Chapter 19.07
GEOLOGICALLY HAZARDOUS AREAS
Sections:
19.07.040 Critical areas reports for landslide hazards.
19.07.010 Applicability.
Geologically hazardous areas may pose a threat to the health and safety of citizens when incompatible development is sited in areas of significant hazard. Such incompatible development may not only place itself at risk, but may also increase the hazard to surrounding development and uses. Areas susceptible to one or more of the following types of geological hazards shall be designated as geologically hazardous areas. The geologically hazardous area provisions in this chapter apply to regulated activities in the following areas:
A. Erosion Hazard Areas. As identified by the U.S. Department of Agriculture Natural Resources Conservation Service (USDA-NRCS) as having “severe” or “very severe” erosion susceptibility.
B. Landslide Hazard Areas. As defined in Chapter 19.02 and mapped as landslides, scarps, and flanks in the Washington State Department of Natural Resources’ (DNR) “2018, Landslide Inventory and Susceptibility of the Columbia Gorge in Clark, Skamania, and Klickitat Counties, Washington.”
C. Seismic Hazard Areas. As defined in Chapter 19.02 and indicated by:
1. A Zone D1 or higher rating as defined by the Seismic Design Category Maps for Residential Construction in Washington by the DNR (2007 or as updated) and defined in the International Residential Code and areas with Site Class C to D, D, D to E, E, and F, as defined by the “Site Class Map of Skamania County, Washington” (September 2004 or as updated), by the DNR and as defined in the International Building Code (IBC).
2. Areas of “low to moderate” or greater liquefaction susceptibility as mapped by the DNR on their “Liquefaction Susceptibility Map of Skamania County, Washington” (September 2004 or as updated).
D. Volcanic Hazard Areas. As defined in Chapter 19.02 and identified on the Volcanic Hazards Zones maps produced by the United States Geological Survey (USGS) for Mount St. Helens and Mount Adams. Volcanic hazard zones regulated under this title include near-volcano hazards and lahar zones on the USGS maps. (Ord. 2020-03, 2-25-20)
19.07.030 Regulations.
A. Permitted Alterations in Geologically Hazardous Areas.
1. Alterations. Alterations of geologically hazardous areas or associated buffers may occur only for activities that:
a. Will not increase the threat of the geological hazard to adjacent properties beyond predevelopment conditions;
b. Will not impact other critical areas adversely;
c. Are designed so that the hazard to the project is eliminated or mitigated to a level equal to or less than predevelopment conditions; and
d. Are recommended by a qualified professional in a signed and stamped geologic hazards assessment or geotechnical engineering report, whichever is required by this chapter.
2. Critical Facilities. Critical facilities, as defined in Chapter 19.02, shall not be sited in geologically hazardous areas unless there is no other practical alternative, as demonstrated in a geotechnical assessment.
3. Utilities Transmission Facilities. Utility facilities that carry liquid petroleum products or any other hazardous substance as defined in Chapter 173-303 WAC may be permitted within geologically hazardous areas only when a qualified professional demonstrates that the design and location of the proposed facility will not cause adverse impacts.
B. Notification of Hazard.
1. Development Notification. As part of any critical areas review approval under this section, the owner of the property subject to development shall record a notice with the Skamania County auditor in the form set forth below:
GEOLOGIC HAZARD AREA NOTICE
Tax Parcel No.:________________________
Address:_____________________________
Legal Description:______________________
Notice: This site lies within a geologic hazard area. Restrictions on use or alteration of the site may exist. For more information, contact the Skamania County Community Development Department.
C. Design Standards – Seismic Hazard Areas.
1. Development proposed in seismic hazard areas shall conform to the provisions of the International Residential Code or IBC that apply to structural standards and safeguards to reduce risks from seismic activity.
2. For sites where a seismic hazard area is the only critical area present, a critical areas report and review are not required.
