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Prepared by: Bolton & Menk, Inc. PROPOSED IMPROVEMENTS 2026 PMP Street & Utility Improvements ǀ 25X.137753 Page 8 normal water level. Submerged pipes that outlet stormwater into static, or landlocked, water bodies typically build up sediment due to standing water. The outlets into Karth Lake were unobstructed likely due to the following: • Sufficient Flow Rate and Velocity - MnDOT recommends RCP stormwater pipes are designed to have a full flow velocity of at least 3 feet per second. This velocity is typically found to ‘self-clean’ stormwater pipes. Sand and small suspended solids are unable to drop out of the stormwater flow. • Depth of Receiving Water - Karth Lake has sufficient depth and volume for water to freely exit each outfall. No underwater deltas have formed creating a hydraulic jump for water and sediment to backflow into the storm outfalls. Additional risks associated with submerged pipes include: • Freeze/Thaw Cycles – Submerged pipes are subject to increased risk of pitting and cracking due to expanding ice in winter months. Throughout the lifespan of the pipe the structural integrity is much lower than a non-submerged pipe. Many factors can contribute to how fast a submerged pipe cracks including water depth, bedding material, pipe diameter, and weather. Ice build-up is another risk that could lead to upstream flooding particularly in snow melt events. If an outlet is completely blocked with ice, snow melt will be unable to discharge from the drainage system and present surface flooding issues. The amount of freezing weather in any winter season will drive the depth of ice in Karth Lake. The deepest outlets are approximately five feet under the ordinary high water level in Karth Lake and are unlikely to fully freeze. The shallower outlets are more susceptible to being encapsulated by fully freezing conditions but the weather will play a factor in the effect freeze/thaw cycles have on backups to the system and long-term effects. • Sediment Build-Up – No sediment build-up is occurring at these submerged outlets, but the risk is higher than non-submerged outlets. It is possible that in a future snowmelt event or large rain event that transports an unusually high amount of suspended solids and organic matter will clog and fill the pipe. This may create flooding upstream of the clogged outlet and present surface flooding issues. That being said, these observations may be due to the fact that the majority of the existing outlets into the lake have existing grit chambers that are properly performing in general to minimize sediment transport. The fact that there are additional proposed improvements to add or improve sediment capture devices prior to outletting stormwater at remaining locations should help alleviate this concern further. No improvements to the storm outlets are proposed and it is recommended to complete regular condition inspections on each apron to address repairs/replacements on an as-needed basis. Additional evaluation can be completed if proactive improvements are desired which would likely include adding an inline storm structure and flattening the outlet apron to be above the ordinary high water level. Stormwater Management 1. Accumulated Sediment Removal A bathymetric survey was performed in April 2025 to assess the sediment load near the five outlets into Karth Lake. The survey found some organic sediment was DRAFT