Evaluating the Effects of Urbanization on Stream Flow and Channel Stability — State of Practice

The hydrology from urban areas has a profound effect on the stability of our urban streams. The urban condition increases peak flows and the volumes of runoff from the watersheds. Detention storage common in urban areas can extend the time that these flows occur in the streams, and possibly change sediment discharge characteristics. The effects on bank stability, aquatic and riparian habitat, sediment transport, and water quality are subjects of current research. Continuous simulation modeling of runoff in urban streams is currently being studied to determine the effects of urbanization on urban streams. The continuous simulation modeling helps characterize the effect of prolonged flows on stream stability. Research has shown that simply limiting peak flows may not be enough to maintain pre-development stability levels. The duration of the flows can affect the stability as much as the magnitude of the flow. The methodology used in developing the continuous simulation hydrology is also extremely important to understand. The determination of bankfull discharge in urban streams is a continuing mystery. Recent research out of Canada has shown that the correlation of bankfull discharge, or morphological forming discharges, to a specific runoff event return interval is not possible. This revelation is extremely important and may invalidate design criteria across the U.S. related to the use of the one-year or two-year return interval flow for stream stability analysis. In addition to bankfull discharge, it has been determined that another discharge should be considered for urban streams. Those discharges relate to the requirements of aquatic habitat at different life stages and are critical to the ecological integrity of the stream system. Design of channels based on morphological forming discharges, in conjunction with these ecological sustaining flows, result in stable channels that provide self-sustaining habitats, diverse enough in structure that they provide essential ecological functions. Finally, effects of upstream detention on stream stability and ecological integrity have long been debated. Studies currently underway seek to answer the question whether, "to detain or not to detain, because detention facilities may be storing sediment leading to channel instabilities, coupled with longer duration storm hydrographs. This paper will summarize the findings of these and other important questions and studies related to stream stability and ecological integrity.