1402 SE EVERETT MALL WAY REGAL CINEMAS 2018-01-01 MF Import

Paul McKee Frem: Jane Zimmerrran Sent: Tuesday, September 21, 2004 7:06 AM To: 'Robert Ashton' Cc: Darren Simpson (E-mail); James Palda (E-maii); Linda Joliannes (t-mail); Boyd McPherson (E-mail); Paul McKee Subject: RE: Fverett Mall Off-Site conveyance ii_ �.01:�-- - - I. have received your e-mail. I apologize for not being at my desk b��ween 1?_:30 ar.d 1:00 ��esterday as I said I would - my attention was naeded elsewhere. '7he main difficulty I see with your proposal to bypass the off-site runoff around the r:s,erett Mall stormwater treatment facilities concerns the difficulty in determining hoi:� much attenuation of stormwater flows is occurring in the existing system. As I've saici, i.he origi,^.al design calculations and criteria tor the east and west pond pre-date the City's microfilm file for the Everett Mall. 47hen the what-is-now Plervyns building and �ssociated parking was developed, documents in the file indicate that the original detetention system calculations could ❑ot be found by either the City or the applicant (the mall cwners). The desire t.o simplify the design, by not modifying che east and ��:est_ rietention system, appears to tiave be�n tha primary reason for the design and constructicn oi the separate pond to the south. Lecause we have no knowledge of what tne "design flows" from the east/west detention crystem were, they would have to be determined as par.t of this project were the off-site. �lows to be bypasse� around the syst2m. This task would involve generating inflow i�:�drographs for the 2-, 10-, and 100-year storms, determining the stage-discharge-storage relationships of the east/west pond system, and routing the inflow hydrographs through th�:� pend system to determine the existing release rates. i,nd once the substantial off-site flows were routed around the pond, and didn't have to c,ass through the existing 29-inch outlet from the west pond, the outlet would have to be modified to provide additionai detention for the Everett Mall runoff in order to compensate for the bypassed flows. i�.t this point, I am thinking back to my 9/14/04 e-mail message and wondering if I��:as (i�terpreted as saying that the wetpond system has to be sized for this off-site runof: and t:hat's why you're 9oing to the trouble to try and bypass the off-site drainage. That isn't the messa9e I intended to convey and I would like to provzde additional clarification. If the off-site runoff is brought into the wetpond system, my main concern is with the sediment loading and the effects on the function of the wetpond system. However, removin9 this 'bff-site" sediment provides a definite benefit to the downstream system. It's true that the wetpond system will function less effecti.vely iE it isn't ..�:.�ed to provide the 6-month, 29-hour storm dead storage volume for the off-site runof?. 'iowever, a less eflecti.ve function is balanced by the removal of the sediment. And I �.hink the benefits of basic treatment (i.e., sediment removal) of the off-site runcf?' �lso ��.::stifies waiving the City requirement for a vegetated swale in series with a��:etpond =or •�i�,� new and!or redeveloped povement... ., I would encourage you to convey the off-siT.e runoff through the wetponciidetcntion �stem. As long as the off-site runoff received basic treatment in the fo�m oi sedimenc .��=moval before it was discharged to the wetpond/detention system, the wetpond volume would .�ct have to include the G-month, 2A-hour storm volume for the off-site basin. City .;r_andards require a"wet" vault/tank for basic sediment removal. However, you may wisi: r_e �>:piore an alternate sediment removing system, such as a StormCeptor. Again, my prim�r,� .;oncern is effective removal of "off-site" sediment prior to discharge to the �aetpond ;:ystem, so that the wetpond system is not compromised by the sedimenc loading. 'i-ir: wetpond dead-stor.age voimae ior this c,rojecc is "only" required *o be t:he 6-nonth, 2�- :.net' runoff volume from the contributin9 F.verett h1a11 area - even it the oti-site runofi ... ...,... ,��J tc nc� wetc�nd.