20D.140.60 Geologically Hazardous Areas.

20D.140.60-010 Classification and Rating of Geologically Hazardous Areas.

To promote consistent application of the standards and requirements of this chapter, geologically hazardous areas within the City of Redmond shall be rated or classified according to their characteristics, function and value, and/or their sensitivity to disturbance.

(1) Geologically Hazardous Area Classifications. Geologically hazardous areas shall be classified according to the criteria in this section.

(a) Erosion Hazard Areas. Erosion hazard areas are lands or areas underlain by soils identified by the U.S. Department of Agriculture Soil Conservation Service (SCS) as having “severe” or “very severe” rill and inter-rill erosion hazards. This includes, but is not limited to, the following group of soils when they occur on slopes of 15 percent or greater: Alderwood-Kitsap (AkF), Alderwood gravelly sandy loam (AgD), Kitsap silt loam (KpD), Everett (EvD) and Indianola (InD).

(b) Landslide Hazard Areas. Landslide hazard areas are areas potentially subject to significant or severe risk of landslides based on a combination of geologic, topographic, and hydrogeologic factors. They include areas susceptible because of any combination of bedrock, soil, slope, slope aspect, structure, hydrology, or other factors. They are areas of the landscape that are at a high risk of failure or that presently exhibit downslope movement of soil and/or rocks and that are separated from the underlying stationary part of the slope by a definite plane of separation. The plane of separation may be thick or thin and may be composed of multiple failure zones depending on local conditions including soil type, slope gradient, and groundwater regime.

Landslide hazard areas include:

(i) Areas of historic failures, such as:

(A) Areas designated as quaternary slumps or landslides on maps published by the United States Geologic Survey (USGS); or

(B) Those areas designated by the United States Department of Agriculture (USDA) Soil Conservation Service (SCS) as having a “severe” limitation for building site development.

(ii) Areas containing a combination of slopes steeper than 15 percent, springs or groundwater seepage, and hillsides intersecting geologic contacts with a relatively permeable sediment overlying a relatively impermeable sediment or bedrock;

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

(iv) Slopes that are parallel or subparallel to planes of weakness in subsurface materials;

(v) Slopes having gradients steeper than 80 percent subject to rockfall during seismic shaking;

(vi) Areas potentially unstable as a result of rapid stream incision, stream bank erosion, and undercutting by wave action; or

(vii) Any area with a slope 40 percent or steeper with a vertical relief of 10 feet or more.

(c) Seismic Hazard Areas. Seismic hazard areas are lands subject to severe risk of damage as a result of earthquake-induced ground shaking, slope failure, settlement, soil liquefaction, or surface faulting.

(2) Classification of geologically hazardous areas shall be determined by the Committee based on consideration of the following factors:

(a) Maps adopted pursuant to this chapter; including the landslide hazard area, erosion hazard area, and seismic hazard areas maps, which identify the approximate location and extent of these hazard areas. These maps shall be used as a general guide only for the assistance of property owners and other interested parties; boundaries are generalized. The actual type, extent, and boundaries of geologically hazardous areas shall be determined in the field by a qualified consultant according to the procedures, definitions, and criteria established by this chapter. In the event of any conflict between the critical area location and designation shown on the City’s map and the criteria or standards of this section, the criteria and standards shall prevail;

(b) Maps published by other governmental agencies such as:

(i) USGS landslide hazard and seismic hazard maps;

(ii) Department of Natural Resources (DNR) seismic hazard maps for western Washington and slope stability maps;

(d) Consideration of the technical reports submitted by qualified consultants in connection with applications subject to these regulations. (Ord. 2259)

20D.140.60-020 Landslide Hazard Area Buffers.

(1) Landslide hazard area buffers shall be measured from the top and toe and along sides of the slope.

(2) Minimum Landslide Hazard Area Buffer. Required buffers shall be 50 feet. The width of the buffer shall reflect the sensitivity of the landslide hazard area in question and the types and density of uses proposed on or adjacent to the geologic hazard. In determining the appropriate buffer width, the Committee shall consider the recommendations contained in any technical report required by these regulations and prepared by an applicant’s qualified consultant.

(3) Buffer Reduction. Buffers may be reduced to a minimum of 15 feet when a qualified professional demonstrates through technical studies that the reduction will adequately protect the proposed and surrounding development from the critical landslide hazard.

(4) Increased Buffer. The buffer may be increased where the Technical Committee determines a larger buffer is necessary to prevent risk of damage to proposed and existing development. (Ord. 2259)

20D.140.60-030 Alteration of Geologically Hazardous Areas – Generally.

Alteration of geologically hazardous areas or their established buffers may only be permitted subject to the criteria in RCDG 20D.140.20-040 and 20D.140.20-050, 20D.140.30-030, 20D.140.40-020, 20D.140.50-020, and 20D.140.60-040. (Ord. 2259)

20D.140.60-040 Alteration of Geologically Hazardous Areas.

