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Cross Drain

7700—Transportation Systems
2500—Watershed and Air Management
July 1998
9877 1804P—SDTDC
Revised for Internet July 2003

Cross Drain Update

Ralph Gonzales
Mechanical Engineer

Cross drain techniques such as open-top cross drains and surface water diverters using conventional materials have been in use for years. New cross drain techniques are emerging due to the availability of new materials and field innovations. Some of these techniques are being developed and have not been applied in the field. The objective of this report is to disseminate information on new materials and techniques. In addition, new, innovative application of existing materials or techniques are reported. Some of the materials presented in this report are subject to local availability.

Report Overview
A synopsis of the information contained in the report is provided in table 1, with further discussion in the following pages. When available, cost information is provided as a reference only, as prices and rates vary from location to location. Definitions of terms used in this document are included in appendix A; a cost summary for open-top pipe culverts is included in appendix B.

Two methods were used to gather information for this report: A market search, including publications in print and on the Internet; and a survey to U.S. Department of Agriculture (USDA) Forest Service personnel. Since the method or technique for using cross drains is specific to local geography and conditions, pertinent local information is included. Both search methods identified available techniques, some of which are not new, but are included to disseminate information and perhaps to stimulate new applications.

Table 1—Summary of cross drain techniques.
Open-top pipe culvert Steel pipe with sections removed to collect and channel surface damage Heavy walled (0.313 in [8 mm]) steel pipe
Portable road spillway Steel grid on top of concrete abuttment Steel concrete
Metal water bar Using a "W" beam guardrail as a water bar Standard "W" beam guardrail
Rubber water diverter Flexible surface drainage structure Converyor belt or rubber skirting
Pre-cast concrete trough Open top drain using concrete Pre-cast concrete
Alternative materials Alternative materials to CMP Polyethylene, used steel pipe
Driveable and durable "hump" Series of water diverters of varying heights Rubber, plastic, concrete, wood, and so on

Open-Top Pipe Culvert
Information for this section was taken from Using Open-Top Pipe Culverts to Control Surface Water on Steep Road Grades by James N. Kochenderfer, Northeastern Forest Experiment Station, General Technical Report NE-194, with the author’s permission. These open-top pipe culverts have been installed and used successfully on “minimum standard” forest truck roads in proximity to the Fernow Experimental Forest near Parsons, WV.

Open-top pipe culverts are effective in controlling surface water on portions of minimum-standard roads where road grades exceed 10 percent. The open-top pipe culvert is not recommended as the primary means of water control but as a supplemental device that can be used on steep road sections where broad base dips are not recommended. Open-top pipe culverts offer land managers an alternative to crowning and ditching roadbeds for water control. Unlike culverts constructed from wood, the open-top pipe culvert is a relatively permanent water control device; however, these culverts may be salvaged and used on other roads as the need arises. The cost of an open-top pipe culvert is comparable to that of a gravel broad-based dip.


Using a chalk line, mark two parallel lines 3 in (75 mm) apart along the length of the steel pipe. The lines create longitudinal borders for the inlet slots. Mark slot locations within the two parallel lines. A welding marker works well for marking the slots. Slots 24-in long by 3-in wide (600-mm by 75-mm) work well with an 8-in (200-mm) diameter pipe (figure 1). Larger slots may be used in larger diameter pipes. Experience has shown that slots this size do not damage tires and are wide enough to allow the pipe to be cleaned. Spacing slots at least 6 in (150 mm) apart will prevent the pipe from collapsing under a heavy wheel load. To provide structural rigidity, leave at least 18 in (450 mm) of solid pipe at both ends.

The open-top pipe culverts have been installed with a downslope skew ranging from 45 to 65° with an average of 54° skew and average road grade of 12 percent. It is important to minimize skew to improve its self-cleaning capability. Skew in this document is measured from the road centerline (figure 2).

Culvert installation may be done manually or with the use of a small dozer. The culvert is installed with the top of the pipe 3 in (75 mm) below the surface. The roadbed is then beveled back about 18 in (450 mm) on each side of the culvert. The skew and depth is better controlled when installed manually. However, a 10-in (250-mm) diameter pipe weighs about 31 lb/ft (46 kg/m) and an 8-in (200-mm) diameter pipe weighs about 25 lb/ft (37 kg/m); therefore, a typical culvert section would weigh between 500 and 600 lb (227 kg and 272 kg). Two people can move and position the culvert by sliding or rolling it. Lifting the culvert requires a small dozer. Once installed, the outfall may be armored with rocks and a half-round plastic pipe.

