Chapter 16C.08 GEOLOGICALLY HAZARDOUS AREAS

Chapter 16C.08
GEOLOGICALLY HAZARDOUS AREAS

Sections:

16C.08.03 Geologically Hazardous Areas Protection Approach.

16C.08.04 Development Review Procedure for Geologically Hazardous Areas.

16C.08.05 General Protection Requirements.

16C.08.01 Purpose and Intent.

(1) Geologically hazardous areas include those areas susceptible to erosion, sliding, earthquake or other geological events. They pose a threat to the health and safety of the citizens of Yakima County when incompatible development is sited in areas of significant hazard. Some risks due to geologic hazards might be capable of mitigation through engineering, design, or modified construction standards so the level of risk is reduced to an acceptable level. However, when mitigation is not feasible, development within geologically hazardous areas is best avoided.

(2) The purposes of this chapter are to:

(a) Minimize risks to public health and safety and reduce the risk of property damage by regulating development on or adjacent to geologically hazardous areas;

(b) Maintain natural geological processes while protecting existing and new development;

(Ord. 5-2017 § 2(C) (Exh. 1) (part), 2017: Ord. 13-2007 §1 (Exh. A)(16C.08.01), 2007).

16C.08.02 Mapping and Designation.

(1) Geologically hazardous areas are areas that are susceptible to one or more of the following types of hazards and are designated as critical areas, based on WAC 365-190-120(3):

(a) Erosion hazards;

(b) Landslide hazards;

(d) Alluvial fan/flash flooding hazards;

(e) Avalanche hazards; and

(f) Stream undercutting hazards;

(g) Seismic hazards (referred to below as earthquake hazards); and

(h) Volcanic hazards;

(2) The approximate location and extent of Erosion Hazard Areas are shown on the County’s critical area map titled “Erosion Hazard Areas of Yakima County.” Erosion hazard areas were identified by using the “Soil Survey of Yakima County Area, Washington” and the “Soil Survey of Yakima Indian Reservation Irrigated Area, Washington, Part of Yakima County.” The analysis utilized the general soil map unit descriptions of severe and very severe hazard of water erosion.

(3) The approximate location and extent of the remaining Geologically Hazardous Areas are shown on the County’s critical area map titled “Geologically Hazardous Areas of Yakima County.” The following geologically hazardous areas, with the corresponding map code in parenthesis, are mapped and classified using the stated criteria based on WAC 365-190-120(3):

(a) Landslide Hazard Areas (LS) – These include places where landslides, debris flows, or slumps have already occurred. Where sliding is presumed to have occurred within 10,000 years or less is shown as High Risk (LS3) on the map. Slides thought to be older than 10,000 years but still capable of movement are shown as Intermediate Risk (LS2). Areas where slides are absent are unlabeled and combined with other Low Risk areas.

(b) Oversteepened Slope Hazard Areas (OS) - These include areas with slopes steep enough to create potential problems. High risk areas (OS3) have a high potential to fail, and include slopes greater than 40%, and consist of areas of rock fall, creep, and places underlain with unstable materials. Intermediate Risk areas (OS2) are less likely to fail but are still potentially hazardous. This category also includes some slopes between 15 and 40%. Low Risk areas, unlikely to fail, are unlabeled and combined with other Low Risk categories.

(c) Alluvial Fan/Flash Flooding Hazard Areas (AF) - These are areas where flash flooding can occur, and are often associated with inundation by debris from flooding. They include alluvial fans, canyons, gullies, and small streams where catastrophic flooding can occur. They do not include all areas where flash flooding may occur with Yakima County. Flooding may also occur in larger streams and rivers, but these are depicted in the “Flood Insurance Study for the Unincorporated Areas of Yakima County,” dated March 2, 1998, with accompanying Flood Insurance Rate Maps (FIRMs) and Flood Boundary and Floodway Maps, and any amendments which may thereafter be made by the Federal Emergency Management Agency, rather than on the Geologically Hazardous Areas Map. High Risk areas (AF3) are those most likely to experience flooding. These areas usually involve larger drainage areas, easily eroded sediments, and steeper gradients. Intermediate Risk areas (AF2) have some potential for flash flooding but involve smaller drainages and flatter slopes. Low Risk areas are where flash flooding is unlikely, are unlabeled and combined with other Low Risk areas on the map.

(d) Avalanche Risk Hazard Areas (AR) - Areas of avalanche hazards are limited (within the mapped boundaries) to areas near the Cascade Crest. High Risk areas (AR3) are those in areas of high snowfall where avalanche scars are visible and slopes are steep to moderately steep. These areas could also be rated OS3. Intermediate Risk areas (AR2) are usually adjacent to AR3 areas but where vegetation is still in place and slopes are moderate. AR2 and AR3 areas are mapped on the basis of aerial photography and observed scars. Climatic data (snowfall, wind direction, etc) are necessary for more detailed mapping. Low Risk areas, where avalanches are unlikely, are unlabeled and combined with other Low Risk geologic hazards.

