Chapter 17.24
GEOLOGICALLY HAZARDOUS AREAS
Sections:
17.24.020 Development standards for landslide hazard areas.
17.24.030 Development standards for erosion hazard areas.
17.24.040 Seismic hazard areas standards.
17.24.050 Review and reporting requirements.
17.24.010 Designation.
The following areas are designated as geologically hazardous areas:
A. Landslide Hazard Areas. Landslide hazard areas include areas susceptible to landslides because of any combination of bedrock, soil, slope (gradient), slope aspect, structure, hydrology, or other physical factors.Potential landslide hazard areas exhibit one or more of the following characteristics:
1. Sensitive Sloped Areas. Slopes exceeding 30 percent with a vertical relief of 10 or more feet except areas composed of competent rock and properly engineered slopes designed and approved by a geotechnical engineer licensed in the state of Washington and experienced with the site;
2. Areas designated by the Soil Conservation Service as having severe limitation for building site development;
3. Areas that have shown evidence of historic failure or instability, including but not limited to back-rotated benches on slopes; areas with structures that exhibit structural damage such as settling and racking of building foundations; and areas that have toppling, leaning, or bowed trees caused by ground surface movement;
4. Slopes greater than 15 percent that have a relatively permeable geologic unit overlying a relatively impermeable unit and having springs or groundwater seepage;
5. Areas potentially unstable as a result of rapid stream incision, stream bank erosion, and undercutting by wave action include slopes exceeding 10 feet in height adjacent to streams, and lakes with more than a 30 percent gradient;
6. Areas located in a canyon or active alluvial fan, presently or potentially subject to inundation by debris flows or catastrophic flooding; and
7. Areas that are at risk of mass wasting due to seismic forces.
B. Erosion Hazard Areas. Erosion hazard areas are those areas of Chehalis containing soils that may experience severe to very severe erosion hazard including those soils groups designated in the Soil Conservation Service “Soil Survey of Lewis County, Washington” as highly erodible land and potentially highly erodible land.
C. Seismic Hazard Areas. Areas subject to severe risk of damage as a result of earthquake-induced ground shaking, slope failure, soil liquefaction or surface faulting including:
1. Areas subject to surface faulting during a seismic event;
2. Areas with underlying deposits indicative of a risk of liquefaction during a seismic event;
3. Areas subject to slope failure during a seismic event;
4. Areas that are at risk of mass wasting due to seismic forces.
Seismic hazards shall be as identified in Washington State Department of Natural Resources seismic hazard maps for Western Washington and other geologic resources. [Ord. 849B § 5, 2009.]
17.24.020 Development standards for landslide hazard areas.
Uses and activities in landslide hazard areas shall conform to the following standards:
A. Protection of Landslide Area and Buffer. The landslide hazard area and associated buffer shall be protected from disturbance, except in compliance with the standards of this section. Modification of topography and vegetation in landslide hazard areas shall be stringently limited to provide multiple benefits of long-term stability of sensitive slopes and related benefits including reduction of erosion potential, reduction of storm water runoff, and preservation of related ecological values. Unless otherwise provided or as part of an approved alteration, removal of vegetation from a landslide hazard area or related buffer shall be prohibited. If the designated landslide hazard and buffer area lacks adequate woody vegetation to provide for stability, the director shall have the authority to require vegetation restoration or other measures to improve slope stability.
B. A buffer shall be established from all edges of landslide hazard areas. The size of the buffer shall be determined as follows to minimize or eliminate the risk of property damage, death, or injury and effects on other elements of the environment resulting from earth movement caused in whole or part by the development:
1. The buffer from the top of a slope shall be designed to protect persons and property from damage due to catastrophic slope failure and slope retreat over the lifetime of the use and provide an area of vegetation to promote shallow stability, control erosion and promote multiple benefits to wildlife and other resources. The minimum dimension of the buffer shall be equal to the greater of:
a. The distance from the top of slope equal to the vertical distance from the toe of slope to the top of slope;
b. The distance from the top of slope equal to the distance from the toe of slope upslope at a slope of 2:1 (horizontal to vertical) to a point that intersects with the site’s ground elevation; or
c. Fifty feet from the top of the slope.