Open-top pipe culverts are being used in central Appalachia. The average culvert length is 20 ft (6 m). The 20-ft (6-m) length allows for enough skew on a 15-ft- (4.6-m-) wide road. Both 8-in- (200-mm-) and 10-in- (250-mm-) diameter pipe were used. The 8-in (200-mm) pipe was preferred because it is generally less expensive, requires a shallower trench, and is easier to maneuver by hand than the larger pipe.

While open-top pipe culverts function well on steeper grades, it is desirable to keep road grades low, both to facilitate water control and to provide maximum use. These culverts also can be used where rocky subgrades might prohibit construction of broad-based dips and on road sections between landings and highways to prevent water from running on to highways.

A tool for cleaning open-top pipe culverts is shown in figure 3.

It is important to space open-top pipe culverts properly so that water can be handled in small amounts. For design, use local cross drain spacing formulas for open-top-drains.

Using a cutting torch to cut slots in a heavy walled pipe.
Figure 1—Using a cutting torch to cut slots in a heavy walled pipe.

Layout and dimensions of an open-top pipe culvert.
Figure 2—Layout and dimensions of an open-top pipe culvert.

A tool for cleaning open-top pipe culverts is made by bending a 5-ft (1.5 m) piece of 0.75-in (19-mm) pipe two ways and welding it to a 4- by 5-in (100- by 127-mm) shaped peice of metal cut from a pipe.
Figure 3—A tool for cleaning open-top pipe culverts is made by bending a 5-ft (1.5 m) piece of 0.75-in (19-mm) pipe two ways and welding it to a 4- by 5-in (100- by 127-mm) shaped peice of metal cut from a pipe.

Portable Road Spillway
The portable road spillway (figures 4a and 4b), a prefabricated, portable, reusable cross drain works on the principle of diverting silt and debris encountered in typical surface runoff into a series of pre-established settling ponds. An illustration of a typical application is shown in figure 5. The settling ponds are constructed on the low side of the road spillway and are made out of native material found at the site. The settling ponds slow the movement of water both from the natural drainage system and from roadway runoff. Sediment and debris are settled out within the ponds before water is discharged to the local water course. Heavy equipment traffic typically associated with logging and mining roads is easily handled by the Portable Road Spillway.


The portable road spillway, distributed by RayMac Environmental Services, consists of a structural steel tubing grid sitting on top of precast L-shaped abutments. It is constructed in three separate sections: two concrete abutments, and one steel top grid. The concrete abutments are made of 3,000 psi (20 MPa) redi-mix concrete reinforced by #5, grade-40 rebar. When fully assembled, each abutment weighs 10,000 lb (4,536 kg). The 2,500-lb (1,134-kg) top grid is constructed of structural grade steel (figure 6).

The entire portable road spillway can be transported to the site in a standard dump truck. An excavator is used to dig a trench across the road to accommodate the spillway and place the abutments. Once the proper separation of abutments has been established, the excavator places the steel grid on top of the abutments and backfills the trench (figure 7).

The steel grid may be removed to clear the trench of obstructions and sludge. Depending on the amount of debris or sediment present, a small excavator may be necessary for cleaning out the trench.

Figure 4a Figure 4B
Figure 4a and 4b—Two views of the Portable Road Spillway.

Typical application of the Portable Road Spillway.
Figure 5—Typical application of the Portable Road Spillway.

Portable Road Spillway assembly.
Figure 6—Portable Road Spillway assembly.

Figure 7—Sectional view of the Portable Road Spillway.

Metal Water Bar
The metal water bar is an innovative example of using a standard “W”-beam guardrail. It combines two common cross drain techniques, the water bar and an open-top drain.


Construct anchors as shown in figure 8 using the 0.25-in (6.4-mm) steel bars. Weld the completed anchors to the “W”-beam guardrail at even intervals as shown in figure 8. Guardrail length comes in standard 12.5-ft (3.8-m) or 25-ft (7.5-m) length. Cut off the excess to achieve an appropriate length, including the additional length needed for the skew angle. The smaller the skew angle, the longer the length has to be over the length of the road. The length will be determined by the following formula:

Length = road width / sine (skew angle) + installation tolerance

When using 12.5-ft (3.8-m) lengths, the two pieces should be butt-welded or overlapped with the downgrade section beneath the upgrade section. All holes on the guardrail must be permanently plugged. All nongalvanized, nontreated areas must be painted with primer.