(e) Stream Undercutting Hazard Areas (SU) - These areas are confined to banks near main streams and rivers where undercutting of soft materials may result. High Risk areas (SU3) include steep banks of soft material adjacent to present stream courses. Intermediate Risk areas (SU2) are banks along the edge of a flood plain but away from the present river course. Low Risk areas are unlabeled and combined with other Low Risk areas on the maps.

(f) Earthquake Activity Hazard Areas (EA) - Recorded earthquake activity in Yakima County is mostly marked by low magnitude events and thus low seismic risk. One exception is an area along Toppenish Ridge where Holocene faulting may have produced earthquakes of as much as magnitude 7. Zones of surficial fault scarps are shown on High Risk areas (EA3) while areas adjacent to the scarps are assigned Intermediate Risk (EA2). The rest of the county is Low Risk, are unlabeled, and combined with other low risk hazards.

(g) Suspected Geologic Hazard Areas (SUS) – These are areas for which detailed geologic mapping is lacking but preliminary data indicate a potential hazard. No risk assessment (1-2-3) is given for these areas. Most are probably OS or LS hazards.

(h) Risk Unknown Hazard Areas (UNK) - In these areas geologic mapping is lacking or is insufficient to make a determination. All of these areas are associated with other classified geologic hazards, and most are located in remote areas of Yakima County.

(4) Volcanic Hazard Areas are not mapped but are defined as areas subject to pyroclastic (formed by volcanic explosion) flows, lava flows and inundation by debris flows, mudflows or related flooding resulting from volcanic activity. Volcanic Hazard Areas in Yakima County are limited to pyroclastic (ash) deposits. While Yakima County contains a portion of Mt Adams and is in close proximity to Mt Rainer, and Mt St. Helens, the threat of volcanic hazards is minimal and limited to ash deposition. The more devastating effects of volcanic activity such as lava flows, and lahars (volcanic landslide or mudflow) are not possible due to intervening ridges. No specific protection requirements are identified for volcanic hazard areas.

(5) This chapter does not imply that land outside mapped geologically hazardous areas or uses permitted within such areas will be without risk. This chapter shall not create liability on the part of Yakima County, any officer, or employee thereof for any damages that result from reliance on this chapter or any administrative decision lawfully made hereunder.

(Ord. 5-2017 § 2(C) (Exh. 1) (part), 2017: Ord. 13-2007 §1 (Exh. A)(16C.08.02), 2007).

16C.08.03 Geologically Hazardous Areas Protection Approach.

(1) Erosion Hazard Areas. Protection measures for erosion hazard areas will be accomplished by implementing the regulatory standards for erosion and drainage control required under YCC Title 13 (Building Code). Any future stormwater program erosion control measures that may be formally adopted by the Board of County Commissioners shall supersede YCC Title 13 erosion control requirements. Standards to meet YCC Title 13 requirements can be met by the application of the best management practices (BMPs) in the Yakima Regional Stormwater Manual (WDOE Publication number 04-10-076) or equivalent manual adopted by Yakima County, or any other approved manual deemed appropriate by the Building Official, including but not limited to applicable Natural Resource Conservation Service (NRCS) Field Office Technical Guide (FOTG) BMPs and the Washington State Department of Transportation Highway Runoff Manual. Application of the Environmental Protection Agency (EPA) “Construction Rainfall Erosivity Waiver” is at the discretion of the Building Official on a case-by-case basis.

(2) Landslide Hazard Areas. Protection measures for landslide hazard areas will be accomplished through the review process of Section 16C.08.04 (Development Review Procedure for Geologically Hazardous Areas), by implementing the development standards of Section 16C.08.05 (General Protection Requirements), and by implementing the appropriate sections of the International Building Code (IBC) as adopted in YCC Title 13 (currently Section 16, Structural Design; Section 18, Soils and Foundations; and Appendix J, Grading).

(3) Alluvial Fan/Flash Flooding Hazard Areas. Protection measures for alluvial fan/flash flooding hazard areas will be accomplished through the review process of Section 16C.08.04 (Development Review Procedure for Geologically Hazardous Areas), by implementing the development standards of Section 16C.08.05 (General Protection Requirements), and by implementing the appropriate sections of the International Building Code (IBC) as adopted in YCC Title 13 (currently Section 16, Structural Design; Section 18, Soils and Foundations; Appendix J, Grading; and Flood Resistant Design and Construction (ASCE-24-98)).

(4) Stream Undercutting Hazard Areas. Protection measures for stream undercutting hazard areas will be accomplished by Critical Areas review for flood hazards, streams, and shoreline jurisdiction, in addition to implementing the appropriate sections of the International Building Code (IBC) as adopted in YCC Title 13 (Flood Resistant Design and Construction (ASCE-24-98)).

