In general, contraction and expansion losses are not used in unsteady flow, and therefore the default coefficients are 0.0. Forces due to contractions and expansion are handled in the momentum equation through pressure force differences. However, because HEC-RAS is a one-dimensional unsteady flow model, the one-dimensional momentum equation does not always capture all of the forces acting on the flow field at a sharp contraction and/or expansion zone. In order to better approximate the forces acting on the water, and the resulting water surface elevation, at a contraction and/or expansion, the user can enter empirical contraction and expansion coefficients for unsteady flow modeling. These coefficients will be multiplied by a change in velocity head, just like in steady flow modeling, but the resulting energy loss gets converted to an equivalent force for placement into the momentum equation.
The user has the ability to view and edit contraction and expansion coefficients for Unsteady Flow hydraulics in a tabular format. This is accomplished by selecting Contraction/Expansion Coefficients (Unsteady Flow) from the Tables menu of the Geometric Data editor. Once this option is selected, a window will appear similar to the one shown in Figure 5-73. The user has the same editing features as described previously for the n values table. See the discussion under Manning's n values, in the previous section, for details on how to edit the data.

Figure 5 73. Contraction and Expansion Coefficients (Unsteady Flow) Table