Effects of Water Table and Tide Cycle on Swash-Backwash Sediment Distribution and Beach Profile Development: ABSTRACT

Abstract

A study of deposition in the swash-backwash zone along a sandy beach indicates that fluctuations in sea-level relative to beach water-table level, resulting from the semi-diurnal tide cycle, show appreciable effects on sediment distribution. In the swash-backwash zone, sediment distribution is dependent on the interaction of swash deposition, swash erosion, backwash deposition and backwash erosion. When the tide level is high and the beach water-table is low, swash deposition and swash erosion predominate; this results in the formation of a thick lens of sediment on the shoreward side of the swash-backwash zone and a scoured area on the surf side of the zone. In contrast, a relatively high water-table results in maximum back-wash erosion and backwash deposition; thick len es of sediment form near the surf boundary. Therefore, as sea-level fluctuates above and below the general water-table level with the tide, the zone of deposition correspondingly shifts its position within the swash-backwash zone and either increases or decreases the gradient of the beach slope. As the tide rises, sediments, deposited by previous swashes are redistributed by the encroaching surf. Above the limit of surf encroachment and in the highest swash-mark area, a berm forms, which displays an onlap-offlap series of End_Page 1759------------------------------ laminae; also, portions of it are cross-laminated. This berm and its internal structure are a result of sedimentation during the flood- to ebb-tide period. End_of_Article - Last_Page 1760------------