Hydrologic modeling of extreme floods using radar rainfall estimates

High resolution radar rainfall fields and a distributed hydrologic model are combined for analysis and monitoring of extreme floods. Hydrologic modeling is based on a Hortonian infiltration model and a network-based representation of hillslope and channel flow. Model analyses are used to examine the hydrology and hydrometeorology of the 27 June 1995 Rapidan River flood which produced a measured peak discharge of 3000 m3 s^-1 at a drainage area of 295 km². The unit discharge of 10.2 m³ s^-1 km^-2 is the largest for the US east of the Mississippi River for basins larger than 100 km². Rainfall estimates at 1 km horizontal scale and 5 min time scale are used to reconstruct flood response to the Rapidan storm at basin scales ranging from 1 to 295 km². Peak storm total rainfall accumulations for the 27 June 1995 storm exceeded 600 mm in a time period of approximately 6 h. Scale dependent flood response is related to the structure and motion of the Rapidan storm and the drainage network structure of the Rapidan River basin. The envelope curve of peak discharge for the Rapidan flood at basin scales less than 295 km², derived from model analyses, is compared with envelope curves, based on extensive indirect discharge measurements, from the 19 July 1942 Smethport, Pennsylvania flood and the 18-19 August 1969 Nelson County, Virginia flood. These three events largely define the envelope curve of flood peaks for the US east of the Mississippi River at basin scales less than 1000 km². Analyses illustrate how radar rainfall estimates can be combined with conventional stream gaging and indirect discharge measurements to enhance monitoring of extreme floods.