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REVIEWED Storm_Drainage_Report+10.13.2021_10.55.40_AM+2460097m J V1 N v FNGINeEP Drainage Memorandum To: City of Edmonds From: Chris Tebaldi, P.E., Tebaldi Engineering, LLC Date: October 4, 2021 Re: Alexander Single Family Residence Addition 10/4/2021 This project is located in City of Edmonds on tax parcel number 00456000001000. The project proposes to construct an addition to the existing single-family residence on the subject property. The existing topography ranges from elevation 358 to elevation 326. The site gently slopes from east to west in the vicinity of the proposed addition and the slope ranges from 0 to 5 percent. The property is currently an existing single- family residence with associated driveway, pavement and landscaping. The proposed addition consists of a re -roof of a portion of the existing roof, additional roof overhang and new deck and stairs. The proposed project is subject to the City of Edmonds stormwater requirements which follow the 2014 Department of Ecology stormwater requirements. The portion of re -roof would not be considered new or replaced impervious since there is no demolition down to the foundation of the structure. The new portion of roof (approx. 568 square feet) is considered new/replaced impervious surface. The proposed deck (approx.. 324 square feet) would not be considered impervious surface. Although, even including the deck, the project would be less than the thresholds that trigger LID and flow control as the total new/replaced impervious surface is less than 2,000 square feet. The proposed roof addition gutters will be directed to the existing downspouts and storm drainage system. www.tebaIdiengineering.com 4625 126t" Ave E, Edgewood, WA 98372 (206) 450-5096 Construction Stormwater Pollution Prevention Plan (SWPPP) 1. Preserve Vegetation/Mark clearing limits: Prior to construction, mark the clearing limits of the area that is to be disturbed in accordance with the approved plans. Make sure to clearly mark trees to remain to protect them from being disturbed during construction. Retain the duff layer, native topsoil, and natural vegetation in an undisturbed state as much as possible. 2. Establish construction access: Install rock construction entrance per the approved site development plans. It is important to make sure you are not tracking sediment and debris offsite during construction. 3. Control flow rates: Ensure that silt -laden (dirty) water is not leaving the project area and that you are not increasing the amount of water that would typically leave the project site. 4. Install sediment controls: Install erosion control measures (e.g. silt fence and inlet protection) per the standard details. Silt fence shall be located along the low sides of the project area per the approved plan in order to prevent sediment and dirty stormwater from leaving the project area. Additional sediment controls may be required by the site inspector. 5. Stabilize soils: Soils cannot be exposed and unworked for longer than 7 days during the dry season (May 1 — September 30) or 2 days in the wet season (October 1 — April 30). Soils can be stabilized several ways, including temporary or permanent seeding, mulching, net and blankets, plastic coverings, sodding, topsoiling/composting, and beauty bark. 6. Protect slopes: Construct cut and fill slopes so that erosion is prevented. Divert stormwater or groundwater away from slopes and disturbed areas with swales, interceptor dikes, and/or pipes. 7. Protect drain inlets: Keep dirty stormwater from entering nearby catch basins and drainages. Protect stormwater inlets (e.g. catch basins, yard drains, culverts, etc.) by installing catch basin inserts, use of sandbags or straw wattles. 8. Stabilize channels and outlets: Stabilize all temporary drainages to prevent erosion. This can be done by using armoring materials (e.g. grass and riprap) adequate to prevent erosion of outlets, slopes, adjacent stream banks, and downstream reaches at the outlets of all conveyance systems. 9. Control pollutants: Handle and dispose of all pollutants so they do not contaminate stormwater. For small projects, controlling pollutants is primarily related to concrete handling. Assure that washout of concrete trucks is performed in designated concrete washout areas only. Do not wash out concrete truck drums or concrete handling equipment onto the ground, or into storm drains, open ditches, streets, or streams. Concrete tools shall be washed in formed areas awaiting concrete or containers. 10. Control dewatering: Foundation, vault, and trench dewatering water shall be treated like other stormwater onsite and can be directed to your sediment control devices, such as silt fence or straw wattles. IL Maintain Best Management Practices (BMPs): Ensure that 13MPs (e.g. silt fence, construction entrance, and inlet protection) are functioning properly throughout the duration of construction. For example, silt fence should be upright and should not have sediment build up greater than one third of the height of the fence. Make sure you are not tracking sediment and debris offsite onto adjacent roads. Temporary erosion and sediment control BMPs shall be removed within 30 days after the site is stabilized or after they are no longer needed. 12. Manage the project: Think about what time of year construction is anticipated to occur and if any adjustments are needed to accommodate weather patterns. Clearing, grading, and soil disturbing should be avoided or limited from October lst through April 30ti' if possible. Install additional items to prevent stormwater pollution as necessary. 13. Protect Permanent Low Impact Development (LID) Best Management Practices No LID facilities are proposed with this scope.