27-Modeling Urban Flooding from Storm Sewers Using Dynamic Wave Theory.

Abstract

Traditionally, urban storm sewers, meant to convey excess runoff from impervious covers efficiently, are designed with the applications of rational formula of hydrology with all of the conceptual limitations of the method. Typically the system is sized such that the hydraulic grade line remains at or below ground surface and thereby the design means to provide a sense of safety. However, this method does not offer the opportunity to evaluate any potential of overland flooding from inadequacy of the system particularly where surcharge occurs by tail water condition. In contrast, hydraulic models utilizing dynamic wave theory, of newly designed or existing systems offer several advantages in this regard. With these models, the overland flow components can be incorporated and hence the degree of surface flooding can be more reasonably assessed. Second, the time dependence of the downstream boundary condition normal during a storm event can be accounted for. Temporal variations of both rainfall and water surface elevation at the outfall control the volume available in the network elements and hence the hydraulics of the system. Most systems which are designed by rational method when modeled with the applications of dynamic wave theory seem to be prone to produce overland flooding. The primary reason for this is the fact that with rational method the entire volume of the system is assumed to be available for peak design flow. However, during an unsteady state flow condition, from both theoretical and practical standpoint, a significant amount of storage of the network system is unavailable to a peak discharge. A few case studies are presented to emphasize the importance of evaluation of storm sewer systems with dynamic wave modeling to assess and reduce the risk of urban flooding. However, differences in results are obtained when commonly available software are used for such purposes. The differences stem from the variations in the computational algorithm used in these software. This poses problems not only in interpretation of the results but also in applying those in forming opinion about improvements of the systems that appear to be inadequate in mitigation of urban fl ooding.