Chapter 20.28
GEOLOGICALLY HAZARDOUS AREAS
Sections:
20.28.010 Designation of geologically hazardous areas.
20.28.020 Critical areas report.
20.28.035 Reduction of storm water impacts.
20.28.040 Modifications and flexibility.
20.28.050 Repair of slope instabilities.
20.28.060 Seasonal restriction and best management practices.
20.28.010 Designation of geologically hazardous areas.
(1) Geologically hazardous areas include areas susceptible to erosion, sliding, earthquake, tsunami, or other geologic events and conditions. Improper and incompatible development sited in these areas can pose a threat to the health and safety of citizens, placing not only itself at risk, but also potentially creating or increasing hazards to surrounding development and land uses. Areas susceptible to one or more of the following types of hazards shall be designated as a geologically hazardous area:
(a) Areas mapped on the city of Oak Harbor geologically hazardous areas map;
(b) Unstable slopes, as defined in OHMC 20.02.020;
(c) Steep slopes, as defined in OHMC 20.02.020; and
(d) Areas of moderate to high liquefaction due to soil type and/or location or seismically induced ground disturbance such as surface rupture, fissuring, and lateral spreading.
(2) As the city is not impacted by mine or volcanic hazards, development in the city shall comply with International Building Code (see OHMC Title 17) requirements with respect to these potential hazards. (Ord. 1801 § 36, 2018; Ord. 1440 § 5, 2005).
20.28.020 Critical areas report.
All single-family residential development within 100 feet of a designated geologically hazardous area and all commercial, industrial, or multifamily developments within 200 feet of a designated geologically hazardous area shall be considered “adjacent” to the geologically hazardous area and shall be required to submit a critical areas report, as described in this section. The director shall approve the critical areas report only if it demonstrates that the proposed development will not increase the risk of harm to public safety or neighboring properties or critical areas. To determine if a critical areas report is required on slopes between 15 and 39 percent, the director may require the applicant to provide a letter prepared by a certified geologist or civil engineer specializing in geotechnical engineering that determines whether springs or ground water seepage is present on the subject slope.
(1) Geotechnical Analysis. Except as provided in subsections (2) and (3) of this section, all development proposals within or adjacent to a designated geologically hazardous area shall submit a site assessment, geotechnical report, grading and erosion control plan and landscape/revegetation plan. This analysis shall contain the following information:
(a) Site Assessment. Along with the standard site plan requirements, the following information shall be provided for the subject property, prepared by a licensed land surveyor:
(i) Topography map at two-foot contour intervals for the entire site, including abutting public rights-of-way, private roads, or access easements;
(ii) Location of all significant trees (evergreen or deciduous trees 12 inches or more in diameter measured four feet above existing grade);
(iii) Location of all manmade drainage structures or features including pipes, drains, catch basins, drainage structures, culverts, and underdrain pipes;
(iv) Location of all frequently flooded areas, as defined in Chapter 17.20 OHMC, and all other critical areas, as defined in this title, including: oak trees, wetlands, fish and wildlife habitat conservation areas, critical aquifer recharge areas, and geologically hazardous areas;
(v) Location of all existing site improvements and the amount of existing hard surface area; and
(vi) Location of all utilities, both above and below ground.
The site plan shall also include a vicinity map, showing the location of the property in relationship to surrounding lots and other critical areas.
