text: The hydraulic radius is one of the properties of a channel that controls water discharge. It also determines how much work the channel can do, for example, in moving sediment. All else equal, a river with a larger hydraulic radius will have a higher flow velocity, and also a larger cross sectional area through which that faster water can travel. This means the greater the hydraulic radius, the larger volume of water the channel can carry.
  Based on the constant shear stress at the boundary assumption, hydraulic radius is defined as the ratio of the channels cross-sectional area of the flow to its wetted perimeter (the portion of the cross-sections perimeter that is "wet"):
  where:
  *  is the hydraulic radius (L);
  *  is the cross sectional area of flow (L);
  *  is the wetted perimeter (L).
  For channels of a given width, the hydraulic radius is greater for deeper channels. In wide rectangular channels, the hydraulic radius is approximated by the flow depth.
  The hydraulic radius is not half the hydraulic diameter as the name may suggest, but one quarter in the case of a full pipe. It is a function of the shape of the pipe, channel, or river in which the water is flowing.
  Hydraulic radius is also important in determining a channels efficiency (its ability to move water and sediment), and is one of the properties used by water engineers to assess the channels capacity.