Letters

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 seaward direction. Ironically, as actually applied in coastal management, the Bruun rule reduces down to a single noninvolved variable: the slope of the shoreface.

We found¹ 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.

Douglas, Leatherman, and Zhang reply: Our paper investigated how increasing sea level will exacerbate the long-term sandy beach erosion that affects nearly 90% of the US coastline.¹ Storms are commonly blamed for the erosion, but 150 years of US shoreline position data do not support that cause.² US sea level is rising at varying rates, so we investigated the regional sensitivity of beach erosion to sea-level rise.

The Bruun model, which relates sea-level rise to erosion rate, works well in wave tanks and lakes. With its assumption of a depth of closure, the model is also reasonably consistent with regional (not local) East Coast erosion trends. We discovered an erosion sensitivity of about two orders of magnitude greater than the rate of sea-level rise, which indicates that a small amount of sea-level rise has a big effect on sandy beaches. Loss of beach width increasingly exposes fixed structures to flooding and the destructive energy of storm waves.