Velocities in Extran

Dear Users,

We have a situation where a steep channel goes into a shallower channel.
The flow goes from super critical to sub-critical. It seems that Extran
output files indicate the peak velocity at the downstream junction of
the steep channel, therefore the reported velocities are slower than
what might be expected in the upper sections of the channel. To
determine the peak velocities in the upper section, we put in a
fictitious junction at the mid point of the steep channel. This did
result in approximately 30% higher velocities.

I'm interested in finding out if there is any recommendations on how we
could determine the actual peak velocity in a channel. I'm currently
running SWMM 4.4h.

Thanks for your help.
Dilan Singaraja

Dilan - the reported channel velocity in the output is the "weighted"
average of the computed velocity at the upstream end and the downstream
end (users manual, eq. 5.4 et seq. I assume the "weights" are 1.0.

The up and downstream end velocities are not in the output. You can
estimate them using the depth at the junctions and a depth-area curve,
along with the discharge; They are based on Q(t) at the beginning of the
time interval and Q(t+delt) at the end of the interval. These calcs can
be done i a spreadsheet.

So you have to call for junction depth in the output and also the
discharge and average velocity listing (versus time).

Supercritical flow results in EXTRAN can be questionable as the Frounde
Number gets above 1.3 to 1.5. You may wish to check the instantaneous
specific energy at the upstream end of the conduit and compare that to
the available energy as indicated by the HGL. However, from your
description this may not be a problem in your case.

Good luck
Ron Kilmartin
ronkilmartin@attbi.com