A. 
Designation. Geologically hazardous areas are areas susceptible to erosion, landsliding, earth-quake activity or other geological processes. Areas susceptible to these types of hazards are hereby designated as geologically hazardous areas and shall be subject to the provisions of this chapter.
B. 
Mapping. The approximate location and extent of geologically hazardous areas are shown on mapping resources that support the city’s identification of critical areas. These resources serve as guidance for the city, applicants, and property owners, and may be updated as new critical areas are identified. They do not provide a definitive critical area designation. Mapping resources include:
1. 
Critical area maps contained in the environmental element of the comprehensive plan;
2. 
The city’s online map portal; and
3. 
The Washington State Department of Natural Resources Geologic Information Portal.
C. 
Classification. Geologically hazardous areas shall be classified as follows:
1. 
Landslide and Erosion Hazard Areas. Erosion hazard areas include those identified by the U.S. Department of Agriculture’s Natural Resources Conservation Service as having a moderate to severe, severe, or very severe erosion hazard because of natural characteristics, including vegetative cover, soil texture, slope, gradient, and rainfall patterns, or human-induced changes to natural characteristics. Landslide hazard areas are areas potentially subject to risk of mass movement due to a combination of geologic, topographic, and hydrologic factors. Landslide and erosion hazard areas include areas with the following characteristics:
a. 
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;
b. 
Slopes that are parallel or subparallel to planes of weakness (such as bedding planes, joint systems, and fault planes) in subsurface materials;
c. 
Slopes having gradients steeper than 80 percent subject to rockfall during seismic shaking;
d. 
Areas potentially unstable because of stream incision and stream bank erosion;
e. 
Areas located in a canyon, ravine, or on an active alluvial fan, presently or potentially subject to inundation by debris flows or flooding;
f. 
Any area with a slope of 40 percent or steeper and a vertical relief of 10 or more feet except areas composed of consolidated rock and properly engineered manmade slopes/retained fill. A slope is delineated by establishing its toe and top and measured by averaging the inclination over at least 10 feet of vertical relief;
g. 
Areas with a severe limitation for building development because of slope conditions, according to the Natural Resource Conservation Service; and
h. 
Areas meeting all three of the following criteria: slopes steeper than 15 percent except that slopes of less than 15 percent may be considered erosion hazard areas if they have certain unstable soil and drainage characteristics; hillsides intersecting geologic contacts with a relatively permeable sediment overlying a relatively impermeable sediment or bedrock; and wet season springs or ground water seepage.
2. 
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. 2004-603 § 2; Ord. 2026-945 § 2 (Att. A))
A. 
Alteration of geologically hazardous areas and buffers shall be prohibited except as expressly allowed in this chapter. The city may approve, condition or deny proposals based on the degree to which risks posed by geologically hazardous areas to public and private property and to health and safety can be mitigated. In an individual case, conditions may include limitations of or on proposed uses, density modification, alteration of site layout and other changes to the proposal determined appropriate by the director. Where the director determines that potential adverse impacts cannot be effectively mitigated, or where the risk to public health, safety and welfare, property, or important natural resources is significant notwithstanding mitigation, the proposal shall be denied. The burden of proof shall be on the applicant.
B. 
Alterations of geologically hazardous areas and their associated buffers may be permitted; provided, that all of the following criteria are met to the director’s satisfaction:
1. 
The alteration will not increase the threat of the geological hazard to adjacent properties beyond predevelopment conditions;
2. 
The alteration will not adversely impact other critical areas;
3. 
The proposed development activity is designed so that the hazard to the project is eliminated or mitigated to a level equal to or less than predevelopment conditions;
4. 
The alteration is certified to be safe as designed and under anticipated conditions by a qualified engineer or geologist, licensed in the state of Washington; and
5. 
The applicant complies with the city’s other clearing and grading regulations.
(Ord. 2004-603 § 2)
A. 
Alteration of land with slopes of 40 percent or greater shall be prohibited.
B. 
