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This section describes the minimum performance standard requirements for geologic hazard areas.

(1) Basic Requirements.

(a) Alteration of geologic hazard critical areas is permitted only if the development proposal can be designed so the hazard to the project and any increase of hazard to adjacent property is eliminated or mitigated, and the development proposal on the site is certified as safe by a geotechnical engineer licensed in the State of Washington.

(b) All proposals involving excavations and placement of fills shall be subject to structural review under Chapter 33, Site Work, Demolition and Construction, of the most current International Building Code.

(c) Essential public facilities as defined by RCW 36.70A.200 shall not be sited within designated geologic hazard areas.

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

(a) Alterations of the buffer and/or geologic hazard area may only occur for activities meeting the following criteria:

(i) No reasonable alternative exists; and

(ii) A geotechnical report is submitted and certifies that:

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

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

(C) That such alterations will not adversely impact other critical areas.

(b) A temporary erosion and sedimentation control plan, prepared in accordance with the requirements of the standard specification of the City of Pasco.

(c) A drainage plan for the collection, transport, treatment, discharge and/or recycle of water in accordance with the standard specification of the City of Pasco.

(d) Surface drainage shall not be directed across the face of a landslide hazard area (including riverine bluffs or ravines). If drainage must be discharged from the hazard area into adjacent waters, it shall be collected above the hazard and directed to the water by a tight line drain and provided with an energy dissipating device at the point of discharge.

(e) All infiltration systems, such as storm water detention and retention facilities, and curtain drains utilizing buried pipe or French drain, are prohibited in landslide hazard areas and their buffers unless a geotechnical report indicates such facilities or systems or the failure of the same will not affect slope stability and the systems are designed by a licensed civil engineer.

(f) A minimum standard buffer width of 30 feet shall be established from the top, toe, and all edges of landslide and erosion hazard areas. Existing native vegetation shall be maintained. The buffer may be reduced to a minimum of 10 feet when an applicant demonstrates the reduction will adequately protect the proposed development, adjacent developments and uses, and the subject critical area. The buffer may be increased by the Department of Community and Economic Development for development adjacent to a river bluff or ravine, or in other areas that circumstances may warrant, where it is determined a larger buffer is necessary to prevent risk of damage to proposed and existing development as in the case where the area potentially impacted by a landslide exceeds 30 feet.

(g) On-site sewage disposal systems, including drain fields, shall be prohibited within landslide and erosion hazard areas and related buffers.

(h) Development designs shall meet the following basic requirements, unless it can be demonstrated that an alternative design provides greater long-term slope stability while meeting all other criteria of this section. The requirement for long-term slope stability shall exclude designs that require periodic maintenance or other actions to maintain their level of function. The basic development design standards are:

(i) Structures and improvement shall be clustered to retain as much open space as possible and to preserve the natural topographic features of the site.

(ii) Structures and improvements shall conform to the natural contour of the slope, and foundations shall be tiered where possible to conform to existing topography.

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

(iv) The use of retaining walls that allow the maintenance of existing natural slope area is preferred over graded artificial slopes.

(v) All development shall be designed to minimize impervious lot coverage.

(3) Flood Hazard Areas. Activities in flood hazard areas shall comply with Chapter 28.28 PMC, Flood Hazard Areas, and the floodplain regulations (PMC Title 24).

(4) Seismic Hazard Areas. Activities on sites containing seismic hazards shall also meet the following requirements:

(a) Mitigation is implemented, which reduces the seismic risk to a level equivalent to that which the activity would experience if it were not located in a seismic hazard area.

(b) Structural development proposals shall meet all applicable provisions of Chapter 16 of the most current addition of the International Building Code (Structural Forces/Structural Design Requirements).

(c) No residential structures or essential public facilities shall be located on a Holocene fault line as indicated by USGS investigative maps and studies.

(5) Mine Hazard Areas. Activities on sites containing mine hazards shall also meet the following requirements:

(a) Mitigation is implemented which reduces the risk from mine hazards to a level equivalent to that which the activity would experience if it were not located in a mine hazard area.

(6) Volcanic Hazard Areas. No additional requirements. [Ord. 4525 § 1, 2021; Ord. 3911 § 2, 2009; Code 1970 § 28.32.070. Formerly 28.32.070.]