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Document Code No.: PUT 8-10 (PR)
Department/Issuing Agency: Department of Natural Resources/ Water and Land Resources Division
Effective Date: February 4, 1998
Approved: /s/ Ron Sims
Type of Action: New

Signed document (PDF, 347 KB)


1.0 SUBJECT TITLE: Cottage Lake Management Plan

2.0 PURPOSE:

2.1 To designate the Cottage Lake watershed (Appendix 9.1) a special management area for total phosphorus loading control and to establish a standard procedure for evaluating drainage plans and related materials for applications to development within the Cottage Lake watershed (within the Bear-Evans Creek basin) for their conformance in meeting a 50 percent phosphorus removal goal. The Sensitive Lake Protection Menu of the 1996 Draft Surface Water Design Manual update (Appendix 9.2) shall be applied currently to all new or redevelopment related stormwater inputs which drain to Cottage Lake, to achieve conformance with the 50 percent total phosphorus loading reduction goal.

3.0 ORGANIZATIONS AFFECTED:

3.1 Landowners and residents of unincorporated King County, Cottage Lake watershed area.

3.2 Applicants for development in the Cottage Lake watershed area.

3.3 Department of Development and Environmental Services.

3.4 Department of Natural Resources, Water and Land Resources Division.

4.0 REFERENCES:

4.1 King County Code 9.08.010 and 9.08.120.

4.2 King County Code 2.98.

4.3 King County Ordinance 12513.

4.4 King County Surface Water Design Manual (1990 and February 1996 draft update) hereinafter referred to as the Design Manual.

5.0 DEFINITIONS:

In addition to the definitions listed in Section 5 of this Public Rule, all definitions included in the Design Manual are hereby adopted by reference.

5.1 "Eutrophic" means a lake trophic status characterized by moderately high algal productivity, more serious oxygen depletion in the bottom waters, some recreational use impairment, summer chlorophyll a concentration greater than 10 micrograms/liter, a summer Secchi depth of <2 meters, and winter total phosphorus concentrations greater than 20 micrograms/liter.

5.2 "Hypereutrophic" means a lake trophic status characterized by high algal productivity, intense algal blooms, fish kills due to oxygen depletion in the bottom waters, frequent recreational use impairment, summer chlorophyll a concentration greater than 10 micrograms/ liter, a summer Secchi depth generally less than 2 meters, and winter total phosphorus concentrations greater than 30 micrograms/liter.

5.3 "Lake Management Plan" means the plan (and supporting documents as appropriate) describing the lake management recommendations and requirements which are formally adopted by rule under procedures specified in K.C.C. 2.98.

5.4 "Mesotrophic" means a lake trophic status characterized by moderate algal productivity, oxygen depletion in the bottom waters, usually no recreational use impairment, summer chlorophyll a concentration averaging 4-10 micrograms/liter, a summer Secchi depth of 2-5 meters, and winter total phosphorus concentrations ranging from 10-20 micrograms/ liter.

5.5 "Mean Annual Storm" means a storm derived rainfall event defined by the U.S. Environmental Protection Agency in the Results of the Nationwide Urban Runoff Program (NURP), 1986. The mean annual storm is defined as the annual rainfall divided by the number of storm events in the year. The NURP studies refer to pond sizing using the Vb/Vr ratio: the ratio of the pond volume (Vb) to the volume of runoff from the mean annual storm (Vr).

5.6 "Oligotrophic" means a lake trophic status characterized by low algal productivity, algal blooms are rare, water clarity is high, all recreational uses unimpaired, summer chlorophyll a concentration average less than 4 micrograms/liter, a summer Secchi depth greater than 5 meters, and winter total phosphorus concentrations ranging from 0-10 micrograms/liter.

5.7 "Phosphorus" means elemental phosphorus and for the purposes of this rule shall be measured as total phosphorus.

5.8 "Phosphorus Concentration" means the mass of phosphorus per liquid volume.

5.9 "Phosphorus Loading" means the total mass of phosphorus in an inflowing stream, surface water runoff, direct discharge through pipes, groundwater, and other sources over a specified time period (often annually).

