University of Twente Student Theses


Systematic assessment of erosion mitigation in a data poor environment : a case study of Lac Cai, Bonaire

Groot, Marijn (2022) Systematic assessment of erosion mitigation in a data poor environment : a case study of Lac Cai, Bonaire.

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Abstract: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 The availability of actual and accurate data was limited and consisted mainly of outdated or inaccurate data.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 vegetation-induced 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. Five different alternatives are investigated by model in order to come up with an alternative measures against the coastal retreat at Lac Cai. The alternative consisted of a reference model without any measure, breakwater, groyne, seagrass implementation and mangroves restoration. Based on the sediment balance and erosive trend over 10 years, the groyne was showing the best results. The implementation of a widespread seagrass meadow with a strongly reduced friction coefficient caused for an overestimation of the vegetation impact. This makes it hard to compare the absolute erosion values but based on the trend this can still be considered as an effective mitigation measure. The mangrove reforestation and expansion towards the sea showed a positive sediment balance with an increasing trend in sedimentation close to the land boundary. The breakwater- and reference model both showed decay in the sediment balance. The alternatives were assessed by six expert with diverse expertise related to coastal engineering. The multi criteria analysis consisted of five criteria which were weighted and scored by the experts: Impact on hazard reduction, Cost, Environmental impact, Local acceptability and Local feasibility. The experts unanimously weighted the reduction impact criterion as most important after which the cost and local feasibility. Despite the fact that the modelling results showed that the groyne construction was the most effective in hazard reduction, it can be concluded from the feedback that mangrove reforestation is the best mitigation alternative. This alternative received the highest points in each criterion scored by the experts. The mangrove restoration is considered feasible and expected to be received optimistically by the local stakeholders while the lifecycle cost are not expected larger than other alternatives. These scores should be verified as well as the impact on hazard reduction should be further investigated by improving the morphological model with more accurate data.
Item Type:Essay (Master)
Faculty:ET: Engineering Technology
Subject:56 civil engineering
Programme:Civil Engineering and Management MSc (60026)
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