Evaluation of Bridge-Scour Research: Abutment and Contraction Scour Processes and Prediction (2011) / Chapter Skim
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From page 36... ...
34 CHAPTER 5. SCOUR DEPTH ESTIMATION FORMULAS Given the complexity of the various scour processes identified in Chapter 4, and the difficulty of including all of those processes in a single empirical formula, it is not surprising that current abutment scour formulas provide scour depth estimates that vary over a wide range of magnitudes.
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35 2 W Bm2Bm1 Bf V1 Floodplain Floodplain Main Channel Main Channel Y1 YF Y2 γ= skew angle Ks = shape factor kf = floodplain roughness Figure 5-1. Definition sketch for abutment terminating in a compound channel.
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36 Table 5-1. Classification of abutment scour parameters.
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37 layer or vice-versa. Fine-grained materials are often found in the river banks and floodplains while sand and gravel may form the erodible boundary of the main channel.
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38 solution to the idealized long rectangular contraction. The principal difference lies in the contraction ratios used; Gill used the full channel contraction ratio (approach channel width divided by bridge opening width)
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39 in a rectangular channel for L/B < 0.4, where B is channel width.
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40 local contraction of flow passing immediately around the abutment, and the turbulence structures generated by the abutment; and, 2. For a severely contracted bridge waterway, α diminishes to a value slightly above 1.
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41 protection, when using CTB compared to riprap aprons. Equations are given to predict the scour depth for spill-through abutments, situated on the flood plain of a compound channel, and the minimum apron width to prevent undermining of the toe at spill-through abutments.
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42 stress, the initial erosion rate is determined from the erodibility curve. Then the maximum scour depth and initial erosion rate are substituted into a standardized hyperbolic time development curve to obtain the scour depth for a specific duration of storm, or for a specified time history of flow taken over the life of the bridge.
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43 formulas essentially treat abutments as a "half pier" for small value of L or as a wide pier in shallow flow for larger values of L Table 5-2.
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44 channels, to obtain the discharge per unit width in the contracted section in ratio to its critical value, q2/qc. The scour methodology of Chang and Davis (1998, 1999)
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46 3. Class III: Wider crossings over braided river channels Class III refers to bridges spanning wide braided river channels, where the river channel can be approximated by a rectangular channel under extreme flood flow conditions.
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47 The following equations in Table A-1 may be considered applicable for estimation of "maximum possible" scour depths at non-erodible abutments/embankments at Class I and Class III crossings: • Liu et al.
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48 To these three scour conditions, a fourth might be added: • Scour Condition AB. This condition is a combination of A and B in which the floodplain as well as the embankment is erodible, and the scour hole on the floodplain can extend into the main channel.
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49 Abutment scour formulas that were developed from experiments in rectangular channels must be used very carefully in compound channel flow. The geometric contraction ratio does not properly represent the flow contraction effect in a compound channel.
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50 Table 5-3. Limitations and experimental databases of abutment scour formula.
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51 The duration of scour experiments has been discussed extensively in the literature. In general, live-bed scour experiments approach an equilibrium state, albeit with fluctuating bedforms, in a relatively short period of time compared to clear-water scour which only approaches equilibrium in an asymptotic manner.
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52 Figure 5-5. Comparison between scour data at a spill-through abutment (with riprap protection extended below the surface of the floodplain)
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53 Ettema et al.
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54 Figure 5-7. Comparison of Briaud et al.
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55 shown in the figure. It is interesting to note that both the Melville formula and the Sturm data for YMAX/YC follow the same trend of an increase to a maximum relative scour depth followed by a gradual decrease as q2/q1 increases.
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56 Figure 5-9. Scour depth trends for Scour Condition B
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57 Figure 5-10. Minnesota River near Belle Plaine, MN for 2001 flood.
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58 5.4.4. Ease of Use of Abutment Scour Formulas Ease of use of abutment scour formulas is a function of how easily and how accurately the important parameters can be estimated.
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