Pipe Culvert Design

A culvert is a hydraulically short conduit, which can be used to convey stream flow underground through a roadway embankment or other flow obstructions, or used as an outlet structure attached to a detention pond. Culverts come in circular and rectangular cross sections, and concrete, corrugated steel, aluminum and plastic materials.

The hydraulics of a culvert are complex since several flow control types may exist. The methods that Pipe Culvert Design uses are from FHWA Hydraulic Design Series No. 5 (HDS-5), Hydraulic Design of Highway Culverts. There are two flow controls: inlet control and outlet control. Under Inlet Control, the culvert's entrance characteristics determine it's capacity, and the culvert is capable of conveying a greater discharge than the inlet will accept. With Outlet Control, the inlet can accept more flow than the culvert can carry because of the head loss due to the friction along the barrel or the high tailwater elevation. Furthermore, because culverts are generally not long enough to achieve uniform flow, the flow profile inside is often gradually or rapidly varied flow. In Pipe Culvert Design Settings tab, you may specify the flow control type to design the culvert. If you choose the Optimum option, both inlet control and outlet control calculations are performed, along with the gradually varied flow analysis. The worst-performing control condition is then used to evaluate the proposed design, i.e. the greater of the inlet control headwater and the outlet control headwater is the controlling headwater. Please refer to HDS-5 for details.

Gradually Varied Flow occurs for free surface flow conditions. This method computes water surface depth curve along the length of the barrel and are developed in an upstream direction by standard step method. In standard step analysis, the reach is divided into a number of sub0reaches. Computations are performed in steps from one section to the next, where the depth is incremented or decremented to yield a correct energy balance in the basic equation.


HY-8 Method version 7.5 was introduced in Carlson 2018. HY-8 automates the design methods described in HDS-5 and in HEC-14. Version 7.5 is the latest version and is available on FHWA website.

One significant difference in HY-8 is inlet control analysis. Carlson computes inlet control headwater strictly by the inlet control questions presented in HDS-5. HY-8 uses polynomial curve fits for inlet control headwater computation. The culvert inlets must have HY-8 Inlet Polynomial Coefficients in order to run HY-8 calculation.

The outlet control utilizes HDS-5 equations, and also conducts more complex water surface analysis based on seven flow profile types.

Please refer to HY-8 User Manuals for more details.




From the Structure menu in the Hydrology Module, choose Pipe Culvert Design to display the design dialog. Click on Load button to load an existing culvert file to view or modify it, or a new file to start a fresh design. From the Solve For list, select the value that you want to calculate. The available values are: Discharge, Headwater and Size. You may choose Size/Discharge or Size/Headwater to compute the exact discharge or headwater values after the size has been solved.

Settings

Under the Settings tab, Control Type is used to calculate culvert flow as described above. Users can also set Slope display format in either ft/ft or percentage.

Culvert Cross Section Tab

 From the Shape list, select the type of culvert that you want to define. The available shapes are Circular and Box. From the Material list, select the material for the culvert such as Concrete or CMP/Aluminum. You can use the Library function next to Material to select the material and manning's n from the pipe material library and to edit or add to this library. If you don't solve for the culvert size, enter the values in the Diameter box or Height and Width boxes depending on the culvert shape. In the Manning's n box, the Manning's n value is usually automatically set after the material type is chosen, but you can also type a different value manually. In the Number of Barrels box, enter the Number of Barrels for the culvert.

The Culvert Inlet list contains the inlet types available for the current culvert shape. It updates with different culvert shapes and materials. All the inlet types are specified in HDS-5. Ke is the entrance loss coefficient, which is depending on the culvert shapes and inlet types. It'll update with different chosen culvert shape and inlet type, you can also type the value in the box. The Library button next to the Ke field opens the Culvert Inlet Library dialog and allows users to create new Culvert Inlet Types.
Culvert Cross Section

Culvert Profile Tab

Inlet Invert is the elevation of the bottom of the culvert at the upstream end, while Outlet Invert is the elevation of the bottom of the culvert at the downstream end. In the Length box, enter the true culvert pipe length. The culvert slope will be calculated after entering the above three values and displayed in the Slope box. You may also change the outlet invert by entering a slope for the culvert. The most efficient way to get the culvert parameters filled is to select the 3D polyline that represents the culvert. Pick button allows you to pick a 3D point that represents the Inlet/Outlet invert.

