Robert W. Carr, P.E.
Stuart G. Walesh, P.E., Ph.D.
Director of Public Works
Village of Skokie, IL
Another rainstorm occurs in the community and results in more basement flooding. Meanwhile, community leaders struggle with complicated options that would eliminate the basement flooding problems, but would also cause financial difficulties for the community. These options are often the separation of the system into storm and sanitary components or the construction of large diameter relief sewers or tunnels. Unfortunately, the high costs of these solutions have often deterred the resolution of combined sewer problems. A carefully engineered system of micro-detention facilities may limit the rate of storm water runoff allowed into a combined system to the maximum hydraulic capacity of the combined sewers and, therefore, eliminate basement flooding at a realistic cost for the community.
The above rainfall to basement flooding scenario described the Village of Skokie in the late 1970s and early 1980s. In 1982, following another major basement flooding storm event, the completely urbanized Village of Skokie decided to begin the construction of a micro-detention flood control project. The Village completed the construction of this micro-detention project in 1999. The basic concept to reduce basement flooding was to limit storm water flow into the combined sewers by a system of flow regulators and temporary storage of excess runoff close to the source, that is, where it falls as precipitation and prior to its entry into the combined sewer system. The system is an optimum combination of on-street storage, detention facilities storage, and relief sewers. Nearly half of the required detention storage volume was accomplished by on-street ponding. The program consists of six components: flow regulators, street berms, storm collector sewers, subsurface storm water detention tanks, surface detention basins, and combined relief sewers. The cost of the recommended system is approximately one-third the cost of a conventional relief sewer system providing similar protection.
Skokie's system is tailored to the combination of its topographic, hydrologic, and hydraulic characteristics. The study techniques, modeling criteria, and storage alternatives can be applied to other urban areas experiencing similar problems with surcharging of combined sewers, storm sewers, or sanitary sewers.
The Village of Skokie is adjacent to and directly north of the City of Chicago. Virtually all of the Village's sewers are combined sewers. Prior to this project, combined sewer surcharging during wet weather caused backups into basements throughout the Village. Undersized trunk and branch sewers caused the surcharging.
The 5,510-acre Village is divided into three sewer districts. The 1,255-acre Howard Street Sewer District (HSSD) serves the southern part of the Village; the 2,300-acre Main Street Sewer District (MSSD) serves the central part of the Village; and the 1,955-acre Emerson and Lake Street Sewer District (ELSSD) serves the northern part. Land use in the Village is 80-percent residential, 10-percent industrial, and 10-percent commercial. The Village population is about 60,000 persons with a growth population density of approximately 15 people per acre.
Precipitation occurs as rain, sleet, hail, and snow and ranges from showers of trace quantities to brief intense storms to longer duration rainfall or snowfall events. Precipitation is distributed throughout the year with an average annual total of 33.3 inches. For a one-hour storm, the 1-, 10-, and 100-year recurrence interval rainfall amounts are 1.49, 1.94, and 2.08 inches, respectively. For a 24-hour storm, the 1-, 10-, and 100-year amounts are 2.21, 3.86, and 6.70 inches, respectively.
Soils in the Skokie area are primarily from glacial deposits of the Pleistocene series. These glacial deposits have an approximate depth of 60 feet and consist of many types of materials. About 25 percent of the Village have sandy soils, while the remainder of the Village has clay soils. Generally, ground water levels are 10 to 15 feet below ground level in sandy areas with the exception of isolated perched lenses of shallower ground water.
The land in the Village generally slopes east toward the North Shore Channel. Slopes vary from 0.1 to 1 percent and the overall slope in many areas of the Village is a flat 0.2 percent. Surface runoff in the Village flows from the front lawn and driveway areas to the street. Flow in the street is along the curb line and gutters to the nearest inlet. Inlets are generally located mid-block and at intersections. Due to the extremely flat conditions, few areas have a continuous drainage pattern from block to block. Trunk sewers in the combined system range in diameter from 30 inches to a maximum of 84 inches. Lateral sewers, which are connected to trunk sewers, range in diameter from 12 to 27 inches. Combined sewage is carried from the Village through three 84-inch trunk sewers to the interceptor sewer owned and maintained by the Metropolitan Water Reclamation District of Greater Chicago (MWRDGC). When the interceptor capacity is exceeded, each trunk sewer overflows first to the MWRDGC Tunnel and Reservoir Plan (TARP) and then to the North Shore Channel.
The main objective of the analysis was to evaluate the various micro-detention flood control system components and develop an optimal system for each district. A four-phase approach was used to identify the individual portions of the system and their relationship to the flooding problems.
The Static Condition Analysis (Phase 1) determined whether flood levels in the North Shore Channel would cause basement flooding. The micro-detention system will not resolve basement flooding resulting solely from backwater from the North Shore Channel. The results of this analysis showed that there were no significant areas in which basement flooding would result solely from high stages in the North Shore Channel.
The Sewer Capacity Analysis (Phase 2) determined the capacity available for carrying storm water runoff. The total hydraulic capacity of the system depends on the quantity of sanitary and storm flows, infiltration and foundation drainage, and the level to which a sewer can surcharge without causing basement flooding or other damage. This capacity is also affected by the backwater effect of downstream sewers. The runoff characteristics and street geometry were evaluated to determine a control system, which would make the most use of streets with significant ponding capacity. The maximum allowable rates at which runoff can be released into the sewer system were determined for specific areas of each district. The maximum allowable regulated runoff rates ranged from 0.08 to 1.0 cfs per acre. Results of this analysis formed the basis for the design of the micro-detention system and for determining the sewer capacity available for those areas which could not be regulated.
