Geologic hazard area classification criteria are listed in the table below, along with the source agencies that provide the guidelines for classification and designation:
Classification and Designation
(1) Erosion Hazard Areas
(a) Areas with soil type possessing erosion hazard of “moderate to severe,” “severe,” or “very severe.” (Classification based on both soil type and slope)
(2) Landslide Hazard Areas
(a) Areas with slopes of 30% or greater slope and with a vertical relief of 10 or more feet;
(b) Areas with slopes steeper than 15% on hillsides intersecting geologic contacts with a relatively permeable sediment overlying a relatively impermeable sediment or bedrock where springs or groundwater seepage is present;
(c) Areas with slopes parallel or subparallel to planes of weakness in subsurface materials (e.g., bedding planes, joint systems, and fault planes);
(d) Areas with slopes having gradients steeper than 80% subject to rockfall during seismic shaking;
(e) Alluvial fans or canyon bottoms presently or potentially subject to inundation by debris flows or catastrophic flooding;
(f) Areas that have shown movement during the Holocene epoch or which are underlain or covered by wastage debris of this epoch;
(g) Evidence of or risk from snow avalanches;
(h) A “severe” limitation for building site development due to slope conditions;
(i) Areas of historic failure such as areas designated as quaternary slumps, earthflows, mudflows, lahars, or landslides on maps or technical reports (e.g., topographic or geologic maps, or other authorized documents).
USGS, WDNR; or other government agencies
(3) Flood Hazard Areas
(a) Areas potentially unstable as a result of rapid stream incision, stream bank erosion, and undercutting by wave action shall be addressed as a flood hazard.
PMC Title 24
(4) Seismic Hazard Areas
(a) Areas subject to severe risk of damage from earthquake-induced ground shaking or soil liquefaction and soil strength loss, including lands designated as alluvium and recessional outwash, surficial geologic units and areas located on or adjacent to a Holocene fault line.
USGS, Wash-DNR; or other government agencies
(5) Mine Hazard Areas
(a) Mine hazard areas are areas directly underlain by, adjacent to or abutting, or affected by mine workings such as adits, tunnels, drifts, or air shafts.
NRCS – U.S. Department of Agriculture, Natural Resources Conservation Service
USGS – United States Geological Survey
Wash-DNR – Washington Department of Natural Resources
[Ord. 4525A § 1, 2021; Ord. 4525 § 1, 2021; Ord. 3911 § 2, 2009; Code 1970 § 28.32.030.]