The purpose of this chapter is to:
A. 
Protect human life and health.
B. 
Regulate uses of land in order to avoid damage to structures and property being developed and damage to neighboring land and structures.
C. 
Identify and map active landslide hazard areas.
D. 
Minimize the ill effects on wetlands and critical fish and wildlife habitat that can result from landslides.
E. 
Establish permit requirement and review procedures for development proposals in areas with potential landslides.
(Ord. 17-513 § 3 (Exh. A))
A. 
Landslide Hazard Areas. Landslide hazard areas are areas potentially subject to mass movement due to a combination of geologic, seismic, topographic, hydrologic, or manmade factors. Landslide hazard areas are identified by the presence of any of the following five indicators.
1. 
Areas of historic failures, including areas of unstable, old and recent landslides or landslide debris within a head scarp.
2. 
Areas with all three of the following characteristics:
a. 
Slopes steeper than 15 percent with a vertical relief of 20 feet or more; and
b. 
Hillsides that intersect geologic contacts with a relatively permeable sediment overlying a relatively impermeable sediment or bedrock; and
c. 
Springs or groundwater seepage.
3. 
Areas exhibiting geomorphological features indicative of past slope failure within the last 10,000 years, such as hummocky ground, back-rotated benches on slopes, tension cracks, etc.
4. 
Any area with a slope of 40 percent or steeper and with a vertical relief of 15 or more feet.
a. 
Slopes may be exempted from the requirements of this section; provided, that it can be demonstrated by a qualified geotechnical professional that such an exemption does not result in an increased risk of landsliding or damage to the subject site, nearby properties, and existing structures. Any associated hazards to proposed structures must be suitably mitigated.
b. 
For the purposes of determining whether a slope is considered to be a landslide hazard area, the horizontal and vertical distance between the top of slope and toe of slope are utilized.
5. 
Areas that are at risk of mass movement due to seismic events.
6. 
Areas potentially unstable as a result of rapid stream incision, streambank erosion, including stream channel migration zones.
B. 
Potential Landslide Hazard Areas. Potential landslide hazard areas, as depicted on city Critical Area GIS database, are those areas where the suspected risk of slope instability and landslide is sufficient to require a geological assessment to determine the potential for active landslide activity. Potential landslide hazard areas are determined by using the following criteria:
1. 
Areas that possess one or more of the landslide hazard area indicators (stratigraphy, topography, emergent groundwater seepage, etc.) as set forth in subsection A of this section and any adjacent area within a distance of 65 feet. These areas include, but are not necessarily limited to, those areas designated on the city’s Critical Area GIS database as moderate or steep slope areas.
2. 
Areas shown on the Washington Geological Information Portal as prepared and maintained by the Washington Department of Natural Resources (WDNR) that are shown under “WGS-Protocol Landslide Mapping.”
C. 
Landslide Hazard Area Categories. Landslide hazard areas shall be classified into categories which reflect each landslide hazard areas past landslide activity and the potential for future landslide activity based on an analysis of slope instability. Landslide hazard areas shall be designated as follows:
1. 
Landslide Hazard Area. A composite of the active landslides and/or unstable areas, including that portion of the top of slope and slope face subject to failure and sliding as well as toe of slope areas subject to impact from down slope run-out, identified and mapped during a geological assessment of a site. An active landslide hazard area exhibits one or more of the following:
a. 
Areas of historical landslide movement on a site which have occurred in the past century including areas identified in subsection B.2 of this section or other mapping sources.
b. 
Unstable areas that exhibit geological and geomorphologic evidence of past slope instability or landsliding or possess geological indicators (stratigraphy, ground water conditions, etc.), as set forth in subsection A of this section, that have been determined through a geological assessment process to be presently failing or may be subject to future landslide activity. The impact of the proposed development activities must be considered in defining the extent of the active areas.
c. 
Interim areas are located between areas identified through the geological assessment process as an active landslide hazard area. Interim areas will be considered part of the active landslide hazard area if the required top of slope or toe of slope landslide hazard area buffer encompasses the area.
2. 
Stable Areas. Areas that have been identified as potential landslide hazard areas, but, through the geological assessment process, meet one of the following conditions:
a. 
