12.11.040 Minimum surface water and storm water requirements.
When drainage review and approval is required the City requires, at a minimum, full compliance with this section.
A. Minimum Requirement No. 1: Preparation of Storm Water Site Plans. All projects meeting the thresholds in LMC 12.11.030, Figure 11.1 or 11.2, shall prepare a storm water site plan for City review. Storm water site plans shall be prepared in accordance with the DOE Stormwater Manual as amended by the Engineering Standards Manual.
B. Minimum Requirement No. 2: Construction Storm Water Pollution Prevention. All new development and redevelopment projects are responsible for preventing erosion and discharge of sediment and other pollutants leaving the site. Projects meeting the thresholds described in LMC 12.11.030, Figure 11.1 or 11.2, shall prepare a construction storm water pollution prevention plan (SWPPP) as part of the storm water site plan (see Minimum Requirement No. 1). In addition, projects greater than one acre that have the potential to discharge to waters of the state, as determined by the City Engineer, shall apply for a construction storm water general permit through the DOE. The permit, as well as guidance for preparing a SWPPP, is available through the DOE.
The SWPPP shall be implemented beginning with initial soil disturbance and until final stabilization. The SWPPP shall meet the following objectives:
1. To implement best management practices (BMPs) to prevent erosion and sedimentation, and to identify, reduce, eliminate or prevent storm water contamination and water pollution from construction activity;
2. To prevent violations of surface water quality, ground water quality or sediment management standards; and
3. To control peak volumetric flow rates and velocities of storm water discharges.
C. Minimum Requirement No. 3: Source Control of Pollution. All known, available and reasonable source control BMPs shall be applied to all projects. Source control BMPs shall be selected, designed, and maintained in accordance with the DOE Stormwater Manual and as amended by the Engineering Standards Manual.
D. Minimum Requirement No. 4: Preservation of Natural Drainage Systems and Outfalls. Natural drainage patterns shall be maintained, and discharges from the project site shall occur at the natural location, to the maximum extent practicable. The manner by which runoff is discharged from the project site shall not cause a significant adverse impact to downstream receiving waters and down gradient properties. All outfalls require energy dissipation.
E. Minimum Requirement No. 5: On-Site Storm Water Management. Projects shall employ on-site storm water management BMPs in accordance with the project thresholds, standards, and lists found in Section 4.5 of Appendix 1 of the Western Washington Phase II Municipal Stormwater Permit (Municipal NPDES Permit) to infiltrate, disperse, and retain storm water runoff on site to the maximum extent feasible without causing flooding or erosion impacts.
F. Minimum Requirement No. 6: Runoff Treatment Project Thresholds. The following require construction of storm water treatment facilities:
1. Projects in which the total of pollution-generating hard surface (PGHS) is 5,000 square feet or more in a threshold discharge area of the project; or
2. Projects in which the total of pollution-generating pervious surfaces (PGPS), not including permeable pavements, is three-quarters of an acre or more in a threshold discharge area, and from which there will be a surface discharge in a natural or manmade conveyance system from the site.
Treatment-type thresholds, facility sizing and facility selection, design and maintenance are as described in Section 4.6 of Appendix 1 of the Municipal NPDES Permit.
G. Minimum Requirement No. 7: Flow Control.
1. Projects shall provide flow control to reduce the impacts of storm water runoff from hard surfaces and land cover conversions. This requirement shall be interpreted as follows within the City of Lakewood:
a. This minimum requirement applies to projects that discharge storm water directly or indirectly through a conveyance or overflow system, into a fresh water body.
b. This minimum requirement applies to projects that discharge storm water to a City-owned conveyance system which subsequently infiltrates storm water. However, for this scenario, the level of control required for such discharges will be approved on a site-by-site basis by the City Engineer. The intent is to ensure that drainage from the project site will not exceed the capacity of the downstream conveyance and/or the downstream infiltration system.
c. This minimum requirement is waived for sites that will infiltrate on site all runoff from hard surfaces and converted vegetation areas.
2. Thresholds. The following require construction of flow control facilities and/or land use management BMPs that will achieve the standard requirements of the DOE Stormwater Manual:
a. Projects in which the total of effective impervious surfaces is 10,000 square feet or more in a threshold discharge area; or
b. Projects that convert three-quarters of an acre or more of native vegetation to lawn or landscape, or convert two and one-half acres or more of native vegetation to pasture in a threshold discharge area, and from which there is a surface discharge in a natural or manmade conveyance system from the site; or
c. Projects that through a combination of hard surfaces and converted vegetation areas cause a 0.10 cubic feet per second or greater increase in the 100-year flow frequency from a threshold discharge area as estimated using the Western Washington Hydrology Model or other approved model and one-hour time steps (or a 0.15 cfs increase or greater using 15-minute time steps).
