The Urban Drainage Network and its Control on Extreme Floods

In urban watersheds, where first order streams have been truncated and replaced by subsurface storm drain networks and vegetation has been replaced by impervious surface, overland travel velocities and runoff volumes are much greater than in predevelopment conditions. Management facilities, such as detention and retention basins, are used to mitigate the adverse impacts of urbanization, including increasing flood peaks and runoff volumes. We present analyses of hydrologic response in Dead Run, a 14.3 sq. km. watershed in the Baltimore metropolitan area, based on a detailed, digital reconstruction of the storm drain network, hydrologic observations in the basin and a hydrologic/hydraulic modeling system. Analyses center on a storm which produced record flooding in Dead Run on 7 July 2004. Model analyses directly incorporate the hydrologic and hydraulic controls of the drainage network (both the surface drainage network and the storm drain system) and those of detention basins. Analyses utilize rainfall observations derived from weather radar and a network of 19 rain gages, as well as stream gaging observations (at 1 - 5 minute time interval) from 6 stations. The effectiveness of the storm drain network to quickly and efficiently remove storm runoff from the ground surface, in conjunction with the stormwater management basin's ability to attenuate flood peaks, is examined at both the local and watershed scale. Analyses focus on events, like the 7 July 2004 event, which have long return intervals.