Hydrometeorology and urban flooding in the Dead Run watershed: the storm of July 7, 2004
 
Miller, A.J.(1), Smith, J.A.(2), Baeck, M.L.(2), Fisher, J. (3) Holland, E.(2), Ballantine, M.(1), Newcomer, T.(1), Meierdiercks, K.(2), Rosenzweig, B.(2)

 
A multicell thunderstorm system on 7 July 2004 produced record flooding in the 14.3 km2 Dead Run watershed, which is an urban tributary of Gwynns Falls. Storm total accumulations of 100 - 130 mm during a period of less than 2 hours resulted in flood peaks that exceeded those produced in 1972 by Hurricane Agnes; a preliminary estimate by USGS using surveyed high-water marks in the vicinity of the Franklintown gage indicates a discharge peak of 265 m3/s (9380 cfs), with a possible range of 215-290 m3/s. Analyses of the 7 July 2004 thunderstorm system focus on the spatial and temporal variability of rainfall at scales relevant to flash flooding in small urban watersheds. Spatial scales of interest extend below 1 sq. km and the time scales of interest are typically less than 60 minutes. Analyses are based on volume scan reflectivity observations from the Sterling, Virginia WSR-88D and BWI TDWR radars, a network of 19 rain gages in the Dead Run watershed and drop spectra observations from a Joss-Waldvogel disdrometer.
 
Availability of high-resolution (~30 cm) orthorectified aerial photographs and 1-m lidar topography data allowed rapid deployment of student field crews during the week after this event to generate a flood-inundation map with surveyed high-water elevations for the entire drainage network downstream of the Baltimore Beltway (I-695) and for selected stream reaches upstream of the Beltway. Partial stage hydrographs recorded at three locations, supplemented by field observations of the time of peak, are used in conjunction with 2d hydraulic modeling analyses to reconstruct the spatial and temporal dynamics of this flood, which inundated areas mapped previously using FEMA guidelines as 100- and 500-year floodplain.
 

 
Keywords: flood, urban hydrology, rainfall, radar, lidar