Mechanisms of berm development and resulting beach growth along a barrier spit complex

Abstract

Weekly topographic profile measurements across a southward migrating recurved‐spit complex throughout a summer period have revealed three different mechanisms of berm development, each reflected by a distinctive sedimentary sequence. Each mechanism dominates berm widening along certain sections of the active spit with transition zones separating each one. Along the straight beach sections where a net longshore transport is well developed, sand accumulates at the distal high‐tide swash mark during neap tide. These sandy accumulations are neap berms which are later redistributed over the main berm by swash occurring at spring high water. The main berm grows vertically and horizontally as a result. To the south, along the middle portion of the recurved spit, swash bars or ridge‐and‐runnel systems actively develop, migrate, and weld onto the established berms. This is the second method of berm widening and results from an excess of sand carried into this portion of the spit due to the steadily decreasing transport of the longshore current system. Berm‐ridges develop along the southernmost portion of the active recurved spit and represent the third and most rapid form of beach progradation. Wide, broad swash bars build nearly up to the spring high tide level. At neap high tide, the swash cannot extend over this feature. Wave energy is expended on the seaward margin of the swash bar initially developing a low‐angle beach face. Rapidly, this beach face steepens and a new berm (beach face and berm top) is developed on top of the swash bar. This berm structure still retains much of its swash bar or ridge appearance, hence the term‘berm‐ridge'. Numerous trenches dug into the beach provide data to model the distribution of primary sedimentary structures in recurved spits. Berm‐ridges are the most important features along rapidly accreting spits, and structures associated with these features are volumetrically the most significant. Berm‐ridges also develop arcuate, vegetated ridges separated by low lying, marsh‐infilled swales. These features are commonly seen within barrier islands and designate former inlets.

Sedimentary structures generated on the foreshore by migrating ridge and runnel systems on microtidal and mesotidal coasts of S. Spain

“…Prograding coasts with a high sediment supply are the favourable sites for the preservation of ridge structures as noted by Van den Berg (1977), Hine (1979) and Beets et al (1981) among other authors. A sufficient supply of sediment and shelter from heavy storms appear to be of prime importan ce for a high preservation potential.…”

“…As shown by Hine (1979), the internal structure of a berm is roade up of a cyclic pattern of horizontal parallellaminations arranged in fining-up sequences which represent the transition froro neap to spring tides. We found that this lateral succession of sequences tends to be overlain by seawardinclined para11el lamination also arranged in fining upward sequences (Fig. …”

“…At low tíde, the ridge is emergent and waves built up in the water ponded landward of the ridge can produce an erosive smoothing of its topo This process is recorded as a low-angle discontinuity surface which separates suceessive bundles oí high-angle íoresets (see also Hine, 1979).…”

“…bedding, eross lamination and low-angle eross bedding whieh may be preserved in accretional coasts (Doeglas, 1954;Wunderlich, 1972;Roep et al, 1975;Van den Berg, 1977;Hine, 1979;Beets et al, 1981). The aim of this paper 1s to dea!…”

Suspended sediment transport and beach dynamics induced by monochromatic conditions, long waves and wave groups

“…The berm formation processes has been previously studied considering the effects of wave height, tidal fluctuations, water level changes induced by storms, sediment fall velocity, grain size or permeability (Bagnold 1940, Hine 1979, Weir et al 2006or Jensen et al 2009). But most of the studies dealing with berm formation, are qualitative studies, based on field experimental data highly affected by the tidal effect on gravel beaches.…”