(b) Geotechnical Report. The report, prepared by a qualified professional, shall include an assessment of the geologic characteristics of the soils, sediments, and/or rock of the project area and potentially affected adjacent properties, and a review of the site history regarding landslides, erosion, and prior grading. Soils analysis shall be prepared in accordance with accepted classification systems in use in the region. The assessment shall include, but not be limited to:
(i) Data regarding underlying geology, slope gradients, soil types, and subsurface information, including boring and/or test pit logs describing soil stratification and results of soil tests conducted;
(ii) Identify any previous landslide activity in the vicinity of the project and provide an assessment of the overall slope stability and the effect the development will have on the slope and adjacent properties over time;
(iii) Recommendations for grading procedures, fill placement, and compaction criteria, temporary and permanent slope inclinations and support, and design criteria for corrective measures and opinions and recommendations regarding the capabilities of the site;
(iv) Evaluation of the seismic stability of the site in drained and saturated conditions, including a statement that the design criteria of proposed structures consider a seismic event with a 10 percent probability of being exceeded in 50 years;
(v) Potential for liquefaction and proposed mitigation measures;
(vi) A description of the hydrology (both surface and subsurface) of the site, including locations of any wetlands, streams, springs, seeps, and ground water along with recommendations consistent with the city’s critical areas regulations for addressing any impacts;
(vii) A recommendation on building site location, foundation type and depths, minimum building setbacks, minimum deck and accessory building setbacks, and if necessary the minimum no-disturbance setback from any geologically sensitive area based upon the geotechnical analysis. The report shall also include recommendations on the design of temporary and permanent retaining structures if any are proposed;
(viii) An estimate of bluff retreat rate that recognizes and reflects potential catastrophic events such as seismic activity or a 100-year storm event; and
(ix) Recommendations and requirements for handling contaminated soils and materials if encountered on the site.
(c) Grading and Erosion Control Plan. The plan shall be prepared by a qualified professional and shall include:
(i) A schedule showing when each stage of the project will be completed, and estimate starting and completion dates, limiting the time that soil is exposed and unprotected to the shortest possible period.
(ii) Measures to be taken for slope stabilization and erosion control, using best management practices as contained in the Washington State Department of Ecology’s 2012 Stormwater Management Manual for Western Washington (as amended December 2014): Volume II, Construction Stormwater Pollution Prevention (Publication No. 14-10-055), or future updated publications or other methodology as approved by the director.
(d) Landscape/Revegetation Plan. A revegetation plan shall be prepared which:
(i) Shows measures to be taken for protection and replacement of the natural vegetative cover;
(ii) Includes a note stating that vegetation trimming debris shall be removed from slopes in such a fashion as to not disturb existing vegetation; and
(iii) Includes a schedule showing when each stage of the project will be revegetated with estimated starting and completion dates.
(2) Geotechnical Letter Requirements. For the following small development applications, a letter prepared by a qualified professional may be prepared in lieu of the full geotechnical reporting requirements:
(a) Building additions less than 30 percent of their entire structural footprint.
(b) Additions to a building’s height where the footprint of the existing structure is not changed.
(c) Earth retaining walls less than 10 feet in height and set back more than 50 feet from the top of a steep slope.
(d) Detached auxiliary buildings such as garages and sheds with no living spaces.
A geotechnical letter shall include an assessment of the existing geologic and geotechnical site conditions, including surface water runoff, ground water, soil types, erosion, and slope stability. The qualified professional shall prepare conclusions and recommendations on the suitability of the proposed development and any mitigation necessary to address existing site conditions that may need to be modified due to the proposed development.
(3) Exceptions. For the following single-family and multifamily residential development applications, the director may waive the requirements for geotechnical analysis or letter, if the development is unlikely to have any impact on a geologically hazardous area:
(a) Single-story additions to a single-family residence less than 200 square feet in size, located so that the addition is no closer than the facade closest to the steep or unstable slope of the existing structure.
(b) Detached auxiliary buildings such as garages and sheds that are 50 feet or more away from a steep or unstable slope.
(c) Decks attached to single-family and multifamily structures where no additional load- bearing weight is added to an adjacent steep or unstable slope. (Ord. 1801 §§ 37 – 40, 2018; Ord. 1784 § 77, 2016; Ord. 1440 § 5, 2005).
20.28.030 Building setback.
(1) New structures and additions to existing structures within or adjacent to a geologically hazardous area shall be set back a minimum of 25 feet from the top of a steep or unstable slope unless a larger setback is recommended in a geotechnical analysis or by the International Building Code. Setbacks may be less than 25 feet from a steep or unstable slope if allowed through the “reasonable use” provisions of OHMC 20.12.060 or supported by a geotechnical report and approved by the director. Decks which add no substantial loading weight to the hazardous area and accessory buildings 120 square feet or less may extend into the setback area to within 10 feet of the top or toe of a steep or unstable slope, unless a larger setback is recommended by the geotechnical analysis or by the International Building Code.
