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Section 8: Drainage Facility Placement

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Overview

This section contains information on the following topics:

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Introduction

In designing drainage systems, the primary objective is to properly accommodate surface runoff along and across highway right-of-way through the application of sound hydraulic principles.

Consideration must also be given to incorporating safety into the design of drainage appurtenances. The best design would efficiently accommodate drainage and be traversable by an out-of-control vehicle without rollover or abrupt change in speed.

To meet safety needs, the designer may use one of the following treatments:

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  • Design or treat drainage appurtenances so that they will be traversable by a vehicle without rollover or abrupt change in speed;
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  • Locate appurtenances a sufficient distance, consistent with traffic volume, from the travel lanes to reduce the likelihood of collision; or
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  • Protect the driver through installation of traffic barrier shielding appurtenances.

The following guidelines are intended to improve roadside safety with respect to facilities accommodating drainage parallel to and crossing under highways. The guidelines apply to all rural, high-speed facilities and other facilities with posted speed limits of 50 mph or more and with rural type (uncurbed) cross sections. Where reference is made to clear zone requirements in these guidelines, see Table 2-12 and the discussions regarding Slopes and Ditches, Roadside Design, and Clear Zone in Section7. Desirable values for clear zone should generally be used and minimum clear zone widths applied where unusual conditions are encountered. Site visits may be appropriate to ascertain terrain conditions and debris potential before arriving at design decisions.

Designers should address and resolve culvert end treatment issues early in project development. If there are doubts about the proper application of criteria on a given project or group of projects, then arrangements should be made for a meeting with the appropriate entities prior to in-depth development of PS&E.

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Design Treatment of Cross Drainage Culvert Ends

Cross drainage culverts are defined as those conveying drainage across and beneath the highway. Selection of an appropriate end treatment is primarily related to culvert size, culvert end location, side slope rate, terrain characteristics, drift conditions, right-of-way availability, and other considerations that may influence treatment selection at individual sites.

Roadside safety performance is related to clear zone width and side slope rate. For a discussion of safety performance and design guidelines related to side slopes, see Section 7, Slopes and Ditches Where right-of-way availability and economic conditions permit, flatter slopes may be used.

Design values for 4R clear zones are shown in Table 2-12 for new location and major reconstruction projects. Within the clear zone, side slopes should preferably be 1V:6H or flatter with 1V:4H as a maximum steepness in most cases.

Small Culverts

A small culvert is defined as a single round pipe with 36-in or less diameter, a single box culvert with span of 36-in or less, or multiple round pipes each with 30-in or less diameter, each oriented normal to the roadway. (Note: For arch pipes, use span dimension instead of diameter.)

When skews are involved, the definition of a small pipe culvert is modified as shown in Table 2-13:

Anchor: #i1067795Table 2-13: Maximum Diameter of Small Pipe Culvert

Skew (degree)

Single Pipe (in)

Multiple Pipe (in)

15

33

30

30

27

24

45

24

21


Small pipe culverts with sloping, open ends have been crash tested and proven to be safely traversable by vehicles for a range of speeds. Small culvert ends should be sloped at a rate of 1V:3H or flatter and should match the side slope rate, thereby providing a flush, traversable safety treatment. Single box culverts normal to the roadway with spans of 36-in or less may be effectively safety treated just as small pipes.

When vulnerable to run-off-the-road vehicles (i.e., unshielded by barrier), sloped ends should be provided on small culverts regardless of culvert end location with respect to the clear zone dimensions. For existing culverts, this often entails removing existing headwalls and may include removing the barrier treatment if it is no longer needed to protect an obstacle other than a culvert end.

For new culverts or existing culverts that may need adjusting, culvert pipe length should be controlled by the intercept of the small pipe and the side slope planes. Side slopes should not be warped or flattened near culvert locations. Also, terrain near the culvert ends should be smooth and free of fixed objects, and headwalls should not be used.

Intermediate Culverts

An intermediate size pipe culvert is defined as a single round pipe with more than 36-in diameter or multiple round pipes each with more than 30-in diameter but having maximum diameter of 60-in. For arch pipes, use span dimension instead of diameter.

