Narrow Results

Open coast storm surge water levels consist of a wind shear forcing component generally referred to as a wind setup; a wave setup component caused by wind induced waves transferring momentum to the water column; an atmospheric pressure head component due to the atmospheric pressure deficit over the spatial extent of the storm system; a Coriolis forced component due to the effects of the rotation of the earth acting on the wind driven alongshore current at the coast; and, if astronomical tides are present, an astronomical tide component (although the tide is not really a direct part of the meteorological driven component of storm surge). Typically, the most important component of a storm surge is the wind setup component, especially on the East Coast of the US and in the Gulf of Mexico. The importance of bathymetry to this wind setup storm surge component is considered herein with special reference to the coastline of Florida where eight Florida transects consisting of a cross-section of bathymetric data perpendicular to the shoreline were investigated. Effects of Coriolis, wave setup, atmospheric pressure head, and astronomical tide are not considered herein but will be addressed in future papers. The present study findings show that the wind setup component can vary over an order of magnitude for the same wind speed depending on the bathymetry leading up to the coast.

A simple empirical model is proposed for predicting extreme wave run-up on natural beaches during severe wave events (deep water wave heights H₀ ≳8 m or return periods of about 50 years). The new model departs from traditional approaches that use the slope of the beach face β_f and the Iribarren number ξ₀ as parameters for predicting run-up and instead uses the distance offshore x_h to water depth h to estimate a near-shore profile slope as S = h/x_h, where the depth of closure is the proposed choice for h. Extreme run-up R_x is then expressed in terms of S as R_x/H₀ = CS^2/3. Observations from recent severe storm events in South Africa are used to estimate the dimensionless coefficient C≃7.5. The data are also compared with those of Holman [1986] and the results verify his regression equations and confirm they are valid for significant wave heights extending to 8.5 m for beach-face slopes around 0.1. The run-up predictions of Holman [1986], Nielsen and Hanslow [1991] and Stockdon et al. [2006] are compared to those of the proposed new model. The results suggest that the new model reduces the uncertainties in predicting wave run-up on natural beaches compared with previous models, and thus enables improved estimates of extreme wave run-up and the upper limit of beach change for coastal planning and management.