Estimation of Combined Wave and Storm Surge Overtopping at Earthen Levees

This Coastal and Hydraulics Engineering Technical Note (CHETN) provides empirical equations for estimating several parameters of unsteady flow resulting from the combination of steady storm surge overflow and overtopping of irregular waves at a trapezoidal-shaped earthen levee. Equations are given for the average overtopping discharge and the cumulative probability distribution of instantaneous overtopping discharge. On the landward-side slope, empirical equations can be used to estimate the mean flow depth, mean flow velocity, root-mean-square wave height, and velocity associated with the overtopping wave front. Worked examples illustrate application of the empirical equations. Earthen levees are used extensively in the United States to protect populations and infrastructure from periodic floods and high water due to storm surges. Overtopping of levees and dikes produces last-flowing, turbulent water velocities on the landward-side slope that can damage the protective grass covering and expose the underlying soil to erosion. If overtopping continues long enough, the erosion may eventually result in loss of levee crest elevation and perhaps breaching of the protective structure. Economics often dictate levee designs with crown elevations having a risk that some wave/surge overtopping will occur during extreme events. In addition, crown elevations for older levee systems may have been established without complete information about possible water levels that might occur during extreme events. Even levees that presently have sufficient freeboard to withstand all but the most extreme storms may become vulnerable to wave overtopping and storm surge overflow in the future if sea level continues to rise at projected rates. The cost and engineering challenges of raising all levees to elevations where overtopping will be within tolerable limits may be insurmountable.