LANDSLIDE HAZARD MAPPING IN PAGE COUNTY, VIRGINIA: PROCESSES, RESULTS AND LESSONS LEARNED

Landslide hazard mapping has been completed for Page County (813 km²), located in the Shenandoah Valley of northwestern Virginia. As part of a one-year FEMA-funded pilot study, the Virginia Division of Geology and Mineral Resources (DGMR) delivered GIS-based hazard maps and data to county officials and the Virginia Department of Emergency Management. These three maps depict: 1) the locations of modern and pre-historic landslide features; 2) areas susceptible to rapid landslide (debris flow) initiation during heavy rainfall; and 3) areas that may be in the path of landslides as they travel downslope. The maps are designed to serve as planning tools for the public and to help guide informed development and emergency management decisions on the local level. As part of this project, a statewide ArcGIS relational geodatabase of various landslide features was also developed and currently contains the locations of 4800+ landslides.

Field data was collected with a GPS unit at 1400 sites to help verify modern landslides and landslide deposits identified using DTM hillshades, several vintages of orthophotography, and scanned aerial photography. Basic soil and geologic information was gathered, including nine sites where soil gradation and Atterberg limits testing was performed. These parameters helped to calibrate the debris flow susceptibility model used to create the second map.

The most significant finding is the lack of modern landslide activity in Page County: only 9 modern landslides were identified. This is surprising as neighboring Madison County experienced 500+ landslides during a 1995 storm. However, evidence of pre-historic landslide activity, mostly in the form of debris and alluvial fans, is apparent and covers nearly 47.8 km² (6%) of the county. In the western portion of the county, the Massanutten Sandstone provides an ample supply of talus, which transitions into block streams and heavily dissected debris fans in varying states of erosion. To the east, the metamorphic rocks of the Blue Ridge produce large composite alluvial and debris fans that are being reworked by modern streams. Modern debris flows were only found in the Blue Ridge portion of the county and initiate in the shales and phyllites of the Harpers and Catoctin Formations.