Part 1: Wilmette Official Answers Readers' Flood Questions

Wilmette's flood survey has conjured in the blog section of our site. A Patch reader and blogger's conversation sparked some interest from Wilmette's Director of Engineering Services, Brigitte Mayerhofer.

Earlier:

Mayerhofer reached out to Patch to answer the concerns raised. Below is the first part of the Q&A. Stay tuned Tuesday for the other half:

1. If the Sanitary sewer system is independent of the Storm Sewer system, WHY DO WE HAVE RAW SEWAGE BACKING UP INTO HOMES DURING SEVERE STORMS ?

The Village of Wilmette has two different systems as it relates to managing storm and sanitary water. The portion of the Village located east of Ridge Road is served by a combined sewer system, in which sanitary and storm flows share a single combined sewer system. West of Ridge Road is served by a separate sewer system in which sanitary and storm flows are collected and conveyed in separate systems. Both systems are tributary to the Metropolitan Water Reclamation District of Greater Chicago’s (MWRDGC) interceptor system. Wastewater flows are treated at the MWRDGC’s Northside Wastewater Treatment Plant. Both systems are relatively old, with portions of the system over 100-years-old.

Sewer backups can occur in both the combined and separate sewer systems. East of Ridge Road, in the combination sewer area, sewer backups can occur when a rainfall event exceeds the capacity of the combined sewer system. In the early 1990’s, the Village invested approximately $30 million in the combined sewer system with the goal of reducing the magnitude and frequency of flooding. A network of relief sewers were built to increase the conveyance capacity of the existing combined sewer system. In addition, drainage berms and inlet restrictors have been installed throughout the area east of Ridge Road to create surface storage of rainwater during heavy rain events. This combination of improvements has proven successful in reducing both the frequency and severity of sewer backups. The relief sewer project is approximately 70 percent complete, with the remaining relief sewer projects programmed in the Village’s capital improvement program.
The sanitary sewers west of Ridge Road function most efficiently during dry-weather days or when there is light precipitation. During heavy rain events groundwater can infiltrate the sanitary sewer through cracks, open joints, manholes and sanitary laterals. A sanitary lateral is the pipe that connects a home with the public sewer. Per Wilmette’s Village Code, sewer laterals are the maintenance responsibility of the home owner.

In addition, inflow of rainwater into the sanitary system can occur through illegal connections and gravity-fed foundation drains. The presence of inflow and infiltration (referred to as I/I) is one reason sanitary backups occur in the separate sewer area. To
address this, the Village invested nearly $10 million in the early 1990’s to build relief sewers on the separate sanitary system. In addition, annual maintenance programs to clean and televise, repair and line the interior of the sanitary sewers has reduced groundwater infiltration and improved the performance of the sewer system. While these steps have improved the efficiency of the sewer system, the Village recognizes additional improvements are necessary. Various short term and long term sewer improvements are outlined in the Village’s capital sewer program.

2. Why do we have such high pressure coming in from the Village Sewer lines when we have a significant slope to the North Shore Channel?
It is correct that pipe flow is a function of diameter of the pipe, pipe roughness and slope. Sewer pipe, however, is a closed conduit with finite capacity. When there is more flow than the pipe can convey, the result is “high pressure” or sewer surcharging. Under normal conditions, all of the Village’s sewer systems operate by gravity.

3. Didn't Wilmette make everyone disconnect the downspouts and storm water discharges from going into the sanitary sewers unless the homeowner could prove otherwise?
Yes. In 1985, the MWRDGC adopted an infiltration/inflow corrective action plan (ICAP) aimed at reducing the amount of excessive I/I in the sanitary sewers. Like other communities tributary to MWRDGC, Wilmette was required to develop a sanitary rehabilitation program to reduce excessive I/I. The Village conducted a comprehensive house sewer inspection program in response to this mandate and downspouts were disconnected from the sanitary system as part of this program.

For more questions regarding this information, contact Brigitte Mayerhofer, P.E., Director of Engineering Services at 847.853.7627. Join the discussion on storm recovery on Wilmette-Kenilworth Patch's Facebook page. 

Stay tuned for part two of the Q&A Tuesday.