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scour depth at a complex pier in a contracted channel. This not primarily cohesive. Nevertheless, these comparisons give
step-by-step procedure has been automated in a computer an indication that the SRICOS-EFA Method may not be lim-
program. ited to cohesive soils. Indeed, the fact that the method is
based on site-specific testing of the erosion function permits
1. Collect the input data: velocity and water depth hydro- incorporating the soil behavior directly in the predictions.
graph, geometry of the pier and of the contracted chan-
nel, erosion functions of the soil layers.
2. Calculate the maximum contraction scour depth for 13.1.11 Future Hydrographs and
the ith velocity in the hydrograph. Scour Risk Analysis
3. Calculate the maximum complex pier scour depth
using the ith velocity in the hydrograph at the pier A novel technique was presented on generating future
location if there is no contraction scour in Step 2, or hydrographs. Indeed, since the SRICOS-EFA Method pre-
the critical velocity for the soil if there is contraction dicts the scour depth as a function of time, it is necessary to
scour in Step 2. input into the program the hydrograph over the design life of
4. Calculate the total pier scour depth as the total of Step the bridge. The proposed technique consists of using a past
2 and Step 3. hydrograph (from a gage station, for example), preparing the
5. Calculate the initial maximum shear stress for pier frequency distribution plot for the floods within that hydro-
scour using the ith velocity in the hydrograph. graph, sampling the distribution randomly and preparing a
6. Read the initial scour rate corresponding to the initial future hydrograph for the required period that has the same
maximum shear stress of Step 5 on the erosion func- mean and standard deviation as the measured hydrograph.
tion of the soil layer corresponding to the current scour This process is repeated 10,000 times and, for each hydro-
depth. graph, a final scour depth (the depth reached at the end of the
7. Use the results of Steps 4 and 6 to construct the hyper- design life of the bridge) is generated. These 10,000 final
bola describing the scour depth versus time for the pier. depths of scour are organized in a frequency distribution plot
8. Calculate the equivalent time for the given curve of with a mean and a standard deviation. That plot can be used
Step 7. The equivalent time is the time required for the to quote a scour depth with a corresponding probability of
ith velocity on the hydrograph to scour the soil to a occurrence, or better, to choose a risk level and quote the cor-
depth equal to the depth scoured by all of the veloci- responding final depth of scour.
ties occurring prior to the ith velocity.
9. Read the additional scour generated by the ith veloc-
ity starting at the equivalent time and ending at the 13.1.12 Example Problems
equivalent time plus the time increment.
A set of example problems was presented to help the reader
10. Repeat Steps 2 to 9 for the (i + 1)th velocity and so on
become more familiar with the SRICOS-EFA Method. Some
until the entire hydrograph is consumed.
examples are performed by hand calculations; some use the
SRICOS-EFA computer program.
13.1.10 Verification of the SRICOS-EFA Method
Several full case histories were identified for verification 13.2 RECOMMENDATIONS
but none could satisfy the requirements necessary to verify
the method developed. Some did not have enough details on It is recommended that
the observed scour depth, some turned out not to be made of
cohesive soil after drilling, some did not have a gage station 1. The proposed method be incorporated in the next ver-
nearby. It was decided to compare the maximum scour depth sion of HEC-18;
for pier and contraction to existing databases. These data- 2. The SRICOS-EFA Method program be transferred to a
bases were mostly in sand, however, and included those col- WindowsTM environment;
lected by Mueller (pier scour), Froehlich (pier scour), and Gill 3. The project be continued to solve abutment scour, the
(contraction scour). The comparisons between the predicted last major unsolved scour problem in cohesive soils; and
and measured scour depths are very satisfactory although it 4. A set of short courses be offered across the country to
is not clear whether they should be or not since the soils were teach the new method and the corresponding program.