(5) Avalanche Hazard Areas. Protection measures for avalanche hazard areas will be accomplished through the review process of Section 16C.08.04 (Development Review Procedure for Geologically Hazardous Areas), by implementing the development standards of Section 16C.08.05 (General Protection Requirements), and by implementing the appropriate sections of the International Building Code (IBC) as adopted in YCC Title 13 (currently Section 16, Structural Design; Section 18, Soils and Foundations; and Appendix J, Grading).

(6) Oversteepened Slope Hazard Areas. Protection measures for oversteepened slope hazard areas will be accomplished through the review process of Section 16C.08.04 (Development Review Procedure for Geologically Hazardous Areas), by implementing the development standards of Section 16C.08.05 (General Protection Requirements), and by implementing the appropriate sections of the International Building Code (IBC) as adopted in YCC Title 13 (currently Section 16, Structural Design; Section 18, Soils and Foundations; and Appendix J, Grading).

(7) Earthquake/Seismic Hazard Area Protection Standards. Protection measures for earthquake/Seismic hazard areas will be accomplished by implementing the appropriate sections of the International Building Code (IBC) as adopted in YCC Title 13 (currently Section 16, Structural Design; Section 18, Soils and Foundations; and Appendix J, Grading).

(8) Suspected Geologic Hazard Areas and Risk Unknown Hazard Areas. Protection measures for suspected geologic hazard areas and risk unknown hazard areas will be accomplished through the review process of Section 16C.08.04 (Development Review Procedure for Geologically Hazardous Areas), by implementing the development standards of Section 16C.08.05 (General Protection Requirements), and by implementing the appropriate sections of the International Building Code (IBC) as adopted in YCC Title 13 (currently Section 16, Structural Design; Section 18, Soils and Foundations; and Appendix J, Grading).

(Ord. 5-2017 § 2(C) (Exh. 1) (part), 2017: Ord. 13-2007 §1 (Exh. A)(16C.08.03), 2007).

16C.08.04 Development Review Procedure for Geologically Hazardous Areas.

(1) The Administrative Official shall make a determination of hazard to confirm whether the development or its associated facilities (building site, access roads, limits of grading/excavation/filling, retaining walls, septic drainfields, landscaping, etc.):

(a) Are located within a mapped geologically hazardous area;

(b) Are within 500 feet of a mapped landslide hazard area;

(c) Are abutting, or adjacent to any other mapped geologically hazardous area and may result in or contribute to an increase in hazard, or pose a risk to life and property on or off the site;

(d) Are located within a distance from the base of an adjacent landslide hazard area that has been determined to be within the runout area of said hazard area;

(e) Are located within the potential run-out path of a mapped avalanche hazard.

(2) Developments that receive an affirmative determination of hazard by the Administrative Official under subsection (1) above must conduct a geologic hazard report as provided in Section 16C.03.18(4) (Supplemental Report Requirements – Geologically Hazardous Areas), which may be part of a geo-technical report required under additional review below:

(a) If the geologic hazard report determines no hazard exists or that the project area lies outside the hazard, then no Geologic Hazard review is needed.

(b) The Administrative Official is authorized to waive further geologic hazard review for oversteepened slope hazards on a determination that the hazards identified in the geologic hazard report will be adequately mitigated under grading or construction permits.

(3) Developments that receive an affirmative determination of hazard, but do not meet the provisions of subsections (2)(a) or (2)(b) above, must:

(a) Obtain a Critical Areas Development Authorization under Chapter 16C.03 (Application and Review Procedures);

(b) Submit a geo-technical report that is suitable for obtaining the grading and construction permits that will be required for development. The geo-technical report should incorporate the submitted assessment, include the design of all facilities and include a description and analysis of the risk associated with the measures proposed to mitigate the hazards, ensure public safety, and protect property and other critical areas; and

(Ord. 5-2017 § 2(C) (Exh. 1) (part), 2017: Ord. 13-2007 §1 (Exh. A)(16C.08.04), 2007).

16C.08.05 General Protection Requirements.

(1) Grading, construction, and development and their associated facilities shall not be located in a geologically hazardous area, or any associated setback for the project recommended by the geo-technical report, unless the applicant demonstrates that the development is structurally safe from the potential hazard, and that the development will not increase the hazard risk onsite or off-site.

(2) Development shall be directed toward portions of parcels, or parcels under contiguous ownership, that are at the least risk of hazard in preference to lands with higher risk, unless determined to be infeasible in the geo-technical report.

(3) The geo-technical report shall recommend methods to ensure the information and education about the hazard and any recommended buildable area for future landowners over the long term.

(4) The applicable requirements of grading and construction permits for developments in hazardous areas must be included in the development proposal and geo-technical report.

(Ord. 5-2017 § 2(C) (Exh. 1) (part), 2017: Ord. 13-2007 §1 (Exh. A)(16C.08.05), 2007).