18E.80.020 Landslide Hazard Areas.
A. Landslide Hazard Areas Indicators. Landslide hazard areas are areas potentially subject to mass movement due to a combination of geologic, seismic, topographic, hydrologic, or manmade factors. Landslide hazard areas can be identified by the presence of any of the following indicators:
1. Areas of historic failures, including areas of unstable, old and recent landslides or landslide debris within a head scarp.
2. Areas with active bluff retreat that exhibit continuing sloughing or calving of bluff sediments, resulting in a vertical or steep bluff face with little or no vegetation.
3. Areas with both of the following characteristics:
a. Slopes steeper than 20 percent with a vertical relief of 20 feet or more (see 18E.80-1 in Chapter 18E.120 PCC); and
b. Hillsides that intersect geologic contacts with a relatively permeable sediment overlying a relatively impermeable sediment or bedrock.
4. Slopes that are parallel or sub-parallel to planes of weakness, such as bedding planes, joint systems, and fault planes in subsurface materials.
5. Areas exhibiting geomorphological features indicative of past slope failure, such as hummocky ground, back-rotated benches on slopes, etc.
6. Areas with tension cracks or ground fractures along and/or near the edge of the top of a bluff or ravine.
7. Areas with structures that exhibit structural damage such as settling and cracking of building foundations or separation of steps or porch from a main structure that is located near the edge of a bluff or ravine.
8. The occurrence of toppling, leaning, bowed, or jackstrawed trees that are caused by disruption of ground surface by active movement.
9. Areas with slopes containing soft or liquifiable soils.
10. Areas where gullying and surface erosion have caused dissection of the bluff edge or slope face as a result of drainage or discharge from pipes, culverts, ditches, and natural drainage courses.
11. Areas where seeps or springs or indicators (e.g., vegetation type) of a shallow groundwater table are observed on or adjacent to the face of the slope.
12. Any area with a slope of 40 percent or steeper and with a vertical relief of 15 or more feet, except those manmade slopes created under the design and inspection of a geotechnical professional or slopes composed of competent bedrock. For the purposes of determining whether a slope is considered to be a landslide hazard area, the horizontal and vertical distance between the top and toe of slope are utilized. (See Figure 18E.80-1 in Chapter 18E.120 PCC.)
13. Areas that are at risk of mass movement due to seismic events.
14. Areas that include alluvial or colluvial fans located at the base of steep slopes and drainages.
B. Potential Landslide Hazard Areas. Potential landslide hazard areas, as depicted on the Critical Areas Atlas-Landslide Hazard Areas Map, are those areas where the suspected risk of slope instability and landslide is sufficient to require a geological assessment to assess the potential for active landslide activity. Potential landslide hazard areas are determined using the following criteria:
1. Areas identified on the Coastal Zone Atlas of Washington, Volume VII, Pierce County as either U (unstable), Urs (unstable recent slide), Uos (unstable old slide), I (intermediate), or M (modified), and any adjacent areas within 300 feet. (See Figure 18E.80-2 in Chapter 18E.120 PCXC.)
2. Areas identified on the Pierce County topographic maps as having slopes greater than 20 percent with a vertical relief of greater than 20 feet and any adjacent areas within a distance of 65 feet (See Figure 18E.80-3 in Chapter 18E.120 PCC.)
3. Areas that possess one or more of the landslide hazard area indicators (stratigraphy, groundwater conditions, etc.) as set forth in PCC 18E.80.020 A. and any adjacent area within a distance of 65 feet.
4. Areas not reflected on the Coastal Zone Atlas that have been determined to be active through a geological assessment process.
5. Areas identified on the Pierce County topographic maps as having slopes greater than 50 percent with a vertical relief of greater than 100 feet and any adjacent areas within a distance of 300 feet.
C. Landslide Hazard Area Categories. Landslide hazard areas shall be classified into categories which reflect each landslide hazard areas past landslide activity and the potential for future landslide activity based on an analysis of slope instability. Landslide hazard areas shall be designated as follows:
1. Active Landslide Areas. A composite of the active landslides and/or unstable areas, including that portion of the top of slope and slope face subject to failure and sliding as well as toe of slope areas subject to impact from down slope run-out, identified and mapped during a geological assessment of a site. An active landslide hazard area exhibits one or more of the following:
a. Areas of historical landslide movement on a site which have occurred in the past century including areas identified on the Coastal Zone Atlas of Washington, Volume VII, Pierce County as Urs (unstable recent slide).
b. Unstable areas that exhibit geological and geomorphologic evidence of past slope instability or landsliding or possess geological indicators (stratigraphy, ground water conditions, etc.), as set forth in 18E.80.020 A., that have been determined through a geological assessment process to be presently failing or may be subject to future landslide activity. The impact of the proposed development activities must be considered in defining the extent of the active areas.
c. Interim areas are located between areas identified through the geological assessment process as an active landslide hazard area. Interim areas will be considered part of the active landslide hazard area if the required top of slope or toe of slope landslide hazard area buffer encompasses the area. (See Figure 18E.80-4 in Chapter 18E.120 PCC.)
2. Stable Areas. Areas that have been identified as potential landslide hazard areas, but, through the geological assessment process, meet one of the following conditions:
a. No indicators as set forth in PCC 18E.80.020 A. actually exist that indicate the potential for future landslide activity to occur.
b. A slope stability analysis has indicated that there is no apparent landslide potential.
c. Adequate engineering or structural measures have been provided in a geological assessment – geotechnical report that mitigates the potential for a future landslide to occur as a result of current or past development activity. The engineering or structural measures must provide a minimum factor of safety of 1.5 static conditions and 1.1 for dynamic conditions. Analysis of dynamic (seismic) conditions shall be based on a minimum horizontal acceleration as established by the current version of the Pierce County Building Code. The engineering or structural measures must be completed, inspected and accepted for the area to be deemed stable. Construction sequencing recommendations must be provided by the geotechnical professional when a proposed development will be constructed concurrently with the engineering or structural measures.
d. A geological assessment has been performed and the results of that assessment indicate that an area is not an active landslide hazard area.
e. Areas that have been determined to be stable or are converted into a stable area by the implementation of engineering or structural measures are not considered a landslide hazard critical area.