SHOREFACE SLOPE AND LONGSHORE SEDIMENT TRANSPORT ALONG A SANDY PARAGLACAL COAST WITH APPLICATION TO COASTAL MANAGEMENT

A one-dimensional, geomorphic process-based model was applied to littoral cell systems along the shores of Cape Cod Bay, Massachusetts, to quantify the directions and volume rates of net longshore sediment transport and delineate sediment sources and sinks at a century scale. Sediment budgets for the individual littoral cells were calculated from cross-shore coastal volumetric change rates determined at 150 m intervals alongshore from contemporary and historical topo-bathymetric data and adjusted to account for on- and offshore sediment losses. Null points in net longshore transport were estimated from geologic indicators, longshore transport gradients and existing literature. Longshore transport diagrams, delineating "source", "fulcrum" and "sink" zones, were produced through integration of the cross-shore volumetric change rates alongshore between the null points.

The longshore transport diagrams revealed a close correspondence between the geomorphic functioning of the sub-systems of the littoral cells - “source”, “fulcrum”, or “sink” - and the shoreface slopes within those sub-systems. A detailed analysis was made of the relationship between shoreface slope and function of one littoral cell, the Brewster cell. Slopes measured seaward of the foreshore (from 0 to -10 m NAVD88) throughout the study area ranged approximately from 0.002 to 0.02, but a distinct pattern was apparent. Slopes within “source” zone ranged approximately between 0.005 and 0.02, while those within “sink” and “fulcrum” zones ranged between 0.002 and 0.005. Also, there was little variance within the population of “fulcrum” and “sink” zone slopes, while variance was high - and increased updrift – in the “source” zone slopes.

These results lead to the suggestion that: (1) the Brewster littoral cell is best characterized by a two-dimensional model having offshore transport in its steep “source” subsystem and along-and-onshore transport in the “fulcrum” and “sink” subsystems, (2) this and similar littoral cell systems self-organize to enhance longshore transport thereby decreasing the work required to shift alongshore form closer to equilibrium, and (3) periodic littoral cell shoreface form/slope monitoring could assist proactive coastal planning of this and similar coastal systems.