Village of Wilmette, Illinois March 13, 2018 Separate Storm Sewer System Study – Questions/Answers 1
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Village of Wilmette, Illinois
March 13, 2018
Separate Storm SewerSystem Study –Questions/Answers
1
Green Infrastructure
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Role of Green Infrastructure
• Green Stormwater Infrastructure (GSI) can reduce volume and peak rate of runoff
• Volume reduction depends on capacity of soil to infiltrate stormwater
• Peak reduction depends on storage capacity of installed systems
• GSI is most effective for smaller, frequent rain events
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Flow
Time
Role of Green Infrastructure
Rain Garden Analysis (CBBEL, 2015)• 10’x 20’ rain gardens• Every residential property in
west Wilmette• Result: 0.2 foot (2.4 inch)
reduction in flood levels for 10-year event
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Source: Clear Choices Clean Water
Green Infrastructure Options –Permeable Pavement
• Permeable pavement absorbs initial rainfall and provides subsurface storage
• One block of residential street can provide about 0.12 acre-feet (38,500 gallons) of storage
• Resiliency and useful life of full width permeable pavements is undetermined
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Green Infrastructure Options –Permeable Pavement
• 60% of streets impacted by Sewer Expansion Option could potentially be restored as permeable pavement
• Full implementation would provide less than 10% of the storage needed for the 10-year storm event and achieve a maximum 6-inch reduction in flood levels.
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Source: Brad Putnam, Ph.D., SC Waterways, Clemson University
Green Infrastructure Options –Permeable Pavement
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Estimated Reduction in Sewer Cost:Less than $5 million
Green Infrastructure Options –Parkway Bioswales
• Bioswales detain tributary runoff and reduce peak rates of runoff
• Potential for Volume reduction depends on soils
• Siting constrained by driveways, trees, utilities
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Green Infrastructure Options –Parkway Bioswales
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Example of Siting Constraints
Green Infrastructure Options –Parkway Bioswales
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Example of Siting Constraints
Estimated Parkway Bioswale Cost: $35,000 - $50,000 per block
Green Infrastructure Options –Summary
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• Green stormwater infrastructure can compliment, but not replace traditional improvements for protection against intense storm events
• Widespread implementation requires careful evaluation of siting constraints, maintenance requirements, durability, and public acceptance
• Investment of an additional 5% could support significant green stormwater infrastructure elements in west Wilmette project
Hybrid Phasing Variations
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Existing Option: Sewer Expansion Plus Neighborhood Storage
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$23 Million
$15 Million
$10 Million
$10 Million
$19 Million
Estimated Total Project Costs (2017 dollars)
$70 Million - $80 million
Hybrid Option Variations
Hybrid Variation Description EstimatedCost
(million)Neighborhood Storage plus Sewer Expansion
All three neighborhood storage projects followed by conveyance elements to deliver 10-year protection
$85 - $100
Neighborhood Storage plus Full Sewer Expansion
All three neighborhood storage projects followed by phased implementation of full conveyance project to provide 25-50 year protection per CBBEL
$120 - $135
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Neighborhood Storage plus Sewer Expansion: 10-year Protection
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$21 Million
$6 Million
$14 Million$15 Million
$19 Million
Estimated Total Project Costs (2017 dollars)
$85 Million - $100 Million
$9 Million
$9 Million
Phasing Analysis Results16
Neighborhood Storage plus Full Sewer Expansion: 25-year Protection
• Phased implementation of Neighborhood Storage Option
• Followed by phased implementation of Full Sewer Expansion Option
• Short-term benefits of storage plus future benefits of conveyance
• Total cost: $120 - $135 million (less than sum of options due to overlap in some areas)
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+
Phasing Analysis Results18
$-
$100
$200
$300
$400
$500
$600
$700
$800
$900
$1,000
$1,100
2019 2021 2023 2025 2027 2029 2031 2033 2035 2037 2039 2041
+$730
Potential Sewer Rate Increases Tied to Debt Issuance for Neighborhood Storage & Sewer Expansion – Phased Over Time
Total construction cost with inflation is approximately $190M constructed over a period of 30 years
+$300Neighborhood Storage
Sewer Expansion with Neighborhood Storage
• Assumes construction inflation of 3%/year and borrowing interest rate of 4.5%, 30-year level debt
• Debt payments conclude in 2076• For every $1m of debt issued, a 1% increase in the interest rate = $230k in additional cost
Other Questions
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How will the three options impact the Park District?
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• Option 1: Sewer Expansion• Easement required for installation of 10-ft x 7-ft box culvert
along the south end of the Wilmette Golf Course• Option 2: Sewer Expansion plus
Neighborhood Storage• Same as Option 1 plus easement required for 10 acre-foot
storage reservoir in Thornwood Park• Option 3: Neighborhood Storage
• Easements required for:• 10 acre-foot storage reservoir under Thornwood Park• 10 acre-foot storage reservoir under Hibbard Park• 12 acre-foot storage reservoir under Centennial Park
Can the Centennial Park Storage Reservoir be moved?
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Storage Facility Parameters
Storage Area: 12 ac-ft
Footprint: 75,000 ft2
Average Ground Elev: + 634
Community Playfield Option23
Community Play Field Storage Facility Parameters
Storage Area: 12 ac-ft
Footprint: 75,000 ft2
Average Ground Elev: + 623
Storage Site Comparison
Centennial Park• Adequate Area• Constrained Site• Proximity to Prairie• Access from ROW• Ground Elev. ~634
Community Playfield• Adequate Area• Flexible Site• Recreational Area• Access Easement Req’d• Ground Elev. ~623
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Playfield site provides greater flexibility and will require less excavation.
Why are buy-outs of vulnerable structures not reasonable?
Buyouts do:• Reduce local damages• Create open space
Buyouts do not:• Reduce flood levels
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Water seeks its own level26
• Wilmette storm sewer system is interconnected• Buyouts in one area will not significantly reduce flooding
in other areas• Centralized storage in public open space is more cost
effective than distributed storage on purchased sites
What is the value of coordinated infrastructure improvements?
• Coordinated utility replacement can save 5%-15%• Areas of savings:
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Typical Utility Cross Section
• Mobilization • Excavation/Backfill• Pavement Restoration • Utility Relocation
Storm SewerWater Main
Sanitary Sewer
What is the value of coordinated infrastructure improvements?
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Improvement OptionNew Storm
Sewer(feet)
Water Main Replacement
(feet)
SanitarySewer
Replacement (feet)
Sewer Expansion 44,000 12,900 11,700
Neighborhood Storage plus Sewer Expansion
29,000 8,400 7,600
Neighborhood Storage 17,500 6,800 100
Cost estimates developed for improvement options include allowances for significant coordination of infrastructure improvements.
Village of Wilmette, Illinois
March 13, 2018
Separate Storm SewerSystem Study –Questions/Answers
1
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