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81 involved in the scour process. In this case, the scour depth tion of time. Because the input for scour calculations is the Z1 (Point A on Figure 9.6b) in Layer 1 is reached after time velocity and not the discharge, it is necessary to transform t1; at that time, the situation is equivalent to having had Layer the discharge data at the gage station into velocity data at the 2 scoured over an equivalent time te (Point B on Figure 9.6c). bridge site. This can be done by using a program such as HEC- Therefore, when Layer 2 starts to be eroded, the scour depth RAS (Hydrologic Engineering Center--River Analysis Sys- versus time curve proceeds from Point B to Point C on Fig- tem, HEC-RAS, 1997), which was developed by the U.S. ure 9.6c. The combined scour process for the two-layer sys- Army Corps of Engineers. In order to run HEC-RAS, several tem corresponds to the path OAC on Figure 9.6d. geographic features are necessary, such as the average slope of In reality, there may be a series of soil layers with differ- the channel bed, the channel cross section, and the roughness ent erosion functions. The computations proceed by stepping coefficient of the riverbed. Figure 9.8 shows the discharge forward in time. The time steps are t long, the velocity is hydrograph, the discharge versus velocity curve (HEC-RAS the one for the corresponding flood event, and the erosion results), and the mean depth velocity at one of the piers ver- function (z versus ) is the one for the soil layer correspond- sus time (velocity hydrograph) for the Woodrow Wilson ing to the current scour depth (bottom of the scour hole). Bridge on the Potomac River in Washington D.C. between When t is such that the scour depth enters a new soil layer, 1960 and 1998. the computations follow the process described in Figure 9.6d. Geometry 9.4 INPUT FOR THE SRICOS-EFA PROGRAM The geometry includes channel geometry and bridge geom- The input includes parameters for the soil, water, and etry. The channel and bridge geometry are used for contrac- geometry of the problem. tion scour evaluation, including the determination of the con- traction ratio. The pier's size, shape, spacing, and angle of Soil Properties attack are used for pier scour calculations. Table 9.1 elaborates on aspects of geometry. In the SRICOS-EFA Method, the soil properties at the bridge site are represented by the soil erosion function, which 9.5 THE SRICOS-EFA PROGRAM is a measure of the erodibility of the soil. The soil erosion function is the relationship between the erosion rate z of the The SRICOS-EFA program automates the SRICOS-EFA soil and the hydraulic shear stress applied on the bottom of Method. The first version of the program was solving the riverbed. It is obtained by performing an EFA test on the soil problem of a cylindrical pier in deep water (Kwak, 1999; sample (Briaud et al., 2002). The erosion function (Figure Kwak et al., 2001). In this study the program was extended 9.7) is needed for each layer within the potential scour depth to predict complex pier scour and contraction including the at the bridge site. superposition of both scour modes. Using the input described in the previous section, the program automates the calcula- Hydrologic Data tions of all of the parameters: transformation of discharge into velocities, maximum shear stress, initial slope of the The water flow is represented by the velocity hydrograph. scour rate versus shear stress curve, maximum scour depth, This hydrograph can be obtained from a nearby gage station. and so on. Then, it proceeds with the techniques described to The hydrograph should last as long as the required period of handle multiflood and multilayer systems. prediction. Furthermore, if the hydrograph obtained from the The program was written in FORTRAN by using Visual gage station does not contain a 100-year flood, it can be spiked FORTRAN 5.0. The flow chart of the program in Figure 9.9 artificially to include such a large event if required by design. gives an overall view of the SRICOS-EFA Method, includ- The hydrograph is typically in the form of discharge as a func- ing all of the equations. As can be seen, there is one branch to handle complex pier scour alone, one branch to handle 20 contraction scour alone, and one branch to handle the con- current occurrence of complex pier scour and contraction Scour Rate (m m /hr) scour. The SRICOS-EFA program is a user-friendly, inter- 15 active code that guides the user through a step-by-step data input procedure except for velocity or discharge data. This 10 program, however, is not in the WindowsTM environment and needs to be implemented in such an environment for easier 5 use. For the hydrograph, the number of velocity or discharge c = 7 N /m 2 data points can be at least several tens of thousands for the 0 time duration corresponding to the design life of bridges and 0 10 20 30 40 50 60 70 80 2 if the velocity data is given on a daily basis. The velocity or Shear Stress (N/m ) discharge data should be prepared in the format of an ASCII Figure 9.7. Typical EFA test result. file or a text document before running the program. The input
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W o o d r o w W ils o n B r id g e H y d r o g r a p h y (fr o m 1 9 6 0 to 1 9 9 8 ) 12000 10000 Discharge (m3/sec) 8000 6000 4000 2000 0 1960 1963 1966 1969 1972 1975 1978 1981 1984 1987 1990 1993 1996 1999 Ye ar R e la tio n s h ip o f D is c h a rg e a n d V e lo c ity (W o o do w W ils o n B ridg e a t P ie r 1 E ) 4 .0 0 3 .5 0 V e lo c it y ( m / s e c ) 3 .0 0 2 .5 0 2 .0 0 1 .5 0 1 .0 0 0 .5 0 0 .0 0 0 5000 10000 15000 20000 25000 3 D i s c h ar g e ( m /s ) W o o d r o w W ils o n B r id g e F lo w V e lo c ity C h a r t (fr o m 1 9 6 0 to 1 9 9 8 ) 3 2 .5 Velocity (m/sec) 2 1 .5 1 0 .5 0 1960 1963 1966 1968 1971 1974 1977 1980 1983 1985 1988 1991 1994 1997 Ye ar Figure 9.8. Example of hydrograph transformation for Pier 1E of the Woodrow Wilson Bridge on the Potomac River in Washington, D.C. TABLE 9.1 Summary of geometry factors Bridge Geometric Factors Channel Geometric Factors Bridge contraction ratio Channel contraction ratio Bridge opening Bridge contraction length Channel contraction length Type, shape Channel water depth Channel Attack angle characteristics Bridge Size, length, width (diameter) Manning coefficient piers Pier spacing Channel hydraulic radius Number of piers Soil stratigraphy
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83 Figure 9.9. Flow chart of the SRICOS-EFA Program.