Chapter 16.50
LANDSLIDE AND EROSION HAZARD AREA
Sections:
16.50.040 Relationship to framework ordinance.
16.50.090 Submittal requirements.
16.50.110 Performance standards.
16.50.130 Subdivision regulations.
16.50.010 Short title.
The ordinance codified in this chapter, together with any amendments, shall be known as the “Landslide and Erosion Hazard Area Ordinance.” (Ord. 1544 § 1 (part), 1992)
16.50.020 General authority.
This chapter is adopted under the authority of RCW 36.70A.050. (Ord. 1544 § 1 (part), 1992)
16.50.030 Purpose.
The purpose of this chapter is to regulate land disturbing activity; to protect lives, property and public infrastructure from impacts associated with construction on steep slopes; reduce erosion impacts associated with construction; reduce sedimentation and water quality impacts associated with uncontrolled surface runoff; and reduce risk of landslide activity; and conform with the Washington State Growth Management Act. (Ord. 1544 § 1 (part), 1992)
16.50.035 Exemptions.
The following activities are exempt from this chapter:
A. Approved mining activities on mineral resource lands pursuant to chapter 16.44 SMC. (Ord. 2071 § 16, 2003)
16.50.040 Relationship to framework ordinance.
The provisions of this chapter shall apply in conjunction with SMC 16.40.030 through 16.40.210. (Ord. 2071 § 17, 2003: Ord. 1544 § 1 (part), 1992)
16.50.050 Applicability.
A. Unless otherwise stated, landslide hazard areas are those areas subject to risk of mass movement and meeting any of the following criteria:
1. Areas of historic land failures, including areas of unstable old and recent landslides;
2. Areas with all three of the following characteristics:
a. Slopes steeper than 15 percent; and
b. Hillsides intersecting geologic contacts with a relatively permeable sediment overlying a relatively impermeable sediment or bedrock; and
c. Any signs of springs or groundwater seepage; and
d. Concave slopes and swales;
3. Slopes that are parallel or subparallel to planes of weakness, such as bedding planes, joint systems, and fault planes, in subsurface materials;
4. Slopes having gradients steeper than 80 percent subject to rockfall during seismic shaking;
5. Areas potentially unstable as a result of rapid stream incision, stream bank erosion, and undercutting by wave action;
6. Any area with a slope of 15 percent or steeper and with a vertical relief of 10 or more feet. A slope is delineated by establishing the toe and top and measured by averaging the inclination over at least 10 feet of vertical relief. Qualifying slopes of 15 percent or greater to less than 25 percent shall be termed “Type II landslide hazard areas” for purposes of this chapter. Qualifying slopes of 25 percent or greater shall be termed “Type I landslide hazard areas”;
7. Areas which have a “severe” limitation for building site development because of slope conditions, according to the U.S. Department of Agriculture’s Natural Resource Conservation Service;
8. Slopes that contain impermeable soils (typically silt and clay) frequently interbedded with granular soils (predominantly sand and gravel);
9. Any area which has indications of mass wasting during the Holocene epoch (from 10,000 years ago to the present) or which is underlain by mass wastage debris of that epoch.
B. Erosion hazard areas are those areas that are identified by the presence of vegetative cover, soil texture, slope, and rainfall patterns, or human-induced changes to such characteristics, which create site conditions which are vulnerable to excessive erosion. Erosion hazard areas are those areas that are classified as having moderate to severe, severe or very severe erosion potential according to the Natural Resource Conservation Service. (Ord. 2071 § 18, 2003: Ord. 1544 § 1 (part), 1992)
16.50.060 Mapping.
Areas meeting the criteria established above may be delineated in the following documents:
A. Soil Survey of Pierce County Area, Washington, 1979, Soil Conservation Service, United States Department of Agriculture (USDA);
B. Areas designated as slumps, earthflows, mudflows, lahars, or landslides on maps published by the United States Geologic Survey or Washington Department of Natural Resources Division of Geology and Earth Resources;
C. Existing or newly developed topographic surveys prepared by the city, USGS, the state, or by applicants or their representatives;
D. The actual presence or location of an active landslide hazard area and/or additional potential landslide hazard areas that have not been mapped, but may be present on or adjacent to a site, shall be evaluated using the site evaluation procedures established in this chapter. (Ord. 2071 § 19, 2003: Ord. 1544 § 1 (part), 1992)
16.50.070 Title notification.
A. All activity in Type I landslide hazard areas shall be accompanied by the recording of a notice with the Pierce County auditor in the form set forth below:
HIGH LANDSLIDE HAZARD AREA NOTICE
Parcel Number:__________________
Address:_______________________
Legal Description:________________ ______________________________
Notice: This site lies within a Landslide Hazard Area as defined by Chapter 16.50, Sumner Municipal Code. The site was the subject of a development proposal for ___________________, Sumner application number ___ filed on____________________. Restrictions on use or alterations of the site may exist due to natural conditions of the site and resulting regulation. Review of such application has provided information on the location of a Landslide Hazard Area and any restrictions on use.
