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A. General. Erosion hazard areas are those areas that because of natural characteristics, including vegetative cover, soil texture, slope, gradient, and rainfall patterns, or human-induced changes to such characteristics, are vulnerable to erosion. Landslide hazard areas are areas potentially subject to risk of mass movement due to a combination of geologic, topographic, and hydrologic factors.

B. Classification.

1. Criteria.

a. Erosion hazard areas are identified by the presence of vegetative cover, soil texture, slope, and rainfall patterns, or human-induced changes to such characteristics, which create site conditions which are vulnerable to erosion. Erosion hazard areas are those areas that are classified as having moderate to severe, severe or very severe erosion potential by the Soil Conservation Service, United States Department of Agriculture (USDA). The geologic units considered as potential erosion hazards within areas of slopes greater than 15 percent may consist of the following: m (modified land), Qal (alluvium), Qw (wetland deposits), Qb (beach deposits), Qtf (tide flat deposits), Qls (landslide deposits), Qf (fan deposits), the Qvr and Qvs series (Vashon recessional outwash, and Steilacoom Gravel), and Qvi (ice contact deposits). These units are identified because of density and composition.

b. Landslide hazard areas are those areas meeting any of the following criteria:

i. Areas of historic failures, including areas of unstable old and recent landslides;

ii. Areas with all three of the following characteristics:

(A) Slopes steeper than 15 percent; and

(B) Hillsides intersecting geologic contacts with a relatively permeable sediment overlying a relatively impermeable sediment or bedrock; and

(C) Springs or ground water seepage;

iii. Slopes that are parallel or sub-parallel to planes of weakness, such as bedding planes, joint systems, and fault planes, in subsurface materials;

iv. Slopes having gradients steeper than 80 percent subject to rockfall during seismic shaking;

v. Areas potentially unstable as a result of rapid stream incision, streambank erosion, and undercutting by wave action;

vi. Areas located in a canyon or on an active alluvial fan, presently or potentially subject to inundation by debris flows or catastrophic flooding;

vii. Any area with a slope of 30 percent or steeper and with a vertical relief of 10 or more feet. A slope is delineated by establishing the toe and top and measured by averaging the inclination over at least 10 feet of vertical relief;

viii. Areas which have a “severe” limitation for building site development because of slope conditions, according to the Soil Conservation Service.

2. Mapping. Areas meeting the criteria established above may be delineated in the following documents:

a. Soil Survey of Pierce County Area, Washington, 1979, Soil Conservation Service, United States Department of Agriculture (USDA);

b. Coastal Zone Atlas for Washington, Washington Department of Ecology;

c. Areas designated as slumps, earthflows, mudflows, lahars, or landslides on maps published by the United States Geological Survey or Washington Department of Natural Resources Division of Geology and Earth Resources;

d. Geologic Map of the Steilacoom 7.5-Minute Quadrangle, Washington 2003. [Ord. 362 § 3, 2004.]