30.62B.340 Landslide hazard areas | Snohomish County Code

30.62B.340 Landslide hazard areas.

(1) Development activities, actions requiring project permits and clearing shall not be allowed in landslide hazard areas as defined in SCC 30.91L.040 unless a deviation is granted by the director when the applicant demonstrates the following criteria are met:

(a) There is no alternate location for the structure on the subject property; and

(b) A geotechnical report demonstrates that building within a landslide hazardous area:

(i) will provide protection commensurate to being located outside of the landslide hazard area; and

(ii) the proposal meets the requirements of SCC 30.62B.320.

(2) In addition to the requirements in SCC 30.62B.320 the following standards and requirements apply to development activities, actions requiring project permits and clearing in landslide hazard areas:

(a) Vegetation shall not be removed from a landslide hazard area, except for hazardous trees based on review by a qualified arborist or as otherwise provided for in a vegetation management and restoration plan;

(b) The factor of safety for landslide occurrences shall not be decreased below the limits of 1.5 for static conditions or 1.1 for dynamic conditions. Analysis of dynamic conditions shall be based on horizontal acceleration as established by the current version of the International Building Code;

(d) Retaining walls that allow for the maintenance of existing natural slope area shall be used wherever possible instead of graded artificial slopes;

(e) Provided there is no practical alternative, utility lines and pipes may be constructed in landslide hazard areas under the following conditions:

(i) the line or pipe shall be located above ground and properly anchored or designed so that it will continue to function in the event of an underlying slide; and

(ii) stormwater conveyance systems shall be designed with high-density polyethylene pipe with fuse-welded joints, or similar product that is technically equivalent; or

(iii) alternatively, utilities may be bored below landslide hazard areas provided they are located beneath the depth of potential slope failure.

(f) Point source discharge of stormwater may be allowed in landslide hazard areas under the following conditions:

(i) the stormwater is conveyed via continuous storm pipe downslope to a point where it does not increase risk to landslide hazard areas or other properties downstream from the discharge;

(ii) the stormwater is discharged at flow durations matching predeveloped conditions with adequate energy dissipation into existing channels; or

(iii) discharge upslope of the landslide hazard area may only occur if:

(A) it is dispersed onto a low-gradient undisturbed setback adequate to infiltrate all surface and stormwater runoff; and

(B) the discharge will not decrease the stability of the slope. (Added by Amended Ord. 06-061, Aug. 1, 2007, Eff date Oct. 1, 2007; Amended by Amended Ord. 15-034, Sept. 2, 2015, Eff date Nov. 1, 2015).