5.10 "Phosphorus Sensitive Lake" means a lake of high resource value which has a combination of water quality characteristics and watershed development potential that makes them particularly prone to eutrophication induced by development.

5.11 "Pollution-generating Impervious Surface" means those impervious surfaces which are subject to vehicular use or storage of leachable material, wastes or chemicals, and which receive direct rainfall or the run-on or blow-in of rainfall.

5.12 "Redevelopment Project" means a project that proposes to add, replace, and/or alter impervious surface (for purposes other than routine maintenance, resurfacing, regrading, or repair) on a site that is already substantially developed (i.e., has 35 percent or more of existing impervious surface coverage).

5.13 "Subject to vehicular use" means a surface whether paved or not, which is regularly used by motor vehicles. This includes roads, unvegetated road shoulders, bike lanes within the roadway, driveways, parking lots, unfenced firelanes, diesel equipment storage yards, and airport runways.

5.14 "Total Phosphorus" means the phosphorus concentration as determined by a state certified analytical laboratory using EPA 365.3 or SM 4500-P-B, E or an equivalent method.

5.15 "Trophic State Index" means a lake classification system which uses algal biomass as the basis for classification which can be independently measured by chlorophyll a Secchi depth, and total phosphorus concentration.

5.16 "Trophic Status" means a classification which defines lake quality by the degree of biological productivity.

5.17 "Vr" means the site runoff volume generated from the mean annual storm in the developed condition.

6.0 POLICIES:

6.1 The Cottage Lake watershed, as generally identified in the Cottage Lake Management Plan, (which is available in summary to all Design Manual subscribers, for purchase at the Water and Land Resources Division, for review at the Department of Development and Environmental Services, or on file and available for review at the King County Woodinville Library), contains a phosphorus sensitive lake and is hereby designated a critical lake watershed. This designation is based on the following:
6.1.1 Existing (1993-1994) annual whole-lake total phosphorus concentration for Cottage Lake is 105 micrograms/liter, winter whole-lake total phosphorus is 87 micrograms/liter;

6.1.2 Whole-lake total phosphorus concentration, chlorophyll a, and Secchi depth indicate that the Cottage Lake system is a hypereutrophic lake;

6.1.3 Unmitigated future phosphorus loading from the Cottage Lake watershed is estimated to reach 355 kilograms per year, compared to 256 kilograms per year currently. This loading increase will exacerbate the existing lake water quality problems, without additional total phosphorus removal via stormwater treatment, in-lake treatments, and watershed best management practices;

6.1.4 Improving existing trophic status is a goal of the management plan;

6.1.5 Improving existing trophic status requires that 50 percent of total phosphorus loading be removed from all new development prior to discharge to any drainage that enters Cottage Lake, and that in-lake measures be implemented as well. The in-lake measures needed include whole-lake buffered alum treatment to reduce the in-lake phosphorus concentration. In addition, aeration of the bottom waters is needed for long-term reduction of phosphorus in the lake.

6.2 The standards specified in Policy 6.4 of this Public Rule shall apply to all new development or redevelopment proposals located within the Cottage Lake watershed which require drainage review as specified in the Design Manual.

6.3 Development proposals within the Cottage Lake watershed may be exempt from these Public Rule requirements if they demonstrate to the satisfaction of the Department of Development and Environmental Services that on-site surface and stormwater runoff drainage does not in fact drain into the Cottage Lake watershed as approximated in Appendix 9.1 of this Public Rule.

6.4 For new or redevelopment projects subject to drainage review which create greater than 5,000 square feet of new and/or replaced pollution generating impervious surface in the Cottage Lake watershed, the following conditions shall apply, unless the conditions identified in Policy 6.3 are documented to the satisfaction of the Department of Development and Environmental Services.

