Per the City’s updated stormwater code, sites that require stormwater management must complete some or all of the minimum requirements (MR) from Appendix 1 of the Phase II Permit. Category 1 project sites (≥ 2,000 and < 5,000 ft2 of impervious surface) are required to complete MR#1 through MR#5; Category 2 project sites (≥ 5,000 ft2 of impervious surface) are required to complete MR#1 through MR#9 (for more information, see Edmonds City Code Section 18.30.060 – Requirements). The most significant change to the Stormwater Code is regarding MR#5, On-Site Stormwater Management. For sites that propose ≥ 2,000 ft2 of impervious surface, there are two options – choosing from a list of LID stormwater management best management practices (BMPs), or conducting an LID performance standard modeling exercise. The list of BMPs (see Table 1 below) are differentiated by the surface type being managed (lawns and landscaped areas; roofs; or other hard surfaces such as driveways). The options must be considered in the order listed, and the first feasible option must be used.
Table 1. List of LID BMPs by surface type
|Lawn and Landscaped Areas
||Other Hard Surfaces
|1. Soil quality and depth
||1. Full dispersion or downspout
2. Rain gardens or bioretention
3. Downspout dispersion
4. Perforated stub-outs
5. Detention pipes or vaults
|1. Full dispersion
2. Permeable pavement or rain
gardens or bioretention
3. Sheet flow dispersion
4. Detention pipes or vaults
A few notes on the list of LID BMPs. Due to the requirement of a substantial onsite vegetated drainage area to implement full dispersion/downspout full infiltration, these BMPs are typically not considered for urban situations such as Edmonds. As for the selection of rain gardens or bioretention, this depends on the project site category - category 1 sites can choose rain gardens or bioretention, while category 2 sites can only choose bioretention. Although rain gardens and bioretention are both landscaped depressions that infiltrate and treat stormwater runoff, rain gardens do not need to be engineered, while bioretention facilities are engineered for specific flow control rates and treatment objectives, with specific soil mixes and even underdrains. And finally, although not LID techniques, detention pipes or vaults are the final option should all other LID BMPs prove to be infeasible. This allowance is a City of Edmonds specific standard.
Another important change involves the tracking of impervious surface area on individual properties. Before 2017, for projects that required City permits, existing (constructed after July 7, 1977 or the date of annexation into the City if after July 7, 1977) unmanaged impervious surfaces would be subject to stormwater management requirements if the total of existing plus proposed impervious surfaces totaled more than 2,000 ft2. Per the updated Stormwater Code, for projects requiring City permits, the City will not be tracking existing unmanaged impervious surfaces. Instead, the City will require that 25% of the existing unmanaged impervious surfaces (or no more than the amount of new plus replaced impervious surface being added) be “retrofitted” to be brought up to current standards. This is in addition to management of any new impervious surface proposed for the project.
Finally, the City is revising the Stormwater Code Reference Documents. These documents include the Stormwater Addendum, LID BMP Infeasibility Criteria, Infiltration Rate Methods, and Project Checklists. The Addendum is complete; the Addendum Appendices are being finalized and will be added to the Addendum as soon as they are completed. Target completion date for the Appendices is February 2017. Please check the City’s Engineering Codes and Design Information web page (http://www.edmondswa.gov/development-a-design-stds-text/engineering-codes-design.html) for up-to-date information. Or contact the Engineering Division at 425-771-0220 or firstname.lastname@example.org.