29.25.070 Geologic hazard areas.
(1) Purpose. The purpose of this section is to reduce the threats to public health and safety posed by geologic hazards within the shoreline jurisdiction. The intent is to reduce incompatible development in areas of significant geologic hazard. Development incompatible with geologic hazards may not only place itself at risk, but also may increase the hazard to surrounding development. Some geologic hazards can be reduced or mitigated by engineering, design, or modified construction or altering mining practices so risks to health and safety are minimized. When technology cannot reduce the risks to acceptable levels, development in the hazard area is best to be avoided.
(2) Geologic Hazard Area Designation. Geologic hazard areas within the City are those areas that are susceptible to significant erosion, landslide, flood hazards, seismic hazards, and surface mine collapse hazards. All areas within the City of Pasco meeting the criteria described in subsection (3) of this section for known or suspected risk or unknown risk, regardless of the presence or lack of any formal identification as such, are designated as critical areas and are subject to the provisions of this section.
(a) Volcanic Hazards. The GMA requires that volcanic hazards be addressed in local critical area regulations. However, since no volcanic hazards exist in the City area, no volcanic hazards regulations are needed.
(b) Flood Hazard Areas. Generally, areas subject to flood hazard conditions are regulated by PMC 29.25.060 and the City of Pasco floodplain regulations (PMC Title 24), which regulate those areas identified and classified by the FEMA on their Flood Hazard Boundary/Flood Insurance Rate Maps.
(3) Geologic Hazard Area Classification and Designation Criteria.
(a) Geologic hazard area classification criteria are listed in the table below, along with the source agencies that provide the guidelines for classification and designation:
Hazard Area | Classification and Designation | Source |
|---|---|---|
Erosion hazard areas | Areas with soil type possessing erosion hazard of “moderate to severe,” “severe,” or “very severe.” (Classification based on both soil type and slope) | NRCS |
Landslide hazard areas | Areas with slopes of 30 percent or greater and with a vertical relief of 10 or more feet; | NRCS |
Areas with slopes steeper than 15 percent on hillsides intersecting geologic contacts with a relatively permeable sediment overlying a relatively impermeable sediment or bedrock where springs or groundwater seepage is present; | NRCS | |
Areas with slopes parallel or subparallel to planes of weakness in subsurface materials (e.g., bedding planes, joint systems, and fault planes); | NRCS | |
Areas with slopes having gradients steeper than 80 percent subject to rockfall during seismic shaking; | NRCS | |
Alluvial fans or canyon bottoms presently or potentially subject to inundation by debris flows or catastrophic flooding; | NRCS | |
Areas that have shown movement during the Holocene epoch or which are underlain or covered by wastage debris of this epoch; | NRCS | |
Evidence of or risk from snow avalanches; | NRCS | |
A “severe” limitation for building site development due to slope conditions; | NRCS | |
Areas of historic failure, such as areas designated as quaternary slumps, earthflows, mudflows, lahars, or landslides on maps or technical reports (e.g., topographic or geologic maps, or other authorized documents). | USGS, WDNR, or other government agencies | |
Flood hazard areas | Areas potentially unstable as a result of rapid stream incision, stream bank erosion, and Undercutting by wave action shall be addressed as a flood hazard | PMC Title 24 |
Note:
NRCS = U. S. Department of Agriculture, Natural Resources Conservation Service
(4) Geologic Hazard Area Rating Criteria. All areas within the City shall be classified by the following risk categories for each geologic hazard type:
Risk Classification | Documentation and Data Sources |
|---|---|
Known or suspected risk | Documentation or projection of the hazard by a qualified expert exists |
No risk | Documentation or projection of the lack of a hazard by a qualified expert exists |
Risk unknown | Data are not available to determine the presence or absence of a geologic hazard |
(5) Determination of Need for Geologic Hazard Area Detailed Study. A geologic hazard area detailed study shall be required if the following indicators are present:
(a) If the project area is listed in the City of Pasco Critical Areas Map as possessing either a known or suspected risk for erosion, landslide, flood, seismic, or mine hazard.
(b) If the project area is listed in the City of Pasco Critical Areas Map as possessing an unknown risk for erosion, landslide, flood, seismic, or mine hazard if any of the following are identified by the applicant or City:
(i) A qualified geologist finds that any of the following exist: evidence of past significant events of the hazard in question on or adjacent to the site; the presence of necessary and sufficient factors for events of the hazard in question on or adjacent to the site; or reasonable uncertainty concerning the hazard or the potential for significant risk to or from the proposed activity; or
(ii) The Shoreline Administrator possesses a reasonable belief that a geologic hazard may exist. Such reasonable belief shall be supported by a site visit and subsequent consultation with a qualified geologist.
