1、 CHAPTER 7 HYDRAULIC ANALYSIS FOR THE LOCATION AND DESIGN OF BRIDGES 2007 by the American Association of State Highway and Transportation Officials. 2007 by the American Association of State Highway and Transportation Officials.CHAPTER 7 TABLE OF CONTENTS 7.1 INTRODUCTION. 7-1 7.2 PLANNING AND LOCAT
2、ION. 7-1 7.2.1 Location of Stream Crossing . 7-2 7.2.2 Structure Type 7-3 7.2.3 Environmental Considerations. 7-3 7.2.4 Coordination with Other Agencies 7-4 7.2.4.1 Water Resources Agencies. 7-5 7.2.4.2 Permits and Approvals . 7-5 7.2.5 Stream Morphology . 7-6 7.2.5.1 Types of Streams 7-7 7.2.5.1.1
3、Braided Streams. 7-7 7.2.5.1.2 Straight Streams. 7-8 7.2.5.1.3 Meandering Streams 7-10 7.2.5.2 Islands, Delta Formations, and Alluvial Fans 7-12 7.2.5.3 Aggradation and Degradation 7-14 7.2.6 Confluences . 7-15 7.2.7 Tidal Areas. 7-17 7.2.8 Floodplain Levees and Encroachments 7-17 7.2.9 Replacement,
4、 Repair, and Rehabilitation. 7-18 7.3 DATA COLLECTION . 7-19 7.3.1 Topographic Features. 7-19 7.3.2 Land Use and Culture 7-19 7.3.3 Hydrologic Data. 7-20 7.3.3.1 Flood Data 7-20 7.3.3.2 Basin Characteristics 7-20 7.3.3.3 Precipitation . 7-22 7.3.3.4 Highwater Information. 7-22 7.3.4 Existing Structu
5、res. 7-23 7.3.5 Channel Characteristics . 7-24 7.3.6 Environmental Data . 7-24 7.3.7 Site Plan . 7-25 7.3.8 Field Reviews. 7-26 7.4 HYDROLOGIC ANALYSIS . 7-26 7.5 HYDRAULIC ANALYSIS OF THE STREAM. 7-28 7.5.1 Stage-Discharge Relationships 7-28 7.5.2 Types of Stage-Discharge Relationships . 7-30 7.5.2
6、.1 Stable 7-30 7.5.2.2 Unstable 7-30 2007 by the American Association of State Highway and Transportation Officials.Highway Drainage Guidelines 7-iv 7.6 STREAM-CROSSING DESIGN . 7-31 7.6.1 Criteria . 7-31 7.6.2 Highway-Stream Crossing Systems. 7-33 7.6.2.1 Highway Profile and Alignment. 7-34 7.6.2.2
7、 Waterway Openings . 7-39 7.6.2.2.1 Location . 7-39 7.6.2.2.2 Size 7-43 7.6.2.2.3 Auxiliary Openings 7-44 7.6.2.2.4 Replacement Bridges . 7-45 7.6.2.3 Structural Alternatives 7-45 7.6.2.3.1 Bridge or Culvert . 7-45 7.6.2.3.2 Piers . 7-47 7.6.2.3.3 Abutments 7-48 7.6.2.3.4 Foundations 7-49 7.6.2.3.5
8、Superstructures. 7-50 7.6.2.4 Channel Modification. 7-54 7.6.3 Analysis of the Stream Crossing System . 7-56 7.6.3.1 Hydraulic Performance of the Crossing System. 7-57 7.6.3.2 Backwater. 7-58 7.6.3.3 Flow Distribution 7-59 7.6.3.4 Velocity 7-60 7.6.3.5 Scour. 7-61 7.6.3.5.1 General Scour 7-63 7.6.3.
9、5.2 Local Scour 7-65 7.6.3.5.3 Natural Scour . 7-65 7.6.3.5.4 In-Stream Borrow Areas, Commercial Mining, and Dredging for Navigation and Flood Control 7-66 7.6.3.5.5 Combined Effects of General, Local, and Natural Scour. 7-68 7.6.3.6 Stream Environment. 7-68 7.6.3.7 Economic Analysis. 7-69 7.6.4 Pro
10、tective and Preventive Measures 7-71 7.6.4.1 Pier Foundations. 7-71 7.6.4.1.1 Preventive Measures at Piers . 7-72 7.6.4.1.2 Protective Measures at Piers 7-75 7.6.4.2 Abutments and Approach Fills. 7-76 7.6.4.2.1 Protective and Preventive Measures along Embankments 7-77 7.6.4.2.2 Protective and Preven
11、tive Measures at Abutments 7-78 7.6.4.3 Bank Stabilization and River Training . 7-81 7.6.4.4 Buoyant, Drag, Debris, and Ice Forces on Bridge Superstructures 7-82 7.6.5 Dolphins and Fender Systems 7-83 7.7 DECK DRAINAGE 7-85 7.7.1 Deck Inlets . 7-85 7.7.2 Bridge End Drains7-86 7.8 DESIGN DOCUMENTATIO
12、N 7-86 7.8.1 Design Data for Permanent Records 7-87 2007 by the American Association of State Highway and Transportation Officials.Hydraulic Analysis for the Location and Design of Bridges 7-v7.8.2 Post-Construction Data 7-87 7.9 CONSTRUCTION-RELATED HYDRAULIC CONSIDERATIONS.7-88 7.9.1 Hydrologic In
13、formation7-88 7.9.2 Cofferdams, Caissons, and Falsework .7-88 7.9.3 Contractor Crossings and Work Areas.7-88 7.9.4 Borrow Areas .7-89 7.10 HYDRAULIC-RELATED CONSTRUCTION CONSIDERATIONS.7-90 7.10.1 Verification of Plans.7-90 7.10.2 Plan Changes7-90 7.10.3 Borrow Areas .7-90 7.10.4 Detours .7-91 7.10.
