Conceptual Model of the Effects of Sea Level Rise on Sandy Coasts

TLDR

The Bruun Rule has dominated sea‑level rise studies of sandy coasts, but its assumptions of net sand transfer and neglect of dune budgets and beach‑dune interactions are likely incorrect. The authors propose an alternative two‑dimensional equilibrium model that serves as a more realistic starting point for shoreline response models incorporating alongshore sediment transport and complex coastal morphology. The model incorporates dune sediment budgets and foredune dynamics, and its validity will be tested by evaluating decadal‑scale volume changes associated with sea‑level rise. Unlike the Bruun Model, it predicts no net nearshore sediment transfer and preserves the foredune through landward migration.

Abstract

Over the past 40 years, consideration of the potential effect of sea level rise on sandy coasts has been dominated by the conceptual model proposed in the Bruun Rule, which is used to predict the horizontal translation of the shoreline associated with a given rise in sea level. A review of the hypotheses that form the basis for this two-dimensional model suggests that the assumption of net sand transfer to the nearshore profile and deposition of a thickness of sediment equal to the rise in sea level is probably incorrect. Moreover, the model omits consideration of a significant component of the coastal sediment budget, namely the dune sediment budget, and the processes associated with beach-dune interaction. An alternative conceptual model is developed on the basis of a two-dimensional equilibrium profile similar to that which forms the basis for the Bruun Model. The proposed model incorporates consideration of the dune sediment budget and foredune dynamics. In contrast to the Bruun Model, it predicts no net transfer of sediment to the nearshore profile and preservation of the foredune through landward migration. It is argued that the model proposed here offers a better starting point for developing more realistic models of shoreline response to sea level rise that incorporate consideration of alongshore sediment transfers and more complex coastal morphology and sediment characteristics. Testing of the validity of the model and its potential use for integrated coastal zone management will require consideration of the volume changes associated with sea level rise on a decadal scale.