Documentation and Analysis of Flash Flood Prone Streams and Subwatershed Basins in Pulaski County, Virginia

Anthony PhillipsDepartment of GeographyVirginia Tech

In Cooperation with:

Research PurposeStatement

Identify, measure, and analyze areas in Pulaski County that are prone to flash flooding using GIS. Results to be compiled in digital form of DVD, as well as in paper form in an operational handbook. All data to be overlaid in Google Earth and accessible from workstations at the NWS.

Research PurposeWhy do we care about flash flooding?

#1 weather-related killer in the United States!Can occur nationwide, in any environment, but primarily affects these areas in Pulaski County:

River floodingSmall stream floodingUrban floodingDry wash flooding

Selecting Flash Flood Prone AreasESRI's ArcGIS

USGS road and stream shapefiles were imported into ArcGIS.Software was used to select points where roads and streams were within 25’ of each other.651 points identified.

FieldworkData collected

Following data collected at each road-stream intersect point:

Road nameRoad typeStream nameStream flow (slow, normal, fast, none)Distance from road to streamAngle of depressionUpstream pictureDownstream picture

FieldworkCalculating the flood stage for each survey point

Distance from road to stream calculated from eye-level to stream edge.

Standard 50’ measuring tape.From same location, angle of depression measured from eye-level to stream edge.

Clinometer

θ°

d

FS= [d x SIN(θ°)] – 6’

6’

FS

Picture by Sarah Prescott

6’

Finalizing the DataWhy only 319 points?

Of the 651 points initially plotted by ArcGIS, 319 were fully documented.

The remaining 332 points were either on private roads or were not likely to have flash flooding.

Finalizing the DataArcGIS and aerial photography of each subbasin

Final data was imported into ArcGIS. Each measured point was color-coded to indicate the flood stage height.

Red: <1.5’Orange: 1.5 – 3.0’Yellow: 3.0 – 5.0’Green: 5.0 – 8.0’Turquoise: 8.0 – 12.0’Blue: >12.0’

Finalizing the DataArcGIS and aerial photography of each subbasin

2007 aerial photography serves as the background for each map.Additionally, each map includes all roads and streams with their associated names, as well as an inset map of surrounding elevation.

Finalizing the DataGoogle Earth

The location and information for each measured point was imported into Google Earth.Additionally, each subwatershed basin was also imported.Points used the same color-coded symbology.

Results and ConclusionsWhat we’ve learned so far…

The project has been a complete success! Areas that are susceptible to flash flooding have been fully documented.All information has been mapped and imported into Google Earth.Information available to the NWS and County of Pulaski in the form of a handbook and DVD.Project expanding to include Montgomery County and Virginia Tech

Questions?

Acknowlegements

Dave Carroll, VT AdvisorSteve Keighton, NWS Science OperationsPeter Corrigan, NWS HydrologistJan Jackson, NWS Senior ForecasterBobby Clark, Pulaski County Emergency

ManagerSarah Prescott, Field AssistantVT McNair SRE Staff