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).