The hydrograph run reaches maximum scour of 2.8 ft (0.85 m) shortly after the peak flow. A

different hydrograph with the same peak discharge but different shape would result in

different amounts of predicted scour. This is because the sediment transport model

computes scour based on the difference between rates of sediment supply and transport

capacity, which vary with discharge. Also shown in Figure 10.35 is the scour at the end of

the hydrograph. Some infilling of the scour hole is predicted to occur during the recession

limb of the hydrograph, and the channel would be expected to return to the original condition

after continued base flow. Because the hydrograph produces less scour than the steady

state analysis, some backwater is predicted for peak flow conditions in this simulation.

Figure10.36 shows a comparison of bridge cross sections from the BRI-STARS simulations.

As expected, the deepest scour is in the center of the main channel. Clear-water contraction

scour also occurs in the left overbank under the bridge (see Figure 10.33 for comparison).

The HEC-18 clear-water contraction scour equation predicted 1.7 ft (0.52 m). The steady

state model reached 0.81 ft (0.25 m) at the end of the 3-day simulation and the hydrograph

model predicted a maximum scour on the left overbank of 0.17 ft (0.05 m). Figure 10.35 also

shows how the scoured areas refill during the recession limb of the hydrograph.

20

15

10

5

Starting Bed

Max. Scour SS

Max. Scour, Hyd.

0

Final Scour, Hyd.

Max. Water Surface

-5

800

900

1000

1100

1200

1300

1400

1500

1600

Figure 10.36 Comparison cross-sections from BRI-STARS.

Although the HEC-18 scour results are larger than those predicted by these BRI-STARS

runs, the HEC-18 equation computes ultimate scour conditions using simplified relationships

for sediment transport. If the BRI-STARS run had been extended, greater scour would have

occurred, and since BRI-STARS does not scour the channel uniformly, the maximum scour

could exceed that predicted by the HEC-18 equations.

The original WSPRO model of this bridge included one cross section downstream (EXIT) and

one cross section upstream (APPROACH) of the bridge (Figure 10.29). These are the

minimum number of sections required for a hydraulic analysis. Sediment transport models

typically require additional sections. In addition to cross sections at the upstream and

downstream bridge faces, the BRI-STARS model included two downstream cross sections

and 18 upstream cross sections spaced at 650 ft (200 m) intervals along the channel.

10.47

Integrated Publishing, Inc. |