Power-based Assessment of the Impact of Urban Runoff on the Stability of Chicago-area Streams

Changes in land cover conditions associated with urbanization fundamentally alter the hydrologic response of watersheds by increasing rates of stormwater runoff. Altered hydrology, in turn, influences hydraulic conditions of urban streams, typically increasing bed shear stress and stream power per unit area. The result is an increased potential for mobilization of channel bed and bank material that can change channel form through erosion or deposition. The purpose of this study is to assess the potential for erosion and deposition in urban streams based on a stream power modeling approach. In the Chicago metropolitan region, enhanced urban flooding from increased runoff has led to the need for increased stormwater control. Hydraulic studies have been conducted to determine, based on HEC-RAS modeling, how different watershed-specific release rates will influence flooding along urban streams in the region. The modeling yields estimates of stream power, which, along with information on particle-size distributions of bed material, can be used to evaluate the potential for transport of different size fractions of bed material for different events. Integrating over time values of stream power in excess of threshold power values for different size fractions yields the total stream energy available for sediment transport at any particular location. Spatial variation in energy available for transport among locations is indicative of channel erosion and deposition. Complex patterns of erosion and deposition revealed by this study provide the basis for continued analysis of stream erosion problem throughout the Chicago area.