The deposition of sand on the updrift side of a groin and the subsequent (and antisymmetric) erosion of sand on the downdrift side of a groin can be determined analytically from a one-line contour model.

The magenta region is the shoreline viewed from above. The vertical black line denotes the groin and the horizontal black line in the center is the original shoreline position.

The variable width denotes the width of the figure, and time indicates the elapsed time in days. The Q shows the area accreted (or eroded) with time. To determine the impounded sand volume, multiply this amount by (B+h*), where B is the berm height of the beach and h* is the depth of closure. Finally, xm is the distance from the groin to a location where the shoreline displacement (at bypassing) is one hundredth the length of the groin. This is a measure of the alongshore influence of the groin.

The animation stops when the groin begins to bypass sand.

Comments: Robert Dalrymple