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OCR for page 19
19 Figure 4.2. Scour due to a sequence of two flood events (small flood followed by big flood). with Flood 2. Therefore, Flood 2 cannot create additional the program is the depth of scour versus time curve over the scour and the scour depth versus time curve remains flat dur- duration of the velocity versus time history. ing Flood 2. If z 1 is smaller than z max 2 then the procedure fol- lowed for the case of a small flood followed by a big flood 4.4 HARD SOIL LAYER applies, and the combined curve is as shown in Figure 4.4. OVER SOFT SOIL LAYER In the general case, the velocity versus time history exhibits many sequences of small floods and big floods. The calcula- The original SRICOS Method (Briaud et al., 1999) was tions for scour depth are performed by choosing an increment developed for a uniform soil. In order to investigate the influ- of time t and breaking the complete velocity versus time his- ence of the difference between a uniform soil and a more real- tory into a series of partial flood events, each lasting t. The istic layered soil on the depth of scour at a bridge pier, the case first two floods in the hydrograph are handled by using the of a two-layer soil profile scoured by a constant velocity flood procedure shown in Figure 4.2 or Figure 4.4, depending on was considered (Figure 4.5). Layer 1 is hard and z 1 thick, the case. Then the process advances by stepping into time and Layer 2 underlies Layer 1 and is softer than Layer 1. The scour considering a new "Flood 2" and a new t* at each step. The depth z versus time t curve for Layer 1 is given by Equation time t is typically one day, and a velocity versus time history 4.4 (Figure 4.5a) and the z versus t curve for Layer 2 is given can be 50 years long. The many steps of calculations are han- by Equation 4.5 (Figure 4.5b). If z 1 is larger than the maxi- dled with a computer program called SRICOS. The output of mum depth of scour in Layer 1, z max 1, then the scour process TABLE 4.1 Properties of the porcelain clay for the flume experiment Liquid Limit, % 34.4 Shear Strength, kPa(lab vane) 12.5 Plastic Limit, % 20.2 Cation Exchange Capacity, (meq/100g) 8.30 Plasticity Index, % 14.1 Sodium Adsorption Ratio 5.00 Water Content, % 28.5 Electrical Conductivity, (mmhos/cm) 1.20 Mean Diameter D50 , (mm) 0.0062 pH 6.00 Sand Content, % 0.0 Unit Weight (kN/m3) 18.0 Silt Content, % 75.0 Specific Gravity 2.61 Clay Content, % 25.0

OCR for page 19
Figure 4.3. Multiflood flume experiment results: a) floods and flood sequence in the experiments, b) experiment results for Flood 1 alone, c) experiment results for Flood 2 alone, d) experiment results for Floods 1 and 2 sequence shown in a) and prediction for Flood 2. Figure 4.4. Scour due to a sequence of two flood events (big flood followed by small flood).