HomeMy WebLinkAbout7e. Current JC GeoHazardsArticle V. Geologically Hazardous Areas
18.22.500 Purpose.
The purpose of this article is to reduce risks to human life and safety and reduce the risk of damage to structures and property from geologic hazards, to allow for natural geologic processes supportive of forming and maintaining fish and wildlife habitat, and to regulate and inform land use and planning
decisions. It is recognized that the elimination of all risk from geologic hazards is not feasible to achieve but the purpose of this article is to reduce the risk to acceptable levels. [Ord. 5-20 § 2 (Appx. A)]
18.22.510 Classification/designation.
Geologically hazardous areas are areas that because of their susceptibility to erosion, sliding, earthquake, or other geological events, are not suited to siting commercial, residential, or industrial development consistent with public health or safety concerns. Unless specifically noted below, principal sources of geologically hazardous areas mapped information are the Washington Department of Natural Resources Geologic Hazard Maps (https://www.dnr.wa.gov/programs-and-services/geology/geologic-hazards/geologic-hazard-maps).
(1) The following are geologically hazardous areas and subject to the standards of this article when mapped as high or moderate geologically hazardous areas:
(a) Erosion hazard areas (as defined in JCC 18.10.050).
(b) Landslide hazard areas (as defined in JCC 18.10.120). Landslide hazard areas include any areas susceptible to landslide because of any combination of bedrock, soil,
slope (gradient), slope aspect, structure, hydrology, or other factors, as follows:
(i) Areas of historic failures, such as:
(A) Areas delineated by United States Department of Agriculture, Natural Resources Conservation Service as having a significant limitation for building site development;
(B) Coastal areas mapped by the Washington Department of Ecology Coastal Atlas as unstable, unstable old slides, and unstable recent slides; or
(C) Areas designated and mapped as quaternary slumps, earthflows, mudflows, lahars, or landslide hazards by the Washington Department of Natural Resources or the United States Geological Survey.
(ii) Areas where all three of the following conditions occur:
(A) Slopes are steeper than 15 percent;
(B) Hillsides intersecting geologic contacts with a relatively permeable
sediment overlying a relatively impermeable sediment or bedrock; and
(C) Spring or groundwater seepage.
(iii) Areas that have shown movement during the Holocene epoch (from 10,000 years ago to present) or have been underlain or covered by mass wastage debris of
this epoch.
(iv) Areas with slopes that are parallel or subparallel to planes of weakness (such as bedding planes, joint systems, and fault planes) in subsurface materials.
(v) Areas with slopes having gradients steeper than 80 percent subject to rockfall during seismic shaking.
(vi) Areas that are potentially unstable as a result of rapid stream incision, stream bank erosion, and undercutting by wave action, including stream channel migration zones.
(vii) Areas that show evidence of, or are at risk from, snow avalanches.
(viii) Areas located in a canyon or on an active alluvial fan, presently or potentially subject to inundation by debris flows or catastrophic flooding.
(ix) Areas with a slope of 40 percent or steeper and with a vertical relief of 10 or more feet, except areas composed of bedrock.
(c) Seismic hazard areas (as defined in JCC 18.10.190).
(d) Channel migration zones (CMZs) (as defined in JCC 18.10.030).
(e) Seiche and landslide generated wave hazard areas (as defined in JCC 18.10.190).
(f) Tsunami hazard areas (as defined in JCC 18.10.200).
(g) Other geologic events, such as coal mine hazards and volcanic hazards, are not a known risk in Jefferson County, and are therefore not subject to review as part of the permitting process. [Ord. 5-20 § 2 (Appx. A)]
18.22.520 Regulated activities.
(1) Any development activity or action requiring a project permit or any clearing within an erosion or landslide area shall:
(a) Comply with the requirements in an approved geotechnical report when one is required, including application of the largest buffer or building setback;
(b) Utilize best management practices (BMPs) and all known and available technology appropriate for compliance with this chapter and typical of industry standards;
(c) Prevent collection, concentration or discharge of stormwater or groundwater within an erosion or landslide hazard area and be in compliance with JCC 18.30.070, Stormwater management standards; and
(d) Minimize impervious surfaces and retain vegetation to minimize risk of erosion or
landslide hazards.
(2) Any development activity or action requiring a project permit or any clearing within an erosion or landslide area shall not:
(a) Result in increased risk of property damage, death or injury;
(b) Cause or increase erosion or landslide hazard risk;
(c) Increase surface water discharge, sedimentation, slope instability, erosion or landslide potential to adjacent downstream and down-drift properties beyond predevelopment conditions;
(d) Adversely impact wetlands, fish and wildlife habitat conservation areas or their buffers; or
(e) Be identified as a critical facility necessary to protect public health, safety and welfare. This includes, but is not limited to, schools, hospitals, police stations, fire departments and other emergency response facilities, nursing homes, and hazardous material storage
or production. [Ord. 5-20 § 2 (Appx. A)]
18.22.530 Protection standards.
(1) Clearing, Grading and Vegetation Removal.
