The purpose of the erosion hazards near sensitive water bodies special overlay district is to provide a means to designate sloped areas posing erosion hazards which drain directly to lakes or streams of high resource value which are particularly sensitive to the impacts of increased erosion and the resulting sediment loads from development.
- Erosion Hazards Near Sensitive Water Bodies SDO
Development Condition Text
21A.38.200 Special district overlay - Erosion hazards near sensitive water bodies.
A. The purpose of the erosion hazards near sensitive water bodies special overlay district is to provide a means to designate sloped areas posing erosion hazards which drain directly to lakes or streams of high resource value which are particularly sensitive to the impacts of increased erosion and the resulting sediment loads from development.
B. The following development standards shall be applied in addition to all applicable requirements of K.C.C. 21A.24 to development proposals located within erosion hazards near a sensitive water bodies district overlay:
1. A no-disturbance area shall be established on the sloped portion of the special district overlay to prevent damage from erosion. Land clearing or development shall not occur in the no-disturbance area, except for the clearing activities listed in subsection a. Clearing activities listed in subsection a shall only be permitted if they meet the requirements of subsection b.
a. Clearing activities may be permitted as follows:
i. for the construction of single family residences on pre-existing separate lots;
ii. for the construction of utility corridors to service existing development along existing rights-of-way including any vacated portions of otherwise contiguous rights-of-way;
iii. for the construction of roads providing sole access to buildable property and associated utility facilities within those roadways; or
iv. for the construction of development within an isolated no-disturbance area of two acres or less in size. The isolated no-disturbance area is either geologically separated from other no-disturbance areas or lies completely within a separate drainage subbasin and is, therefore, hydrologically isolated from the rest of the no-disturbance area.
b. The clearing activities listed in subsection a. may be permitted only if the following requirements are met:
i. a report which meets the requirements of K.C.C. 21A.24.120 shall show that the clearing activities will not subject the area to risk of landslide or erosion and that the purpose of the no-disturbance area is not compromised in any way;
ii. the clearing activities shall be mitigated, monitored and bonded consistent with the mitigation requirements applicable to sensitive areas regulated in K.C.C. 21A.24;
iii. the clearing activities are limited to the minimal area and duration necessary for construction; and
iv. the clearing activities are consistent with K.C.C. 21A.24.
2. The upslope boundary of the no-disturbance area lies at the first obvious break in slope from the upland plateau over onto the steep valley walls. The downslope boundary of this zone includes those areas designated as erosion or landslide hazard areas pursuant to K.C.C. 21A.24.220 and 21A.24.280. The sensitive areas folio indicates the general location of these hazard areas, but it cannot be used to specify the areas' precise boundaries. Maps of the approximate boundaries of these no-disturbance zones shall be available at the department. Single family or multi-family residential density from the no-disturbance area may be reallocated onto any buildable portion of the site pursuant to K.C.C. 21A.12.080, or transferred to other sites pursuant to K.C.C. 21A.36;
3. New development proposals for sites which drained predeveloped runoff to the no-disturbance zone shall evaluate the suitability of onsite soils for infiltration. All runoff from newly constructed impervious surfaces shall be retained on-site unless this requirement precludes the ability to meet minimum density requirements in K.C.C. 21A.12. When minimum density cannot be met, runoff shall be retained on-site as follows:
a. Infiltration of all site runoff shall be required in granular soils as defined in the King County Surface Water Design Manual.
b. Infiltration of downspouts shall be required in granular soils and in soil conditions defined as allowable in the Surface Water Design Manual when feasible to fit the required trench lengths onsite;
c. When infiltration of downspouts is not feasible, downspout dispersion trenches shall be required when minimum flow paths defined in the Surface Water Design Manual can be met onsite or into adjacent open space; and
d. When dispersion of downspouts is not feasible, downspouts shall be connected to the drainage system via perforated pipe.
4. For the portions of proposed subdivisions, short subdivisions and binding site plans that cannot infiltrate runoff up to the 100-year peak flow, at least 25 percent shall remain undisturbed and set aside in an open space tract consistent with K.C.C. 21A.24.150-180; and
5. For the portions of all development proposals that cannot infiltrate runoff up to the 100-year peak flow, no more than 35 percent of the gross site area shall be covered by impervious surfaces. For new subdivisions and short subdivisions, maximum lot coverage should be specified for subsequent residential building permits on individual lots.
6. If the application of this section would deny all reasonable use of property, the applicant may apply for a reasonable use exception pursuant to K.C.C. 21A.24.070B.
7. The director may modify the property specific development standards required by B.1 through B.5 of this section, when a development proposal complies with the following:
a. The proposed development is subject to public/private partnerships such as an approved community block grant or other such water quality program designed to improve water quality in the basin,
b. The proposed development is designated by King County, in consultation with the Lake Sammamish Management Committee, as a demonstration project designed to implement best management practices and state of the art technology that assures the greatest possible improvement to water quality, and
c. A site specific study is conducted by the applicant and approved by the director, which demonstrates that the proposed development substantially increases water quality by showing the following:
(1) water quality on-site is improved;
(2) the development project will not subject downstream channels to increased risk of landslide or erosion;
(3) the development project will not subject the nearest sensitive water body to additional erosion hazards; and
(4) the project is consistent with element a. and b. above, and provides predictable improvements to the water quality of Lake Sammamish. (Ord. 12823 § 15, 1997).
August 18, 1997
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