MICROMANAGEMENT OF STORMWATER FOR WET WEATHER CONTROL

Persistent basement flooding caused by excess stormwater runoff often forces the leaders of a city to take a hard look at options for solving the problem. Community officials usually are presented with complicated solutions that would eliminate the flooding but also would cause financial difficulties for the community.These options usually involve 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 often have deterred the resolution of combined sewer problems. A carefully engineered system of micro-detention facilities may limit the rate of stormwater runoff allowed into a combined system to the maximum hydraulic capacity of the combined sewers, thereby eliminating basement flooding at a realistic cost for the community.Wet weather problems such as basement flooding often are caused by peak rates of stormwater runoff, not necessarily by the runoff volume. Wet weather flooding and pollution problems frequently would not occur, or would be much less severe, if the peak flows could be lessened. Peak flows are often the principal culprit, not the volume of stormwater runoff.Stormwater micromanagement works by temporarily storing stormwater in many varied locations on the surface (off-street and on-street) and, as needed, below the land surface, near to where it falls as precipitation. The idea is to accept the volume of stormwater runoff into the sewer system but greatly reduce the peak rate of stormwater entry.Numerous components may comprise a micromanagement system, including: downspout disconnection to slow down, more widely distribute, and temporarily intercept stormwater; offstreet surface storage of stormwater (conventional detention/retention) with regulated outflow; on-street surface storage with regulated outflow achieved by a combination of on-street berms and catchbasin flow restrictors; and subsurface storage of stormwater with regulated outlets control using restrictors. These subsurface storage facilities range from oversized sewer segments to large tanks.In the late 1970s and early 1980s, the 22.3 km2 (8.6 mi2) village of Skokie, Illinois, suffered many episodes of basement flooding caused by the inability of its undersized trunk and branch sewers to handle excess stormwater. In 1982, the completely urbanized village decided to begin the construction of a micro-detention flood control project. The village completed construction of a system to handle the 10-year recurrence interval storm in 1999.Skokie's stormwater management system is an optimum combination of on-street storage, detention facility 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 stormwater detention tanks, surface detention basins, and combined relief sewers.The main objective of the analysis was to evaluate the various micro-detention flood control system components and develop an optimal system for the Village. A four-phase approach was used to identify the individual portions of the system and their relationship to the flooding problems. The four phases shown below will be described in the paper: Static Condition Analysis (Phase One), Sewer Capacity Analysis (Phase Two), Street Ponding Analysis (Phase Three) and Storage Alternative Analysis (Phase Four).The watershed modeling and analysis provided an understanding of the hydrologic-hydraulic behavior of the combined sewer systems for the Village. The maximum rates at which stormwater 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 sewers 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 was developed from the above analyses.Ultimately, the complete village of Skokie micro-detention stormwater system included 2900 flow regulators, 871 roadway berms, 106 storage facilities, and 28,401 m (93,180 ft) of storm collector and combined relief sewers. The estimated cost of these facilities was 70 million, about one-third the cost of conventional sewer separation.The test is in the doing. As described in the paper, a stormwater micromanagement project has solved community-wide basement flooding problems at great cost savings. In addition, there has been widespread citizen acceptance. Furthermore, potential difficulties have not occurred. That is, there have been no vehicular traffic (including emergency vehicle) problems, pavement has not deteriorated, icing has not occurred and there has been no excessive maintenance. On August 2, 2001, the Chicago area, including Skokie, was hit by a large storm (about a 35-year recurrence event). Although the system was designed to handle a 10-year recurrence storm event, the storm caused only minor flooding problems, as compared to previous large events before the start of the project. Also, Skokie residents experienced less damages and inconvenience when compared to other nearby communities.