Chapter 14.11
GEOLOGICALLY HAZARDOUS AREAS

Sections:

14.11.010    Purpose.

14.11.020    Designation.

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    Native growth protection easement/critical area tract.

14.11.090    Critical area report.

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.

A. As part of the implementation of approved development applications and alter­ations, geologically hazardous areas and any associated buffers that remain undeveloped pursuant to the critical area regulations, in accordance with Chapter 14.05 NBMC, shall be designated as native growth protection easements (NGPE).

B. When the subject development is a for­mal subdivision, short subdivision (short plat), binding site plan, master site plan, contract rezone, site plan/design review, or planned residential development (PRD), the geologi­cally hazardous area(s) and any buffers shall be placed in a critical area tract and designated as an NGPE, as described in Chapter 14.05 NBMC. (Ord. 1243 Exh. B (part), 2006).

14.11.090 Critical area report.

A. When required, a critical area report for a geologically hazardous area shall be pre­pared by an engineer or geologist, licensed in the state of Washington, with experience ana­lyzing geologic, hydrogeologic, and ground water flow systems, and who has experience preparing reports for the relevant type of haz­ard.

B. In addition to the requirements of Chapter 14.05 NBMC, critical area reports are required for geologically hazardous areas and shall include the following additional informa­tion:

1. On the site map:

a. All geologically hazardous areas within or adjacent to the project area or that have potential to be affected by the proposal;

b. The top and toe of slope (note: these should be located and flagged in the field subject to city staff review).

2. In the report:

a. A geological description of the site;

b. A discussion of any evidence of existing or historic instability, significant ero­sion or seepage on the slope;

c. A discussion of the depth of weathered or loosened soil on the site and the nature of the weathered and underlying base­ment soils;

d. An estimate of load capacity, including surface and ground water condi­tions, public and private sewage disposal system, fill and excavations, and all structural development;

e. Recommendations for building limitations, structural foundations, and an esti­mate of foundation settlement;

f. A complete discussion of the potential impacts of seismic activity on the site;

g. Recommendations for manage­ment of stormwater for any development above the top of slope;

h. A description of the nature and extent of any colluviums or slope debris near the toe of the slope in the vicinity of any pro­posed development; and

i. Recommendations for appropri­ate building setbacks, grading restrictions, and vegetation management and erosion control for any proposed development in the vicinity of the geologically hazardous areas. (Ord. 1243 Exh. B (part), 2006).

1

See city website for current version.


2

See city website for current version.