Coastal sand dunes as geomorphological systems

Abstract

SynopsisThe importance of aeolian deposition is clearly shown by the size and bulk of coastal dunes. Sand dunes occur where there is a large supply of sand, a wind to move it, and a place in which it can accumulate. A dune classification is presented which takes into account the origin, internal geometry and surface geomorphology of coastal dunes. Since the main element that distinguishes coastal dunes from desert dunes is vegetation, the relative amount of vegetation may be used as a typology. Four dune types are distinguished: vegetated dunes, parabolic dunes, medanos (i.e. large sand hills devoid of vegetation), and artificially-inseminated dunes. Vegetated and medano dunes are the end members, with parabolic dunes in between. Parabolic dunes are “anchored” by vegetation, but the centre of the dunes has migrated in the down-wind direction. The artificially-inseminated dunes are formed by vegetation plantings, fencing, or other artificial means, but with natural sand accumulation around these obstacles. These now account for a very substantial portion of the world's coastal dunes.The role of wind and its relation to the internal geometry (i.e. cross-bed dip and direction) are discussed in some detail, with examples from Brazil, U.S. east and south coasts, Israel and China. Differences in these characteristics are detailed, and related to the different modes of formation of the four dune types.The role of the wind in transporting and depositing sand in coastal areas is being quantified through both wind tunnel and field transport measurements. An example from a field study along the coast of Israel illustrates the differences between desert and coastal dune transport, where the role of vegetation and beach topography must be taken into account. From the Israeli study, and others, it appears that transport in coastal dunes is reduced by one-third to one-half of that in deserts, due to these factors.Coastal dunes provide a useful, and often necessary, buffer against storm waves and the presently rising sea level. They form primarily through vertical sand accretion trapped by the sensitive dune vegetation. Provided that sufficient space exists between the high tide line and developed areas, planned dunes can be easily formed with the aid of plantings and fencing.