BACKGROUND

DuPage County, in northeastern Illinois, is part of the expanding Chicago Metropolitan area. The county has three major watersheds: Salt Creek and the East Branch and the West Branch of the DuPage River (Figure 1). Salt Creek is an urbanized watershed. The East Branch of the DuPage River is now experiencing rapid urban development. The West Branch is largely undeveloped. Today, Salt Creek has serious flooding problems. Early development during the 1950s and 1960s occurred without storm water detention or floodplain regulations. Rapid development in the 1970s and 1980s converted much of the remainder of the watershed to urban land uses (1). Now, many people live and work in flood-prone areas. Average annual flood damages on the main stem of Salt Creek exceed $1 million a year (2). To reduce damage from floods several major flood control projects are planned for Salt Creek. In August 1987 a night of extreme rainfall (> 9 in. at O'Hare Airport) caused record flooding on Salt Creek. This flood galvanized regional support for strong storm water management artd flood control measures. The Illinois General Assembly soon passed legislation authorizing county governments to mitigate the effects of urban development through the use of countywide storm water management planning. By 1991 DuPage County had developed a storm water and floodplain ordinance. To prevent future problems within the county's urbanizing watersheds, the ordinance requires significant on-site detention storage for new development, compensation for lost depressional storage, and no increase in flood elevations and damages off-site. The ordinance also requires that new hydrologic and hydraulic methodologies be used for estimating floodplain limits. The following sections describe these methods.