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Figure 1.1. Typical bridge with potential contraction and pier scour.
of time is usually sufficient to generate the maximum scour numerical simulations, fundamental principles in method
depth in cohesionless soils. This means that only the peak development, and verification of the method against avail-
velocity needs to be used in the calculations of scour depth for able data. The review of existing knowledge avoided dupli-
cohesionless soils and that such a scour depth is the maximum cation of effort and helped in establishing a solid foundation.
scour depth for that velocity. Typically, the velocities used are The flume tests gave the equations for the maximum scour
the 100- and 500-year flood velocities. In cohesive soils, depth and the influence of various factors. The flume tests
scour and erosion rates can be 1,000 times slower than in also gave a calibration basis for the numerical simulations.
cohesionless soils and a few days may generate only a small These numerical simulations were used to generate the equa-
fraction of the maximum scour depth. Therefore, for cohe- tions for the maximum initial shear stress at the initiation of
sive soils it becomes necessary to consider the rate of ero- scour. The method was assembled by linking the calculated
sion and the cumulative effect of multiple floods. initial erosion rate (given by the numerical simulation results
and the results of the EFA test) to the calculated maximum
1.5 APPROACH SELECTED TO SOLVE scour depth (given by the flume tests results) through the use
THE PROBLEM of a hyperbolic model. The multiflood hydrograph and mul-
tilayer soil were included through simple accumulation algo-
The approach selected to solve the problem of predicting rithms. Verification was based on comparison with existing
the scour depth versus time for complex piers in a contracted databases as well as performing calculations for sample cases
channel and for a given velocity hydrograph was based on a and evaluating the reasonableness of the results based on
combination of existing knowledge review, flume tests, experience.