Standard Step Backwater Method (used for Energy Balance Method computations)

Refer to Chapter 7 for the
Standard Step Backwater Method
. Figure 9-12 shows the relative location of section geometry for profile computations. B
d
 and B
u
 refer to the bridge geometry at the downstream and upstream inside faces, respectively.
Relative Location of Section Geometry (click in image to see full-size image)
Figure 9-12. Relative Location of Section Geometry
  1. Solve the energy equation (step backwater) between cross section 2 and the downstream bridge face (B
    d
    ). Use the water surface at cross section 2 determined from the previous backwater profile computations.
  2. Proceed with the standard step backwater calculations from the downstream bridge face to the upstream face. Use the bridge geometry superimposed on cross sections 2 and 3 respectively.
  3. Approximate the effects of piers and impingement of flow on the lowchord by reducing the section area and increasing the wetted perimeter accordingly.
  4. Similarly, consider roadway overflow as open channel flow. Proceed with calculations from the upstream bridge face (B
    u
    ) to cross section 3.
  5. As indicated in the previous
    Flow Zones and Energy Losses
    subsection, proceed with calculating the remainder of the bridge impact from cross section 3 upstream using step backwater calculations.
Under the right circumstances, you can consider the energy balance method for low flow and high flow.