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Publication Number: FHWA-RD-06-138
Date: October 2006

Effects of Inlet Geometry on Hydraulic Performance of Box Culverts

CHAPTER 7. CONCLUSIONS AND RECOMMENDATIONS

Since most highway designers use HY-8 or a similar program for hydraulic design of culverts,highway agencies do not receive much benefit from research results until the results are finally coded into a computer program. Even the Corps of Engineers Hydrologic Engineering Center River Analysis System (HECRAS) program, used for most water surface profile studies, includes the HY-8 logic for culvert evaluations. The HY-8 program is currently being revised to incorporate recent research results and to allow for user-defined design coefficients.

As it is currently coded, HY-8 does not allow for user-defined design coefficients, which restricts practitioners from using research results until they are finally coded into a program. Since HY-8 and other similar programs are based on HDS-5, the best current approach for advancing the research to an implementation stage is to develop coefficients and performance curves in a format that is equivalent to the HDS-5 format, to derive fifth-order polynomials to facilitate coding inlet control performance curves in a computer program, and to identify which results will have the most impact on the applicability or clarity of HDS-5.

HDS-5 is the most widely recognized publication available on culvert hydraulics. All of the inlets tested in this study were inlets covered in HDS-5 with various combinations of entrance improvements. HDS-5 does not include thumbnail sketches of the inlet configurations that go with the descriptions, and it is sometimes difficult to visualize some of the details of the inlets. Nevertheless, experimental results in this study were compared to published coefficients in HDS-5 in cases where the inlets appeared to match the descriptions in HDS-5.

FINDINGS AND CONCLUSIONS

The major findings and conclusions of this study are:

Figure 83. Graph. PC and FC single barrel models (sketches 1, 7, 11 in figure 93).

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Figure 84. Graph. PC and FC multiple barrel models (sketches 1, 2, 7, 8, 11, 12 in figure 93).

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Figure 85. Graph. Combined corner fillet data, FC-S-0 and PC-A models (sketches 7, 10, 11, 14 in figure 93).

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Figure 86. Graph. Combined multiple barrel data, FC-0 models (sketches 7, 8 in figure 93)

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Figure 87. Graph. Combined multiple barrel data, FC-30 models (sketches 1, 2 in figure 93).

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Figure 88. Graph. Combined multiple barrel data, PC models (sketches 11, 12 in figure 93).

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Figure 89. Graph. Combined span-to-rise data, FC-S-0 models (sketches 7, 9 in figure 93).

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Figure 90. Graph. Combined span-to-rise data, FC-S-30 models (sketches 1, 3 in figure 93).

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Figure 91. Graph. Combined span-to-rise data, PC models (sketches 10, 13 in figure 93).

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Figure 92. Graph. Skewed and nonskewed headwalls, FC-T-30 models (sketches 4, 5 in figure 93).

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RECOMMENDATIONS

The following recommendations are for consideration by practitioners involved in field cast and precast culvert installations:

The following recommendations are for consideration as enhancements to future editions of hydraulic design manuals such as HDS-5 and future generations of hydraulic design software such as HY-8:

Table 11. Design coefficients suggested for future editions of HDS-5.
Description Source Ke K1 M1 K2 M2 c Y

Box, reinforced concrete

     30° to 75° flared wingwalls

Chart 8/1, HDS-5

 

 

 

0.469

0.696

0.033

0.751

     30° to 75° flared wingwalls, Square-edged crown

Table 12, HDS-5

0.4

 

 

 

 

 

 

     30° to 75° flared wingwalls, Crown edge rounded or top edge beveled

Table 12, HDS-5

0.2

 

 

 

 

 

 

30° flared wingwalls, top edge beveled at 45°

     Single barrel

Sketch 1, figure 93

0.26

0.005

1.05

0.44

0.74

0.04

0.48

     2, 3, and 4 multiple barrels

Sketch 2, figure 93

0.32

 

 

