REVISITING THE BRUUN RULE

The Bruun Rule (BR) (Bruun, 1954, 1962, 1982 and others) allows calculation of beach response to relative sea level rise which would otherwise be impossible. However, even though the BR was developed to account for shoreline response to sea level rise, it does not account for landward sediment transport such as occurs during major storm events. Landward sediment transport and deposition is Nature’s way of maintaining land elevation relative to rising sea level and is accomplished dominantly by overwash and Aeolian processes. Although landward transport is taken into consideration in the modified “Barrier Island BR” as presented by Dean and Maurmeyer (1983), it has not been discussed for the more general case of a mainland or barrier shoreline which may have landward transport limited to a specified cross-shore distance as contrasted to the barrier case in which the entire island translates upward and toward the bay or lagoon without change of form.

The hypothesis examined herein is that landward sediment transport in the nearshore plays a substantial role in long-term nearshore response. Evidence supporting this hypothesis includes the aforementioned washover deposits, the de Beaumont theory (1845) of barrier island formation, general lack of nearshore sediment deposits as predicted by the BR and sediment sorting across the nearshore.

Generalization of the original BR to account for landward sediment transport in the nearshore and on the upland results in the same form of relationship as the original BR except now the characteristics of the landward transport are required to determine the shoreline recession. Thus the implications of the generalized BR to account for landward transport are that in order to better quantify the nearshore long-term response to sea level rise, the characteristics of overwash during storms and long-term Aeolian transport must be much better understood.