Install the metal water bar with a maximum 60° downslope skew. The installation can either be done manually or with the help of a dozer for the heavy lifting. Measure the maximum height of the metal bar assembly, once constructed. This will determine the depth of the trench to be dug. The depth of the trench should be about 3-in (75-mm) deeper than the maximum height of the assembly. A trench wider than the width of the water bar assembly is necessary not only to more accurately position the water bar, but also to allow for a margin of error. The water bar is installed with the top of the “W” beam about 3-in (75-mm) below the road surface. Once installed, the road could be beveled back about 18 in (450 mm).

A rock outfall may be constructed at the end of the water bar using 3- to 12-in- (75- to 300-mm-) diameter rock. The outfall should be approximately 2-ft (0.6-m) wide and at least 6-in (150-mm) deep.

Metal water bar construction details.
Figure 8—Metal water bar construction details.

Rubber Water Diverters
Rubber skirting or used conveyor belts are utilized to make water diverters. The water diverters direct water off the surface of the road. Like the other cross drains, the skew angle is critical to the function of the water diverter. Rubber diverters require minimal maintenance; however, to reduce possible damage by grading operations, use an object marker to identify location of diverters.


Secure the rubber skirting on the 4-in (100-mm) face of the pressure treated timber, using the lag screws and washers. Figure 9 illustrates the construction of the diverter.

An alternate method of construction uses conveyor belting.


The bottom of the conveyor belt is “sandwiched” between the boards.

Install the rubber diverter with a maximum 60° downslope skew. A trench is dug approximately 36-in (900-mm) wide. The diverter is installed so that approximately 3 to 4 in (75 to 100 mm) is above the road surface. The density of the backfill must equal or exceed the density of the surrounding material. The backfill material must be either the same material as the road or be crushed aggregate. Figure 10 provides installation details.

Rubber water diverter detail.
Figure 9—Rubber water diverter detail.

Installation detail of rubber water diverter.
Figure 10—Installation detail of rubber water diverter.

Precast Concrete Trough

This cross drain device could be classified under open-top drains. Similar to the devices in the open-top category, this concrete trough allows surface water to accumulate through the open top (figure 11).


The soil around both sides of the cross drain must be compacted. The concrete trough must be installed with a maximum 60° skew and at least a 4 percent fall.

Construction and installation details of pre-cast concrete trough.
Figure 11—Construction and installation details of pre-cast concrete trough.

Alternative Culvert Materials to Corrugated Metal Pipe

Polyethylene Pipe
Polyethylene pipe has approximately twice the service life of corrugated metal pipe and is lighter and easier to install. The anticipated service life of high density polyethylene (HDPE) is approximately 75 years. Corrugated steel has an anticipated service life of 40 years. HDPE is strong enough to endure soil pressures at a depth of up to 100 ft, and is durable enough to handle runoff containing abrasive bedload.

Two polyethylene products were evaluated for this project: Advance Drainage Systems (ADS) N-12 and ADS N-12 HC. ADS N-12 is a HDPE drainage pipe available in diameters ranging from 4 to 36 in (100 to 900 mm). The pipe is a combination of an angular corrugated exterior for strength and a smooth inner wall for maximum flow capacity. ADS N-12 HC comes in 10-in (250-mm), 12-in (300-mm), 15-in (380-mm), 18-in (450-mm), 24-in (600-mm), 30-in (760-mm), 36-in (900-mm), 42-in (1-m) and 48-in (1.2-m) diameters. The N-12 HC has smooth inner and outer walls and a “honeycomb” wall section for structural strength and ring stiffness. The HDPE pipe withstands vertical pressure by transferring the load to the surrounding soil. N-12 and N-12 HC will support HS-20 live loads under 12 in (300 mm) of cover. This is equivalent to values specified for corrugated metal and concrete pipe. HS-20 loading designation is specified by American Association of State Highway and Transportation Officials. The HS-20 live loading is comparable to a 3-axle truck with an 8,000-lb (3,630-kg) load on the front axle and a 32,000-lb (14,500-kg) load on the two rear axles. Maximum cover will vary with conditions, but can usually extend from 30 to 50 ft (9 to 15 m). Table 2 provides a weight comparison of HDPE, clay or concrete, and corrugated metal pipe. Figure 12 graphically represents corrosion resistance (recommended pH range).

