Chapter 16.28
GEOLOGICALLY HAZARDOUS AREAS
Sections:
16.28.040 Contents of critical area reports.
16.28.050 Substantive requirements.
16.28.010 Designation.
A. Erosion hazard areas are areas identified by the U.S. Department of Agriculture’s Natural Resources Conservation Service as having a moderate-to-severe, severe, or very severe rill and inter-rill (sheet wash) erosion hazard.
B. Class 1 landslide hazard areas meet the criterion in either subsection (B)(1) or (2) of this section:
1. Areas with slopes that are 40 percent or greater with a vertical relief of 10 or more feet as determined below:
a. The slope shall be measured by establishing its toe and top and averaging the inclination over at least 10 feet of elevation difference.
b. The “toe” of a slope means a distinct topographic break in slope that separates slopes inclined at less than 40 percent from slopes inclined at 40 percent or more. Where no distinct break exists, the toe of a slope is the lowermost limit of the area where the ground surface drops 10 feet or more vertically within a horizontal distance of 25 feet.
c. The “top” of a slope is a distinct topographic break in slope that separates slopes inclined at less than 40 percent from slopes inclined at 40 percent or more. Where no distinct break exists, the top of a slope is the uppermost limit of the area where the ground surface drops 10 feet or more vertically within a horizontal distance of 25 feet.
2. Areas with slopes that are greater than 25 percent but less than 40 percent with a vertical relief of 10 or more feet as calculated in subsection (B)(2)(a) of this section and containing a minimum of two of the characteristics in subsection (B)(2)(b) of this section:
a. The slope shall be measured by establishing its toe and top and averaging the inclination over at least 10 feet of elevation difference consistent with the following:
i. The “toe” of a slope means a distinct topographic break in slope that separates slopes inclined at less than 25 percent from slopes inclined at 25 percent to 40 percent. Where no distinct break exists, the toe of a slope is the lowermost limit of the area where the ground surface drops 10 feet or more vertically within a horizontal distance of 25 feet.
ii. The “top” of a slope is a distinct topographic break in slope that separates slopes inclined at less than 25 percent from slopes inclined at 25 percent to 40 percent. Where no distinct break exists, the top of a slope is the uppermost limit of the area where the ground surface drops 10 feet or more vertically within a horizontal distance of 25 feet.
b. Additional limiting characteristics shall include the following:
i. Groundwater seepage or springs present on the slope, areas underlain by impermeable silts or clays, or mappable emergent water; or
ii. Areas having a moderate or high susceptibility to a shallow landslide; or
iii. Areas having a moderate or high susceptibility to a deep landslide; or
iv. Areas having a moderate to high susceptibility to liquefaction.
3. Repealed by Ord. 1615.
C. Class 2 landslide hazard areas meet one of the following criteria:
1. Areas with slopes greater than 25 percent but less then 40 percent with a vertical relief of 10 or more feet as calculated in subsection (B)(2)(a) of this section and have less than two of the characteristics listed in subsection (B)(2)(b) of this section.
2. Areas with slopes of 15 percent to 25 percent with a vertical relief of 10 or more feet measured between the toe and top of the slope and containing two or more of the characteristics in subsection (B)(2)(b) of this section. The following shall be used to determine the toe and top of slope:
i. The “toe” of a slope means a distinct topographic break in slope that separates slopes inclined at less than 15 percent from slopes inclined at 15 percent to 25 percent. Where no distinct break exists, the toe of a slope is the lowermost limit of the area where the ground surface drops 10 feet or more vertically within a horizontal distance of 65 feet.
ii. The “top” of a slope is a distinct topographic break in slope that separates slopes inclined at less than 15 percent from slopes inclined at 15 percent to 25 percent. Where no distinct break exists, the top of a slope is the upper-most limit of the area where the ground surface drops 10 feet or more vertically within a horizontal distance of 65 feet.
3. All engineered slopes with at least 10 feet in vertical elevation change including, but not limited to, those slopes in Sky Island, Panorama Heights and Panorama West subdivision.
D. Seismic hazard areas are areas designated by the Washington Department of Natural Resources as having a moderate-to-high or a low-to-moderate susceptibility to liquefaction.
E. Volcanic hazard areas include those areas subject to Case 1 lahar or within the lateral blast zone of Mount Rainier. (Ord. 1641 § 7, 2020; Ord. 1615 § 14, 2019; Ord. 1189 § 1, 2006; Ord. 1070 § 2, 2004).
16.28.020 Mapping.
The following maps, which may be continuously updated, may be used as a guide for locating geologically hazardous areas.
