Reduction of wave overtopping by Vetiver grass

Abstract

In order to protect the area inside the coastal flood defend system, the height of a sea dike should be sufficient. Huge sea dikes with revetment protection which are made from traditional hard material such as block concretes or big rock has given good results. However they are quite costly to implement and materials are not always available. In order to reduce total cost, the traditional revetment could be replaced by cheaper materials. A combination between hard and soft materials is a good alternative solution. In a number of tropical countries Vetiver grass (Vetiveria zizanioides) is well-known as bioengineering species in stabilizing inner slopes, reducing run-off and controlling soil loss. Recently, it has been planted on outer slope as sea dike protection. However the understanding of the processes and properties between waves and Vetiver grasses are still limited. In this research Vetiver grass is investigated as outer slope protection in order to reduce wave overtopping discharges. In consequence sea dike crest can be reduced. Beside it, this research also focuses on addressing the hydraulic effects of flow and Vetiver hedges. Physical model is conducted full-scale of Vetiver grass and wave parameters in front of the hedge. Mature Vetiver grasses are used for testing and wave is simulated by dam-break method. Experimental results have shown that resistance between flow and Vetiver hedge depends on grass density. In this research, Mannings coefficient is used to describe the resistance; it varies with the changing of flow depth. One interested characteristic of Vetiver grasses showed in this experiment is that its ability to withstand flow of backwater which reaches depths up to nearly 0.4m. The reduction of wave overtopping of more than 60% is measured. The roughness coefficient of Vetiver grass that depends on grass density is found to be varying between 0.33 and 0.41. An example with the use of Vetiver grass on a sea dike in Vietnam is worked out, by use of the results from the physical model. This example shows that a reduction of 0.5m of the crest height is feasible for upgrading the present sea dike in Nam Dinh, Vietnam. It corresponds with a reduction of 12,6% of the costs in case of two Vetiver hedges are planted on the outer slope. This case shows that Vetiver grass is a good solution for sea dike in order to reduce wave run-up on the outer slop and decreases the cost for upgrading sea dike.