Intermediate size single box culverts are defined as those having only one barrel with maximum height of 60-in. Cross sectional area of the single box or individual pipe normally should not exceed 25-ft2.

The openings of intermediate size single barrel box and pipe culverts are too large to be safely traversed by a vehicle. Recommended safety treatment options should be considered in the following order:

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  1. Provide sloped ends with safety pipe runners.
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  2. Provide flat side slopes and locate the ends outside the clear zone.
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  3. Use barrier to shield culvert ends.

Sloped end treatments with safety pipe runners are preferred from a safety standpoint and are generally cost effective for both new and existing intermediate size culverts, regardless of end location with respect to clear zone criteria. These end treatments should be sloped at a rate of 1V:3H or flatter and should match the side slope rate thereby providing a flush, traversable safety treatment. Length of new culverts should be governed by the locations of the side slope plane/culvert intercepts rather than by clear zone. Terrain near the culvert end should be smoothly shaped and traversable, and headwalls should not be used.

For existing intermediate size single barrel box and pipe culverts, no treatment is necessary for culvert end offsets beyond the clear zone and below the traffic volume threshold as shown in Table 2-12. Where an improved design is warranted using Table 2-12, the removal of headwalls and installation of sloped ends with safety pipe runners is the preferred safety treatment.

In certain situations (e.g., culvert skew exceeds 15 degrees or severe debris problems) treatment with safety pipe runners may be impractical. For these conditions, locating intermediate size culvert ends to meet desirable clear zone values (see Table 2-12) is preferred over shielding with barrier. Designs having flared wing walls with safety pipe runners oriented parallel to the stream flow and spaced at 30-in maximum center to center thereby can minimize debris problems.

Large Culverts

Large multiple box culverts are defined as those with more than one barrel and a total opening (i.e., distance) of 20-ft or less between extreme inside faces as measured along the highway centerline. Large single pipes or single box culverts are defined as those with diameter or height exceeding 5-ft or cross-sectional area exceeding 25-ft2.

From a safety standpoint alone, treatment for both new and existing installations should be considered in the following order:

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  1. Provide safety pipe runners.
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  2. Meet or exceed desirable clear zone value.
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  3. Shield with barrier.

Designers should carefully consider several factors before opting to use safety pipe runners. Where a defined channel is present, it may be impossible or impractical to shape the terrain near the culvert end to provide for vehicular traversability. Such circumstances would dictate that a more suitable, culvert end treatment be selected.

Meeting clear zone criteria does not eliminate the obstacle of the culvert end, rather the obstacle is placed at a location where it is less likely to be struck. Although not as desirable as providing a traversable culvert end, it is preferred over barrier treatment where there is sufficient right-of-way and where the cost of providing the necessary culvert length is reasonable. Where the cost of added length for new culverts or of extension of existing culverts is three or more times the cost of shielding with barrier, treatment with barrier becomes an attractive alternative.

For low-volume conditions (less than 750 current ADT), the treatment option that has the lowest initial construction cost is generally the most cost-effective design if an improved design is warranted.

Bridge Class Drainage Culverts

Bridge class culverts are defined as those having an opening (i.e., distance) of more than 20-ft between the extreme inside faces as measured along the highway centerline. Bridge class culverts require protection whether they are inside or outside the clear zone. Exceptions to this requirement can be obtained by approval of a Design Exception or Design Waiver Request by the Bridge Division (see the Bridge Railing Manual for specific exception criteria).

Recommended treatment options are in the following priority:

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  1. Safety treat culvert ends.
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  2. Shield with appropriate barrier or attenuator. Table 2-14 provides guidelines for installing guardrails and bridge rails.

See discussion on cost comparison under Multiple Box Culverts for considerations when deciding on whether to safety treat or shield the culvert ends.