Alteration of slopes less than 40 percent, including slopes of 15 percent or less that have unstable soil or drainage characteristics, may be permitted pursuant to an approved critical area report and mitigation plan that certifies the following to the director’s satisfaction:
1. 
The development will not increase or concentrate surface water discharge or sedimentation to adjacent sites beyond predevelopment conditions; and
2. 
The development will not decrease slope stability on the development site or on adjacent sites.
C. 
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 through a geotechnical study 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:
1. 
The proposed development shall not increase the risk of landslide occurrences;
2. 
Structures and improvements shall avoid geologically hazardous areas and other critical areas;
3. 
Structures and improvements shall minimize alterations to the natural contour of the slope. Foundations shall be tiered where possible to conform to existing topography;
4. 
Structures and improvements shall be located, and clustered, if appropriate, to preserve the most critical portion of the site and its natural landforms and vegetation;
5. 
The proposed development shall not result in greater risk or a need for increased buffers on neighboring properties;
6. 
The use of engineered retaining walls that allow the maintenance of existing natural slope area is preferred over graded artificial slopes. Engineered retaining walls shall not exceed 15 feet in height and preferably should be less than eight feet in height. Engineered retaining walls over eight feet in height shall be allowed by the director only when it can be demonstrated that no other reasonable alternative exists. Riprap retaining walls should not exceed eight feet in height. Wherever possible, retaining walls should be designed as structural elements of the building foundation;
7. 
Development shall be designed to minimize impervious lot coverage. Use of common access drives and utility corridors is encouraged; and
8. 
Disturbed areas of a site not used for buildings, roads and other improvements shall be replanted promptly pursuant to an approved landscape plan.
D. 
Native 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. Limited pruning or selective removal of dead, diseased or damaged branches; limited removal of specified branches that block views; and topping as shown on a landscape plan may be approved by the director if the activity will not adversely affect slope stability. Identification of significant vegetation shall be based upon the tree species, location and condition in addition to size. Significant habitat should be preserved to the greatest extent feasible.
E. 
Seasonal Restriction. Clearing shall be allowed only from April 1st to October 31st of each year; provided, that the city may extend or shorten the designated dry season on a case-by-case basis depending on actual weather conditions.
F. 
Utility Lines and Pipes. Utility lines and pipes shall be permitted in landslide and erosion hazard areas pursuant to MCMC § 18.06.430. 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. Above ground utility lines and pipes shall be located and designed to minimize potential risks associated with tree fall.
G. 
Stormwater conveyance shall be allowed only by use of high-density polyethylene pipe with fuse-welded joints, or similar product that is technically equal or superior.
H. 
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:
1. 
The discharge is conveyed via continuous storm pipe downslope to a point where there are no erosion hazards areas downstream from the discharge; or
2. 
The discharge is released at flow durations matching predeveloped conditions, with adequate energy dissipation, into existing channels that previously conveyed stormwater runoff in the predeveloped state; or
3. 
The discharge is dispersed 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.
I. 
Access roads and utilities may be permitted within the erosion or landslide hazard area and associated buffers if the director determines based on an approved critical area report and mitigation plan that the road will not increase the risk to adjacent sites and that no other feasible alternative exists.
J. 
Erosion control plans shall be required for all regulated activities within landslide and erosion hazards areas. The erosion control plans shall be consistent with the provisions of Chapter 15.10 and 15.12 MCMC and prepared pursuant to a plan approved by the city engineer. A master drainage plan shall be prepared for large projects as required and approved by the city engineer.
K. 
Prohibited Development. On-site sewage disposal systems, including drain fields, shall be prohibited within landslide and erosion hazard areas and related buffers.
L. 
A monitoring plan shall be prepared and implemented for permitted construction activities permitted in landslide and erosion hazard areas.
(Ord. 2004-603 § 2; Ord. 2026-945 § 2 (Att. A))
Activities, development and uses on sites containing erosion or landslide hazards shall meet the following buffer requirements:
A. 
