Catchment

BSc International Land and Water Management Wageningen University

Supervisors:
WU Supervisor: Klaas Metselaar
Host supervisor: Sabine Engel

Abstract
Mangrove forest systems and coral reefs are vital ecosystems with ecological, economic, and societal importance. However, they face threats such as excessive sediment inflow from human activities, endangering their health and resilience. This report explores sediment-related issues in the Lagun Bay catchment area on Bonaire and proposes dams as sediment control measures. Potential soil erosion was assessed and dams were designed to reduce sediment transport effectively. However, maintenance is essential for dam functionality. Though not definitive, these dam designs offer potential solutions. Sustainable land and coastal management, coupled with improved data and regular maintenance, can protect these valuable ecosystems and preserve biodiversity. Further research is needed to enhance data quality and expand the study area.

Master Thesis Water Systems and Global Change Group in partial fulfilment of the degree of Master of Science in 2023 master program Climate Studies at Wageningen University, the Netherlands. This master thesis is a contribution in the context of the BO project: Duurzaam regenwaterbeheer BES-eilanden.

Abstract This study examines the impact of climate change and water management choices on infiltration and runoff in the case of the Salinja di Vlijt catchment on Bonaire. Climate change will affect precipitation, temperature will increase, sea level will rise (at least 0.2 meters), and tropical storms will be more intense. This will influence the catchments hydrology – runoff will decrease in base flow and increase in peak flow. The current water management measures – drain pipes, channels, reservoirs, and dams – lead to nuisance flooding and partly are in need of maintenance. Evaluating different water management scenarios, rain barrels and rooftop disconnection systems with water storage can reduce runoff and improve infiltration, while additional infiltration trenches decrease catchment discharge and increase infiltration in specific sub-catchments. Extra reservoirs have limited impact on overall runoff and infiltration. Implementing selected measures can help mitigate the effects of changing urbanization and climate change in the catchment. Further research is required to explore the combined effects of measures and assess spatial feasibility. This study contributes valuable insights for effective water management strategies in the Salinja di Vlijt catchment, and the approach presented can be applied to other catchments.

This research was conducted as part of my bachelor internship for the Tropical Forestry specialization of the forest and nature management studies at Van Hall Larenstein, University of Applied Sciences. The internship was conducted at Mangrove Maniacs, an Bonaire based NGO that mainly works to restore the mangrove forests on the island. Field work was conducted between May and July 2022.

Catchment areas with potential water flows made with ESRI ArcGis hydrology tools using the improved ASTER DEM as an input.

69 watersheds have been identified in the downloadable GIS shapefile.

Read this report for more details.

Smalls islands are especially vulnerable to climate change and land  use changes due to the competing needs for limited resources. To support the NEXUS approach we need evidence based monitoring tools that can provide policy makers, conservation managers, entrepeneurs, scientists and the general public with information on the state, pressures and associated changes in the environment. Satellite imagery can provide synoptic information at appropriate
spatial and temporal resolutions that can support evidence based monitoring. Only at very detailed levels information might be added by using airplanes or drones. Remotely sensed information can help to provide information on e.g. land cover and associated dynamics such as urban sprawl, mapping habitats such as mangroves and coral reefs, surveying terrain conditions such as soil moisture conditions and erosion hazards associated within catchments, sea level rise and changing coastlines, and on many aspects of the vegetation (natural and agriculture), such as plant traits, phenology and plant growth. Remotely sensed information can in general make field surveys and monitoring more effective, and can thoroughly support decision making.

Deterioration of mangrove forests is occurring globally. The loss of mangrove habitats causes a decline in fishery resources, livelihood and biodiversity loss. Deterioration is also been seen in the mangrove forest of Lac Bay on Bonaire. Especially at the landside of the mangroves hyper saline conditions are found and Rhizophora mangles as well as Avicennia germinans trees are dying. The deterioration of the mangroves at the landside is partly compensated by expansion of the mangroves at the sea side. The main causes for the seaward shift seem to be, 1) less fresh water inflow during rainy season, 2) increased sediment transport towards Lac, 3) reduced interaction (tidal flow) between sea side and land side. This project has focussed on the dynamics of the fresh water fluxes toward the mangroves and the associated sediment transport. During two major rain events water discharge and sediment transport was measured for two catchments. Based on these measurements the total runoff amount and amount of sediments transported during the rainy season was determined. It can be concluded that runoff have brought only relatively minor quantities of fresh water to Lac. However, extreme events or extreme annual rainfall sums (1980, 1981, 1985, 1988, 2004, 2005) might have caused extreme runoff amounts and in addition extreme sediment load transports toward the mangrove system. Along with the deterioration of the channels and the associated water circulation, this might have caused root smothering and mangrove die off. This mangrove die off is probably strengthened by a relatively dry period with relatively less freshwater transport by surface runoff toward the mangrove system which enhances the hyper saline waters in the back of the system. Although deterioration of the landscape occurs since the 17th century, due to cutting down forests and overgrazing by goats and sheep’s, heavy rainfall events will not only lead to an increase in water supply but also to more extreme sediment transport rates toward the mangrove system. In addition, also removal of dams would cause an increase in surface runoff towards the mangrove system and an increase in sediment transport. Monitoring rainfall, surface runoff, and sediment transport over a large number of years together with monitoring the growth and the health of the mangrove forest would give more insight in how the mangrove systems reacts on hydrological changes over a number of years.