9512 EVERGREEN WAY 2022-08-29 - Laserfiche WebLink

MINIMUM REQUIREMENT NO. 3: SOURCE CONTROL OF POLLUTION The proposed development will incorporate Source Control BMPs from the SWMMWW Volume II §4.1 as needed throughout construction to reduce the risk of pollutants entering public waterways. Additionally, the project will incorporate the following Operational and Structural BMPs from the SWMMWW Volume IV: • Applicable (Mandatory) Operational Source Control BMPs • S411 BMPs for Landscaping and Lawn/Vegetation Management • S417 BMPs for Maintenance of Stormwater Drainage and Treatment Systems • S421 BMPs for Parking and Storage of Vehicles and Equipment • S430 BMPs for Urban Streets (for parking lot sweeping) MINIMUM REQUIREMENT NO. 4: PRESERVATION OF NATURAL DRAINAGE SYSTEMS AND OUTFALLS Stormwater on the site currently flows overland, is captured by a network of catchbasins and pipes, and then is discharged to the Evergreen Way stormdrain system. The proposed development maintains this pattern with the pump station discharging to Evergreen Way. Additionally, overflows will be directed to Evergreen Way similar to the current overflow path. MINIMUM REQUIREMENT NO. 5: ON -SITE STORMWATER MANAGEMENT According to the SWMMWW, the project follows List #2: Onsite Stormwater Management BMPs for Projects Triggering Minimum Requirements #1 through #9. The project is proposing bioretention to meet these requirements. See section 5 for an LID site assessment. MINIMUM REQUIREMENT NO. 6: RUNOFF TREATMENT Runoff will be treated to enhanced basic water quality standards. Treatment will occur in a Filterra unit and bioretention cell. Enhanced Basic water quality treatment targets total suspended solids and metals. The treatment facilities are designed to treat up to the 91 st percentile, 24-hour runoff volume as estimated by MGSFlood, an approved continuous runoff model. Refer to Section 5 for additional information. MINIMUM REQUIREMENT NO. 7: FLOW CONTROL As the project is adding more than 10,000 square feet of hard surfaces, the development is required to meet the standard flow control requirement. This requirement states, "Stormwater discharges shall match developed discharge durations to pre -developed durations for the range of pre -developed discharge rates from 50% of the 2-year peak flow up to the full 50-year peak flow." Per correspondence with city staff, the onsite and frontage improvements are considered separate projects with respect to minimum requirement thresholds. As the frontage improvements are only adding approximately 3,000 square feet of hard surface, frontage flow control is not required. See Appendix B1 for correspondence with city staff and Section 5 for additional information. MINIMUM REQUIREMENT NO. 8: WETLANDS PROTECTION No wetlands or streams are contained within the project site. Evergreen Branch Library Expansion — Dykeman, Inc. 3