16.55.390 Geologically Hazardous Areas.
(1) General Designation. Geologically hazardous areas in shoreline jurisdiction include areas susceptible to erosion, sliding, earthquake, or other geological events. They pose a threat to the health and safety of citizens when incompatible development is sited in areas of significant hazard. Such incompatible development may not only place itself at risk, but also may increase the hazard to surrounding development and use. Areas susceptible to one or more of the following types of hazards shall be designated as a geologically hazardous area:
(a) Erosion hazard;
(b) Landslide hazard;
(c) Seismic hazard;
(d) Mine hazard;
(e) Volcanic hazard; and
(f) Other geological events including mass wasting, debris flows, rock falls, and differential settlement.
(2) Designation of Specific Hazard Areas.
(a) Erosion Hazard Areas. Erosion hazard areas are those areas identified by the U.S. Department of Agriculture - Natural Resources Conservation Service (USDA-NRCS) as having a "moderate to severe,” “severe," or “very severe” rill and inter-rill erosion hazard. Rill erosion tends to occur on slopes, particularly steep slopes with easily-erodable soils or poor vegetation. Erosion hazard areas also include those areas with a slope greater than fifteen percent (15%).
(b) Landslide Hazard Areas. Landslide hazard areas are areas potentially subject to landslides based on a combination of geologic, topographic, and hydrologic factors. They include areas susceptible because of any combination of bedrock, soil, slope (gradient), slope aspect, structure, hydrology, or other factors. Example of these may include, but are not limited to the following:
(i) Areas of historic failures, such as:
aa. Those areas delineated by the USDA-NRCS as having a "severe" limitation for building site development for factors other than slope for one or more types of building development;
bb. Those areas mapped by the Department of Natural Resources (slope stability mapping) as unstable (“U” or class 3), unstable old slides (“UOS” or class 4), or unstable recent slides (“URS” or class 5); or
cc. Areas designated as quaternary slumps, earthflows, mudflows, lahars, or landslides on maps published by the U.S. Geological Survey or Department of Natural Resources;
(ii) Areas with all three of the following characteristics:
aa. Slopes steeper than fifteen percent (15%);
bb. Hillsides intersecting geologic contacts with a relatively permeable sediment overlying a relatively impermeable sediment or bedrock; and
cc. Springs or ground water seepage;
(iii) Areas that have shown movement during the Holocene epoch (from 10,000 years ago to the present) or that are underlain or covered by mass wastage debris of that epoch;
(iv) Slopes that are parallel or subparallel to planes of weakness (such as bedding planes, joint systems, and fault planes) in subsurface materials;
(v) Slopes having gradients steeper than eighty percent (80%) subject to rock fall during seismic shaking;
(vi) Areas potentially unstable because of rapid stream incision, streambank erosion, and undercutting by wave action;
(vii) Areas that show evidence of, or are at risk from snow avalanches;
(viii) Areas located in a canyon or on an active alluvial fan, presently or potentially subject to inundation by debris flows or catastrophic flooding; and
(ix) Any area with a slope of forty percent (40%) or steeper and with a vertical relief of ten or more feet except areas composed of consolidated rock.
(c) Seismic Hazard Areas. Seismic hazard areas are areas subject to severe risk of damage as a result of earthquake induced ground shaking, slope failure, settlement, soil liquefaction, lateral spreading, or surface faulting. One indicator of potential for future earthquake damage is a record of earthquake damage in the past. Ground shaking is the primary cause of earthquake damage in Washington. The strength of ground shaking is primarily affected by:
(i) The magnitude of an earthquake;
(ii) The distance from the source of an earthquake;
(iii) The type or thickness of geologic materials at the surface; and
(iv) The type of subsurface geologic structure.
Settlement and soil liquefaction conditions occur in areas underlain by cohesionless, loose, or soft-saturated soils of low density, typically in association with a shallow ground water table.
(d) Mine Hazard Areas. Mine hazard areas are those areas underlain by, or affected by mine workings such as adits, gangways, tunnels, drifts, or airshafts, and those areas of probable sink holes, gas releases, or subsidence due to mine workings. Steep and unstable slopes may be created by open mines (e.g. open basalt rock pits, rock quarries, sand and gravel pits). Factors that should be considered include: proximity to development, depth from ground surface to the mine working, and geologic material.
(e) Volcanic Hazard Areas. Volcanic hazard areas are areas subject to pyroclastic flows, lava flows, debris avalanche, inundation by debris flows, lahars, mudflows, or related flooding resulting from volcanic activity.
(f) Other Hazard Areas. Geologically hazardous areas shall also include areas determined by the SMP Administrator to be susceptible to other geological events including mass wasting, debris flows, rock falls, and differential settlement.
(3) Classification. The level of risk for each geologic hazard type is described in this subsection. Documentation of specific areas in which a known or suspected risk exists for each of the following hazard areas is provided in the pertinent City critical area map. The provisions of this Section apply only to those areas for which a known or suspected risk exists in shoreline jurisdiction.