2. The minimum buffer from the bottom of a slope shall provide for safety of persons and property from the run-out resulting from slope failure and shall be the greater of:
a. The height of the slope; or
b. Fifty feet from the toe of the slope.
3. Buffer Reduction. The buffer may be reduced to a minimum of 10 feet based on analysis of specific development plans provided by a qualified professional that demonstrates to the director’s satisfaction that the reduction will adequately protect the proposed development, adjacent developments, and uses and other nearby critical areas.
4. Increased Buffer. The buffer may be increased where the director determines a larger buffer is necessary to prevent risk of damage to proposed and existing development.
C. Development Standards.
1. Division of land within or adjacent to landslide hazard areas and associated buffers shall be clustered to avoid landslide hazard areas and associated buffers. 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 the landslide area and buffer with provision for drainage, erosion control and related features that will not adversely affect the stability of the landslide area.
2. Alteration of a landslide hazard area and buffer in order to accommodate structures or land alteration may be authorized only in cases where the director finds that reasonable development cannot be accommodated on portions of the site not subject to landslide hazards and buffers, and if analysis by a qualified professional establishes compliance with the following standards, based on specific development plans:
a. The proposed development will not result in a risk of landslide that may affect development on the subject property or other properties in the vicinity, and will not result in a greater risk or a need for increased buffers on neighboring properties. For unconsolidated deposits, development shall not decrease the factor of safety for landslide occurrences below the limits of 1.5 for static conditions and 1.2 for dynamic conditions. Analysis of dynamic conditions shall be based on a minimum horizontal acceleration as established by the current version of the International Building Code.
b. Measures to maintain slope stability, such as drainage systems, must be of a design that will assure operation without facilities requiring regular maintenance that would jeopardize stability if the facility fails.
c. The development will not increase erosion or sedimentation risk on the site.
d. The development will not increase surface water discharge or sedimentation to adjacent properties beyond pre-development conditions.
e. Such alterations will not adversely impact other critical areas.
f. Structures shall be located on the least sensitive portion of the site and clustered where possible to reduce disturbance and removal of vegetation.
g. Grading shall minimize alterations to the natural contour of the slope.
h. Foundations should conform to the natural contours of the slope and foundations should be stepped/tiered where possible to conform to existing topography of the site.
i. Retaining walls shall be preferred over cut and fill and shall be incorporated into structures wherever feasible.
j. Landslide hazard areas on unconsolidated deposits with a gradient of 40 percent where the toe of slope is within the buffer area of a wetland, stream, pond or lake are not eligible for alteration of landslide hazard areas or but may be subject to alteration of buffers, subject to compliance with the standards of this chapter.
3. Critical facilities, including, but not limited to, schools, nursing homes, hospitals, police, fire and emergency response installations, and installations that produce, use, or store hazardous materials shall not be located in landslide hazard areas if there is a feasible alternative location outside the hazardous areas that would serve the intended service population. A facility may be allowed only subject to the standards in subsection (C)(2) of this section.
4. Utility lines and pipes shall be permitted in erosion and landslide hazard areas only when the applicant demonstrates that no other practical alternative is available. The line or pipe 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. Storm water conveyance shall be allowed only through a high-density polyethylene pipe with fuse-welded joints, or similar product that is technically equal or superior.
5. Point discharges from surface water facilities and roof drains onto or upgradient from an erosion or landslide hazard area shall be prohibited.
6. Roads, driveways and other vehicular access, trails and walkways, may be permitted only if the applicant demonstrates that no other feasible alternative exists, including through the provisions of Chapter 8.24 RCW and subject to the standards in subsection (C)(2) of this section. If access through a hazard area is granted, exceptions or deviations from technical standards for width or other dimensions, and specific construction standards to minimize impacts may be specified. Access roads and trails shall be engineered and built to standards that avoid the need for major repair or reconstruction beyond that which would be required in non-hazard areas and shall be:
a. Located in the least sensitive area of the site.
b. Designed to minimize topographic modification with low gradients and/or parallel to the natural contours of the site.
c. Retaining walls shall be preferred over cut and fill slopes to minimize topographic modification.
d. Clearing and grading shall minimize ground disturbance to the maximum extent feasible to accommodate allowed development and generally shall not extend more than 10 feet beyond the approved development.
