607 RIVERSIDE RD 2021-01-29 - Laserfiche WebLink

CITY OF EVERETT fig P REVIEWED FOR PERMIT PVJ180 -0 PROJECT MEMO telAkE 7- -ig OHBL TO: Erik Emerson DATE: 7/5/2018 FROM: Bart Brynestad PROJECT NO.: Glacier Peak at Riverside(PW1801-037) Tacoma-(253)383-2422 PROJECT NAME: 2170746.10 SUBJECT: Response to Comments received 7/3/2018 This memorandum summarizes our responses to your comments provided via email from Sabrina Fandler on 7/3/2018. Your comments are provided verbatim with our responses below. 1. In my previous comments I noted that a Rational Method analysis must be provided in accordance with DCSS Section 4-2.2(1) and(3). Section 4-2.2(3). The report includes a well laid out analysis of the 25 year storm backwater condition, but does not include a 100 year analysis as required per section 4-2.2(3). Since the 25 year backwater analysis shows that the system is at capacity with a small amount of ponding it is necessary to run the 100-year backwater analysis. Although ponding is allowed in the 100 year event we need to be reasonably sure that the ponding will not cause significant safety concerns or property damage. Especially of concern are the following locations: • CB's#8 and#9 have water levels close to the rim elevations in the 25 year analysis. If these CB's overflow the water will immediately run down the entrance slope into the street system, a system that has little to no excess capacity available. • CB's#11 and#12 overflow in the 25 year analysis. The analysis needs to show that the 100 year ponding will not create a dangerous condition for drivers or overflow to unwanted locations. Response: To accommodate the 100-year conveyance standards I upsized pipes between SDCB#6 and SDCB#11.This ensures that there is less than 4" of ponding for this event(only SDCB#12).The 25-year model was also updated to reflect these upsized pipes and there is neither overtopping nor ponding during that event.Attached are the updated drainage plan sheet and the updated rational method model results for the 25-year and 100-year events.The system now complies with the City's conveyance standards. 2. In addition, although it wasn't previously commented on many of the pipes are quite shallow and likely don't meet the recommended cover. If it is necessary to upsize some of the pipes to convey the 100 year flows it may be necessary to change the pipes to ductile iron. The engineer should verify the pipe materials for the upper end of the system, especially in consideration of the use of the site by large trucks. Response: Both pipes have been checked for appropriate cover. Pipes near the southwest driveway are now ductile iron to accommodate a minimum cover of 1.0'at this location.This depth is acceptable for ductile iron pipe. Some other shallow CPEP pipes along the west side of the building have at least 1.0' of cover,which is acceptable for ADS N-12 (CPEP) per the manufacturer's recommendation.Additionally,these shallow pipes are all on private property as part of a private drainage system. BEB/ ql). 0 tit JUL 10 2018 : (______,,L,,,, ,, CITY OF EVERETT Permit Services Page 1 of 1