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Geologically hazardous areas
What is a geologic hazard?
Due to Bainbridge Island’s unique landscape, geologic hazards can include erosion hazard areas, landslide hazard areas, and seismic hazard areas (including fault and liquefaction hazard areas).
- Erosion hazards include areas containing soils which, according to the USDA Natural Resource Conservation Service Soil Services, may experience significant erosion. This can also include coastal erosion-prone areas and channel migration areas.
- Landslide hazards include areas which are at risk of mass movement due to a combination of geologic, topographic, and hydrologic factors. There are several characteristics which can designate a slope as a landslide hazard area listed within the Bainbridge Island Municipal Code. Most commonly this is a slope greater than 40% that rises more than 10’ vertically. The “Steep Slope Analysis” layer on the City of Bainbridge Island Critical Areas Web Map Application will show areas identified as steep ( >40%, colored red) or moderate (15-39% slope, orange). Orange colored areas require additional criteria to be characterized as landslide hazards. See references linked in the section below and choose the one that governs the proposed site based on being inside or outside the shoreline jurisdiction.
- Fault hazards include areas of known surface rupture or significant surface deformation as a result of an active fault movement, including 50 feet on either side.
- Liquefaction hazards include areas of cohesionless, loose or soft saturated soils of low density in association with a shallow groundwater table that are subject to settlement and/or liquefaction from ground shaking.
Do you have a geologically hazardous area on your property?
Visit the City of Bainbridge Island Critical Areas Web Map Application to determine if you may have a hazard that requires further analysis as part of your project. Layers for each of the hazards listed above can be found within the map.
Geological hazard assessments
A Geological Hazards Assessment is required for all projects proposing work (structures, stairs, paths, drain fields, utility pipes, vegetation removal, etc.) within geologically hazardous areas and landslide hazard area setbacks in accordance with Bainbridge Island Municipal Code, Section 16 Environment. The proposed work is normally reviewed for compliance with the Municipal Code during construction permit review (Building, Grade & Fill, Right-of-Way, etc.), except for projects that require separate land use permitting prior to applying for construction permits, where the Geological Hazard Assessment is required for submittal with the applicable land use permit(s).
These assessments are to be completed by a licensed Geotechnical Engineer or Engineering Geologist per the relevant Bainbridge Island Municipal Code section. An assessment completed by a Geologist may be acceptable provided it does not contain recommended methods for mitigating identified impacts other than avoidance, structural impacts to, or suitability of civil works. Engineering Geologists may not provide Engineering recommendations or design recommendations but may contribute to a complete geotechnical report that is co-sealed by a Geotechnical Engineer.
Construction in geologically hazardous areas and setbacks
Once it has been determined that a proposed project requires a Geological Hazards Assessment per BIMC, there are three main “steps” to complete during the permitting and construction phases of the project. COBI has created forms for each step to help simplify the process and provide applicants and their consultants with the guidance to provide COBI the information needed to properly complete all phases of their project.
Construction in Geologically Hazardous Areas: Step 1 – Initial Permit Submittal (Revised May 2025)
This document provides additional information on what is required for an initial Geological Hazard Assessment. This assessment is typically completed prior to the design of the project as the Geotechnical Engineer provides information on the potential impacts of structure locations, foundation design, stormwater runoff, etc. Previously this was a standalone form, but in practice the information required on the previous form was routinely provided in an initial report already, so this Step 1 form became guidance on what should be included in the initial Geotech report submitted with the application (land use or construction permit)
As the initial Geological Hazard Assessment is typically completed prior to or concurrent with the design of structures, utility layouts, etc. the step 2 form is required as part of an updated assessment or as a standalone submittal document with the Engineer of record’s seal and signature. This form confirms that the Geotechnical Engineer of record has reviewed the most current site plans and that they conform with their original or any additional geotechnical engineering recommendations.
Similar to the step 2 form, the step 3 form is required as part of an updated assessment or can be completed as a standalone submittal document with the Engineer of records seal and signature. This form is required in order to complete the “DE – GEOTECH STEP 3” construction inspection and certifies that the Engineer of record has inspected all geotechnical features recommended in their original report(s). This form also includes any operation and maintenance recommendations from the Engineer of record as applicable.
