Ten Rules of Thumb for Culvert Crossings

Spending the time to achieve proper drainage crossings of ranch and forest roads is good business. As consultants for ranch, forest, rural subdivision, and commercial site road design, we are called upon to advise on new construction and, unfortunately, sometimes on the repair of culvert road crossings. Forest access roads are essential to forest management, insect and fire control, timber stand improvement, inventory, recreation, and harvesting. Crossings of live and ephemeral drainages are most commonly accomplished with buried culverts. An overwhelming number of these are galvanized or aluminized corrugated metal pipe (CMP). The most common failures for forest roads are improper drainage control and culvert blowout. It stands to reason that a properly designed and constructed culvert will not only stand the test of weather and time but also will be in place when needed the most.

This culvert was set too shallow without a proper tail ditch.A culvert smaller than 18 in. might be the correct size, but size is not everything in the design of culverts. Pound for pound, most commonly available 18-in. CMPs are the strongest pipes to put in service. The gauge of the steel often is the same for pipes from 18 in. all the way up to 60 in. Smaller sizes, such as 8, 12, and 15 in., often are made of thinner-gauge steel. The 18-in. pipe offers the added benefit of being easier to clean if it becomes plugged than pipes of smaller diameters. Pop cans, leaves, and pinecones easily can render useless a 6- or 8-in. culvert. The cover over the pipe is critical to the performance of the pipe in service. The cover spreads the downward load of vehicles’ wheels over a greater footprint, preventing the collapse that can occur when a heavy force is applied to a focused point on the pipe. The cover must be clean: free of large sticks, stones, and trash. We often specify that no debris, including stones, larger than 3 in. in diameter be allowed in the backfill. Manufacturers identify minimum tover depths over the top centerline for their products – generally 18 in. of clean cover. More cover is preferred, and we often specify at least 1.5 times the minimum to ensure that adequate cover is achieved. Warranty of the installation might not be possible if the manufacturer’s minimum requirements are not met. Rule 2. Measure the Cross-Sectional Area of the Culvert Crossing to Obtain the Area of Flow for the Spring StormThis rule comes with several caveats. All culverts should be sized according to their intended use and the amount of protection they are to provide. On primary roads, over live streams where residential and emergency traffic travels, a professional engineer trained in hydrology and hydraulics should be employed to design the crossing; rules of thumb do not apply in these situations. The rules of thumb for sizing can be applied on woodlot, ranch, farm, and private roads; in minor drainages; and across small streams where seasonal access is required.The idea is to design a crossing that will function in the spring melt and summer afternoon thunderstorms in an average year. The method we employ is to obtain the stream’s average area beneath the scour mark or high-water mark that results from years of storms. This mark can appear as a cut bank, a vegetation line, several years’ collection of flotsam arranged as a high-water mark, stains on boulders from storm events, or the limits of the exposed sand- or gravel-armored stream bottom. It generally is the visually observed “channel” limits where a casual observer easily could identify where water previously has flowed.

This is the same double culvert after some needed attention. The brush was cleared, the riprap added, and the culvert approach and departure channel reshaped before the snow.Sizing riprap can be fairly complicated if standard engineering calculations are to be completed because of the level of detail needed about the site to fill the variables in the equations. Often professional judgment is substituted for site-specific measurements to complete the equations, which would not be an option for someone who is not trained in these matters. There is a rule-of-thumb alternative.If the area has natural rock occurring in the stream as in many northern, eastern, and western states, a quick and easy way to size riprap is based on the energy of the stream to move it. Go to the stream and walk in the bottom stepping heel to toe. Count, measure, and record the average diameter of 100 stones you encountered on the end of the left toe only. Record all the measurements, not just the large ones. Generally, 0.5-, 0.75-, and 1-in. classifications to 10 in. followed by 2-in. classifications to 24 in. are plenty. Naturally, anything larger than about 6 in. will need to be measured in place. Drop the two smallest diameters and add the two largest to the list. Add the entire list of diameters together and divide by 100 to get the average-size stone that nature has left in the creek for bottom armor. The idea is to size median stones in the riprap to be slightly larger than the average stone left behind after years of storms have scoured the channel. Commercially availably riprap median sizes range from 6 to 36 in. in a mixture of smaller and larger pieces. The size classes available from quarries usually are in 6-in. groups (6, 12, 18, 24, and so on). About 50% of the stones in a delivery will be larger and 50% smaller, but the median stone will be what you ordered. Whatever size you estimate from your count, go up to the next size to be safe. Always use a graded material, one with many differing stone sizes. This aids in interlocking the material, filling the voids, and dispersing the jet of water through the riprap to help prevent erosion of the underlying soils (piping).The depth of the riprap is key in providing the necessary protection. Engineers generally stipulate that the depth be at least twice the stone size (e.g., 12-in. stones that are 24 in. deep). Because the riprap must be flush to the dry gulch or live stream bottom, overexcavation will be required. Under the riprap, place a 6-in. bed of pea gravel or a strong nonwoven geotextile filter fabric. If bedding or fabric is not used, piping will occur and the riprap will appear to sink into the ground over time, thereby losing its effectiveness. If the project is in an area frequented by the public, it is best to use stones at least 12 in. in diameter or larger to prevent vandalism. Smaller stones tend to “migrate” to campgrounds, become souvenirs, or be changed into “tosserights” (“Bobby ‘tossed her right’ through the window, Dad!”) and disappear. Generally five times the culvert diameter will be adequate for the length of the riprap needed upstream and downstream of the culvert. A width strategy that usually works is to place the stones so that they come up the sides of the channel to at least the halfway point on the diameter of the pipe.Rule 9. Check the Condition of the Crossings Frequently and Clear the Openings of DebrisWe often specify that a fence post or a Karsonite marker with a reflector be placed on the bank above the culvert openings for easy identification in the dark and when there is snowpack. The culvert cannot operate properly if blocked. Snow and ice can block an opening just as easily as twigs and leaves can. When culverts are marked, snowplow operators can be alerted so as not to block or damage the ends. It is easy to routinely observe the opening of a marked culvert when driving by. Rule 10. Know Your LimitationsIf the job seems too big, get help. Damage to downstream areas from an improperly designed crossing is the landowner’s responsibility. In many instances, designs on live streams require federal, state, and local permits. Restricting the stream to a culvert can block fish passage to upstream feeding and spawning areas. Most professionals will offer general advice and guidance gratis over a cup of coffee and can quickly assess your need for further engineering and habitat design studies. Summary Designing and building proper roadway crossings will greatly increase the service life of the piping and roadway section. Reduced maintenance and replacement costs are the monetary savings for following these rules of thumb. Christopher M. Crowley is an SAF Certified forester, president of the Colorado Forestry Association, and a forest hydrologist in the Central Rockies for TetraTech-RMC.

Dennis Will, a forester with the Colorado Forest Service, and Colby Hayden, P.E., with TetraTech-RMC, were consulted in writing this article.