Chapter 17.11
GEOLOGICALLY HAZARDOUS AREAS
Sections:
17.11.050 Geotechnical reports.
17.11.060 Performance standards.
17.11.070 Geotechnical assessments.
17.11.080 Buffer requirements.
17.11.090 Modifications to buffer width.
17.11.100 Building setback and construction near buffer.
17.11.110 On-site sewage disposal.
17.11.010 Purpose.
The intent of the classification and designation of geologically hazardous areas is to classify and designate areas on which development should be prohibited, restricted or otherwise controlled because of danger from geologic hazards. For the purpose of this chapter, geologically hazardous areas include areas susceptible to erosion, sliding, earthquake, or other geological events. (Ord. 1566-05 § 28, 2005; Ord. 1117 § 2, 1992).
17.11.020 Definitions.
A. “Alluvial geological unit” means recent stream, lake, swamp and beach deposits of gravel, sand, peat and silt.
B. “Buffer” is an area contiguous with a critical area that is required for the integrity, maintenance, function, and structural stability of the critical area.
C. “Clearing” means the removal of timber, brush, grass, ground cover, or other vegetative matter from a site which exposes the earth’s surface.
D. “Erosion” means the wearing away of the earth’s surface as a result of movement of wind, water, or ice.
E. “Erosion hazard areas” are areas that because of natural characteristics, including vegetative cover, soil texture, slope, gradient, and rainfall patterns, or manmade changes to such characteristics, are vulnerable to erosion.
F. “Geologically hazardous areas” are those areas that, because of their susceptibility to erosion, sliding, earthquake, or other geological events, are not suited to the siting of commercial, residential, or industrial development land uses because of concerns for public health, or safety.
G. “Geotechnical assessment” means an assessment prepared by a geologist or geotechnical engineer licensed as a civil engineer with the state of Washington, detailing the surface and subsurface conditions of a site and delineating the areas of a property subject to geologic hazards.
H. “Geotechnical report” means a report prepared by a geologist or geotechnical engineer licensed with the state of Washington as a civil engineer, which evaluates the site conditions and mitigating measures necessary to insure that the risks associated with geologic hazards are eliminated on the site proposed to be altered.
I. “Ground amplification” means an increase in the intensity of earthquake induced ground shaking which occurs at a site where thick deposits of unconsolidated soil or surficial geologic materials are present.
J. “Landslide” means the abrupt down slope movement of soil, rocks, or other surface matter on a site. Landslides include, but are not limited to, slumps, mudflows, earthflows, rockfall, and snow avalanches.
K. “Landslide hazard areas” are areas potentially subject to risk of mass movement due to a combination geologic, topographic, and hydrologic factors.
L. “Liquefaction” means a process by which a water saturated granular (sandy) soil layer loses strength because of ground shaking commonly caused by an earthquake.
M. “Recessional outwash geologic unit” means sand and gravel materials deposited by melt-water streams from receding glaciers.
N. “Seismic hazard areas” are areas subject to severe risk of damage as a result of earthquake induced ground shaking, slope failure, settlement, or soil liquefaction.
O. “Toe of slope” is a distinct topographic break in slope at the lower most limit of an area where the ground surface drops 10 feet or more vertically within a horizontal distance of 25 feet.
P. “Top of slope” is a distinct topographic break in slope at the upper most limit of an area where the ground surface drops 10 feet or more vertically within a horizontal distance of 25 feet. (Ord. 1117 § 3, 1992).
17.11.030 Hazardous areas.
A. Erosion hazard areas are:
1. Those areas that, because of natural characteristics, including vegetative cover, soil texture, slope, gradient, and rainfall patterns, or manmade changes to such characteristics, are vulnerable to erosion; or
2. Those areas identified by the United States Department of Agriculture Soil Conservation Service as having a “severe” rill and inter-rill erosion hazard.