(a) Minor pruning of vegetation for view enhancement may be allowed through consultation with the department. The thinning of limbs on individual trees is preferred to topping of trees for view corridors. Total buffer thinning shall not exceed 25 percent and no more than 30 percent of the live tree crowns shall be removed.
(b) Vegetation shall not be removed from a landslide hazard area, except for hazardous trees based on review by a qualified arborist or as otherwise provided for in a vegetation management and restoration plan.
(c) Seasonal Restrictions. Clearing and grading shall be limited to the period between May 1st and October 1st, unless the applicant provides an erosion and sedimentation control plan prepared by a professional engineer licensed in the state of Washington that specifically and realistically identifies methods of erosion control for wet weather
conditions.
(d) Only the clearing necessary to install temporary erosion control measures will be
allowed prior to clearing for roads and utilities construction.
(e) The faces of cut and fill slopes shall be protected to prevent erosion as required by the engineered erosion and sedimentation control plan.
(f) Clearing for roads and utilities shall be the minimum necessary and shall remain within marked construction limits.
(g) Clearing for overhead power lines shall be the minimum necessary for construction
and will provide the required minimum clearances for the serving utility corridor.
(2) Existing Logging Roads. Where existing logging roads occur in geologically hazardous areas, a geological assessment may be required prior to use as a temporary haul road or permanent access road under a conversion or COHP forest practices application.
(3) The department may also require:
(a) Clustering to increase protection to geologically hazardous areas; or
(b) Enhancement of buffer vegetation to increase protection to geologically hazardous areas.
(4) The provisions in this section are in addition to those required in JCC 18.30.060, Grading and excavation standards, and JCC 18.30.070, Stormwater management standards.
(5) If there is a conflict between applicable published standards, the more restrictive protection requirement applies.
(6) Drainage and Erosion Control.
(a) An applicant submitting a project application shall also submit, and have approved, a
stormwater site plan that meets all the requirements of JCC 18.30.070(4)(e), when the project application involves either of the following:
(i) The alteration of a high or moderately high geologically hazardous area or its critical area buffer; or
(ii) The creation of a new parcel within a high or moderately high geologically hazardous area; and
(b) Shall discuss, evaluate, and recommend methods to minimize sedimentation of adjacent properties during and after construction.
(c) Surface drainage shall not be directed across the face of a marine bluff that is mapped as high or moderate erodible or susceptible to landslide or debris flow. The applicant must demonstrate that the stormwater discharge cannot be accommodated on site or upland by evidence presented in a geological assessment as required by JCC 18.22.945, unless waived by the administrator. If drainage must be discharged from a bluff to
adjacent waters, it shall be collected above the face of the bluff and directed to the water
by tight line drain and provided with an energy dissipating device at the shoreline, above ordinary high water mark (OHWM).
(d) The applicant must clearly demonstrate in the geological assessment that stormwater quantity, quality, and flow path post-construction will be comparable to pre-construction conditions.
(7) Vegetation Retention. The following provisions regarding vegetation retention shall apply:
(a) During clearing for roadways and utilities, all trees and understory vegetation lying outside of approved construction limits shall be retained to the maximum extent
practicable;
(b) Clearing limits, as shown on the approved site plan, shall be marked by orange construction barrier fencing to be installed prior to beginning any clearing, grading, or other land-disturbing activities;
(c) Vegetation within identified clearing limits may be removed upon permit issuance. All vegetation within the high or moderate geologically hazardous area or its critical area buffer, but outside the marked clearing limits, shall be retained. Cleared vegetation shall not be placed within a high or moderate geologically hazardous area unless it is used as part of a mitigation plan consistent with other critical area mitigation provisions, is reviewed by the geotechnical professional, and is approved by the administrator; and
(d) Within a high risk CMZ, vegetation removal shall not be allowed. Vegetation removal outside of a high risk CMZ shall not be reviewed under this article. Should this provision conflict with other vegetation retention requirements specified elsewhere within this title,
the more restrictive protection requirement applies.
(8) Erosion and Landslide Hazard Development Standards.
(a) Development activities or actions requiring project permits or clearing shall not be allowed in landslide hazard areas unless a geotechnical report demonstrates that building within a landslide hazard area will provide protection commensurate to being located outside the landslide hazard area and meets the requirements of this section. This may include proposed mitigation measures.
(b) Top of Slope Building Setback. All development activities or actions that require project permits or clearing in erosion and landslide hazard areas shall provide native vegetation from the toe of the slope to 25 feet beyond the top of slope, with an additional minimum 15-foot building and impervious surface setback, unless otherwise allowed through a geologic assessment. The minimum building and setback shall be increased from the top of the slope as follows:
(i) For high landslide hazard areas, the setback shall be equal to the height of the
slope (1:1 horizontal to vertical) plus the greater of one-third of the vertical slope
height or 25 feet.
(ii) For moderate landslide hazard areas, the setback shall be 40 feet from the top of slope.
(9) Geotechnical Report for Toe of Slope Building Setback May Be Required. A geotechnical report may be required based on slope height and stability indicators.