D. Design Standards – Erosion Hazard Areas.
1. Erosion and Control Plan. For any development proposal on a site containing an erosion hazard area, an erosion control plan shall be required addressing temporary erosion from construction activities as well as the potential for erosion after construction and any necessary mitigations, unless the applicant submits a letter from a qualified professional which documents that no erosion hazard is present. The erosion control plan shall be prepared in compliance with requirements set forth in Section 24.02.090 and shall follow all relevant best management practices for erosion and sedimentation control outlined in the most current version of the Stormwater Management Manual of Western Washington (SMMWW). The erosion control plan shall also demonstrate that clearing, grading, and impervious surfaces have been minimized and that the proposed development will not increase erosion hazards after construction. Erosion control plans shall be prepared by a qualified professional.
2. For sites where an erosion hazard area is the only critical area present, a critical areas report and review are not required. When other critical areas are present on the site any required critical areas reports shall address how the regulated activity and site alterations may impact the erosion hazard area.
E. Design Standards – Landslide Hazard Areas.
1. Applicants proposing regulated activities within mapped landslide hazard areas or areas located within one hundred feet of mapped landslide hazard areas must complete a geotechnical letter documenting whether the hazard is present. The county may require a geotechnical letter for areas located more than one hundred feet away from a mapped hazard if, in its professional judgement, a further investigation is required to determine if a hazard is present. Conversely, the county may waive this requirement for areas located less than one hundred feet away from a mapped hazard.
2. For sites confirmed to contain a hazard, the applicant must provide a geotechnical assessment consistent with Section 19.07.040 discussing whether the activity will increase the risk of erosion or landslide and, if so, whether mitigations are necessary. If the applicant demonstrates through a geotechnical assessment that the regulated activity and any site alterations will not impact the erosion or landslide hazard area, then the requirement to submit a geotechnical report will be waived and a critical areas review is not required.
3. A geotechnical report will be required when the assessment does not clearly demonstrate that the regulated activities and site alterations do not impact the landslide hazard area; the geotechnical assessment recommends additional geotechnical study; and/or the regulated activity is within an identified active landslide hazard area.
4. Buffers. A buffer shall be established from all edges of landslide hazard areas.
a. Buffer Size. A buffer shall be established from the edge of all landslide hazard areas with the width of the buffer determined based on the recommendations of a geotechnical assessment or report.
b. The buffer shall be clearly staked before and during any construction or clearing.
c. All portions of landslide hazard areas and buffers shall be designated as landslide protection areas.
5. Design and Development Standards. Development or alterations within a landslide hazard area and/or buffer shall be designed to meet the following requirements. Deviations from one or more of these standards may be permitted where it can be demonstrated by a qualified professional that an alternative design provides equal or greater protection of the critical area and proposal. The basic development design standards are:
a. Structures and improvements shall be located to avoid landslide hazard areas and other critical areas;
b. Structures and improvements shall minimize alterations to the natural contour of the slope, and foundations shall be tiered where possible to conform to existing topography;
c. Structures and improvements shall be located to preserve the most critical portion of the site and its natural landforms and vegetation;
d. The proposed development shall not result in an increase in surface water discharge or sedimentation to adjacent properties.
F. Design Standards – Volcanic Hazard Areas.
1. The following uses are allowed in volcanic hazard areas without review:
a. Agriculture and forestry activities;
b. Open space and passive recreational uses, which may include hiking, boating, fishing, etc.;
c. Maintenance and repair of existing structures, roads, trails, educational facilities, and serviceable structures;
d. Structures less than two hundred square feet in area, which are not subject to building permit requirements; and
e. Exploratory or scientific research, fish and wildlife enhancement projects, or similar activities.
2. For all other uses, development proposals must include an evacuation and emergency management plan. At a minimum, the evacuation and emergency management plan must demonstrate that the evacuation route has been determined to not contain any other potential natural hazards, such as landslide or flood hazards, that could cause a blockage or destruction of the evacuation route during an event (i.e., seismic event triggers a landslide that results in the evacuation route becoming impassible). An existing area-wide evacuation and emergency management plan that includes the proposed development site can be used to satisfy this requirement.
