GEOLOGIC EFFECTS OF LONGSHORE SAND TRANSPORT

Forty years of personal observation suggest that an upper limit to the longshore transport rate along a segment of a sandy shore can be given from the size of the water body facing the shore, and the exposure of that shore to prevailing winds. In units of cubic meters per yr directed parallel to the shore, typical maximum rates would be: 1000 for large rivers and moderately sized bays or lakes; 7500 to 75,000 for very large bays and very large lakes; 75,000 to 250,000 for open ocean coasts where prevailing winds blow from land; and 400,000 or more for open ocean coasts facing into prevailing winds. A sand layer one meter thick over 100 sq km area, or a sand spit 7km long, 2km wide, and 7 meters thick could result from transport at 100,000 cubic meters per yr for 1000 yrs. The above rates assume abundant sand and unidirectional longshore transport. Sand sources are transient on geological time scales, and most open coasts have significant longshore transport in both of the longshore directions. In higher latitudes, ice reduces fetch and armors the shore in the stormy season. In lower latitudes, carbonate cementation removes sand from the supply. Waves in the surf zone are efficient at sorting sediment in the cross-shore direction, say 1km, but inefficient at sorting the sand, even in the much longer shore-parallel direction, say tens of km. On most open coasts, net longshore transport of sand, per unit time and width, is many times greater than net cross-shore transport, per unit time and width. Yet the bedforms indicate only cross-shore transport. Longshore currents are weak, but persistent. Cross-shore transport is energetic, but oscillatory. As a result, the universe of sand grains nudged down the coast by the longshore currents come to rest in strata formed by cross-shore waves, a result illustrated by the recent work of Moore and others (2004) on a segment of the Pacific shore of Washington state.