HydroCAD Stormwater Modeling - Since 1986

Automatic Pipe & Culvert Sizing

HydroCAD-7 provides a comprehensive set of tools for automatic pipe and culvert sizing.  These options are located on the Resize tab of the Calculation Settings screen.  (This tab is available only when the Storage-Indication method is selected for routing.)

bulletFor a pipe reach, the sizing operation normally ensures that the maximum depth of flow within the pipe is slightly less than the pipe diameter. This causes the pipe to be sized for pipe-full Manning's flow, assuming that the pipe entrance doesn't restrict the flow.
bulletFor a culvert outlet, the sizing operation normally ensures that the maximum culvert headwater does not exceed the culvert crown. (If the culvert has a reverse slope, the outlet crown is used instead of the inlet.)

Alternatively, pipes and culverts may be sized so that the user-defined Flood Elevation is not exceeded. This allows the water level to be maintained below the crown, or for a culvert to be sized for a specific headwater. (If the flood elevation is not defined for a particular pond, the culvert crown will always be used as the maximum.)

Other options are available on the Resize tab, and are fully described in the associated help page.


Background Information

HydroCAD-5 included a capability for automatic sizing of pipe reaches.  This feature sets the pipe diameter according to Manning's equation, so that the pipe will flow full under the calculated peak inflow.  When using this feature, the following factors must be kept in mind:

A pipe reach assumes normal Manning's, and doesn't consider potential inlet control or tailwater factors.  In general, reaches are implemented in HydroCAD for the purpose of modeling long sections of open-channel flow governed by Manning's equation.  A pipe reach is not designed to model pipes that may operate under inlet or outlet control, or with a headwater.  In general, these situations are better modeled as a pond with a culvert outlet.

In order not to encourage inappropriate use of pipe reaches, the automatic sizing feature was removed in HydroCAD-6.  It is our experience that many situations that have been modeled as a pipe reach would be better handled by using a pond with a culvert outlet.

With the release of HydroCAD-7, a complete set of pipe and culvert sizing tools has been added as described at the top of this page.  These tools allow for automatic resizing of culverts, which are generally more appropriate than the use of a pipe reach.

For additional details read about pipes and storm sewers.

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