Extreme floods in the Moores Run watershed
 
Miller, A.J., Smith, J.A., Baeck, M.L., Nelson, P.A., Holland, E., Meierdiercks, K.L., Diehl, J.O., Ballantine, M.
 
Urban watersheds are characterized by high percentages of impervious cover and efficient storm drain networks, with exaggerated hydrologic response to high-intensity, short-duration summer thunderstorms. In our study of small watersheds in the Baltimore metropolitan area, the end member in terms of flood magnitude and response time is the Moores Run watershed (drainage area 9.1 km2). Moores Run drains an urban neighborhood that dates back to the early 20th century, and there is no surface drainage in the northern half of the watershed. Flood peaks generally occur within 20 minutes after the initial rise of the hydrograph; a small tributary of Moores Run with a drainage area of 0.5 km2 often reaches peak discharge within 1 to 5 minutes. The flood of record on the main stem occurred in June 2003, with discharge exceeding 150 m3/s and possibly as high as 220 m3/s. This event falls on the envelope curve for flood peaks in the mid-Atlantic region and corresponds to floods at comparable drainage area in other environments with recurrence intervals of 100-500 years. Flood-producing rainfall over the watershed lasted approximately 20 minutes, with rainfall intensities (125-150 mm/hr for 5-15 minutes) that are expected to recur with a frequency of 2 to 5 years. Flood peaks exceeding 100 m3/s in response to similar storms have occurred five times in the past six years. Ongoing upgrades to the storm-drain network are expected to increase the efficiency of the system with the possibility of even larger and more frequent flood peaks.
 
Keywords: floods, urban, streams, watersheds
 

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