Effects of Debris on Bridge Pier Scour (2010) / Chapter Skim
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From page 41... ...
41 3.1 Introduction This chapter presents guidelines for assessing the potential for debris production and accumulation at a given bridge site, an overview of laboratory testing of a variety of debris configurations on model bridge piers, and an appraisal of the results. The summary of the current state of practice in Chapter 2 is combined with the testing results to provide an improved methodology for quantifying the depth and extent of scour at bridge piers resulting from the accumulation of organic debris.
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the data sheet entries and the flowcharts that are included within the guidelines. A considerable amount of information can be acquired prior to a site evaluation.
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Direct Evidence. The primary method of debris production is through bank erosion that results in woody vegetation being introduced into the channel.
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erosion, which is the primary method of debris introduction to a channel. These channel changes include the following: • Downcutting or incision • Lateral migration • Channel widening • Channelization • Dams and diversions • Widespread timber harvesting in the basin • Existing or potential changes in land use practices Some of the general upstream watershed characteristics that may have an indirect impact on debris production are documented under Item B1a, Lines 8–14, of the Field Data Sheets (Appendix D, Part 1)
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Direct evidence of a low potential for debris delivery includes the following: • The upstream channel is narrower and/or shallower during flood flows than most of the debris produced. • Debris is absent after floods in typical debris accumulation sites other than bridges (e.g., on bars and along the outer bank of meander bends)
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In the eastern United States, channels that are 40 to 200 ft (12 to 60 m) wide transport logs with an estimated design (or key)
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tively confined path within the channel. This path is closely related to the thalweg.
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Phase 2. Also, debris accumulation problems for an existing bridge can be tied to the location categories of those bridge elements identified under Phase 2 where debris accumulates, and this information could then be used to evaluate potential countermeasures and/or develop appropriate maintenance programs.
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ment to these types of piers under various flow conditions can significantly influence their debris-trapping potential. Where flow is skewed, the apertures may trap debris that would normally pass between the piers if they were aligned to flow.
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one end of the debris downward toward the streambed or flood plain, which can cause the other end to project upward into the substructure as shown in Figure 3.9. For this reason, the height of the vertical gap (i.e., vertical gap width)
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High LOW Potential LOCATION CATEGORY LOCATION CATEGORY MEDIUM Potential HIGH Potential HIGH, CHRONIC Potential YES NO PIER OR SUBSTRUCTURE DESIGN SOLID WITH APERTURES Potential for Delivery of Debris Low High Low Low High DEBRIS PATH DEBRIS PATHCHANNEL FLOOD PLAIN/ TOP BANK CHANNEL FLOOD PLAIN/ TOP BANK Potential for Delivery of Debris Potential for Delivery of Debris PIER OR SUBSTRUCTURE SECTION IN SHELTERED LOCATION CATEGORY? Figure 3.10.
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or chronic rating for debris accumulation potential that is based on a high or chronic rating for only one or two bridge elements. Therefore, it is recommended here that each individual element and gap/span should be treated as a separate entity in an effort to reduce design and construction costs, while maintaining structural integrity.
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dent on both banks upstream of the bridge and are the primary mode of debris delivery to the channel. Bridge WEL 50/67A has three sharp-nosed piers about 18 in.
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54 Rectangular Triangular Profile Geometry Source State Stream Number of Photos Physio- graphic Region Eco- region Single Log Multi Mass Multi Mass Conical Rectangular Inverted Cone Pacific Coast Arroyo Grande 11 24 260 X X Hopper Creek 3 24 M260 California (Central & South Coast) Salsipuedes Creek 5 24 M260 X California (Mountain)
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Rectangular Triangular Profile Geometry Source State Stream Number of Photos Physio- graphic Region Eco- region Single Log Multi Mass Multi Mass Conical Rectangular Inverted Cone Kentucky Unknown 2 X X Texas Clear Fork Brazos Rv. 6 12 310 Debris Tennessee Harpeth River 8 11 220 Free, Inc.
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of debris accumulation length upstream of the pier and debris accumulation width relative to pier width. Table 3.3 provides a list of sites from the photographic archive that were used to acquire measurements of relative debris accumulation width and length.
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submerged, depending on flow depth. Figures 3.23 and 3.24 display schematics of the various submergence possibilities.
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factor to the overall equation (such as the Kw factor for wide piers) or an adjustment to the pier width used as an input variable to the equation (similar to the HEC-18 complex pier approach)
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59 Figure 3.16. Mass of logs debris accumulation.
