OVERVIEW OF THE CAUSES OF URBAN STREAM INSTABILITIES, EXAMPLES OF INSTABILITIES, AND THE CHALLENGES IN THEIR RESTORATION

Key drivers of urban stream instabilities are stormwater mismanagement and loss of floodplains. Urban runoff modifies the channel-forming flow in a stream, and a common first response is downcutting. Advanced downcutting leads to bank failures and channel widening. Both downcutting and widening increase channel sediment loads. The increased sediment loads: can result in downstream instabilities; reduce water quality; and can destroy benthic habitat. The downcutting process prevents the higher flows from getting on to the former floodplain. Even if these areas were formerly categorized as floodplains, the general public falls into a state of complacency and begins to locate infrastructure in the former floodplain. With more of the flood flows now constrained to within the former bankfull channel, the downcutting process is further aggravated, thereby accelerating the process.

If a floodplain still exists, the best solution for restoration is to put the stream back up on to the floodplain. However because of all of the horizontal and vertical controls in urban areas as well as individual property issues, the second best solution is more common, and that is to widen the existing channel, in order to create the necessary floodplain width. This leaves the former floodplain as a terrace. Because of private property concerns with channel widening, a third, and the most common solution for urban stream restoration is to “harden” the existing channel and its alignment. Historically this has meant rip rap, gabions, trash, or concrete, however more modern channel-training structures improve habitat and riparian corridor movement, while at the same time stabilize banks and provide for the movement of water and sediment.

This urbanization process and impairment of our streams has occurred over the past three centuries. These impairments will not be undone in just one generation, nor will entire streams be restored all at once. Solutions need to be mindful that what we do today may be the first necessary step in a process of restoration. Various examples will be shown to demonstrate the causes, consequences, and types of solutions that are employed for urban stream restoration.