Bottomless Culvert Scour Study: Phase II Laboratory Report

PDF Version (3.67 mb)

 

FOREWORD

The bottomless culvert study described in this report was conducted at the Federal Highway Administration (FHWA) hydraulics laboratory in response to a request by the Maryland State Highway Administration (MDSHA) in a partnership arrangement in which MDSHA shared the cost of the study. A primary objective of this study was to validate or improve an existing methodology developed by MDSHA for estimating scour in bottomless culverts. The study included experiments to determine stability of rock riprap and to test effectiveness of rock cross vanes and other measures to reduce scour at the foundations of bottomless culverts. This report will be of interest to hydraulic engineers and bridge engineers who are involved in selection and design of structures for small stream crossings. It is being distributed as an electronic document through the Turner-Fairbank Highway Research Center Web site (www.fhwa.dot.gov/research/tfhrc/).

Gary L. Henderson, P.E.
Director, Office of Infrastructure
Research and Development

NOTICE

This document is disseminated under the sponsorship of the U.S. Department of Transportation in the interest of information exchange. The U.S. Government assumes no liability for the contents or use thereof. This report does not constitute a standard, specification, or regulation.

The U.S. Government does not endorse products or manufacturers. Trade and manufacturers' names appear in this report only because they are considered essential to the object of the document.

Quality Assurance Statement

The Federal Highway Administration (FHWA) provides high-quality information to serve Government, industry, and the public in a manner that promotes public understanding. Standards and policies are used to ensure and maximize the quality, objectivity, utility, and integrity of its information. FHWA periodically reviews quality issues and adjusts its programs and processes to ensure continuous quality improvement.

Technical Report Documentation Page

1. Report No. FHWA-HRT-07-026	2. Government Accession No.	3. Recipient's Catalog No.
4. Title and Subtitle Bottomless Culvert Scour Study: Phase II Laboratory Report		5. Report Date February 2007
		6. Performing Organization Code
7. Author(s) Kornel Kerenyi, J. Sterling Jones, and Stuart Stein		8. Performing Organization Report No.
9. Performance Organization Name and Address GKY and Associates, Inc. 5411-E Backlick Road Springfield, VA 22151		10. Work Unit No. (TRAIS)
		11. Contract or Grant No.
12. Sponsoring Agency and Address Office of Engineering Research and Development Federal Highway Administration 6300 Georgetown Pike McLean, VA 22101-2296		13. Type of Report and Period Covered Laboratory Report May 2002–November 2005
		14. Sponsoring Agency Code
15. Supplementary Notes Contracting Officer’s Technical Representative (COTR): J. Sterling Jones, HRDI-07 The Maryland State Highway Administration (MDSHA) provided technical assistance and partial funding for this study. Dr. Fred Chang was instrumental in setting up the experimental plan and provided a data analysis strategy. Dr. Larry Arneson and Jorge E. Pagán-Ortiz provided technical review of this document.
16. Abstract Bottomless culverts are three-sided structures that use the natural channel for the bottom. These structures could be used to convey flows from one side of a highway to the other. As such, they are an environmentally attractive alternative to box, pipe, and pipe arch culvert designs. Bottomless culverts range in size from less than a meter (1.5 feet) to more than 10 meters (35 feet) in width. The failure of such a structure could have severe consequences similar to the failure of a bridge. On the other hand, since the cost of the foundation and scour countermeasures represents a significant portion of the cost of this type of structure, overdesign of these elements can add significantly to the cost of the project. Several dozen physical modeling configurations of bottomless culverts were tested, and the resulting scour at the entrance along the foundation and outlet was measured. Predictive equations for estimating scour depth were developed and compared to MDSHA methodology. These equations will provide guidance for the design of footing depths for bottomless culverts. The study was conducted in two phases. The first phase focused on measuring maximum scour depths at the culvert entrance and developing an analysis procedure using methods found in the literature to approximate prescour hydraulic parameters that drive the analysis. No fixed-bed experiments were conducted in the first phase to measure actual prescour hydraulic parameters. No submerged entrance experiments were conducted in the first phase. The second phase expanded the investigation to include scour measurements at the outlet, submerged entrance scour measurements, and detailed velocity and depth measurements with a prescour fixed bed at locations where maximum scour occurred. Additional tests were conducted to evaluate the use of various measures to reduce scour including wingwalls, pile dissipators, riprap, and cross vanes. Phase I results are reported in Kerenyi, K., Jones, J.S., and Stein, S., Bottomless Culvert Scour Study: Phase I Laboratory Report, FHWA-RD-02-078, 2003.
17. Key Words: Scour, culverts, hydraulics, physical model.		18. Distribution Statement No restrictions. This document is available to the public through the National Technical Information Service (NTIS), Springfield, VA 22161.
19. Security Classification (of this report) Unclassified	20. Security Classification (of this page) Unclassified	21. No. of Pages 77	22. Price N/A

