The former Altenheim property contains 11 acres of which 8 would be developed under the plan. That entire 11-acre area therefore could presently in theory contain up to 298,694 gallons of water in a single one-inch rainfall event. Open land helps to absorb large event rainfalls and help reduce the possibility of over stressing storm sewers and preventing flooding of private properties. The storm sewer systems are built to handle normal events, but we are having more abnormal rainfalls each decade: what were once 10-year events are now occurring more frequently. Having open land for absorption of storm water becomes a critical component when these 10-year weather events occur.
Of course there is also the issue of our underground water table, and the storage of water in our soils for long term environmental health. if we average the wettest and driest years in Chicago, we get 36.54-inches of rain; a 2011 Chicago Tribune statistic shows 35.82 as an average, the numbers are similar, so we can go with that lower number.
One acre-inch of rain equals 3,630 cubic feet or 27,154 gallons. So multiplying that 298,694 gallons times the 35.82 statistic gives us a total average annual rainfall of 10,699,219.08 gallons on that 11-acre property. That’s over 10 and a half million gallons of water that is currently captive on this undeveloped site that would potentially need to be absorbed by the Forest Park storm system and spread over Forest Park over the course of a year! If that area is developed, the risk increases that that event water will flow into our basements or yards or streets is increased. If the entirety of the eight acres is developed, that leaves only three acres of open space to absorb a discharge of 7,781,250 gallons over the course of a year. …
A one-inch rainfall event over the entirety of Forest Park represents 41,708,544 gallons of water. Much of Forest Park is impervious surface: roads, sidewalks, roofs. Without knowing the exact percentage of permeable to impermeable surface, or each surface’s degree of permeability, we can safely consider that much of that will be discharged rather than absorbed.
By developing 8 acres of the 11 of the Altenheim property, we are adding more gallons of water – that’s almost 8 million gallons of water! – to our storm sewers and basements. Is this in the public good? Will the additional tax revenue from the currently proposed development benefit the community as a whole? Is the developer being asked to construct the property in a manner that uses best practices for water management such as permeable surfaces? The village council needs to consider all these things when making a decision regarding new development of open land in respect to all of our homes in the village. Are there ways to offset this water accumulation aside from environmentally unfriendly detention basins? Yes. Work by Conservation Design Forum, who designed Chicago City Hall’s green roof, has shown us that this is possible. And necessary, if we are determined to go that route.
In the meantime, let’s please consider what each of us can do to mitigate flooding, while continuing to hold our officials accountable regarding the writing and enforcement of reasonable regulations for developers. Lawns vary in their degrees of permeability, but rain gardens and native-plant-rich gardens are a resourceful way to absorb and divert runoff. These and other solutions should be incorporated before permits are written.
Any development of the former Altenheim property needs to take into account the needs of the entire village including management of storm water, pollution, safety, and meeting state and federal water management initiatives. Planned and approved development must include provision for rainfall to be considered a resource and not a waste product if we are fulfilling our long-term community responsibility and not just our short-term financial revenues. If we don’t, we will end up with something that will cost us time and money wasted in pumping sewage out of our basements with a net loss of property value rather than gain.
Kathy Marie Garness, Forest Park