Chapter 13.13 CRITICAL AREAS—GEOLOGICALLY HAZARDOUS AREAS

Chapter 13.13
CRITICAL AREAS—GEOLOGICALLY HAZARDOUS AREAS

Sections:

13.13.005 Purpose, goals, and policies.

13.13.010 Designation of geologically hazardous areas.

13.13.020 Designation of specific hazard areas.

13.13.030 Mapping of geologically hazardous areas.

13.13.040 Critical area report—Additional requirements for geologically hazardous areas.

13.13.050 Performance standards—General requirements.

13.13.060 Performance standards—Specific hazards.

13.13.005 Purpose, goals, and policies.

Purpose. It is the intent of the city to reduce the threat posed to the health and safety of its citizens from commercial, residential or industrial development that may be sited in areas of significant geologic hazard. In some cases, it is recognized that risks from geologic hazards can be reduced or mitigated to acceptable levels through engineering design or modified construction practices. In other cases where technological efforts are not sufficient to reduce associated risks, building is best avoided. The following reflect the goals and policies of the community:

(a) Goal. Provide appropriate measures to either avoid or mitigate significant risks that are posed by geologic hazard areas to public and private property and to public health and safety.

(b) Policies. When probable significant adverse impacts from geologically hazardous areas are identified during the review of a development application, documentation that fully addresses these potential impacts and identifies alternative mitigation measures to eliminate or minimize the impacts will be required. (Ord. 1505 § 5 (part), 2018).

13.13.010 Designation of geologically hazardous areas.

Geologically hazardous areas include areas susceptible to erosion, sliding, earthquake, or other geological events. They 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 also may increase the hazard to surrounding development and use. Areas susceptible to one or more of the following types of hazards shall be designated as a geologically hazardous area:

(a) Erosion hazard;

(b) Landslide hazard;

(d) Other geological events including mass wasting, debris flows, rock falls, and differential settlement. (Ord. 1505 § 5 (part), 2018).

13.13.020 Designation of specific hazard areas.

(a) Erosion Hazard Areas. Erosion hazard areas are those areas which, according to the U.S. Department of Agriculture’s Natural Resources Conservation Service Soil Survey Program, may experience significant erosion. Erosion hazard areas include areas likely to become unstable, such as bluffs, steep slopes, and areas with unconsolidated soils. Cities may consult with the Natural Resources Conservation Service for data to help identify erosion hazard areas (WAC 365-190-030(5) and 365-190-120(5)).

(b) Landslide Hazard Areas. Landslide hazard areas are areas potentially subject to landslides based on a combination of geologic, topographic, and hydrologic factors. They include areas susceptible because of any combination of bedrock, soil, slope (gradient), slope aspect, structure, hydrology, or other factors. Examples of these may include but are not limited to the following:

(1) Areas of historic failures, such as:

(A) Those areas delineated by the U.S. Department of Agriculture’s Natural Resources Conservation Service as having a severe limitation for building site development;

(B) Those areas mapped by the Washington State Department of Natural Resources (slope stability mapping) as unstable (U or Class 3), unstable old slides (UOS or Class 4), or unstable recent slides (URS or Class 5); and

(C) Areas designated as quaternary slumps, earthflows, mudflows, or landslides on maps published by the U.S. Geological Survey or Washington State Department of Natural Resources;

(2) Areas with all three of the following characteristics:

(A) Slopes steeper than fifteen percent;

(B) Hillsides intersecting geologic contacts with a relatively permeable sediment overlying a relatively impermeable sediment or bedrock; and

(3) Areas that have shown movement during the Holocene epoch (from ten thousand years ago to the present) or that are underlain or covered by mass wastage debris of that epoch;

(4) Slopes that are parallel or subparallel to planes of weakness (such as bedding planes, joint systems, and fault planes) in subsurface materials;

(5) Slopes having gradients steeper than eighty percent subject to rock fall during seismic shaking;

(6) Areas potentially unstable because of rapid stream incision, stream bank erosion, and undercutting by wave action;

(7) Areas that show evidence of or are at risk from snow avalanches;

(8) Areas located in a canyon or on an active alluvial fan, presently or potentially subject to inundation by debris flows or catastrophic flooding; and

(9) Any area with a slope of forty percent or steeper and with a vertical relief of ten or more feet except areas composed of consolidated rock. A slope is delineated by establishing its toe and top and is measured by averaging the inclination over at least ten feet of vertical relief.