0.47

0.68

0.04

0.62

     2:1 to 4:1 span-to-rise ratio

Sketch 3, figure 93

0.20

 

 

0.48

0.65

0.041

0.57

     15° skewed headwall, multiple barrels

Sketch 4, figure 93

0.36

 

 

0.69

0.49

0.029

0.95

     30° to 45° skewed headwall, multiple barrels

Sketch 5, figure 93

0.45

 

 

0.69

0.49

0.027

1.02

0° flared wingwalls, extended sides

     Square-edged at crown

Chart 8/3, HDS-5

0.7

 

 

0.55

0.64

0.05

0.55

     Square-edged at crown

Sketch 6, figure 93

0.79

0.055

0.68

0.55

0.64

0.047

0.55

     45° straight bevel at crown, 0- and 6-inch corner fillets

Sketch 7, figure 93

0.48

 

 

0.56

0.62

0.045

0.55

     45° straight bevel at crown, 2, 3, and 4 multiple barrels

Sketch 8, figure 93

0.52

 

 

0.55

0.59

0.038

0.69

     45° straight bevel at crown, 2:1 to 4:1 span-to-rise ratio

Sketch 9, figure 93

0.37

 

 

0.61

0.57

0.041

0.67

     Crown rounded at 8-inch radius, 0- and 6-inch corner fillets

Sketch 10, figure 93

0.24

 

 

0.56

0.62

0.038

0.67

     Crown rounded at 8-inch radius, 12-inch corner fillets

Sketch 11, figure 93

0.3

 

 

0.56

0.62

0.038

0.67

     Crown rounded at 8-inch radius, 12-inch corner fillets, 2, 3, and 4 multiple barrels

Sketch 12, figure 93

0.54

 

 

0.55

0.60

0.023

0.96

     Crown rounded at 8-inch radius, no fillets, 2:1 to 4:1 span-to-rise ratio

Sketch 13, figure 93

0.30

 

 

0.61

0.57

0.033

0.79

Note: The source column refers to two documents: HDS-51 and figure 93 in this report. Empty cells mean no data are available.1 inch = 2.54 cm

Table 12. Fifth-order polynomial coefficients.
DescriptionSourceabCdef

Box, reinforced concrete

     30° to 75° flared wingwalls

Chart 8/1, HDS−5

0.175897

0.20233

0.139818

−0.071970

0.014148

−0.000930

     30° to 75° flared wingwalls, Square-edged crown

Table 12, HDS−5

 

 

 

 

 

 

     30° to 75° flared wingwalls, Crown edge rounded or top edge beveled

Table 12, HDS−5

 

 

 

 

 

 

30° flared wingwalls, top edge beveled at 45°

     Single barrel

Sketch 1, figure 93

0.163450

0.127103

0.256193

−0.131630

0.025211

−0.001600

     2, 3, and 4 multiple barrels

Sketch 2, figure 93

0.112542

0.375074

0.002657

−0.026380

0.006867

−0.000470

     2:1 to 4:1 span-to-rise ratio

Sketch 3, figure 93

0.182681

0.209471

0.158774

−0.092970

0.019381

−0.001310

     15° skewed headwall, multiple barrels

Sketch 4, figure 93

0.182031

0.686256

−0.277040

0.082113

−0.01167

0.000654

     30° to 45° skewed headwall, multiple barrels

Sketch 5, figure 93

0.363958

0.283523

0.069040

−0.044221

0.008965

−0.000595

0° flared wingwalls, extended sides

     Square-edged at crown

Chart 8/3, HDS−5

 

 

 

 

 

 

