500 SE EVERETT MALL WAY A BASE FILE 2018-01-01 MF Import

. TO: FROM: DATE: SUBJECT: PUBL/C WORKS MEMORANDUM Kirk Brooks/�� /y�,pbl2r Jane Zimmerman July 11, 1997 Drainage Calculation Review, Robinson Retail I ' . The storm detention system design and calculations must account for any changes due to clearing of phase II, even if impervious surfaces won't be constructed at ihis time. Since the pond will be reconstructed and relocated at z future da[e, and since a site plan (and impervious surface calculations) have not been developed for phase II, the s[ormwater facilities should be designed for the land alteration and construction that will occur for phase I only. It is unclear from the calculations whether the stage- storage relationship and orifice design are valid for the reduced-area pond. In addition, the vege[ated swale will collect any runoff from the remainder of the site and convey it to the pond, even if phase II hasn't been constructed. The calculations must reflect this contribution of runoff', in both the detention and treatment design. Existing conditions should be documented, as they are, to allow the determination of the allowable detention releases for the stormwater facilities which will serve the entire site. City standards require a two-cell wetpond design, with 3 to 6 feet of dead storage provided in the first cell (called the open water cell in the Ciry's standardsj or in a wetvault. The volume and depth nf dead storage provided in the open water cell must be stated in the calculation package. A maximum of 85% of the wetpond design volume may be associated with the open water cell. The minimum volume that must be provided in the open water cell can be calculated using [he City's wetvault standards. The volume and depth of dead storage provided in the wetland treatment cell must also be stated. A minimum of 15% of the required permanznt storage volume, with a