(c) Seismic Hazard Areas. Seismic hazard areas are areas subject to severe risk of damage as a result of earthquake-induced ground shaking, slope failure, settlement, soil liquefaction, lateral spreading, or surface faulting. One indicator of potential for future earthquake damage is a record of earthquake damage in the past. Ground shaking is the primary cause of earthquake damage in Washington. The strength of ground shaking is primarily affected by:

(1) The magnitude of an earthquake;

(2) The distance from the source of an earthquake;

(3) The type or thickness of geologic materials at the surface; and

(4) The type of subsurface geologic structure.

Settlement and soil liquefaction conditions occur in areas underlain by cohesionless, loose, or soft-saturated soils of low density, typically in association with a shallow ground water table.

(d) Other Hazard Areas. Geologically hazardous areas shall also include areas determined by the director to be susceptible to other geological events including mass wasting, debris flows, lahars, mudflows or related flooding resulting from volcanic activity, mine hazards, rock falls, and differential settlement. (Ord. 1505 § 5 (part), 2018).

13.13.030 Mapping of geologically hazardous areas.

(a) The approximate location and extent of geologically hazardous areas are shown on the critical area maps adopted with the ordinance codified in this chapter and listed below. The critical area maps listed below are available from the city and/or the listed governmental agency and include:

(1) U.S. Geological Survey topographical maps;

(2) Washington State Department of Natural Resources seismic hazard maps for Eastern Washington;

(3) Washington State Department of Natural Resources slope stability maps;

(4) Federal Emergency Management Administration flood insurance maps; and

(5) Locally adopted maps.

(b) These maps are to be used as a guide for the city, project applicants and/or property owners and may be continuously updated as new critical areas are identified. They are a reference and do not provide a final critical area designation. (Ord. 1505 § 5 (part), 2018).

13.13.040 Critical area report—Additional requirements for geologically hazardous areas.

The following requirements for geologically hazardous area critical area reports are in addition to the requirements for critical area reports set forth in Section 13.08.100:

(a) Area Addressed in Critical Area Report. The following areas shall be addressed in a critical area report for geologically hazardous areas:

(1) The project area of the proposed activity; and

(2) All geologically hazardous areas previously identified by the city within two hundred feet of the project area or that have potential to affect or be affected by the proposal.

(b) Geological Hazards Assessment. A critical area report for a geologically hazardous area shall contain an assessment of geological hazards including the following site- and proposal-related information at a minimum:

(1) Site and Construction Plans. The report shall include a copy of the site plans for the proposal showing:

(A) The type of impacts, if any, that the project will either experience or cause in relation to any other critical area so identified under this section;

(B) Proposed development, including the location of existing and proposed structures, fill, storage of materials, and drainage facilities;

(D) Clearing limits;

(2) Assessment of Geological Characteristics. The report shall include an assessment of the geologic characteristics of the soils, sediments, and/or rock of the project area and potentially affected adjacent properties, and a review of the site history regarding landslides, erosion, and prior grading. Soils analysis shall be accomplished in accordance with accepted classification systems in use in the region. The assessment shall include, but not be limited to:

(A) A description of the surface and subsurface geology, hydrology, soils, and vegetation found in the project area and in all hazard areas addressed in the report;

(B) A detailed overview of the 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 of geologically hazardous areas; and

(3) Analysis of Proposal. The report shall contain a hazards analysis including a detailed description of the project, its relationship to the geologic hazard(s), and its potential impact upon the hazard area, the subject property, and affected adjacent properties; and

(4) Minimum Buffer and Building Setback. The report shall make a recommendation for the minimum no-disturbance buffer and minimum building setback from any geologic hazard based upon the geotechnical analysis.

(c) Incorporation of Previous Study. Where a valid critical areas 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 report may be incorporated into the required critical area report. The applicant shall submit a hazards assessment detailing any changed environmental conditions associated with the site.

(d) 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 lifespan 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.

(e) Additional Analysis to Be Included in a Critical Area Report for Geologically Hazardous Areas. Parameters for design of site improvements, including appropriate foundations and retaining structures, should include allowable load and resistance capacities for bearing and lateral loads, installation considerations, slope stability and estimates of settlement performance, vegetation management, erosion control, and damage control. (Ord. 1505 § 5 (part), 2018).

