10-02-2023 Council Work Session Packet

Home Search

Ms. Katie Larsen 09/27/2023 Page 2 Glamos Wire Storage Expansion Engineering Review 5. Verify whether there is an existing storm sewer structure east of what is shown on the plans. The storm sewer is not shown starting at a structure. (BM 9/21/2023) Drain tile from the previous rain garden enters the storm sewer structure from the east. There is no other storm sewer structure east of the catch basin manhole. 6. The fence and vehicle gate are shown crossing the drainage swale at the northwest corner of the storage area. Verify that these do not impede drainage or will collect debris that would impede drainage. (BM 9/21/2023) The vehicle gate has been deleted. A note has been added to Sheet C1.01 calling for a clearance of 0.5 feet between the channel bottom and the base of the fence. A summary of the open channel depth is 0.65 feet. The depth at the upstream side of the fence will increase 0.29 feet as the hydraulic control changes from open channel to orifice control. The 100-year depth just upstream of the fence will be 0.94 feet, less than half-full in the channel. If debris collects at the corssing it will be removed by Tomas Company maintenance staff. • Stormwater Management The stormwater management for the storage expansion consists of an existing rain garden and proposed sedimentation basin. Based on the HydroCAD modeling and water quality calculations provided in the updated stormwater report, this project meets City requirements for rate control and water quality treatment. Pre- and Post- Development Discharge Rates (cfs) Condition 2-Year 10-Year 100-Year 10-Day Snowmelt Existing 22.91 36.73 70.61 Not provided Proposed 20.88 33.02 61.51 Not provided Comments: 1. The applicant shall clarify the proposed rim elevation of the pond OCS. The detail shows an elevation of 910.4 and the drainage plan sheet shows 908.1. (BM 9/21/2023) The Rim elevation has been updated in the plans. 2. The applicant shall revise the BMP design in the northwest corner of the basin. Locating an inlet that flows over the OCS and into the basin will result in flow shortcutting and potential long-term damage to the structure. It is recommended to shift the inlet location, OCS, or both to avoid this risk. (BM 9/21/2023) The 18” inlet pipe to the OCS has been realigned and extended to draw water from the southwestern corner of the pond. 3. The applicant shall revise the riprap shown on the plans to cover the surface down to the normal water level of the pond. Current plans limit riprap to an elevation of 908, leaving approximately 10’ of unarmored ground at risk of erosion from flows into the basin. (BM 9/21/2023) Riprap has been extended to the normal water level on Sheet C3.01