Spokane Municipal Code

Title 17E Environmental Standards

Chapter 17E.040 Spokane Geologically Hazardous Areas

Section 17E.040.030 Geologically Hazardous Areas

Geologically hazardous areas shall include both erosion and landslide hazard areas and be determined by the following characteristics:

1. Erosion hazard areas are susceptible to severe erosion and may require mitigation measures, engineering solutions or restrictions to development to protect public safety. Erosion hazard areas are defined as "at least those identified by the U.S. department of agriculture natural resource conservation service (NRCS) as having a severe rill and interrill erosion hazard." The NRCS has compiled a table that identifies all soils in the City of Spokane having a severe rill or interrill erosion hazard. This Building Site Development Water Erosion Hazard Table and associated map will be used to classify erosion hazard areas. Erosion hazard areas are also defined as those cutbank areas within a river or stream meander that area highly susceptible to bank carving. A variety of techniques may be used to identify cutbanks along the outside banks or river and stream meanders. Erosion also occurs through the slow process of channel migration. The channel migration zone (CMZ) is the area where the active channel of a stream is prone to movement over time. Channel migration is usually found along a small percentage of the entire stream network length; however effective management of ecological functions in the CMZ is critical to reduce flood hazards, erosion and habitat loss, and to avoid the need for future shoreline stabilization.

2. Landslide hazard areas are potentially subject to landslides based on a combination of geologic, topographic and hydrologic factors. These include areas susceptible to landslides because of any combination of bedrock, soil, slope, structure, hydrology or other factors. Classifications of landslide hazard areas include:

1. 1. slopes greater than eighty percent subject to rockfall during seismic shaking;

1. 2. any area with a slope of thirty percent or greater;

1. 3. areas with all three of the following characteristics:

1. 1. 1. Slopes greater than fifteen percent.

1. 1. 2. Steep hillsides intersecting permeable sediment overlying an impermeable sediment or bedrock; and

1. 1. 3. Evidence of perennial or intermittent springs or ground water seepage.

1. 4. slopes that are parallel or sub-parallel to planes of weakness (such as bedding-planes, joint systems and fault planes) in subsurface materials;

1. 5. areas of previous failures identified by the NRCS as having a severe limitation for building site development;

1. 6. areas of previous failures designated on department of natural resources (DNR) maps as landslides;

1. 7. areas potentially unstable as a result of bank carving and erosion or areas located in a canyon or on an active alluvial fan subject to inundation by debris flows or catastrophic flooding;

1. 8. areas of the Latah formation (sedimentary layers of clay interlain between basalt flows) that are subject to landslides;

1. 9. areas of uncompacted fill;

1. 10. sloped areas exhibiting recent erosion or mass-wasting landslide activity such as gullies, piping and surfaces devoid of all vegetation;

1. 11. sloped areas greater than fifteen percent with previous levels of development that may have changed sloped stability. Slope characteristics may have changed due to removal of vegetation, the removal and disturbance of soil or a change in surface geology, and modification to underlying geology. Slopes may also experience increased water content and corresponding increase in weight and change in soil friction characteristics due to increased irrigation.

1. 12. sloped areas exhibiting high rates of creep as evidenced by trees with curved trunks, fence posts angled downslope or retaining walls that are angled downslope or broken.

3. The City of Spokane is not in an area of severe risk for seismic hazards; therefore, no designation of these areas is warranted at this time. All building activity is subject to the provisions of the International Building Code which provides structural safeguards to reduce the risks from seismic activity.

4. Other geological hazard areas include volcanic and mine hazards. Initial research and investigation has determined that no mine hazards exist in the City of Spokane. In the past, the City has been impacted by volcanic ash, but this is not considered a geological hazard and does not warrant classification or designation for the purpose of this chapter.

Date Passed: Monday, December 3, 2007

Effective Date: Sunday, January 6, 2008

ORD C34149 Section 8