B. Landslide hazard areas are:
1. Areas potentially subject to risk of mass movement due to geologic, topographic, or hydrologic factors.
2. Areas meeting the following criteria:
a. Areas delineated by the United States Department of Agriculture, Soil Conservation Service, as having a severe limitation, because of slope conditions, for building site development; or
b. Areas with all three of the following characteristics:
i. Slopes greater than 15 percent; and
ii. Hillsides intersecting geologic contacts with relatively permeable sediment overlying a relatively impermeable sediment of bedrock; and
iii. Springs or groundwater seepage; or
c. Areas potentially unstable as a result of rapid stream incision or stream bank erosion; or
d. Areas with visible signs of earth movement such as rockslides, earthflows, mudflows, and landslides; or
e. Those areas mapped by the Department of Ecology (Coastal Zone Atlas) or 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” of Class 5); or
f. Areas designated as quaternary slumps, earthflows, mudflows, lahars, or landslides on maps published by the U.S. Geological Survey or Department of Natural Resources; or
g. 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; or
h. Slopes that are parallel or subparallel to planes of weakness (such as bedding planes, joint systems, and fault planes) in subsurface materials; or
i. Slopes having gradients steeper than 80 percent subject to rock fall during seismic shaking; or
j. Areas located on an alluvial fan, presently or potentially subject to inundation by debris flows or catastrophic flooding; or
k. Any area with a slope of 40 percent or steeper and with a vertical relief of 10 or more feet, except areas composed of consolidated rock. A slope is delineated by establishing its toe and top and measured by averaging the inclination over at least 10 feet of vertical relief. (Ord. 1566-05 § 29, 2005; Ord. 1117 § 4, 1992).
17.11.037 Other hazard areas.
Other hazard areas subject to the provisions of this chapter include those areas determined by the community development director to be susceptible to other geologic events including mass wasting, debris flows, rock falls, and differential settlement. (Ord. 1566-05 § 30, 2005).
17.11.040 Regulation.
For all regulated activities proposed within designated geologically hazardous areas, in addition to a critical areas report as required by FMC 17.05.085, a geotechnical report shall be prepared by a geologist or geotechnical engineer licensed in the state of Washington with experience analyzing geologic, hydrologic, and groundwater flow systems. If an applicant can demonstrate, through submittal of a geotechnical assessment, that no landslide or erosion hazards exist on site, the requirement for a geotechnical report may be waived by the community development director. (Ord. 1566-05 § 31, 2005; Ord. 1117 § 5, 1992).
17.11.050 Geotechnical reports.
A. If a geotechnical report is required it shall contain, at a minimum, the following information:
1. Site geology information required:
a. Topographic data: contour map of proposed site at a scale of one inch equals 200 feet, slopes shall be clearly delineated for the ranges between 15 and 29 percent and 30 percent and greater, including figures for area coverage of each slope category on the site.
b. Subsurface data: boring logs and exploratory methods, soil and rock stratigraphy, groundwater levels including seasonal changes.
c. Site history: description of any prior grading, soil instability, or slope failure.
d. Seismic hazard: data concerning the vulnerability of the site to seismic events.
2. Geotechnical engineering information required:
a. Slope stability studies and opinion of slope stability;
b. Proposed angles of cut and fill slopes and site grading requirements;
c. Structural foundation requirements and estimated foundation settlements;
d. Soil compaction criteria;
e. Proposed surface and subsurface drainage including infiltration testing and site assessment for on-site stormwater management bmps;
f. Lateral earth pressures;
g. Erosion vulnerability of site;
h. Suitability of on-site soil for fill;
i. Laboratory data and soil index properties for soil samples; and
j. Building limitations.
3. Site Evaluation. Evaluation of the ability of the site to accommodate the proposed activity.
B. Where a valid geotechnical report has been prepared within the last five years for a specific site, and where the proposed activity and surrounding site conditions are unchanged, said report may be utilized and a new report may not be required. The applicant shall submit a geotechnical assessment detailing any changed environmental conditions associated with the site. (Ord. 1954 § 25, 2016; Ord. 1117 § 6, 1992).
