What is the purpose of these regulations?
The intent of the regulations for landslide hazard areas is to ensure that land divisions in areas of potential landslide hazard are designed based on detailed knowledge of the site conditions in order to reduce the risk of private and public losses as a result of landslides.
Where do these regulations apply?
These regulations apply to land division proposals where any portion of the site is within a potential landslide area, as identified on the City’s Potential Landslide Hazard Areas Map, available on PortlandMaps.
What are the land division requirements if my property is in the potential landslide hazard area?
If any part of your site is a potential landslide hazard area, your application must include a Landslide Hazard Study and address the standards or approval criteria of Portland City Code Chapter 33.632. The proposal must locate the lots, buildings, services, and utilities on parts of the site that are suitable for development in a manner that reasonably limits the risk of a landslide affecting the site, adjacent sites, and sites directly across a street or alley from the site.
Site improvements may be required to limit risk. Alternative development options, such as reduced density and housing types may also be required.
Frequently Asked Questions
How will I know if my property is in a potential landslide hazard area?
You can determine whether your site is within the potential Landslide Hazard Area or within other hazard areas on PortlandMaps by searching for your site address and then scrolling through the information on the right side of the screen. Alternatively, call 503-823-7300 or schedule a 15-minute appointment with a City Planner.
How was the potential landslide hazard area map created?
The map was created from the following sources:
- Oregon Department of Geology and Mineral Industries (DOGAMI) Interpretative Map Series IMS-22: GIS Overview Map of Potential Rapidly Moving Landslide Hazards in Western Oregon.
- IMS-57 Landslide Inventory Map of Central and Eastern Multnomah County.
- IMS-57 Deep Landslide Susceptibility Map of Central and Western Multnomah County, high and moderate susceptibility areas.
- IMS-57 Shallow Landslide Susceptibility Map of Central and Western Multnomah County, high susceptibility areas modified to include only areas with an average 30 percent slope or greater.
If my site has a potential landslide hazard area, how are the density calculations impacted for the site?
For single-dwelling zones:
- Minimum density: The area of the site within the potential landslide hazard area is subtracted from the total site area when calculating minimum density (see Portland City Code Chapter 33.610).
- Maximum density: You may still propose the maximum number of lots allowed by your site area, as long as your proposal and landslide hazard study demonstrate that your site can safely accommodate the dwellings and accompanying services.
For multi-dwelling zones, the minimum density may be reduced through the land division to facilitate suitable development.
If my property has no obvious landslide hazard, but it is still shown within the potential landslide hazard area, do I still need to meet these requirements?
Yes. A landslide hazard study is still required. However, the level of investigation and analysis is expected to be commensurate with the level of hazard, the complexity of site conditions, and the complexity of the proposed development.
Landslide Hazard Study Requirements – Portland City Code Chapter 33.730.060.D
If any part of the site is in a potential landslide hazard area as shown on Map 632-1, Potential Landslide Hazard Areas, the application must include a Landslide Hazard Study prepared by a Certified Engineering Geologist and a Geotechnical Engineer. The study must be based on current geotechnical professional standards and practices. If the study was prepared more than 2 years ago, an addendum must be provided that confirms that the study and any recommendations reflect current site conditions.
The Landslide Hazard Study must identify landslide hazard areas within the site and identify the part or parts of the site suitable for development in terms of the risk of a landslide affecting the site, adjacent sites, and sites directly across a street or alley from the site. The Landslide Hazard Study should make recommendations for the layout and design of the land division and development. On sites where the proposal is to meet the standards of 33.632.100.B. the Landslide Study must provide the rationale for slope stability conclusions.
The Landslide Hazard Study should include:
- Identification of the safest portion of the site and the portion(s) of the site that are suitable for development in a manner that reasonably limit the risk of landslide potential on the site and other properties in the vicinity;
- Hazardous or no-build areas within the land division site, if any;
- Recommended building setback distances from slopes, if any;
- Recommendations for the location of driveway and/or street locations;
- Utility trench locations;
- Retaining walls, associated drainage and discharge systems, if any;
- Estimated effect of tree removal on slope stability;
- Estimated effect of the development on stormwater and groundwater runoff, as well as evaluation of runoff and stormwater disposal from adjacent property as they relate to slope stability and landslide hazard;
- Recommendations for stormwater and groundwater disposal methods, and
- Recommendations and corresponding design calculations for slope stabilization.
If any of the above items are not applicable to the proposal, the Landslide Hazard Study must describe why they are not applicable. The study must provide adequate detail to show the design of all proposed structures and improvements and must include a statement of on-site slope stability after the proposed development is complete. The study may also include:
- Review of LIDAR or aerial photography including stereo views,
- Review of geologic literature or previous reports,
- Site reconnaissance including mapping of observable geologic features or hazards,
- Additional field explorations as necessary,
- Laboratory testing,
- Subsurface exploration logs, and
- Slope stability calculations.