Corrugated Steel Pipe are usually the cheapest, most effective way to convey water underneath a roadway. In this article, I will outline the steps to designing a corrugated steel culvert.
I will be using our in-house design software, CulvertPro. If you choose to use other software to do the calculations (or do them by hand) the mechanics will be different but the steps should generally be the same.
STEP 1: Hydrology. The output of this step is a design flow. If you have hydrology software, this is the place to use it. If you don’t, CulvertPro has some simple calculators, but they will not be as sophisticated as most hydrology software.
Navigate to the Rational Method Calculator and enter the following inputs:
- Runoff Coefficient (C): This coefficient takes into account all of the hydrological abstractions, soil types and antecedent conditions. Click on C-Table in the Information Panel to get a table listing various values for this coefficient. Also, use the Hybrid Runoff Coefficient Calculator if you need
- Rainfall Intensity (i): The intensity of the rainfall assumed to occur over the entire drainage basin for at least the time of concentration of the watershed. The Time of Concentration is defined as the time it takes for the last drop of water in the drainage basin to reach the site, and the handy Time of Concentration Calculator in the Information Panel contains the three most used methods.
- Drainage Area (A): The size of the drainage basin.
The calculator performs a rational method analysis and produces a flow rate in m3/s or ft3/s. The Rational method is best suited for small drainage areas because it assumes a uniform rainfall over the entire area. Larger than about 15 mi2, the results will be higher than you’d expect.
Because we want to use two different methods (for comparison), navigate to the SCS Method Calculator, and enter the following inputs:
- Drainage Area: The size of the drainage basin.
- Curve Number: Look this one up on the CN Table in the Information Panel.
- Time of Concentration: Same as for the Rational Method. Use the Time of Concentration Calculator in the Information Panel if you need to.
- Rainfall Intensity: Together with the rainfall duration, this defines the design storm. The two variables are usually obtained from Intensity-Duration-Frequency curves for the area, which are usually produced by the relevant government agency. The designer chooses a return period to design for (such as a 1:50 year storm) and the axes represent each of the values, intensity and duration.
- Rainfall Duration: Together with the rainfall intensity, this defines the design storm. See above.
The SCS Method Calculator produces a flow rate in m3/s or ft3/s, and the designer must choose a value after considering these results.
STEP 2: Channel Hydraulics. The output of this step is a channel flow depth, which in the next step will be called “Tailwater.” Navigate to the Tailwater Calculator and enter the following inputs:
- Flow: The design flow, chosen from the Rational or SCS Method Calculator (or other hydrology software).
- Channel Slope: Find the average channel slope in the reach of the culvert. By definition, you will need to have some type of channel survey to get an accurate channel slope. If you don’t, you can estimate it but this has a decent impact on the overall results. In the open prairies, you’re likely to see less than 0.005 ft/ft. In hilly terrain, 0.005 to 0.020 ft/ft, and above 0.02 ft/ft is usually only found in steep, hilly terrain or mountains.
- Manning’s n: The roughness coefficient of the stream. Use the handy Manning’s n table in the Information Panel to help you.
- Channel Shape and cross-section properties: This is where you tell CulvertPro the channel geometry. Make it as average as possible, and don’t use cross-section properties for an area that has been influenced by man made features (like an existing culvert).
The Tailwater Calculator produces a channel flow depth.
STEP 3: Culvert Hydraulics and Culvert Sizing. Armed with your design flow and channel flow depth, we proceed to the meat and potatoes of the culvert design. The previously calculated channel flow depth is also called the “tailwater,” as it is assumed at the downstream end of the culvert. Navigate to the Culvert Hydraulics Calculator and enter the following inputs:
- Culvert Shape: Self Explanatory. Either Round, Box, Horizontally Ellipsed, or Pipe Arch.
- Culvert Length: You can assume this one, as it has very little impact, or you can use the Culvert Length Calculator (the 4th Calculator from the left).
- Flow: The design flow, calculated in step 1.
- Tailwater Depth: The tailwater calculated in step 2.
- Upstream Invert Elevation: The elevation of the upstream invert of the culvert.
- Downstream Invert Elevation: The elevation of the downstream invert of the culvert.
- Culvert Burial: The depth that the inverts will be “buried” below the streambed.
- Manning’s n of Culvert: Use the handy table in the Information Panel to find out the roughness coefficient of the culvert type you are using.
- Entrance Loss Coefficient: Use the handy table in the Information Panel to find out the Entrance Loss Coefficient for the culvert type you are using.
- End Section: Either conforming to the slope (bevelled), projecting from the slope (straight end), or headwall (flat vertical surface around culvert opening).
The culvert hydraulics calculator produces a headwater elevation and inlet/outlet extreme velocities, which can be compared against design criteria for your area.
STEP 4: Structural Design. CulvertPro does not have a structural design calculator as of this writing. Most jurisdictions have design tables which specify the culvert thickness given its size and height of fill above it. You can also see my overview of structural design of CSP culvert using the AASHTO LRFD Bridge Design Specifications here.
These steps are covered in more detail in the CulvertPro Manual. Good Luck!