PHYSICAL PROCESSES AND MORPHOLOGIC CHANGES IN HAMPTON-SEABROOK HARBOR, NEW HAMPSHIRE: NATURAL AND ANTHROPOGENIC ALTERATIONS

Hampton-Seabrook Harbor, New Hampshire, is a classic mesotidal, back-barrier system with strong tidal currents, extensive sand shoals, and fully developed salt marshes. The back-barrier system is strongly affected by sedimentation and shoaling problems, as well as rapidly migrating channels and shifting sand bodies that result in shoreline erosion, navigation problems, and loss of anchorage. As a result, Hampton-Seabrook Harbor has been significantly modified by engineering activities including construction of jetties, periodic dredging, and a reconfiguration of a major tidal channel and shoal system. In 2005, as part of the National Shoreline Erosion Control and Demonstration Program, one of the major tidal channel systems was modified by the U.S. Army Corps of Engineers to alleviate shoreline erosion and navigation problems. A tidal channel that naturally cut through a large intertidal shoal in the harbor was closed off with bulkheads and a new channel dredged to handle the tidal prism. Following the placement of the bulkheads, a study was conducted to assess morphologic changes, tidally and meteorologically forced water levels, tidal currents in selected channels and adjacent to the bulkheads, and bottom sediment characteristics. Results of the field program show that water level fluctuations at the stations monitored in the harbor are dominantly controlled by eight tidal constituents (99% of the variance), with the M2 and N2 constituents accounting for 96% of the variance. However, the M2, as well as several higher harmonic amplitudes (M4, M6 and M8) increase up-estuary, accompanied by an 11 to 15 minute tidal lag. Despite the strong control of the water level changes by the tidal constituents, meteorological conditions cause aperiodic storm surges as demonstrated by water levels in the upper estuary being ~50 cm higher than predicted. The tidal forcing in Hampton-Seabrook Estuary results in strong tidal currents with peak observed velocities on the order of 50 to 75 cm/s, with significant vertical and cross-channel flows (~10 to 20 cm/s). The strong tidal currents and transport of coarser-grained sediments result in channel instability, shoal development, and rapid changes in the morphologic character of the system.