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HomeMy WebLinkAboutSmall Site Stormwater Quality Treatment PERMIT CENTER TECHNICAL MEMO ON SMALL SITE STORMWATER TREATMENT MEASURES Informational Brochure October 2004 Prepared by Development Section Engineering Division Public Works Department 25 West Main St. Auburn, WA 98001-4998 (253) 931-3010 FAX (253) 931-3053 The following technical memo was created to aid Developers and Contractors in design and construction of water quality and quantity control measures for small sites. A small site is defined as containing less than 20,000 square feet of impervious area. A site’s impervious area includes pavement and roof areas. This summary includes information from Auburn’s Design Standards and Standard Details. This technical memo is not intended to replace Chapter 6 – Storm Drainage of the Design Standards. Its intended use is for small site’s where it is not feasible to install standard storm retention or detention facilities. The treatment methods described herein are subject to additional requirements depending upon site-specific uses. Impervious surfaces subject to high vehicular use and/or discharging to sensitive areas may be required to provide additional measures from those described below. Treatment of Storm Water 1. Bio-Swales Bio-swales are generally the most cost effective water quality treatment method for small sites. Bio-swales should be designed using the following criteria: A. Bio-swales should be designed to treat sixty-four percent (64%) of the two (2) year post-development storm to insure that it takes storm water at least nine minutes to travel from one end to the other. For example, for a flow of approximately one tenth of a cubic foot per second (0.1cfs) this can be achieved with a bio-swale one hundred feet (100’) long and two feet (2’) wide with a slope of two percent (2%). By changing the slope and/or the configuration of the bio-swale, the area needed can be adjusted to best fit the site. B. After water has passed through the bio-swale it is then collected and directed to either a detention facility, a retention facility (infiltration trench), or if permitted, the City system. C. Bio-swales shall be designed above seasonal high ground water elevations. D. In areas with high infiltration rates, liners or impermeable material shall be used to preclude infiltration prior to traveling the full distance of the bio- swale. E. Bio-swales shall be established using sod. Routine maintenance needs to be done to insure that the vegetation is healthy and the bio-swale is functioning as designed. Page 1 of 6 REF. H\DEVELOPMENT\HANDOUT\Technical Memo on Small Site Storm Water (10-04) Bio-swales may not be suitable for all small sites. Below are two conditions that need to be taken into account when determining if a bio-swale is the appropriate treatment method for a particular site. 1. Bio-swales require a certain amount of area that would then be unusable for other purposes. Small sites tend to be limited in the amount of area available for storm systems. 2. Bio-swales require a change in elevation from the entering point to the exit. Depending on the layout of the storm water conveyance system, this elevation change could create a swale that is too deep to easily maintain. 2. Pollution Prevention Devices Pollution Prevention Devices may be used for sites that have been determined to be unsuitable for Bio-swales. Pollution prevention devices refer to any type of structure where by the use of filters, baffles, weirs, and/or other mechanical means, storm water is cleaned as it travels through a structure. A couple of the more common makes of these TMTM TM devices are StormFilter Stormcepterand Vortechnics. These devices are designed to remove suspended solids and floating contaminants from storm water. Since this is a new technology, the City is not able at this time to recommend a specific device or set forth standards within which these devices should be implemented. The engineer should present information supporting the use of a specific devise. (see attached) 3. Down Turned Elbows Down turned elbows will only be allowed for sites with impervious area (excluding roofs) of less than 2600 square feet or with site retrofitting where other water quality systems are determined by the City to be impractical. For these sites water quality shall be addressed as shown in the City of Auburn’s standard detail STORM-03. (see attached) Dispersal of Storm Water Infiltration is the preferred method of dispersal of pretreated storm water on small sites. Where infiltration rates are too low to allow for the practical dispersion of storm water, connection to the City system will be allowed when available. In these cases, the onsite storm system shall be designed such that the maximum post development discharge to the City system is limited to one tenth of a cubic foot per second (0.1cfs) or the pre-developed condition, which ever is greater. The system shown in Storm-03 may be used as a water quantity device on small sites where this issue arises. On sites where infiltration and a City connection are not practical a detention pond may be required. Page 2 of 6 REF. H\DEVELOPMENT\HANDOUT\Technical Memo on Small Site Storm Water (10-04) Page 3 of 6 REF. H\DEVELOPMENT\HANDOUT\Technical Memo on Small Site Storm Water (10-04) Page 4 of 6 REF. H\DEVELOPMENT\HANDOUT\Technical Memo on Small Site Storm Water (10-04) Page 5 of 6 REF. H\DEVELOPMENT\HANDOUT\Technical Memo on Small Site Storm Water (10-04) Page 6 of 6 REF. H\DEVELOPMENT\HANDOUT\Technical Memo on Small Site Storm Water (10-04)