(2) Signage. After completion of the project, the top of the steep or unstable slope shall be identified with signage as approved by the director, as a critical area or native growth protection area, in accordance with OHMC 20.12.140(2). (Ord. 1959 § 2, 2022; Ord. 1801 §§ 41, 42, 2018; Ord. 1440 § 5, 2005).
20.28.035 Reduction of storm water impacts.
Storm water impacts shall be addressed in compliance with geotechnical recommendations, city code, and the Washington State Department of Ecology’s 2012 Stormwater Management Manual for Western Washington (as amended December 2014) (Publication No. 14-10-055), or future updated publications, and other applicable regulations. Storm water drainage in areas of steep or unstable slopes shall be designed in such a manner that storm water does not create stability or erosion impacts. Surface drainage shall be directed away from landslide and erosion hazard areas. When no other solution is feasible, surface drainage piping may be located on the face of a geologically hazardous area when contained in a pipe slope drain (closed, nonleaking pipe) in such a way that erosion will not be exacerbated. At no time shall concentrated storm water runoff be allowed to flow uncontained over a steep or unstable slope or impact a neighboring property. (Ord. 1801 § 43, 2018).
20.28.040 Modifications and flexibility.
Minor alterations on steep or unstable slopes or associated setbacks may be allowed by the director where all of the following standards have been met:
(1) A site assessment has been submitted showing that the proposal will have no adverse impact on the stability or erosion susceptibility of the slope;
(2) The impacted area totals no more than 20 percent of the entire site;
(3) The modification will not increase surface water discharge or sedimentation to adjacent properties or critical areas beyond predevelopment conditions;
(4) The activity will not adversely impact other critical areas;
(5) The development will not decrease slope stability on adjacent properties. (Ord. 1801 § 44, 2018; Ord. 1784 § 78, 2016; Ord. 1440 § 5, 2005).
20.28.050 Repair of slope instabilities.
Repair of slope instabilities and slope failures on an emergency basis shall be allowed by the director as needed to correct an immediate danger to the public health, welfare and safety. The director shall use the guidance of this chapter when evaluating the necessary repairs and add mitigation measures as appropriate to ensure that the intent of this chapter has been met. (Ord. 1440 § 5, 2005).
20.28.060 Seasonal restriction and best management practices.
Clearing and grading within the wet weather months (October through April) shall be allowed in or adjacent to geologically hazardous areas only with the approval of the director. The developer shall fully implement a wet weather construction plan using at a minimum the current best management practices as contained in the Washington State Department of Ecology’s 2012 Stormwater Management Manual for Western Washington (as amended December 2014): Volume II, Construction Stormwater Pollution Prevention (Publication No. 14-10-055), or future updated publication. If the wet weather construction plan is not implemented or turbid water leaves the site, construction shall be stopped immediately until proper erosion control devices are implemented and established. Best management practices include, but are not limited to:
(1) Exposed soils shall be protected from the forces of rain and flowing water within two days during the winter season and seven days during the summer season.
(2) Erosion control devices shall include as appropriate silt fences, straw mats, hay bales, filter fabrics, plastic sheeting, mulch, retention of vegetative buffers, and soil stabilization plant materials.
(3) Development shall be phased to limit the area of exposed soils to no more than two acres at a time.
(4) Water flows shall be directed away from steep or unstable slopes. At no time shall water be allowed to flow freely over steep or unstable slopes.
(5) Vegetation removal or planting on steep slopes shall be conducted by hand or by nonimpacting procedures as approved by the director. Heavy equipment shall not be allowed on steep or unstable slopes. (Ord. 1801 § 45, 2018; Ord. 1784 § 79, 2016; Ord. 1440 § 5, 2005).