Backwash-and-swash-oriented current crescents: indicators of beach slope, current direction and environment

Abstract

Backwash-and-swash-oriented current crescents, which are confined to the swash zone of beach environments, are reported, with their detailed morphology and formation mechanism, from the siliciclastic fine sandy beaches of Sagar Island and Lower Long Sand, two tidal islands of the Ganga estuary, northeast India.Morphologically, the swash-oriented arms of these current crescents are shorter and less prominent than the backwash-oriented arms. The shorter arms open landward whereas the longer arms open seaward. They generally form around obstacles like armoured mud balls, polychaete tubes, tussocks etc. which are partially set in a scour implying greater anchoring or a semipermanent nature. These current crescents occur on fine to very find sandy beaches (Mz = 2.9 to 3.1 Φ) with a beach slopes of 1:50 to 1:90 (1.4-0.7°). They are very commonly associated with current lineations. The arms of crescents parallel current lineations and are at right angles to the shoreline.In contrast, ordinary backwash-oriented current crescents form on steeper beach slopes, 1:9 to 1:25 (6−3°), in fine to medium sand(Mz = 1.8 to 2 Φ). They occur around tiny obstacles which are easily swept away by wave swash and backwash.