A geological assessment has been performed and the results of that assessment indicate that an area is not a landslide hazard area set forth in subsection C.1 of this section.
b. 
Areas that have been determined to be stable or are converted into a stable area by the implementation of engineering or structural measures are not considered a landslide hazard critical area.
c. 
Adequate engineering or structural measures have been provided in a geotechnical report that mitigates the potential for a future landslide to occur as a result of proposed, current, or past development activity that also complies with the following criteria:
i. 
The engineering or structural measures must provide a minimum factor of safety of 1.5 static conditions and 1.1 for dynamic conditions. Analysis of dynamic (seismic) conditions shall be based on a minimum horizontal acceleration as established by the current version of the International Building Code.
ii. 
Construction sequencing recommendations must be provided by the geotechnical professional when a proposed development will be constructed concurrently with the engineering or structural measures.
iii. 
Demonstrate how modification of the Landslide Hazard Area will not result in any adverse impacts on adjacent properties (within 100 feet).
iv. 
The proposed development and stabilization measures comply with EMC §§ 14.90.040(A) and (B)(1).
v. 
The engineering or structural measures must be completed, inspected and accepted by the city for the area to be deemed stable.
(Ord. 17-513 § 3 (Exh. A); Ord. 25-0695, 12/23/2025)
A. 
General Requirements.
1. 
For the purposes of regulating this chapter, “director” means the Public Works Director or their designee, and “department” means the Public Works Department.
2. 
The city’s Critical Area GIS Database provides an indication of where potential landslide hazard areas are located within the city. The actual presence or location of landslide hazard areas that have not been mapped, but may be present on or adjacent to a site, shall be determined using the geological assessment procedures established in this chapter.
3. 
The department will complete a review of the available GIS data and other source documents for any proposed regulated activity to determine whether the site is, or may be, located within a landslide hazard area or potential landslide hazard area. Identification of a landslide hazard area or potential landslide hazard area may also occur as a result of field investigations conducted by department staff.
4. 
When the department’s maps or sources indicate that the site for a proposed regulated activity is or may be located within a landslide hazard area or potential landslide hazard area, the department shall require the submittal of a geological assessment as outlined in subsection B of this section.
5. 
Unless otherwise stated in this chapter, the critical area protective measure provisions contained in EMC § 14.10.070 shall apply.
B. 
Geological Assessment. A geological assessment is a site investigation process to evaluate the on-site geology affecting a subject property.
1. 
A geotechnical professional shall complete a geological assessment to determine whether a landslide hazard area or potential landslide area exists on or within 65 feet of the site. The following are acceptable geological assessments:
a. 
Geotechnical Verification (EMC § 14.90.060A). When a geotechnical professional finds that there is no landslide hazard area within or within 65 feet of the site.
b. 
Geotechnical Report (EMC § 14.90.060B). When a geotechnical professional finds that a landslide hazard area exists on or within 65 feet of the proposed project area or when a geotechnical professional determines that mitigation measures are necessary to construct or develop within a potential landslide hazard area.
2. 
All geotechnical assessments shall include the following information:
a. 
All the required Critical Area Report requirements per EMC § 14.10.080C.
b. 
Shall be prepared, signed, and dated by a geotechnical professional.
c. 
Shall include a field investigation and may also include review of public records and documentation, analysis of historical air photos, LiDAR mapping, published data and references, etc.
d. 
A determination of which areas on the site meet the criteria for a riverine erosion hazard area and soil erosion hazard area.
e. 
Provide all the required information pursuant to EMC § 14.90.060.
f. 
The director may require that the geotechnical assessment include analysis of abutting properties to ensure no adverse impacts will occur as a result of the requested development.
g. 
The director may require a hydrogeologic assessment (EMC § 14.60.040) to accompany the geotechnical assessment if adverse impacts are anticipated.
3. 
A geological assessment of a specific site may be valid for a period of up to five years when the proposed land use activity and surrounding site conditions are unchanged. However, if any environmental conditions associated with the site change during that five-year period, the director may require the applicant to submit an amendment to the geological assessment.
4. 
The department shall review the geological assessment and either:
a. 
Accept the geological assessment; or
b. 
Reject the geological assessment and require revisions or additional information.
