Urban Runoff Simulation Using HEC-RAS and the Modified Rational Method

Abstract In this paper, we deal with simulation of surface rain-runoff in urban areas, in particular densely built ones, where impermeable surfaces prevail and prevent rainwater infiltration to the ground. Our aim is to check whether simple computational methods and freely available computational tools can adequately describe rain-runoff along streets. The basic idea is that streets can be simulated as open channels and adjacent neighbourhoods as hydrological basins, feeding them. Then, the Modified Rational method can be combined with HEC-RAS to calculate water depths at selected street cross-sections. To test the procedure, we have selected an area of the historical centre of Thessaloniki, Greece, which is densely built. Two streets have been considered as the main open channels. Using detailed topographical maps and findings of in situ survey, we delineated the respective drainage areas. Special care was needed in street junctions, to decide on inflows and outflows. Moreover, to highlight the importance of using low environmental impact techniques in the urban fabric, application of rain gardens in suitable locations was proposed, with the aim of reducing the runoff volume and, consequently, local inundation problems during the peaks of extreme rain events. Finally, we have compared the calculated water depths with those obtained using the Manning formula for open channel flows. Our results show that the free hydraulic model HEC-RAS, although developed for simulation of flows in natural watercourses, can be used successfully in the simulation of stormwater runoff at the urban street scale and for planning local flood alleviation measures.