Ten Rules of Thumb for Culvert Crossings

Sept. 1, 2003
By Christopher M. Crowley

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.

Over many years of firsthand observation of installed culverts, 10 items stand out as good rules of thumb. These rules address pipe sizing, installation, pipe material, erosion protection, and maintenance.Rule 1. Use a Pipe No Smaller Than 18-In. Diameter With 18 In. of Clean, Compacted Cover
This culvert was too small and not long enough for the traffic to make the turn. Its crushed end will not pass storm flows.
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.
We take the average depth multiplied by the average width to obtain the cross-sectional area and divide by four to determine the rough diameter of pipe needed to pass the average storm. The diameter of pipe(s) used must add up to the total diameter needed without using a pipe taller than the average depth.Always measure the slope of the channel because slope affects the capacity of the pipe. A simple way to measure is to pull a 10-ft. rope taut downstream with a line level bubble attached to obtain level and measure the feet of rise from the channel bottom to the end of the string. Calculate the feet per foot of fall by dividing the length (10 ft.) by the rise. Generally pipe slopes are kept to something less than 0.25 ft. per foot, or 2.5%. The steeper the pipe, the more capacity it will have, because the velocity within the pipe is increased. Increased velocity generally requires more energy dissipation at the outfall end (see Rule 8). If you calculate that something more than a 48-in. pipe is needed or that the fall is greater than 2.5 ft., it is strongly recommended that the opinion of a professional be obtained. A structure’s failure could cause significant flood damage downstream.Crossings that carry storm drainage from one side of a roadway to another to “relieve” the roadside ditch are called relief culverts and should be sized to turn out 75% of the ditch flow. These culverts should be placed at least every 500 ft. where the road is in a cut or the ditch continues to follow the road. Watch your roadway after construction and place additional relief culverts upstream of where water crosses the roadway. Generally, using 18-in. pipes as relief culverts systemwide is considered acceptable. Remember to place them low enough in the roadside ditch to allow for a bypass equal to or greater than 25% of the full ditch capacity. Rule 3. Place Multiple Culverts at Least One Culvert Diameter Apart
PVC is not UV light resistant and will become brittle. The pipe is also too small and set too high to drain the ditch.
The embankment soils are too steep and were not compacted, which has led to filling in the orifice.Backfill of soils under, adjacent to, and atop culverts is extremely important in realizing their full service life. Pipes should be separated enough so that the soil can be worked into a tight bond with a compactor. Where fill is loosely placed or voids occur in the backfill next to the culvert, a phenomenon called piping can occur. Piping means that water enters the space between the culvert and the backfill, saturates the soil, and slowly washes away fine particles. Over time this action creates larger voids and erodes the soil away until a failure occurs, either when a storm washes out the remaining soil and causes a blowout or when a vehicle sinks into a hole and crushes the culvert. Rule 4. Compact Clean Soil Tightly in and Around Culverts and the Cover MaterialYou can rent a “wacker” for a few dollars at most rental centers. A wacker is a gas-operated, vibratory, single-operator compactor that can be brought to the site in a pickup truck. You can build a great road and design an incredible culvert crossing, but without compaction of the backfill, you will get to do it all over again. Poor compaction and debris-laced fill are the most commonly diagnosed failures in culvert crossings.Careful placement and compaction of clean fill is needed to install a culvert properly. Installing a granular backfill of pea gravel, often called pipe bedding, in the trench to a depth of 6 in. under the pipe and on both sides up to the midway point aids in draining groundwater and seepage away from the outer walls of the pipe and reliably assists in preventing piping failures.Rule 5. Construct the Road Section Low or Allow for Overtopping to One SideThe sizing of the culvert crossing is for an average year storm. Blockage, larger storms, rapid snowmelt, and debris flows all endanger the roadway and crossing. A prudent designer purposely will depress the road grade to allow for a storm-flow section equal to twice the cross-sectional area developed in Rule 2 to pass over the roadway. The site geometry will determine whether the overtopping section should be directly over the culvert or whether placing it to one side will be adequate. If possible, the overtop section should be placed to one side to prevent the erosive forces of the overtopping mixture of water and debris from destroying the downstream embankment face directly over the culvert. Overtopping to one side often causes the water to flow on a thinner fill section or, even better, over stable native ground. If the overtopping section is strategically placed and sufficiently armored, the access road will still be in place once the floodwaters have passed. Rule 6. Use Maximum Sideslopes of 2:1 (H:V) and a Road Surface Width of at Least 12 Ft. to Calculate the Pipe LengthA common problem with culvert installations is trying to get by with a short pipe length. The design needs to take into account the sideslopes, depth of cover, and roadway width to calculate the necessary length of pipe. Sideslopes steeper than 2:1 generally are not stable and erode quickly in the first few years. Steep slopes are difficult to establish vegetation on and hard to maintain. The slope should meet the bottom of the pipe and not the top; the latter causes a very steep point over the pipe that easily is eroded. Add together the diameter of the pipe twice, the slope length (cover times slope) twice, and the roadway width to determine the pipe width. For pipes set in steeply sloped channels, more length is needed, but in general this approach works for a majority of the crossings. If possible, always install a factory-made flared end section on the pipe, which aids in outfall dispersal of the stormwater and makes road embankment sideslopes above the pipe more stable.
Rule 7. Consult a Professional When Working With Special-Use PipesSpecial-use pipes include arch pipe, elliptical reinforced concrete (RCP), noncorrugated steel, and PVC. Special-design pipes, such as arched and elliptical, generally are more expensive than standard CMP culverts and therefore should be applied carefully. In addition, RCP pipes can be more technically challenging to install properly. RCP is heavy and usually must be delivered to the site on a large truck. Mild steel and some easily found tubing from metal yards or discards from industrial applications usually are not up to vehicle loading pressures and likely not as corrosion-resistant as coated CMP. Industrial pipes sometimes can cause a discoloration of water through rusting or chemical coatings. PVC waterline pipes generally are not recommended because they are not ultraviolet- (UV-) resistant and will become brittle after extended exposure to sunlight. PVC pipes do not have as high an elasticity coefficient as other available materials and might shatter or splinter instead of crimping or bending when heavily loaded.Although multipurpose CMP is by far the most widely used pipe for this application, the use of high-density polyethylene, or HDPE, piping for roadway projects is on the rise. These pipes are UV- and corrosion-resistant, come in all the standard sizes with end sections and adapters such as CMP, and are cost-competitive. The drawback is that these pipes cannot withstand wildland or prescribed fire. They are very flexible, however, and can be reused because of their durability. Rule 8. Add Riprap Protection to the Upstream and Downstream Approaches to CulvertsCommonly, riprap or concrete rubble is installed on the ends of culverts to provide scour protection. Unfortunately it often is placed incorrectly or is too small to provide the needed level of protection.
These culverts are properly separated to better handle flows.
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.