Alluvial rivers are the authors of their own bankfull geometry. Many relations have been proposed to allow computation of bankfull geometry from a specified bankfull discharge. As for bankfull discharge itself, a common strategy is to declare that it is the flood discharge with the 1.5 year recurrence interval and then walk away from the problem. Yet in the broad sense, bankfull discharge is a dependent variable, just as bankfull depth and width. Here we take a step back, and specify the flow duration curve, in addition to bed material load and wash load supply. The river can increase its channel depth through overbank deposition, and can decrease it by floodplain shaving, according to which the channel preferentially migrates into older, higher floodplain. Overbank deposition is favored by discharges sufficiently high to spill onto the floodplain. Shaving is favored by flood flows that are nevertheless mostly in-channel, allowing the thread of high velocity to hug and erode outer banks. We present a model which includes both bed material load and wash load, and which seeks a balance between shaving and overbank deposition by integrating over the flow duration curve. This analysis allows computation of bankfull discharge. In the case of the lower Minnesota River, Minnesota, we find that bankfull discharge corresponds to a flow that is exceeded about 2.5 % of the time.
Dept. of Civil and Environmental Engineering and
Dept. of Geology
University of Illinois Urbana-Champaign
|Introduction||Gary Parker University of Illinois At Urbana-Champaign||20|
|Discussant||Gary Parker University of Illinois At Urbana-Champaign||20|
|Panelist||Gary Parker University of Illinois At Urbana-Champaign||20|
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