Wave overtopping simulator on a 1/15 slope protected by two local grass species

Abstract

The phenomenon of wave overtopping during storms was simulated by the Wave Overtopping Simulator on a 1/15 grass covered slope. The four 'Wave Overtopping Simulator' tests were done within the framework of the Research project 'Super sea dike with high safety level and environmental friendly' funded by Viet Nam government. The main objective of these tests was to test the resistance of the gentle slope (steepness of 1/15) protected with local grass against wave overtopping with the Wave Overtopping Simulator. The super dike model was built up at the outdoor laboratory of the Viet Nam academy for water resources, in Yen Binh ward, Thach That district, Ha Noi. In June 2012, the super dike slope covered with one year grass mat was tested with the simulator. The front velocity of overtopping flows were estimated at three positions along the slope using digital camcorders. In general, increasing volume generates higher front velocity and it is likely to become larger downward the slope. Under attack of overtopping flows generated by the simulator, the super dike slope covered with Bermuda and Carpet grass was damaged moderately. Damage was in the form of a shallow erosion which took place around the dike crest and toe (geometric transitions). The mechanism is defined as 'roll up' or 'turf set-off', the grass mat of about 5 to 7 cm thick is lifted and rolled up gradually to expose the underneath soil body. Small damaged spots were observed around two obstacles of YB1 and YB4 sections therefore, influence of these objects on slope performance was unclear. Man-made eroded spots were introduced at two positions on YB3 slope by removing the grass turf which was about 10 cm thick of the top layer including most of roots. Applying discharges of 40, 60, 80 and 100 l/s per m with a wave height of 2.0 m caused a gradual extension of the artificial damage downward while depths and widths remained the same. Apparently, the destructive impact of overtopping flow is most effective in its direction, from crest to toe. The super dike slope was 1/15 of inclination and protected by two local grass species, Bermuda and Carpet. When being tested, the grass mats were about one year old. The maximum discharge of the simulator 100 l/s per m was applied for 6 to 10 hours, thus resulting in moderate damage to the grass cover. These damaged spots were limited within 7 to 10 cm under the slope surface and insufficiently significant that might threaten the function of the super dike. The simulator was designed to test the normal sea (river) dike slopes with a steepness of 1/3 to 1/6 under a mild wave condition, significant wave heights not above 2.0 m. Therefore, to assess the super dike slope which is unbreachable even in extreme condition, a device with much higher capacity is clearly required.