Hydraulic Design of Highway Culverts - DOT On-Line Publications

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Table 1--Factors Influencing Culvert Performance. Factor 8 Inlet Control Outlet Control Headwater Elevation X X Inlet Area X X Inlet Edge Configuration X X Inlet Shape X X Barrel Roughness X Barrel Area X Barrel Shape X Barrel Length X Barrel Slope * X Tailwater Elevation X *Barrel slope affects inlet control performance to a small degree, but may be neglected. Figure I-14--Typical Outlet Control Flow Conditions
3. Headwater. Energy is required to force flow through a culvert. This energy takes the form of an increased water surface elevation on the upstream side of the culvert. The depth of the upstream water surface measured from the invert at the culvert entrance is generally referred to as headwater depth (Figures I-13 and I-14). A considerable volume of water may be ponded upstream of a culvert installation under high fills or in areas with flat ground slopes. The pond which is created may attenuate flood peaks under such conditions. This peak discharge attenuation may justify a reduction in the required culvert size. 4. Tailwater. Tailwater is defined as the depth of water downstream of the culvert measured from the outlet invert. Figure I-14 It is an important factor in determining culvert capacity under outlet control conditions. Tailwater may be caused by an obstruction in the downstream channel or by the hydraulic resistance of the channel. In either case, backwater calculations from the downstream control point are required to precisely define tailwater. When appropriate, normal depth approximations may be used instead of backwater calculations. 5. Outlet Velocity. Since a culvert usually constricts the available channel area, flow velocities in the culvert are likely to be higher than in the channel. These increased velocities can cause streambed scour and bank erosion in the vicinity of the culvert outlet. Minor problems can Figure I-15--Culvert Performance Curve occasionally be avoided by increasing the barrel roughness. Energy dissipaters and outlet protection devices are sometimes required to avoid excessive scour at the culvert outlet. When a culvert is operating under inlet control and the culvert barrel is not operating at capacity, it is often beneficial to flatten the barrel slope or add a roughened section to reduce outlet velocities. 6. Performance Curves. A performance curve is a plot of headwater depth or elevation versus flow rate. The resulting graphical depiction of culvert operation is useful in evaluating the hydraulic capacity of a culvert for various headwaters. Among its uses, the performance curve displays the consequences of higher flow rates at the site and the benefits of inlet improvements. In developing a culvert performance curve, both inlet and outlet control curves must be plotted. This is necessary because the dominant control at a given headwater is hard to predict. Also, control may shift from the inlet to the outlet, or vice-versa over a range of flow rates. Figure I-15 illustrates a typical culvert performance curve. At the design headwater, the culvert operates under inlet control. With inlet improvement the culvert performance can be increased to take better advantage of the culvert barrel capacity. 9
Page 1: Publication No. FHWA-NHI-01-020 Sep
Page 4 and 5: Acknowledgements This document’s
Page 6 and 7: (Page intentionally blank) iv
Page 8 and 9: TABLE OF CONTENTS (Cont.) III. CULV
Page 10 and 11: TABLE OF CONTENTS (Cont.) F. Safety
Page 12 and 13: LIST OF FIGURES (Cont.) Figure III-
Page 14 and 15: LIST OF TABLES Table 1. Factors Inf
Page 16 and 17: ELhf ELhi ELho ELht ELO ELsf ELSO E
Page 18 and 19: GLOSSARY (Cont.) p Wetted perimeter
Page 20 and 21: (Page intentionally blank) xviii
Page 22 and 23: B. Overview of Culverts A culvert i
Page 24 and 25: Figure I-9--Side-tapered inlet Figu
Page 26 and 27: . Partly Full (Free Surface) Flow.
