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HomeMy WebLinkAboutDick Lanyons Evanston Stormwater Management Master Plan Article1 Stormwater Experts Help Plan for Evanston’s Future Stormy Weather A Synopsis of the Evanston Stormwater Management Master Plan Prepared by the Evanston Utilities Commission A little over 40 years ago, a person could float down Sherman Avenue on a raft while other Evanstonians cleaned out and disposed of floodwater damaged goods from their basements. The 1990 Relief Sewer Program, completed in 2008, was a big success and its $210 million cost ended persistent flooding over the years in the city. But it’s no time to rest on our laurels. Climate change may have history repeat itself. Evanston’s first Climate Action Plan was adopted in 2008 as the Relief Serwer Program was concluded. Five years later, as some of the climate action goals were achieved, a storm on April 18, 2013, caused extensive flooding throughout the city, as shown on Figure 1. Work continued to refine the city’s approach to climate change and the formation of a resident-led effort to develop the Climate Action and Resiliency Plan (CARP) adopted in November 2018. One action item included in CARP was “Prepare a comprehensive plan for stormwater management that goes beyond baseline regulatory requirements and includes green infrastructure with the goal of eliminating CSOs.” Today, CSOs, combined sewer overflows, have become a thing of the past with implementation of the Metropolitan Water Reclamation District’s (MWRD) regional Tunnel and Reservoir Plan. Tunnel and Reservoir Plan (TARP) | MWRD The 30-mile-long Mainstream 2 Tunnel serving Evanston begins in Wilmette and ends in Hodgkins at the McCook Reservoir. Tunnel conveyance and reservoir storage have successfully eliminated frequent CSOs. However, the CARP action item stressed the need for resiliency in the face of increased severity of storms due to climate change. Figure 1. Reported Flooding Resulting from the Storm on April 18, 2013. 3 In April 2020, the Evanston City Council approved engaging Hey and Associates (Hey) to prepare the Stormwater Management Master Plan (SMMP). Hey prepared a detailed hydrologic and hydraulic (H&H) model of rainfall’s interaction with the land surface and routing stormwater runoff through the city’s complex sewer infrastructure. The model was used to analyze historic and statistical storms to determine how well the infrastructure performs and the potential flood risk for various storm durations and frequencies. Figure 2. Stormwater drainage. Blue areas are served by storm sewers. Brown areas are served by combined sewers in dry and wet weather. Green areas have relief sewers to relieve combined sewers of excess stormwater. 4 Two types of sewers handle Evanston’s stormwater; combined sewers and storm sewers. Combined sewers convey dry weather sewage and wet weather combined sewage and stormwater. To help combined sewers handle stormwater, relief sewers were built in the 1990 Relief Sewer Program. Combined and relief sewers are the most prevalent sewer types as shown by the brown (combined) and green (relief) areas in Figure 2. Blue are those areas served by city storm sewers that convey only stormwater from where the rain falls to the North Shore Channel (NSC) or to Lake Michigan in two small areas. Flow in storm sewers is not tainted by sewage; hence it can be discharged, properly permitted by the Illinois Environmental Agency, to surface receiving waters. Most storm sewers were constructed as part of the city’s 1990 Relief Sewer Program. Green are those areas served by city relief sewers that convey excess stormwater to the MWRD Mainstream Tunnel. This excess stormwater can be tainted by sewage; hence it must be controlled to avoid discharge to surface waters. In an extreme event when Mainstream Tunnel is full, the excess flow will be discharged to the NSC and reported by the MWRD as a combined sewer overflow. Some city relief sewers are tunnels about 60 feet below ground level. The MWRD Mainstream Tunnel is 200 feet below ground level and runs beneath the NSC. Brown are those areas served by city combined sewers, the oldest sewers, some exceeding 100 years. These sewers were built at a time when streets were narrow, there were fewer parking lots, and fewer automotive vehicles. The combined sewers don’t have the capacity to safely drain away intense storm rainfall and that is why the relief sewers were built. Along with the relief sewers, street stormwater inlets at the curb in the brown area have restrictors installed that limit the rate of flow into the combined sewer. This temporarily stores rainfall runoff on the street to reduce the incidence of stormwater and sewage backing up into basements. Temporarily flooded streets are to be expected in the brown areas