Classification | Documentation and Data Sources |
|---|---|
Known or Suspected Risk | Documentation or projection of the hazard by a qualified professional exists. |
Low or No Risk | Documentation exists by a qualified professional regarding low hazard risk or lack of hazard. |
Risk Unknown | Documentation, data, or projection of the hazard risk by a qualified professional are not available or sufficient to determine the presence or absence of a geologic hazard. |
(a) Erosion Hazard Areas – Known or Suspected Risk. The Thatuna Soil Series within the City is identified as having a potentially severe erosion hazard. The Thatuna Series within the City consists of: the Thatuna Silt Loams on slopes of seven to twenty-five percent (7-25%) and twenty-five to forty percent (25-40%) and the Thatuna-Tilma Silt Loams on slopes of seven to twenty-five percent (7-25%).
Soil Series Name | Slope (%) | Erosion Hazard |
|---|---|---|
Thatuna Silt Loam | 7 – 25 | Moderate to Severe |
Thatuna Silt Loam | 25 – 40 | High to Severe |
Thatuna-Tilma Silt Loams | 7 – 25 | Moderate to Severe |
(b) Landslide Hazard Areas – Known or Suspected Risk. Areas of severe limitations to building development as identified by the USDA-NRCS for reasons other than slope for one or more types of building development include the soil series in the following table.
Soil Name | Risk Level: Reason | ||
|---|---|---|---|
Dwellings w/o Basements: | Dwellings w/ Basements | Small Commercial Buildings | |
Caldwell | Severe: floods | Severe: floods, wetness | Severe: floods |
Garfield | Severe: shrink-swell, low strength | Severe: shrink- swell, low strength | Severe: shrink- swell, low strength |
Gwin-Tucannon | Severe: slope | Severe: depth to rock, slope | Severe: slope |
Konert | Severe: floods, wetness, shrink-swell | Severe: floods, wetness, shrink- swell | Severe: floods, wetness, shrink- swell |
Latah | Severe: floods, wetness, shrink-swell | Severe: floods, wetness, shrink- swell | Severe: floods, wetness, shrink- swell |
Naff | Severe: slope, shrink-swell | Severe: slope, shrink-swell | Severe: slope, shrink-swell |
Thatuna-Tilma | Severe: shrink-swell, slope, wetness | Severe: wetness, shrink-swell, slope | Severe: shrink- swell, wetness, slope |
Tucannon | Severe: slope | Severe: depth to rock, slope | Severe: slope |
(c) Seismic Hazard Areas – Low or No Risk. There are no known specific faults that are likely to create a significant seismic hazard within the City.
(d) Mine Hazard Areas – Known or Suspected Risk. No subsurface mines exist in the City. Open mines such as basalt rock pits and sand/gravel pits that exist within the City are mine hazard areas.
(e) Volcanic Hazard Areas – Low or No Risk. Pullman is not located within a volcanic hazard zone. If a volcanic eruption were to occur in the western part of Washington, the only anticipated impact in Pullman would be ash deposition.
(f) Other Hazard Areas. Other geologically hazardous areas may be designated by the City if documentation thereof is available.
(4) Mapping.
(a) The approximate location and extent of potential geologically hazardous areas are shown in the pertinent City critical area map. The hazard areas outlined on this map are based on the following data:
(i) Department of Natural Resources slope stability maps (slope);
(ii) USGS 10-meter Digital Elevation Model (slope);
(iii) USDA-NRCS Soil Survey of Whitman County, Washington: WA075/Sheet 99/109 (soil type);
(iv) International Building Code (IBC) Ch. 16, Sec. 1613 (earthquake loads);
(v) Additional data as determined necessary by the City.
(b) The City critical area map regarding geologically hazardous areas is to be used as a guide for the City, project applicants, and property owners, and will be periodically updated as new information becomes available. This map provides a general reference and does not provide a final critical area designation.
(5) Existing Regulations Pertaining to Geologically Hazardous Areas. The following regulations and procedures are in place for the City to minimize the undesirable impacts that could be associated with geologically hazardous areas:
(a) IBC Appendix J: Grading.
(b) IBC Section 1803: Excavation, Grading and Fill.
(c) IBC Section 1612: Flood Loads.
(d) City of Pullman Design Standards.
(e) Landslide Hazard Areas.
(i) IBC Chapter 18: Soils and Foundations.
(ii) IBC Appendix J: Grading.
(iii) City of Pullman Design Standards.
(iv) City of Pullman Policy Regarding Footings and Soil Types.
(v) City of Pullman Design Guidelines for Typical Residential Footings on Poor Soils.
(f) Seismic Hazard Areas.
(i) IBC Chapter 16, Section 1613: Earthquake Loads.
(ii) IBC Chapter 18: Soils and Foundations
(g) Mine Hazard Areas.
(i) Chapter 17.125 Pullman City Code: Conditional Use Permit.
(ii) Chapter 78.44 RCW – Surface Mining.
(6) New development and lots. In any geologically hazardous area, new development and creation of new lots that would cause foreseeable risk from geological conditions after application of the provisions referenced in Subsection 16.55.390(5) during the life of the development is prohibited. (Ord. 16-3 §40, 2016).