RAINFALL-RUNOFF MODELLING TO ASSESS FLOOD HAZARD ON SINT EUSTATIUS, CARIBBEAN NETHERLANDS
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The Caribbean island of Sint Eustatius experiences regular flooding during intense rainstorms. Among the negative effects are large sediment deposits and deep ponds on roads, as well as erosion on unpaved roads and in gullies. To study the causes of runoff generation, rainfall-runoff models were run in the raster-based rainfall-runoff model “LISEM”. Topsoil infiltration rates and observations of the local geology and geomorphology were obtained through fieldwork. Design storms for the model were based on annual extreme rainfall events. Furthermore, a new digital terrain model (DTM) was created, which provided sufficient detail to model runoff through urban areas. By comparing the results of using different resolution DTMs, the effects of the resolution on the performance of the model could be assessed. For coarser resolutions, the flow would spread across a wider area. This caused an overestimation of infiltration, and an underestimation of flow velocities. In addition, the coarser resolution DTMs were less capable of modelling the small-scale topography. Especially in urban areas, this allowed water to run off pavements onto permeable soils, thus again overestimating infiltration. In the urban area, runoff was primarily generated on roads and other paved surfaces, and to a lesser extent on compacted unpaved roads. To control erosion, runoff on unpaved roads should be limited as much as possible. This can be achieved by confining runoff to paved roads, and by creating infiltration pits next to unpaved roads. Regularly spaced drains in paved roads can reduce peak discharges on the road, and prevent flooding of downslope areas. Collected runoff can be stored in large infiltration basins or temporary storage facilities. Flooding on the main road was mostly facilitated by elevated roadsides, which prevented water from running off to the sides. Ponding in these areas can be easily prevented by installing drains or breaks in the roadsides. Active gullies were mainly fed by runoff from an impermeable rock outcrop, but peak and total discharges were also greatly affected by the presence of impermeable rock layers below a shallow topsoil, as this limited the amount of water that could infiltrate. Issues occur where gullies flow directly onto the main road. This can be resolved by installing culverts below the road, or by redirecting gullies towards an existing culvert.
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