14、5 Environmental and Ecological Aspects7-91 7.10.6 Feedback.7-92 7.11 MAINTENANCE-RELATED HYDRAULIC CONSIDERATIONS.7-92 7.12 HYDRAULIC-RELATED MAINTENANCE CONSIDERATIONS.7-93 7.12.1 Maintenance Inspections 7-93 7.12.2 Flood Damages.7-93 7.12.2.1 Remedial Construction and Repair 7-94 7.12.2.2 Recurrin
15、g Damage. 7-94 7.13 REFERENCES7-95 2007 by the American Association of State Highway and Transportation Officials.Chapter 7 Hydraulic Analysis for the Location and Design of Bridges 7.1 INTRODUCTION Bridges serve a variety of highway purposes including the elimination of conflicts with traffic and o
16、ther modes of transportation, such as rail, marine, air, and pedestrian. Bridges enable watercourses to maintain the natural function of flow conveyance and sustain aquatic life. Bridges are also important and expensive highway-hydraulic structures and are vulnerable to failure from flood-related ca
17、uses. To minimize the risk of failure, the hydraulic requirements of stream crossings must be recognized and considered in all phases of highway development, construction, and maintenance. Features that are important to the hydraulic performance of a bridge include the approach fill alignment, skew,
18、 and profile; bridge location, skew, and length; span lengths; bent and pier location and design; and foundation and superstructure configuration and elevations. These features of a highway-stream crossing are usually the responsibility of location, design, and bridge engineers; however, the integri
19、ty and safety of the facility are often as dependent upon competent hydraulic design as on competent structural and geometric design. In this chapter, the hydraulic engineering aspects of bridged stream crossings are discussed, proceeding from hydraulic considerations in planning and location throug
20、h studies necessary for design and construction to hydraulic considerations in maintenance and operation. Tidal areas, such as bays and estuaries, are not discussed in detail although much of the discussion is applicable to these areas. Another chapter will discuss tidal hydraulics. Structures measu
21、ring more than 6.1 m (20 ft) along the roadway centerline are conventionally classified as bridges. However, structures designed hydraulically as bridges are treated in this chapter, regardless of length. Discussion of the hydraulics of bridges will include consideration of the total crossing, inclu
22、ding approach embankments and structures on the floodplains. 7.2 PLANNING AND LOCATION The general stream-crossing location is selected during the planning and location phase of highway project development. The final location should be selected only after detailed survey information has been obtaine
23、d and preliminary hydraulic studies completed. Although not the sole consideration in bridge location, hydraulic aspects should receive major attention in the initial planning of the 2007 by the American Association of State Highway and Transportation Officials.Highway Drainage Guidelines 7-2 highwa
24、y. The location and alignment of the highway can either exaggerate or improve hydraulic problems at the crossing. 7.2.1 Location of Stream Crossing Some of the factors considered in the selection of bridge locations are: the safety of the highway user, user costs, vertical and horizontal highway ali
25、gnment, construction and maintenance costs, foundation conditions, availability of funds, traffic needs, navigation requirements, development within the highway corridor, social costs, natural resources, political considerations, public opinion, stream regime, environmental considerations, and hazar
26、ds from floods. Obviously, the most favorable hydraulic conditions cannot always be achieved when other considerations are in conflict. To illustrate, additional construction and maintenance costs are often accepted to achieve a satisfactory resolution of the conflict between these other factors and
27、 a more desirable hydraulic condition. Other factors, however, should not be given such weight that the integrity of the hydraulic design and the safety of the bridge are compromised. Hydraulic considerations in site selection are numerous because of the many variations in flow conditions encountere
28、d and the many water-related environmental considerations. Flow may be in an incised stream channel, or the stream may have floodplains that are several kilometers miles wide. Floodplains may be clear or heavily vegetated, symmetrical about the stream channel or highly eccentric, clearly defined by
29、natural topography or man-made levees, or indeterminate. Flow may be almost uniformly distributed across the floodplains or concentrated in swales in the overbank areas. Flow direction often varies with stage so that a bridge substructure oriented for one stage would be incorrectly oriented for anot
30、her stage. Flow direction in overbank areas is often unrelated to that in the main or low-flow channel. In some instances, the floodplains convey a large proportion of the total flow during extreme floods, and the stream channel conveys only a small proportion. Stream channels may be braided, straig
31、ht, or meandering, poised, aggrading or degrading; and bends may be relatively stable or moving outward and downstream. Confluences, man-made stream controls, channelization, drift, and ice are additional hydraulic considerations that should be addressed in site selection. 2007 by the American Assoc