Construction in the zone of influence
As per BIMC Shoreline Master Program, Geologically Hazardous Areas: Chapter 16.12.030.B.5 and BIMC Critical Areas, Geologically Hazardous Areas: Chapter 16.20.130 a Zone of Influence shall be established 300 feet upslope from slopes greater than 40 percent and 200 feet upslope from slopes greater than 15 percent but less than 40 percent that are determined to be geologically hazardous areas to assess changes in land use and hydrology that may affect the stability of the geologically hazardous area.
Projects proposing changes in land use or hydrology within the zone of influence shall have the stormwater site plan and the stormwater pollution prevention plan (erosion control plan) for the project reviewed by a licensed geotechnical engineer to determine if there are any potentially adverse impacts to the geologically hazardous area(s).
In lieu of a full report or addendum the Zone of Influence Review form may be used to meet this requirement, however, any work within the geologically hazardous area or its setbacks will require a full geological hazard assessment per BIMC. A full geological hazard assessment may also be required if the geotechnical engineer or the city engineer determines that there are potential adverse impacts to the geologically hazardous area(s).
Third-party geotechnical review
To protect public health, safety and welfare, the director may require third party review of any geological hazards assessments or geotechnical reports in cases where there may be potential for substantial damage to life, property or the environment should the proposed engineering solution fail. For projects within or adjacent to a landslide hazard area, a reduction in the standard setback requires that the geological hazards assessment shall require a third-party independent review at the cost to the applicant per BIMC Critical Areas, Geologically Hazardous Areas: Chapter 16.20.130.
If a third-party geotechnical review is required, the applicant must submit a completed third-party review authorization form along with a $2,500 deposit to the permit specialists, noting the relevant permit number(s). Development Engineering staff will then forward the relevant documents to one of the Geotechnical Engineers on retainer for third-party review. This review process typically takes 2-3 weeks once all documentation and deposit have been received. At the conclusion of the review COBI staff will provide the applicant with any relevant comments from the third-party reviewer which may require further response from the applicant’s Geotechnical Engineer.
Stormwater Pollution Prevention Plan (SWPPP)
Projects creating ground disturbance on site shall implement best management practices (BMPs) for control of erosion and sedimentation through an approved stormwater pollution prevention plan (SWPPP). Smaller projects often can satisfy this requirement with the use of the COBI B109d SWPPP Narrative form outlining how the project will provide the necessary BMPs on site.
Sites containing geologically hazardous areas must use greater caution during construction to prevent erosion or potential impacts to the hazardous areas. Projects within the Zone of Influence established by BIMC 16.20.130.E.7 & 16.12.030.B.5.k.iv(A)(7) shall have the SWPPP for the project reviewed by a Geotechnical Engineer to determine if there are any potentially adverse impacts to the landslide hazardous areas. Projects within landslide or erosion hazardous areas shall have the SWPPP prepared by a licensed Civil Engineer per BIMC and have the plan reviewed by a Geotechnical Engineer to determine if there are any potentially adverse impacts to the landslide hazardous areas.
Additional requirements
Additional items may be required as a result of proposed construction in a geologically hazardous area that are beyond the scope of the Development Engineering purview. Commonly this includes, but is not limited to: Notice to Title, Public Noticing, SEPA determination and general compliance with critical areas/shoreline code. These areas are typically reviewed and approved by Planning and Community Development / Planning staff.
Bainbridge Island Municipal Code considerations
Projects should consider the relevant section(s) of the BIMC regarding the Geologically Hazardous Area based on whether the proposed scope of work takes place within the Shoreline Jurisdiction, generally defined as being 200’ from the Ordinary High-Water Mark (OHWM). Projects within the shoreline jurisdiction must comply with the Shoreline Master Program (SMP) portion of the Geologically Hazardous Areas code while projects outside the shoreline jurisdiction must comply with the Critical Areas portion of BIMC.