THE ROLE OF WIND DIRECTION IN EOLIAN TRANSPORT ON A NARROW SANDY BEACH

Comparison of eolian transport during five high-velocity wind events over a 29 day period on a narrow estuarine beach in Delaware Bay, New Jersey, USA, reveals the temporal variability of transport, due to changes in direction of wind approach. Mean wind speed measured 6 m above the dune crest for the five events ranged from 8.5 to 15.9 ms(-1). Mean wind direction was oblique to the shoreline (63 degrees from shore-normal) during one event but was within 14 degrees of shore-normal during the other events. Eolian transport is greatest during low tide and rising tide, when the beach source area is widest and when drying of surface sediments occurs. The quantity of sediment caught in a vertical trap for the five events varied from a total of 0.07 to 113.73 kg m(-1). Differences in temperature, relative humidity and moisture and salt content of surficial sediments were slight. Mean grain sizes ranged from 0.33 to 0.58 mm, causing slight differences in threshold shear velocity, but shear velocities exceeded the threshold required for transport during all events. Beach width, measured normal to the shoreline, varied from 15.5 to 18.0 m; beach slope differed by 0.5 degrees. The oblique wind during one event created a source width nearly double the width during other days. Beach slope, measured in the direction of the wind, was less than half as steep as the slope measured normal to the shoreline. The amount of sand trapped during the oblique wind was over 20 times greater than any other event, even those with higher shear velocities. The ability of the beach surface to supply grains to the air stream is limited on narrow beaches, but increased source width, due to oblique wind approach, can partially overcome limitations of surface conditions on the beach.