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From page 59... ... 59 5.1 Introduction Chapter 5 provides two approaches for assessing the conditional probability that the design scour depth will be exceeded for a given design flood event. Either approach can be used to estimate this probability for each of the three individual scour components. Read the entire page →
From page 60... ... 60 Reference Guide for Applying Risk and Reliability-Based Approaches for Bridge Scour Prediction 5.2.2 Calibration of Level I Statistical Parameters The Level I approach provides an easy-to-apply method to allow the engineer to control the level of safety to use when designing a foundation for scour. The calibration of the probability values and scour factors requires knowledge of the appropriate bias and COV values, which may depend on the bridge foundation and channel geometric and site conditions. Read the entire page →
From page 61... ... Probability-Based Scour Estimates 61 of hydrologic uncertainty. The values in Table 5.2 show the 100-year discharges used for the typical bridges and correspond to the values shown in Table 3.2 and Table 3.4. Read the entire page →
From page 62... ... 62 Reference Guide for Applying Risk and Reliability-Based Approaches for Bridge Scour Prediction a total design scour of 15.22 ft. Considering the bias in the scour equations, the results of the Monte Carlo simulation indicate expected scour of 4.89 ft of pier scour, 7.42 ft of contraction scour, and 12.31 ft of total scour. Read the entire page →
From page 63... ... Probability-Based Scour Estimates 63 scour and a very large value of COV, which was 0.21 from the model (equation) and increased to 0.37 for this bridge associated with hydraulic uncertainty. Read the entire page →
From page 64... ... 64 Reference Guide for Applying Risk and Reliability-Based Approaches for Bridge Scour Prediction contraction) and standard deviation of 2.85 ft (0.792 + 2.742) Read the entire page →
From page 65... ... Probability-Based Scour Estimates 65 of the simulations. For pier scour, whether the HEC-18 or Florida DOT equation is used, there is very little difference in the scour factors among the 27 simulations. Read the entire page →
From page 66... ... 66 Reference Guide for Applying Risk and Reliability-Based Approaches for Bridge Scour Prediction results shown in Figure 5.2(a) and the statistics shown in Figure 5.2(b) Read the entire page →
From page 67... ... Probability-Based Scour Estimates 67 Figure 5.3 shows the scour factors for contraction scour. Pier size was considered a secondary influence with contraction scour; therefore, the nine conditions represent bridge size and hydrologic uncertainty. Read the entire page →
From page 68... ... 68 Reference Guide for Applying Risk and Reliability-Based Approaches for Bridge Scour Prediction Section 5.4.3) Read the entire page →
From page 69... ... Probability-Based Scour Estimates 69 (a) Read the entire page →
From page 70... ... 70 Reference Guide for Applying Risk and Reliability-Based Approaches for Bridge Scour Prediction 5.4.1 Step-by-Step Procedure for Level II Analysis A Level II analysis involves developing the statistical distribution of each scour component and total scour at a particular bridge site. This type of analysis may be required if the site conditions differ significantly from the conditions used to develop the Level I tables presented in Appendix B Read the entire page →
From page 71... ... Probability-Based Scour Estimates 71 then the model uncertainties (bias and COV) from the laboratory data analysis presented in Chapter 4 should be used. Read the entire page →
From page 72... ... 72 Reference Guide for Applying Risk and Reliability-Based Approaches for Bridge Scour Prediction 0.68 x 0.16 = 0.109) Read the entire page →
From page 73... ... Probability-Based Scour Estimates 73 significantly, the computed range of pier scour was 4 ft for the HEC-18 equation and was only 1.6 ft for the Florida DOT equation. For this range of hydraulic conditions, the range of computed scour from the Florida DOT equation is very small, indicating that the Florida DOT equation is less sensitive to hydraulic conditions. Read the entire page →
From page 74... ... 74 Reference Guide for Applying Risk and Reliability-Based Approaches for Bridge Scour Prediction in each case the design value is very close to the mean of the calculated values. This is expected because in the Monte Carlo simulation velocity and depth are distributed around the basemodel results. Read the entire page →
From page 75... ... Probability-Based Scour Estimates 75 result in less required scour (11.2 x 1.17 = 13.1 ft) to achieve the same reliability as the HEC-18 equation (13.7 x 1.04 = 14.2 ft) Read the entire page →
From page 76... ... 76 Reference Guide for Applying Risk and Reliability-Based Approaches for Bridge Scour Prediction 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 0 10000 20000 30000 40000 50000 60000 70000 80000 Co nt ra ct io n Sc ou r( ) RoadOvertoppingDischarge (cfs) Read the entire page →
From page 77... ... Probability-Based Scour Estimates 77 with and without road overtopping flow. The design scour is 5.3 ft, which is centered within the distributions. Read the entire page →
From page 78... ... 78 Reference Guide for Applying Risk and Reliability-Based Approaches for Bridge Scour Prediction would require that contraction scour be multiplied by a factor of 1.8, resulting in a design scour of 9.7 ft if road overtopping is not considered. With road overtopping at this bridge, a b of 2 would require multiplying contraction scour by 1.6, giving 8.5 ft of scour to be used for design. Read the entire page →
From page 79... ... Probability-Based Scour Estimates 79 0% 10% 20% 30% 40% 50% 60% 70% 80% 90% 100% 0 500 1000 1500 2000 2500 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 M or e Fr eq ue nc y Abutment Scour () Frequency of Computed Scour Frequency after Bias and COV Cumulave % of Computed Scour Cumulave % after Bias and COV Figure 5.10. Read the entire page →

From page 59...

... 59 5.1 Introduction Chapter 5 provides two approaches for assessing the conditional probability that the design scour depth will be exceeded for a given design flood event. Either approach can be used to estimate this probability for each of the three individual scour components.