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DIVISION OF GEOLOGY AND MINERAL RESOURCES

Landslide Hazard Mapping in Nelson and Albemarle Counties

A landslide is a downslope movement of a mass of soil, and/or bedrock material. Landslides result from a diverse assortment of ground movement processes and produce diagnostic landforms. The primary driving force for a landslide is gravity, but other factors may contribute to the failure of a slope. Landslides in Virginia are usually triggered by heavy rainfall, rapid snow melt, or oversteepening of slopes by stream incision. Human activities that alter natural slopes and water drainages can also greatly increase the likelihood of landslides. Landslides are capable of destroying buildings, knocking out power and telephone lines, and blocking transportation routes.

Some landslides move slowly, but often the movement can occur quickly and without warning. Soil creep and slumping cause property damage gradually, whereas swift-moving rockslides and debris flows can sweep away people, buildings, roads, and vehicles. In the United States, landslides kill twenty-five to fifty people and cause between $2 and $4 billion in property damage annually (year 2010 dollars).

Past landslides in Nelson and Albemarle

On the night of August 19-20, 1969, torrential rains dumped up to 27 inches of water on portions of Nelson County, southern Albemarle County, and nearby counties over a period of about 8 hours. A rare combination of moisture and circulation allowed for rapid intensification of rainfall as a weakened Hurricane Camille, having made landfall along the Gulf Coast 2 days prior, met air nearly saturated with moisture in the region southeast of Nelson County. Flood damage from the deluge extended all the way down the James River to Richmond, but the bulk of the affected area was in Nelson County. In addition to the flooding, hillsides in the area experienced over 3,700 landslides, typically debris flows and slides (see image below). These slides account for approximately 2,000 acres worth of damage by landslide scarring and inundation.

Debris flow chutes soon after Hurricane Camille, in Nelson County

Debris flow chutes soon after Hurricane Camille, in Nelson County

The property damage and loss of life from Hurricane Camille was extensive in Virginia. At least 150 people died in the storm and flood, 125 in Nelson County alone. Property damage amounted to $140 million (1969 dollars), primarily in losses of cropland (including orchards), residences, personal property, highways, and railroads.

House destroyed by Hurricane Camille flood waters in the Davis Creek Community, in Nelson County

House destroyed by Hurricane Camille flood waters in the Davis Creek Community, in Nelson County

In 1995, another severe storm caused landslides in western Albemarle and Madison counties. That event caused property damage in the amount of $112 million (1995 dollars). Over 70 landslides occurred in the North Fork of the Moormans River within Shenandoah National Park. Significantly, debris from these landslides surged downstream into the Sugar Hollow Reservoir, the main water source for Charlottesville. The sedimentation from that debris reduced the reservoir’s holding capacity by 15%.

Recent flooding in Albemarle County has reminded us of the impact that severe storm events can have on the flanks of the Blue Ridge Mountains. 2018 was one of the wettest years on record for this part of Central Virginia.

Prehistoric landslide deposit (center top) exposed at the North Fork of the Moormans River, Shenandoah National Park.

Prehistoric landslide deposit (center top) exposed at the North Fork of the Moormans River, Shenandoah National Park.

Mitigating Risk

Landslide damage poses significant risk to public health and safety, and is a recurring problem. Identifying areas prone to future landslide hazards, especially debris flows like those experienced during Hurricane Camille in 1969 and the severe thunderstorm in western Albemarle County in 1995, can help inform citizens and allow emergency personnel to target specific areas in need of early warning and evacuation procedures during future severe rainstorm events (greater than 5 inches in 24 hours). No landslide hazard maps currently exist for Nelson and Albemarle Counties.

Landslide Mapping 2019-2021

The Virginia Department of Mines, Minerals, and Energy, Division of Geology and Mineral Resources (DGMR) has received funding from the Federal Emergency Management Agency (FEMA) Pre-Disaster Mitigation Grant Program, through the Virginia Department of Emergency Management (VDEM) to complete a landslide hazard mapping study for western Nelson and Albemarle Counties. The study, entitled “Enhancement of Landslide Hazard Risk in State and Local Hazard Mitigation Plans” (PDMC-PL-03-VA-2017-00) will identify areas and infrastructure at greater risk and communicate these risks to VDEM, local emergency management, county officials, and residents to prepare for future storm events. This mapping can also be used to improve statewide and local Hazard Mitigation Plans.

Comprehensive landslide mapping can reduce the potential for future damages by providing county planners and emergency personnel with information about areas that could be prone to landslides during heavy rainfall events. This mapping is intended to assist local officials in adopting strategies to reduce landslide risks in their communities and improve warning and evacuation plans for residents.

Study Area

This project is focused in western Albemarle and Nelson Counties, an area with recently released 2016 LiDAR (Light Detection and Ranging) digital elevation data with a 1-meter resolution. This high-resolution data allows for precise mapping of landslide features, identification of potential landslide initiation and inundation areas, and more focused and efficient field work by geologists. The communities of Crozet, Beech Grove, Nellysford, and Wintergreen Resort are included in the project area.