(Ord. 17-513 § 3 (Exh. A); Ord. 25-0695, 12/23/2025)
A. 
General. The following performance standards apply to all landslide hazard area(s) or associated buffer(s):
1. 
The removal and disturbance of vegetation and grading activities shall be limited to the area of the approved development and shall not be allowed during the wet season from November 1st until May 1st unless adequate provisions for wet season erosion have been addressed in the geotechnical report and approved by the department.
2. 
Surface drainage from developed areas, including downspouts and runoff from paved or unpaved surfaces upslope, shall not be directed through a landslide hazard area or its associated buffer unless it is conveyed in conformance with the provisions in EMC §§ 14.90.030 and 14.90.020C.2.c.
3. 
Stormwater retention facilities, including infiltration systems utilizing perforated pipe, are prohibited unless the development complies with EMC § 14.90.020C.2.c.
4. 
Subdivision, Short Plat, Binding Site Plans (EMC Title 16).
a. 
Land that is located wholly within a landslide hazard area or its buffer may not be subdivided.
b. 
Land that is located partially within a landslide hazard area or its buffer may be divided; provided, that each resulting lot has sufficient buildable area outside of, and will not affect, the landslide hazard or its buffer.
c. 
Subdivision access roads and utilities may be permitted within the landslide hazard area and associated buffers if the city determines that no other feasible alternative exists.
d. 
The subdivision shall comply with EMC § 14.10.070.
5. 
Landslide hazard areas that are directly adjacent to any riparian areas, or wetlands, may be subject to additional buffer requirements and standards. See Chapter 14.50 EMC, Fish and Wildlife Habitat Conservation Areas, and Chapter 14.40 EMC, Wetlands, for additional details.
6. 
Prohibited Development. On-site sewage disposal systems, including drain fields, shall be prohibited within landslide hazard areas and related buffers unless compliant with subsection B of this section.
B. 
Landslide Hazard Areas Alterations. Alterations of a landslide hazard area and/or buffer may only occur for the following activities and criteria:
1. 
The activities listed in subsections (B)(2) through (B)(4) of this section shall provide a geotechnical report that make the following findings:
a. 
Will not increase the threat of the geological hazard to adjacent properties beyond predevelopment conditions;
b. 
Will not increase the threat of the geological hazard to adjacent properties beyond predevelopment conditions;
c. 
Will not adversely impact other critical areas;
d. 
Are designed so that the hazard to the project is eliminated or mitigated to a level equal to or less than predevelopment conditions; and
e. 
Are certified as safe as designed and under anticipated conditions by a geotechnical professional, licensed in the state of Washington.
f. 
Critical facilities shall not be sited within geologically hazardous areas unless there is no other practical alternative.
2. 
Stormwater Conveyance. Stormwater conveyance shall be allowed when it is conveyed through a high-density polyethylene stormwater pipe with fuse-welded joints and when no other stormwater conveyance alternative is available. The pipe shall be located on the surface of the ground and be properly anchored so that it will continue to function in the event of an underlying slide.
3. 
Utility Lines. Utility lines will be permitted when no other conveyance alternative is available. The line shall be located above ground and properly anchored and/or designed so that it will continue to function in the event of an underlying slide.
4. 
Roads, Bridges, and Trails. Roads, bridges, and trails shall be allowed when all of the following conditions have been met:
a. 
Mitigation measures are provided that ensure the roadway prism and/or bridge structure will not be susceptible to damage from landslide-induced ground deformation or impact/coverage by landslide debris. Mitigation measures shall be designed for static and seismic loading conditions in accordance with the most recent version of the American Association of State Highway and Transportation Officials (AASHTO) Manual.
b. 
The road is not a sole access for a development.
c. 
Trails shall be designed and constructed using an engineered drainage system or other methods to prevent the trail surface from becoming a drainage course.
(Ord. 17-513 § 3 (Exh. A); Ord. 25-0695, 12/23/2025)
A. 
Determining Buffer Widths.
1. 
The buffer width shall be measured on a horizontal plane from a perpendicular line established at the edge of the landslide hazard area limits, both from the top and toe of the slope.
2. 
A buffer of undisturbed vegetation shall be required for a landslide hazard area. The required buffer width is the greater amount of the distances described in this chapter:
a. 