3. Standard Flow Control Requirement. Storm water discharges shall match developed discharge durations to predeveloped durations for the range of predeveloped discharge rates from 50 percent of the two-year peak flow up to the full 50-year peak flow. The predeveloped condition to be matched shall be a forested land cover unless:
a. Reasonable, historic information is provided that indicates the site was prairie prior to settlement (modeled as “pasture” in the Western Washington Hydrology Model); or
b. The drainage area of the immediate stream and all subsequent downstream basins have had at least 40 percent total impervious area since 1985. In this case, the predeveloped condition to be matched shall be the existing land cover condition. The map in Appendix I-G of the DOE Stormwater Manual depicts those areas which meet this criterion. Where basin-specific studies determine a stream channel to be unstable, even though the above criteria is met, the predeveloped condition assumption shall be the “historic” land cover condition or a land cover condition commensurate with achieving a target flow regime identified by an approved basin study.
This standard requirement is waived for sites that will reliably infiltrate on site all the runoff from hard surfaces and converted vegetation areas.
4. Alternative Requirement. An alternative requirement may be established through application of watershed-scale hydrological modeling and supporting field observations. Possible reasons for an alternative flow control requirement include the following:
a. Establishment of a stream-specific threshold of significant bed load movement other than the assumed 50 percent of the two-year peak flow;
b. Zoning and land clearing ordinance restrictions that, in combination with an alternative flow control standard, maintain or reduce the naturally occurring erosive forces on the stream channel; or
c. A duration control standard is not necessary for protection, maintenance, or restoration of designated beneficial uses or Clean Water Act compliance.
See Section 7, Basin/Watershed Planning, of Appendix 1 of the Municipal NPDES Permit for details on how alternative control requirements may be established.
5. Additional Requirement. Flow control BMPs shall be selected, designed, and maintained in accordance with Volume III of the DOE Stormwater Manual or an approved equivalent.
H. Minimum Requirement No. 8: Wetlands Protection.
1. Applicability. The requirements below apply only to projects whose storm water discharges into a wetland, either directly or indirectly through a conveyance system. These requirements shall be met in addition to meeting Minimum Requirement No. 6, Runoff Treatment Project Thresholds; and Minimum Requirement No. 7, Flow Control; and comply with the provisions of Chapter 14.162 LMC, Wetlands Areas.
2. Standard Requirement. Projects shall comply with Guide Sheets No. 1 and No. 3 in Appendix I-D of the DOE Stormwater Manual. The hydrologic analysis shall use the existing land cover condition to determine the existing hydrologic conditions unless directed otherwise by a regulatory agency with jurisdiction.
3. Additional Requirements. The standard requirement does not excuse any discharge from the obligation to apply whatever technology is necessary to comply with state water quality standards, Chapter 173-201A WAC, or state ground water standards, Chapter 173-200 WAC. Additional treatment requirements to meet those standards may be required by federal, state, or local governments.
Storm water treatment and flow control facilities shall not be built within a natural vegetated buffer, except for necessary conveyance systems as approved by the City Engineer or as allowed in wetlands approved for hydrologic modification and/or treatment in accordance with the Guide Sheet No. 2 in Appendix I-D of the DOE Stormwater Manual.
An adopted and implemented basin plan prepared in accordance with provisions of Section 7 of Appendix 1 of the Municipal NPDES Permit may be used to develop requirements for wetlands that are tailored to a specific basin.
I. Minimum Requirement No. 9: Operation and Maintenance. An operation and maintenance manual consistent with the provisions in Volume V of the DOE Stormwater Manual shall be provided for all proposed storm water facilities and BMPs. The party (or parties) responsible for maintenance and operation shall be identified in the operation and maintenance manual. At private facilities, a copy of the manual shall be retained on site or within reasonable access to the site, and shall be transferred with the property to the new owner. For short plats and subdivisions, a plat agreement to follow the long-term operation and maintenance plan shall be signed. For public facilities, a copy of the manual shall be retained in the appropriate department. A log of maintenance activity that indicates what actions were taken shall be kept and be available for inspection by the City. [Ord. 648 § 5, 2016; Ord. 501 § 3, 2009.]