CSO control is no longer mere engineering
When water pollution caused beach closings at Lake St. Clair, north of Detroit, public attention focused on the problems of urban water management and the combined sewer over-flow (CSO) issue.
The River Rouge basin, south of Lake St. Clair, has experienced severe pollution problems in the recent past. The river’s water quality has been so degraded that it is listed by the Michigan Department of Natural Resources as one of the most toxic sites in the state — 28th out of 2,700 sites in a 1994 survey conducted by the department.
Fecal coliform is the biggest water quality problem; eighty-six percent of samples taken in 1987-1988 exceeded the accepted water quality standard, and there were significant dissolved oxygen and total solids violations.
The Rouge CSO problem is not unique. But proposed solutions, which involve a combination of engineering, public policy changes, land-use modification and community action, represent a break from traditional control methods that are both innovative and adaptable to individual communities.
These control methods involve a variety of non-structural and structural alternatives. In Portland, Maine, for instance, a proposed plan calls for industrial pretreatment to remove pollutants at the source. It includes street sweeping, sewer cleaning and flow control using vortex valves.
The plan incorporates watershed management techniques, such as wetlands construction, the use of planted grassed-in areas and construction of retention basins, although this accounts for less than one-fourth of the total plan cost.
Indeed, as the 1993 floods in the Midwest demonstrated, the days of relying purely on engineering solutions to water management problems are over. At Lake St. Clair, the diversity of the Rouge watershed ensures that no single formula for water management is appropriate for all parts of the basin. Thus, any approach to the problem must begin with a thorough analysis of the watershed’s physiography — soils, slopes, vegetation and water features– and land-use impacts of stream hydrology, including baseflow (groundwater contributions), overland flow and stormwater loading.
The resolution of the CSO issue is being presented as a tradeoff between retention basins and sewer separation, although neither alternative considers source reduction — decreasing the amount of water released to sanitary and storm sewers by using low-flush toilets, disconnecting downspouts from storm drains and increasing stormwater retention.
Under a proposed plan, 10 or more additional basins will be constructed in southeastern Michigan at a cost of an estimated $1 billion. In addition to the cost, the plan may be handicapped by the availability of land; CSO retention basins have not been and cannot be sized to accommodate all storms.
Storm magnitude and frequency is another problem with the basin approach. Currently, two trends in the Rouge watershed are producing large 10-year runoffs. First, continued land, use development and the construction of roads, sidewalks and other impervious surfaces are generating more runoff per rainstorm.
And, since 1960 there has been significant land-use development in the Rouge basin through expansion into vacant lands and the infilling and reuse of existing urban lands. Consequently, the size of the 10-year storm in terms of stormwater added to the sewer system is getting larger.
Also, the magnitude and frequency of rainstorms in southeastern Michigan are increasing with urban development, a trend documented by the Southeast Michigan Council of Governments. By the time basins are completed in 2005, physical conditions within the watershed may well have changed significantly, and more and bigger basins will be needed.
A new approach to the overflow problem must consider ecological, behavioral, economic, legal and geographical factors and involve longterm planning. Stormwater management should accommodate the entire spectrum of stormwater and sewage flows, including the volumes produced, the amounts discarded from properties and the rates at which stormwater and wastewater are delivered to streams.