sediment along the Cape (as a series of maps), which can inform sound design and planning decisions.
Essentially, direct oscillatory wave action (waves repeatedly breaking at a non-perpendicular angle to the beach) and wave-generated currents combine to transport sediments in a (net) singular shore-parallel direction. Where wave approach and landforms differ, the direction of longshore transport changes and the boundary of a littoral cell is created. Coastal structures can disrupt longshore transport, accumulating sediments on the updrift side and eroding sediments on the downdrift side (the depositional shadow), and threaten the sustainability of our beaches. Tidal inlets, barrier beaches, and sandy points also can be indicators of sediment transport direction along the coast.
Using air photo interpretation of the accumulation patterns at 645 jetties, groins, inlets, sand bars and spits, WHSG and CCCE identified over 30 littoral cells throughout Cape Cod’s ocean and bay shorelines. The littoral cell delineations should help resource managers determine the impact area of coastal projects.
This qualitative study of longshore sediment transport is exactly what is needed to get some momentum for regional sediment management on Cape Cod. Now that littoral cells have been defined, the next step is to quantify rates of sediment transport. This can be accomplished using wave and sediment transport models (provided more wave data is collected for the region), and initial studies could be strategically targeted to highly erosional areas.
Woods Hole Group congratulates WHSG and CCCE on this accomplishment, and thanks them for the opportunity to review and comment on the report.