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90 Hydrograph (Add 500year flood) Scour Depth Vs. Time (Add 500year flood) 18000 12 15000 10 Streamflow (m 3 /s) Scour Depth (m) 12000 8 9000 6 6000 4 3000 2 0 0 1960 1970 1980 1990 2000 1960 1970 1980 1990 2000 Time (Year) Time (Year) (a) Hydrograph (b) Scour Depth Vs. Time Curve Figure 11.1. Woodrow Wilson measured hydrograph spiked with a 500-year flood. and standard deviation deviation Q have, on average, a maximum value of about 12,000 m3s-1, which suggests that such a distribution gives an Q 2 Log 1 + adequate representation of the extrema. Figure 11.3 shows an y = (11.2) Q example of a generated future hydrograph and the associated scour depth history as predicted by the SRICOS-EFA Method. with Q and Q being the mean and the standard deviation of daily stream-flow, respectively. 11.3 RISK APPROACH In the case of the Woodrow Wilson Bridge, stream-flow TO SCOUR PREDICTIONS data is available at the Little Falls Station (USGS #01646500) on the Potomac River, approximately 13 km upstream from Many equally possible future hydrographs such as the one the bridge. Correction of the measured stream flow is applied in Figure 11.3 are generated by the random sampling process. by multiplying the values by the drainage area ratio. The cor- For each hydrograph, the SRICOS program generates a scour rection is on the order of 3%. Figure 11.2 shows the original depth history, including a final depth of scour, d, at the end of hydrograph and the corresponding prediction of scour depth the project life. These values of the final depth of scour can history using the SRICOS-EFA Method. The mean and stan- be organized in a frequency distribution. Figure 11.4 shows dard deviation of Q in the period of record 19312000 are the probability distributions obtained for the example of the Q=327 m3s-1, and Q=467 m3s-1, respectively, while the max- Woodrow Wilson Bridge at the end of a chosen bridge life, Lt. imum discharge in the 70-year-long record was 12,056 m3s-1. This analysis can be used to estimate the level of risk, R, Synthetic hydrographs of the same length generated by sam- associated with the choice of different design values of scour pling from a log-normal distribution of mean Q and standard depth and project lives. By definition, the risk level is the prob- Original Hydrograph Original Scour Depth vs. Time 18000 12 15000 10 Streamflow (m 3 /s) Scour Depth (m) 12000 8 9000 6 6000 4 3000 2 0 0 1930 1940 1950 1960 1970 1980 1990 2000 1930 1940 1950 1960 1970 1980 1990 2000 Time (Year) Time (Year) (a) Hydrograph (b) Scour Depth vs. Time Figure 11.2. Original hydrograph and scour depth versus time near Woodrow Wilson Bridge site.