Chapter 23.80 GEOLOGICALLY HAZARDOUS AREAS

23.80.050 Special study and report requirements – Geologically hazardous areas.

Part I. Designation, Rating and Mapping

23.80.000 Geologically hazardous areas compliance requirements flowchart.

23.80.010 Designation, rating and mapping – Geologically hazardous areas.

Geologically hazardous areas include areas susceptible to erosion, land 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:

23.80.020 Designation of specific hazard areas.

A. 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. Erosion hazard areas are also those areas impacted by shoreland and/or stream bank erosion. Within the city of Edmonds erosion hazard areas include:

1. Those areas of the city of Edmonds containing soils that may experience severe to very severe erosion hazard. This group of soils includes, but is not limited to, the following when they occur on slopes of 15 percent or greater:

2. Any area with slopes of 15 percent or greater and impermeable soils interbedded with granular soils and springs or ground water seepage; and

3. Areas with significant visible evidence of ground water seepage, and which also include existing landslide deposits regardless of slope.

B. 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. Within the city of Edmonds landslide hazard areas specifically include:

1. Areas of ancient or historic failures in Edmonds which include all areas within the earth subsidence and landslide hazard area as identified in the 1979 report of Robert Lowe Associates and amended by the 1985 report of GeoEngineers, Inc.;

2. 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 (as defined in Figure 1 in subsection (B)(1) of this section) and is measured by averaging the inclination over at least 10 feet of vertical relief or 25 feet of horizontal distance. Benches, steps and variations in gradient shall be incorporated into a larger slope if they do not meet criteria defining toe and/or top depicted in Figure 1 in subsection (B)(1) of this section (see also Figure 2 at the end of this subsection). If the toe or top of a slope is located off of a subject property, then the location of the toe or top shall be delineated 200 horizontal feet from the property boundary or at its natural location, whichever is closer to the subject parcel (see Figure 2 at the end of this subsection);

3. Any area potentially unstable as a result of rapid stream incision or stream bank erosion; and

4. Any area located on an alluvial fan, presently subject to, or potentially subject to, inundation by debris flow or deposition of stream-transported sediments.

C. 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. 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 ground water table. [Ord. 3527 § 2, 2004].

23.80.030 Mapping of geologically hazardous areas.

A. The approximate location and extent of geologically hazardous areas are shown on the city of Edmonds critical areas inventory. In addition, resources providing information on the location and extent of geologically hazardous areas in Edmonds include:

2. U.S. Geological Survey geologic maps, landslide hazard maps, and seismic hazard maps;

3. Washington State Department of Natural Resources seismic hazard maps for Western Washington;

B. The critical areas inventory and the resources cited above are to be used as a guide for the city of Edmonds development services department, 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. 3527 § 2, 2004].

Part II. Allowed Activities – Geologically Hazardous Areas

23.80.040 Allowed activities – Geologically hazardous areas.

The following activities are allowed in geologically hazardous areas as consistent with ECDC 23.40.220, Allowed activities, Chapter 19.10 ECDC, Building Permits – Earth Subsidence and Landslide Hazard Areas, and Chapter 18.30 ECDC, Storm Water Management, and do not require submission of a critical area report:

A. Erosion and Landslide Hazard Areas. Except as otherwise provided for in this title, only those activities approved and permitted consistent with an approved critical areas report in accordance with this title shall be allowed in erosion or landslide hazard areas.

B. Seismic Hazard Areas. The following activities are allowed within seismic hazard areas:

1. 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;

2. Additions to existing single-story residences that are 250 square feet or less; and

Part III. Additional Report Requirements – Geologically Hazardous Areas

23.80.050 Special study and report requirements – Geologically hazardous areas.

Critical area report requirements for geologically hazardous areas are generally met through submission to the director of one or more geotechnical engineering reports. In addition to the general critical areas report requirements of ECDC 23.40.090, critical areas reports for geologically hazardous areas must meet the requirements of this section and Chapters 18.30 and 19.10 ECDC as applicable. Critical areas reports for two or more types of critical areas must meet the report requirements for each relevant type of critical area. Geotechnical report(s) submitted for the purpose of critical areas review are required as necessary in addition to reports, data and other information mandated per ECDC Titles 18 and 19.