D. A qualified professional, licensed in the state of Washington, shall review projects in geologically hazardous areas to ensure that they are properly designed and constructed. [Ord. 849B § 5, 2009.]
17.24.030 Development standards for erosion hazard areas.
A. Within erosion hazard areas disturbance of natural vegetation shall be limited. The following chart sets forth the maximum disturbance allowed on a site:
|
Slope |
Disturbance Allowed |
|
0 to 15 percent |
100 percent |
|
15 to 25 percent |
60 percent |
|
25 to 40 percent |
45 percent |
|
Greater than 40 percent |
0 percent |
1. The overall disturbance allowed on development sites which have any combination of the above slope categories shall be determined by the following formula: (square footage of the site having 0 to 15 percent slopes) times 1.00 plus (square footage of site having 15 to 25 percent slopes) times .60 plus (square footage of site having 25 to 40 percent slopes) times .45 equals total allowable site disturbance.
2. Areas protected as critical areas by other provisions of this chapter shall be eliminated from the calculation of allowed site disturbance in this subsection.
3. The total allowable site disturbance limits shall be applied to the entire site and shall include all disturbance over the life of the project.
4. The disturbed area of the site shall be located within areas of the least sensitive portions of the site.
5. Areas to be preserved as undisturbed shall be located on site plans and protected from disturbance during construction and use. Areas to be preserved in subdivisions shall be indicated on the face of the plat in accordance with CMC 17.21.085, Notice on title. Disturbance limits shall be observed in subsequent development of lots.
6. Disturbance limits shall not be applied to existing single-family residential lots less than 20,000 square feet in size that were created prior to the adoption of the ordinance codified in this chapter.
B. Structures shall be located on the least sensitive portion of the site and clustered where possible to reduce disturbance and removal of vegetation.
C. Grading shall minimize alterations to the natural contour of the slope. Building foundations shall conform to the natural contours of the slope and be stepped/tiered to conform to existing topography of the site.
D. Retaining walls shall be preferred over cut and fill for roads, parking lots and structures. Structures on slopes in excess of 25 percent shall incorporate earth retaining structures in buildings rather than employing free-standing earth retention structures. Clearing and grading shall minimize ground disturbance to the maximum extent feasible and generally shall not extend more than 10 feet beyond the approved development.
E. All structures or impervious surface improvements shall a be required to have on-site drainage systems to meet the specifications of the public works department to control conveyance of storm water to avoid erosion hazard areas. Point discharges or overland dispersion systems from surface water facilities and roof drains onto or upstream from an erosion or landslide hazard area shall be prohibited from discharging onto slopes in excess of five percent. Conveyance should be provided to the foot of slopes.
F. Roads, driveways and other vehicular access, trails and walkways shall be:
1. Located in the least sensitive area of the site.
2. Designed to minimize topographic modification with low gradients and/or parallel to the natural contours of the site.
3. Retaining walls shall be preferred over cut and fill slopes to minimize topographic modification.
G. Logging activity allowed on slope exceeding 25 percent shall be partial cutting only and not clear cutting. “Partial cutting” is defined here as per WAC 222-16-010. In addition, subsequent harvest shall not create a condition inconsistent with that definition. Timber harvest in these areas shall be consistent with all applicable laws including but not limited to Chapter 222-30 WAC, Timber Harvesting, Chapter 222-34 WAC, Reforestation, and Chapter 222-38 WAC, Forest Chemicals. [Ord. 849B § 5, 2009.]
17.24.040 Seismic hazard areas standards.
Development may be allowed in seismic hazard areas when all of the following apply:
A. If evaluation of site-specific subsurface conditions by a qualified professional demonstrates that the proposed development site is not subject to the conditions indicating seismic risk, the provisions of this section shall not apply.