Anchor: #i1571549Table 2-14: Treatment Barrier Rail for Bridge Class Culverts

Depth of Cover (D)

Treatment

D < 9-in

Bridge Railing

9-in ≤ D < 36-in

Steel post welded to base plate and bolted to culvert ceiling (Low-fill culvert post option on Guardrail standard)

D ≥ 36-in1

Standard Guardrail

Note:

  1. Refer to Bridge Division for further guidance.


An additional option for shorter bridge class culverts is the use of the Long Span Guardrail when the clearance requirements on the standard are met.

Where guardrail is carried across a bridge class culvert, side slopes should be designed to provide for lateral support of the guardrail. See Figure 2-8, and the current guardrail standard for specific application.

Use of Guardrail at Culverts. (click in image to see full-size image) Anchor: #UJJIVNWVgrtop

Figure 2-8. Use of Guardrail at Culverts.

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Parallel Drainage Culverts

The inlet and outlet points of culverts handling drainage parallel to the travel lanes, such as at driveways, side roads, and median crossovers, are concerns in providing a safe roadside environment. Flow quantities for parallel drainage situations are generally low with drainage typically accommodated by a single pipe. The following guidelines apply to driveway, side road, and median crossover drainage facilities:

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  • Within the clear zone, there should be no culvert headwalls or vertical ends. Outside the clear zone, single pipe ends preferably should be sloped although not required;
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  • Where used, sloped pipe ends should be at a rate of 1V:6H or flatter. The sloping end may be terminated and a vertical section introduced at the top and bottom of the partial pipe section as shown in Figure 2-9;
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  • Median crossover, side road, and driveway embankment slopes should be 1V:6H maximum steepness, with 1V:8H preferred, within the clear zone dimensions;
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  • Where greater than 30-in diameter pipe ends are located within the clear zone, safety pipe runners should be provided with a maximum slope steepness of 1V:6H with 1V:8H preferred. Typical details for a driveway, side road, or median crossover grate are shown in Figure 2-10. Cross pipes are not required on single, small (30-in or less diameter) pipes regardless of end location with respect to clear zone requirements; however, the ends of small pipes should be sloped as described above and appropriate measures taken to control erosion and stabilize the pipe end. Multiple 30 in. pipes require cross pipes;
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  • The use of paved dips, instead of pipes, is encouraged particularly at infrequently used driveways such as those serving unimproved private property; and
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  • For unusual situations, such as driveways on high fills or where multiple pipes or box culverts are necessary to accommodate side or median ditch drainage, the designer should consider the alternatives available and select an appropriate design.

Use of Sloping Pipe Ends without Cross Pipes. (click in image to see full-size image) Anchor: #i1049629grtop

Figure 2-9. Use of Sloping Pipe Ends without Cross Pipes.

Use of Sloping Pipe Ends with Cross Pipes. (click in image to see full-size image) Anchor: #GFNSAQMFgrtop

Figure 2-10. Use of Sloping Pipe Ends with Cross Pipes.

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Side Ditches

For side ditches, attention to cross section design can reduce the likelihood of serious injuries during vehicular encroachments. Ditches with the cross sectional characteristics defined in Table 2-15 are preferred and should especially be sought when ditch location is within the clear zone requirements. Where conditions dictate, such as insufficient existing right-of-way to accommodate the desired ditch cross section or where ditches are located outside the clear zone requirements, other ditch configurations may be used. Typically, guardrail is not necessary where the desired ditch cross sections are provided. For additional information on general applications of roadside barriers, see Appendix A, Section 2.

Anchor: #i1067831Table 2-15: Desirable Ditch Cross Sections

Given Front Slope (Vertical:Horizontal)

Desirable Maximum Back Slope (Vertical:Horizontal)

V-Shaped

Trapezoidal-Shaped1

1V:8H

1V:3.5H

1V:2.5H

1V:6H

1V:4H

1V:3H

1V:4H

1V:6H

1V:4H

1V:3H

Level

1V:8H

Notes:

  1. Trapezoidal channel bottom widths equal to or greater than 4-ft.


Ditches that include retards to control erosion should be avoided inside the clear zone requirements and should be located as far from the travel lanes as practical unless the retardant is a rock filter dam with side slopes of 1V:6H or flatter. Non-traversable catch or stilling basins should also be located outside the clear zone requirements.

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