Signature of owner(s)
(NOTARY ACKNOWLEDGEMENT)
B. All activity in erosion hazard areas shall be accompanied by the recording of a notice with the Pierce County Auditor in the form set forth below:
EROSION HAZARD AREA NOTICE
Parcel Number:__________________
Address:_______________________
Legal Description:________________ ______________________________
Notice: This site lies within an Erosion Hazard Area as defined by Chapter 16.50, Sumner Municipal Code. The site was the subject of a development proposal for ___________________, Sumner application number ___ filed on____________________. Restrictions on use or alterations of the site may exist due to natural conditions of the site and resulting regulation. Review of such application has provided information on the location of an Erosion Hazard Area and any restrictions on use.
Signature of owner(s)
(NOTARY ACKNOWLEDGEMENT)
(Ord. 1544 § 1 (part), 1992)
16.50.080 Plat notification.
A. For all proposed short subdivisions and subdivision proposals within Type I landslide hazard areas, the applicant shall include a note on the face of the plat as set forth below:
Notice: This site lies within a Landslide Hazard Area as defined by the Sumner Municipal Code. Restrictions on use or alterations on the site may exist due to natural conditions of the site and resulting regulation.
B. For all proposed short subdivisions and subdivision proposals within erosion hazard areas, the applicant shall include a note on the face of the plat as set forth below:
Notice: This site lies within an Erosion Hazard Area as defined by the Sumner Municipal Code. Restrictions on use or alterations on the site may exist due to natural conditions of the site and resulting regulation.
(Ord. 1544 § 1 (part), 1992)
16.50.090 Submittal requirements.
A. For all nonexempt activity proposed within a Type I landslide hazard area, a site evaluation shall be submitted; provided, where an applicant can demonstrate through submittal of a geotechnical letter, that there are no Type I landslide hazard areas on-site, the requirement for the site evaluation as set forth in subsection B of this section may be waived. The geotechnical letter shall include at a minimum the following:
1. A brief description of the project (including the proposed land use) and a description of the area to be developed.
2. A paragraph that states the following specific language:
The services described in this report were prepared under the responsible charge of (Individual’s Name). (Individual’s Name) meets the qualifications contained in Title 16, Section 16.50.090 to prepare a landslide hazard geological assessment. (Individual’s Name) understands the requirements of the current Landslide and Erosion Hazard Area Chapter 16.50 and the definitions of the applicable terms contained within Chapter 16.04. Individuals under the responsible charge of (Individual’s Name) have performed a landslide hazard geological assessment, conducted a field investigation, and researched historic records on or in the vicinity of the above referenced site. In my opinion, the scope of services completed for this project is adequate to meet the requirements of the Sumner Municipal Code and it does not appear that an active landslide hazard area exists on site.
3. The geotechnical letter shall be prepared under the responsible charge of an appropriately licensed geotechnical professional(s) and be signed, sealed and dated by the geotechnical professional(s).
B. The site evaluation shall address the existing geologic, topographic, and hydrologic conditions on a site, including an evaluation of the ability of the site to accommodate the proposed activity. The site evaluation shall include at a minimum the following:
1. Topographic data showing the site with a maximum five-foot contour interval. Slopes shall be clearly delineated for the ranges between 15 and 24 percent, and 25 percent or greater, including calculations for areal coverage of each slope category on the site. When site conditions indicate the necessity, the department may require the topographic data to be field surveyed and/or may require that a contour interval of one foot be used.
2. Site history data describing prior uses, grading, soil instability, or slope failures on the property.
3. Geotechnical report prepared by a professional engineer, geologist, engineering geologist, or hydrogeologist, licensed by the state of Washington with expertise in geotechnical engineering. The report shall include the following:
a. Results from boring logs, exploration pits, and any other exploration methods;
b. Data concerning the vulnerability of the site to unusual seismic events;
c. Slope stability analysis and opinion(s) regarding the stability of the slope;
d. Proposed angles of cut and fill slopes and site grading requirements;
e. Structural foundation requirements and estimated foundation settlements;
f. Soil compaction criteria;
g. Proposed surface and subsurface drainage and calculations regarding design;
h. Lateral earth pressure values and calculations for all lateral pressure walls;
i. Suitability of on-site material for fill;
j. Laboratory data and soil index properties for soil samples; and
k. Any additional information necessary for the director to determine the stability of the site and adjacent properties.
