An analysis of the downward-flushing flow on the crest of a levee under combined wave and surge overtopping

Abstract

In extreme storm conditions, a levee would be subject to a combination of wave overtopping and surge overflow, namely combined wave and surge overtopping, which is easier to induce catastrophic levee failures than common wave overtopping. One of the significant differences between combined wave and surge overtopping and wave overtopping is that downward-flushing flows are formed on the levee crest during combined wave and surge overtopping due to plunging breakers. Downward-flushing flow might induce lining damage and thus lead to the failure of levees. In this paper, the downward-flushing flow on the levee crest are investigated based on the measurements of full-scale flume tests on combined wave and surge overtopping. Firstly, the occurrence probabilities and velocity magnitudes of downward-flushing flow are analyzed on the basis of the time series of instantaneous two-dimensional flow velocity measured on the levee crest. A general portray of the downward-flushing flow is given based on the analysis. Furthermore, the individual downward-flushing flow processes induced by individual waves are distinguished from the time series of flow velocity and studied in different flow directions. Empirical formulae are proposed to estimate the occurrence probabilities, mean velocities and velocity distributions of individual downward-flushing flow processes. The results can provide references for laboratory and field tests on levee linings under extreme overtopping conditions.