05-05-25 - Council Work Session Agenda

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BMP Option Ditch 100-yr HWL HWL Decrease (ft) TSS Removal (lb/yr) TP Removal (lb/yr) Existing 892.15 N/A N/A N/A Concept 1 891.44 0.72 758 4.21 Concept 2 891.29 0.87 1,416 9.05 Concept 3 891.31 0.85 1,297 8.37 Concept 4 891.31 0.85 5,688 19.75 A major component of assessing BMP cost efficiency is the estimated cost divided by the provided pounds of TP removal. The proposed improvements were evaluated with a 25-year lifespan. Costs over the lifetime included full construction cost, 5 years of vegetation maintenance, annual BMP maintenance, and any major maintenance costs. These costs and the price per pound of TP are summarized in the table below. Annual TP 5-year Annual BMP Major 25-year BMP Removal Construction Vegetation Maintenance O&M Cost/Pound Option (Ib) Cost Maintenance Cost Cost TP Cost Concept 1 4.21 $170,250 $42,880 $100 $0 $2,049 Concept 2 9.05 $410,000 $42,880 $200 $0 $2,024 Concept 3 8.37 $455,350 $42,880 $200 $0 $2,405 Concept 4 19.75 $601,000 $42,880 $300 $45,000 $1,410 All concepts proposed provide improvements in varying degrees. Concept 1 provides lower benefits but comes at a lower cost and would entail a smaller amount of disturbance to existing site conditions. Concept 2 maximizes basin size and associated pollutant removals, but at an increased cost and with no broader improvements to the park. Concept 3 provides large basin sizes, ditch realignment similar to a natural channel, and potential recreational benefits, but with a greater cost and major site disturbance. Concept 4 provides similar benefits in addition to the best cost per pound of TP removal, but at a significant upfront cost and higher long term maintenance needs. This report recommends the City consider the improvements in Concept 4. The proposed work would allow for high degrees of wetland improvements in large basins that would also serve to reduce flood levels. Ditch realignment would further serve to enhance these natural resources benefits while also providing options for a greenway trail system and educational opportunities around the merits of stormwater treatment and wetland ecosystems. Finally, the proposed IESF would provide exceptional pollutant treatment and the best price per pound of TP removed. With the wide degree of benefits and greatest price -quantified water quality improvement, Concept 4 would also be a strong candidate to receive grant funding to help the City offset its construction costs. However, WSB will be happy to support the City in pursuing further design of any concept proposed. Shenandoah Improvements Feasibility Report City of Lino Lakes, MN WSB Project No. 018901-000 Page 20