Pink hatch marks identify the extent of the study area

Pink hatch marks identify the extent of the study area

Methods

This project will be completed in four phases:

  1. Remote sensing of modern and prehistoric landslides in the study area
  2. Geologic field mapping of landslide prone areas
  3. Landslide susceptibility mapping and modeling
  4. Presentation of data products and results to the planning community and the public.

Landslide features are outlined in red in this LiDAR-derived slope-shade basemap. These features are at Fortune’s Cove in Nelson County, Virginia.

Landslide features are outlined in red in this LiDAR-derived slope-shade
basemap. These features are at Fortune’s Cove in Nelson County, Virginia.

Remote sensing

One-meter resolution LiDAR collected in 2016 is available for the study area from the U.S. Geological Survey. Our staff will create high-resolution base maps from this publicly available LiDAR and use them to view a detailed representation of the earth’s ground surface. For more information about LiDAR, visit our webpage.

Left: An aerial view of Fortune's Cove taken days after Hurricane Camille. View to the north. Right: Landslide features in Fortune’s Cove, as they appear on the 2016 LiDAR basemap

Left: An aerial view of Fortune's Cove taken days after Hurricane Camille. View to the north.
Right: Landslide features in Fortune’s Cove, as they appear on the 2016 LiDAR basemap.

Using the LiDAR and several vintages of aerial photography, we will visually identify pre-existing landslide features present in the study area and inventory their location, extent, and other geomorphic characteristics. All landslide data will be entered into a comprehensive ArcGIS (geographic information system) geodatabase.

Bouldery landslide debris photographed along the upper trail in Fortune's Cove.

Bouldery landslide debris photographed along the upper trail in Fortune's Cove.

Field mapping

Preliminary LiDAR interpretation allows for targeted, detailed field mapping in the study area. Studying landslides up close helps geologists identify causes, model movement, determine soil properties, and constrain the geologic conditions where landslides have happened. The goal of gathering this information is to better understand where landslides are likely to occur in the future.

Landslide susceptibility modeling

Once existing landslide features are adequately mapped, geologists will use ArcGIS to identify potential landslide hazard areas. These maps will highlight at-risk areas based on factors such as slope and soil properties identified from the newly mapped pre-existing landslides in this area. DGMR has used similar modelling methods to produce hazard maps for other areas in western Virginia.

Deliverables

Final deliverables will include a series of GIS data layers and maps showing at-risk areas, a complete explanation of the data layers, and a list of recommendations for local planners and emergency management officials to lessen the impact of future landslides. The spatial data will be available directly to the public through the DGMR website.

Other resources

What is a landslide? (USGS)
Landslides in Virginia (DGMR)
Details of Hurricane Camille
Debris-flow hazards in the Blue Ridge of Virginia (USGS)

Selected References:

American Geosciences Institute, How much do landslides cost the U.S. in terms of monetary losses?: https://www.americangeosciences.org/critical-issues/faq/how-much-do-landslides-cost-terms-monetary-losses (accessed April 2019).

Boyer, John, 2019, Year of extremes: Richmond and Virginia’s biggest weather stories of 2018: Richmond Times-Dispatch, January 2nd: https://www.richmond.com/weather/year-of-extremes-richmond-and-virginia-s-biggest-weather-stories/article_a2c08eb0-07ec-5f10-b582-b66904c53a08.html (accessed Mar. 2019).

Gryta, J. J., and Bartholomew, M.J., 1989, Factors influencing the distribution of debris avalanches associated with the 1969 Hurricane Camille in Nelson County, Virginia in Schultz, A.P., and Jibson, R.W., eds., Landslide processes of the Eastern United State and Puerto Rico: Geological Society of America Special Paper 236, p. 15-28.

Morgan, B.A., and Wieczorek, G.F., 1996, Debris flows and landslides resulting from the June 27, 1995, storm on the North Fork of the Moormans River, Shenandoah National Park, Virginia: U.S. Geological Survey Open-File Report 96-503, 21 p.

Williams, G.P., and Guy, H.P., 1973, Erosional and Depositional Aspects of Hurricane Camille in Virginia, 1969: U.S. Geological Survey Professional Paper 804, 80 p.

Virginia Division of Geology and Mineral Resources, Landslides. Available at https://www.dmme.virginia.gov/DGMR/landslides.shtml (accessed Mar. 2019).

Virginia Division of Mineral Resources, 1969, Natural features caused by a catastrophic storm in Nelson and Amherst counties, Virginia:  Virginia Division of Mineral Resources Virginia Minerals, Special Issue, 20 p.

Witt, AC., and Whitehead, D., 2017, Fortune’s Cove and the 1969 Hurricane Camille Debris Flows: 3rd North American Symposium on Landslides field trip guide, 26 p.