Chapter 14.90
LANDSLIDE HAZARD AREAS

Sections:

14.90.010    Purpose.

14.90.020    Landslide hazard areas.

14.90.030    Landslide hazard area review procedures.

14.90.040    Landslide and erosion hazard area standards.

14.90.050    Buffer requirements.

14.90.060    Appendices.

14.90.010 Purpose.

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)).

14.90.020 Landslide hazard areas.

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.

B. Potential Landslide Hazard Areas. Potential landslide hazard areas, as depicted on the geologically hazardous areas map, are those areas where the suspected risk of slope instability and landslide is sufficient to require a geological assessment to assess 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 geologically hazardous areas map as moderate or steep slope areas. (Ord. 17-513 § 3 (Exh. A)).

14.90.030 Landslide hazard area review procedures.

A. General Requirements.

1. The city’s geologically hazardous areas map 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.

2. The department will complete a review of the geologically hazardous areas map 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.

3. 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.

4. Unless otherwise stated in this chapter, the critical protective measure provisions contained in Chapter 14.10 EMC shall apply.

B. Geological Assessment. A geological assessment is a site investigation process to evaluate the on-site geology affecting a subject property. The findings of the geological assessment shall be presented in a landslide hazard geotechnical verification or geotechnical report.

1. Geological assessments shall be submitted to the department for review and approval together with a landslide hazard area application and associated fee.

2. A geological assessment shall include a field investigation and may include the use of historical air photo analysis, LiDAR mapping, review of regional geologic mapping, review of public records and documentation, and interviews with adjacent property owners, etc.

3. The geological assessment shall include the following information and analysis:

a. A determination of which areas on the site or within the vicinity of the site meet the criteria for a landslide hazard area as set forth herein.

b. Consider the run-out hazard of landslide debris to the proposed development that starts upslope (whether part of the subject property or on a neighboring property) and/or the impacts of landslide run-out on downslope properties.

c. The geological assessment shall include a detailed review of the field investigations, published data and references, data and conclusions from past geological assessments, or geotechnical investigations of the site, site-specific measurements, tests, investigations, or studies, as well as the methods of data analysis and calculations that support the results, conclusions, and recommendations.

d. All of the information required per EMC 14.10.080(C).

4. Geological assessments shall be prepared, signed, and dated by a geotechnical professional. The format shall be pre-approved by the department.

5. A geotechnical professional shall complete a field investigation and geological assessment to determine whether or not a landslide hazard area is likely to exist within 300 feet of the site. Where access to off-site properties is not available to the geotechnical professional, evaluation of off-site landslide hazards must include review of regional geologic mapping and LiDAR based topographic mapping.

a. The geological assessment shall be submitted in the form of geotechnical verification when the geotechnical professional finds that no landslide hazard area exists within 300 feet of the project area.

b. The geological assessment shall be submitted in the form of a geotechnical report when the geotechnical professional finds that a landslide hazard area exists within 300 feet of the proposed project area or when a geotechnical professional determines that mitigation measures are necessary in order to construct or develop within a potential landslide hazard area.

6. Geological assessments that do not contain the required information will be returned to the geotechnical professional for revision.

7. 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.

8. A geological assessment for 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 applicant may be required to submit an amendment to the geological assessment. (Ord. 17-513 § 3 (Exh. A)).

14.90.040 Landslide and erosion hazard area standards.

A. Landslide Hazard Areas. Any development, encroachment, clearing and grading, building structures, impervious surfaces, or vegetation removal shall be prohibited within landslide hazard areas and associated buffers except as specified in the following standards:

1. Stormwater Conveyance. Stormwater conveyance shall be allowed when it is conveyed through a highdensity 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.

2. 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.

3. 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. The removal or disturbance of vegetation and grading activities shall be prohibited during the wet season from November 1st until May 1st.

B. Landslide Hazard Management Areas. All regulated activities may be allowed in areas located within 300 feet of a landslide hazard area subject to the following standards:

1. The department reviews and approves a geological assessment – geotechnical report and determines that the potential landslide hazard area is stable.

2. The proposed development is located outside of a landslide hazard area and any required buffer.

3. The proposed recommendations and mitigation measures contained within the geotechnical report are adequate to reduce or mitigate risks to health and safety.

4. The proposed development shall not decrease the factor of safety for landslide occurrence below the limits of 1.5 for 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.

5. 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.

6. 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.

7. Stormwater retention facilities, including infiltration systems utilizing perforated pipe, are prohibited unless the slope stability impacts of such systems have been analyzed and mitigated by a geotechnical professional and the impacts have been determined to be negligible.

8. The proposed development shall not create a need for larger landslide hazard area buffers and setbacks on neighboring properties unless approved through a notarized written agreement with the affected property owner(s).

9. The proposed development shall be sited far enough from regressing slope faces to project 120 years of useful life for the proposed structure(s) or infrastructure.

10. Any proposed lots must be completely located outside any identified landslide hazard areas or their associated buffers.

11. 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, Critical Fish and Wildlife Habitat Areas, and Chapter 14.40 EMC, Wetlands, for additional details. (Ord. 17-513 § 3 (Exh. A)).

14.90.050 Buffer requirements.

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. A distance of one-third the height of the slope at the top of the active landslide hazard area and a distance of one-half the height of the slope at the bottom of an active landslide hazard area.

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. (Ord. 17-513 § 3 (Exh. A)).

14.90.060 Appendices.

A. Geological Assessment – Landslide Hazard Geotechnical Verification.

B. Geological Assessment – Landslide Hazard Geotechnical Report.

APPENDIX A

GEOLOGICAL ASSESSMENT – LANDSLIDE HAZARD GEOTECHNICAL VERIFICATION

A. A geotechnical verification shall include the following:

1. The general critical areas report requirements.

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.

B. The geotechnical professional who prepared the verification document shall stamp the verification with their license stamp or seal.

C. Geotechnical verifications shall be in conformance with a format that is pre-approved by the department.

APPENDIX B

GEOLOGICAL ASSESSMENT – LANDSLIDE HAZARD GEOTECHNICAL REPORT

A. At a minimum, a geotechnical report shall include the following:

1. The general critical areas report requirements.

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 appendix. 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-ofthe-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 critical fish and wildlife habitat 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 appendix.

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.

B. The geotechnical report shall be prepared by an engineering geologist and shall be co-written by both an engineering geologist and professional engineer where both geological interpretations and engineering analyses and designs are necessary or prudent in the mitigation of the landslide hazard.

C. The geotechnical professional(s) who prepared the geotechnical report shall stamp the report with their license stamp or seal.

D. 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.

E. Geotechnical reports shall be in conformance with a format that is pre-approved by the department. (Ord. 17-513 § 3 (Exh. A)).