Chapter 17.115
CRITICAL AREAS – GEOLOGICALLY HAZARDOUS AREAS – SPECIFIC STANDARDS*

Sections:

17.115.010    Purpose and intent.

17.115.020    Relationship to general standards for critical areas.

17.115.030    Designation.

17.115.040    Classification.

17.115.050    Mapping.

17.115.060    Allowed activities.

17.115.070    Critical area reports – Specific requirements for geologically sensitive areas.

17.115.080    Performance standards.

*Prior legislation: Ords. 929 and 1110.

17.115.010 Purpose and intent.

It is the purpose of this chapter to provide development criteria to the underlying zoning districts to assure that growth occurs in such a manner as to protect the natural and topographic character and identity of these areas, environmental resources, the aesthetic qualities and restorative value of lands, and the public health, safety, and general welfare by ensuring that development does not create soil erosion, silting of lower slopes, slide damage, flooding problems, and severe cutting or scarring. It is the intent of this chapter to encourage a sensitive form of development and to allow for a reasonable use that complements the natural and visual character of the city of Stanwood. (Ord. 1164 § 4, 2004).

17.115.020 Relationship to general standards for critical areas.

This chapter contains standards specific to geologically hazardous areas, which in most cases should be considered in conjunction with the standards and administrative rules in Chapter 17.114 SMC. Please refer to that chapter for general standards and administration. In the event of a conflict between this chapter and Chapter 17.114 SMC, the regulations in this chapter shall prevail. (Ord. 1164 § 4, 2004).

17.115.030 Designation.

(1) 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;

(c) Seismic hazard; and

(d) Other geological events including tsunamis, volcanic hazards, and differential settlement.

(2) Erosion Hazard Areas. Erosion hazard areas are at least those areas identified by the U.S. Department of Agriculture’s Natural Resources Conservation Service as having a “moderate to severe,” “severe,” or “very severe” rill and inter-rill erosion hazard. On the city’s critical areas maps, these are shown as areas of moderate or steep slopes. Erosion hazard areas are also those areas impacted by shore land and/or stream bank erosion.

(3) 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:

(a) Areas of historic failures, such as 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) Areas with all three of the following characteristics:

(i) Slopes steeper than 15 percent; and

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

(iii) Springs or groundwater seepage;

(c) Areas that have shown movement during the Holocene epoch (from 10,000 years ago to the present) or that are underlain or covered by mass wastage debris of that epoch;

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

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

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

(g) Any area with a slope of 40 percent or steeper and with a vertical relief of 10 or more feet except areas composed of consolidated rock. A slope is delineated by establishing its toe and top and measured by averaging the inclination over at least 10 feet of vertical relief.

(4) 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:

(a) The magnitude of an earthquake;

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

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

(d) 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 groundwater table.

(5) Tsunami Hazard Areas. Tsunami hazard areas are coastal areas and large lake shoreline areas susceptible to flooding and inundation as the result of excessive wave action derived from seismic or other geologic events.

(6) Lahar Hazard Areas. Areas susceptible to mud or debris flows from volcanic eruptions (Glacier Peak).

(7) Other Hazard Areas. Geologically hazardous areas shall also include areas determined by the city to be susceptible to other geological events including mass wasting, debris flows, rock falls, and differential settlement. (Ord. 1164 § 4, 2004).

17.115.040 Classification.

All geologic hazard areas should be classified according to the following categories for each geologic hazard type.

Classification	Documentation and Data Sources
Known or Suspected Risk	Documentation or projection of the hazard by a qualified professional exists.
Risk Unknown	Documentation or projection of the lack of hazard by a qualified professional exists, or data are not available to determine the presence or absence of a geologic hazard.

(Ord. 1164 § 4, 2004).

17.115.050 Mapping.

(1) The approximate location and extent of geologically hazardous areas are shown on the adopted critical area maps (see the Natural Features Element of the Stanwood Comprehensive Plan). The adopted critical area maps include:

(a) U.S. Geological Survey landslide, seismic, and volcanic hazard maps;

(b) Department of Natural Resources seismic hazard maps for Western Washington, as applicable;

(c) Department of Natural Resources slope stability maps, as applicable;

(d) National Oceanic and Atmospheric Administration tsunami hazard maps (pending); and

(e) Locally adopted maps.

(2) These maps are to be used as a guide for city staff, 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. 1164 § 4, 2004).

17.115.060 Allowed activities.

(1) See SMC 17.114.140 for permitted activities in geologically hazardous areas.

