Maintenance of the shoreface by wave orbital currents and mean flow: Observations from the Long Island Coast

Abstract

Waves and currents were observed for a total of 36 days in August 1976 and 1977. Calculations of wave transport of sediment, using a Bagnold equation for transport and second order Stokes theory, show that wave orbital current asymmetry results in landward sediment transport in 10 m or less water depth, but does not significantly effect transport at greater depths. Calculations of sand transport using the Madsen‐Grant formulation show that during a 36 day observation period, sediment in less than 10 m of water generally moved landward into the breaker and then along coast with the wave‐driven littoral flow. The calculations show that below 10 m the generally upwelling coast‐parallel mean flow did not move sediment. However, during two days of a mild summer "northeaster" storm, a downwelling coastal jet developed, with 60 cm sec−1 maximum current. The calculations show that if this flow had been combined with the intense wave regime of the succeeding several days, sediment would have been entrained across the lower as well as the upper shoreface. Movement would have been offshore and alongshore. Such storm transport is believed to be typical of the more intense winter storms, and to be responsible for the erosional shoreface retreat that has caused up to a kilometer of shoreline retreat on the Long Island Coast in the last century.