The purpose of this chapter is to designate geologic sensitive areas in the city and to regulate development activities in or near geologic sensitive areas to safeguard the public health, safety and welfare.

Several geologic conditions influence development on or adjacent to slopes, including: slope inclination, soil types, underlying geology, ground water and seepage, surface water runoff and vegetative cover. Therefore, for the purposes of this chapter, a geologic sensitive areas map has been prepared for the city that will be used to determine when additional site analysis will be required as a condition of development. This map is to be used as a trigger point for further site analysis and does not mean that properties that fall within a geologic sensitive area are unbuildable. Development within the city will be allowed, conditionally allowed, or prohibited, based on the city's review of the findings of a geotechnical report of the site prepared by either an engineering geologist licensed in Washington State and/or a registered professional civil engineer licensed in the state of Washington (herein referred to as "licensed professional") which addresses each of the items in this chapter.

Development within these geologic sensitive areas should be evaluated based on their impacts on the surrounding terrain and geological conditions, not in isolation. Management of the entire system should be considered during the development process.

Areas subject to erosion rated moderate to severe or higher by the U.S. Department of Agriculture's Natural Resource Conservation Service;

Areas subject to erosion caused by streams, surface drainage, or along the shoreline;

Areas within a stream's channel migration zone;

Areas mapped on the city of Mukilteo's landslide hazard map having a moderate or higher rating;

Areas that are found to have, based on a site specific inspection, all of the following characteristics:

Areas that are underlain or covered by mass wastage debris or landslide materials;

Areas of known landslides, earth movement, or containing evidence of past landslides or earth movement;

Areas of steep slopes; slopes that have forty percent (forty percent or a twenty-two-degree angle) or steeper gradients and having a vertical relief greater than ten feet, excluding constructed slopes;

Areas subject to liquefaction due to soil type and/or location or seismically induced ground disturbance such as surface rupture, fissuring, and lateral spreading;

Areas that have soil types that fall within soil category II or III per the Preliminary Surficial Geologic Map of the Mukilteo and Everett Quadrangle, Snohomish County Washington, 1976; and/or

Areas that are subject to tsunami wave action.

Soil map identification from Preliminary surficial geologic map of the Mukilteo and Everett Quadrangle, Snohomish County, Washington, 1976.

City of Mukilteo Landslide Hazard Map;

City of Mukilteo Soil Identification Map;

U.S. Department of Agriculture's Natural Resource Conservation Service Maps;

Coastal Zone Atlas (for marine bluff hazards);

U.S. Geological Survey Landslide Hazard, Seismic Hazard, and Volcano Hazards Maps;

Washington State Department of Natural Resources Seismic Hazard Maps for Western Washington;

Washington State Department of Natural Resources Slope Stability Maps;

National Oceanic and Atmospheric Administration Tsunami Hazard Maps; and

Federal Emergency Management Agency Flood Insurance Maps.

Full Geotechnical Analysis. All development proposals which include lands that are within the designated geologic sensitive area shall submit a geotechnical report and site assessment. The report and site assessment shall be provided at the applicant's expense and shall contain the following information:

Geotechnical Letter Requirements. For the following single-family residential development applications, a letter prepared by a licensed professional may be prepared in lieu of the full geotechnical reporting requirements. A geotechnical letter summation shall include an assessment of the existing geological and geotechnical site conditions including surface water runoff, ground water, soil types, erosion, and slope stability. The licensed professional shall prepare conclusions and recommendations on the suitability of the proposed development and any mitigation necessary to address existing site conditions that may need to be modified due to the proposed development.

Exceptions. For the following single-family and multifamily residential development applications, the public works director may waive the requirements for a full geotechnical analysis or letter if the development has no impact on a geologic sensitive area:

Additional Review. The public works director may require a higher level of analysis than indicated in subsections B and C of this section if there is a localized condition on the site that cannot be adequately addressed in a letter report.

Applicants proposing development on undeveloped lots within a geologic sensitive area shall comply with the building setback and buffer recommendation presented in the geotechnical analysis. In no case shall the critical slope setback be less than twenty-five feet from a steep slope (forty percent or twenty-two-degree angle) unless allowed through the "reasonable use" provisions of the Mukilteo Municipal Code and supported by a geotechnical report and approved by the public works director. Decks which add no substantial loading weight to the sensitive area and accessory buildings one hundred and twenty square feet or less may extend into the setback area to within ten feet of the top or toe of a steep slope.