The Street Ponding Analysis (Phase 3) determined the location and extent of the micro-detention facilities to be located on village residential streets (intentional street ponding) which can be achieved through flow regulators and minor street grade modifications, called berms (Figure 1). The release rates into the combined sewer were regulated to use the available street storage capacity. Therefore, on streets with limited storage potential, a higher release rate was used and conversely, on streets which could pond large storm water volumes, a lower release rate was used. The Phase 2 and the Phase 3 analyses were conducted concurrently with each analysis providing input to the other. The product of this analysis included delineation of street ponding elevations in allowable areas and identification of the volume of additional runoff, which must be detained in off-street locations.
The Storage Alternative Analysis (Phase 4) was conducted to determine the degree of runoff regulation, which would be required to maintain flows within the capacity of the existing system. The results of the previous three analyses were used in Phase 4. Off-street detention was used to store runoff in excess of street ponding capacity and where street ponding was not feasible to store runoff in excess of the regulated runoff rate. Potential facility locations and sites were evaluated based on estimated construction costs. Open turf lined basins are least costly to construct but require greater land area than subsurface tanks. Gravity operated systems are preferable to pump systems to avoid electrical and mechanical operation and maintenance costs. However, deeper facilities with pumped outlets require less land area than shallower facilities, which drain to existing sewers by gravity. Vacant lands, Village-owned lands, park lands, and school district lands were considered first as locations for detention facilities. Where no off-street land was available, construction of subsurface storage beneath roadways was considered.
In most on-street ponding areas, excess detention storage was accomplished with subsurface detention facilities in the street right-of-way. Where possible, excess runoff from ponding areas was conveyed to more cost-effective detention facilities. In many locations, oversized storm sewers with a restricted outlet were found to be more cost effective than collector storm sewers to a subsurface detention facility. In some locations within the Village, combined relief sewers were found to be more cost effective than detention facilities. These sewers provide the Village with additional capacity to the MWRDGC interceptor and deep tunnel system.
The watershed modeling and analysis provided an understanding of the hydrologic-hydraulic behavior of the combined sewer systems for each district. The maximum rates at which storm water can be released to the combined sewer system without causing sewer surcharging and the elevation and location of berms to create maximum safe street ponding were determined. The locations and volumes of off-street detention facilities and relief sewer needed to complete the micro-detention system were evaluated. The micro-detention system components necessary to provide an equal degree of protection from sewer backups for all areas of the Village were identified. The recommended plan for each sewer district was developed from the above analyses. A comparison of the Skokie sewer system with and without the micro-detention project is shown on Figure 2.
Design criteria developed for the recommended micro-detention system plan include: 1) design for the 10-year recurrence interval storm; 2) reduce sewer surcharging to prevent sewer backup into the basement; 3) use available street ponding capacity without causing flood damage to adjacent private developments; 4) minimize street flooding on state and county highways and arterial Village streets; 5) favor gravity-drained facilities over pumped facilities; 6) store excess runoff first on Village streets, second in Village off-street areas, and last in underground storage facilities; and 7) disconnect downspouts from the sewer system and discharge to land surface.
The constructed micro-detention system for the HSSD consists of 484 flow regulators, 180 roadway berms, and nine storage facilities. These facilities include eight subsurface tanks and one surface basin. The micro-detention system includes 2,000 feet of storm collector sewer and 8,000 feet of combined relief sewer. In the HSSD, the total storage volume required was 895,200 cubic feet-777,500 cubic feet (87 percent) stored on the street and 117,700 cubic feet (13 percent) stored in detention facilities.
The constructed micro-detention system for the MSSD consists of 825 flow regulators, 370 roadway berms, 21 subsurface tanks, 6 surface basins, and 20 in-pipe storage facilities for a total of 52 facilities. In addition to the storage facilities, 50,630 feet of storm collector sewers and 11,600 feet of combined relief sewers will be constructed. In the MSSD, a total storage volume of 3,757,900 cubic feet is required-1,763,500 cubic feet (47 percent) in on-street storage and 1,994,400 cubic feet (53 percent) in storage facilities.
The constructed micro-detention system for the ELSSD consists of 700 flow regulators, 321 roadway berms, 26 subsurface tanks, 11 surface basins, and 8 in-pipe storage facilities. In addition to the 45 storage facilities, 11,600 feet of storm collector sewer and 9,300 feet of combined relief sewers will be required. In the ELSSD, a total storage volume of 2,244,800 cubic feet is required-761,600 cubic feet (34 percent) in on-street storage and 1,483,200 cubic feet (66 percent) in storage facilities.
The complete Village of Skokie micro-detention storm water system includes 2,900 flow regulators, 871 roadway berms, 106 storage facilities and 93,180 feet of storm collector and combined relief sewers. The estimated cost of these facilities is $70 million, about one-third the cost of conventional sewer separation.
Experience with micromanagement of storm water indicates that acceptance by community officials and citizens and effective performance of the system require careful attention to many and varied project elements. These success factors are:
Based on the experience to date, micromanagement of storm water for wet weather control has great potential. It can be retrofitted into existing urban areas and can be included as a basic element in new urban storm water management design.
The Village of Skokie is not the only community with problems resulting from an undersized combined sewer system. The Skokie micro-detention system is tailored to the combination of topographic, hydrologic, and hydraulic characteristics in Skokie, Illinois. However, the study techniques, modeling criteria, and storage alternatives can be applied to other urban areas experiencing similar problems with surcharging of combined sewers, storm sewers, or sanitary sewers. A well-engineered micro-detention flood control system eliminates adverse impacts and is virtually unnoticeable to residents on both wet and dry days.
For more information, contact Robert W. Carr at 414-225-5135 or firstname.lastname@example.org; Stuart G. Walesh at 219-464-1704 or email@example.com; or Dennis York at 847-933-8271 or firstname.lastname@example.org.