Calculation of Open Channel Flow Hydraulic Radius: Calculate using Trapezoid Area

Calculation of the hydraulic radius for various channel cross section shapes is an important part of using the Manning equation for open channel flow calculations. Three common open channel cross sections, the rectangle, trapezoid, and triangle, are covered in this article. The hydraulic radius for open channel flow is defined as the cross sectional area of flow divided by the wetted perimeter. The calculation of trapezoid area, triangle area and rectangle area are important along with determination of the wetted perimeter of each of these shapes. A simplified expression for hydraulic radius of a trapezoidal channel is given from expressions for the trapezoid area and wetted perimeter. The rectangle and triangle area are used similarly.