A physical model simulation of combination of a geo-bag dike and mangrove vegetation as a natural coastal protection system for the Indonesian shoreline

Abstract

To mitigate coastal erosion, coastal regions worldwide focus on sustainable coastal development practices. To this end, many countries use natural protective methods, including the use of vegetation such as mangroves. However, this practice is challenging. For example, mangrove seedlings are often damaged by sea waves before they can grow strong enough to protect coastal lines, and hence they require protection for at least two years after planting. In this study, a coastal protection system combining a main natural structure and a temporary artificial structure is proposed. The effectiveness of this method is related to the deep-water steepness of the incoming waves and the transmitted wave parameters. The transmitted waves consist of two parts: waves transmitted after the water has passed through the artificial system, and those transmitted after it has passed through the mangroves. The first part affects the juvenile mangroves, whereas the latter affects the shoreline. To identify natural coastal protection methods applicable to Indonesia, wave steepness data specific to Indonesian waters were derived using data from an online open database. After weighing the alternatives for temporary human-made structures, the geo-bag dike was chosen. The effects of various geo-bag-dike configurations and geo-bag unit weights on wave height reduction were quantified. Laboratory experiments were conducted on a narrow wave flume using a mangrove model as the main structure and geo-textile-geo-bag models as temporary artificial structures. Based on the dimensionless wave steepness data, the wave conditions of Indonesian waters were produced in the laboratory. This study determined the most effective geo-bag-dike configuration to reduce the wave height, which affects juvenile mangroves, and to decrease the height of transmitted waves after they have passed through mangroves, which cause shoreline erosion. The results reveal that the effectiveness of a temporary geo-bag-dike structure significantly depends on the incoming wave height and the dimensionless wave steepness. The steeper the waves are, the greater the reduction in the transmitted wave height. The results also showed that the reduction in wave height due to the presence of a geo-bag-dike structure varied between 2% and 65% in front of the juvenile mangroves and between 8% and 71% after the waves passed through them. The most effective geo-bag-dike configuration uses relatively heavy geo-bag units with a moderate dike slope of 1:2.0. The reduction significantly depended on the wave parameters, slope, and unit weight.