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2717 Roekefeller Av�nu� <br /> p O M E GA EvereK, WA 98201 <br /> � ENGINEERING, INC. a2s.sa�.sazo <br /> fax 425.259.1958 <br /> ('w ( I03- dd � <br /> April 6, 2011 <br /> City of Everett <br /> 3200 Cedar Street <br /> Everett, WA 98201 <br /> RE: Drainage Letter for PSKC Part,ing Lot Revisio�s. <br /> Dear Staff, <br /> This letter is intended to provide the necessary information to describe the changes to the <br /> original site plan and support the engineering plans submitted for this project. This site is <br /> located east of Nassau Street and North of Pacific Avenue, in Everett. The site has recently <br /> contained two single-family residences which have been removed and currently contains <br /> landscaping and limited impervious surfaces. The proposed project will provide a total of 18 <br /> parking stalls. Runoff from the existing parking lot is collected by a catch basin with a <br /> downturned elbow and conveyed west to the existing storm/sewer system within Nassau Street. <br /> All runoff from the impervious surfaces on the final project will be conveyed to the existing <br /> storm/sewer system within the roadway to the west. <br /> The origi�al application showed a parking lot with approximately 6,000 sf of impervious <br /> surfaces. Based on the City's definition of new impervious surfaces within the 2010 stormwater <br /> manual this would have required flow control. Therefore, tfie parking �ot has been revised to <br /> reduce the total effective impervious surface area to 4,890 sf with an additional 200 sf of <br /> grasscrete which is not consideied an effective impervious surface and is therefore removed <br /> from the area calculations. As a result this project now proposes to construct less than 5,000 sf <br /> of effective impervious surtaces and is exempt from flow control. However, on-site stormwater <br /> manayement EMPs are provided to meet the conditions/intent of Minimum Requirement#5. <br /> This project proposes to finish removing the existing improvements on lhese lots and construct <br /> a parking lot. Approximately 4,890 sf of impervious surfaces and 200 sf of grasscrete or an <br /> equivalent product will replace the previous homes and driveways of similar areas. The actual <br /> impervious areas that were on this site prior to the demolition of the existing homes is not known <br /> but in reviewing the aerial photos and observing the site today it appears that a similar amount <br /> of impervious surfaces were previously on this site. <br /> For this project two catch basins will be instailed to collect the runoff from the parking and <br /> landscaping areas. In addition, curb cuts will be placed 5-feet O.C. along the western parking <br /> curb iine to disperse flows from some of the parkina lot over the amended soils in the <br /> landscaped area between the parking and the R.O.W. CB #1 will be installed with a tee on the <br /> outfall to prohibit any vapors from the combined storm/sewer system from backing up into the <br /> new drainage system. A detail has been provided on sheet C3 of the engineering plans <br /> s��hmitted for this project. The new 6" PVC pipe connecting CB #1 to the combined storm/sewer <br /> system has been analyzed for capacity to ensure it can accommodate the anticipated flows. <br /> In order to remain conservative, it has been assumed that 100°/a of the parking lot runoff will be <br /> collec?ed and con�deyed through the CB #1 outfall pipe. This pipe is a 6" PVC pipe with a slope <br /> � <br /> /' <br />