(2) In addition to the standards in SMC 17.114.140, trails proposed to be located in landslide or erosion hazard areas shall be constructed in a manner that does not increase the risk of landslide or erosion and in accordance with an approved geotechnical report.

(3) Moderate and Severe Erosion Hazards and Landslide Hazards on Moderate and Steep Slopes. Except as otherwise provided for in this chapter and Chapter 17.114 SMC, only those activities approved and permitted consistent with an approved critical area report in accordance with this chapter shall be allowed in moderate or severe erosion hazard areas or in landslide hazard areas with moderate or steep (severe) slopes.

(4) The following activities are allowed within slight erosion, low landslide, all volcanic, and all seismic hazard areas without a critical areas report:

(a) Construction of new buildings with less than 2,500 square feet of floor area or roof area, whichever is greater, and which are not residential structures or used as places of employment or public assembly;

(b) Single-Family Infill Development. Construction of a single-family unit, additions, and accessory structures that do not impact streams, wetlands, or their buffers; and

(c) Installation of fences.

(5) Frequently Flooded Areas. Properties located in frequently flooded areas are subject to the regulations in Chapter 17.120 SMC.

(6) For single-family permits meeting the criteria of subsection (4) of this section, the city shall require, prior to occupancy, recording of a covenant on the title of the property for which a building permit is issued, stating as follows:

Persons with interest in this property are advised that this property is potentially subject to flooding, geologic (seismic), erosion, and volcanic lahars (mudflow) hazards.

(7) Critical facilities shall not be permitted on seismic faults or tsunami inundation areas. If such a prohibition is unreasonable, an allowance for critical facilities in tsunami areas can be made with the following specific conditions:

(a) Construction of new critical facilities shall be permissible if no feasible alternative site is available.

(b) Critical facilities shall have the lowest floor elevated three feet or more above the level of the base flood elevation (100-year flood).

(c) Floodproofing and sealing measures must be taken to ensure that toxic substances will not be displaced by or released into flood waters.

(d) Access routes elevated to or above the level of the base flood elevation shall be provided to all critical facilities to the extent possible. (Ord. 1164 § 4, 2004).

17.115.070 Critical area reports – Specific requirements for geologically sensitive areas.

In addition to the general requirements of SMC 17.114.160, critical area reports for geologically hazardous areas must meet the requirements of this section.

(1) Prepared by a Qualified Professional. A critical areas report for a geologically hazardous area shall be prepared by a geotechnical engineer or geologist, licensed in the state of Washington, with experience analyzing geologic, hydrologic, and groundwater flow systems; or by a geologist who earns his or her livelihood from the field of geology and/or geotechnical analysis, with experience analyzing geologic, hydrologic and groundwater flow systems, who has experience preparing reports for the relevant type of hazard.

(2) Erosion and Landslide Hazard Areas. In addition to the basic critical area report requirements, a critical area report for an erosion hazard or landslide hazard area shall include the following information at a minimum:

(a) Site Plan. The report shall include a copy of the site plan for the proposal showing:

(i) The height of slope, slope gradient, and cross section of the project area;

(ii) The location of springs, seeps, or other surface expressions of groundwater on or within 200 feet of the project area or that have the potential to be affected by the proposal; and

(iii) The location and description of surface water runoff.

(b) Hazards Analysis. The hazards analysis shall specifically include:

(i) A description of subsurface conditions based on data from site-specific explorations;

(ii) Descriptions of surface and groundwater conditions, public and private sewage disposal systems, fills and excavations, and all structural improvements;

(iii) An estimate of slope stability and the effect construction and placement of structures will have on the slope over 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) Consideration of the run-out hazard of landslide debris and/or the impacts of landslide run-out on down slope properties;

(vi) A study of slope stability including an analysis of proposed cuts, fills, and other site grading;

(vii) Recommendations for building siting limitations;

(viii) An analysis of proposed surface and subsurface drainage, and the vulnerability of the site to erosion.

(c) Geotechnical Engineering Report. The technical information for a project within a landslide hazard area shall include a geotechnical engineering report prepared by a licensed engineer that presents engineering recommendations for the following:

(i) Parameters for design of site improvements including appropriate foundations and retaining structures. These should include allowable load and resistance capacities for bearing and lateral loads, installation considerations, and estimates of settlement performance;

(ii) Recommendations for drainage and subdrainage improvements;

(iii) Earthwork recommendations including clearing and site preparation criteria, fill placement and compaction criteria, temporary and permanent slope inclinations and protection, and temporary excavation support, if necessary; and

(iv) Mitigation of adverse site conditions including slope stabilization measures and seismically unstable soils, if appropriate.

