A sand budget for the Alexandria coastal dunefield, South Africa

Abstract

ABSTRACTThe sand in the Alexandria coastal dunefield is derived from the sandy beach which forms the seaward boundary of the dunefield. Sand is blown off the beach onto the dunefield by the high‐energy onshore‐directed dominant wind. The dunefield has been forming over the past 6500 years.Sand transport rates calculated from dune movement rates and wind data range from 15 to 30 m3 m ‐1 yr‐1 in an ENE direction. The sand transport rate decreases with increasing distance from the sea due to a reduction in wind speed resulting from the higher drag imposed upon the wind by the land surface. Aeolian sand movement rates of this order are typical of dunefields around the world.The total volume of sand blown into the dunefield is 375 000 m3 yr‐1. Sand is being lost to the sea by wave erosion along the eastern third of the dunefield at a rate of 45 000 m3 yr ‐1. The dunefield thus gains 330 000 m3 of sand per year. This results in dunefield growth by vertical accretion at about 1.5 mm yr‐1 and landward movement at about 0.25 m yr‐1. The dunefield is a significant sand sink in the coastal sand transport system.The rate of deposition in coastal dunefields can be 10 times as high as rates of deposition in continental sand seas. The higher rate of deposition may result from the abundant sand supply on sandy beaches, and the higher energy of coastal winds.Wind transport is slow and steady compared to fluvial or longshore drift transport of sediment, and catastrophic aeolian events do not seem to be significant in wind‐laid deposits.