Mountain Flash Flooding and Its Impacts Across The Blacksburg Weather Forecast Office County Warning Area

Anthony PhillipsDepartment of Geography

Virginia Tech

Results: Case Study Cont.

Located near the mouth of the study basin (.9 mi away) is a USGS automated river gauge station. The station records the water height and flow under the bridge in Snowville. In less than 15 minutes the gauging station reported elevated river levels from the upstream thunderstorm event.

Pictures taken from the flooded basin show creeks and streams out of their banks as well as a roadway under water and impassable (bottom-left photo). Hollows that were normally dry were seen with massive amounts of runoff (bottom-right photo).

In total, over 3.5” of rain fell across the study basin in less than four hours. For this day in March 2007 only ~2” of rain was needed in ~four hours to produce flash flooding.

Results: Case Study

On March 28, 2007 a flash flood ripped through the town of Snowville in southern Pulaski County, VA causing over $5,000 in property damage. This thunderstorm system was fully documented by the author and a full evaluation was conducted on the local watershed basin.

Figure 3. Watershed basin evaluated in study

Introduction

Flash flooding in the mountainous regions of the Blacksburg Weather Forecast Office (WFO) County Warning Area (CWA) occurs very quickly and can be of serious danger to life and property. Physical and cultural geography both play a role in how these events unfold and how they are handled by the public.

Figure 1. Blacksburg NWS coverage area and elevationReferences

"HDSS Access System." National Climatic Data Center (NCDC). 30-Apr-2009. NOAA. 30-Apr-2009

"Little River at Graysontown, VA." National Water Information System: Web Interface. 29-Apr-2009. USGS. 30-Apr-2009

"NWS Blacksburg, VA." National Weather Service Forecast Office. 29-Apr-2009. NOAA. 28-Apr-2009

"Storm Events." National Climatic Data Center (NCDC). 10-Nov-2008. NOAA. 30-Apr-2009

Acknowledgments

•Peter Corrigan, NWS Blacksburg, VA•Jan Jackson, NWS Blacksburg, VA•Sarah Prescott, NRCC•All those who distributed and took part in the Severe Weather Survey through survey.vt.edu

Analysis

Analysis was conducted in regards to the following:

•A case study of a mountain flash flood in Pulaski and Montgomery counties in Southwest Virginia

•Flash Flood/Flood records from all mountainous counties and the town of Clifton Forge, VA during a ten year period from 1998 to 2008

•Flooding from hurricane Jeanne in 2004 along Little River in Snowville, VA…a mountainous location

•Results from a survey conducted by students and faculty in the College of Natural Resources, Virginia Tech

•A recently published report from the Blacksburg NWS regarding climatology of flooding and flash flooding in their coverage area

Results: Flash Floods, 1998-2008

After analyzing flash flood records for the past ten years several conclusions were made: Flash flooding resulted in $38,520,000 in property damage and $50,000 in crop damage in the mountains of the Blacksburg CWA & only one death occurred. Additionally, there were no injuries reported.

Figure 2. The timeframe of mountain flash floods

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Timeframe of Flash Flood ReportsMountains of Blacksburg WFO CWA

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More Results & Conclusions

In a recent survey, 115 random students and faculty at Virginia Tech were asked various questions regarding flash flooding and safety. When asked what they would do when confronted by a swollen creek below a bridge, 57% said they would turn around and find an alternate route but 34% said they would continue across. What those 34% don’t realize is that the bridge surface may still be intact but the supporting structure may not be there any longer. Also of note: 47% of the participants said they would not know exactly where to go during a severe weather event.

In conclusion, flash flooding does cause substantial damage in the mountains of the Blacksburg WFO CWA and some people are still unaware of its risks…it is the #1 weather-related killer.

© David Evenson