Standard Step Method • Direct Step method is not suitable if we want to determine the flow depth at specified locations or if the channel is non-prismatic (i.e., channel cross section and/or bottom slope vary with distance) • The standard step method is a suitable method when we want to determine the flow depth at specified locations/ channel is non-prismatic/channel cross sections are available only at some specified locations. • HEC-RAS is based on the standard step method.
Standard Step Method • The flow depth for a specified discharge in a given channel at section 1 (distance ) is known; we want to determine the flow depth at distance (section 2). • (Eq. 6-22) • 6-22 equation may be solved by trial-error, Newton-Raphson or bisection methods. (Choudhry, 2008)
Example • A trapezoidal channel having a bottom slope of 0.001 is carrying a flow of . The bottom width is 10.0 m and the side slope are 2H to 1V. At the downstream end, a control structure raises the water depth to 5 m. Determine the water surface level at 1,2, and 4 km upstream of control structure. The Manning n for the flow surface is 0.013, and the elevation of the channel bottom at the downstream end is 0.0.