SUCCESSES AND FAILURES OF STORM WATER MANAGEMENT

The post-world war II era was marked by the establishment of a national highway system and rapid expansion of urban and suburban developments. These two land-use changes have increased the water supply to our rivers. As early as the 1960s it was recognized that unmitigated storm water runoff caused widespread stream incision and widening and a fundamental change in channel substrate. These changes in the physical stream characteristics degrade in-stream habitat and impair ecological function of urban streams. Early storm water management principles such as peak shaving sought to reduce flooding and mitigate erosive flows. These treatments have not always been successful. It has been argued theoretically that peak shaving would not eliminate bed erosion because of the increased duration of flows that, although low in magnitude, still exceed the critical shear stress threshold of the bed. Moreover, in-line ponds tend to sequester sediment leaving downstream reaches starved of bed load. Field observations, although rare, indicates that storm water management is largely unsuccessful at mitigating downstream erosion.

A study conducted in Apple Creek, a small, urbanized watershed in northeastern Wisconsin, provides an interesting contrast to previous work. Storm water management here has been remarkably successful despite the use of dated peak-shaving techniques and in-line ponds that store bed load. The reasons for channel stability here include a relatively low valley slope and a resistant channel and bank substrate. Although these ponds serve as water quality and well as quantity control, they may be over designed in regard to erosion mitigation.

Based on this work and comparison with previous studies it seems that storm water management plans need to go beyond hydrologic and hydraulic modeling and must account for the character of the channel substrate and banks and the upstream sediment supply. Without consideration of these important components many designs are unlikely to meet their goals. Moreover, some river systems, like Apple Creek, may be particularly resistant to channel change and could therefore utilize less costly mitigation methods.