(6) Geologic Hazard Area Detailed Study Requirements. The minimum requirements for a geologic hazard area detailed study include the following in addition to the basic requirements identified in PMC 29.25.020(10):
(a) Basic Requirements. A geologic hazard area detailed study shall meet the following:
(i) The detailed study shall be prepared by a qualified professional engineer or geologist. Evidence of qualifications shall be provided with the detailed study.
(ii) A map, of a scale no smaller than one inch equals 200 feet, of the site and the extent of the geologic hazard area as determined by the criteria in subsection (3) of this section.
(iii) An assessment of the geologic characteristics and engineering properties of the soils, sediments, and/or rock of the subject property and potentially affected adjacent properties, and a review of the site history regarding landslides, erosion, and prior grading. The study shall include a soils analysis consistent with the accepted regional taxonomic classification system, and a description of the vulnerability of the site to seismic events. Documentation of data and methods shall be included.
(iv) A geotechnical analysis, including a detailed description of the proposed project, its relationship to the geologic hazard(s), and its potential impact upon the hazard area, the subject property and affected adjacent properties shall be included.
(v) A mitigation plan, if appropriate, prepared by a professional engineer or geologist under the supervision of a professional engineer qualified to prepare a detailed study. The mitigation plan shall include a discussion on how the project has been designed to avoid and minimize the impacts discussed in the geotechnical analysis (see mitigation standards provided in PMC 29.25.020(13)). The plan shall make a recommendation for the minimum building setbacks from any geologic hazard based on the geotechnical analysis. The plan shall also address the potential benefit of mitigation on the hazard area, the subject property, and affected adjacent properties.
(vi) Where more than one geologic hazard exists within, adjacent to, or impacts, or is impacted by, the activity site, then only one detailed study is required to be completed to conduct a geologic hazard critical area review of the activity. The critical area review report shall meet all of the requirements of each critical area type, but may present a unified mitigation plan.
(vii) Where a valid geotechnical 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 detailed study. The applicant shall submit a geotechnical assessment detailing any changed environmental conditions associated with the site.
(b) Erosion and Landslide Hazard Areas. In addition to the requirements of subsection (6)(a) of this section, an erosion hazard or landslide hazard area detailed study must also meet the following requirements:
(i) The map shall depict the height of slope, slope gradient, and cross section of the site. The site plan shall also include the location of springs, seeps, or other surface expressions of groundwater. The site plan shall also depict any evidence of surface or storm water runoff.
(ii) A description of load intensity, including surface and groundwater conditions, public and private sewage disposal systems, fills and excavations, and all structural development.
(iii) An estimate of slope stability and the effect construction and placement of structures will have on the slope during the estimated life of the structure.
(iv) An estimate of the bluff retreat rate that recognizes and reflects potential catastrophic events, such as seismic activity or a 100-year storm event.
(v) An assessment describing the extent and type of vegetative cover.
(vi) The geotechnical analysis shall specifically include:
(A) Slope stability studies and opinion(s) of slope stability;
(B) Proposed angles of cut and fill slopes and site grading requirements;
(C) Structural foundation requirements and estimated foundation settlements;
(D) Soil compaction criteria;
(E) Proposed surface and subsurface drainage;
(F) Lateral earth pressures;
(G) Vulnerability of the site to erosion;
(H) Suitability of on-site soil for use as fill; and
(I) Building limitations.
(vii) Mitigation proposals shall include the location and methods of drainage, surface water management, locations, and methods of erosion control, a vegetation management and/or restoration plan, and/or other means for maintaining long-term stability of slopes.
(c) Flood Hazard Areas. Flood Hazard Areas are addressed through PMC 29.25.060, Flood hazard areas, and the City of Pasco floodplain regulations (PMC Title 24). If evidence exists that the proposed development area is subject to flood hazards that are not indicated on the City’s Flood Hazard Boundary maps, and site characteristics do not warrant an erosion or landslide hazard detailed study, the City may require additional analysis and preparation of a mitigation plan to determine if the site is suitable for development.
(d) Seismic Hazard Areas. In addition to the basic requirements, subsection (6)(a) of this section, a detailed study for a seismic hazard critical area shall also meet the following requirements:
(i) The site map shall show all known and mapped faults in the project vicinity.
(ii) The geotechnical analysis shall include a complete discussion of the potential impacts of seismic activity reasonably probable on the site (e.g., forces generated and fault displacement).
(e) Mine Hazard Areas. In addition to the basic requirements subsection (6)(a) of this section, a detailed study for a mine hazard critical area shall also meet the following requirements:
(i) The site plan shall delineate the existence of mine workings adjacent to or abutting the site, or nearby mine workings, which may impact the site; and
(ii) The geotechnical analysis shall include a discussion of the potential for subsidence on the site.
(f) Volcanic Hazard Areas. The City is located in an area of minimal risk from volcanic hazard areas.
(7) Performance Standards Minimum Requirements. This section describes the minimum performance standard requirements for geologic hazard areas.