(d) Erosion and Sediment Control Plan. For any development proposal on a site containing an erosion hazard area, an erosion and sediment control plan shall be required. The erosion and sediment control plan shall be prepared in compliance with requirements set forth in the city’s stormwater management regulations (Chapter 17.140 SMC).

(e) Drainage Plan. The report shall include a drainage plan for the collection, transport, treatment, discharge and/or recycling of water prepared in accordance with the city’s surface water management plan. The drainage plan should consider on-site septic system disposal volumes where the additional volume will affect the erosion or landslide hazard area.

(f) Mitigation Plans. Hazard and environmental mitigation plans for erosion and landslide hazard areas shall include the location and methods of drainage, surface water management, locations and methods of erosion control, a vegetation management and/or replanting plan and/or other means for maintaining long-term soil stability.

(g) Monitoring Surface Waters. If the city determines that there is a significant risk of damage to downstream receiving waters due to potential erosion from the site, based on the size of the project, the proximity to the receiving waters, or the sensitivity of the receiving waters, the critical area report shall include a plan to monitor the surface water discharge from the site. The monitoring plan shall include a recommended schedule for submitting monitoring reports to the city.

(3) Seismic Hazard Areas. In addition to the basic report requirements, a critical area report for a seismic hazard area shall also meet the following requirements:

(a) The site map shall show all known and mapped faults within 200 feet of the project area or that have potential to be affected by the proposal.

(b) The hazards analysis shall include a complete discussion of the potential impacts of seismic activity on the site (for example, forces generated and fault displacement).

(c) A geotechnical engineering report shall evaluate the physical properties of the subsurface soils, especially the thickness of unconsolidated deposits and their liquefaction potential. If it is determined that the site is subject to liquefaction, mitigation measures appropriate to the scale of the development shall be recommended and implemented.

(4) Tsunami Hazard Areas. In addition to the basic report requirements, a critical area report for a tsunami hazard area shall also meet the following requirements:

(a) Site Plan. The site plan shall show all areas within 200 feet of the project area that have the potential to be inundated by wave action derived from a seismic event;

(b) Hazards Analysis. The hazards analysis shall include a complete discussion of the potential impacts of the tsunami hazard on the site; and

(c) Emergency Management Plan. The emergency management plan shall include plans for emergency building exit routes, site evacuation routes, emergency training, notification of local emergency management officials, and an emergency warning system.

(5) Volcanic and Other Geologically Hazardous Areas. In addition to the basic report requirements, the city may require additional information to be included in the critical area report when determined to be necessary to review the proposed activity and the subject hazard. Additional information that may be required includes, but is not limited to:

(a) Site Plan. The site plan shall show all hazard areas located within 200 feet of the project area or that have the potential to be affected by the proposal; and

(b) Hazards Analysis. The hazards analysis shall include a complete discussion of the potential impacts of the hazard on the project area and of the proposal on the hazard. (Ord. 1164 § 4, 2004).

17.115.080 Performance standards.

(1) Erosion and Landslide Hazard Areas. Activities on sites containing moderate or high erosion or landslide hazards shall meet the following requirements:

(a) Buffer Required. A buffer shall be established from all edges of moderate or high erosion or landslide hazard areas. The size of the buffer shall be determined by the city to eliminate or minimize the risk of property damage, death or injury resulting from erosion and 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.

(i) Minimum Buffer. At the base of a slope, the buffer shall be equal to one-half the height of the slope (as measured vertically from the toe to the top of the slope). For slopes less than 100 percent, the setback shall be measured from the toe of the slope. For slopes greater than 100 percent, the buffer shall be measured from a projection of a 100 percent slope from the top of the slope rather than the actual toe of the slope. See illustration under subsection (1)(a)(ii) of this section.

(ii) At the top of a slope, the setback should be equal to one-third the height of the slope. For slopes less than 100 percent, this buffer shall be measured from the top of the slope. For slopes greater than 100 percent, this buffer shall be measured from the projection of a 100 percent slope originating at the toe.

 

Figure 17.115.080A

(iii) Buffer Reduction. The buffer may be reduced to a minimum of 25 feet when a qualified professional demonstrates to the city’s satisfaction that the reduction will adequately protect the proposed development, adjacent developments and uses and the subject critical area.

(iv) Increased Buffer. The buffer may be increased where the city determines a larger buffer is necessary to prevent risk of damage to proposed and existing development.

