Assessment of Flash Flood Hazard Maps Using Different Threshold Values and Indices Methods
Abstract: This paper presents an integrated approach for preparation of flash flood hazard maps using different threshold values and indices methods. The methods are based on the influence of the main physical-geographical factors on the rainfall-runoff processes.
The approach utilizes the ROFFG threshold runoff values for the small sub-basins configured within the Romanian Flash Flood Guidance System (ROFFG), together with robust runoff coefficient estimates for selected rainfall scenarios. Calculation of Flash Flood Potential employs a dimensionless index based on several geographical factors determined in GIS (raster format at 30 meters cell size) that influence the surface runoff. The index and general rainfall-runoff analysis in representative gauged sub-basins (area < 200 km2) are used for the general validation of the results of the ROFFG threshold runoff method. The results of the ROFFG method highlight the existence of a high hazard caused by flash floods in 2401 basins, which cover about 61754 km2 (25% of the total area of Romania). The Flash Flood Potential Index (FFPI) method highlights too high and very high values of FFPI in 2805 small basins covering an area of about 80000 km2 (approximately 33% of the total area of Romanian area). Both methods indicate that the highest flash floods hazards occur in the mountain and hilly areas.