(10) Larger Native Vegetation Width May Be Required. The department may require a larger native vegetation width than the standard buffer distance as determined above if any of the following are identified through the geological assessment process:
(a) The adjacent land is susceptible to severe erosion and erosion control measures will not effectively prevent adverse impacts;
(b) The area has a severe risk of slope failure or downslope stormwater drainage
impacts; or
(c) The minimum native vegetation width or building setback requirement may be decreased if a geotechnical report demonstrates that a lesser distance, through design and engineering solutions, will adequately protect both the proposed development and the erosion or landslide hazard area. The department may decrease the setback when such a setback would result in a greater than 1:1 slope setback.
(11) Seismic Hazard Development Standards.
(a) Development activities or actions requiring a project permit occurring within 200 feet of a “high hazard” seismic hazard area may be allowed with an approved geotechnical report that confirms the site is suitable for the proposed development and addresses any fill or grading that has occurred on the subject parcel.
(b) Development activities or actions requiring a project permit within a seismic hazard area shall be in accordance with JCC Title 15.
(12) Reducing Buffer Widths. The administrator may reduce geologically hazardous area buffers as
follows:
(a) Buffers may be reduced by up to 25 percent with a geotechnical report prepared by a geotechnical professional with a state stamp; provided, the geotechnical report identifies recommendations for preventing or minimizing risks post-development.
(b) All buffer reductions 25 percent or greater and all development within a high or moderate geologically hazardous area shall require a geotechnical report prepared by a geotechnical professional. The administrator may require a third-party review based on JCC 18.22.930 or the applicant enters into an indemnity and hold harmless agreement with the county that is approved by the county’s risk manager and the prosecuting attorney. If the administrator requires a third-party review of the geotechnical report, the administrator shall be responsible for identifying and transmitting the geotechnical report to the third-party reviewer.
(13) Increasing Buffer Widths. Buffer widths may be increased on a case-by-case basis, as
determined by the administrator, to protect the functions and values of a geologically hazardous area
and to reduce risks to public safety and welfare. Information that may be used to support this determination, includes but is not limited to:
(a) The landslide area is unstable and active.
(b) The adjacent land is susceptible to severe landslide or erosion, and erosion control measures will not effectively protect the proposed project from the risks posed by the landslide hazard area.
(c) The adjacent land has minimal vegetative cover.
(14) Landslide Hazard Areas – Additional Standards. The following activities may be allowed in active landslide hazard areas when all reasonable measures have been taken to minimize risks and other adverse effects associated with landslide hazards, and when the amount and degree of the
alteration are limited to the minimum needed to accomplish the project purpose:
(a) The standards of subsection (1) of this section shall apply.
(b) Developments that will not increase the threat to the health or safety of people and will not increase potential for landslides on or off the site and meet the reasonable economic use exception in JCC 18.22.260.
(c) Utility lines and pipes that are above ground, properly anchored or designed so that they will continue to function in the event of a slope failure or movement of the underlying materials and will not increase the risk or consequences of static or seismic slope instability or result in a risk of mass wasting. Such utility lines may be permitted only when the applicant demonstrates that no other feasible alternative is available to serve the affected population.
(d) Access roads and trails that are engineered and built to standards that minimize the need for major repair or reconstruction beyond that which would be required in nonhazard areas. Access roads and trails may be permitted only if the applicant demonstrates that
no other feasible alternative exists, including through the provisions of Chapter 8.24 RCW. If such access through critical areas is granted, exceptions or deviations from technical standards for width or other dimensions and specific construction standards to minimize impacts, including drainage and drainage maintenance plans, may be required.
(e) Stormwater conveyance through a properly designed stormwater pipe when no other stormwater conveyance alternative is available. The pipe shall be located above ground and be properly anchored or designed so that it will continue to function in the event of a slope failure or movement of the underlying materials and will not increase the risk or consequences of static or seismic slope instability or result in increased risk of mass wasting activity.
(15) Seismic Hazard Areas – Standards. Development may be allowed in seismic hazard areas
when all of the following apply:
(a) The standards of subsection (1) of this section shall apply.
(b) Structures in seismic hazard areas shall conform to applicable analysis and design criteria of Chapter 18.15 JCC.
(c) Public roads, bridges, utilities, and trails shall be allowed when there are no feasible alternative locations, and geotechnical analysis and design are provided that minimize potential damage to roadway, bridge, and utility structures, and facilities will not be susceptible to damage from seismically 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 another appropriate document.
(16) Tsunami Hazard Areas – Standards.
(a) The standards of subsection (1) of this section shall apply.
(b) For development within tsunami hazard areas the proposed development shall be designed to provide protection from the tsunami hazard that meets the projected hazard
on the Washington Department of Natural Resources Tsunami Inundation Maps.
(c) For other low-lying coastal areas not included on the inundation maps, development shall be designed to provide protection for debris impact and an inundation as determined by current Washington Department of Natural Resource modeling, unless other measures can be shown to provide equal or greater protection. [Ord. 5-20 § 2 (Appx. A)]