Weighing the options to improve stormwater management and alleviate flooding in west Wilmette

Currently, the Village Board and staff are considering three projects that will deliver varying levels of flood protection to neighborhoods west of Ridge Road. Following is a summary of the improvements currently under consideration. In the coming weeks, the Village will be providing more information about these options to residents and is asking for your feedback.

Option 1: Sewer Expansion

The main component of this option involves installing 44,000 feet of new storm sewer. New and larger storm sewer protects the largest numbers of homes and properties from flooding. In a 10-year storm (storm with a 10-percent chance of occurring in any given year) this option will eliminate flooding for 95 percent of the flood prone homes west of Ridge Road. This alternative is so effective because it provides new, larger storm sewers to more quickly convey water to the stormwater pumping station, taking full advantage of currently unused capacity of the pump station. Other benefits include reduced flood depths and duration during more intense storms, fewer sanitary sewer backups (as a result of better stormwater management) and reduction in private property damage caused by standing water. While this option delivers the most relief to the highest number of residents compared to the other two options, it is also the most expensive. Installing the new pipe would also have the most intensive impacts to heavily travelled Wilmette roads during the installation of large storm sewer pipes, which could take up to five years or longer if it is phased in over time.

Highlights

  • 44,000 feet of new storm sewer
  • 95% reduction in vulnerable structures (approx. 295 homes protected) in significant rain events
  • 97% reduction in vulnerable properties (approx. 1,226 properties protected) in significant rain events
  • Estimated cost in 2017 dollars: $80-$95 million
  • Projected Construction Inflation Cost: Up to $108 million
  • Option 1 Map

Option 2: Sewer Expansion and Stormwater Storage Hybrid

This option incorporates components of the expansion and storage options. It improves stormwater conveyance and reduces flooding in southwest Wilmette through construction of 29,000 feet of new storm sewer (instead of 44,000 in Option 1) and improves flooding in the Kenilworth Gardens neighborhood through construction of detention storage under Thornwood Park. This option is slightly less expensive ($70-$80 million) than option 1 because it requires a reduced amount of new storm sewer. It protects the second largest number of homes and properties from flooding compared to options one and three. In a 10-year storm (storm with a 10-percent chance of occurring in any given year) Option 2 will eliminate flooding for 81 percent of flood prone homes west of Ridge Road. Another benefit of Option 2 is that the conveyance (new pipe) and detention storage could be built at the same time.

Highlights

  • 29,000 feet of new storm sewer and 10-12 acre-feet of storage under Thornwood Park
  • 81% reduction in vulnerable structures (approx. 253 homes protected) in significant rain events
  • 79% reduction in vulnerable properties (approx. 997 properties protected) in significant rain events
  • Estimated cost in 2017 dollars: $70-$80 million
  • Projected Construction Inflation Cost: Up to $91 million
  • Option 2 Map

Option 3: Stormwater Storage

The Stormwater Storage option includes the construction of neighborhood stormwater storage facilities beneath Thornwood Park, Hibbard Park and Centennial Park. The storage option also includes 17,500 feet of new storm sewer to help convey the stormwater to the three storage areas. This is the least expensive option at an estimated cost of $48-$55 million. It is slightly less effective than the expanded sewer and hybrid options because it primarily benefits the neighborhoods directly served by the proposed storage areas. This option will protect 71 percent of flood prone homes in a 10-year storm (storm with a 10-percent chance of occurring in any given year). A benefit of this option is that there is significant flexibility in how it is constructed. All three storage areas could be built simultaneously, or they could be phased over a longer period.

Highlights

  • Stormwater storage under Thornwood Park, Hibbard Park and Centennial Park
  • 71% reduction in vulnerable structures (approx. 220 homes protected) in significant rain events
  • 67% reduction in vulnerable properties (approx. 855 properties protected) in significant rain events
  • Estimated cost in 2017 dollars: $48-55 million
  • Projected Construction Inflation Cost: Up to $60 million
  • Option 3 Map

What if the Village decides not to proceed with any of these projects?

The Village believes that providing a basic level of flood protection is a core service along with maintaining and investing in other types of infrastructure that enhances the quality of life for all Wilmette residents. The Village has made every effort to exhaust all possible project alternatives in an effort to identify lower cost options. Based on hydraulic analysis performed by two independent engineering firms, the Village has reached the conclusion that there are no other effective or affordable engineering solutions to provide flood relief west of Ridge Road than those currently under consideration.

Accordingly, if none of the three projects are selected, the existing conditions will remain and homes and streets will continue to flood during significant rain events.

If existing conditions remain in place, the Village will look to solutions which may incrementally improve the situation over time, but will provide little to no flood relief during significant rain events. These actions may include increased private property regulations during construction to require onsite stormwater detention, increasing the size of sewer mains during street construction projects, and changes to zoning laws to reduce the amount of impervious surface on a property.