Mapping runoff and erosion to reduce urban flooding and sediment flow towards sea. A case study for the Playa catchment, Bonaire

Past deforestation, overgrazing and urbanization have led to an increase in surface runoff and erosion in the Playa catchment, Bonaire. Together with the lack of sufficient spatial planning, this has led to increased urban flooding and larger sediment flows into the ocean causing harm to the island’s famous coral reefs. For this research, an event-based, spatially-distributed erosion and runoff model (Kineros2) was used to map and quantify the sources and routing of this runoff and sediment. The effect of several physical factors such as rainfall intensity, rainfall duration, initial soil moisture content and vegetation cover were tested to understand their effects on runoff and erosion. Furthermore, three different management scenarios were simulated: decreasing the amount of pavement, reducing the number of reservoirs and reducing the grazing pressure.

The results show that the highest rates of soil loss are found in the uplands. However due to the high sediment trapping efficiency of the many reservoirs, most of this upland sediment is trapped and therefore does not reach the sea. Most surface runoff is produced in central Kralendijk. This is also what leads to flooding as the capacity of the drainage system in central Kralendijk is too small to effectively drain the area. Large intensity rainfall events (usually occurring in October) are most problematic.

Reducing the grazing pressure and therefore increasing the vegetation cover was found to be effective in decreasing the rate of soil loss. This however did not translate into significantly lower sediment yields at the outlets – which was much more influenced by the loss of agricultural reservoirs. Reducing the amount of pavement in central Kralendijk did not have a significant effect in reducing runoff – flooding can therefore only be effectively tackled by increasing the capacity of the drainage network.