A Landowner's Guide to Building Forest Access Roads

The importance of proper planning for stream crossings cannot be overstated. If stream crossings are not planned and located before road construction begins, you have set the stage for serious problems in the future, including unintended damage to other resources. Requirements for stream crossings vary from State to State. Often a permit is required; check with the water division of your local natural resources agency. This section covers the simple designs and installation methods for culverts and fords. If you have fish in your stream, also see the section on Protecting Fish Habitat. When selecting the best method to build a road across a stream, the following factors must be considered:
	Stream size
	Debris potential
	Foundation conditions
	Construction cost
	Maintenance costs
	Road use and life.
Three methods are recommended for crossing a stream: bridges, pipe culverts, and fords. Water bars, broad-based dips, and open top and pole culverts should never be used to cross streams. These methods are used to improve drainage (see the section on Road Drainage Methods). Bridges are not covered here as their construction and design are beyond the scope of this guide. Portable bridges have been widely used in temporary and permanent applications, and are available from several manufacturers. Consult a professional engineer if you are considering a bridge.

Pipe culverts are used primarily to channel water across roadways from uphill drainages or roadside ditches. Spacing would be the same as for water bars (see Water Bars under Road Drainage Methods for spacing).

Historically, pipe culverts were steel or aluminum; however, polyurethane culverts have recently been introduced. These double wall constructed pipes are lighter, easier to handle, and can be cut to length with a handsaw. They may be worth considering, if the price is comparable.

Use no smaller than a 15-inch pipe (Helvey and Kochenderfer 1988). If there is evidence of a defined stream channel, use at least an 18-inch pipe. A drainage table provides help in determining the proper size culvert (Tables 8 and 9). An example of how to use the table is provided in the box; however, it is generic. Table 10 can also be used to determine proper culvert size and is easier to use (Helvey and Kochenderfer 1988). The method in Table 10 was developed for Appalachian forests.

Use of Drainage Table

The following example illustrates how to use the drainage table (Table 8) and choose pipe size (Table 9). Note: you will need information on slope, soils, and cover.

Example: The area to be drained is 70 acres on steep slopes with heavy soils and moderate cover. In Table 8 under C opposite 70, find area required—10.3 square feet. Under the area table for round pipe (Table 9), this falls between a 42-inch and a 48-inch pipe. Use 42-inch pipe with an area of 9.6 square feet. If a wood or other type of box culvert is planned, one 3 feet by 3.5 feet would furnish the required area.

Table 9. Size of round pipe needed for area of waterway listed in Table 8
Area (square feet)		Pipe diameter (inches)

1.25		15
1.80		18
3.10		24
4.90		30
7.10		36
9.60		42
12.60		48
15.90		54
19.60		60
23.80		66
28.30		72
33.20		78
38.50		84
44.20		90

Source: Figure 45, Haussman and Pruett 1978, p. 36

Table 8. Drainage table based on Talbot’s formula for rainfall of 2½ inches per hour
Area required for waterway*
Acres	Impervious 100% runoff	Steep slopes Heavy soils Moderate cover		Moderate slopes Heavy to light soils Dense cover		Gentle slopes Agricultural soil & cover		Flatland Pervious soils
	†C=1.00	C=.80 C=.70		C=.60 C=.50		C=.40 C=.30		C=.20

.s	square feet
2	1.0	0.8	0.7	0.6
4	1.7	1.4	1.2	1.0
6	2.3	1.9	1.6	1.4	1.2	0.9	0.6
8	2.9	2.3	2.0	1.7	1.4	1.2	0.9
10	3.4	2.7	2.4	2.0	1.7	1.4	1.0	0.6
20	5.8	4.6	4.0	3.5	2.9	2.3	1.7	0.7
30	8.0	6.3	5.4	4.8	4.0	3.2	2.4	1.2
40	9.8	7.8	6.8	5.9	4.9	3.9	3.0	1.6
50	11.6	9.3	8.0	7.0	5.8	4.6	3.5	2.0
60	13.4	10.7	9.2	8.0	6.7	5.3	4.0	2.3
70	15.0	12.0	10.3	9.0	7.5	6.0	4.5	2.7
80	16.6	13.3	11.5	10.0	8.3	6.6	5.0	3.0
90	18.2	14.6	12.5	11.0	9.1	7.2	5.4	3.3
100	19.7	15.8	13.5	11.8	9.8	7.8	5.8	3.6
150	26.9	21.2	18.5	16.0	13.3	10.7	8.0	3.9
200	33.2	26.8	22.9	20.0	16.7	13.3	10.0	5.4
250	39.5	31.5	27.1	23.8	19.7	15.7	11.8	6.6
300	45.7	36.1	31.0	27.1	27.0	18.0	13.5	7.9
350	51.0	40.6	35.0	30.5	25.3	20.2	15.0	10.1
400	56.0	45.0	39.0	33.9	28.0	22.2	16.7	11.2
450	61.7	49.7	42.0	37.0	30.6	24.2	18.0	12.3
500	66.8	52.8	46.0	40.0	33.2	26.5	19.8	13.2
600	77.0	61.6	52.5	46.0	38.0	30.3	22.8	15.3
700	86.0	68.4	59.5	52.0	43.0	34.0	25.8	17.2
800	96.0	76.1	65.8	57.0	47.5	38.0	28.5	19.0
900	104.0	83.0	71.7	62.2	51.9	41.5	31.1	20.8
1000	113.0	90.0	77.7	68.0	56.5	45.0	33.7	22.4

Source: Figure 44, Haussman and Pruett 1978, p. 36 *See Table 9 for size of pipe needed. † C is the constant factor based on a combination of how much water the soil can hold, slope, and cover. C=.70 is adequate for most conditions prevailing in the Northeast. C=1.00 represents complete runoff of precipitation.