Table 2—Weight comparison of three pipe types by inside diameter
Inside Diameter in inches (mm) ADS N-12/N-12 HC HDPE Pipe in lb/ft (kg/m) Clay or Concrete in lb/ft (kg/m) Corrugated Metal in lb/ft (kg/m)
15 (   380)   4.6 (  7) 103 (   153) 12.9 (19)
18 (   450)   8.4 (12.5) 131 (   195) 15.8 (23.5)
24 (   600) 11.5 (17) 217 (   323) 19.4 (29)
30 (   760) 15.4 (23) 384 (   571.5) 30.0 (45)
36 (   900) 18.1 (27) 524 (   780) 36.0 (54)
42 (1,000) 26.5 (38) 650 (   967) 57.0 (85)
48 (1,200) 32.0 (48) 780 (1,161) 65.0 (97)

Used Gas Pipe
Used gas pipe has been utilized in the Allegheny National Forest of Region 9, where the pipe is available locally. The wall thickness on the steel pipe is four times greater than that of conventional corrugated metal pipe (CMP). Thicker walls allow the pipe to be installed in areas where the minimum coverage of 12 in (300 mm) for CMP or HDPE pipe is difficult to achieve. Although the procurement and installation costs are higher than for new CMP, the anticipated service life is longer.

Driveable and Durable “Hump”
This cross drain technique remains in the concept stage as illustrated in figure 13 and is included in this report to generate interest and possible implementation. Like the rubber water diverter, the hump diverts surface flow off the road while requiring minimal modification to the road profile. Several stages would allow for sedimentation while still preserving diversion capability and extending periods between required maintenance. Hump length and height should be tailored to road grade, climate, expected flows, soil type, and design vehicle. Possible materials to test include rubber, rubber strapping, plastic, concrete, metal, and wood.

The application of cross drain techniques will have varying results due to local geographical conditions. The techniques are presented to provide information on products that have been successful in other areas and also to stimulate innovative applications.

Corrosion resistance 9recommended pH range).
Figure 12—Corrosion resistance 9recommended pH range).

A dreveable and durable "hump".
Figure 13—A dreveable and durable "hump".


Appendix A
Armoring—protective covering, such as rock, vegetation, or engineered materials used to protect stream banks, fill- or cut-slopes, or drainage structure outflows from erosion.

Cross Drain—a ditch relief culvert or other structure or shaping of the traveled way designed to capture and remove surface water from the traveled way or other road surfaces.

Crown—traveled-way surface shaping with the high point in the middle, causing surface runoff to flow both toward the uphill shoulder or ditch and toward the downhill shoulder.

Culvert—a conduit or passageway under a road or other obstruction designed for the passage of water, debris, sediment, and fish, backfilled with embankment material.

Inslope—traveled way surface shape with high point on downhill shoulder, causing runoff to flow toward the toe of the backslope or inboard ditch.

Manning’s Roughness Coefficient—dimensionless number indicating surface roughness. A lower number indicates a smoother surface.

Outfall—the outlet end of a culvert.

Outslope—traveled-way surface shaping with the high point on the uphill shoulder, causing surface runoff to flow toward and over the downhill shoulder.

Pipe—a culvert that is circular in cross section.
Road—a general term denoting a mode for travel by vehicles greater than 50 in (1.3 m) in width.

Roadbed—the graded portion of a road between the intersection of subgrade and side slopes, excluding that portion of the ditch below the subgrade.

Sediment—deposition of materials eroded and transported from locations higher in the watershed.

Service Life—the length of time for which a facility is expected to provide a specified service.

Skew—the angle of deviation from a reference line. In this document, the reference line is the road centerline.

Subgrade—the layers of roadbed that come up to the top surface, upon which subbase, base, or surface course is constructed. For roads without base course or surface course, that portion of roadbed prepared as the finished wearing surface.

Surface Drainage—the concentration and flow of surface water on roads and related surfaces and in ditches.

Appendix B

Cost Summary for the Open-Top Pipe Culvert
Unit Cost ($)
Total Cost ($)
Culvert Preparation
20.0 ft
1.0 hr
55.0 ft³
Installation (manual)
1.0 hr
0.25 ton
Crew vehicle
10.0 mile
Installation (bulldozer)
Dozer and operator
1.0 hr
1.0 hr
0.25 ton
Crew vehicle
10.0 mile
Culvert outlet protection
3.0 hr
Crew vehicle
6.0 mile
Half-round plastic
10.0 ft
1.0 hr
Crew vehicle
6.0 mile

TD logo
For Additional Information Contact:
Project Leader
San Dimas Technology & Development Center
444 East Bonita Avenue, San Dimas CA 91773-3198
Phone 909-599-1267; TDD: 909-599-2357; FAX: 909-592-2309
E-mail: mailroom_wo_sdtdc@fs.fed.us

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