An excess-work approach to assessing channel instability potential within urban streams of Chicago, Illinois: Relative importance of spatial variability in hydraulic conditions and stormwater mitigation

Stormwater management in urban environments typically involves regulation of release rates of stored water from control structures to mitigate enhanced peak flows that can cause damaging flooding. The extent to which this mitigation influences stream geomorphic stability remains largely unexplored. Moreover, few, if any, studies have examined how instability is related to hydraulic effects of in-channel structures within urban stream systems. This paper assesses the potential for channel instability under existing conditions in two urban streams in Cook County, Illinois using a stream-power modeling approach. It also evaluates the impact of watershed-specific release rates intended to mitigate flooding from future development on instability potential. The analysis utilizes hydrologic and hydraulic modeling to estimate stream power per unit area for 2-year and 50-year storm events for both the existing base condition and for four release-rate scenarios. Stream power exceeding the critical power required to mobilize channel bed material is integrated over time to determine excess work. Results show that the spatial distribution of excess work for the base condition varies by more than nine orders of magnitude within individual reaches, confirming high potential for instability. Release-rate scenarios both increase and decrease the magnitude of excess work relative to the base conditions within specific reaches of the two streams but do not alter substantially the high variability in excess work. The results demonstrate that instability potential in these urban fluvial systems is governed primarily by spatial variability in hydraulic properties associated with fragmentation of the streams by multiple in-channel structures.