Measurements of Surf Zone Currents and Waves In Support of Madaket and Sankaty Head, Nantucket, Beach Nourishment

Coastal sand bluffs and cliffs on the Atlantic seaboard of North America frequently regress, forcing home drawbacks and lighthouse relocation such as that facing homeowners at Madaket and the Sankaty Light on Nantucket. Whether the dominant cause of bluff and cliff calving is erosion of beach at the base from high tide waves or exceptional high water currents or if it is rainfall induced, increased sand on the beach is unlikely to be harmful and may reduce the rate of regression. Evidence of sand spit building from a wrecked clam boat at Sankaty Head beneath the lighthouse suggests that the regression rate might be reduced to the point where relocation of the light is less urgent if barges could be sunk to cause sand to accrete and build the beach at this point. Measurement of suspended sand turbidity, along shore current, and waves at 3 m and 4 m depth is being undertaken with two monopod emplacements of MAVS current and wave sensors. Presently these are installed off Madaket at the western end of the island where beach regression is severe. These instruments are powered from shore and return data by cable at 4 Hz for access by Internet. A webcam at the Madaket site supplements the data record on the Internet so that conditions of tidal state, sand bar building and exposure, breaker line, and sand nourishment can be monitored remotely. Surf zone wave and current measurements are generally high maintenance installations with wave forces tending to uproot and overturn support structures. Cable connections can be dragged alongshore and broken. In this installation, a monopod was employed to reduce the horizontal area of the support structure subject to upward force by waves and chain-weighting buried the cable to reduce lateral drag. Two locations offshore from the region of concern permit shear in the alongshore current to be detected and difference in wave states captured. Winter storms are important causes of beach sand loss or possibly deposition and this set of o- bservations is designed to correlate current and wave state with such events. Northeast storms are assumed to be the most serious drivers of beach processes here on the coast of Massachusetts but these are typical of sand beaches from Maine to Florida. Observation of the Valentine´s Day northeast storm of 2007 represents these processes at Madaket and the extension of these measurements to Sankaty Head may be useful for other locations where cliff regression has endangered lighthouses. Remediation by barge sinking for beach nourishment is planned to follow with similar monitoring of current, waves, and beach condition.