Boundary County

Definition, Description and Potential Damage The Boundary County Hazard Identification and Risk Analysis define a landslide as a mass movement of rock, earth or debris down a slope. The steeper the slope, the more likely a slide will occur. However, this is dependent upon the geology and soils associated with a particular site. Loose soils perched on steep slopes with little or no vegetative cover are highly susceptible to landslides. Human activity can add to this problem by creating less stable situations. For example, highway construction can increase the potential for landslides by restructuring the slope and the combination of materials. Increasing development can increase runoff and saturate soils.

For a landslide to occur there must be a trigger. The most likely trigger for landslides is a combination of precipitation and human activities. Inherent weaknesses in the rock or soil often combine with one or more triggering events, such as heavy rain, snowmelt, or changes in ground water level. Late spring to early summer is slide season, particularly after days and weeks of greater than normal precipitation. It not only triggers slides but also changes the shape of rock and earth as it runs off into streams. Vegetation can help with the retention of runoff and serve as a barrier to sliding. Fire damage can reduce the potential for this vegetation to serve as a barrier to slides. Much of the county is exposed to wildfires, which could increase the potential for slides. In fact, many slides occur as a result of some other natural hazard.

National Atlas.gov and FEMA’s risk map (Map 6-1, pg. 6-3) indicate that the landslide risk in Boundary County is low.

Landslide History The Map of Historical Landslides (Map 6-2, pg. 6-4) from Boundary County’s 2001 risk assessment shows several types of landslides that occurred in the 20^th Century. Most of these occurred adjacent to roads where erosion and steep slopes have combined to produce these slides.

In 1998, a landslide closed US Highway 95 at the northern city limits of Bonners Ferry. Highway 95 is the only route into Canada and Montana from Boundary County. As a result, this slide affected all traffic flow, both north- and southbound, for several months.

Table 6-1, below, is a summary of significant landslide events that have occurred in Boundary County within the last 50 years. As there is no current data tracking system in place for detailed descriptions, this list has been compiled using historical records.

DATE	NOTES	EFFECTS
March 1954	In Bonners Ferry	Two Fatalities, Highway 95
March 1959	Mud Slide	Train Derailment
February 1961	Mud Slide
May 1961	Mud Slide	Road Closures
May 1965	Mud Slide	Roads blocked, washed out
March 1972	Mud Slide	Covers House
June 1974	Mud Slide	45,000 cubic yards
January 1981	Mud Slide	Minor Damage
March 1997	Mud Slide	Damage roads, injuries
October 1998	Mud Slide	Destroyed Highway 95, Railroad. School Closures

Critical Facility Exposure Assessment Most of the property exposed to danger from landslides lies along the major roadways in the county, with the exception of communication repeaters. The major roadways have received attention and monitoring regarding landslides. There has been no recent landslide that has affected any critical facility, except the recent Highway 95 landslide in Boundary County.

Summary Landslides are a potential hazard and can cause disruptions by blocking roads, destroying homes and other property. Boundary County must insure that proper development standards are instituted to reduce the potential for landslides due to development in the county, particularly on steep slopes.

1. Develop subdivision design standards for steep slopes to reduce the potential for landslide events during construction and post development.

2. Require site design review of all development projects proposed on slopes to insure stability, based on known information, evaluation of slope and soil stability characteristics.

3. Identify and map key locations adjacent to existing roads that are highly vulnerable to landslides.

Supporting Agencies: Boundary County Road and Bridge, Idaho Department of Transportation, National Resources Conservation District, and US Forest Service

4. Prohibit future construction of public critical facilities, businesses with hazardous materials storage and private roads on slopes that may create a hazard. Evaluate project sites on an individual basis to assess risk due to soil stability characteristics and slope stability.

5. Support programs that assist Boundary County Road and Bridge with restoration of degraded slopes through re-vegetation and slope stabilization efforts in landslide hazard areas adjacent to county road and bridges.

Supporting Agencies: Boundary County Office of Emergency Management, highway districts

6. Have information at building and planning departments on spray barriers, netting for rock slide prone slopes, and cement barriers available to contractors to minimize landslide occurrences on private roads.

Resources/Funding: Educational materials from Federal Emergency Management Agency, American Planning Association, Soil Conservation District

7. Support local, state and federal reforestation efforts on steep slopes after wildfire to minimize mud and landslides.

Supporting Agencies: Boundary County Office of Emergency Management, highway districts, Idaho Department of Transportation, Idaho Department of Lands, US Forest Service, fire services, National Resources Conservation Service and Soil Conservation District

8. Provide information to property owners on retrofits of buildings constructed on soft soils in high risk landslide locations.

Resources/Funding: Educational materials from Federal Emergency Management Agency, American Planning Association and Soil Conservation District

9. Identify landing zones for emergency services in key landslide prone areas to expedite emergency services.

Supporting Agencies: Boundary County Office of Emergency Management, fire services, emergency medical services, highway districts, Idaho Department of Transportation, MedStar, emergency responders.

All Hazards Mitigation Plan