22.80.130 Geologically hazardous areas

22.80.130 Geologically hazardous areas.

A. Designation. 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 may also increase the hazard to surrounding development and uses. Areas susceptible to one or more of the following types of hazards shall be designated as a geologically hazardous area:

1. Erosion hazard;

2. Landslide hazard;

3. Seismic hazard; and

4. Other geological events including tsunami, mass wasting, debris flows, rock falls, and differential settlement.

1. 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 “severe” or “very severe” rill and inter-rill erosion hazard.

2. 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 failure, such as:

i. Those areas delineated by the U.S. Department of Agriculture’s Natural Resources Conservation Service as having a “severe” limitation for building site development; or

ii. Areas designated as quaternary slumps, earthflows, mudflows, lahars, or landslides on maps published by the U.S. Geological Survey or Department of Natural Resources;

b. Areas with all three of the following characteristics:

i. Slopes steeper than fifteen percent; and

ii. Hillsides intersecting geologic contacts with a relatively permeable sediment overlaying a relatively impermeable sediment or bedrock; and

iii. Springs or groundwater seepage;

c. Areas that have shown movement during the Holocene epoch (from ten thousand 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 faults) in subsurface materials;

e. Slopes having a gradient steeper than eighty percent subject to rock fall during seismic shaking;

f. Areas potentially unstable because of rapid stream incision, stream bank erosion, and undercutting by wave action;

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

h. Any area with a slope of forty percent or steeper and with a vertical relief of ten or more feet except areas composed of consolidated rock. A slope delineated by establishing its toe and top and measured by averaging the inclination over at least ten feet of vertical relief.

3. Seismic Hazard Areas. Seismic hazard areas are subject to severe risk of damage as a result of earthquake-induced ground shaking, slope failure, settlement, soil liquefaction, lateral spreading, or surface failure. 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 and thickness of geologic materials at the surface; and

d. The type of subsurface geological structure.

1. The approximate location and extent of geologically hazardous areas are shown on the adopted critical areas maps. The adopted critical areas maps include:

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

b. Department of Natural Resources seismic hazard maps of Western Washington, as they become available;

c. Department of Natural Resources slope stability maps, as they become available;

d. Federal Emergency Management Administration flood insurance maps; and

e. Locally adopted maps.

2. These maps are to be used as a guide for the city of Monroe, 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.

D. Best Available Science. Any approval of alterations of impacts to a geologically hazardous area or any associated buffers shall be supported by the best available science.

E. Native Growth Protection Easement/Critical Area Tract. As part of the implementation of approved development applications and alterations, geologically hazardous areas and any associated buffers that remain undeveloped pursuant to the critical areas regulations, in accordance with MMC 22.80.080, Protection and mitigation measures, shall be designated as native growth protection easements (NGPE). When the subject development is a formal subdivision (plat) or a short subdivision (short plat), the geologically hazardous area(s) and any buffers shall be placed in a critical areas tract instead of an NGPE, as described in MMC 22.80.080, Protection and mitigation measures.

F. Allowed Activities. The following activities are allowed in geologically hazardous areas and do not require submission of a critical areas report:

1. Erosion and Landslide Hazard Areas. Except as otherwise provided for in this chapter, only those activities approved and permitted consistent with an approved critical areas report in accordance with this chapter shall be allowed.

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

a. Construction of new buildings and/or additions will be reviewed on a case-by-case basis.

b. Installation of fences.

3. Other Hazard Areas. The following activities areas are allowed within other geological hazard areas:

a. Construction of new buildings and/or additions will be reviewed on a case-by-case basis.

b. Installation of fences.

1. Alterations of 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 predevelopment 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 predevelopment conditions; and

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

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

a. Buffers Required. A buffer shall be established for all edges of 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 areas report prepared by a qualified professional.

b. Minimum Buffers. The minimum buffer shall be equal to the height of the slope or fifty feet, whichever is greater.

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

d. Increased Buffer. The buffer may be increased when the zoning administrator determines a larger buffer is necessary to prevent risk of damage to proposed and existing development.

e. 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 the predevelopment condition;

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

iii. Such alteration will not adversely impact other critical areas.

I. 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 requirements 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 decrease the factor of safety for landslide occurrences below the limits of one and one-half for static conditions and one and two-tenths for dynamic conditions. Analysis of dynamic conditions shall be based on a minimum horizontal acceleration as established by the current version of the International Building Code;

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

3. Structures and improvements shall minimize alterations to the natural contours of the slope and foundations shall be tiered where possible to conform to existing topography;

4. Structures and improvements shall be located 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 retaining walls that allow the maintenance of existing natural slopes is preferred over graded artificial slopes; and

7. Development shall be designed to minimize impervious lot coverage.

J. Vegetation. Vegetation shall be retained unless it can be shown that the removal will not increase the geologic hazards, and a vegetation management plan is submitted with the request.

K. 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 the actual weather conditions, except that timber harvest, not including brush clearing or stump removal, may be allowed pursuant to an approved forest practices permit issued by the State Department of Natural Resources.

L. Utility Lines and Pipes. Utility lines and pipes shall be permitted in the 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 be properly anchored and/or designed so that it will continue to function in the event of an underlying slide. Storm water conveyance shall be allowed only through a high density polyethylene pipe with fuse-welded joints, or similar product that is technically equal or superior.

M. Point Discharge. 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. Conveyance via continuous storm pipe downslope to a point where there are no erosion hazard areas downstream from the discharge; and

2. 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.

N. Subdivisions. The division of land in erosion or landslide hazard areas and associated buffers is subject to provisions established for all critical areas in MMC 22.80.080, Protection and mitigation measures.

O. Prohibited Development. On-site sewage disposal systems, including drain fields, shall be prohibited within erosion and landslide hazard areas and associated buffers. (Ord. 005/2019 § 10 (Exh. B))