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56 CHAPTER 7 THE SRICOS-EFA METHOD FOR MAXIMUM CONTRACTION SCOUR DEPTH 7.1 EXISTING KNOWLEDGE bed shear stress exceeds the critical shear stress of the cohe- sive soils, contraction scour develops and the contraction Contraction scour refers to the lowering of the river bot- scour profile looks like that shown in Figure 7.1. This profile tom due to the narrowing of the flow opening between two identifies two separate scour depths: the maximum contrac- abutments or between two bridge piers. In cohesionless soils tion scour depth zmax, which occurs xmax after the beginning of equations are recommended by HEC-18 (1995) for live bed the start of the contracted channel, and the uniform scour and clear water contraction scour depths. These equations depth zunif, which occurs after that. involve one soil parameter: the mean grain size. The studies As described in this chapter, a series of flume tests were per- on cohesionless soils include those of Straub (1934), Laursen formed to develop equations to predict the values of zmax, xmax, (1960, 1963), Komura (1966), Gill (1981), Lim (1998), Chang and zunif in cohesive soils. One of the inputs was the mean (1998), and Smith (1967). depth velocity of the water. The velocity that controls the con- For cohesive soils, two methods were found in the literature: traction scour was the velocity V2 in the contracted channel; Chang and Davis, and Ivarson. Both methods are empirically this velocity can be estimated by using the velocity in the based and have neither a time nor stratigraphy component. The uncontracted channel and the contraction ratio B2/B1 or by studies on contraction scour in cohesive soils are even fewer using a program such as HEC-RAS to obtain V2 directly. These in number than the studies on pier scour in cohesive soils. two approaches were developed in the analysis of the flume Chang and Davis (1998) proposed a method to predict clear tests. In Chapter 8, the contraction scour rate issue is addressed water contraction scour at bridges. They assumed that the through numerical simulation to obtain the initial shear stress maximum scour depth is reached when the critical flow veloc- before scour starts. ity for the bed material is equal to the average velocity of the flow. The critical velocity is defined as the velocity that causes 7.3 FLUME TESTS AND MEASUREMENTS the incipient motion of the bed particles. The method makes use of Neill's (1973) competent velocity concept, which is tied The flume used for the contraction tests was the 0.45-m- to the mean grain size D50. A series of equations to predict the wide flume because initial tests in the 1.5-m-wide flume led total depth of flow, including the maximum contraction scour to very large amounts of soil loss. The budget and the sched- depths, is proposed. The equations involve two parameters: ule did not allow the use of such large quantities of soil. The the unit discharge at the contraction and the mean grain size. parameters influencing the contraction scour were the mean Ivarson et al. (1996) (cited from Ivarson, 1998) developed depth approach velocity V1, the contraction ratio B2/B1, the an equation to predict contraction scour for cohesive soils approach water depth H1, the contraction or abutment transi- based on Laursen's non-cohesive soils contraction scour tion angle , the contraction length L, and the soil properties. equation. Ivarson et al. set the shear stress in the contracted In this research, a Porcelain clay, as described in Chapter 4, section equal to the critical shear stress at incipient motion. was used for all flume tests. The erosion function of the clay For cohesive soils, they use the relationship between critical has been described previously in Section 5.5 and Figure 5.6. shear stress and unconfined compressive strength (Flaxman, The most important parameters were considered to be V1, 1963). Ivarson et al. propose an equation that includes the fol- B2/B1, and H1 and led to the main part of the equation. The lowing parameters: the undrained shear strength of the soil, influence of the transition angle and the contraction length the water depth in the approach uncontracted channel, the dis- L were incorporated through the correction factors. There- charge per unit width in the contracted channel, and Man- fore, the tentative form of the equation to predict the con- ning's Coefficient in the contracted channel. traction depth Z (Zmax or Zunif) is Z = K K L f V1 , 2 , H1 B (7.1) 7.2 GENERAL B1 This chapter addresses the problem of contraction scour Where, K and KL are the correction factors for transition (Figure 7.1). When contraction of the flow occurs and if the angle and contraction length, respectively.