Figure 10.33. Revised plot of total scour for example problem.
10.3.5 Level 3 - BRI-STARS Analysis
The contraction scour computed in Section 10.3.4 using the HEC-18 (Richardson and Davis
2001) live-bed and clear-water scour equations could also be determined using the Level 3
approach of applying a sediment transport model such as BRI-STARS (see discussion in
Section 5.6.2). Contraction scour occurs in a bridge when the sediment supplied from
upstream is less than the sediment transport capacity in the bridge opening. The HEC-18
equations compute the ultimate amount of contraction scour that would occur if the hydraulic
condition persists until the transport capacity in the bridge opening matches the upstream
sediment supply.
Two BRI-STARS models were developed for the comprehensive scour example presented in
HEC-18 and summarized above. The first BRI-STARS model simulated a constant flow of
30,000 cfs (850 m3/s), the 100-year flow, for a 3-day period. The second model was run for
a 3-day hydrograph that peaks at 30,000 cfs (850 m3/s). The hydrograph is presented in
Figure 10.34. The models used 1.2 hour computation intervals for a total of 60 time steps for
each of the 3-day simulations (Arneson 2001).
Figure 10.35 shows the comparison thalweg profiles in the vicinity of the of the bridge, which
is located between stations 1400 and 1450. Water surface and bed profiles are shown for
the steady state (SS) and hydrograph (Hyd.) runs. The starting profile is a uniform slope of
0.002 ft/ft (m/m). For the steady state run, scour increases for the entire simulation and
would continue well beyond the 3-day simulation time. At the end of the steady state
simulation the maximum scour is approximately 4.5 ft (1.4 m) as compared with the HEC-18
(see Section 10.3.4) result of 9.2 ft (2.8 m). For the steady state run, the starting water
surface profile shows approximately 1.4 ft (0.43 m) of backwater. At the maximum scour
condition, however, there is virtually no backwater because the scour enlarges the bridge
opening and reduces the head loss caused by the structure.
10.45
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