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
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.