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



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.