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

DETERMINING THE CRITICAL VELOCITY OF GRASS SODS FOR WAVE OVERTOPPING BY A GRASS PULLING DEVICE Roel Bijlard, Delft University of Technology, roelbijlard@gmail.com Gosse Jan Steendam, INFRAM International, gosse.jan.steendam@infram.nl Henk Jan Verhagen, Delft University of Technology, h.j.verhagen@tudelft.nl Jentsje van der Meer, Van der Meer Consulting bv, jm@vandermeerconsulting.nl INTRODUCTION There is a shift in the approach for designing coastal structures in the Netherlands, such as dikes or levees. In the past dikes were designed on the probability of exceedance of the water level during specific incoming (wave) storm conditions. In the near future the design criterion will be the probability of flooding of the hinterland. In order to determine this flood probability, the strength of the dike has to be known at which failure occurs. During extreme storm conditions waves will overtop the crest which can lead to erosion of the grass sod on the landward slope. This can finally result in instability of the dike and flooding of the hinterland. Past research focused on the erosion of the grass sod during different wave overtopping conditions, see Steendam 2014. The last few years many tests have been performed with the Wave Overtopping Simulator. During these tests the Cumulative Hydraulic Overload Method has been developed, see Van der Meer 2010 and Steendam 2014. With this method an estimation of the critical velocity of the grass sod has to be made. The critical velocity is a strength parameter for a grass sod on a dike during loads induced by overtopping wave volumes. SOD PULLING TESTS For safety assessments it would be beneficial if there is also an easier way to determine the critical velocity of the grass sod. However, it is important to measure the actual strength of the grass cover, so a visual inspection cannot be satisfactory. The sod pulling test is developed in order to investigate the resistance of the grass cover. It lifts the grass sod perpendicular to the slope out of the sod and measures the force as a function of the deformation. In order to lift the sod, a pull frame is anchored into the top layer with pins. This frame then is lifted out of the grass sod by a hydraulic cylinder. In order to insert the pins into the sod, the soil has to be excavated on two sides (condition 2 test) or on all 4 sides (condition 4 test). This has the disadvantage that the strength of an intact sod cannot be measured directly. So a methodology is developed to estimate the strength of an intact grass sod from the measured data. A further introduction on the sod pulling tests is given in Steendam 2014. The goal is to rewrite the measured forces from the sod pulling test into a critical velocity so that the Cumulative Hydraulic Overload Method can be used for determining the flooding probability of a dike. Some of the locations tested with the wave overtopping simulator have also been tested for the strength of the grass cover with the sod pulling tests. The two methods use the same failure mechanism of the grass, erosion of the grass sod. The top layer of a dike consists of soil and roots growing in multiple directions. The roots anchor the grass into the soil and can deform centimeters without tearing. Pressures acting on the grass cover will first break the weakest roots, but the forces will be redistributed to other roots. Only when a critical amount of roots are broken, the redistribution stops and the grass cover will fail. CONCLUSION It is possible to rewrite the measured forces with the sod pulling tests into a critical grass normal stress (σgrass.c), which is one of the input parameters for determining the critical velocity of a grass sod, see Hoffmans 2012. The equation also uses the pore water pressure (pw), the relative turbulence intensity (r0) and the density of the water (ρ). When the critical velocity resulting from this equation is compared with the determined critical velocity during the wave overtopping simulations, there is good correspondence between the values for the five tested locations. So the sod pulling test could provide results that are reliable enough to determine the critical velocity of a dike section. Further elaboration and scientific background will follow in the paper after the conference. REFERENCES Hoffmans (2012): The influence of turbulence on soil erosion. Eburon, Delft. Steendam, van Hoven, van der Meer, Hoffmans (2014): Wave Overtopping Simulator tests on transitions and obstacles at grass covered slopes of dikes, proc. ICCE 2014 Seoul. Van der Meer, Hardeman, Steendam, Schuttrumpf, Verheij (2010): Flow depths and velocities at crest and inner slope of a dike, in theory and with the Wave Overtopping Simulator, Proc. ICCE 2010, Shanghai.