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HydroCAD Stormwater Modeling - Since 1986

Pipe FAQ
Storm Sewers Arch Pipes

Modeling Pipes & Culverts

What types of pipes can I model?

bulletBox and circular pipes can be modeled in all versions of HydroCAD.
bulletElliptical and arch pipes were added in HydroCAD 9.0, and can be approximated in earlier versions.  Details here.
bulletPartially-filled pipes were added in HydroCAD-9.1.
bulletHydroCAD-10 provides a built-in lookup table for standard sizes of round, elliptical, and arch pipes.
bulletHydroCAD-10 build 8 adds true-ellipse pipes, ellipse pipes with a vertical major axis, and arched conduits (box with a curved roof)

All these shapes are available for pond storage, culvert outlet, pipe reach, or Tc flow segment, as discussed below.

Arch Pipe

What's the best way to model a pipe?

There are several ways to model a pipe, depending on the effects of the pipe on your drainage network, and the objectives of your analysis:

1) If the pipe always operates under normal open channel flow (Manning's flow), you can model it as a separate pipe reach.  However, this approach does not evaluate inlet conditions, which often control the flow.  A pipe reach is useful for long pipes with significant storage volume, which can produce significant attenuation of the peak flow.  A short pipe reach will often have no effect on the inflow hydrograph, so there may be little benefit to including it in your model.

2) Another option for pipes operating under open-channel conditions is to model them as a flow segment within a subcatchment.  This allows for the pipe's attenuation effects by including it in the subcatchment's time-of-concentration.

3) The most complete solution is to model the pipe as a culvert outlet on a pond, even if the "pond" is simply a catch basin or roadway impoundment.  A culvert outlet provides a more complete analysis of flow conditions, including inlet losses, headwater, and tailwater.  The culvert analysis can also accommodate a level pipe or even an adverse slope.

4) If the discharge is limited by a separate outlet control device, such as an orifice, the overall structure should generally be modeled as a pond, using the "pipe storage" option for the pond storage.  The outlet configuration would include the orifice (since it controls the flow) but not the pipe, since it's being modeled as storage and doesn't limit the outflow.

5) A pipe connecting two ponds can be modeled as a culvert outlet on the upper pond.  This situation also requires a tailwater-sensitive routing procedure.  But if the pipe is sufficient to equalize the ponds at all times, a more stable routing will be obtained by modeling both storage volumes as a single pond, including the pipe as part of the pond storage as described in scenario number 4, above.

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