A buffer shall be established from the top, toe, and edges of all slope or erosion or landslide hazard areas with 10 feet or more of vertical elevation change. The size of the buffer shall be sufficient, as determined by the director, to eliminate or minimize the risk of damage to persons or property resulting from landslide and erosions caused in whole or part by the development, shall be based upon the director’s review of and concurrence with the geotechnical report prepared by a qualified professional, and shall be consistent with the following criteria:
1. 
For slopes between 16 and 39 percent, the minimum buffer shall be equal to one-half the height of the slope or 25 feet, whichever is greater. The buffer may be reduced by 25 percent or to a minimum of 25 feet, whichever is greater, when a qualified professional demonstrates to the director’s satisfaction that the reduction will adequately protect the proposed development, adjacent areas, developments, uses, and the subject critical area.
2. 
For slopes equal to or greater than 40 percent, the minimum buffer shall be equal to the height of the slope or 25 feet, whichever is greater. The buffer may be reduced by 25 percent when a qualified professional demonstrates to the director’s satisfaction that the reduction will adequately protect the proposed development, adjacent areas, developments, uses, and the subject critical area.
3. 
For slopes with a vertical elevation of 25 feet or less, the minimum buffer shall be equal to one-half the height of the slope, regardless of the slope percent; provided, that there are no other factors that pose a slope stability risk.
B. 
The minimum buffer area shall be comprised of undisturbed natural vegetation consisting of trees or dense woody vegetation and have adequate drainage.
C. 
To improve the functional attributes of the buffer, the director may require that the buffer be enhanced through planting.
D. 
Based on the findings of the geotechnical report, the director may require that the buffer be increased where a larger buffer is necessary to prevent risk of damage to adjacent areas and proposed and existing development.
E. 
The edge of the buffer area shall be clearly staked, flagged, and fenced prior to any site clearing and construction. The buffer boundary markers shall be clearly visible, durable, and permanently affixed to the ground. Site clearing shall not commence until the applicant has submitted written notice to the department that buffer requirements of this chapter are met. Field marking shall remain until all construction and clearing phases are completed, and final approval has been granted by the city. The buffer shall be maintained and preserved through a protective easement or other appropriate permanent protective covenant consistent with MCMC § 18.06.610 and 18.06.820.
F. 
Structures shall be set back in accordance with MCMC § 18.06.840 such that construction activities and outdoor living areas do not infringe upon the required buffer edge.
(Ord. 2004-603 § 2)
A. 
A critical areas report for geologically hazardous areas must meet the general critical area report requirements of MCMC § 18.06.530 and the geotechnical report requirements of this chapter. A geotechnical report for a geologically hazardous area shall also meet all of the following additional standards:
1. 
It shall address the area of the proposed activity and all geologically hazardous areas within 200 feet of the project area or that have potential to affect or be affected by the proposal;
2. 
It shall contain an assessment of geological hazards including at a minimum all of the following information:
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; an assessment of the geologic characteristics and engineering properties of the soils, sediments, and/or rock of the project area and potentially affected adjacent properties; a review of the site history regarding landslides, erosion, and prior grading; and an evaluation of the vulnerability of the site to seismic and other geologic events. Soils analysis shall be performed in accordance with accepted classification systems in use in the region.
b. 
A recommendation for the minimum buffer and minimum building setback from all geologic hazards based upon the geotechnical analysis.
c. 
When hazard mitigation is required, the report shall specifically address how the activity maintains or reduces the preexisting level of risk to the site and adjacent properties on a longterm 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.
B. 
Where a valid geotechnical 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 area report. Further updated analysis may be required if site-specific conditions so warrant in the director’s discretion. The applicant shall submit a geotechnical assessment detailing any changed environmental conditions associated with the site.
C. 
Geotechnical studies for two or more types of hazard areas shall meet the report requirements for each relevant type.
D. 
Monitoring Surface Waters. If the director 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 director.
(Ord. 2004-603 § 2)