Observations of the Flow Field near the Nose of a Buoyant Coastal Current

Abstract

Low-salinity water from Chesapeake Bay forms an intermittent buoyant gravity current that propagates more than 100 km southward along the coast. During five events when wind and surface gravity-wave forcing were weak, the buoyant coastal current 90 km south of Chesapeake Bay was less than 5 km wide, was 5?10 m thick, and propagated alongshore at ?50 cm s?1. The density decreased 2?3 kg m?3 over a few hundred meters at the nose of the buoyant coastal current, which was located about 1 km offshore in ?8 m of water. Water up to 4 km ahead of the advancing nose was displaced southward and offshore (maximum velocities near the nose of 20 and 10 cm s?1, respectively). The southward alongshore current increased abruptly to ?50 cm s?1 at the nose and continued to increase to a supercritical maximum of ?70 cm s?1 about 1 km behind the nose. An onshore flow of between 5 and 15 cm s?1, which extended at least 5 km behind the nose, supplied buoyant water to the onshore region of weak, subcritical alongshore flow. The observed flow structure is qualitatively similar to theoretical predictions and laboratory measurements of buoyant gravity currents propagating along a sloping bottom.