B. If a site is subject to seismic risk, the applicant shall implement appropriate engineering design based on analysis by a qualified professional of the best available engineering and geological practices that either eliminates or minimizes the risk of structural damage or injury resulting from seismically induced settlement or soil liquefaction, including compliance with the following criteria:
1. Subdivision within seismic hazard areas shall assure that each resulting lot has sufficient buildable area outside of the hazard area or that appropriate limitations on building and reference to appropriate standards are incorporated into subdivision approval and may be placed as restrictions on the face of the plat;
2. Structures in seismic hazard areas shall conform to applicable analysis and design criteria of the International Building Code;
3. Public roads, bridges, utilities and trails shall be allowed when there are no feasible alternative locations and geotechnical analysis and design are provided that ensure the roadway, bridge and utility structures and facilities will not be susceptible to damage from seismic induced ground deformation. Mitigation measures shall be designed in accordance with the most recent version of the American Association of State Highway and Transportation Officials (AASHTO) Manual or other appropriate document.
C. The director may waive or reduce engineering study and design requirements for alterations in seismic hazard areas for:
1. Mobile homes;
2. Additions or alterations to existing structures that do not increase occupancy or significantly affect the risk of structural damage or injury; and
3. Buildings that are not dwelling units or used as places of employment or public assembly. [Ord. 849B § 5, 2009.]
17.24.050 Review and reporting requirements.
A. When critical area maps or other sources of credible information indicate that a site proposed for development or alteration is or may be located within a geologically hazardous area, the director shall have the authority to require the submittal of a geological hazard assessment report.
B. A geological hazard assessment report is an investigation process to evaluate the geologic characteristics of the subject property and adjacent areas. The geological assessment shall include field investigation and may include the analysis of historical aerial photographs, review of public records and documentation, and interviews with adjacent property owners. The report shall include the following:
1. A description of which areas on the site, surrounding areas that influence or could be influenced by the site, or areas within 300 feet of the site meet the criteria for geological hazard.
2. A scaled site plan showing:
a. The type and extent of geologic hazard areas, and any other critical areas, and buffers on, adjacent to or that are likely to impact or influence the proposal, including properties upslope of the subject site;
b. The location of existing and proposed structures, fill, access roads, storage of materials, and drainage facilities, with dimensions indicating distances to the floodplain;
c. The existing site topography preferably accurate to within two-foot contours; and
d. Clearing limits.
3. A description of the site features, including surface and subsurface geology, hydrology, soils, and vegetation found in the project area and in all hazard areas addressed in the report. This may include surface exploration data such as borings, drill holes, test pits, wells, geologic reports, and other relevant reports or site investigations that may be useful in making conclusions or recommendations about the site under investigation.
4. A description of the processes affecting the property or affected by development of the property including soil erosion, deposition, or accretion.
5. A description of the vulnerability of the site to seismic and other geologic processes and a description of any potential hazards that could be created or exacerbated as a result of site development.
C. If development is proposed in an area subject to geologic hazards, the assessment shall include:
1. A description and analysis of the level of risk associated with development that complies with prohibitions and buffers associated with this code;
2. A description and analysis of the level of risk associated with alternative proposals for development within or with less setback from the area of geological hazard including risk to future occupants of the subject property, adjacent property, other critical areas and the general public safety;
3. A description and analysis of the level of risk associated with the measures proposed to mitigate the hazards, ensure public safety, and protect property and other critical areas, including the risk of failure if structures, drainage systems or other facilities are not monitored, maintain, or cease to function as designed for any reasons;
4. A description and analysis of the level of risk associated with increased erosion or sedimentation risk on the site and potential effects on adjacent properties, water bodies and wetlands;
5. For projects in or affecting landslide hazard areas the report shall also include:
a. Assessments and conclusions regarding slope stability for both the existing and developed conditions including the potential types of landslide failure mechanisms (e.g., debris flow, rotational slump, translational slip, etc.) that may affect the site. 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;
b. Description of 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 and critical areas;
c. For proposed development on unconsolidated deposits, analysis of whether the development results in a factor of safety for landslide occurrences below the limits of 1.5 for static conditions and 1.2 for dynamic conditions. Analysis of dynamic conditions shall be based on a minimum horizontal acceleration as established by the current version of the International Building Code;
6. For projects in seismic hazard areas, the report shall also include a detailed engineering evaluation of expected ground displacements or other liquefaction and/or dynamic settlement effects and proposed mitigation measures to ensure an acceptable level of risk for the proposed structure type or other development facilities such as access roads and utilities. [Ord. 849B § 5, 2009.]