Form DOT F 1700.7 (8-72) Reproduction of completed page authorized

---

Metric Conversion Chart

---

TABLE OF CONTENTS

1. INTRODUCTION
2. EXPERIMENTAL APPROACH
   • TEST FACILITIES AND INSTRUMENTATION
     • Hydraulic Flume
     • Electromagnetic Velocity Meter Operation
     • Particle Image Velocimetry
     • Postprocessing and Data Analysis
   • MODEL BOTTOMLESS CULVERT SHAPES
     • Phase I
     • Phase II
   • EXPERIMENTAL PARAMETERS
     • Approach Flow and Sediment Sizes
     • Outlet Scour
     • Riprap Experiments
     • Cross Vane Analysis
     • Test Matrix
3. THEORETICAL BACKGROUND
   • CLEAR WATER SCOUR
     • Representative Velocity
     • Critical Velocity
     • Adjustment for Spiral Flow at Culvert Toe
     • Adjustment for Pressure Flow at a Submerged Culvert
   • SCOUR PROTECTION: RIPRAP ANALYSIS
4. RESULTS
   • CLEAR WATER SCOUR EXPERIMENTS
     • Representative Velocity
     • Spiral Flow Adjustment Factors
     • Pressure Flow Adjustment Factors
   • OUTLET SCOUR EXPERIMENTS
     • Flow Conditions
     • Wingwalls
     • Scour Countermeasures
   • RIPRAP STABILITY DESIGN COEFFICIENTS
   • USE OF CROSS VANES FOR INLET SCOUR CONTROL
5. CONCLUSIONS
6. SCOUR CALCULATION EXAMPLES
   • USING k_s AS A FUNCTION OF V_RA, V_CL, AND F₁
   • USING k_s AS A FUNCTION OF V_RM, V_CN, AND Q_blocked

