Anticipating urban flash flooding using basin upstream rainfall (BUR) and Google Earth

Flash flooding is a phenomenon that typically occurs over a short period of time, and influences a relatively small geographic area. While an individual basin may not receive enough rainfall to produce flooding, the contribution of increased streamflow from upstream basins may be a major contribution for flash flooding that might not otherwise occur. As a result, forecasters must account for the routing of water over more than just an individual basin. The integration of Basin Upstream Rainfall (BUR) into operational use would provide forecasters with a new radar rainfall tool for flash flood detection.

In addition, the effects of flash floods can be exacerbated when the streamflow of water is restricted or unexpectedly augmented by runoff from another basin. Streamflow can be restricted by urban drainage systems and the damming of debris at bridges or culverts. A second possibility for streamflow alteration may occur when flow from two basins intersects into a single channel. In urban areas, these ‘flashpoints' may be the most dangerous locations for the public in a flooding situation.

When the remnants of Hurricane Ivan struck Pittsburgh, PA in September of 2004, several flashpoints significantly impacted the public. Some, but not all, of these flashpoints were anticipated as possible problem locations before flooding actually began. In post-event analysis, it was found that any location where a major tributary entered the mainstream usually had some sort of flooding. BUR would provide a quantitative measure of the impacts of the merging stream channels. This paper outlines a procedure for proactively determining potential flashpoints before a flash flooding event occurs, provides a method for displaying flashpoints in Google Earth, and demonstrates how this knowledge can be passed onto the public to meet the NWS mission of “protecting life and property.”