Modeling Pipes & Culverts
What types of pipes can I model?
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What's the best way to model a pipe?
There are four basic 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.
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.