Culvert Sizing
Culverts are a common method to cross streams. The culvert size selection may vary slightly with differing landowner objectives, but the primary objective is to convey water beneath a road bed. All culvert, regardless of size, inherently have an associated risk of washout/failure. However, as the size of the culvert selected increase, the risk of perceived failure decreases. Landowners and forestland managers should make an informed decision with the information available to them.
One of the most important factors when implementing a culvert stream crossing is to determine the proper size given the watershed characteristics. Stream gauge data, historical records, or other hydrological data are useful to help understand flow dynamics, but are often not available in many forest management situations. Without hydrologic data, Talbot’s Formula method has been a generally acceptable method for culvert sizing. It is one of the simplest and easiest to implement when few data records exist.
A = C * M0.75
Where A = cross sectional area of a drainage needed. C = runoff coefficient constant. This is based on a combination of soil absorptive capacity, slope, and cover. M = acres of upstream watershed. Talbot’s formula is used for a maximum of 4 inches of rainfall per hour, which is the rate used in the formula above. Note that the culvert sizing recommendations in the BMP manual are at a rate of 2.5 inches of rainfall per hour.
Check out Talbot’s Formula Excel Spreadheet
This user-friendly interactive spreadsheet allows anyone to enter a given upstream acreage and runoff coefficient (based on easy-to-use descriptors) to calculate a culvert pipe(s) size needed to discharge stormwater runoff, based on rainfall received.