Radar analyses of extreme rainfall and flooding in urban drainage basins

The Charlotte, North Carolina metropolitan area has experienced extensive urban and suburban growth during the past 40 years, resulting in increasing flood hazards in the region. Record flooding in the urban core of Charlotte occurred on 23 July 1997 from a storm that produced rainfall accumulations of more than 250 mm during an 18 h period, more than doubling the 24 h rainfall maximum in Charlotte, and causing $60 million in property damage and three fatalities. Analyses of the 23 July 1997 storm and flood are based on rainfall and discharge observations from dense networks of rain gages and stream gages maintained by the U.S. Geological Survey and rainfall estimates from two WSR-88D weather radars, both located approximately 150 km from the urban core of Charlotte. This wealth of observations provides an opportunity to address hydrometeorological questions concerning: (1) the accuracy of radar rainfall estimates for extreme, flood-producing rainfall; (2) the space-time variability of extreme, flood-producing rainfall in urban environments; and (3) the effects of urbanization on extreme flood response in urban environments. It is shown that bias-corrected radar rainfall estimates for the 23 July 1997 storm are quite accurate and provide the capability for resolving the fundamental rainfall forcing associated with regional variation in extreme flood response in urban landscapes. Extreme flood response in urban watersheds is characterized by pronounced nonlinearities in runoff production for rainfall accumulations exceeding 50 mm. Extreme flood response is also characterized by large spatial heterogeneities that are tied to the history of urban development. Case study analyses of four additional flood events in the Charlotte metropolitan area are used to assess the robustness of conclusions derived from analyses of the most extreme event in the region and to examine the transition to "upper tail" properties of extreme flood response.