A review of biological and geomorphological processes involved in the initiation and development of incipient foredunes

Abstract

SynopsisThe evolution, ecological processes, aerodynamics, and morphology of incipient or new foredunes on the upper beach is reviewed. Four types of incipient foredunes may be distinguished, namely: type 1, those initiated by shadow dune formation within zones of discrete individual pioneer annuals (e.g. Cakile spp.), and perennials (e.g. Spinifex spp.; Ammophila spp.); type 2, those initiated by dunelet or hummock formation within discrete colonies of perennial grasses and herbs; type 3, those initiated by sand deposition within laterally extensive colonies of pioneer seedlings; and type 4, those initiated by sand deposition within a laterally extensive plant rhizome cover.Type 1 dunes are dominated by high, local flow deceleration and 3-D flow separation. Shadow dunes form within and downwind of the discrete plants. If colonised by annual plants, invasion by perennial plants is necessary for survival. Later lateral plant spread and accretion produces hummocky terraces and ridges. Type 2 dunes (dunelets) are characterised by local landward flow deceleration and marginal high velocity side flows. Sand deposition results in the formation of low, discrete, semi-circular convex mounds (seedlings), or more elongate hummocks and mounds (rhizomes). Types 3 and 4 dunes are characterised by laterally extensive, often homogeneous vegetation canopies. High canopies display more rapid landward flow deceleration than low canopies, thus narrow, asymmetric ridges and longer convex ridges result respectively. High plant densities result in rapid down-canopy flow deceleration, maximum traction load retardation and the formation of narrow asymmetric ridges. Dune height decreases and dune length increases as plant densities decrease. Maximum deposition zones occur for each wind velocity range. Whether ramp, terrace or ridge morphologies are formed depends on mode of beach colonisation, plant density and distribution, sand volume, wind speed, and plant species type and morphology.A range of variables which affect foredune morphology and evolution, including plant canopy density, height and distribution, wind velocity, and various ecological environmental processes are examined.