6.4.1 All proposed projects meeting the threshold given in 6.4 shall provide a stormwater treatment facility option as specified below. These facilities or facility combinations are sized to remove 50 percent of the total phosphorus loading due to development in the Cottage Lake Watershed. When the Design Manual update is adopted, these facilities choices shall be replaced by the sensitive lake protection menu, providing the menu goal for total phosphorus removal remains 50 percent or greater. In the interim, the treatment standard for phosphorus sensitive lakes can be fulfilled immediately by implementing one of the following options:
6.4.1.1 A wetpond or combined detention/wetpond with a permanent pool volume equal to 4.5 times the volume of runoff (Vr) from the mean annual storm (pond volume = 4.5 Vr). The wetpond sizing criteria are described in Appendix 9.3.

6.4.1.2 A two-facility treatment train which involves the use of a biofiltration swale, filter strip, wetpond, basic combined detention and wetpond, or wetvault followed by a linear sand filter, sand filter vault, or basic sand filter (Appendix 9.2). All facilities should be designed using the criteria detailed in Chapter 6 of the February 1996 draft Design Manual. A storage volume of 3Vr applies to the wetpond, combined detention/wetpond, and wetvault options;

6.4.1.2.1 A phosphorus credit may be accrued to substitute for the second water quality facility, provided enough credits are earned. Credit earning actions include: (1) leaving 20-65 percent of the site in undisturbed native vegetation; (2) directing runoff to grassy areas with level spreading; (3) covered parking or covered waste disposal and recycling areas isolated from stormwater conveyance system; and (4) covered vehicle washing areas connected to the sanitary sewer system. Each credit action has a range of possible points associated with it. These credits are detailed in Appendix 9.2.

6.4.1.3 For properties located at least a quarter of a mile from Cottage Lake and where soils are suitable, on-site infiltration of storm water runoff may be used. General requirements for infiltration facilities are included in Appendix 9.4. Design criteria are detailed in Chapter 6 of the February 1996 Draft Design Manual.

6.4.1.4 As additional treatment options and designs for phosphorus removal become available, alternative treatment systems may be proposed using the experimental variance process in the Design Manual. The 50 percent total phosphorus removal goal must be demonstrated as part of the variance application.

6.5 If any portion of this Public Rule or its application to any person or property is held invalid, the remainder of this Public Rule or the application of the provision to other persons or property shall not be affected.

6.6 This Public Rule is exempt from the rule of strict construction and shall be liberally construed to give full effect to the objectives and purposes for which it was adopted.

7.0 PROCEDURES:

Action By: DDES

Action:

7.1 Identifies that the project is in the Cottage Lake watershed.

7.2 Informs applicant that the project is located in the Cottage Lake Management Plan Public Rule area.

Action By: Applicant

Action:

7.3 Submits additional information which either:
7.3.1 Demonstrates that the project qualifies for the exemption in Policy 6.3; or

7.3.2 Complies with Policy 6.4.

Action By: DDES

Action:

7.4 Reviews the materials submitted in step 7.3 and determines if it complies with the Cottage Lake Management Plan Public Rule.
7.4.1 Returns inadequate material to the applicant with specific instructions to comply with step 7.3.

7.4.2 Forwards adequate engineering plans to be processed according to the established review process.

8.0 RESPONSIBILITIES:

8.1 The Water and Land Resources Division is responsible for:
8.1.1 Amending this Cottage Lake Management Plan Public Rule if needed.

8.1.2 Updating the Cottage Lake Management Plan Public Rule as new information becomes available.

8.2 The DDES is responsible for:

8.2.1 Identifying each project which is within the Cottage Lake Management Plan Public Rule area.

8.2.2 Determining if the material submitted by the applicant complies with either Policy 6.3 or Policy 6.4 of this Public Rule.

8.3 The Applicant is responsible, at a minimum for:

8.3.1 Demonstrating that the project complies with Policy 6.3 or with the requirements in Policy 6.4 of this Public Rule.

9.0 APPENDICES:

9.1 Map of the Cottage Lake Management Plan Area. Approximate borders of this management area are also indicated on the Cottage Lake Watershed Map at the DDES permit center.

9.2 Section 6.1.2: Sensitive Lake Protection Menu (including Credit Earning Actions).

9.3 Section 6.4: Wetpool Water Quality Facility Designs (including Wetpond Sizing).

9.4 Section 5.4: Infiltration Facilities.

Please call 206-296-6519 if you would like to receive the appendices.