A. Preparation by a Qualified Professional. A critical areas report for a geologically hazardous area shall be prepared by an engineer or geologist licensed in the state of Washington, with experience analyzing geologic, hydrologic, and ground water flow systems, and who has experience preparing reports for the relevant type of hazard. Critical areas studies and reports on geologically hazardous areas shall be subject to independent review pursuant to ECDC 23.40.090(B).

B. Area Addressed in Critical Areas Report. The following areas shall be addressed in a critical areas report for geologically hazardous areas:

2. All geologically hazardous areas within 200 feet of the project area or that have the potential to be affected by the proposal.

C. Geological Hazards Assessment. A critical areas report for a geologically hazardous area shall contain an assessment of geological hazards including the following site- and proposal-related information at a minimum:

1. Site and Construction Plans. The report shall include a copy of the site plans for the proposal showing:

a. The type and extent of geologic hazard areas, any other critical areas, and buffers on, adjacent to, within 200 feet of, or that are likely to impact the proposal;

b. Proposed development, including the location of existing and proposed structures, fill, storage of materials, and drainage facilities, with dimensions indicating distances to the floodplain, if available;

c. The topography, in two-foot contours, of the project area and all hazard areas addressed in the report; and

2. Assessment of Geological Characteristics. The report shall include an assessment of the geologic characteristics of the soils, sediments, and/or rock of the project area and potentially affected adjacent properties, and a review of the site history regarding landslides, erosion, and prior grading. Soils analysis shall be accomplished in accordance with accepted classification systems in use in the region. The assessment shall include, but not be limited to:

a. A description of the surface and subsurface geology, hydrology, soils, and vegetation found in the project area and in all hazard areas addressed in the report;

b. A detailed overview of the field investigations, published data, and references; data and conclusions from past assessments of the site; and site-specific measurements, tests, investigations, or studies that support the identification of geologically hazardous areas; and

c. A description of the vulnerability of the site to seismic and other geologic events;

3. Analysis of Proposal. The report shall contain a hazards analysis including a detailed description of the project, its relationship to the geologic hazard(s), and its potential impact upon the hazard area, the subject property, and affected adjacent properties; and

4. Minimum Buffer and Building Setback. The report shall make a recommendation for the minimum no-disturbance buffer and minimum building setback from any geologic hazard based upon the geotechnical analysis.

D. Incorporation of Previous Study. Where a valid critical areas 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 required critical areas report. The applicant shall submit a hazards assessment detailing any changed environmental conditions associated with the site.

E. Mitigation of Long-Term Impacts. When hazard mitigation is required, the mitigation plan shall specifically address how the activity maintains or reduces the preexisting 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). Proposed mitigation techniques shall be considered to provide long-term hazard reduction only if they do not require regular maintenance or other actions to maintain their function. Mitigation may also be required to avoid any increase in risk above the preexisting conditions following abandonment of the activity.

F. Additional Technical Information Requirements for Erosion and Landslide Hazard Areas. In addition to the basic critical areas report requirements for geologically hazardous areas provided in subsections A through E of this section, technical information for erosion and landslide hazard areas shall meet the requirements of Chapter 19.10 ECDC and include the following information at a minimum:

1. Site Plan. The critical areas report shall include a copy of the site plan for the proposal showing:

b. The location of springs, seeps, or other surface expressions of ground water on or within 200 feet of the project area or that have the potential to be affected by the proposal; and

2. Hazards Analysis. The hazards analysis component of the critical areas report shall specifically include:

b. A description of subsurface conditions based on data from site-specific explorations;

c. Descriptions of surface and ground water conditions, public and private sewage disposal systems, fills and excavations, and all structural improvements;

d. 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;

e. An estimate of the bluff retreat rate that recognizes and reflects potential catastrophic events such as seismic activity or a 100-year storm event;

f. Consideration of the run-out hazard of landslide debris and/or the impacts of landslide run-out on down-slope properties;

g. A study of slope stability including an analysis of proposed cuts, fills, and other site grading;

i. An analysis of proposed surface and subsurface drainage, and the vulnerability of the site to erosion;

3. 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:

a. 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;

c. 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

d. Mitigation of adverse site conditions including slope stabilization measures and seismically unstable soils, if appropriate;

4. 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 Chapter 18.30 ECDC.

