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(This is the third in a series of occasional articles about water.)
More frequent and intense storms are in the forecast for Evanston in coming years, according to climate change projections for this area. Such heavy downpours can overwhelm the local sewer system, leading to flooded streets and yards, wet basements and even discharge of untreated water into Lake Michigan.
Evanston’s Sewer System
Like many older cities, Evanston has a combined sewer system that carries both sewage and stormwater. While adequate for the flow of wastewater, the pipes in much of the system are too small to handle stormwater effectively. In years past, this led frequently to sewage backups in Evanston basements after heavy rains.
To address this problem, the City undertook a major sewer improvement program from 1991 to 2008. A partial relief sewer system of pipes larger in diameter was installed on roughly one-third of the City’s streets to carry stormwater to the Metropolitan Water Reclamation District’s (MWRD) deep tunnel. In addition, restrictors were installed in the drainage structures on most streets to slow the flow of stormwater into the combined sewer system.
On streets where the relief sewer system is in place, rainwater flows into the sewer and is carried away to the deep tunnel. On streets where only the combined sewer is installed, the restrictors keep the runoff on the street surface and out of the combined sewer system. The stormwater gradually flows downhill for up to two blocks where it drains into the relief sewer system. It should be entirely gone within two hours after the rain stops.
Evanston’s sewer improvements and the progress made on the not-yet-finished deep-tunnel project have reduced significantly the incidence of basement and street flooding and discharges into Lake Michigan. Nonetheless, it is not feasible to build a system large enough to handle all the runoff from heavier rains. And the more intense storms that are expected with the changing climate increase the likelihood of more flooding and combined sewer overflows into the lake.
Rainwater as an Asset
Green infrastructure practices – using such features as trees, rain gardens, green roofs and permeable pavement – tackle this problem by managing rainwater where it falls, thus reducing the runoff that would otherwise flow into the sewer system. These green solutions reduce the risk of flooding and drainage overflows and provide multiple other benefits as well, including:
• Increased groundwater recharge
• Reduced energy usage
• Reduced water pollution
• Improved air quality
• Reduced atmospheric CO2
• Improved community livability
• Improved wildlife habitat
Green Infrastructure at Home
Evanston property owners can be an important part of the solution by incor-porating one or more of these green infrastructure features in their landscaping.
Tree planting. Trees reduce and slow the flow of stormwater runoff by holding rain temporarily on their leaves and branches, allowing some to evaporate while the rest drips slowly to the ground. In addition, tree roots break up the soil, increasing the amount of water it can absorb. Among other benefits, trees reduce energy use by shielding buildings from summer sun and winter winds, improve air quality by absorbing air pollutants and store CO2.
Rain gardens and bioswales. These landscape features collect runoff from nearby impermeable surfaces and allow it to infiltrate into the soil. Rain gardens are shallow depressions, often planted with deep-rooted native plants that soak up rainwater from a roof, driveway, or lawn. Bioswales are vegetated open channels typically placed alongside parking areas, streets or sidewalks. Both of these features increase groundwater recharge and provide wildlife habitat. Bioswales also filter the pollutants (particularly automotive pollutants) that run off paved surfaces, keeping them out of the sewer system.
Water harvesting. Disconnected downspouts keep roof runoff out of the sewer system and available for irrigation on site. Rain barrels and cisterns also capture rainwater for irrigation and other potential uses on the property. By treating rainwater as a resource, these practices reduce the amount of water that needs to be treated and supplied to users and cleaned up afterwards. This in turn saves energy at Evanston’s water treatment plant and at MWRD facilities.
Green roofs. Landscaped roof surfaces retain rainwater, most of which returns to the atmosphere. Green roofs also cut building energy use by providing additional insulation, decrease the urban heat island effect by reducing the surrounding air temperature, increase wildlife habitat, and provide outdoor areas for people to enjoy.
Permeable paving. Using permeable materials – such as porous concrete, porous asphalt, or permeable pavers – for sidewalks, driveways, and parking areas allows rainwater to seep into the ground, thus recharging the groundwater and reducing runoff. Permeable paving also helps reduce water treatment needs by retaining rainwater on site.