Chapter 14.11
GEOLOGICALLY HAZARDOUS AREAS
Sections:
14.11.030 Designation of specific geologic hazard areas.
14.11.050 Performance standards.
14.11.060 Special provisions – Erosion and landslide areas.
14.11.070 Design standards – Erosion and landslide hazard areas.
14.11.080 Repealed.
14.11.090 Repealed.
14.11.010 Purpose.
To prevent incompatible development activity from being conducted in or near geologically hazardous areas in order to reduce the risk to public health and safety. (Ord. 1243 Exh. B (part), 2006).
14.11.020 Designation.
Geologically hazardous areas 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 may also increase the hazard to surrounding development and uses. Areas susceptible to one or more of the following types of hazards shall be designated as geologically hazardous areas:
A. Erosion hazard;
B. Landslide hazard (including steep slopes);
C. Seismic hazard; and
D. Other geological events including mass wasting, debris flows, rock falls, and differential settlement.
Figure 6, Seismic Hazards1 depicts areas subject to soil liquefaction in an earthquake and Figure 7, Erosion and Debris Flow2 identifies geologically hazardous areas in the North Bend area. These maps may be periodically revised by the city to add or remove areas based on additional information. (Ord. 1243 Exh. B (part), 2006).
14.11.030 Designation of specific geologic hazard areas.
A. The adopted critical areas maps include:
1. U.S. Geological Survey (USGS) landslide hazard, seismic hazard, and volcanic hazard maps;
2. Washington State Department of Natural Resources (WDNR) seismic hazard maps of Western Washington, as they are available;
3. WDNR slope stability maps, as they are available; and
4. Local geologic hazard maps, as adopted.
These maps are to be used as a guide for the city of North Bend, project applicants, and/or property owners, and may be continuously updated as new critical areas are identified. They are a reference and do not provide a final critical area designation.
B. Other geologic hazardous areas regulated by this chapter include:
1. Erosion Hazard Areas. Erosion hazard areas are at least those areas identified by the U.S. Department of Agriculture’s Natural Resources Conservation Service (NRCS) as having “severe” or “very severe” rill and inter-rill erosion hazard.
2. 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. Examples of these may include, but are not limited to, the following:
a. Areas of historic failure, such as:
i. Those areas delineated by the NRCS as having a “severe” limitation for building site development; or
ii. Areas designated as quaternary slumps, earth flows, mudflows, debris flow, or landslides on maps published by the USGS, WDNR, or the city of North Bend;
b. Areas with all three of the following characteristics:
i. Slopes steeper than 15 percent; and
ii. Hillsides intersecting geologic contacts with a relatively permeable sediment overlaying a relatively impermeable sediment or bedrock; and
iii. Springs or groundwater seepage;
c. 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;
d. Slopes that are parallel or sub-parallel to planes of weakness (such as bedding planes, joint systems, and faults) in subsurface materials;
e. Slopes having a gradient steeper than 80 percent subject to rock fall during seismic shaking;
f. Areas potentially unstable because of rapid stream incision, stream bank erosion, and undercutting by water action;
g. Areas located in a canyon or on an active alluvial fan, presently or potentially subject to inundation by debris flows or catastrophic flooding; and
h. 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 delineated by establishing its toe and top and measured by averaging the inclination over at least 10 feet of vertical relief.
3. Seismic Hazard Areas. Seismic hazard areas are subject to severe risk of damage as a result of earthquake-induced ground shaking, slope failure, settlement, soil liquefaction, lateral spreading, or surface failure. The strength of ground shaking is primarily affected by:
a. The magnitude of an earthquake;
b. The distance from the source of an earthquake;
c. The type and thickness of geologic materials at the surface; and
d. The type of subsurface geological structure. (Ord. 1243 Exh. B (part), 2006).
14.11.050 Performance standards.
A. All projects shall be evaluated to determine whether the project is proposed to be located in a geologically hazardous area, the project’s potential impact on the geologically hazardous area, and the potential impact of the proposed project. The director may require the preparation of a critical area report to determine the project’s ability to meet the performance standards.
B. Alterations to erosion or landslide hazardous areas or associated buffers may only occur for activities that:
1. Will not increase the threat of the geological hazard to adjacent properties beyond predevelopment conditions;
2. Will not adversely impact other critical areas;
3. Are designed so that the hazard to the project is eliminated or mitigated to a level equal to or less than predevelopment conditions; and
4. Are certified as safe as designed and under anticipated conditions by a qualified geotechnical engineer or geologist, licensed in the state of Washington.
