Estimating Bankfull Discharge
Bankfull discharge is determined using various techniques.
Another field-based technique is to use a Manning Equation estimate based on survey data, as follows: estimate Bankfull water surface slope, derive the hydraulic radius from the ratio of Bankfull cross-sectional area and Bankfull wetted perimeter, and compute channel roughness using look up tables or analytic techniques.
A third method is to analyze U.S. Geological Survey (USGS) Form 9-207 data that shows field measured velocity data, some of which often corresponds with a Bankfull flow. It is not recommended that estimates of Bankfull Discharge come from peak discharge probability plots, where an Bankfull probability is presumed, and the associated discharge is defined as Bankfull.
Image courtesy of USGS