A. U.S. Geological Survey landslide hazard, seismic hazard, and volcano hazard maps;
B. Washington State Department of Natural Resources seismic hazard maps for Western Washington;
C. Washington State Department of Natural Resources slope stability maps;
D. Locally adopted maps. (Ord. 1070 § 2, 2004).
16.28.030 Exemptions.
In addition to those listed in BLMC 16.20.070, the following developments shall be exempt from this chapter:
A. Minor developments as determined by the director(s).
B. Sustainable selective-cut forest practices in which:
1. The property is being managed according to a plan, approved by the city or the Washington State Department of Natural Resources, to achieve and retain at least 75 percent tree canopy at all times, in all areas;
2. Logs are removed by methods that do not unduly damage the forest floor, such as by low-ground-pressure tracked machines; and
3. City tree removal permits and Washington State Department of Natural Resources forest practice permits are obtained if required. See also Chapter 16.64 BLMC. (Ord. 1635 § 5, 2020; Ord. 1189 § 2, 2006; Ord. 1070 § 2, 2004).
16.28.040 Contents of critical area reports.
In addition to the requirements of BLMC 16.20.090, critical area reports for geologically hazardous areas shall include, where applicable:
A. Site history regarding landslides, erosion, and prior grading;
B. Topography in suitable contour intervals;
C. Height of slope, slope gradient, slope stability, and slope retreat rate recognizing potential catastrophic events;
D. Description of the geology (including faults), hydrology (including springs, seeps, and surface runoff features), soils (including, in seismic hazard areas, thickness of unconsolidated deposits and liquefaction potential), and vegetation;
E. Type, extent, and severity of geologic hazard(s);
F. Analysis of the proposal’s risk from geologic hazard and the proposal’s potential for exacerbating off-site hazards consistent with the following:
1. Calculations of stability and bearing capacity shall explicitly assume a seismic event consistent with local geotechnical practice; and
2. Demonstration that the proposed development will not decrease the factor of safety for landslides below the limits of 1.5 for static conditions and 1.1 for dynamic conditions. Analysis of dynamic conditions may be based on a minimum horizontal acceleration as established by the International Building Code;
G. Recommended conditions of approval, which may include:
1. Clearing, fill, and hard-surfacing limits, slope stabilization measures, and vegetation management plan;
2. Limitation on clearing during the rainy season, generally from October 1st to May 1st;
3. Design parameters of foundations and retaining structures; and
4. Drainage plan and erosion and sediment control plan in compliance with city stormwater management regulations; and
H. Overview of field investigations, exploration logs, measurements, references, and past assessments of the site.
I. If a geotechnical report contains specific recommendations to mitigate hazards, the geotechnical engineer shall monitor construction sufficiently to ensure compliance with said recommendations, and prior to final permit approval shall submit to the city a monitoring report verifying compliance. The cost of the monitoring shall be borne by the applicant. (Ord. 1615 § 15, 2019; Ord. 1189 § 3, 2006; Ord. 1070 § 2, 2004).
16.28.050 Substantive requirements.
In addition to the substantive requirements of BLMC 16.20.130, the following requirements shall apply to geologic hazard areas:
A. Developments may occur in erosion hazard, seismic hazard, and volcanic hazard areas to the extent that the development does not increase the long-term risk of, or exposure to, geological hazard on site or off site.
B. Class 1 landslide hazard areas shall be subject to the following standards:
1. All development is prohibited within a Class 1 landslide hazard area;
2. No structure or impervious surface is allowed within 50 feet of the top and toe of Class 1 landslide hazard area, unless reductions are supported by an approved geotechnical report;
3. Native vegetation shall be in place within 25 feet of the top and toe of a Class 1 landslide hazard area, unless modifications are supported by an approved geotechnical report.
C. Class 2 landslide hazard areas shall be subject to the following standards:
1. No development may occur in a Class 2 landslide hazard area unless supported by a valid critical area report and without increasing the long-term risk of or exposure to geological hazard on site or off site;
2. No structure or impervious surface is allowed within 25 feet of the top and toe of Class 2 landslide hazard area, unless reductions are supported by an approved geotechnical report;
3. Native vegetation shall be in place within 15 feet of the top and toe of a Class 2 landslide hazard area, unless modifications are supported by an approved geotechnical report.
D. Hazard mitigation shall not rely on actions that require extensive maintenance.
E. Development near a landslide hazard area shall:
1. Repealed by Ord. 1615;
2. Repealed by Ord. 1615;
3. Cluster structures and improvements as necessary to avoid hazard areas;
4. Use retaining walls that allow the retention of existing natural slopes when possible rather than graded artificial slopes;
5. Place utility lines and pipes in landslide hazard areas only when no other alternative is available and when the line or pipe can be installed above ground in such a manner as to remain intact without leaks in the event of a slide;
6. Discharge water from surface water facilities and roof drains onto or upstream from a landslide hazard area only if:
a. Discharged at flow durations matching predeveloped conditions, with adequate energy dissipation, into existing channels; or
b. Dispersed upslope of the steep slope onto a low-gradient undisturbed buffer of adequate infiltrate capacity without increasing saturation of the slope; and
7. Locate any on-site sewage drain fields outside the hazard area and related buffers. (Ord. 1615 § 16, 2019; Ord. 1189 § 4, 2006; Ord. 1070 § 2, 2004).