A METHODOLOGY FOR MAPPING AREAS UNDER TORRENTIAL FLOOD RISK: CASE STUDY - THE REBROVAČKI BROOK BASIN / BANJA LUKA MUNICIPALITY (B&H)

Torrential floods are one of the most common and destructive natural hazards that cause great material damage and numerous environmental problems. This article is focused on presentation of a new methodological approach to mapping torrential flood risk that could potentially be applied to other torrential rivers in Bosnia and Herzegovina. A hybrid digital terrain model was developed using geodetic surveys of cross-sections of river channels and river valleys and the existing digital terrain model. This was a basis for hydraulic modelling in HEC-RAS software. Hydrological modelling determined flood flows of 100 and 500-year return periods, ranging from 23m3/s to 88m3/s. Hydraulic modelling determined depths, velocities and extent of flooding for different return periods, values of erosion and sediment accumulation, and shear stresses along the Rebrovački Brook. Maximum velocities of 100-year flood flows range up to 11.07m/s and up to 12.41m/s for 500-year flood flow. Depth of 100-year flood flows range up to 3.96m and 4.67m for 500-year flood flow. For 100-year return period the flood extent is 34.53ha, while for 500-year return period it is 46.11ha. Shear stresses of the Rebrovački Brook for 100-year flood flows range to a maximum of 3,482N/m2, while they are maximum 5,348N/m2 for 500-year return period. Maximum values of erosion and sediment accumulation of 100-year flood flow for the Rebrovački Brook are 0.59m for erosion and 1.50m for sediment accumulation, while for 500-year return period they are 3.50m for erosion and 1.66m for sediment accumulation. During the preparation of the flood hazard maps, a new formula was proposed which determined the flood hazard rating of the Rebrovački Brook for 100 and 500-year floods. The resulting maximum hazard ratings of 68.90 for 100-year flood and 69.28 for 500-year flood are several times higher than the values obtained on alluvial rivers. The flood risk factors for the categories of population and economy in the Rebrovački Brook basin were determined on the basis of the obtained flood hazard rating and corrected weighted factors for the calculation of flood risk. Risk maps for floods of different return periods of the Rebrovački Brook showed that most of the floodplain is currently in a zone of negligible or low risk. However, given the current trend of city expansion and relocation of economic activities from the centre to the suburbs, this is slowly changing to high and extreme risk. The proposed methodology for developing torrential flood hazard and risk map, tested in several other basins in Bosnia and Herzegovina, gave acceptable outcomes according to the validation results, and also provides a good basis for numerous studies and projects in flood risk management, environmental protection, spatial planning, and other areas of human action.