Determining the critical velocity of grass sods for wave overtopping by a grass pulling device

Abstract

The strength of the grass sod is an important factor for the stability of a dike in the Netherlands during wave overtopping conditions. Many tests have been performed the last few years with the Wave Overtopping Simulator, leading to the Cumulative Overload Method and a critical velocity. This velocity is a strength parameter of grass on a dike under loads induced by overtopping wave volumes. A new method has been developed to determine this critical velocity, by measuring the force while lifting the grass sod perpendicular to the slope out of the sod. This force is rewritten into the critical grass normal stress which is one of the input parameters for determining the critical velocity of a grass sod. When the critical velocity resulting from this method is compared with the determined critical velocities with the Wave Overtopping Simulator, there is good correspondence between the results for the tested locations. Therefore the sod pulling test could provide results that are reliable enough to determine the critical velocity of a dike section.