(1) The City shall approve, condition or deny proposals in a geologically hazardous area as appropriate based upon the effective mitigation of risks posed to property, health and safety. The objective of mitigation measures shall be to render a site containing a geologically hazardous site as safe as one not containing such hazard. Conditions may include limitations of proposed uses, modification of density, alteration of site layout and other appropriate changes to the proposal. Where potential impacts cannot be effectively mitigated, or where the risk to public health, safety and welfare, public or private property, or important natural resources is significant notwithstanding mitigation, the proposal shall be denied.

(2) Landslide Hazard Areas. Development shall be prohibited in landslide hazard areas except for the installation and construction of streets and/or utilities, subject to the criteria below. The Technical Committee shall refer the proposed project to the Hearing Examiner for review and approval.

(a) The proposed street and/or utility is identified in an adopted plan as of October 1, 1997, such as the Comprehensive Plan, Capital Facility Plan, Transportation Improvement Plan or other Utility Facility Plan. As new or amended plans are prepared and adopted, streets and utilities shall be located to avoid impact to landslide hazard areas. Where no reasonable alternative to locating in landslide hazard areas exists, review and approval of the plan shall include a discussion of alternatives and rationale for planning streets and utilities in landslide hazard areas.

(b) Alternative locations which avoid impact to landslide hazard areas are evaluated and are determined to be economically or functionally infeasible.

(c) There is a geotechnical evaluation to identify the risks of damage from the proposal, both on-site and off-site, to ascertain that the proposal will not increase the risk of occurrence of the potential geologic hazard; and to identify measures to eliminate or reduce risks.

When no alternative exists, the impact shall be minimized by limiting the magnitude of the proposed construction to the extent possible. Any impacts shall be rectified by repairing, rehabilitating, restoring, replacing or providing substitute resources consistent with the mitigation and performance standards contained in RCDG 20D.140.10-110 and 20D.140.10-120.

(3) Erosion Hazard Areas. Alteration of an erosion hazard area may only occur for activities for which a geotechnical analysis is submitted and certified that:

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

(b) The development will not decrease slope stability on the subject and adjacent properties; and

(4) Seismic Hazard Areas.

(a) For one-story and two-story residential structures, the applicant shall conduct an evaluation of site response and liquefaction potential based on the performance of similar structures under similar foundation conditions; and

(b) For all other proposals, the applicant shall conduct an evaluation of site response and liquefaction potential including sufficient subsurface exploration to provide a site coefficient (S) for use in the static lateral force procedure described in the International Building Code.

(5) When development is permitted in a geologically hazardous area by these regulations, an applicant and/or its qualified consultant shall provide assurances which include the following:

(a) A letter from the geotechnical engineer and/or geologist who prepared the studies required by these regulations that risks of damage from the proposal, both on-site and off-site, are minimal subject to the conditions set forth in the report, that the proposal will not increase the risk of occurrence of the potential geologic hazard, and that measures to eliminate or reduce risks have been incorporated into its recommendations; and

(b) A legal statement which shall be recorded and noted on the face of the deed or plat, and executed in a form satisfactory to the City, characterizing the site as being located in a geologically hazardous area and that there may or may not be risks associated with development of such site. (Ord. 2259)

20D.140.60-050 Geologically Hazardous Area Performance Standards.

(1) Relevant performance standards from RCDG 20D.140.20-060 and 20D.140.20-070 and 20D.140.30-040, as determined by the Committee, shall be incorporated into mitigation plans.

(2) Development within a geologically hazardous area shall meet the following basic requirements unless it can be demonstrated that an alternative design that deviates from one or more of these standards provides equivalent or greater long-term slope stability. The following performance standards shall be reflected in proposals within landslide and erosion hazard areas:

(a) Geotechnical studies shall be prepared by a qualified consultant to identify and evaluate potential hazards and to formulate mitigation measures;

(b) Construction methods will reduce or not adversely affect geologic hazards;

(c) 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;

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

(e) Structures and improvements shall be clustered to avoid geologically hazardous areas;

(f) 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;

(g) Development shall be designed to minimize impervious surface coverage;

(h) Disturbed areas should be replanted as soon as feasible pursuant to an approved landscape plan;

(i) Clearing and grading regulations as set forth by the City shall be followed;

(j) Use of retaining walls that allow maintenance of existing natural slope areas are preferred over graded artificial slopes;

(k) Temporary erosion and sedimentation controls, pursuant to an approved plan, shall be implemented during construction;

(l) A master drainage plan shall be prepared for large projects as required by the City Engineer;

(m) A monitoring program shall be prepared for construction activities permitted in geologically hazardous areas;

(n) Development shall not increase instability or create a hazard to the site or adjacent properties, or result in a significant increase in sedimentation or erosion; and

(o) 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 storm water runoff in the predevelopment 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 storm water runoff, and where it can be demonstrated that such discharge will not increase the saturation of the slope. (Ord. 2259)