Patterns of Bedform Migration and Mean Tidal Currents in Hampton-Seabrook Inlet, NH
Hampton Harbor is the second largest estuary system in New Hampshire, and is an important economic and ecological resource in the state. The only connection between the estuary and the open ocean is though the Hampton-Seabrook inlet, which is characterized by large tidal ranges (up to 4 m), strong mean currents (up to 1.5 m/s), and coherent seafloor bedforms (ranging in size from mega-ripples to sand dunes). Large amplitude sand dunes (up to 2 m) create dynamic navigational hazards in the inlet channel, and need to be dredged regularly. Historical evidence indicates that bedforms redevelop after dredging, suggesting a strong link between seafloor topography and currents. The relationship between bedform migration and mean tidal currents in the Hampton-Seabrook inlet is examined over a month long period in the fall of 2011.
Nine multibeam echosounder surveys were conducted in the inlet navigation channel, and coincident mean currents were measured throughout the inlet using an acoustic current profiler mounted on a movable personal watercraft. A series of 8 sand dunes migrated steadily onshore, up to 2 m/day, during neap tides and steadily offshore, up to 7 m/day, during spring tides. Spatial variation in bottom roughness showed that mega-ripples continually evolved both spatially and temporally over the study period. Net offshore movement of sediment over the study period indicated that the spring-ebb tides dominated sediment transport. Calculations of Shields parameters were always greater than critical values, suggesting sediment transport was always being initiated. Additionally, estimates of shear stress from vertical current profiles validated observations of bedform migration.
Lindsay McKenna is pursuing an M.S. in Earth Sciences: Mapping Option. Lindsay graduated with a B.S. in Geological Sciences from Brown University in 2007. She completed her senior thesis on “Coastal Dynamics of Annawamscutt Beach.” Before coming to CCOM, Lindsay worked as a geologist at an environmental consulting firm in the NYC metro area. Lindsay is an active member of the Surfrider Foundation and enjoys training for triathlons.