(a) Basic Requirements.
(i) Alteration of geologic hazard critical areas is permitted only if the development proposal can be designed so the hazard to the project and any increase of hazard to adjacent property is eliminated or mitigated, and the development proposal on the site is certified as safe by a geotechnical engineer licensed in the State of Washington.
(ii) All proposals involving excavations and placement of fills shall be subject to structural review under Chapter 33, Site Work, Demolition and Construction, of the most current International Building Code.
(iii) Essential public facilities as defined by RCW 36.70A.200 shall not be sited within designated geologic hazard areas.
(b) Erosion and Landslide Hazard Areas. Activities on sites containing landslide or erosion hazards shall also meet the following requirements:
(i) Alterations of the buffer and/or geologic hazard area may only occur for activities meeting the following criteria:
(A) No reasonable alternative exists; and
(B) A geotechnical report is submitted and certifies that:
•The development will not significantly increase surface-water discharge or sedimentation to adjacent properties beyond predevelopment conditions;
•The development will not decrease slope stability on adjacent properties; and
•That such alterations will not adversely impact other critical areas.
(ii) A temporary erosion and sedimentation control plan, prepared in accordance with the requirements of the standard specification of the City of Pasco.
(iii) A drainage plan for the collection, transport, treatment, discharge and/or recycle of water in accordance with the standard specifications of the City of Pasco.
(iv) Surface drainage shall not be directed across the face of a landslide hazard area (including riverine bluffs or ravines). If drainage must be discharged from the hazard area into adjacent waters, it shall be collected above the hazard and directed to the water by a tightline drain and provided with an energy dissipating device at the point of discharge.
(v) All infiltration systems, such as storm water detention and retention facilities, and curtain drains utilizing buried pipe or French drain, are prohibited in landslide hazard areas and their buffers unless a geotechnical report indicates such facilities or systems or the failure of the same will not affect slope stability and the systems are designed by a licensed civil engineer.
(vi) A minimum standard buffer width of 30 feet shall be established from the top, toe, and all edges of landslide and erosion hazard areas. Existing native vegetation shall be maintained. The buffer may be reduced to a minimum of 10 feet when an applicant demonstrates the reduction will adequately protect the proposed development, adjacent developments and uses, and the subject critical area. The buffer may be increased by the Shoreline Administrator for development adjacent to a river bluff or ravine, or in other areas that circumstances may warrant, where it is determined a larger buffer is necessary to prevent risk of damage to proposed and existing development, as in the case where the area potentially impacted by a landslide exceeds 30 feet.
(vii) On-site sewage disposal systems, including drainfields, shall be prohibited within landslide and erosion hazard areas and related buffers.
(viii) Development designs shall meet the following basic requirements, unless it can be demonstrated that an alternative design provides greater long-term slope stability while meeting all other criteria of this section. The requirement for long-term slope stability shall exclude designs that require periodic maintenance or other actions to maintain their level of function. The basic development design standards are:
(A) Structures and improvement shall be clustered to retain as much open space as possible and to preserve the natural topographic features of the site.
(B) Structures and improvements shall conform 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 use of retaining walls that allow the maintenance of existing natural slope area is preferred over graded artificial slopes.
(E) All development shall be designed to minimize impervious lot coverage.
(c) Flood Hazard Areas. Activities in flood hazard areas shall comply with PMC 29.25.060, Flood hazard areas, and the City of Pasco floodplain regulations (PMC Title 24).
(d) Seismic Hazard Areas. Activities on sites containing seismic hazards shall also meet the following requirements:
(i) Mitigation is implemented, which reduces the seismic risk to a level equivalent to that which the activity would experience if it were not located in a seismic hazard area.
(ii) Structural development proposals shall meet all applicable provisions of Chapter 16 of the most current edition of the International Building Code (Structural Forces/Structural Design Requirements).
(iii) No residential structures or essential public facility shall be located on a Holocene fault line as indicated by USGS investigative maps and studies.
(e) Mine Hazard Areas. Activities on sites containing mine hazards shall also meet the following requirements:
(i) Mitigation is implemented which reduces the risk from mine hazards to a level equivalent to that which the activity would experience if it were not located in a mine hazard area.
(f) Volcanic Hazard Areas. No additional requirements.
(8) Long-Term Mitigation and Restoration Standards.
(a) The mitigation plan shall specifically address how the activity maintains or reduces the pre-existing 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). Mitigation techniques providing long-term hazard reduction are those that do not require periodic maintenance or other actions to maintain their function.
(b) Mitigation may be required to avoid any increase in risk above the pre-existing conditions following abandonment of the activity.
(c) Any required restoration shall meet the long-term hazard reduction standards. In the case of restoration, long-term shall be defined as the equivalent of natural function. [Ord. 4314 § 2, 2016; Code 1970 § 29.01.560.]