     Square-edged at crown

Sketch 6, figure 93

0.278122

−0.002930

0.448521

−0.228310

0.044973

−0.00299

     45° straight bevel at crown, 0- and 6-inch corner fillets

Sketch 7, figure 93

0.244295

0.129322

0.339620

−0.189660

0.038635

−0.0026

     45° straight bevel at crown, 2, 3, and 4 multiple barrels

Sketch 8, figure 93

0.164261

0.43842

−0.03128

−0.01919

0.006288

−0.00046

     45° straight bevel at crown, 2:1 to 4:1 span-to-rise ratio

Sketch 9, figure 93

0.254968

0.273934

0.154712

−0.09911

0.020506

−0.00135

     Crown rounded at 8-inch radius, 0- and 6-inch corner fillets

Sketch 10, figure 93

0.203424

0.31092

0.10783

−0.07609

0.015779

−0.00102

     Crown rounded at 8-inch radius, 12-inch corner fillets

Sketch 11, figure 93

0.203424

0.31092

0.10783

−0.07609

0.015779

−0.00102

     Crown rounded at 8-inch radius, 12-inch corner fillets, 2, 3, and 4 multiple barrels

Sketch 12, figure 93

0.10315

0.619895

−0.23147

0.071093

−0.01075

0.000628

     Crown rounded at 8-inch radius, no fillets, 2:1 to 4:1 span-to-rise ratio

Sketch 13, figure 93

0.199715

0.446748

−0.02414

−0.02334

0.006761

−0.00048

Note: The source column refers to two documents: HDS-51 and figure 93 in this report. Empty cells mean no data are available.1 inch = 2.54 cm

Figure 93. Thumbnail sketches of inlets recommended for implementation.

a. Sketch 1
30°-flared wingwalls; top edge beveled at 45°
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b. Sketch 2
30°-flared wingwalls; top edge beveled at 45°; 2, 3, and 4 multiple barrels
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c. Sketch 3
30°-flared wingwalls; top edge beveled at 45°; 2:1 to 4:1 span-to-rise ratio
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d. Sketch 4
30°-flared wingwalls; top edge beveled at 45°; 15° skewed headwall with multiple barrels
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e. Sketch 5
30°-flared wingwalls; top edge beveled at 45°; 30° to 45° skewed headwall with multiple barrels
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f. Sketch 6
0°-flared wingwalls (extended sides); square-edged at crown
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g. Sketch 7
0°-flared wingwalls (extended sides); top edge beveled at 45°; 0- and 6-inch corner fillets
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h. Sketch 8
0°-flared wingwalls (extended sides); top edge beveled at 45°; 2, 3, and 4 multiple barrels
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i. Sketch 9
0°-flared wingwalls (extended sides); top edge beveled at 45°; 2:1 to 4:1 span-to-rise ratio
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j. Sketch 10
0°-flared wingwalls (extended sides); Crown rounded at 8-inch radius; 0- and 6-inch corner fillets
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k. Sketch 11
0°-flared wingwalls (extended sides); Crown rounded at 8-inch radius; 12-inch corner fillets
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l. Sketch 12
0°-flared wingwalls (extended sides); Crown rounded at 8-inch radius; 12-inch corner fillets; 2,3, and 4 multiple barrels
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m. Sketch 13
0° -flared wingwalls (extended sides); Crown rounded at 8-inch radius; 12-inch corner fillets; 2:1 to 4:1 span-to-rise ratio.
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n. Sketch 14
0°-flared wingwalls (extended sides); top edge beveled at 45°; 12-inch corner fillets
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Note: Extended center walls do not affect flow conditions.1 inch = 2.54 cm

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The Federal Highway Administration (FHWA) is a part of the U.S. Department of Transportation and is headquartered in Washington, D.C., with field offices across the United States. is a major agency of the U.S. Department of Transportation (DOT).
The Federal Highway Administration (FHWA) is a part of the U.S. Department of Transportation and is headquartered in Washington, D.C., with field offices across the United States. is a major agency of the U.S. Department of Transportation (DOT). The hydraulics and hydrology research program at the TFHRC Federal Highway Administration's (FHWA) R&T Web site portal, which provides access to or information about the Agency’s R&T program, projects, partnerships, publications, and results.
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