2.10 Chapter Summary

Subcritical and supercritical regimes, normal and critical depths, alternate and sequent depths, along with many other terms and variables defined in Chapter 2, are important to compute successful solutions for most open channel problems. The computation of normal and critical depths and knowing how depth changes with distance along the channel are important keys in determining the shape of a water surface profile. Gradually varied flow profiles may be classified and sketched if normal, critical, and actual depths are known.

Numerical computation of a gradually varied water surface profile requires the continuity, energy, and Manning equations to obtain an adequate solution. The momentum equation is used for situations in which the energy equation is not adequate, such as where the flow changes from subcritical to supercritical and for analysis of hydraulic jumps. These equations are most often applied to steady, gradually varied, or rapidly varied flow situations to compute a depth or a water surface elevation for a specified discharge. Significant data are needed for these computations, including cross-section geometry, discharge, Manning's n, expansion and contraction coefficients, boundary conditions, and flow regime. Most computations are for subcritical flow, but the equations and procedures are equally applicable to supercritical flow.

The standard step method is the procedure most often used to compute water surface profiles, using the concept of conveyance to determine the distribution of flow in a cross section and the friction slope. The computation process is iterative, due to the nonlinearity of the equations, requiring a trial-and-error solution at every cross section. HEC-RAS is a popular computer program for performing these calculations.

Chapter 2 supplies the building blocks, in the form of equations and concepts, to help the reader gain a basic understanding of open channel hydraulics. Building on this introductory material, the following chapters describe the application of this information to develop water surface profile simulation models.