4. Location of all vegetation, including location and description of all trees and shrubs over three inches diameter measured five feet above the base of the trunk.
5. Grading plans showing all proposed grading activity, timing of construction, location of existing and proposed structures, location of underground utilities, location of any required buffers or conservation easements, and location and nature of any off-site improvements which are associated with the activity. (Ord. 2071 § 20, 2003: Ord. 1544 § 1 (part), 1992)
16.50.100 Regulations.
The provisions of this section apply in all Type I landslide hazard areas:
A. Any disturbance of the earth shall be limited to no more than 25 cubic yards of cut and fill provided such grading is in conjunction with construction of access roads, pedestrian paths and utility corridors. Essential public facilities may disturb more than 25 cubic yards, but shall not exceed the minimum necessary for the public facility.
B. Any clearing of natural or planted vegetation shall be prohibited, except that up to 2,500 square feet of clearing may be allowed in order to construct access roads, pedestrian paths and utility corridors provided no removal of trees greater than six inches in diameter shall occur. Essential public facilities may clear more than 2,500 square feet, but shall not exceed the minimum necessary for the public facility.
C. Upon a showing that the trees are not necessary to preserve slope stability or reduce erosion, the director may permit a maximum of 40 percent of the trees greater than six inches in diameter to be removed in conjunction with construction.
D. Based on the information provided in the site evaluation, project plans, and geotechnical report, the director may approve, conditionally approve or deny the proposal. Any conditions shall be necessary to ensure the stabilization of the site during and/or following construction.
E. Water tanks shall be screened or use a minimally invasive design to reduce visual impacts to adjacent property owners and major transportation thoroughfares. (Ord. 2321 § 1, 2010: Ord. 1544 § 1 (part), 1992)
16.50.110 Performance standards.
The following standards shall apply to all actions in Type I and Type II landslide hazard areas:
A. All disturbed areas on the site, including areas proposed for disturbance, shall be controlled in a manner sufficient to control drainage and prevent erosion during construction consistent with chapter 16.05 SMC, and revegetated as soon as possible to promote drainage control and prevent erosion during and after construction. In cases where erosion potential could threaten the stability of the site, the director may require a revegetation plan be submitted and implemented prior to permit issuance.
B. The director may restrict development coverage and construction activity areas to the most level, environmentally suitable and naturally stable portion of the site. Grading activities may be restricted beyond those required by SMC 16.50.100 if necessary to ensure stability of the site.
C. Impervious surfaces shall only be located within the site’s development coverage and construction activity areas and shall be limited to a maximum of 40 percent of the lot area. The maximum lot coverage restrictions may be waived by the city council in approving a planned residential development pursuant to the zoning code.
D. All drainage systems and discharge points associated with actions shall be approved by the city engineer.
E. All grading in all landslide hazard areas shall be stabilized by October 1st of each year and may not resume until April 1st of the following year; provided, that if the applicant submits documentation to substantiate that adverse impacts will not result from construction and site activity between these dates, the director may authorize certain activity.
F. Construction shall adhere to a prepared schedule to be approved with the construction plans.
G. Construction and site actions shall conform to best management practices for the types of construction or activity. (Ord. 2071 § 21, 2003: Ord. 1544 § 1 (part), 1992)
16.50.120 Buffers.
A. A buffer, consisting of undisturbed natural vegetation and measured horizontally from all sides of Type I landslide hazard areas, shall be provided. The buffer width shall be as required by the International Building Code as adopted by the city.
B. In order to increase the functional attributes of the buffer, the director may require that the applicant enhance the buffer with native vegetation.
C. The edge of the buffer shall be clearly staked, flagged, and fenced prior to any site clearing or construction. Field marking shall be shown on the construction plans and shall remain in place, in functional condition, for the duration of construction.
D. The director may require additional building setbacks or buffers if recommended by the geotechnical report.
E. Buffers shall be shown or described on all plats recorded in conjunction with development. (Ord. 2439 § 15, 2013; Ord. 1544 § 1 (part), 1992)
16.50.130 Subdivision regulations.
Where a site is proposed for short subdivision or subdivision, up to 50 percent of the total site’s area which is designated as being a Type I landslide hazard area (25 percent slope or greater), may be permitted for use in calculating minimum lot area for the proposed lots. (Ord. 2071 § 22, 2003: Ord. 1544 § 1 (part), 1992)
16.50.140 Erosion control.
All actions shall conform to the city requirements for erosion control established in chapter 16.05 SMC. Erosion control plans and the requirements of that section apply regardless of the exemption status resulting from this title. (Ord. 2071 § 23, 2003: Ord. 1544 § 1 (part), 1992)