THE CAMERON RUN FLOOD OF JUNE 25, 2006 – HYDROLOGIC RESPONSE TO SEDIMENT AGGRADATION IN A FLASHY URBAN STREAM, NORTHERN VIRGINIA

On June 25, 2006, up to 5 inches of rain fell on the Cameron run watershed of Northern Virginia within a 3-hour period. Subsequently the USGS gaging station for Cameron Run at Alexandria (USGS 01653000) recorded the highest peak natural flow (16,500 cfs) since the start of recording in 1957. Widespread flooding occurred throughout the Cameron Run watershed, but was concentrated in the downstream area near its confluence with the Potomac River. This investigation focuses on a commercial property located near the piedmont/coastal plain break of slope in Alexandria, VA. This recently-constructed facility experienced rapid flooding, including inundation of an underground parking garage that could have resulted in fatalities if it occurred during business hours.

A 1976 USGS flood plain delineation for the subject portion of the Cameron Run watershed placed the 100 year flood plain at an elevation of 15.8 feet; the nearby I-495 bridge was designed for a maximum water elevation of 17 feet. Comparison of 1976 and 1982 data sources to a recent bathymetric survey reveals more than five feet of sediment aggradation had also occurred in places since construction of the I-495 overpass. The June 2006 flood, which approached 100 year flood flow (depending upon calculation method), exceeded 24 feet elevation at the subject property

We modified an existing HEC-RAS model completed by the Virginia DOT (in 2001) to simulate the 2006 flood under approximate 1976, pure trapezoidal cross-section, and current conditions. These model runs confirmed that under existing conditions, the flood elevations associated with the 25-year, 50-year, and 100-year recurrence intervals all exceed the 1976 calculated 100-year flood elevation. Our study indicates that a number of factors contributed to the June 2006 flood, including straightening and channelization of the lower Cameron Run reach, backwater effects of the I-495 overpass, and 30 years of sediment aggradation at the coastal plain / piedmont break in slope. Analysis of gage data indicates that maximum peak flows have also been increasing since 1957, suggesting that continued urbanization of the watershed also contributed to this flood.