Evaluation of Bridge Scour Research: Pier Scour Processes and Predictions (2011) / Chapter Skim
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From page 46... ...
44 CHAPTER 4 PARAMETER FRAMEWORK 4.1 Introduction Quantitative explanation of how pier flow field, erodibility of foundation material, and erosion processes influence pier scour depth requires a framework of parameters linking variables influencing pier scour. The framework's central parameters can be determined by considering the variables associated with scour at a cylindrical pier in a single stratum of non-cohesive sediment.
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From page 47... ...
45 The parameter framework must be consistent and comprise relevant parameters. As several variables exert multiple influences often interlinked, parameter sets can be formed of less relevant parameters.
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46 Vc = critical shear velocity for bed sediment entrainment; D and σg = median size and geometric standard deviation of the foundation material particle size distribution; ρs = sediment density; and c = a parameter describing cohesiveness of the material. The ensuing discussion focuses on coheshionless foundation material, leaving consideration of material cohesiveness to Chapter 5, which addresses complications at pier sites.
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47 Some of the parameters in Eq.
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48 σs is geometric standard deviation of bed particles, and characterizes sediment uniformity tV/a characterizes (in conjunction with other parameters) the temporal development of scour associated with pier flow field and nature of foundation material Before proceeding to discuss parameter influences, it is useful to make several comments: 1.
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49 considerations. Moreover, acquiring the data to establish the quantitative relationships of parameter influences quickly entails very extensive programs of laboratory experiments and field observations.
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50 water scour at reduced flow velocities, lesser scour depths are developed. The lines have been plotted assuming a linear relation between clear-water scour depth and flow velocity.
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51 geometry. Figure 3-6, in Chapter 3, illustrates a typical scour formation at a wide pier.
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52 methods yield scour depth estimates that exceed observed depths at wide piers. Piers tens of feet wide, though, are reported as creating scour holes considerably shallower (relative to pier width)
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53 Figure 4-3 Influence of sediment coarseness on local scour depth at piers for clear-water scour conditions (Melville and Coleman 2000) Figure 4-4 Influence of sediment coarseness on local scour depth at piers at different flow intensities for live-bed scour conditions (Melville and Coleman 2000)
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54 Figure 4-5 Local scour depth variation with sediment coarseness (Melville and Coleman 2000) However, for much larger values of a/D, representative of prototype sized piers founded in sandy materials, recent data by Sheppard et al.
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55 Figure 4-6 Influence of sediment size a/D50 on local scour depth ys /a (Lee and Sturm 2008) 4.4.3 Pier Face Shape, Ω As scour depth is consequent to the flow field a pier develops, the shape of a pier's face affects scour depth.
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56 The recommended shape factors for cylindrical piers given in Table 4-2 are normally used (e.g., Melville and Coleman 2000, Richardson et al. 2001, Sheppard et al.
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57 Table 4-3 Comparison of local scour depths for the pier shapes shown in Figure 4-8 (Mostafa 1994) Shape (Figure 4-17)
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58 The scour depth related to effective pier width, though, may not increase, because other parameters may exert influences; e.g., y/a and a/D, as discussed in subsequent subsections. Research since 1990 indicates that the use of the curves is not without complication in this regard (e.g., Ettema et al.
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59 Figure 4-10 Local scour depth variation with flow intensity, V/Vc (Melville and Coleman 2000) Under clear-water conditions, the local scour depth in uniform sediment increases almost linearly with velocity to a maximum at the threshold velocity.
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60 1984; Melville 1984; Raudkivi, 1986; Melville and Sutherland, 1988; and Dongol, 1994)
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61 Figure 4-11 Influence of flow intensity on local scour depth in uniform sediment (Melville and Coleman 2000) Figure 4-12 Influence of flow intensity on local scour depth in non-uniform sediment (Melville and Coleman 2000)
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62 Figure 4-13 Influence of sediment non-uniformity on local scour depth at piers subject to clear water scour (Melville and Coleman 2000) The broader context of the data is given in Figure 4-14, which schematically summarises the trends associated with varying intensity of bed material motions, V/Vc.
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63 Figure 4-14. Local scour depth variation with sediment non-uniformity (Melville and Coleman 2000)
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64 Ettema et al.
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65 scaling of the flow velocity in the field. Hence, the Froude number used in laboratory experiments may be larger than that for the corresponding field conditions.
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66 4.4.9 Time Rate of Scour, tV/a Though the objectives set for the present evaluation do not expressly include evaluation of the time-rate of scour development, it is useful to mention that differences in pier flow fields and in foundation material affect the rate of scour development. The parameter tV/a relates scour duration to pier width a, and approach velocity V
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67 Several papers outline the difficulties of scour estimation for these conditions; e.g., Hughes (1999) , Normets et al.
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68 For example, Hopkins and Beckham's (1999) comprehensive study of rock scour at bridge piers and abutments in Kentucky shows that for scour of weak rock may occur over several years before becoming noticeably severe.
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69 bed flows, t* is expected to rapidly decrease again, as shown by the dashed line (refer also to Figure 4-10)
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70 Figure 4-18 Equilibrium time-scale variation with flow shallowness, flow intensity and sediment coarseness
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71 4.5 Data Quality and Gaps The parameter trends are based largely on data obtained from laboratory flume experiments, and to a far lesser extent on field measurements at actual pier sites. The flume experiments normally use similitude considerations to interpret the data obtained.
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