Table 10. Culvert sizes for drainage areas ranging from 10 to 200 acres for the Central Appalachians. To use this table, determine the size of the drainage area above the stream crossing and the expected life of the culvert (recurrence interval 10, 20, or 50 years). The 20-year values are adequate in most cases within the Central Appalachians. The 50-year values should be used in more northern locations.

	Recurrence interval (years)
Area (acres)		10	20	50

10		16	16	18
20		18	18	20
30		18	20	24
40		20	22	26
50		22	24	28
60		22	24	28
70		24	26	30
80		24	26	30
90		24	28	32
100		26	28	34
125		28	30	36
150		28	32	38
175		30	34	40
200		32	36	42

Source: Table 3, Helvey and Kochenderfer 1988, p. 125

Figure 16. It is important to plan for the failure of a stream crossing, to reduce the amount of sediment that would enter the stream channel should the crossing fail. (Redrawn from Furniss et al. 1991, p. 310)

Stream crossings, such as culverts, can be considered dams that are designed to fail. The risk of culvert failure is substantial for most crossings, so how they fail is critical. In the upper sketch in Figure 16, the crossing has failed and the road grade has diverted the stream down the road, resulting in severe erosion and downstream sedimentation. Such damage to aquatic habitats can persist for many years. Stream diversions are easy to prevent, as illustrated by the lower sketch, in which the road grade was such that a failed crossing caused only the loss of some road fill (Furniss et al. 1991).

Figure 17. A cross section of a culvert at A-A shows the recommended structure of fill material installed around a culvert pipe. (Redrawn and adapted from Figures 6-5 and 6-6, Wisconsin Department of Natural Resources 1995, p. 26)

Culverts should be installed as the road work progresses. The culvert and its related drainage features should be installed in the following order:
1.	Place debris and slash to be used as a filter system, if needed.
2.	Construct sediment ponds, if needed.
3.	Complete downstream work first, such as energy dissipating devices and large rock riprap.
4.	Route stream around work area until pipe is installed.
5.	Construct pipe inlet structure.
6.	Install culvert pipe.

A culvert inlet should be placed on the same level as the stream bottom. In some instances where the culvert inlet has to be lower than the drainage gradient, a drop box can be constructed. This box, which is a place for sediment to settle out before water enters the culvert, needs frequent maintenance.

Install culvert pipes as near as possible to the gradient of the natural channel and so there is no change in the stream bottom elevation (Figure 17 top). Culverts should not cause damming or pooling. Seat the culvert on firm ground and compact the earth at least halfway up the side of the pipe to prevent water from leaking around it. Pipe culverts must be adequately covered with fill; the rule is a minimum of 12 inches or half the culvert diameter, whichever is greater (Figure 17 bottom).

Culverts not installed at the same level as the stream cause water to back up.

Figure 18. Use stabilization practices on soil exposed at stream crossings during construction, until the soil is permanently stabilized. (Detail A-A redrawn and adapted from Figure 6-13a, Wisconsin Department of Natural Resources 1995, p. 35)

The following are additional guidelines for installing culverts in streams:
	Limit construction activity in the water to periods of low or normal flow. Minimize use of equipment in streams.
	Use soil stabilization practices on exposed soil at stream crossings. Seed and mulch and install temporary sediment control structures, such as silt fences made of straw bales or geotextiles immediately after road construction, to minimize erosion into streams (Figure 18). Maintain these practices until the soil is permanently stabilized.
	Use materials that are clean, nonerodible, and nontoxic.
	To prevent erosion and under-cutting of the inlet end of the culvert, provide a headwall. Sandbags containing some cement mixed with the sand, durable logs, concrete, or hand-placed riprap are suitable (Figure 19).

Keep culverts clear and free of debris so water can pass unimpeded at all times. Culvert failure is caused by blockage with debris as often as by the culvert’s capacity being exceeded. For this reason, avoid leaving excess amounts of woody debris in stream channels where it can float downstream and lodge in culverts. All culverts should be checked after major storms and at least twice per year—in spring and fall (Helvey and Kochenderfer 1988). Maintenance is especially important in areas where beavers are present.

Figure 19. Install riprap to prevent erosion at the inlet to a culvert pipe.

A ford is an alternative way to cross a water course under the following circumstances:
1.	The streambed has a firm rock or coarse gravel bottom, and the approaches are low and stable enough to support traffic.
2.	Traffic is limited to low volumes of light vehicles.
3.	Water depth is less than 3 feet.
4.	If corduroy, coarse gravel, or gabion is used to create a driving surface, it should be installed flush with the streambed to minimize erosion and to allow fish passage.
5.	Crossings should be at right angles to the stream.
6.	Stabilize the approaches by using nonerodible material. The material should extend at least 50 feet on both sides of the crossing.

Fords can be an economical method of crossing streams under certain low water conditions and when properly located and designed.


Undersized culverts can become plugged with sediment		A culvert not installed at the existing stream gradient can degrade the stream channel.