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Table 3.4 shows a summary of the observed debris dimensions contained in Table 3.3 and the range of debris dimensions for the laboratory physical modeling. All of the dimensions were normalized by the pier width so the field conditions could be used to develop a realistic range of laboratory runs.
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Flume Description All laboratory tests were conducted under clear-water conditions with a sand bed 1.5 ft (0.46 m) thick.
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62 Figure 3.26. Collapsed debris pile with an inverted cone geometry.
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63 H d L LOOKING DOWNSTREAM H d Pier width = a W PROFILE Pier length = a Flow Figure 3.28. Conical shape definition sketch.
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All piers were designed and constructed in two sections. The lower piece remained secured to the floor of the flume while the upper section could be removed at the pre-test sand level in order to easily re-level the bed after each test.
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65 60 m 13 m Mobile data acquisition cartConcrete cap PLAN VIEW 10 m10 m 10 m17 m Pier APier BPier CPier D Point gauge assembly with velocity probe Tailgate PROFILE flow 0.3 m 0.5 msand bed 2.4 m Figure 3.31. Schematic plan and profile of testing flume.
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pier and between each pier to quantify the water surface elevation and the velocity profile. Velocity data were collected and recorded with a SonTek Acoustic Doppler Velocimeter (ADV)
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and at positions of 7 ft (2 m)
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P6 1 2 3 4 5 9 8 7 15 16 17 18 11 12 13 14 22 21 20 19 10 37 38 39 40 33 34 35 36 28 23 31 30 29 44 43 42 41 32 50 46 47 48 49 45 53 52 51 59 60 61 62 55 56 57 5854 66 65 64 63 72 67 75 74 73 81 82 83 84 77 78 79 80 88 87 86 P P FLOW Pier DP Pier C Pier B Pier A P P P P P P P P P P Water Surface and Velocity at 20%, 40%, 60%, and 80% of Depth Water Surface and Velocity at 60% of Depth Pier 24 25 26 27 68 69 70 71 85 76 Figure 3.35. Schematic of data collection locations in 8 ft (2.4 m)
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transducers, Figure 3.37, collected a depth measurement through the transmission of a sonar pulse from the bottom of the transducer to the surface of the sand bed every 30 seconds. The data were sent to a data logger and then saved to a computer data file.
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the debris cluster. A schematic of the transducer orientation around a suspended debris cluster at a pier is presented in Figure 3.39a.
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71 b. Transducer array used for automated scour hole mapping.
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Figure 3.45 illustrates the tank and an example debris cluster being tested to determine the porosity. 3.5.3 Baseline Tests Bed Material and Incipient Motion The sand comprising the bed material in the test flume was characterized by a d50 grain size of approximately 0.70 mm.
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73 Figure 3.44. Typical photograph showing 2 in.
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0.047 for the initiation of movement, the critical velocity is estimated to range from 1.5 to 1.6 ft/s (0.45 to 0.49 m/s)
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of baseline scour for all pier shapes. Figure 3.47 shows the results for square piers after 72 hours of testing; the upper segment of the pier has been removed for data collection purposes and ambient bed elevation is represented by the top of the lower segment of the pier.
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3.5.4 Tests with Debris The goal of the laboratory plan was to develop a series of tests for a wide range of debris configurations that could be run quickly and efficiently. Tests were performed for single debris clusters at individual piers.
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77 LOOKING DOWNSTREAM H d Pier width = a W PROFILE H d Pier length = a L Flow Figure 3.49. Rectangular debris geometry and associated variables.
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Table 3.10 provides the results (maximum scour at the pier only) of tests performed with rectangular debris configurations.
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from the debris configuration presented in the previous figure after 8 hours of testing at 1 Vc; the upper segment of the pier has been removed for data collection purposes and ambient bed elevation is represented by the top of the lower segment of the pier. Table 3.13 provides the results of tests performed with triangular/conical debris configurations at a pier.
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80 Run No. Pier V/Vc Duration (h)
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81 LOOKING DOWNSTREAM H d Pier width = a W PROFILE H d Pier length = a L Flow Figure 3.56. Triangular/conical debris geometry and associated variables.
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82 Figure 3.57. Triangular/conical debris cluster before Test 007_02A, mounted such that the top surface of the debris was located at the water surface.
