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.
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Dennis Will, a forester with the Colorado Forest Service, and Colby Hayden, P.E., with TetraTech-RMC, were consulted in writing this article.
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This is a question with a hundred or more variables. Where I live, we use a rule of thumb to reality check runoff calculations, which is 1 CFS per acre is usually on the high end. So calculate the contributary area which would lead to the pipe. Include the roof of the pole building and any area upstream of the barn where water could flow overland and eventually reach your pipe.
Secondly, the diameter of the culvert is less important than the slope you install it on. So will it be flat as a pancake? 2%? 5%? 10%?
Generally, 2' of cover will be safe for any kind of pipe, assuming you bed and backfill properly (compact every 12"). You will probably be finding PVC or HDPE (ADS or Hancor) are the most cost effective.
If you want to know how culverts are sized professionally, then I'll tell you they're made as big as the agency can afford. Which is never nearly big enough; but a harsh reality in a Country where the infrastructure is in shambled.
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