Separate Storm Sewer System Study Questions/Answers

Transcription

1 1 Separate Storm Sewer System Study Questions/Answers Village of Wilmette, Illinois March 13, 2018

2 Green Infrastructure 2

3 3 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 Flow Time

4 4 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 Source: Clear Choices Clean Water

5 5 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

6 6 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. Source: Brad Putnam, Ph.D., SC Waterways, Clemson University

7 7 Green Infrastructure Options Permeable Pavement Estimated Reduction in Sewer Cost: Less than $5 million

8 8 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

9 9 Green Infrastructure Options Parkway Bioswales Example of Siting Constraints

10 10 Green Infrastructure Options Parkway Bioswales Example of Siting Constraints Estimated Parkway Bioswale Cost: $35,000 - $50,000 per block

11 11 Green Infrastructure Options Summary 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

12 Hybrid Phasing Variations 12

13 Existing Option: Sewer Expansion Plus Neighborhood Storage 13 $19 Million $23 Million $10 Million $15 Million $10 Million Estimated Total Project Costs (2017 dollars) $70 Million - $80 million

14 14 Hybrid Option Variations Hybrid Variation Description Estimated Cost (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 year protection per CBBEL $120 - $135

15 Neighborhood Storage plus Sewer Expansion: 10-year Protection 15 $19 Million $21 Million $9 Million $9 Million $6 Million Estimated Total Project Costs (2017 dollars) $85 Million - $100 Million $14 Million $15 Million

16 Phasing Analysis Results 16

17 Neighborhood Storage plus Full Sewer Expansion: 25-year Protection 17 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)

18 Phasing Analysis Results 18

19 $1,100 $1,000 Potential Sewer Rate Increases Tied to Debt Issuance for Neighborhood Storage & Sewer Expansion Phased Over Time +$730 $900 $800 $700 $600 $500 $400 $300 $200 Sewer Expansion with Neighborhood Storage Neighborhood Storage Total construction cost with inflation is approximately $190M constructed over a period of 30 years +$300 $100 $ 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

20 Other Questions 20

21 21 How will the three options impact the Park District? 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

22 22 Can the Centennial Park Storage Reservoir be moved? Storage Facility Parameters Storage Area: 12 ac-ft Footprint: 75,000 ft 2 Average Ground Elev: + 634

23 23 Community Playfield Option Community Play Field Storage Facility Parameters Storage Area: 12 ac-ft Footprint: 75,000 ft 2 Average Ground Elev: + 623

24 24 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 Playfield site provides greater flexibility and will require less excavation.

25 25 Why are buy-outs of vulnerable structures not reasonable? Buyouts do: Reduce local damages Create open space Buyouts do not: Reduce flood levels

26 26 Water seeks its own level 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

27 27 What is the value of coordinated infrastructure improvements? Water Main Storm Sewer Sanitary Sewer Typical Utility Cross Section Coordinated utility replacement can save 5%-15% Areas of savings: Mobilization Excavation/Backfill Pavement Restoration Utility Relocation

28 28 What is the value of coordinated infrastructure improvements? Cost estimates developed for improvement options include allowances for significant coordination of infrastructure improvements. Improvement Option New Storm Sewer (feet) Water Main Replacement (feet) Sanitary Sewer 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,

29 1 Separate Storm Sewer System Study Questions/Answers Village of Wilmette, Illinois March 13, 2018