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From page 15... ... 15 CHAPTER 2. Background Research 2.1 Introduction Numerous idealized experimental studies of scour around bridges, conducted since the late 1950s, have generated largely empirical formulas for scour depth estimation at bridge foundations. Read the entire page →
From page 16... ... 16 Variables are defined in Figure 2-1. Lb W LpVf2, qf2 Vf1, qf1 La Vm1, qm1 Vm2, qm2 Bf BmMain-channel Flood-plain hbYf1 Ym1 YfoYf2max Vp Approach Bridge Main-channel Flood-plain Flood-plain Section A-A Plan Figure 2-1. Read the entire page →
From page 17... ... 17 K is a factor accounting for the abutment alignment to the flow, aL is the length of the embankment and abutment, F is the approach flow Froude number, 50d is median sediment grain size, g is geometric standard deviation of the sediment size distribution, and "+1" is added for the factor of safety (FS) Read the entire page →
From page 19... ... 19 Table 2-1. Abutment shape factors for Melville scour formula Abutment Shape Ks Vertical-wall Square End 1.00 Semi-circular End 0.75 45° Wingwall 0.75 Spill -through 0.5:1.0 (H:V) Read the entire page →
From page 20... ... 20 Figure 2-2. Typical cases of abutment positions in compound channels (reproduced from Melville and Coleman (2000) Read the entire page →
From page 21... ... 21 In a 3.3-m wide by 5.2-m long flume, Sturm and Janjua (1994) conducted a series of experiments with a compound channel in which the ratio of abutment length to floodplain width varied from 0.24 to 0.62, and M varied from 0.64 to 0.91. Read the entire page →
From page 22... ... 22 result is consistent with that of Melville and Coleman (2000) in that the abutment shape effect becomes immaterial for long embankments due to dominance of contraction effects in comparison to turbulence. Read the entire page →
From page 23... ... 23 Scour Condition A Scour Condition B Scour Condition C Figure 2-3. Abutment scour conditions: Scour Condition A - bank failure and failure of the abutment face, Scour Condition B - failure of the abutment face, and Scour Condition C - breaching of the approach embankment (Ettema et al. Read the entire page →
From page 24... ... 24 An important finding was definition of a geotechnical limit to the maximum scour depth. Due to sliding or even outflanking of the earthfill embankment, the contracted flow through the bridge opening is relieved and maximum scour depth is reduced. Read the entire page →
From page 25... ... 25             1 76 1 2 7/3 76 11maxmax f f c f BTBffs q q mCYYYd   (2-17) in which, maxsd = potential scour depth, maxY = maximum flow depth where maximum scour depth is located, 1mY = initial flow depth in the approach main channel, 1fY = initial approach flow depth on the floodplain, TAC = coefficient of turbulent influence for abutment scour in Scour Condition A, TBC = coefficient of turbulent influence for abutment scour in Scour Condition B, Am and Bm are the values 2max qq for the Scour Condition A and B, respectively, maxq = unit discharge coinciding with the location of deepest scour in the main channel, 111 mmm BQq  , 2mq = estimate of the mean value of the unit discharge through the bridge opening in the main channel, f = shear stress in approach flow section on the floodplain, c = critical shear stress for sediment movement, 1fq = average flow rate per unit width on the approach flow floodplain, and 2fq is the average flow rate per unit width at the bridge opening floodplain. Read the entire page →
From page 26... ... 26 7/6 1 1 1 2 0 1 16.0 1 2max2 75.2                        fc f f f f f f f fo f V V q q Y Y q q Y Y (2-18) For bankline abutments (BLA) Read the entire page →
From page 27... ... 27 been of concern for decades and analytical solutions have been developed for an idealized long contraction both in clear-water and live-bed scour conditions. As observed by Melville and Coleman (2000) Read the entire page →
From page 28... ... 28 sediment transport formula depending on whether sediment load is mostly bed-load, mixed load, or mostly suspended load, 1mB = approach-flow main channel width, and 2mB = contraction main channel width. Read the entire page →
From page 30... ... 30 Figure 2-5. Definition diagram for pressure-flow scour at bridges (HEC-18) Read the entire page →
From page 31... ... 31 In HEC-18, Arneson et al. Read the entire page →
From page 32... ... 32 sets of data are plotted, the new live-bed data and data from Umbrell et al. Read the entire page →
From page 33... ... 33 The equation of the solid (envelope) line is 5.20.145.0 14.04.075.0 1 1 11 1       c s cc s V V Y d V V V V Y d (2-34) Read the entire page →
From page 34... ... 34 The literature search undertaken for NCHRP Project 24-32 revealed that there is very little information (predictive equations and data) explicitly on scour at wide piers and long skewed piers. Read the entire page →
