EROSION AND OVERTOPPING OF A GRASS DIKE LARGE SCALE MODEL TESTS

A comprehensive test series at full-scale was performed on a grass dike to obtain quantitative information on the wave loading, erosion rate, residual strength and overtopping under design storm conditions. The purpose was to establish design and assessment criteria for this type of dike, which exists in many places in The Netherlands. The particular dike tested exhibited, in general, a considerable resistance to erosion. However, such a non-homogeneous surface is subject to weak spots which can erode very quickly. The most critical region, where the wave loading is the highest, has been identified. The residual strength of the underlying layer of clay is defined as the time it takes for erosion to progress through the entire layer and expose the sand core of the dike. For the dike tested, the residual strength of the 0.8 m thick clay layer is estimated to be 10 hours under design conditions. Measurements of the average overtopping rate and the volumes per overtopping wave were compared to existing formulae, developed from the results of small-scale tests. The comparison was favourable, indicating that scale effects are not substantial for these quantities. INTRODUCTION Along many rivers and some sections of sea coast in The Netherlands a grass dike protects the low-lying polder land. Interest in grass dikes is growing, as a result of increasing environmental, economic and aesthetic concerns. In spite of this, no method was available to assess the strength of a grass layer for design or repair 1) DELFT HYDRAULICS, P.O. Box 152, 8300 AD Emmeloord, The Netherlands 2) Dutch Ministry of Public Works, Road and Hydraulic Engineering Division, P.O. Box 5044, 2600 GA Delft, The Netherlands 2639 2640 COASTAL ENGINEERING 1994 purposes. In recent years the strength characteristics of a grass dike have been systematically studied in order to answer the following questions; Is the strength mostly determined by the grass itself, the root system or the underlying soil? How does the grass layer behave during long-term wave and current loading and how long does it take before the grass layer can be considered to have failed? What is the residual strength of the dike, ie. how long does it take, after failure of the grass layer, until the sand core of the dike becomes exposed? In 1992 a large-scale testing programme was created in order to answer these questions. By also measuring the overtopping rate, these tests then also provided the opportunity to compare recently derived overtopping formulae with full scale measurements. The aim of the study was: 1. to study the behaviour of the water motion (velocity, pressure distribution, layer thickness and runup) over the grass slope. 2. to measure the erosion resistance of the grass layer. 3. to measure the average overtopping rate and the overtopping volume per wave, for a wide range of wave and water level conditions. 4. to measure the residual strength of the underlying layer of clay. The project was commissioned by the road and hydraulic engineering division of the Dutch Ministry of Public Works and was performed by DELFT HYDRAULICS, Delft Geotechnics and the Agricultural University of Wageningen. The tests were performed in de Delta Flume of DELFT HYDRAULICS. This flume is 250 m long, 5 m wide, 7 m deep. Waves can be generated up to 3 m in height. With this facility waves occurring under design storm conditions for the north coast of Holland could therefore be generated.