STORM WATER FLOW CONTROL/DETENTION DESIGN REQUIREMENTS

Transcription

1 City of Mercer Island 9611 SE 36 th Street Mercer Island, WA (206) FAX (206) STORM WATER FLOW CONTROL/DETENTION DESIGN REQUIREMENTS General Requirements This guidance document applies only to projects or common developments disturbing less than one (1) acre of land when flow control is required. The applicable design manual is the 1992 Stormwater Management Manual for the Puget Sound Basin as prepared by the Washington State Department of Ecology (1992 Manual). Applicants have the option of using the 2005 version of the manual. Is stormwater detention (flow control) required for my project? Yes, if: The new construction/redevelopment project creates more than 500 square feet of impervious surface or The addition/alteration creates more than a 500 square foot net increase in impervious surface. No, if: Construction results in less than 500 square feet of impervious surface area. The stormwater runoff from the site must be managed properly either through use of low impact development (LID) techniques appropriate for the site, infiltration, collection and conveyance to an existing drainage system, or other acceptable manner. The project discharges runoff directly to Lake Washington. The applicant s downstream analysis performed by a civil engineer confirms that the entire downstream system is comprised of pipes/roadside ditches and the proposed project discharges will not exceed the conveyance capacity of the downstream system. Designing your detention system All stormwater detention system designs must be prepared by a professional engineer licensed by the State of Washington. 1. Construction that results in 500 to 5,000 square feet of impervious surface area Size flow control according to Table 2 (Standard Detention Pipe Design for Projects Between 500 SF and 5,000 SF Impervious Area) or through a specific facility design based on the 1992 Manual prepared. The configuration of the detention system shall be as shown on Attachment 1 (Standard Detention Systems Worksheet) or as specifically designed by the engineer for the site. 2. Construction that results in more than 5,000 square feet of impervious surface area Size flow control according to the 1992 Manual. The applicant may pay a fee-in-lieu-of constructing a stormwater detention system when allowed by the City Engineer. The fee will not be an option when in the opinion of the City Engineer, undetained runoff from the project may adversely impact an existing stormwater system problem (MICC15.11). Areas Where Infiltration and Low Impact Development (LID) Techniques are Acceptable The City encourages the use of infiltration and LID techiques to reduce runoff rates when designed properly and constructed in appropriate loctions. Infiltration should only be installed in locations where soils have suitable infiltration potential and will not affect slope stability. Table 1 identifies areas where infiltration/lid is generally not allowed without site specific geotechnical analysis and

2 approval. The 2005 Manual identifies certain construction materials and LID techniques that are eligible for partial credits toward reducing the size of flow control. (see below) Table 1 Infiltration/LID Limitations Erosion Hazard or Landslide Hazard Areas. Locations that do not provide suitable infiltration potential according to Mercer Island s Erosion Hazard Assessment Map available on the City s website at Setbacks for Erosion Hazard or Landslide Hazard Areas. Setbacks are typically calculated as ten (10) times the height of the Erosion Hazard (or Landslide Hazard Area slope) to a maximum of 500 feet. Within 5 feet of property line (excluding property line abutting the right-of-way) without written agreement from neighboring property owner. Within 10 feet (horizontal) of buried utilities that are within three feet (vertical) of the bottom of the finished infiltration facility. Where groundwater is within one foot (vertical) of the bottom design elevation of an infiltration facility. Within 10 feet (horizontal) of underground storage tanks. Within areas of streams or delineated wetlands (for buffer requirements see MICC). Within 100 feet of drinking water suppy wells or springs. Within 100 feet upslope from a septic tank or drain field. Within 100 feet of confirmed or suspected contaminated sites. Within a groundwater protection area. Reducing Detention System Size Through Infiltration/LID Figure 1 provides a flow chart showing steps for applying consideration of infiltration/lid to reduce detention system size and drawing submittal requirements. For projects between 500 square feet and 5,000 square feet, infiltration/lid is encouraged but not required. Applicants may use Infiltration/LID techniques as described in the 2005 Manual. Unless site runoff is completely infiltrated, credits shall be as follows: 50% of the area receiving infiltration/lid shall be considered as grass in the postdeveloped condition. The City may consider applying a greater credit if the applicant can demonstrate that the BMP will effectively infiltrate a greater percentage of runoff in the long term. Infiltration/LID design methods shall be designed in accordance with the 2005 Manual. The Standard Detention System worksheet (Attachment 1) must be submitted on 18 x 24 (minimum) size sheets and be prepared by a Professional Engineer. Sizing Detention Systems for Projects Adding Over 5,000 Square Feet of Impervious Area up to Less Than One Acre of Land Disturbance 1. The detention facility shall be sized using the SBUH methodology in accordance with the 1992 Manual and meet the following standards: a. The peak outflow for the 2-year, 24-hour storm from the detention facility shall match 50% of the peak flow for 2-year, 24-hour storm from the site under existing conditions. b. The peak outflows for 10-year, 24-hour and 100-year, 24-hour storms shall match the peak flows from the site under existing conditions for these same storm events. 2. Existing site conditions shall be calculated as 2 nd growth forest. 3. Increase the calculated detention volume by a correction factor according to the impervious cover of the site. Refer to Figure III-1.1 of the 1992 Manual to determine the appropriate correction factor. 4. Use design guidelines in the 1992 Manual to design the detention facility. A copy of the 1992 Manual is available for download at

3 Sizing Detention Systems for Projects Disturbing Greater Than One Acre of Land Disturbance Detention facilities shall be designed in accordance with the 2005 Manual. Figure 1 Does applicant want to implement infiltration/lid and is it feasible (check Table 1)? No Yes Go to Table 2 Determine amount of new impervious to receive infilitration/lid (e.g. 2,000 sq ft out of 4000 sq ft of new impervious area) Calculate new "net" impervious area and soil type Apply 50% credit for those services and determine target surfaces for Flow Control (e.g. 4,000 less 50% x 2,000 = 3000 sf). Note if the targeted surface is less than 500 sf, no flow control required. Select pipe diameter and determine required length (from Table 2) Using soil type and pipe size, determine orifice diameters and elevations (from Table 2) Complete "Standard Detention Systems" worksheet

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5 TABLE 2 - STANDARD DETENTION PIPE DESIGN FOR PROJECTS BETWEEN 500 SF AND 5,000 SF IMPERVIOUS AREA (WITH 120% CORRECTION FACTOR) New Impervious Area (sf) 500 to 1,000 sf 1,001 to 2,000 sf 2,001 to 3,000 sf 3,001 to 4,000 sf 4,001 to 5,000 sf Detention Pipe Size (in.) and Length (ft) Detention Pipe Size (in.) and Length (ft) Detention Pipe Size (in.) and Length (ft) Detention Pipe Size (in.) and Length (ft) Detention Pipe Size (in.) and Length (ft) Soil Type* 36" 48" 60" 36" 48" 60" 36" 48" 60" 36" 48" 60" 36" 48" 60" B C Detention Pipe Size (in) (feet) Outlet Size and Design Height for Type B Soils Only (feet) (feet) (feet) (feet) Detention Pipe Size (in) (feet) Outlet Size and Design Height for Type C Soils Only (feet) (feet) (feet) (feet) Minimum diameter = 0.5 inches * Geotechnical Analysis Required Basis of Sizing Assumptions: - Based on the Stormwater Management Manual for Puget Sound Basin (1992 Manual) - Converting 2nd growth forest to impervious - Assumes 0.5 foot sediment storage in detention pipe - Moderate slope - Includes Volume Correction Factor, assuming 120% safety factor