Assessing Urban Flood Risk with Probabilistic Approaches

Flooding is a serious natural disaster in urban areas. Moreover, the consequences of land use change and rainfall can affect the flood process in urbanised catchments. Fluvial flooding can be seen at downstream locations due to the high and fast discharge from sub-catchments. In addition, pluvial flooding can be seen at settlements are situated on the floodplains by river channel at downstream locations, due to the impermeable surfaces and insufficient drainage capacity. Therefore, the combined pluvial and fluvial flooding can be observed on the floodplains of urban stream basins. Although, flood risk can be severe for these places, research on combined fluvial and pluvial flooding is very rare.
One of these places is Wortley Beck catchment, Leeds, UK. To observe the interaction between fluvial and pluvial flooding, the floods were modelled for different land-use scenarios and rainfall events for an urbanised and ungauged catchment. The inflow hydrographs and rainfall hyetograph were designed by using the ReFH rainfall-runoff method. 1D and 2D hydrodynamic models were used to simulate fluvial and pluvial flooding.
The outcomes were peak flow values and probabilistic inundation maps with maximum water depth values. The peak flow values were used to investigate the relationship of return period between rainfall and flow by using the FEH statistical model. The effects of the land use change and rainfall on the flood risk were observed from the maps. In addition, the flood extent of combined pluvial and fluvial flooding was observed from these maps. Water depth values in the inundation area by combined flooding were computed. Hence, fluvial flooding in combination with pluvial flooding was observed to have a higher flood risk in the urban stream basins. These outcomes can be used to manage flood risk due to land use change in the future for ungauged catchments by National and Local Governments.