32、iation of State Highway and Transportation Officials.Hydraulic Analysis for the Location and Design of Bridges 7-3Many aspects of the environmental assessment made in connection with site selection are also related to the hydraulic design of a stream crossing. These include the effects that adoption
33、 of the site under study would have on the aquatic life in the stream; other developments, such as by shoaling near a domestic or irrigation water supply intake; and the effects that the proposed highway facility would have on wise use of the flood hazard area in the floodplain. There are also natio
34、nal objectives that must be considered in site selection, the most obvious being navigation clearance requirements for the safety of marine traffic and for the safety of the bridge. Another is the national objective to reduce the rate of annual increases in flood losses by precluding the unwise use
35、of floodplains. Highway-stream crossings should be located and designed so as to aid in the wise management of the nations streams and floodplains and in the efforts to achieve the objective of reducing flood losses. The preservation of wetlands is also a national objective that must be considered i
36、n the selection of a stream crossing. The importance of wetlands is recognized because of their high productivity of food and fiber; beneficial effects on flooding, pollution, and sediment control; and the wildlife habitat provided. Stream-crossing locations must be selected so that important wetlan
37、ds will not be destroyed or their value diminished unnecessarily. Not all of the above will apply to each stream crossing or bridge location, but many of the more important site considerations are hydraulic or water related. Crossing location alternatives often do not include a desirable site from t
38、he hydraulic design viewpoint, but the difficulties involved can often be reduced by attention to the hydraulic-related considerations. 7.2.2 Structure Type The final selection of structure type should not be made prior to the completion of detailed surveys and necessary scour, hydrologic, and hydra
39、ulic studies. Structure selection is a design decision, and preliminary decisions should not be binding on the engineer. Where final structure-type selection is necessary to satisfy the requirements of the environmental assessment, public hearings, or right-of-way acquisition, all available informat
40、ion should be furnished to the engineer and the decision made in consideration of all factors that influence the selection. 7.2.3 Environmental Considerations This chapter does not include a definitive discussion of the environmental considerations in site selection and bridge design. Because of the
41、 many and complex considerations involved, discussion here is limited to a broad approach to the environmental concerns that must be addressed in the selection and design of a stream crossing. Refer to Chapter 10 for more discussion on environmental considerations. The environmental effects of const
42、ruction activities may be classified as the hydrologic, physical, chemical, aesthetic, and biologic effects on water quality. The concept of water quality, to have meaning, must be related to a particular water use. Water quality, per se, cannot be quantified, but the hydrologic, physical, chemical,
43、 and biological components do have quantitative units that may be used to measure the relative effects of site selection on water quality. Hydrologic considerations, as used by ecologists, include both the hydrology and hydraulics of the crossing. Hydrology at a crossing is unlikely to be a factor i
44、n site selection, but the hydraulics may be an overriding concern. The environmental considerations for the hydraulic and physical aspects of 2007 by the American Association of State Highway and Transportation Officials.Highway Drainage Guidelines 7-4 water quality at alternative sites are the same
45、 concerns that the hydraulics engineer has historically addressed in evaluating the relative merits of alternative locations. These include the effects of the crossing on velocities, water surface profiles, velocity and flow distribution, scour, bank stability, sediment transport, aggradation and de
46、gradation of the channel, and the supply of sediment to the stream or water body. The hydraulics engineer must evaluate the potential effect of these factors on the crossing and the potential effects of the crossing site on the environment. Effects of a highway on the chemical quality of surface wat
47、ers are not ordinarily a consideration in site selection, though it is possible that contaminants in the form of minerals or from a sanitary landfill would be exposed in one location and not at an alternative site. There is some concern for chemical quality at crossing sites, particularly near publi
48、c water supply intakes, due to the risk of toxic material spills. The probability of such spills should be considered in weighing factors that influence site selection. The use of concrete for construction in the stream, deicing salts, fertilizers, growth inhibitors and other chemicals would not usu
49、ally vary appreciably among alternative sites, but the adverse effects of normal usage of such materials may vary among alternatives and, therefore, be a factor in site selection. Aesthetic considerations include effects on the visual, odor, and taste qualities of surface waters. The aesthetic quality of surface waters should be considered in site selection where potable water supplies, water contact sports, and fisheries a
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