Movable-Bed Model Investigation of Groin Notching

Groin notching, the purposeful lowering of a portion of the structure to promote controlled movement of sand alongshore, has been implemented for evaluation along the New Jersey coast by the U.S. Army Corps of Engineers. Notching increases the longshore sand transport through the structure that can reach the downdrift beach while maintaining some local level of protection. Notching is typically more economical than shortening of an existing structure. Because little is known about the functioning of notched groins, a movable-bed physical model study was conducted to examine processes and responses at a single groin installed in a large wave basin. Cases investigated were: a long, impermeable groin; a short groin (as compared to the long groin), and three notched-groin variations as a notch at the breaker line, notch in the mid-surf zone, and notch in the swash zone. The purpose of the short and notched groins was to allow controlled bypassing of sand transported alongshore by waves breaking at an oblique angle. The various groin configurations produced different patterns of the nearshore current and sand transport, inducing different beach and shoreline responses. Groin influence on breaking wave height and sand suspension was limited to the vicinity of the structure. Factors that determined the functioning were the distribution patterns of the longshore and cross-shore sand transport, permeability of the groin, groin length, and location and depth of the notch. A notch located in the swash zone produced the greatest direct benefit to the subaerial beach and was considered the most efficient. It is desirable to allow some passing of the longshore current through the outer surf zone to reduce the development of a rip current. The rip current that tends to persist on the updrift side of a groin transports sand offshore and acts to produce a net loss of beach sand to the offshore region. Onshore transport is necessary for the sand that is transported to the offshore by a rip current to nourish the downdrift beach, and a net onshore-transport condition may not always occur on all coasts.