4220 COLBY AVE B 2020-05-05 - Laserfiche WebLink

Proposed Site Information: 1. How will your project change the existing conditions? The proposed project will extend the existing alley storm drain pipe to the south property line and then a private short storm drain extension, to included an inline grass filter swale, along the south property line with a catch basin to be placed at the existing low point at mid property line. The proposed new parking lot will drain to the grass filter using a section of notched concrete curb to spread the runoff along the grass swale, together with channel drain fronting the proposed building entrance with outfall to the grass filter swale. The grass swale overflow is then directed to the new storm drain extension. This will intercept and confine all site runoff within the property, benefiting the existing south neighboring building,in addition to providing a minimal level of water quality treatment. 2. Please provide the following quantities of proposed hard surfaces in square feet: Proposed Roof Area: 2,116 sf Proposed Other Hard Surfaces(pavement,gravel,etc.): 5,086 sf 3. How will you manage rooftop runoff after the project is completed? All roof downspouts from thc existing and thc proposcd addition will arc to be collected in a perimeter tightline and discharged at the grass filter or at the south catch basin. See evaluation below under Plan for Permanent On-Site Stormwater Management 4. How will you manage stormwater runoff from hard surfaces(driveways, paved pathways, patios,etc.)? Evaluate each type of hard surface separately. The proposed paved parking lot slopes to southeast where interceptor extruded concrete curbing is to intercept the runoff directing it to a channel drain fronting the proposed building first level westerly entrance and/or over the pavement through a notched curb section perpendicular to sheet flow over the grass swale. Plan for Permanent On-Site Stormwater Management: You are required to "infiltrate, retain, and disperse stormwater" on your site to the maximum extent you can without causing erosion or flooding. The goal is to get stormwater runoff in contact with vegetated surfaces as much as possible before discharging to downstream conveyance systems. See Volume Ill, Chapter 12 of the 2010 Storm water Management Manual for stormwater facility requirements: https://everettwa.gov/DocumentCenter/Home/View/1214 1. Determine which options you must use for managing stormwater from rooftops via downspout disconnect by considering the following Best Management Practices (BMPs) in order: • BMP 12.30 On-site Infiltration Areas • BMP 12.40 Raingardens • BMP 12.50 Downspout Dispersion Notes: a. If location criteria can be met, but sizing criteria cannot,you must implement the BMPs to the maximum extent you can without causing erosion or flooding. Pre-sized facilities have been designed to meet 100%mitigation. Facilities may be able to be installed to achieve some stormwater mitigation. Page 3 of 7 CITY OF EVERETT 13200 Cedar Street I Everett, WA 98201 ( 425-257-8810 I everettwa.gov