HR: 0830h
AN: H31C-0482    [PDF]
TI: High-Magnitude Short-Duration Floods in Small Urban Watersheds
AU: * Miller, A J
EM: miller@umbc.edu
AF: Department of Geography and Environmental Systems, University of Maryland Baltimore County, 1000 Hilltop Circle, Baltimore, MD 21250 United States
AU: Smith, J A
AF: Department of Civil and Environmental Engineering, Princeton University, E-209 Engineering Quad, Princeton, NJ 08544 United States
AU: Baeck, M L
AF: Department of Civil and Environmental Engineering, Princeton University, E-209 Engineering Quad, Princeton, NJ 08544 United States
AU: Nelson, P A
AF: Department of Civil and Environmental Engineering, Princeton University, E-209 Engineering Quad, Princeton, NJ 08544 United States
AU: Holland, E
AF: Department of Civil and Environmental Engineering, Princeton University, E-209 Engineering Quad, Princeton, NJ 08544 United States
AU: Meierdiercks, K L
AF: Department of Civil and Environmental Engineering, Princeton University, E-209 Engineering Quad, Princeton, NJ 08544 United States
AU: Diehl, J O
AF: Department of Geography and Environmental Systems, University of Maryland Baltimore County, 1000 Hilltop Circle, Baltimore, MD 21250 United States
AU: Ballantine, M
AF: Department of Geography and Environmental Systems, University of Maryland Baltimore County, 1000 Hilltop Circle, Baltimore, MD 21250 United States
AB: 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, MD metropolitan area, the availability of high-resolution weather radar data (5-minute time interval, 1-km$^{2}$ grid) and high-resolution lidar data (approximately 1-m point spacing with 15-30 cm vertical accuracy) in watersheds with stream gages recording at 1-5 minute time intervals allows detailed reconstruction and modeling of extreme flood events that is not possible in most other environments. The end member in terms of flood magnitude and response time is the Moores Run watershed (drainage area 9.1 km$^{2}$). 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 km$^{2}$ typically reaches peak discharge within 1 to 5 minutes. The flood of record on the main stem occurred in June 2003, with discharge exceeding 150 m$^{3}$/s and possibly as high as 220 m$^{3}$/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 m$^{3}$/s in response to similar storms have occurred four 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. The channel of Moores Run (up to 3-4 m deep and 7-10 m wide) is grossly enlarged by comparison with streams of comparable drainage area in rural settings, and the bed is armored by cobbles and boulders. Moores Run appears stable and highly resistant to the shear stresses imposed by floods, presumably as a result of adjustments that have occurred over multiple decades of recurring high flows. Lidar data, supplemented by field-survey data, are being used to characterize longitudinal trends in channel and floodplain topography for use in hydrologic and hydraulic modeling. Models will allow us to simulate hydrologic response and shear-stress distribution along the channel for floods produced by 15-minute rainfall rates ranging from 25 to 250 mm/h.
DE: 1815 Erosion and sedimentation
DE: 1821 Floods
DE: 1860 Runoff and streamflow
SC: Hydrology [H]
MN: 2003 Fall Meeting