during intense rainfall. Prior to using the Hey H&H model to assess sewer system performance, the model was calibrated and verified using data from a three-month monitoring program in which rainfall was measured at six local rain gauges and flow in sewers was gauged at 23 flow measurement locations. Fortunately, two significant storms were captured, providing useful data to calibrate and verify the model. The historic storm of September 2008, which lasted three days and dumped seven inches of rain on Evanston, was modeled. MWRD facilities and the NSC were at maximum capacity restricting city sewer outfalls. Modelling results showed city sewers running full throughout the city, but more prominently in areas east of the NSC. Surface flooding was limited to a few small areas near the NSC. Modeling the April 2013 storm, which dumped 4.75 inches of rain over two days while MWRD facilities and the NSC were at maximum capacity, showed similar results. With the model showing sewers running full during these events, one would expect a large number of reports of basement and surface flooding for these storms, but that was not the case. Either residents didn’t report basement flooding or, it is more likely that many residences were protected with onsite flood protection technologies. 5 The model was also run using statistical rainfall data to determine the risk of flooding for storms that occur from every 2-years to 100-years. The risks of flooding were similar to the results explained for the two historic storms mentioned above. The complete Hey report can be accessed at cityofevanston.org/home/showpublisheddocument/90359/638236461218630000 The foregoing results indicated that Evanston’s sewer infrastructure is performing as expected and there was no immediate needs for improvements. However, Hey was tasked to include three conceptual projects to alleviate surface flooding. Two involved the construction of storm sewers and the third suggested the use of green infrastructure. Street flooding along Darrow Avenue near Emerson Street could be relieved by a storm sewer discharging to the NSC. Street flooding over a large area north and east of James Park could be relieved by a network of storm sewers discharging to the NSC. Street flooding in the vicinity of Davis Street and Dewey Avenue could be relieved by detention storage in a bioswale constructed along the west side of Darrow Avenue as part of the Mason Park improvement project. Overflow from the bioswale could be discharged to an existing storm sewer. Hey also had several recommendations for improving Evanston’s drainage regulations and initiatives to floodproof private property. Regulatory recommendation updates included: • Stormwater performance criteria for single residential lots. • Require detention storage for buildings smaller than 5,000 square feet. • Require detention storage when parking lots are resurfaced. • Include residential lots in the fee-in-lieu of detention program. • Update the stormwater control fact sheet. The private property plumbing code should be updated to include: • Require backflow preventers or overhead sewers for building modifications. • Require all downspouts and sump pump discharges to be disconnected from the sewer. Investigate the authority needed to implement the following: • Require backflow prevention and overhead sewers upon ownership transfer. • Require basement flooding inspection and or remediation upon ownership transfer. • Require downspout disconnection upon ownership transfer. • Require lateral sewer inspection upon ownership transfer. • Require periodic inspection of sump pump discharge location. • Provide private property incentives for onsite flood protection technologies. Initiatives that private property owners can take for flood protection included: • Installation of overhead sewers or sewer backflow prevention valves. • Downspout and stormwater sump pump discharge disconnection from sewer. • Reconfigure lot drainage to move stormwater away from residential structures. • Residential structure floodproofing and foundation wall sealing. 6 • Onsite rain gardens or bioswales to receive and contain lot drainage. Hey also recommended that the city continue sewer system monitoring to update and verify H&H model performance, and periodically use the model to assess sewer system performance when flooding occurs. When assessments indicate the need for sewer system improvements, the city should use the H&H model to evaluate sewer system improvement alternatives including: • Sewer conveyance capacity improvements. • Sewer system outlet improvements or modifications. • Surface or underground stormwater detention storage structures. • Green infrastructure. In meeting the challenge of climate change, the H&H model is a valuable tool for the city to use in facing the future.