3. Critical areas reports are not required for volcanically hazardous areas. (Ord. 2020-03, 2-25-20)
19.07.040 Critical areas reports for landslide hazards.
A. Applicants proposing regulated activities in mapped landslide hazard areas must complete a geotechnical letter and/or assessment and report as required by Section 19.07.030 and consistent with the requirements in this section.
B. Site Plan(s) – Assessments and Reports. The following information must be included on all site plans in geotechnical assessments and reports. For sites determined not to contain or be adjacent to a geologically hazardous area, the administrator will accept a letter from a qualified professional documenting that no hazards are present. For sites with documented hazards, a site plan, included with the geotechnical assessment or report, must be provided with the following information:
1. The type and extent of geologic hazard areas and buffers on, adjacent to, within two hundred feet of, or that are likely to impact the proposal;
2. Proposed development activity area, including the location of existing and proposed structures, septic drain field and reserve areas, clearing limits, fill locations, storage of materials, and drainage facilities;
3. Dimensions of the proposed development activity to the property lines, critical areas, landslide scarp and slide mass;
4. A contour map of the proposed site, at a scale of one inch equals twenty feet or other scale as deemed appropriate by the administrator. Slopes shall be clearly delineated for the ranges between fifteen percent and twenty-five percent, twenty-five percent and forty percent, and forty percent or greater, including the height of slope, slope gradient, and cross section of the project area;
5. The location and type of surface water runoff features, including the location of springs, seeps, or other surface expressions of groundwater on or within two hundred feet of the project area or that have potential to be affected by the proposal; and
6. When site-specific conditions indicate the necessity, the administrator may require the topographic data to be field surveyed.
C. Geotechnical Assessment. A geotechnical assessment is an overview of site conditions and a professional evaluation of the need for additional studies prior to development on the property. The geotechnical assessment is intended to be a limited study of geological hazards (i.e., landslide, seismic, and erosion hazards) at a site, and does not include the collection of any subsurface soil, rock, and groundwater data. If determined to be necessary, the qualified professional will recommend the preparation of a detailed geotechnical report that includes the investigation of subsurface soil, rock, and groundwater conditions to better assess the geological site hazards. The geologic assessment shall include, but not be limited to:
1. A field reconnaissance of the site and vicinity;
2. A detailed description of the project;
3. Site plan of the area, including all items specified in subsection (B) of this section;
4. Feasibility of the site for the proposed development activity;
5. A description of the surface and subsurface geology, hydrology, soils, and vegetation found in the project area;
6. A discussion of the project and its relationship to the geologic hazards found on site, which may include the type and extent of geologic hazard areas and buffers on, adjacent to, within two hundred lateral feet of, or that are likely to impact the proposal or be impacted by the proposal;
7. An overview of any field investigations, published data, and references; data and conclusions from past assessments of the site; and site-specific measurements, tests, investigations, or studies that support the identification or lack of geologically hazardous areas;
8. If applicable, an identification of any areas of the site recommended to be avoided by any aspect of the proposed development;
9. If necessary, identification of mitigation measures needed to address any anticipated geologic problems, including necessary erosion control and drainage plans, seasonal grading and clearing restrictions, the necessity for retaining vegetation for slope stability, and limitations on the placement of utility lines, pipes, and sewerage disposal systems;
10. If applicable, development recommendations for the proposed activities. These recommendations should include, but are not limited to, structure and septic system setbacks from geologic hazard areas if less than or greater than the minimum established buffer or if they do not meet IBC setback requirements, foundation design, filling and excavation, erosion control, drainage, and site preparations;
11. Discussion regarding the need for follow-up studies that should be conducted, such as geotechnical engineering reports, additional subsurface exploration, or more extensive soil reports; and
