MAGNITUDE, FREQUENCY AND SPATIAL PATTERN OF URBAN FLOODS

Comparison of peak flow time series for small (5-15 km²) watersheds in the Baltimore metropolitan area reveals several trends in flood magnitude and frequency. Peak unit discharge values of 10 m³s^-1km^-2 or more occur exclusively in response to short-duration summer thunderstorms and have recurrence intervals of the order of 2 to 5 years in watersheds with high percent impervious cover and dense storm-drain networks. Occurrence of extreme flood peaks follows distinct spatial gradients and may be tied in part to the age and style of urban development and associated stormwater control infrastructure, in combination with regional physiographic patterns. Observations and hydraulic modeling of a series of flood events during the summers of 2003-2006 allow reconstruction of the influence of bridges, culverts, road embankments and floodplain modification on spatial patterns of overbank flow and floodplain inundation. Flood magnitude-frequency relationships for urban streams are strongly affected by model-based adjustments to stage-discharge relationships on the downstream sides of bridges. Such adjustments are particularly important at sites where short lag times, rapid changes in stage and high velocities preclude direct measurement of flood peaks using conventional methods.