In this article, I will explain the process of sizing a culvert. I will be using CulvertPro, our in-house culvert design software.
This article is meant for technical professionals designing bridge sized culverts larger than approximately 1.5 m (5 ft) in diameter. If you are designing a small culvert, please refer to our article How to Design a Culvert.
The overall process is to start with a channel flow depth in absence of the culvert, and choose a culvert size based on how high the flow will back up. The steps are as follows:
1. Determine the Design Flow
Measured in m3/s or ft3/s, this is the flow you will want to pass through the culvert at the given return period. You can use any hydrological software package or CulvertPro, which contains a Rational Method calculator and an SCS Method calculator. The determination of a design flow is outside the scope of this article.
2. Determine the Tailwater
Tailwater is defined as the depth of flow of the channel in absence of a culvert.
Using some sort of channel hydraulics software, hopefully one that uses the Manning’s equation, you can use the flow determined in step 1 together with the average channel properties to determine the flow depth of the channel at that flow. If the channel has been surveyed, you would have these values. Make sure they are all averages though (no minimums or maximums at this step).
3. Determine the Headwater and Velocities
To use CulvertPro, you will need the following information:
- Design Flow: From step 1.
- Tailwater: From step 2.
- Culvert shape and size (see our standard size list for CSP)
- Culvert length
- Invert Elevations
- Culvert burial (how far below the streambed the culvert invert is)
- Manning’s roughness coefficient of culvert (looked up on a table in the sidebar)
- Entrance Loss coefficient (looked up on a table in the sidebar)
- End section type (either straight or conforming to the slope)
CulvertPro will give you the headwater elevation and velocity at the upstream end, which can be compared to your design criteria. Usually the design criteria consists of having a culvert flowing full (headwater elev. = top of culvert) or not overtopping the road (headwater elev. = road shoulder) at the design flow. If you’ve got other infrastructure that will be flooded you might want to consider that as a maximum headwater elevation. Also, creating extreme velocities is also undesirable. You probably want to keep the extreme velocities (calculated with CulvertPro) less than about 2.5 times the normal channel velocity.
Let me know how it goes in the comments section below.