A New Approach for Generating Flow Directions in Urban Watersheds

Flow directions are important because they may be used to derive flow paths. These parameters are requisite in terrain-based flood mapping methods. Prior algorithms of flow direction generation are based in digital elevation models alone. Implicit in these methods is the assumption that elevation is the sole determinant in flow directions; however, such assumption may not be valid in complex landscapes such as in urban areas, where flow directions may be better described by the two-dimensional shallow water equations. A new approach for generating flow directions is therefore described. In the proposed approach, flow directions were derived using a hyper-resolution, hydrodynamic flood simulation instead of digital elevation models alone. With a city block in Austin, Texas, as a test case, results were compared to the widely used D∞ algorithm. It was concluded that elevation difference still greatly affects flow paths; nevertheless, flow is still a complex phenomenon in an urban landscape. The result therefore supports continued application of terrain-based models in rural areas but suggests that the use of hydrodynamic models is a more robust procedure in generating flow directions and flow paths and therefore deserves consideration for future research.