Fifty feet from all edges of the active landslide hazard area limits; or
b. 
The distance from edge of the landslide hazard area as recommended by the geotechnical professional in a geotechnical assessment based on the site-specific conditions.
B. 
Modification of Buffer Widths. The department may require a larger buffer width than the buffer distance, as determined in subsection A of this section, if any of the following are identified:
1. 
The adjacent land is susceptible to severe erosion and erosion control measures will not effectively prevent adverse impacts.
2. 
The area has a severe risk of slope failure or downslope stormwater drainage impacts.
3. 
Downslope stormwater drainage impacts.
(Ord. 17-513 § 3 (Exh. A); Ord. 25-0695, 12/23/2025)
A. 
Geotechnical verification. A geotechnical verification shall include the following:
1. 
The general critical areas report requirements pursuant to EMC § 14.10.080.
2. 
A description of the surface and subsurface geology, hydrology, soils, and vegetation at the site and a list of the landslide hazard area indicators that were found on or in the vicinity of the site.
3. 
A summary of the results, conclusions, and recommendations resulting from the geological assessment of the landslide hazards on or in the vicinity of the site.
4. 
An accurate site plan drawn at a scale of one inch equals 20 feet, one inch equals 30 feet, one inch equals 50 feet (or other scale deemed appropriate by the department) is required. The department may require that the site plan information listed below be based on a field survey by a licensed surveyor. The site plan shall include:
a. 
The limits and location of any active landslide hazard area.
b. 
The limits and location of the required landslide hazard buffer.
c. 
The location of any existing and proposed structures, utilities, on-site septic systems, wells, and stormwater management facilities.
d. 
The full geographical limits of the proposed project area or area to be developed.
e. 
A dimension of the closest distance between the identified active landslide hazard area boundary and the project area.
f. 
Existing topography on the site presented in two-foot contours.
g. 
Property lines for the site.
h. 
North arrow and plan scale.
5. 
The geotechnical verification shall be prepared under the responsible charge of an appropriately licensed geotechnical professional(s) and signed, sealed and dated by the geotechnical professional who prepared the geotechnical verification.
6. 
Geotechnical verifications shall be in conformance with a format that is pre-approved by the department.
B. 
Geotechnical Report. At a minimum, a geotechnical report shall include the following:
1. 
The general critical areas report requirements pursuant to EMC § 14.10.080.
2. 
A description of the surface and subsurface geology, hydrology, soils, and vegetation of the site and a list of the landslide hazard area indicators that were found on or in the vicinity of the site.
3. 
A summary of the results, conclusions, and recommendations resulting from the geological assessment of the landslide hazards on or in the vicinity of the site.
4. 
An accurate site plan drawn at a scale of one inch equals 20 feet, one inch equals 30 feet, one inch equals 50 feet (or other scale deemed appropriate by the department) is required. The department may require that the site plan information listed below be based on a field survey by a licensed surveyor. The site plan shall include:
a. 
The limits and location of any landslide hazard area within the site. Delineation of the landslide hazard area limits shall identify any areas of historic landslide activity.
b. 
The limits and location of the required landslide hazard buffer.
c. 
The location of any existing and proposed structures, utilities, on-site septic systems, wells, and stormwater management facilities.
d. 
The full geographical limits of the proposed project area or area to be developed.
e. 
Location and unique identifier of geotechnical borings, CPT soundings, or other surveys or explorations used to characterize subsurface conditions.
f. 
Extent of cross-section(s) used to evaluate the three-dimensional subsurface geologic and groundwater conditions at the site.
g. 
Extent of cross-section(s) used in the evaluation of slope instability.
h. 
Existing topography on the site presented in two-foot contours.
i. 
Property lines for the site.
j. 
North arrow and plan scale.
5. 
Subsurface characterization data must be provided. The data shall be based on both existing and new information that may include soil borings, test pits, geophysical surveys, or other appropriate subsurface exploration methods, development of site-specific soil and/or rock stratigraphy, and measurement of groundwater levels including variability resulting from seasonal changes, alterations to the site, etc.
a. 
