When using either the Rational Method or Modified Rational Method, Bentley StormCAD has an option to ignore the time of flow through long lengths of carrier pipes (i.e. pipes with no contributing sub-catchment) when calculating the system time / time of concentration. This allows modelers to eliminate the flow reductions that would otherwise occur as a result of the increased system time.

The following diagrams show different configurations of carrier and non carrier pipes in a simple network, and their effect on flow.

The diagram above shows how the system time or time of concentration (ToC) changes through a network when there are no carrier pipes. At a minimum the ToC increases by the pipe travel time at each node in the network (it may increase by more if a downstream catchment has a larger time of concentration).

At the junction of branch 2 with branch 1, the longer of the two ToC's is used for the downstream pipe. This is one of the basic principles of Rational Method hydraulics.

In the diagram above the time in pipe of the carrier pipe is ignored when calculating the flow for the following pipe, so the running ToC for the downstream node of the carrier pipe is the same as the upstream node.

The next pipe downstream is not a carrier pipe. The ToC at its downstream node therefore includes the previously ignored travel time in the carrier pipe.

In the diagram above the ToC does not increase in the consecutive carrier pipes.

In the diagram above the ToC does not increase in the consecutive carrier pipes. At the junction branch 1 has the highest carrier ToC (10.2 minutes), but branch 2 has the highest running ToC (10.9 minutes). Therefore the carrier pipes downstream of the junction use the carrier ToC from branch 1 (10.2 minutes).

For the first non carrier and subsequent pipes (including any subsequent carrier pipes) the ToC is taken from the running ToC of branch 2. This is calculated as 10.9 + 0.6 + 0.8 + 0.3 = 12.6 minutes.

Note: the above diagrams are intended to explain the effect of carrier pipes on the node ToC's so subcatchments areas and the resulting flows have not been shown.

Summary

The rules for system time (time of concentration) in carrier and non-carrier pipe cases are when the 'Ignore Travel Time in Carrier Pipes' calculation option is set to TRUE are:

• When calculating the running ToC for the nodes, the pipe travel time is ignored for carrier pipes.
• The running ToC for the downstream node of the carrier pipe is the same as the upstream node.
• If the downstream pipe from a carrier pipe is not a carrier pipe, the running ToC will include the `time in pipe' from the previous carrier pipe(s).
• If there are several incoming pipes, and none of them are carrier pipes, then the highest incoming ToC is used as the ToC for the outgoing pipe.
• If there are several incoming pipes and all of them are carrier pipes, and the outgoing pipe is a carrier, then the highest carrier pipe ToC is used as the ToC for the outgoing pipe.
• If there are several incoming pipes and they are a mixture of carrier and non carrier pipes, and the outgoing pipe is a carrier pipe, then the highest running ToC is used for the ToC for the outgoing pipe.