Mechanisms of Sand Loss from an Estuarine Groin System Following an Artificial Sand Fill

摘要

Following the placement of 410,590 cu m of sand fill in a groin system at Willoughby Spit, Virginia, in August and September, 1984, the volume of fill retained in a test compartment was observed to decrease slowly with time as a result of sediment movement in two shorewise zones designated herein as Belt 1 (nearshore and dominated by wave and swash action) and Belt 2 (beyond the groin ends and dominated by action of strongly asymmetrical tidal currents). In the first belt, sediment is moved primarily within a compartment along the shoreline when water levels are low, i.e., below groin tops, or from compartment to compartment by groin overwashing when the water level is sufficiently elevated. Findings of the present study show that the intensity of sediment transport decreases with elapsed time after the fill as conditions of static equilibrium are slowly approached. An equation for post-fill shoreline history was developed based on this fall-off in sediment transport intensity and was constructed so as to include the main mechanism of sand loss. If T-head groins were to be installed at the seaward end of each groin, the effective life of a new fill would be increased by reducing offshore losses of sand by inhibiting the action of rip currents along groin walls.