Beyond Bankfull

Beyond Bankfull

Its time to move on. For too long, channel assessment and design have been based on some notion of a “correct” discharge that just fills the channel. This discharge might be determined from a variety of metrics, including flood frequency, drainage area, and various field indicators. A weakness in this approach is immediately evident from changing fashion: the correct flow magnitude over the years has decreased gradually, then suddenly, from order two-year flood to flows that occur every year. The fundamental problem is in the linkage between channel dimension and desired attributes of channel behavior. Begin with a “correct” discharge and one can only hope that all good things will follow, whether channel dynamics, habitat, aquatic life, or appearance. This is backwards. Correct assessment and restoration practice begins with objectives and then seeks to determine the channel dimension and behavior that best meets those objectives. The starting point is to ask: “what is the supply of water and sediment and what do you wish to do with them”? There are two basic elements to channel behavior: the mobility of the bed material (zero mobility = threshold channel) and the balance between sediment supply and transport capacity (the Lane Balance provides a conceptual, but not predictive model of sediment surplus or deficit). Channels fall along the full spectrum of static to mobile beds and may accumulate or evacuate sediment. These are the essential “what do you wish to do with them” attributes which then support actual stream objectives such as ecosystem change, habitat type and amount, infrastructure and flood protection, recreational uses, and appearance.

The solution to the threshold and mobile-bed problems can be evaluated on a single chart with two curves – one threshold, one mobile-bed – giving channel slope as a function of channel width. The mobile-bed solution should not be found for a single “correct” discharge (and associated sediment supply), but should be the slope needed to transport the supplied sediment over the full range of flows. This is the graded slope from the fluvial geomorphological canon. Once the threshold and graded slopes (and widths) are determined, channel assessment and design can proceed to consider the desired channel behavior as well as the risk associated with uncertainty in the inputs and calculations. Such an approach might appear to be fraught with uncertainty and freighted with effort. Wouldn’t it be simpler to pick a currently popular “correct” discharge and go from there? Indeed, it is possible to demonstrate that many situations do not require extensive sediment transport estimates. Any stream channel must deal with its water and sediment supply, regardless of one’s thought process. This is true of a channel viewed through the lens of a “correct” discharge as well as a channel for which channel dimension is determined based on the water and sediment supply. The difference is that consideration of transport and channel behavior is explicit in the latter case, rather than hoping for the best.