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

(i) The development will not increase surface water discharge or sedimentation to adjacent properties beyond pre-development conditions;

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

(iii) Such alterations will not adversely impact other critical areas.

(c) Design Standards. 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 standards are:

(i) 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 Uniform Building Code;

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

(iii) 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;

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

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

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

(vii) Development shall be designed to minimize impervious lot coverage.

(d) Vegetation Shall Be Retained. 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.

(e) Seasonal Restriction. Clearing shall be allowed only from May 1st to October 1st of each year; provided, that the city may extend or shorten the dry season on a case-by-case basis depending on actual weather conditions, except that timber harvest, not including brush clearing or stump removal, may be allowed pursuant to an approved forest practice permit issued by the Department of Natural Resources.

(f) 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.

(g) 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:

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

(ii) Discharged at flow durations matching predeveloped conditions, with adequate energy dissipation, into existing channels that previously conveyed stormwater runoff in the predeveloped state; or

(iii) 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.

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

(i) Land that is located wholly within an erosion or landslide hazard area or its buffer may not be subdivided. Land that is located partially within an erosion or 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 erosion or landslide hazard or its buffer.

(ii) Access roads and utilities may be permitted within the erosion or landslide hazard area and associated buffers if the city determines that no other feasible alternative exists.

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

(2) Tsunami Hazard Areas (Unknown Hazard). Activities on sites containing areas susceptible to inundation due to tsunami hazards shall require an evacuation and emergency management plan. Uses in all tsunami hazard areas shall adhere to the following standards, in addition to the other requirements of this chapter:

(a) All new construction shall be located landward of the reach of mean high tide.

(b) All new construction and substantial improvements shall be elevated on pilings and columns so that:

(i) The bottom of the lowest horizontal structural member of the lowest floor (excluding the pilings or columns) is elevated one foot or more above the base flood level; and

(ii) The pile or column foundation and structure attached thereto is anchored to resist flotation, collapse and lateral movement due to the effects of wind and water loads acting simultaneously on all building components. Wind and water loading values shall each have a one percent chance of being equaled or exceeded in any given year (100-year mean recurrence interval).

(c) Design Must Be Certified. The structural design, specifications and plans for a proposed activity within a tsunami hazard area shall be developed, reviewed, and certified by a registered professional engineer or architect that the design and methods of construction to be used are in accordance with accepted standards of practice for meeting the standards of this section.

(d) Space Below Lowest Floor Shall Be Free of Obstruction. The space below the lowest floor of all new construction and substantial improvements shall be either free of obstruction or constructed with nonsupporting breakaway walls, open wood lattice-work, or insect screening intended to collapse under wind and water loads without causing collapse, displacement, or other structural damage to the elevated portion of the building or supporting foundation system. For the purposes of this section, a breakaway wall shall have a design safe loading resistance of not less than 10 and no more than 20 pounds per square foot. Use of breakaway walls which exceed a design safe loading resistance of 20 pounds per square foot (either by design or when so required by local or state codes) may be permitted only if a registered professional engineer or architect certifies that the designs proposed meet the following conditions:

(i) Breakaway wall collapse shall result from water load less than that which would occur during the base flood; and

(ii) The elevated portion of the building and supporting foundation system shall not be subject to collapse, displacement, or other structural damage due to the effects of wind and water loads acting simultaneously on all building components (structural and nonstructural). Maximum wind and water loading values to be used in this determination shall each have a one percent chance of being equaled or exceeded in any given year (100-year mean recurrence interval).

(e) Use of Breakaway Walls Shall Be Limited. If breakaway walls are utilized, such enclosed space shall be used solely for parking of vehicles, building access, or storage, and shall not be used for human habitation.

(f) Manufactured Homes. Manufactured homes to be placed or substantially improved in tsunami hazard areas shall meet the requirements of this chapter.

(g) Recreational Vehicles. Recreational vehicles placed on sites within tsunami hazard areas shall meet the requirements of this chapter.

(3) Seismic and Other Hazard Areas. Alterations of these geologically hazardous areas or associated buffers may only occur for activities that:

(a) Will not increase the threat of the geological hazard to adjacent properties beyond pre-development conditions;

(b) Will not adversely impact other critical areas;

(c) Are designed so that the hazard to the project is eliminated or mitigated to a level equal to or less than pre-development conditions; and

(d) Are certified as safe as designed and under anticipated conditions by a qualified engineer or geologist, licensed in the state of Washington. (Ord. 1164 § 4, 2004).