• APPENDIX A. SCOUR MAPS
• APPENDIX B. OUTLET SCOUR RESULTS
• REFERENCES

LIST OF FIGURES

1. Photo. View of the flume in the Hydraulics Laboratory.
2. Photo. Rectangular culvert.
3. Photo. Riprap test for a rectangular culvert.
4. Diagram. Flow concentration and separation zone.
5. Diagram. Definition sketch before scour for unsubmerged flow conditions.
6. Diagram. Definition sketch after scour for unsubmerged flow conditions.
7. Definition sketch after scour for submerged flow conditions
8. Diagram. Side view after scour for submerged flow conditions (Section A-A′ in figure 7).
9. Graph. Chang’s approximations to Neill’s competent velocity curves.
10. Graph. Calibration of C in equation 4.
11. Graph. Calibration of k_s as a function of V_RA, V_CL, and F₁.
12. Graph. Validation of y_max using k_s as a function of V_RA, V_CL, and F1.
13. Graph. Calibration of k_s as a function of V_RM, V_CN, and Q_blocked.
14. Graph. Validation of y_max using k_s as a function of V_RM, V_CN, and Q_blocked.
15. Graph. Calibration of k_p when k_s is a function of V_RA, V_CL, and F₁.
16. Graph. Validation of y_max using k_p when k_s is a function of V_RA, V_CL, and F₁.
17. Graph. Calibration of k_p when k_s is a function of V_RM, V_CN, and Q_blocked.
18. Graph. Validation of y_max using k_p when k_s is a function of V_RM, V_CN, and Q_blocked.
19. Photo. Outlet prior to scour test.
20. Image. Velocity distribution for unsubmerged culvert with 45-degree wingwalls at entrance.
21. Image. Turbulent shear map for outlet with no wingwalls.
22. Image. Scour map for outlet with no wingwalls.
23. Image. Turbulent shear map for outlet with streamlined wingwalls.
24. Image. Scour map for outlet with streamlined wingwalls.
25. Photo. Outlet scour after test.
26. Photo. 45-degree inlet wingwalls before scour.
27. Photo. 45-degree inlet wingwalls after scour.
28. Photo. 8-degree inlet wingwalls before scour.
29. Photo. 8-degree inlet wingwalls after scour.
30. Photo. No wingwalls.
31. Photo. Truncated, circular wingwalls before scour.
32. Photo. Truncated, circular wingwalls after scour.
33. Photo. Elongated, streamlined wingwalls before scour.
34. Photo. Elongated, streamlined wingwalls after scour.
35. Photo. Short, streamlined bevel wingwalls after scour.
36. Photo. Wingwalls with 8-degree flare (rough joint) before scour.
37. Photo. Wingwalls with 8-degree flare (rough joint) after scour.
38. Photo. Wingwalls with 8-degree flare (smooth joint) before scour.
39. Photo. Wingwalls with 8-degree flare (smooth joint) after scour.
40. Photo. 45-degree wingwalls after scour.
41. Photo. Pile dissipators.
42. Diagram. Plan view of pile dissipators.
43. Photo. Culvert outlet prior to pile dissipator test.
44. Photo. Outlet scour area without protective pile dissipators.
45. Photo. Outlet scour area with protective pile dissipators.
46. Diagram. Countermeasure installation for MDSHA Standard Plan (top view).
47. Diagram. Countermeasure installation for MDSHA Standard Plan (Section A-A from figure 46).
48. Photo. Culvert inlet before Standard Plan test.
49. Photo. Culvert barrel before Standard Plan test.
50. Photo. Culvert outlet before Standard Plan test.
51. Photo. Shifted riprap in culvert inlet after Standard Plan test.
52. Photo. Shifted riprap in culvert barrel after Standard Plan test.
53. Graph. Calibrated function for KVM.
54. Graph. Calibration function for K_RIP.
55. Graph. Validation of D₅₀ for riprap sizing.
56. Diagram. Culvert with a cross vane.
57. Diagram. Experimental arrangement of culvert with a cross vane.
58. Photo. Fabrication of the cross vane.
59. Photo. Cross vane installed at inlet of experimental culvert.
60. Image. PIV image of flow field at culvert entrance showing spiral current in corners.
61. Graph. Cross vane results.
62. Diagram. Scour map (top) and profile (bottom), culvert submerged, February 11, 2003.
63. Diagram. Scour map (top) and profile (bottom), free surface, February 25, 2003.
64. Diagram. Scour map (top) and profile (bottom), free surface with circular bevel at exit, March 25, 2003.
65. Diagram. Scour map (top) and profile (bottom), free surface with streamlined bevel at exit, April 7, 2003.
66. Diagram. Scour map (top) and profile (bottom), free surface with short streamlined bevel at exit, April 29, 2003.
67. Diagram. Scour map (top) and profile (bottom), free surface with wingwalls at outlet, July 22, 2003.
68. Diagram. Scour map (top) and profile (bottom), free surface with 8-degree wingwalls at outlet, August 6, 2003.
69. Diagram. Scour map (top) and profile (bottom), free surface with 8-degree wingwalls at outlet (smooth walls), October 7, 2003.
70. Diagram. Scour map (top) and profile (bottom), free surface with 8-degree wingwalls at outlet and inlet (smooth walls), December 9, 2003.
71. Diagram. Scour map (top) and profile (bottom), submerged with 8-degree wingwalls at outlet and inlet (smooth walls), December 16, 2003.
72. Diagram. Scour map (top) and profile (bottom), submerged with 45-degree wingwalls at outlet and inlet, October 27, 2004.
73. Diagram. Scour map (top) and profile (bottom), submerged with 45-degree wingwalls at outlet and inlet and Chang’s pile dissipater at outlet, November 10, 2004.
74. Diagram. Scour map (top) and profile (bottom), MDSHA Standard Plan, submerged with 45-degree wingwalls at outlet and inlet, March 19, 2004.

