Systematic assessment of erosion mitigation in a data poor environment. A case study of Lac Cai, Bonaire
Lac Cai is one of the lagoons in the Dutch Caribbean with a high environmental value which attracts a lot of tourism to the area. Lac Cai is included in the Bonaire National Marine Park (STINAPA) and in order to preserve the ecological variety inside the lagoon, the allowed recreational purposed are delineated in a zonation plan. Cai Beach is one of only two beaches around the lagoon destined for general beach recreational purposes and therefor an attractive location for visitors. Cai is however only accessible over a small road which is endangered by coastal erosion. In an attempt to mitigate the erosion in front of the entrance road, a rock formation parallel to the coast was built in 2019 to serve as breakwater. Unfortunately, this construction did not have the desired effect and erosion continues. The objective of this research is to detect the driving processes causing this erosion and to find a fitting solution by modelling alternative measures in Delft3D and systematically assess these alternatives with the use of a Multi Criteria Analysis.
The availability of actual and accurate data was limited and consisted mainly of outdated or inaccurate data. The coastal evolution was analysed with the use Google Earth images between the years 2002 and 2019 which showed a coastal retreat of approximately 30 meters. With the use of DelftDashboard, a SWAN model was setup with global bathymetry data from the Gebco8 dataset and interpolated with the measured bathymetry data from inside the bay. The wave input is retrieved from the global offshore waveseries of ERA5 which is schematized into a wave climate with the Energy Flux Method. The results of the SWAN model showed that incoming waves enter the shoreline in an oblique angle from the North-East which creates wave-induced longshore sediment transport. Furthermore it was encountered that the adjacent profiles located north of the study area were composed of unerodable reefal limestone terraces which restricts the amount of sediment supply along the coast. Also, the analysis of old aerial photos showed a degradation of the mangrove fringe in front of the entrance road to Cai beach which is considered to have a correlation with the coastal retreat.
Delft3D is used for the setup of a FLOW-model in order to investigate the correlation between mangrove loss and erosion and analyse the long-term morphodynamics. The development of the model is done by a sensitivity analysis and initial simulations are carried out to develop a more realistic bathymetry, starting with an equilibrium coastline profile based on the Dean’s method.
The correlation between mangrove deforestation and erosion is analysed by simulating the current mangrove fringe, the historic mangrove fringe and a situation without mangroves. These scenarios are simulated with the use of three different vegetation modelling methods: Vegetation-induced bed roughness, Trachytopes and Digital Point Model (DPM). The impact of the different methods could not clearly be distinguished but the model results did show less coastal erosion in situation with present mangrove fringes. The first method showed the most impact while also, the vegetationinduced bed roughness did not overestimate the impact of the vegetation. Therefore it seemed to be the most appropriate method for the purposes of the subsequent alternative measurement study.