Landslide Hazard Area
Snohomish County updated its Critical Area Regulations CAR in September 2015. This update included changes to the definition of a landslide hazard area and to the development regulations addressing landslide hazard areas. The updates to regulations in landslide hazard areas are located in Chapter 30.62B SCC - Geologically Hazardous Areas.
Development activities are not allowed in landslide hazard areas unless certain criteria are met and no other alternative is available. A geotechnical evaluation and analysis is required of the landslide hazard area in proximity to new development activity. There are also notice, disclosure, and covenant requirements for development activities in a landslide hazard area.
The Washington Geological Survey and the Oregon Department of Geology and Mineral Industries have created a handout that defines and explains landslides. You can find it online (PDF).
Find answers to some key questions about landslide hazard areas below. If you have any questions, please contact the Snohomish County Planning and Development Services Permit Center at (425) 388-3311.
What is a landslide hazard area?
Landslide hazard areas are lands that are subject to mass earth movement due to a combination of geologic, topographic, and hydrologic factors with a vertical height of 10 feet or more. The criteria that defines a landslide hazardous area includes land within a distance from the top of the slope equal to the height of a slope or within a distance of the toe of the slope equal to two times the height of the slope.
The full definition of a landslide hazard areas can be found in SCC 30.91L.04. Click the image to the right for a larger version.
Where are landslide hazard areas?
Snohomish County has developed maps showing areas of the County that could be a potential landslide hazard area.
How do I find out if my property is in a landslide hazard area?
If steep slopes 33% or greater are located on the project parcel, and/or within 200 feet of the proposed disturbance area, the county will require that the steep slopes be evaluated by a geotechnical engineer (project proponents) to determine if they meet the definition of a landslide hazard area per SCC 30.91L.040.
Click on the picture to the left to go to our Landslide Hazard Area Map Series to determine if your property is in a potential landslide hazard area. Users can further explore landslide data and maps by utilizing our PDS GIS page. This page includes links to our interactive maps, downloadable products, map services and other county GIS products.
The maps are for informational purposes only. All parcels that are identified as being in a potential landslide hazard area must be verified by the project proponent.
The map is based on the adopted definition SCC 30.91L.040. The map utilizes elevation values, slope angles and steep slopes over 30%. Limitations on geologic data may affect the accuracy of the map. Not all factors (seeps, geologic contacts, unstable soils) that could constitute a potential landslide hazard can be mapped on a county wide basis.
Areas that may be mapped as a landslide hazard area may have site specific conditions that would exclude them from being considered a landslide hazard area (e.g., The site may have steep slopes but is underlain by bedrock).
There also may be geologic factors where it is determined that a parcel is within a landslide hazard area, even if it is not shown on the map. [See SCC 30.62B.390]
What other resources are available?
The Washington Geological Survey develops, produces, and publishes a large variety of maps and reports on the hazards faced by our state, including mapped landslides. This Department of Natural Resources webpage contains a collection of these resources and a brief description of each item. Detailed information can be found in the map or report. All of the resources can also be found on their Publications and Maps page, through the Washington Geology Library, or on the Geologic Information Portal.
What if my property is within a landslide hazard area?
- If site disturbance is proposed within a landslide hazard area, a geotechnical report, pursuant to SCC 30.62B Geologically Hazardous Areas, is required.
- The site-specific geotechnical report will identify what mitigation measures are needed if construction is allowed within the landslide hazard area.
- Development is not allowed in a landslide hazard area unless there is no other location and the conditions of SCC 30.62B.340(1) are satisfied.
- If the property is partially within the landslide hazard area, development may occur on that portion of the property outside of the landslide hazard area.
- Before any permits can be issued, a covenant (PDF) that runs with the land must be recorded that states that the property is within a landslide hazard area, describes the hazards, and the owner accepts the responsibility for the risks, informs future purchasers, and provides indemnification. [SCC 30.62B.160(4)]