13.13.050 Performance standards—General requirements.

(a) Alterations of geologically hazardous areas or associated buffers may only occur for activities that:

(1) Will not increase the threat of the geological hazard to adjacent properties beyond predevelopment conditions;

(2) Will not adversely impact other critical areas;

(3) Are designed so that the hazard to the project is eliminated or mitigated to a level equal to or less than predevelopment conditions; and

(4) Are certified as safe as designed and under anticipated conditions by a qualified engineer or geologist, licensed in the state of Washington.

(b) Critical Facilities Prohibited. Critical facilities, such as hospitals and emergency response centers, shall not be sited within geologically hazardous areas unless there is no other practical alternative. (Ord. 1505 § 5 (part), 2018).

13.13.060 Performance standards—Specific hazards.

(a) Erosion and Landslide Hazard Areas. Activities on sites containing erosion or landslide hazards shall meet the standards of Section 13.13.050, Performance standards—General requirements, and the specific following requirements:

(1) Buffer Requirement. A buffer shall be established from all edges of landslide hazard areas. The size of the buffer shall be determined by the director to eliminate or minimize the risk of property damage, death, or injury resulting from landslides caused in whole or part by the development, based upon review of, and concurrence with, a critical area report prepared by a qualified professional;

(A) Minimum Buffer. The minimum buffer shall be equal to the height of the slope or fifty feet, whichever is greater;

(B) Buffer Reduction. The buffer may be reduced to a minimum of ten feet when a qualified professional demonstrates to the director’s satisfaction that the reduction will adequately protect the proposed development, adjacent developments and uses, and the subject critical area;

(C) 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;

(2) Alterations. Alterations of an erosion or landslide hazard area and/or buffer may only occur for activities for which a hazards analysis is submitted and certifies that:

(A) The development will not increase surface water discharge or sedimentation to adjacent properties beyond predevelopment conditions;

(B) The development will not decrease slope stability on adjacent properties; and

(3) Design Provisions. Development within an erosion or landslide hazard area and/or buffer shall be designed to meet the following basic requirements unless it can be demonstrated that an alternative design that deviates from one or more of these standards provides greater long-term slope stability while meeting all other provisions of this chapter. The requirement for long-term slope stability shall exclude designs that require regular and periodic maintenance to maintain their level of function. The basic development design provisions are:

(A) The proposed 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) Structures and improvements shall be clustered to avoid geologically hazardous areas and other critical areas;

(C) 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;

(D) Structures and improvements shall be located to preserve the most critical portion of the site and its natural landforms and vegetation;

(E) The proposed development shall not result in greater risk or a need for increased buffers on neighboring properties;

(F) The use of retaining walls that allow the maintenance of existing natural slope area is preferred over graded artificial slopes; and

(G) Development shall be designed to minimize impervious lot coverage;

(4) Vegetation Retention. Unless otherwise provided or as part of an approved alteration, removal of vegetation from an erosion or landslide hazard area or related buffer shall be prohibited;

(5) Utility Lines and Pipes. 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. Stormwater conveyance shall be allowed only through a high-density polyethylene pipe with fuse-welded joints, or similar product that is technically equal or superior;

(6) Point Discharges. Point discharges from surface water facilities and roof drains onto or upstream from an erosion or landslide hazard area shall be prohibited except as follows:

(A) Conveyed via continuous storm pipe downslope to a point where there are no erosion hazard areas downstream from the discharge;

(B) Discharged at flow durations consistent with the city’s public works standards for stormwater runoff control, with adequate energy dissipation, into existing channels that previously conveyed stormwater runoff in the predeveloped state; or

(C) Dispersed discharge upslope of the steep slope onto a low-gradient undisturbed buffer demonstrated to be adequate to infiltrate all surface and stormwater runoff, and where it can be demonstrated that such discharge will not increase the saturation of the slope;

(7) Subdivisions. The division of land in landslide hazard areas and associated buffers is subject to the following:

(A) Land that is located wholly within a landslide hazard area or its buffer may not be subdivided. 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;

(B) 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; and

(8) Prohibited Development. On-site sewage disposal systems, including drain fields, shall be prohibited within erosion and landslide hazard areas and related buffers.

(b) Seismic Hazard Areas. Activities proposed to be located in seismic hazard areas shall meet the standards of Section 13.13.050, Performance standards—General requirements. (Ord. 1505 § 5 (part), 2018).