12. Demonstration that all applicable development standards are met.
D. Geotechnical Engineering Report. A geotechnical engineering report will be required for any development proposal on a site containing an active landslide hazard area or landslide and erosion hazard that the geotechnical assessment identified for further geotechnical analysis. It is the responsibility of the qualified professional to provide a report and design recommendations that are appropriate for existing site conditions and the proposed development. The geotechnical engineering report shall be completed and stamped by a qualified professional and shall present results and engineering recommendations. The following information must be included in all geotechnical engineering critical areas reports:
1. The requirements listed within the geotechnical assessment criteria.
2. Site history – description of any prior grading, soil instability or slope failure.
3. Subsurface data – logs of borings, test pits, and other exploratory methods; soil and rock stratigraphy; groundwater levels, including seasonal changes and laboratory tests.
4. The results of any laboratory tests – laboratory data and soil index properties for soil samples.
5. The effect construction and placement of structures will have on the slope over the estimated life of the structure.
6. Vulnerability of the site to erosion.
7. The location, dimensions, and estimated depth of any area(s) of slope instability on the site.
8. Description of analysis performed and results of that analysis.
9. An estimate of the bluff retreat rate that recognizes and reflects potential catastrophic events, such as seismic activity or a one-hundred-year storm event.
10. Assumed or established site and subsurface conditions used in the slope stability analysis (e.g., slope dimensions of any landslides, thickness and strengths of soil and rock units, depth to failure plane if any, groundwater levels, etc.).
11. Consideration of the run-out hazard of landslide debris and/or the impacts of landslide run-out on downslope properties.
12. Parameters for design of site improvements, including:
a. Building limitations;
b. Suitability of on-site soil for use as fill;
c. Soil compaction criteria;
d. Proposed angles and heights of cut and fill slopes;
e. Appropriate foundations and retaining structures. These should include allowable bearing capacities and lateral earth pressures appropriate for the design of foundations and walls, installation considerations, and estimates of settlement performance;
f. Recommendations for drainage and subdrainage and utility line improvements for collection, transport, treatment, discharge, and/or recycling of water, including consideration of on-site septic system disposal volumes where the additional volume will affect the landslide hazard area;
g. Earthwork recommendations, including clearing and site preparation criteria, fill placement, temporary and permanent slope inclinations and protection, and temporary excavation support, if necessary;
h. Mitigation of adverse site conditions, including slope stabilization measures and seismically unstable soils, if appropriate; and
i. Stability.
i. For terrain containing no obvious instability or identified landslides, the proposed site grading or structures shall not reduce the existing slope stability on the development site or adjacent sites to less than a factor of safety of 1.5 for static conditions and 1.2 for dynamic conditions.
ii. For terrain containing active or inactive landslides or for proposed structures, cuts, fills or roads near designated or identified areas of instability, the proposed development shall not decrease the factor of safety below the limits of 1.5 for static conditions and 1.2 for dynamic conditions at or adjacent to the development location.
iii. The administrator may consider lower factors of safety, if the qualified professional provides a detailed explanation that satisfies the administrator regarding why lower factor of safety values present an acceptable level of risk to the development.
iv. Designs that require regular or periodic maintenance to maintain their level of function are excluded from long-term slope stabilization measures.
E. Incorporation of Previous Study. Where a valid critical areas assessment or geotechnical engineering report has been prepared within the last five years for a specific site, and where the proposed land use activity and surrounding site conditions are unchanged, said assessment or report may be incorporated into the required critical area assessment. The applicant shall submit a hazards assessment detailing any changed environmental conditions associated with the site.
F. Mitigation of Long-Term Impacts. When hazard mitigation is required, the mitigation plan shall specifically address how the activity maintains or reduces the preexisting level of risk to the site and adjacent properties on a long-term basis (equal to or exceeding the projected life span of the activity or occupation). Proposed mitigation techniques shall be considered to provide long-term hazard reduction only if they do not require regular maintenance or other actions to maintain their function. Mitigation may also be required to avoid any increase in risk above the preexisting conditions following abandonment of the activity. (Ord. 2020-03, 2-25-20)