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Ms. Katie Larsen <br />04/10/2024 <br />Page 4 <br />Lino Lake Tech Center Engineering Review <br />5. (WSB 3/6/2024) It is recommended to model the downstream BMP in order to <br />understand potential tailwater effects and impacts to high water levels. <br />(Elan 3/26/2024) The plan has been modified since your last review. The outlet is <br />now directed to the wetland. We would be happy to look at the tailwater condition if <br />you are able to provide a hydrograph for the wetland. We could also create an <br />assumed synthetic hydrograph if the HWL is known. <br />(WSB 4/5/2024) Based on wetland size and elevation, this tailwater effect is not a <br />concern. <br /> <br />6. (WSB 3/6/2024) Exfiltration is not allowed in modeling in order to more conservatively <br />estimate high water levels. <br />(Elan 3/26/2024) This comment is not understood. The required WQ volume is met <br />without discharge. <br />(WSB 4/5/2024) Correct, WQ volume is not a concern with this comment. Infiltration <br />BMPs are requested to be modeled without exfiltration outletting in the case of long- <br />term clogging to ensure rate control and HWLs are still within allowable ranges. <br /> <br />7. (WSB 3/6/2024) Applicant to model the constructed BMP overflow. Current modeling <br />indicates that this overflow would be used and impact outflow rates in 100-yr event. <br />(Elan 3/26/2024) The overflow continues to be directed to the existing basin with an <br />overflow elevation equal to the calculated HWL. No discharge via the overflow is <br />intended for the 100-year or lesser events. <br />(WSB 4/5/2024) HydroCAD modeling shows a 100-yr HWL of 901.99 and the EOF <br />on sheet C-201 is called out as 901.75, meaning it would see discharge for the 100- <br />yr event. Additionally, more flows could be routed over it based on the exfiltration <br />outlet removal of Comment 6. <br /> <br />8. (WSB 3/6/2024) This review finds the minor increase in outflow rates during 10-year <br />storm event is within modeling error. However, the City will defer to RCWD <br />comments if they find this increase unacceptable. <br />(Elan 3/26/2024) Runoff is reduced in all events in the updated model submitted to <br />RCWD. <br />(WSB 4/5/2024) This comment remains open based on the items discussed in <br />Comments 6 and 7. <br /> <br /> <br />9. (WSB 3/6/2024) Based on pervious area berm flowing to CB 4, a Tc of 10 minutes is <br />more appropriate and might ease pipe capacity limitations. <br />(Elan 3/26/2024) The pipes are sized for a 10-year event with a 5 minute Tc. There is <br />no capacity issue. <br /> <br /> <br />10. (WSB 3/6/2024) Applicant to confirm constructability of CB 4. Current rim and inverts <br />don’t appear to leave enough concrete leg above pipe cutout. <br />(Elan 3/26/2024) The storm sewer system has been renumbered since there was a <br />STMH 4 and CB4. CB 4 is now CB 5. It is a 2’ x 3’ structure with 2.06’ of cover over <br />the pipe. This structure is constructable. <br /> <br /> <br />11. (WSB 3/6/2024) Applicant to confirm constructability of V-notch in BMP OCS. Detail <br />appears to indicate notch continues up to structure rim, which would impact mounting <br />of skimmer grate. <br />117