“Basically, this is an outgrowth of the initial study,” said Trustee Julie Wolf, Chair of the Municipal Services Committee, referring to the initial Separate Storm Sewer System Stormwater Management Report. The initial report, released in January 2015, included a proposal for a $75 million upgrade to prevent flooding, which caused “sticker shock” according to Trustee Wolf, and the board decided to keep searching for cheaper alternatives.
Focus of the Studies
Both the January report and the new studies are focused on improvements for Wilmette’s separate storm sewer system, which handles runoff after rainstorms in the area west of Ridge Road. The system functions independently from the sanitary sewer that removes unclean waste water from toilets, sinks, etc. on the west side.
(Note that east Wilmette has a combined sewer that mixes runoff with waste water.)
Results of January Report: Bottleneck is Storm Sewer Line Capacity
For the January storm sewer report, Christopher B. Burke Engineering used hydraulic modeling to analyze Wilmette flooding. The goal was to identify improvements to prevent flooding during storms with 4.47 inches of rainfall within 24 hours, an intensity expected only once every 10 years.
The study found that west Wilmette floods during 10-year rain events not because the storm-water pump station lacks capacity (at 585 cubic feet per second), and not because the two outfalls lack capacity to convey storm water to destination waterways (at 980 cfs).
Instead, the current limiting factor is the capacity of the storm sewer lines (at 300 cfs). There are not enough pipes, of large enough diameter, to clear all the water from the west side of the village.
January Recommendation and Alternative Options
The main recommendation of the January report is to prevent 10-year flooding by directly attacking the primary problem:
# 1 Recommendation: Increase Capacity of the Relief Storm Sewer System
- Install 42,000 feet of new trunk and lateral lines.
- Add a 6th pump to the existing Pump Station as an essential backup.
- Streets and ground surfaces will be cleared during a 10-year rain event.
- More intense rainfalls will be less damaging (e.g., 100-year storms would damage 370, not 700, structures).
- Estimated construction cost = $75 million.
Pros: Resolves the main issue identified in the study (that is, the lack of sewer line capacity). Prevents flooding during 10-year rain events.
Cons: Initial construction costs are high.
The January report described two additional options that either do not directly tackle the main bottleneck (storm sewer capacity), or do not fully resolve 10-year flooding:
Option 2: Centralized Stormwater Storage at Community Playfield
- Build an 18-million-gallon underground storage tank for storm runoff.
- Locate the tank under the Community Playfield (behind Wilmette Junior High School).
- Add 35,000 feet of trunk and lateral sewer lines to divert runoff to the tank.
- Estimated construction cost = $70 million.
Pros: Underground, smaller scale project.
Cons: Not much cheaper than fully upgrading the sewer lines.
Option 3: Neighborhood Stormwater Storage
- Install several small below-ground storage tanks to hold storm runoff.
- Locate the tanks in Thornwood, Centennial, and Hibbard Parks.
- Estimated construction cost = $44 million.
Pros: Smaller scale project.
Cons: Not a “10-year solution” for all homes; flooding would still occur.
The January storm sewer study included a chart comparing the existing system with the #1 recommendation (highlighted in red) and alternative options:
New Studies to Evaluate Lower-Cost Alternatives
In a quest to avoid a $75 million price tag, the board authorized payment of $56,300 to Burke Engineering to evaluate a number of lower-cost alternatives. None of these alternatives will resolve 10-year flooding events for the entire west side. However, Option 3 could be effective in a small locale.
1st New Option: Ponding on Right-of-Way
- Partially increase sewer line capacity.
- For 10-year rain events, allow water to “pond” on roadways.
- Apply this “ponding” model to the initial options to identify adjusted costs/benefits.
Pros: Lower initial construction costs for a smaller project.
Cons: During 10-year rain events, water would cover some roads, parkways, and sidewalks.
2nd New Option: Above-Ground Storage at Community Playfield
- Store excess runoff in an above-ground basin.
- Excavate and grade to create the basin in the Community Playfield.
- Landscape, terrace, and add amenities to make the basin usable during fair weather.
Pros: May cost less than the initial proposal for a below-ground tank.
Cons: May still be costly, might not solve 10-year rain events, and makes fields unusable during heavy rainfall.
3rd New Option: Local Solution for Southwest Corner
- Reduce flooding in southwest Wilmette (25 acres at Lockerbie Lane/LeClaire Avenue).
- Improve drainage in the area.
- Connect to a Glenview storm sewer, also currently considered for an upgrade.
Pros: Could solve flooding issues in southwest Wilmette.
Cons: Requires coordination; does not resolve the rest of Wilmette’s flooding issues.
Next Steps At End of June
The new studies (paid for using bonds issued by the village for work on sewers and flooding) should be completed by the end of June. After that, the Wilmette village engineer, the Municipal Services Committee, and the Wilmette Village Board will decide which, if any, of the improvements should be implemented.
UPDATE: Go to the Village of Wilmette webpage on the September 24 storm sewer meeting to get more information about the event, along with copies of the studies, and the meeting information packet.
Wilmette Village Board Meeting, April 14, 2015, which includes debate and approval of new storm sewer studies.