A common coastal engineering problem is erosion due to increasing long shore sediment transport in the longshore direction. In such a situation, more sediment leaves an area on the downdrift side (i.e. in the direction of the long shore current), than enters the area on the updrift side.

A possible solution to this coastal erosion problem is to build a structure which diminishes the sediment transport. If the diminished transport rate out of the area equals the input rate then erosion is stopped. Such a construction might be a series of groynes. The groynes extend into the surf zone and hence inhibit long shore
sediment transport. However, protection of the area with groynes might move the erosion problem elsewhere. Downdrift of the protected zone the erosion is likely to increase, since the artificially diminished transport causes reduced sediment supply to the downdrift zone (see Fig. 63 through Fig. 65).