Storm causes flooding of streets, sidewalks and yards. Photo by Mary Mumbrue

When the rains came down and the floods came up last Thursday, those who were not pumping out their basements might have thought of building an ark. Water – up to a foot deep in places – covered yards, streets and sidewalks and flooded basements from the nearly four inches of rain that had fallen between midnight and 8 a.m. Earlier that week, the City had received 1.6 inches of rain.

Lightning strikes took out power in two neighborhoods. In southwest Evanston 384 Commonwealth Edison accounts were without power from about 4 a.m. to 7 a.m. on April 18. In a central/south neighborhood 98 ComEd accounts were without power from about 5 a.m. until noon. Attention to a third outage, reported shortly after 9 a.m. near the intersection of Golf Road and Crawford Avenue, was on hold, as no crews were available to send there, said City Utilities Department Director Dave Stoneback.

Along Central Street, the pressure of the water in the sewers dislodged manhole covers, causing water to flow onto the street. By mid-morning, the Utilities Department had received 50 complaints of basement flooding and 45 complaints about street/alley flooding, said Mr. Stoneback. These complaints came from all parts of the City, Mr. Stoneback said, adding that surrounding communities “were experiencing the same problem.”

By early afternoon, water was receding from streets and power restored, but water was still a problem for some residents.

Street-flooding during extraordinary rains is anticipated, said Mr. Stoneback. The City’s combined sewer system is designed “to keep stormwater on the streets” and dangerous sewage off of streets and out of basements during a serious storm. The system works by closing interceptors when the larger pipes are full, thus preventing below-ground spillovers. This forces stormwater, rather than sewage, to back up into streets. Most Evanston residences have two separate sewer pipes. One, the private sewer lateral, carries sewage; the other carries stormwater. It is the homeowner’s responsibility to keep these pipes free of roots and clogs, according to the City. When these pipes leading from homes or apartment buildings are free of roots and clogs, there is generally no backup of either stormwater or sewage into basements. Flooding basements are often the result of clogged laterals or stormwater pipes rather than a problem farther down the line, said Mr. Stoneback. He added that the City was checking to see that there were no clogs in the City’s pipes.

A memo from Mr. Stoneback sent early in the day on April 18 describes the workings of the combined sewer system. “The combined sewer system carries water from Evanston and other surrounding communities to Metropolitan Water Reclamation District’s sewer system, which includes large diameter interceptors that carry the sewage to the treatment plant.

Mr. Stoneback’s memo also said, “When the District’s interceptors are filled to capacity, they then discharge to the deep tunnel, which stores storm water. However, when the deep tunnel is full, as it is currently, the excess storm water flows to the North Shore Channel [the canal]. When the Channel level rises, valves are opened at the Wilmette Harbor, allowing water in the Channel to flow into Lake Michigan.  The water level in the Channel rose quickly last night, and the valves at the Wilmette Harbor were opened at 1:20 a.m. this morning [April 18].”

In other words, the combined sewers can overflow into the interceptors, the interceptors into the deep tunnel, the deep tunnel into the North Shore Channel and the Channel into Lake Michigan. When the locks are opened and the sewer’s contents are allowed to flow into Lake Michigan, additional anti-bacterial chemicals are put into the lake, said Mr. Stoneback.

The City added free special pick-ups this week for only storm-damaged bulky items. Because of the anticipated volume of pickups of flood debris, the City said it is possible that recycling collections could be delayed by as much as two days.