Full Equations (FEQ) Model for the Solution of the Full, Dynamic Equations
of Motion for One-Dimensional Unsteady Flow in Open Channels and Through
Control Structures
Whitaker (1968, p. 212) makes a comment about this issue in a chapter on macroscopic balances (his term for 1-D analysis):
The student should be forewarned that the methods to be studied in this chapter, and subsequent ones, are approximate; in general, there will be no "right" answers. There will often, however, be a "best" answer, and as often as possible we shall try to determine the best answer by comparing our results with experiments. In attacking this chapter, we should remember the macroscopic balances are perhaps the most powerful tool the engineer possesses for solving the often ill-defined problems of everyday practice. Judicious application of these equations comes only with experience and practice. At best, the student can hope to understand the development of the equations and gain some insight regarding the difficulties that may be encountered in their application.
In the methods used in FEQ and in applications of FEQ, 1-D analysis is pushed to its limits; therefore, simplifications involved in 1-D analysis need to be thoroughly understood. Consideration also must be given to the requirements on the hydraulic geometry to meet the assumptions of 1-D analysis and to development of a description of a stream channel that is consistent with these assumptions. An appreciation for the approximations of the hydraulic geometry is necessary to prevent overconfidence in the results of the FEQ analysis.