10330 4TH AVE W 2016-01-01 MF Import

For the computer model, the existing conditions were considered as they were prior to the development of the existing South Pointe Assisted ;_iving Facility and the developed conditions conside�ed both the e�tisting South Point Assisted Living Facility and the proposed expansion as the new developed. Using a one inch minimum orifice, a Flow restrictor has been designed to release detained runoff under developed conditions 3l 50% of the two year storm and 100% of the l0-yr and I00-yr storm events under predeveloped conditions Water quality treatment for the proposed project wil; occur place in the wet pond. As prescribed by the City of Everett, the wet pond is a two cell facility with the first cell being three feet and the second cell being two feet, and wii! retain the volume of runoti generated during the six month storm event under developed conditions (?.,269 ft'). Runoff enters the pond in the northwest corner (Cell "A") of the open water cell, and is conveyed to vegetated treatment cell via a notch in the baftle wall. The volumes for each of the water quality cells was calculated using the "average end area" method and the stage/area relationship. The open water cell volume is 1,126 ft', or 40% of the total water quality volume provided, and the water treatment cell volume is 1,748 ft�, for a total retained volume of 2,874 ft�. Per the C.O.E. Stormwater Management Manual, Section 3-3.5.1, the water treatment cell will be planted with wetland vegetation at a density of one plant per 2.25 ft2. The following table presents the plant species, number to be planted, and the elevation rage over which each species is to be planted: Runoff rates and volume calculations were performed using SBUH and SCS Curve Number methodologies in "StormSHED" by Engenious Systems. The proposed detention system incor�orates a 33% volume safety factor (C.O.E. Stormwater Management Manual Figure I-3.1 and a 54% impervious area under developed conditions) and one foot of free board. The release rates, detained volumes, and associated stages for the two, 10, and 100 year events, as well as the flow restrictor specifications, are in Section [I of this report. UPSTREAM/DOWNSTREAM CONDITIONS: The proposed development is southwest of the intersection between SW Everett A1all Way and 4'� Ave W, with a high point near the northeast comer of the subject property. Consequently, runoff from directions of SW Everett Mall Way and 4�' Ave W is topographically prevented J: REM)Rl'S— 99I78�9927RD.Aoc 3 S��