Shoreline Reaction to an Impermeable Groin,

Examination of data shows that the experimental groin affected the shoreline for 2,300 ft upcoast predominantly updrift side analyzing the dimensions of the groin, the effective length was 2,040 ft. This generally compares with the rule-of-thumb expressed in the Shore Protection Manual SPM 1973, i.e., the shore-line length affected by a groin should equal 2 or 3 times the groin length from berm crest to the seaward end. The volume of impounded material trapped by the experimental groin between Nov 70-Oct 71 was 244,610 cu yds the calculated volume of material trapped by the groin was 285,000 cu yds. This generally compares with the rule-of-thumb in SPM The relationship of 1 sq ft change in beach surface area equals 1 cu yd of beach material. It has been shown that accretion is not confined to the area shoreward of the groins end. Immediately after construction, most of the available material was impounded within the limits of the groin, but as the system filled the accretion extended offshore. This accretion influenced waves breaking further offshore thereby reducing wave energy reaching the foreshore resulting in a stable foreshore topography.