GIS

Mapping the bathymetry of Bonaire through the use of satellite data

Abstract

Bonaire is home to a wide range of biodiversity, and especially in the shallow coastal waters where coral reefs occur. Coral reefs provide many important ecosystem services and should therefore be protected. However, they are threatened due to many causes like global warming and diseases. Therefore, knowledge about their habitat, the shallow coastal waters, is crucial in order to ensure the conservation of this organism. This knowledge is attainable through the use of satellite data, which is called satellite-derived bathymetry (SDB). The basic principle behind SDB is the relationship between the attenuation of radiance, on one hand, and the depth and wavelength in the water column on the other. This research aims to investigate the possibilities of satellite-derived bathymetry for the island of Bonaire. Furthermore, it explores which method achieves the most accurate bathymetric models and to what extent accurate estimation of bathymetry is possible.  

For this research, the bathymetry was calculated with an empirical approach that makes use of insitu measurements and a ratio between the green and blue band. However, in order to be able to apply this formula, the data first had to be preprocessed. These preprocessing steps included the masking of land/clouds and a sun glint correction. The masking was done through thresholds of reflectance values in the visible bands. The masked images were then deglinted. After deglinting, the data was ready for the calculation of the bathymetry. This was done using two formulas: SDBA and SDBB. The formula of SDBA was calibrated using all in-situ depths less than 30 metres whereas SDBB was calibrated using all depths less than 20 metres.  

The results were validated by determining the Root Mean Square Error (RMSE) for the different depth classes: 1-10m, 10-20m, and 20-30m. However, since this research mainly focuses on shallow coastal waters, especially the 1-10m depth class was interesting. The results showed that the models created with SDBB  were more accurate for depth class 1-10m, compared to the results with SDBA. The average RMSE of depth class 1-10m for the most accurate method was 4.11m. The most accurate bathymetric model had an RMSE of 3.58m for depth class 1-10m. However, the RMSEs for the other depth classes showed that this method is not applicable for accurate results in deeper depth classes.  This research showed that there are possibilities for the island of Bonaire regarding satellite-derived bathymetry. However, more research needs to be done in order to create more accurate results and be able to circumvent limitations.  

Date
2023
Data type
Research report
Theme
Research and monitoring
Report number
WUR MSc Thesis - GIRS-2023 -29
Geographic location
Bonaire

Mapping of the Potential Erosion in the Catchment of Lac Bay

Abstract: Over the past decades coastal ecosystems have been increasingly threatened and have been reducing at alarming rates. Similar is happening on Bonaire, where increased sediment transport and decrease in the freshwater inflow is responsible for high mortality of the mangrove forest in Lac Bay. Factors, such as Bonaire’s arid climate, past deforestation and overgrazing by feral animals have left the island bare which furtherly increases erosion and sediment rich runoff towards the bay. To come up with interventions to reduce mangrove mortality quantitative and qualitative data on the erosion potential and on the rainfall runoff relationship is required. This research provides information on spatial distribution of potential erosion rates in the catchment of Lac Bay, using the RUSLE equation. Moreover, a portable mini rainfall simulator is used to estimate the surface runoff coefficient and to validate RUSLEs potential erosion rates. Unfortunately, no correlation has been found between the measured data and the estimated soil erosion rates. Median annual potential soil loss is 19,3 t ha1 corresponding to annual soil loss of 41678 t. Spatial variation of potential erosion rates is homogeneous, implying catchment wide conservation measures. Measures such as reduced grazing could decrease the potential erosion rates in Lac Bay up to 5%, by increasing vegetation cover in the catchment. Structural measures such as earth dams could furtherly reduce sedimentation at the point of deposition, however before implementation further research needs to be conducted on the impact of such structures on freshwater inflow to the bay. The median runoff coefficient is 0,24 with 76% of the catchment having runoff coefficients between 0,16 and 0,33.

Date
2022
Data type
Research report
Theme
Research and monitoring
Report number
YWU 8081
Geographic location
Bonaire
Author