Geotechnical borings or CPT soundings will be advanced to a depth sufficient to characterize geologic conditions within and below the existing or potential landslide mass.
b. 
Other methods used for subsurface characterization shall be assigned a unique identifier, and the basic data presented in appropriate graphical and/or tabular format.
c. 
The three-dimensional subsurface conditions at the site shall be presented using one or more cross-sections showing location and depth penetration of geotechnical borings, CPT soundings, or other subsurface characterization methods, interpretation of the geometry of major soil units, and projected location of the static groundwater surface determined from the subsurface exploration. The cross-sections shall be presented at a scale of one inch equals 20 feet, one inch equals 30 feet, one inch equals 50 feet (or other scale deemed appropriate by the department). Each cross-section shall have a legend with a description of the various major soil units.
6. 
A detailed description of any prior grading activity, soil instability, or slope failure.
7. 
Where deemed appropriate by the geotechnical professional, assessments and conclusions regarding slope stability for both the existing and developed conditions shall be presented and documented. These assessments and conclusions shall include the information provided below in this section. The project geotechnical professional must provide justification for not including a slope stability analysis if one is excluded. The city's geotechnical professional reserves the right to request a slope stability analysis based on site conditions. If a dispute arises between the project geotechnical professional and the city's geotechnical professional regarding the need for a slope stability analysis, then the city reserves the right to require an independent, third party review to be paid for by the applicant to resolve the dispute.
a. 
Determination of the potential type(s) of landslide failure mechanisms, debris flow, rotational slump, or translational slip that may affect the site.
b. 
Quantitative stability evaluation of slope conditions of the various failure mechanisms using state-of-the-practice modeling techniques. Limiting equilibrium methods of analysis shall state the stability conditions as a factor of safety. The most unstable failure geometry(ies) shall be presented in the form of a cross-section(s), with the least stable failure geometry for each failure mechanism clearly indicated. The stability evaluation shall also consider dynamic (earthquake) loading, and shall use a minimum horizontal acceleration as established by the current version of the International Building Code.
c. 
An analysis of slope regression rate shall be presented in those cases where stability is impacted or influenced by erosional processes (e.g., wave cutting, stream meandering, etc.) acting on the toe of the slope.
8. 
Mitigation recommendations using engineered measures to protect the proposed structure(s) and any adjacent structures, infrastructure, adjacent wetlands, or fish and wildlife habitat conservation area from damage or destruction as a result of proposed construction activities shall be designed by a professional engineer. Design plans and detailed geotechnical recommendations may be provided in a document separate from the geotechnical report. When appropriate, such recommendations/plans may include, but are not necessarily limited to:
a. 
Design plans and associated design calculations for engineered structures or drainage systems (e.g., structural foundation requirements, retaining wall design, etc.).
b. 
Recommendations and requirements pertaining to the handling of surface and subsurface runoff in the developed condition.
c. 
Identification of necessary geotechnical inspections to assure conformance with the report mitigation and recommendations.
d. 
Proposed angles of cut and fill slopes, site grading requirements, final site topography shown as two-foot contours, and the location of any proposed structures, on-site septic systems, wells, stormwater management features, or facilities associated with the development detailed within the body of the report and shown on a site map at the same scale as that required in subsection A.4 of this section.
e. 
Soil compaction criteria and compaction inspection requirements.
f. 
An analysis that indicates how the proposal meets the standards outlined in this chapter.
g. 
Structural foundation requirements and estimated foundation settlement shall be provided if structures are proposed.
h. 
Lateral earth pressures.
i. 
Suitability of on-site soil for use as fill.
j. 
Mitigation measures for building construction on each lot for short plats, large lots, or formal plats such that additional geotechnical professional involvement is minimized during building construction.
9. 
The geotechnical report shall be prepared under the responsible charge of an appropriately licensed geotechnical professional(s) and be signed, sealed and dated by the geotechnical professional who prepared the geotechnical report.
10. 
The department may request a geotechnical professional to provide additional information in the geotechnical report based upon existing conditions, changed conditions, or unique circumstances occurring on a case-by-case basis.
11. 
Geotechnical reports shall be in conformance with a format that is pre-approved by the department.
(Ord. 17-513 § 3 (Exh. A); Ord. 25-0695, 12/23/2025)