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
Distance (ft)
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
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