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The Bruun rule, widely used to predict transgression due to sea level rise on decade to century timescales based on a fixed nearshore profile, neglects the influence of inland topography and substrate lithology, leading to physically unreasonable predictions on longer timescales. We use a new approach, the shoreline Exner equation, to model shoreline transgression on wave‐dominated coasts over timescales of decades to millennia. Our results show that interactions between nearshore processes and inland topography, neglected by Bruun‐style models, drive morphologic evolution which modulates shoreline retreat. Analytical solutions suggest that while short‐term shoreline retreat will sometimes follow the Bruun rule, long‐term transgression will always follow the slope of the inland topography. Moreover, our results show that the slope of the inland landscape, relative to the nearshore slope, exerts a first‐order control on coastal morphology, such that steep coasts tend to form cliff‐backed beaches while gentle coasts tend to form barrier island‐lagoon systems. However compositional variations between the inland landscape and nearshore system can alter this pattern.
Wolinsky et al. (Fri,) studied this question.