The City can do a few things, including:
Placing restrictors—known as “Rain Blockers”—in CB’s all over town. These plastic devices slow down the flow of water from the street. It goes into the sewer only as the sewer is able to handle it. The streets are turned into temporary reservoirs (for 3-4 hours), and it gradually drains. If the restrictors are missing, your sewer will surcharge and push water into your basement.
For more information, visit http://www.cityofchicago.org/city/en/depts/water/supp_info/basement_floodingpartnership.html