New lots developed as part of a subdivision, short subdivision, binding site plans and those lots reconfigured as part of a boundary line adjustment, shall be created in such a manner so that:

Utilities. Utility installation and repair projects within a geologic sensitive area shall comply with the reporting requirements contained in Section 17.52A.040, Analysis required. Geotechnical analysis letters may be allowed for projects less than two hundred feet in length. Exemptions may be allowed by the public works director for less than fifty feet. Utilities shall be designed according to the site specific geotechnical analysis and to meet typical public safety standards that include but are not limited to: use of polyethylene-encased ductile iron restraint joint pipes, isolation valves, pipe anchors, under drains, borings, uses of high density polyethylene pipe (HDPE), flexible expansion joints, minimized clearing, replanting of native vegetation, and installation of erosion control measures.

Modifications to slopes of forty percent or greater may only be allowed where the disturbed slope is not connected to a larger ravine system, is a manmade slope, or the area has been previously regraded and landscaped;

A site assessment has been submitted showing that the proposal will have no adverse impact on the stability or erosion susceptibility of the adjacent sensitive slope area;

The impacted area totals no more than twenty percent of the entire site;

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

The activity will not adversely impact other critical areas as regulated by Chapters 17.52 through 17.52E;

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

Stormwater runoff from any new impervious surface shall be collected in a detention system and directed to an enclosed drainage system. Where minor additions of less than one thousand square feet of new impervious areas are proposed to existing developed properties that do not have detention facilities, the stormwater runoff shall be directed to the city's storm drainage system or be designed for natural infiltration or dispersion. At no time shall concentrated stormwater runoff be allowed to flow directly over a steep slope or impact a neighboring property.

Alder, willow and bitter cherry and other similar trees may be cut and removed from the site in a method determined by the planning director and public works director; however, the stumps and root systems shall be left undisturbed to protect the slope from erosion. Deep rooted bushes or ground cover such as ocean spray, snow berry, salal or evergreen huckleberry shall be planted around the stump of the tree to establish erosion control functions that the tree once provided.

Trees (such as big leaf maple, vine maple, Pacific madrone, red cedar and Douglas fir) which help to stabilize bluffs, offer wildlife habitat, and keep soils from being oversaturated with water may not be cut down, except with the submittal of a geotechnical report as approved by the public works director to maintain slope stability. Trees may be trimmed and pruned consistent with standards in Chapter 17.59.

Himalayan blackberry, Scot's broom, thistle and other similar invasive plants (including those listed by the Snohomish County noxious weed control board) may be removed manually from a steep slope with no or minimal soil disturbance, but the slope must immediately be replanted with native shrub species such as Oregon grape, salal and evergreen huckleberry.

Unless otherwise provided or as part of an approved alteration, removal of vegetation from erosion or landslide hazard or related buffer shall be prohibited.

The proposed development shall provide a safety factor of 1.5 for static conditions and 1.2 for dynamic conditions for seismic occurrences. Analysis of dynamic conditions shall be based on a minimum horizontal acceleration as established by the current adopted version of the International Building Code;

Structures and improvements shall be clustered to avoid geologic sensitive areas and other critical areas;

Structures and improvements shall minimize alterations to the natural contour of the slope and foundations shall be tiered to conform to existing topography;

Structures and improvements shall be located to preserve the most sensitive portion of the site and its natural landforms and vegetation;

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

Single-family residential development shall be designed so that the impervious lot coverage does not exceed fifty percent of the site; and

Stormwater runoff shall be collected, detained, and released in accordance with the city's stormwater detention requirements. At no time shall concentrated stormwater runoff be allowed to flow directly over a steep slope or impact a neighboring property.

An owner of a site or property containing a geologic hazard area may be permitted to transfer the density attributable to the geologic hazard area portion of the property to another nonsensitive portion of the same site or property subject to the limitation of this section.

Up to one hundred percent of the density that could be achieved on the geologic sensitive area portion of the site can be transferred to the non-sensitive portion of the property, subject to: