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30 CHAPTER 5 THE SRICOS-EFA METHOD FOR MAXIMUM SCOUR DEPTH AT COMPLEX PIERS 5.1 EXISTING KNOWLEDGE 5.3 FLUMES AND SCOUR MODELS An extensive review of the literature on the topic of pier The in-floor concrete flume, which is 1.5 m wide, 30.48 m scour in cohesive soils led to very few publications. Studies long, and 3.48 m deep, was used to conduct the complex pier related to the maximum scour depth included those of Hosny scour tests. The wooden flume, which is 0.45 m wide, 36 m (1995), Ivarson (1998), and Molinas et al. (1999). Hosny ran long, and 1.22 m deep, was used for the contraction scour tests. a large number of flume tests on prepared samples of clay These two flumes form a closed system (Figure 5.2) in which and sand mixtures. He recommended multiplying factors to the water is recirculated without any fresh water being added. include in the HEC-18 equation for the maximum scour False bottoms were designed to make sure that the start and depth in cohesionless soils. These factors are based on sim- end transitions would not affect the velocity distribution in the ple soil properties and lead to maximum scour depths smaller test area. For the 1.5-m-wide flume, the slopes of the ramps at than the cohesionless soil values by 10% to 40%. Ivarson the two ends of the false bottom are 1:3 (vertical to horizontal) (1998) developed a modification factor K4 for the HEC-18 to guarantee a smooth transition. The distance between the two equation for maximum scour depth in cohesionless soils. soil tanks is 7.6 m to make sure there is no interaction between The K4 factor is a multiplier in the HEC-18 equation and them. Trial tests were conducted before starting the scour tests, makes use of the undrained shear strength of the cohesive and the velocity distributions were measured along the center- soil. Molinas et al. (1999) presented a modification of the line of the channel to confirm the validity of the design. In the Hosny (1995) factors. Studies related to numerical simula- 1.5-m-wide flume, the soil tank is 0.6 m deep and 1.5 m long tions were more numerous. for the front tank and 0.6 m deep and 1.2 m long for the rear tank. The false bottom is built with plastic plates and supported by aluminum frames. 5.2 GENERAL The false bottom in the 0.45-m-wide flume was designed for the contraction scour tests. A smooth transition between Chapter 4 described the SRICOS method for cylindrical the uncontracted and contracted channels was constructed. piers in deep water. This chapter deals with piers that can be The soil tank is 2.0 m long and 0.3 m deep to provide enough rectangular, square, or cylindrical; are attacked by the flow space for both the long contraction flow and the contraction at a non-zero angle between the flow direction and the main scour hole. This false bottom is made of plywood. axis of the pier; and are in shallow water. The influence of Two types of pier models were used in the complex pier pier spacing also is discussed. Figure 5.1 shows the definition scour tests as shown in Figure 5.3. The cylindrical piers were of the parameters involved with these influencing factors. cut from PVC pipes with three different diameters: 273 mm, The approach consisted of using the solution for the case of 160 mm, and 61 mm. The rectangular piers were made of the cylindrical pier in deep water: plywood with the same width (61 mm) and different lengths: 61 mm, 122 mm, 244 mm, and so on. For piers with projec- Zmax ( mm ) = 0.18 Re 0.635 (5.1) tion Width B larger than 160 mm, strutted frames were needed to fix the piers so they would neither sway nor be flushed away during scouring. The abutment models were all and developing correction factors to include the effect of the made of plywood and are shown in Figure 5.4, which also various situations deviating from that case. Since the case of shows the transition inlets for the contraction scour tests. the cylindrical pier in deep water was developed on the basis of two fundamental equations (i.e., maximum scour depth and initial maximum shear stress), two sets of correction fac- 5.4 MEASURING EQUIPMENT tors had to be developed. The correction factors for the max- imum scour depth were developed on the basis of flume tests; The Acoustic Doppler Velocimeter (ADV), as shown in the correction factors for the initial maximum shear stress Figure 5.5, uses acoustic techniques to measure the velocity were developed on the basis of numerical simulations. in a remotely sensed volume so that the measured flow is
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31 B F low V H Flow V B B' (a) Water Depth Effect g B S L (b) Pier Spacing Effect (d) Attack Angle Effect B F low V L (c) Pier Shape Effect Figure 5.1. Parameter definition for complex pier scour. (5) (3) (6) ((3)) (3) (2) (4) 0.45m Flume 1.50m Flume (7) (14) (8) (9) (10) (11) (12) (13) (13) (2) (4) (4) (1) Tail Gate (4) Soil Tank (7) Computer (10) Pumps (13) Piers (2) False Bottom (5) Carriage (8) Water Fall (11) Measuring Cage (14) Mini Pump (3) Contraction Abutments (6) ADV and Point Gage (9) Switch (12) Screen Wire Figure 5.2. Diagram of the flume system (not to scale).