From page 35... ... 35 These two equations are used in the present project when considering combined local scour and contraction scour at bridge piers. According to recommendations in HEC-18, total scour at bridge piers is estimated by simply adding the contraction scour and local scour, inherently assuming that the scour processes for each are independent. Read the entire page →
From page 36... ... 36 2.6 Field Studies Sources of field data that might be employed in this research were explored. Criteria for suitable field data to be used were established as:  Continuous, real-time measurements of scour and the velocity field during several storm events;  Documentation of the occurrence of more than one type of scour;  Detailed bathymetry of the flow approach section, the bridge section, and the exit section as required for HEC-RAS modeling;  Detailed geometric data for the bridge;  Continuous measurement of discharge at a USGS rated gauging station during the storm events at the bridge;  Size distribution and other geotechnical properties of collected field samples of the soil materials at the bridge foundation augmented by geotechnical boring data obtained during the bridge design phase. Read the entire page →
From page 37... ... 37 affected directly the foundation of the bridge. The failure of the bridge was caused by the combined effect of local scour, bend scour, and contraction scour, together with the impinging jet scour and long-term bed degradation. Read the entire page →
From page 38... ... 38 References Scour Type Channel Shape Flow Type Hydraulic &soil data Measurements P A C V Q Soil Shatanawi et al. Read the entire page →
From page 39... ... 39 river crossing is located at the sharp bend of a compound channel with an asymmetric floodplain. Because the bridge crossing is located at a sharp bend, the concentrated channel flow is directed towards one particular pier, which would not normally experience the observed high velocity if the channel were straight. Read the entire page →
From page 40... ... 40 Figure 2-10. Aerial photograph of James River bridge near Mitchell, South Dakota (Rossell and Ting 2013) Read the entire page →
From page 41... ... 41 Lombard and Hodgkins (2008) and Zhang et al. Read the entire page →
From page 42... ... 42 Figure 2-11. Minnesota River near Belle Plaine. Read the entire page →
From page 43... ... 43 results shown in Figure 2-14 for the 1998 flood of 1840 m3/s show good agreement with the prototype measurements of total scour at the main bridge pier. However, between the piers, contraction scour is not predicted correctly by the model. Read the entire page →
From page 44... ... 44 Bent #4 Bent #2 Bent #3 70 75 80 85 90 95 100 0 20 40 60 80 100 120 140 Cross-section station (m) Read the entire page →
From page 45... ... 45 Figure 2-15. Measured contraction scour depths for Ocmulgee R Read the entire page →
From page 46... ... 46 Figure 2-16. Laboratory model of Towaliga River Bridge (Hong and Sturm, 2010) Read the entire page →
From page 47... ... 47 2.8 Scour Component Interaction Direct or indirect research on the interaction between local scour and contraction scour started as early as the work of Laursen and Toch (1956) Read the entire page →
From page 48... ... 48 energy (TKE = bK ) near the bed in the vicinity of the scour hole for F, SO, and OT flows. Read the entire page →
From page 49... ... 49 flow domain. The size of the grid spacing is very important with respect to accuracy and convergence of the numerical algorithms utilized in the approximation and solution of the governing equations. Read the entire page →
From page 50... ... 50 et al. Read the entire page →
From page 51... ... 51 Figure 2-19. Simulated water surface (top) Read the entire page →
From page 52... ... 52 The advantage of using LES for the simulation of such complex flows is that the method is able to capture the instantaneous turbulent flow and thus, when time-averaged, provides accurate flow statistics. After successful validation of their simulation Kara et al. Read the entire page →
From page 53... ... 53 about its evaluation is needed when pressure scour and overtopping are components of the total scour. At a more fundamental level, the contribution of turbulence to the amplification factor is not well known, and the question of whether its evaluation can be done using surrogate variables needs to be explored. Read the entire page →

From page 15...

... 15 CHAPTER 2. Background Research 2.1 Introduction Numerous idealized experimental studies of scour around bridges, conducted since the late 1950s, have generated largely empirical formulas for scour depth estimation at bridge foundations.