Advantages of adding “basin upstream rainfall” (BUR) to the flash flood monitoring and prediction (FFMP) program

Flash floods occur across a wide variety of watershed areas and time durations. The great majority of flash floods occur in watersheds less than 64.8 km2 (25 mi2) in area and in time durations of less than two hours. The “County Layer” of the Flash Flood Monitoring and Prediction Program(FFMP) is uniquely designed to detect these small scale, short time duration flash floods. The “County Layer” contains the most detailed watershed segments of the FFMP stream database in a “single” county display. Flash floods can and do occur on much larger watersheds in the range of 129.5 to 1294.9 km2 (50 to 500 mi2) with time durations of 3 to 6 hours. These larger scale flash floods, often caused by inland tropical storms or training of fast moving thunderstorms over the same geographic area, can produce devastating flash floods over large watersheds. Support for these larger scale floods by FFMP is considerably less robust than the detection support for smaller scales storms in the “County Layer”.

The addition of “Basin Upstream Rainfall” (BUR) to the FFMP basin table for a “single” county would greatly enhance FFMP's ability to detect flash flooding in larger watersheds. In 2005, FFMP first introduced five aggregated basin layers to aid in the detection of flooding in larger watersheds. Unfortunately the aggregated basin layers are not accessible in a single county display, but only in a table that contains all the aggregated basins in the entire county warning area (CWA). This makes individual basins difficult to locate. There is no direct link between the commonly used “county layer” and the larger watersheds. By adding the BUR to the FFMP basin table as an optional column, support for the larger streams would be available in the “county layer”. A user would no longer have to exit the “county layer” in order to display the aggregated basin layers, saving precious time when analyzing flash flood threats.

The BUR is simply the average basin rainfall for the entire upstream contributing area of a single FFMP basin segment. This upstream area, defined as flow accumulation in the NSSL FFMP stream database, can be displayed for any FFMP segment with the “click” menu by selecting “upstream basin trace”. The average basin rainfall for each small FFMP basin segment is currently displayed in the FFMP basin table in the “QPE” column. A column could be added to the FFMP basin table as “BUR” to display the average basin rainfall of the upstream contributing area. This new “BUR” column could replace the “RATIO” column, which is of limited hydrologic value.

This paper will demonstrate the utility of the proposed “BUR” column by showing BUR results for some historic floods in the Pittsburgh CWA including the Shadyside, Ohio flood of 1990, the Etna, PA flash flood of 1986, and the record flooding during Hurricane Ivan in 2004.