When you use the and the starting water surface elevation is unknown or indefinite, you can use a computer to calculate several backwater profiles based on several arbitrary starting elevations for the same discharge. If you plot these profiles ,as shown in Figure 7‑16, they will tend to converge to a common curve at some point upstream because each successive calculation brings the water level nearer the uniform depth profile.

The purpose of plotting the curves and finding the convergence point is to determine where the proposed structure site is in reference to the convergence point. If the site is in the vicinity or upstream of the convergence point, you have started the calculations far enough downstream to define a proper tailwater from an unknown starting elevation. Otherwise, you may have to begin the calculations at a point further downstream by using additional cross sections.