Exploring spatial relationships between stream channel features, water depths and flow velocities during flash floods using HEC-GeoRAS and Geographic Information Systems

Water depths and flow velocities decisively influence the damage caused by flash floods. Geographic Information System (GIS) is a powerful and useful tool, allowing the spatial analysis of results obtained by hydraulic modelling, namely from the HEC-RAS/HEC-GeoRAS software. The GIS spatial analysis performed in this study seeks to explain and quantify the spatial relationships between the stream channel features and flow components during flash flood events. Despite these relationships are generically known, there are few studies exploring this subject in different geographic contexts. A 1D hydraulic model was applied in a small watershed in Portugal, providing good results in the definition of floodable areas, water depths and longitudinal velocities. No direct relationship was found between water depths and velocities in the floodable areas; however, negative strong correlations were found between the two flow components along the stream centerlines. Bed slope, channel and flood width, and roughness prove to be highly relevant on the longitudinal variations of water depths and velocities and on the location of maximum values. Increasing peak discharges and return periods ( R T ) can change the relationships between water depths and velocities at the same location. Results can be improved with more accurate elevation data for stream channels and floodplains.