LIST OF TABLES

1. Test matrix for bottomless culvert experiments.
2. Unsubmerged scour equations.
3. Submerged scour equations for culverts with wingwalls.
4. Inlet wingwall test configurations.
5. Outlet wingwall test configurations.
6. Tests using pile dissipators.
7. Tests using MDSHA Standard Plan methods.
8. Outlet scour results summary.

LIST OF SYMBOLS

A_k

dimensionless ratio: area of approaching flow directly above culvert divided by total area of flow approaching culvert.

A_CULV

cross sectional area of flow in the culvert.

C

calibration coefficient for determining V_RM.

D

height of culvert at approach prior to scour.

D₅₀

sediment size.

E

Ishbash constant.

F₁

Froude number at culvert approach.

F_o

Froude number in contraction zone.

g

acceleration of gravity.

k_p

empirical coefficient needed to explain additional scour depth caused by pressure flow at a submerged culvert.

k_s

empirical coefficient needed to explain additional scour depth caused by spiral flow at culvert toe.

k_v

ratio of velocity at the culvert toe to the mean velocity in the contracted section.

k_vadj

k_v with a calibration coefficient, C.

K_RIP

coefficient used to size riprap for scour.

K_u

6.19 for SI units, or 11.17 for U.S. customary units.

K_U

0.55217 for SI units, or 1.0 for U.S. customary units.

K_U1

0.3048^(0.65−x) for SI units, or 1.0 for U.S. customary units.

K_U2

0.788 for SI units, or 1.0 for U.S. customary units.

K_VM

coefficient relating local bed velocity in experiments to average velocity in contraction zone.

N_SC

computed sediment number for distributed flow.

q₁

unit discharge in the approach section.

q₂

unit discharge in the contracted section.

q_R

assumed representative unit discharge across the scour hole at the beginning of scour.

Q

volumetric flow rate.

Q_blocked

portion of approach flow that is to one side of channel centerline and blocked by the embankment as flow approaches culvert.

SG

specific gravity of riprap.

R_Qblocked

dimensionless ratio that includes Q_blocked and y₂.

V_AC

average velocity in the contracted zone prior to scour in the vicinity of the upstream corner of a culvert.

V_C

critical velocity at which incipient sediment motion occurs.

V_CL

Laursen’s critical velocity.

V_CN

Neill’s critical velocity.

V_eff

effective velocity that accounts for turbulence and vorticity in the mixing zone at the upstream corner of a culvert.

V_LB

local velocity along the bed prior to scour in the vicinity of the upstream corner of a culvert.

V_max

maximum velocity that rolls out the stones lying among others on a slope.

V_min

minimum velocity that removes the loose stones lying on top of fill.

V_R

representative (local) velocity at culvert entrance.

V_RA

average velocity.

V_RP

representative velocity from potential flow principles.

V_RM

measured velocity.

w_a

width of approach channel.

w_CULV

width of culvert.

y₀

water depth at the culvert entrance before scour occurs.

y₁

water depth in the approach channel at a distance three times wCULV upstream of the culvert entrance.

y₂

equilibrium water depth after scour hole develops.

y_max

maximum water depth in the culvert after scour hole develops.

y_s

maximum depth of scour in the culvert.

ABBREVIATED GLOSSARY

ASCE

American Society of Civil Engineers

EGL

energy grade line

HGL

hydraulic grade line

MDSHA

Maryland State Highway Administration

PIV

particle image velocimetry

SI

International System of Units

VI

virtual instruments

ww

wingwall

Contents | Next

Page Owner: Office of Research, Development, and Technology, Office of Infrastructure, RDT Scheduled Update: Archive - No Update Technical Issues: TFHRC.WebMaster@dot.gov Topics: research, infrastructure, hydraulics Keywords: research, infrastructure, hydraulics, Scour, culverts, hydraulics, physical model TRT Terms: research, hydraulics, hydrology, fluid mechanics, earth sciences, geophysics Updated: 04/23/2012