Culvert Profile

Tailwater Tab

There are six ways to calculate culvert tailwater: Constant Tailwater, Rectangular Channel, Triangular Channel, Trapezoidal Channel, Irregular Channel, and Tailwater Elev./Discharge Rating.

The Constant Tailwater method allows you to specify a single elevation for the culvert design regardless of the culvert discharge. This option is useful when the culvert is discharging into a large reservoir since the change in water surface elevation due to the culvert discharge is negligible.

The Rectangular/Triangular/Trapezoidal/Irregular Channel tailwater methods allow you to specify the channel downstream of the culvert. The culvert taiwater elevation will vary with discharge, and will be equal to the channel invert elevation plus the normal depth of flow in the channel for the current discharge. Channel Data will become available for input if chosen.

The Tailwater Rating method allows you to input a discharge/tailwater elevation table. Linear interpolation and extrapolation are used to calculate the tailwater for discharges not specified in the table.

Tailwater

Overtopping  Analysis Tab

When the culvert headwater begins to rise above the elevation of the roadway, overtopping will occur. Overtopping flow is modeled as a special type of weir flow expressed by the general broadcrested weir equation. Enter values for Crest Length, Crest Elevation, Weir Coefficient in the dialog and overtopping flow can be calculated for the current culvert headwater elevation.

Overtopping Analysis

Calculation

In the Discharge box, enter the flow rate in the culvert.  In the Headwater Elev box, enter the water surface elevation at the upstream end of the culvert. You can also enter the Headwater Depth then the program will add the depth value to the inlet invert to get the headwater elevation. If you are solving for either discharge or headwater, the corresponding box will be disabled. Section Size Library allows you to specify as many as available pipe sizes for solving for culvert size. After the hydraulic calculation, the smallest, large enough, available pipe size will be chosen. Please refer to the Pipe Size Library command for defining pipe sizes.

When the Nominal HW/D Ratio is greater than 0, the headwater is always set by the ratio and pipe size. The HW/D result is the ratio of the actual headwater depth and the selected pipe size.

Click on Solve button, depending on Solve For selection, the discharge, headwater elevation, culvert size and the tailwater elevation are calculated and displayed in the dialog correspondingly. The Outlet Velocity and Flow Depth are calculated and shown, and the Control Type is also illustrated.

Outputs

After culvert calculation, click on the Show HGL button to show the culvert water surface profile, normal flow level, critical flow level, as well as headwater elevation and tailwater elevation.

HGL Dialog

Click on the Stage-Discharge Result button to display the stage-discharge curve in the Stage-Discharge Result Dialog. From this dialog, you can view the stage-discharge curve, write the result to a stage-discharge file(.STG), and draw the graph into the CAD graphic. When you click on the Draw button, the Stage-Discharge Curve Settings dialog displays from where you can define how to plot the text and graph on screen.
Stage-Discharge Limits


Stage-Discharge Dialog

Click on Generate Report button, the program will present a report screen that contains detailed information regarding the design parameters and the calculations. The report window provides the options of printing, drawing the report in AutoCAD or storing the report to a file. Shown below is an example.
Culvert Report


When HY8 method is used, click on HY8 Report button to generate the HY8 PDF report that based on the HY8 PDF report template. Shown below is an example.





A rating table presents the discharge-headwater relationship in the tabular form. It can be displayed in a Microsoft spreadsheet or a standard report. Click on Generate Rating Table button to open Rating Limits dialog. In the Variant list, select the independent variable. The available variables are Discharge and Headwater. Enter data in the Minimum, Maximum and Increment boxes. Enter the Tailwater Elev and select the decimal setting from the Decimals list. When you finish entering data, click on OK button to calculate the rating table. A rating table example is shown below in the standard report format. The first column Discharge is an independent variable, and the other columns are computed variables.
Culvert Rating Table Limits


Culvert Rating Table in Report Format

Draw Pipe Detail button plots a fully annotated standard detail, with user-controlled inlet and outlet slope entries and scaling.
Draw Pipe Detail Settings

Pulldown Menu Location: Structure
Keyboard Command: culvert
Prerequisite: a culvert file

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