Watershed delineation for runoff estimations to culverts in the Swedish road network : a comparison between two GIS based hydrological modelling methods and a manually delineated watershed

Abstract: The anticipated climate changes are predicted to alter the precipitation patterns in Sweden. Increased episodes of intense short-duration rainfall and higher intensities of day long-duration rainfall are expected. Road sections are often a target of flooding, causing impassable roads, due to the lack of capacity of drainage systems. One of the factors determining the water flow to a depression is the size of the contributing watershed. In this study, catchments derived from two flow routing methods, the deterministic eight-node algorithm (D8) and the Triangular Form-based Multiple flow algorithm (TFM), are compared to a manually measured watershed, to investigate the runoff to a single culvert. For further comparison of the modelled catchments the elevation of the data is modified into different height intervals. The deviations between the modelled catchments to the manually delineated watershed are noticeable regarding both size and shape. The differences between the modelled watersheds are minor concerning size, shape and location. Based on the six digital elevation models tested in this study, the TFM catchments covered a greater area in comparison to the D8 catchments when the vertical resolution is high, whilst the D8 watersheds cover a greater area when the vertical resolution is low. According to the estimated 50-, 100- and 200-year flows, the investigated culvert is not likely to be flooded.