Hydraulic radius¶
The hydraulic radius is a measure of the effect of the wall/bed resistance on the water volume. The larger the hydraulic radius, the larger amount of water is affected by each metre of the wall; there is thus less friction and the reach will convey water more efficiently.
The hydraulic radius is traditionally defined as¶
Equation: Hydraulic radius defined as the ratio of flow area to wetted perimeter.
where A is the cross section and P is the length of the wetted perimeter. The wetted perimeter is the portion of the cross section perimeter that is currently wet.
The traditional definition of the hydraulic radius has some properties that are not desirable in all situations. Therefore a number of variations exist, and the hydraulic radius used can be one of three different types:
- Resistance radius
- Hydraulic radius, effective area
- Hydraulic radius, total area
The two types of hydraulic radii, effective area and total area, are alike except when specifying a relative resistance number which varies along the cross section profile, either as distributed resistance numbers or using flow bank zones.
The hydraulic radius is calculated from the raw data. The following sections describe how the hydraulic radius is calculated.