Flash flood spatial analysis using hydraulic modelling and Geographic Information Systems

The relationships between flow components and stream channel features during flash floods are theoretically well known and proven under controlled environments but are rarely explored and quantified for case studies in different geographic contexts. This research is focused on the spatial relationships between water depths and flow velocities for several return periods (R_T), and how these are influenced by stream channel features. Spatial analyses were performed in Geographic Information Systems (GIS) using the hydraulic modelling results (HEC-RAS/HEC-GeoRAS) in a section of a small watershed in Portugal, which is frequently affected by flash floods. This section comprises about 1000 meters of the main watercourse (Barcarena stream) and the last 350 meters of one of its main tributaries (Massamá stream).

The relationships between water depths and flow velocities are not particularly evident in the floodable areas, although correlation coefficients increase with increasing return periods (0.39 for 5-year R_T; 0.50 for 100-year R_T). Water depths tend to grow with increasing flow velocities and vice versa. Nevertheless, this trend changes when high values of water depth or velocity are reached, preventing higher correlations. This inversion is explained by modifications in channel geometry, morphology or slope, the presence of confluences and obstacles, and flood width/overbank flooding. Unlike what happens with the entire floodable area, strong negative correlations between water depths and flow velocities were found along the stream centrelines. Correlation coefficients of -0.78 and -0.83 (2-year R_T), and -0.66 and -0.87 (100-year R_T) were determined for the Barcarena and Massamá streams, respectively. The more direct relationship in the tributary can be explained by the narrower channel when compared to the main watercourse and by the limited overbank flooding. Bed slope, channel and flood width, and roughness are highly relevant on the longitudinal variations of water depths and velocities and on the location of their maximum values. The relationships between water depths and velocities can also change in result of increasing peak discharges and return periods.

The 1D hydraulic model provided good results in the definition of floodable areas, water depths and longitudinal flow velocities. Lateral velocities are correctly represented in straight sections or in mildly curved bends, which are present in most of the study area, but there are errors in the sharp bends and at the confluence of the Barcarena and Massamá streams. The lack of hydrometric data compromises the calibration and validation of the velocity results. The non-existence of LiDAR elevation data in the floodplains and the lack of elevation data along the stream channels compromise the quality of the DSM. However, it was possible to overcome the lack of elevation data along the stream channels by including the position of the thalwegs in the DSM through the Topo to Raster tool of ArcMap. This guarantees the transversal and longitudinal variations of elevation, improving the hydrologic modelling results in areas with scarce or no elevation data along the channels. The obtained results demonstrate the usefulness of GIS to represent hydraulic modelling results and perform spatial analysis for flood events and other natural hazards.