SHORELINE EVOLUTION AT JETTIED TIDAL INLETS TOWARD AN EQUILIBRIUM CONFIGURATION: WELLS INLET, ME, AND MERRIMACK RIVER INLET, MA

It has been assumed that the movement of sediment around a tidal inlet is halted or greatly reduced once jetties are constructed. Angular wave approach produces longshore currents that transport sediment to the updrift side of the jetties and removes sediment from the downdrift beach. Although this pattern occurs at many sites, there are numerous examples of stabilized inlets that exhibit a different pattern. The Merrimack River Inlet in Newburyport, MA and Wells Inlet in Wells, ME are both located along mixed energy (tide-dominated) coasts and have reached a state of quasi-equilibrium.

Shoreline data using previous historical studies and more recent vertical aerial photographs provide a means of documenting shoreline evolution from pre-jetty construction through the present time. This information was put into a Geographic Information System to facilitate data analysis and illustrate the extent of shoreline evolution adjacent to each inlet.

Preliminary analysis of erosional-depositional changes at each inlet suggests that the dominant southerly longshore transport system has not produced significant accretion on the updrift beaches. This pattern suggests that sediment bypassing is active at both inlets whereby sand is moved past the inlet or into the main channel. Sediment bypassing is most active during high energy events. Storm waves produce strong longshore currents, which push water to elevated levels against the updrift jetty. This process creates a strong rip current that transports wave suspended sediment offshore. Sediment may then be moved along the ebb tidal delta and transferred to the downdrift shoreline by onshore wave-generated and flood tidal currents. Another method of sediment bypassing involves a breaching of a new main ebb channel through the ebb tidal delta. Sediment accumulating on the updrift side of the jetty eventually deflects the main ebb channel towards the downdrift beach. When a more efficient channel is cut through the ebb tidal delta during a high energy event, sediment can be transferred to the downdrift side and transported onshore by wave action. The pathways and processes whereby sediment bypasses jettied inlets are poorly understood and additional field work will aid in determining the exact mechanisms involved.