Like the riprap calculator, the scour equations get most of their required data directly from the HEC-RAS simulation (with the exception of the bankfull data required by Neill). The only three user parameters required are Radius of Curvature, d50, and a categorical assessment of the bend severity.
Radius of Curvature:
The Radius of Curvature for the Scour Calculation is the same as the riprap calculation. See the Radius of Curvature section in the Riprap section for a discussion of how to compute this parameter. This parameter should match the Radius of Curvature in the riprap calculator.

The d50 is only used in the Lacey Equation. It is the median grain size of the bed sediment.

Bend Severity:
Finally, the Scour calculator requires a qualitative, categorical, assessment of the reach sinuosity. The bend scour equations use the Radius of Curvature, but the general scour equations still require some information about the channel alignment. Most of the general scour equations use a qualitative factor (Z) to scale scour to one of three discrete bend-severity categories: Straight, Moderate, or Severe.

Howard et al. (2021) has quantitate guidance for these categories, based on the ratio of the Radius of Curvature and Width (R/W):

Channel Description
(for Z parameter in Table 1)

R/W Range
(ratio of radius of curvature to width)


RW<3 or 4


3 or 4<RW<10



The riprap calculator reports the R/W ratio and the recommended sinuosity category below the dropdown box where users can select it to guide this choice.