17.11.060 Performance standards.
The community development director shall evaluate all geotechnical reports for landslide and erosion hazard areas to insure that the following standards are met:
A. Location and Extent of Development.
1. Development must be located to minimize disturbance and removal of vegetation;
2. Structures must be clustered where possible to reduce disturbance and maintain natural topographic character; and
3. Structures should conform to the natural contours of the slope and foundations should be tiered where possible to conform to existing topography of the site.
B. Design of Development.
1. All development proposals shall be designed to minimize the footprint of the building and other disturbed areas;
2. All development proposals shall be designed to minimize coverage of lot with impervious materials;
3. Roads, walkways, and parking areas should be designed to parallel the natural contours of the site; and
4. Access shall be in the least sensitive area of the site.
C. Additional standards for slopes 30 percent or greater: All proposed development on slopes 30 percent or more should be avoided. (Ord. 1117 § 7, 1992).
17.11.070 Geotechnical assessments.
A. Should the applicant question the presence of landslide or erosion hazard areas on the site, the applicant may submit a geotechnical assessment prepared by a geologist or geotechnical engineer licensed as a professional civil engineer in the state of Washington.
B. The geotechnical assessment shall include at a minimum the following:
1. A discussion of the surface and subsurface geologic conditions of the site;
2. A site plan of the area delineating all areas of the site subject to landslide and erosion hazards based on mapping and criteria referenced in above. A map meeting the criteria set forth in FMC 17.11.050 above shall be included.
C. If the geotechnical assessment demonstrates, to the satisfaction of the community development director, that the proposed site is not located in any landslide and erosion hazard areas, then the requirements of this chapter shall not apply. (Ord. 1117 § 8, 1992).
17.11.080 Buffer requirements.
A. A buffer, consisting of native vegetation, and measured in a perpendicular distance from all edges, shall be required from the top of slope, toe of slope, and all sides of all landslide or erosion hazard areas that measure 10 feet or more in vertical elevation change from top to toe of slope, as identified in the geotechnical report, maps, and by field checking. Minimum buffer distance requirements for buffer areas are determined by the director to eliminate or minimize the risk of property damage, death or injury resulting from erosion and landslides caused in whole or in part by development, based upon review of a concurrence with the critical area report and geotechnical report prepared by a qualified professional.
1. The minimum buffer shall be equal to the height of the slope or 50 feet, whichever is greater.
2. The buffer may be reduced to a minimum of 10 feet when a qualified professional demonstrates to the director’s satisfaction that the reduction will adequately protect the proposed development, adjacent developments and uses and the subject critical area.
3. The buffer may be increased where the director determines a larger buffer is necessary to prevent risk of damage to proposed and existing development:
|
|
Vertical relief |
Vertical relief |
|---|---|---|
|
|
<75 feet |
=>75 feet |
|
a. Residential uses |
60 feet |
75 feet |
|
b. Commercial and industrial uses |
75 feet |
100 feet |
B. The buffer shall be clearly staked before any construction or clearing. The city shall inspect the site to confirm that the buffer requirements of this chapter have been met before any construction or clearing begins. The buffer area shall remain in a natural state, and deed instruments shall dedicate the critical area and its buffer, as such, to run with the land. (Ord. 1566-05 § 32, 2005; Ord. 1117 § 9, 1992).
17.11.090 Modifications to buffer width.
When the geotechnical report suggests that a lesser buffer distance, and design and engineering solutions will meet the intent of this chapter, such reduced buffer width and design and engineering solutions may be permitted. Should the report indicate that a greater buffer than required above is needed to meet the intent of this chapter, the greater buffer shall be required. (Ord. 1117 § 10, 1992).
17.11.100 Building setback and construction near buffer.
A minimum setback of 15 feet shall be maintained for the construction of any impervious surface(s) greater than 100 square feet of base coverage. Clearing grading and filling near the buffer shall only be allowed if the applicant can demonstrate that vegetation in the buffer will not be damaged. (Ord. 1117 § 11, 1992).
17.11.110 On-site sewage disposal.
On-site sewage disposal systems are prohibited within designated landslide and erosion hazard areas. (Ord. 1117 § 12, 1992).