G. Limited Report Requirements for Stable Erosion Hazard Areas. At the director's discretion, detailed critical areas report requirements may be waived for erosion hazard areas with suitable slope stability. Report requirements for stable erosion hazard areas may be met through construction documents that shall include at a minimum an erosion and sediment control plan prepared in compliance with requirements set forth in Chapter 18.30 ECDC.

H. Seismic Hazard Areas. In addition to the basic critical areas report requirements for geologically hazardous areas provided in subsections A through E of this section, a critical areas report for a seismic hazard area shall also meet the following requirements:

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

2. 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).

3. 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. [Ord. 3527 § 2, 2004].

Part IV. Development Standards – Geologically Hazardous Areas

23.80.060 Development standards – General requirements.

A. Alterations of geologically hazardous areas or associated buffers may only occur for activities that:

1. Will not increase the threat of the geological hazard to adjacent properties beyond predevelopment conditions;

3. Are designed so that the hazard to the project is eliminated or mitigated to a level equal to or less than predevelopment conditions; and

4. Are certified as safe as designed and under anticipated conditions by a qualified engineer or geologist, licensed in the state of Washington.

B. Critical Facilities Prohibited. Critical facilities shall not be sited within geologically hazardous areas unless there is no other practical alternative. [Ord. 3527 § 2, 2004].

23.80.070 Development standards – Specific hazards.

A. Erosion and Landslide Hazard Areas. Activities on sites containing erosion or landslide hazards shall meet the requirements of ECDC 23.80.060, Development Standards – General Requirements, and the specific following requirements:

1. Buffer Requirement. A buffer shall be established from all edges of landslide hazard areas. The size of the buffer shall be determined by the director consistent with recommendations provided in the geotechnical report to eliminate or minimize the risk of property damage, death, or injury resulting from landslides caused in whole or part by the development, based upon review of and concurrence with a critical areas report prepared by a qualified professional.

a. Minimum Buffer. The minimum buffer shall be equal to the height of the slope existing within the project area or 50 feet, whichever is greater;

b. Buffer Reduction. The buffer may be reduced to a minimum of 10 feet when a qualified professional demonstrates to the satisfaction of the director that the reduction will adequately protect the proposed development, adjacent developments and uses and the subject critical area;

c. Increased Buffer. The buffer may be increased where the director determines that a larger buffer is necessary to prevent risk of damage to proposed and existing development;

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

a. The development will not increase surface water discharge or sedimentation to adjacent properties beyond predevelopment conditions;

b. The development will not decrease slope stability on adjacent properties; and

3. 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 title. 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:

a. 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. If stability at the proposed development site is below these limits, the proposed development shall provide practicable approaches to reduce risk to human safety and improve the factor of safety for landsliding. In no case shall the existing factor of safety be reduced for the subject property or adjacent properties;

b. Structures and improvements shall be clustered to avoid geologically hazardous areas and other critical areas;

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

f. The use of retaining walls that allow the maintenance of existing natural slope area is preferred over graded artificial slopes; and

4. Vegetation Retention. 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;

5. Seasonal Restriction. Clearing shall be allowed only from May 1st to October 1st of each year; provided, that the director 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 city of Edmonds or the Washington State Department of Natural Resources;

6. 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:

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

b. Discharged at flow durations matching predeveloped conditions, with adequate energy dissipation, into existing channels that previously conveyed storm water runoff in the predeveloped state; or

c. 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; and

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

B. Earth Subsidence and Landslide Hazard Area. In addition to the requirements of this chapter, development proposals for lands located within the earth subsidence and landslide hazard area as indicated on the critical areas inventory shall be subject to the provisions of Chapter 19.10 ECDC.

C. Seismic Hazard Areas. Activities proposed to be located in seismic hazard areas shall meet the standards of ECDC 23.80.060, Development Standards – General Requirements. [Ord. 3527 § 2, 2004].