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where: Kt = Fraction of ultimate scour reached at time t (dimensionless) t = Elapsed time from start of scour, days te = Time to ultimate (equilibrium)
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84 Velocity ratio V/Vc Run No. Pier Target Meas.
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Equilibrium scour depths for baseline (no-debris) conditions must be identified for each of these pier types in order to assess the effect of debris on scour.
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table also shows the measured scour depths at the end of each test and the ultimate scour depth estimated by the Melville method. Scour created by rectangular debris clusters typically resulted in a trough upstream of the pier created by the plunging flow, as shown in Figures 3.65 and 3.66.
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motion of the bed material. The remaining 11 tests were conducted at a nominal velocity of 1.0 Vcrit.
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roughness elements. Dye tests performed at triangular clusters showed that at the centerline of the debris mass, flow tended to plunge beneath the debris but was also shed very readily around the sides of the debris, as seen in the figure.
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As with the rectangular debris tests, lateral extent of scour created by triangular debris clusters was directly related to the width of the cluster. However, the lateral extent of scour caused by a triangular debris cluster was shown to be greater than that of a rectangular one.
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Figure 3.70. Typical scour pattern at square pier with triangular debris cluster.
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depth of scour at the pier. The factors examined in this study included the following: • Shape: Rectangular or triangular • Size: Width, length, and thickness • Location: Surface (floating)
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• Triangular debris shapes: – 0.7 Vcrit: 3 tests – Vcrit: 11 tests Most of the tests (35 tests) were conducted with the top surface of the debris at the water surface, forming a "raft." Selected tests were also performed with the debris located in the center of the water column, resting on the bed, or buried into the bed.
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A modification to the equivalent width equation was therefore proposed and tested against the laboratory data. The proposed modification is denoted as "ad " to distinguish it from the Melville and Dongol "be" and is given as: where: Kd1 = Dimensionless coefficient optimized from laboratory test data Kd2 = Dimensionless exponent optimized from laboratory test data L = Length of debris upstream from pier face, ft (m)
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predict ultimate clear-water scour at the pier face, with the equivalent pier width calculated using the modified version of the Melville–Dongol equation (Equation 3.12)
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95 Figure 3.77. Example of a smooth, impermeable debris shape.
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program indicate that the lateral extent of the scour caused by floating debris rafts is directly proportional to the width of the raft. The impact of the lateral scour extent on adjacent piers or abutments was not directly investigated in this study program.
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3.8 Incorporating Debris in Hydraulic Models HEC-RAS (Brunner 2008) is a one-dimensional model that is the primary tool for simulating hydraulic conditions at bridges.
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grade upstream of the bridge by 0.06 ft (18.3 mm)
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caused by the obstruction. To model debris in FESWMS, the user should calculate the total obstructed area of the pier and debris, divide by the flow depth, and use the resulting width as the pier width.
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but care must be taken to assess whether changes in span length, deck height, pier type, pier orientation, or pier placement would affect potential debris characteristics. For a new bridge crossing, the assessment should include other nearby bridges and differences in reach conditions for debris production and delivery as well as differences in structure type when determining the debris characteristics.
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Step 6 Compute pier scour without debris: Alternatively, the pier scour can be computed using the projected width and excluding K2 from the pier scour equation. The projected width of the pier without debris is: Step 7 Determine the effective pier width with debris for the maximum debris dimensions.
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Calculate scour for ad equal to the largest computed value of 5.2 ft (1.58 m) excluding K1 and K2 from the pier scour equation.
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debris that will eventually create a scour hazard; however, this approach is often not practical given limited maintenance resources. Guidance for inspectors includes the following elements: • Inspection of debris conditions will usually consist of simple observations.
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3.10.3 Maintenance Obviously, removing debris from a pier before it becomes a large, fully developed mass is desirable, but such removal is often impractical from a management and operations standpoint. As discussed in HEC-9 (Bradley et al.
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3.11 Implementation Plan 3.11.1 The Product As described in more detail in the preceding sections, the products of this research include practical guidelines for predicting debris hazards at bridges and methods for predicting the depth, shape, and extent of scour at bridge piers resulting from debris accumulations. 3.11.2 The Market The market or audience for the results of this research will be hydraulic engineers and maintenance and inspection personnel in state, federal, and local agencies with a bridge-related responsibility.
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and decentralized target audience. For example, recommendations from this study could be considered for the next edition of HEC-18, "Evaluating Scour at Bridges," and NHI Course No.
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