Page 30 and 31: D. Economics The hydraulic design o
Page 32 and 33: For gaged sites, statistical analys
Page 34 and 35: volume of the remaining runoff hydr
Page 36 and 37: Figure II-4--Flood hydrograph shape
Page 38 and 39: Figure II-6--Cross section location
Page 40 and 41: . Culvert Length. Important dimensi
Page 42 and 43: HYDROLOGY Peak Flow Check Flows Tab
Page 44 and 45: Figure III-1--Types of inlet contro
Page 46 and 47: Figure II-2--Flow contractions for
Page 48 and 49: The flow transition zone between th
Page 50 and 51: Figure III-6--Culvert with Inlet Su
Page 52 and 53: Condition III-7-A represents the cl
Page 54 and 55: Outlet control flow conditions can
Page 56 and 57: 2 2 Vu Vd HW o + = TW + + HL (6) 2g
Page 58 and 59: This approximate method works best
Page 60 and 61: Figure III-12--Weir Crest Length De
Page 62 and 63: Critical depth is used when the tai
Page 64 and 65: 4. Add the culvert flow and the roa
Page 66 and 67: NOTE: If the nomographs are put int
Page 68 and 69: Figure III-19--Critical Depth Chart
Page 70 and 71: (1) If the Manning’s n value give
Page 72: Example Problem #1 (SI Units) A cul
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Example Problem #2 (SI Units) A new
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Example Problem #3 (SI Units) Desig
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Example Problem #4 (SI Units) An ex
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CHART 51A Figure III-21--Inlet Cont
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2. Outlet Control. a. Partly Full F
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Backwater Calculations From hydraul
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English Units INLET CONTROL: AD 0.
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A. Introduction IV. TAPERED INLETS
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height by more than 10 percent (1.1
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A slope-tapered inlet has three pos
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The mitered face slope-tapered inle
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La is the approximate length of the
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E. Design Methods Figure IV-9--Tape
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a. Complete Design Data. Fill in th
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H 1 i. For FALL < D/4, use side-tap
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3. Example Problems a. Example Prob
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. Example Problem #1 (English Units
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103
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105
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c. Example Problem #2 (SI Units). F
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Conclusions: A side-tapered inlet a
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111
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G. Circular Pipe Culverts 1. Design
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Double barrel slope-tapered inlets
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117
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. Example Problem #3 (English Units
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121
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A. The Routing Concept V. STORAGE R
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C. Application to Culvert Design Fi
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Figure V-6--Peak Flow Reduction Bas
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I + I + ( 2s / ∆t − O) = ( 2s /
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Table 5--Inflow Hydrograph, Example
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Figure V-10—Culvert Design Form f
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Figure V-14--Storage vs. Outflow Re
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Table 5 --Inflow Hydrograph, Exampl
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Figure V-18--Culvert Design Form fo
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Figure V-22--Storage vs. Outflow Re
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A. Introduction VI. SPECIAL CONSIDE
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With minor modifications, the culve
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If headwater and flow consideration
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Figure VI-6--Subatmospheric Pressur
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Figure VI-7--Fish Baffles in Culver
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A popular method of providing for f
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passage of the peak flow. Methods f
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Figure VI-15--Sediment Deposition i
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1. Skewed Barrels. The alignment of
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Skewed inlets slightly reduce the h
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arrel for pipes or the flow per met
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Culvert shapes are as important in
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• Flood plain ordinances or other
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Figure VI-28--Guardrail Adjacent to
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Both of the above equations are emp
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Tables, charts, and formulas are av
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3. Endwalls and Wingwalls. Culvert
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2. Hydraulic Considerations. Long s
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conditions. Variations in the concr
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REFERENCES (www.fhwa.dot.gov/bridge
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28. "Design Approaches for Stormwat
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54. "Hydraulic Analysis of Pipe-Arc
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ADDITIONAL REFERENCES (In Alphabeti
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"Risk Analysis For Hydraulic Design
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A. Introduction APPENDIX A DESIGN M
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Chart No. 1 Shape and Material Circ
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NOTE: The rest of this Appendix A i
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Figure A-1--Dimensionless Performan
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developed for circular and elliptic
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APPENDIX B HYDRAULIC RESISTANCE OF
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D. Corrugated Metal Culverts The hy
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Curves are shown for 2-2/3 by 1/2 i
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F. Spiral Rib Pipe Spiral rib pipe
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APPENDIX C CULVERT DESIGN OPTIMIZAT
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C. Inlet Control Performance Curves
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Figure C-4--Optimization of Perform
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E. Tapered Inlet Face Control Perfo
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APPENDIX D DESIGN CHARTS, TABLES, A
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Chart Corrugated Metal Box Culverts
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Chart Circular Tapered Inlets 55A,
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Table 12--Entrance Loss Coefficient
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CHART 1B 225
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