Questioning the Rules in Coastal Erosion

We take issue with the Physics Today article (February 2004, page 24 ) that praises the work of Keqi Zhang, Bruce Douglas, and Stephen Leatherman in documenting and promoting use of the Bruun rule to predict the impact of sea-level rise on shoreline erosion. We contend that the rule, a simple mathematical model, ¹ has no basis in geologic or oceanographic reality but survives because of its simplicity, the lack of another approach, and a religious-like belief in the concept.

The Bruun rule doesn’t work in the context of our modern understanding of shoreface processes. For example, many shorefaces—the dynamic zones between the continental shelf and the beach—are not simply surfaces of sand but rather are underlain by rock or mud. In addition, sand-transporting bottom currents of many kinds occur on shorefaces and these are not considered in the model.

A particular absurdity of the rule is the assumption of a “sediment fence,” called closure depth, at the base of the shoreface; beyond that depth, significant amounts of sand are assumed not to flow in a sea-ward direction. Ironically, as actually applied in coastal management, the Bruun rule reduces down to a single noninvolved variable: the slope of the shoreface.

We found ¹ that the rule was being applied in at least 26 countries on six continents as a coastal management tool that adds a meaningless element to an already highly politicized process. The Physics Today article does coastal management a major disservice by reporting favorably on a rule that doesn’t work.