C. Vegetation in erosion or landslide hazardous areas shall be retained unless it can be shown that the removal will not increase the geologic hazards and a vegetation management plan is submitted with the request.
D. Approved clearing and grading in erosion and landslide hazardous areas shall only be allowed from May 1st to October 1st of each year; provided, that the city may extend or shorten the dry season on a case-by-case basis depending on the actual weather conditions, except that timber harvest, not including brush clearing or stump removal, may be allowed pursuant to an approved forest practices permit issued by WDNR.
E. Access roads and utilities may be permitted within the erosion or landslide hazard area and associated buffers if the city determines that no other feasible alternative exists.
F. Utility lines and pipes shall be permitted in the erosion or landslide hazard areas only when the applicant demonstrates that no other practical alternative is available. The line or pipe shall be located above ground and be properly anchored and/or designed so that it will continue to function in the event of an underlying slide. Stormwater conveyance shall be allowed only through a high-density polyethylene pipe with fuse-welded joints, or similar product that is technically equal or superior.
G. Sheet flow discharges from impervious surfaces and point discharges from surface water facilities and roof drains onto or upstream from an erosion or landslide hazard area shall be prohibited except that conveyance is allowed via continuous storm pipe downslope to a point where there are no erosion hazard areas downstream from the discharge.
H. The division of land in erosion or landslide hazard areas and associated buffers is subject to provisions established for all critical areas in Chapter 14.05 NBMC. (Ord. 1243 Exh. B (part), 2006).
14.11.060 Special provisions – Erosion and landslide areas.
Activities on sites containing erosion or landslide hazards shall meet the following requirements:
A. Buffers Required. A buffer shall be established for all edges of erosion or landslide hazard areas. The size of the buffer shall be determined by the director, based on the recommendations of the critical areas study, to eliminate or minimize the risk of property damage, death, or injury resulting from erosion and landslides caused in whole or in part by the development, based upon review of and concurrence with a critical area report prepared by a qualified professional.
B. Minimum Buffers. The typical buffer for landslide hazard areas is the height of the slope or 50 feet, whichever is greater. The buffer may be reduced or enlarged depending upon site-specific conditions and the nature of the hazard, as analyzed by a qualified geologist. There is no minimum buffer for erosion areas. Erosion protection shall be based on site-specific analysis to achieve no net loss or impact to the erosion area. Best management practices (BMPs), mitigation, monitoring, and where necessary an erosion and control plan.
C. Buffer Reduction. The buffer may be reduced to zero 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.
D. Increased Buffer. The buffer may be increased when the director determines a larger buffer is necessary to prevent risk of damage to proposed and existing development.
E. Alterations. Alterations of an erosion or landslide hazard area and/or buffer may only occur for activities for which a geotechnical analysis is submitted and certifies that:
1. The development will not increase the surface water discharge rate or sedimentation to adjacent properties beyond the predevelopment condition;
2. The development will not decrease slope stability on adjacent properties; and
3. Such alteration will not adversely impact other critical areas. (Ord. 1243 Exh. B (part), 2006).
14.11.070 Design standards – Erosion and landslide hazard areas.
Development within an erosion or landslide hazard area and/or buffer shall be designed to meet the following basic requirements unless it can be demonstrated that an alternative design that deviates from one or more of these standards provides greater long-term slope stability while meeting all other provisions of this chapter. The requirements for long-term slope stability shall exclude designs that require regular and periodic maintenance to maintain their level of function. The basic development design standards are:
A. Structures and improvements shall be clustered to avoid geologically hazardous areas and other critical areas;
B. Structures and improvements shall minimize alterations to the natural contours of the slope and foundations shall be tiered where possible to conform to existing topography;
C. Structures and improvements shall be located to preserve the most critical portion of the site and its natural landforms and vegetation;
D. The proposed development shall not result in greater risk or a need for increased buffers on neighboring properties;
E. The use of a retaining wall that allows the maintenance of existing natural slopes are preferred over graded artificial slopes; and
F. Development shall be designed to minimize impervious lot coverage. (Ord. 1243 Exh. B (part), 2006).
14.11.080 Native growth protection easement/critical area tract.
Repealed by Ord. 1688. (Ord. 1243 Exh. B (part), 2006).
14.11.090 Critical area report.
Repealed by Ord. 1688. (Ord. 1243 Exh. B (part), 2006).
See city website for current version.
See city website for current version.