Sagged sewer sections

Good Morning SWMM Users. My review of a CCTV video inspection has
identified a section of sewer between two nodes that is sagged such that
a section of pipe flows full under "normal" conditions. In your
experience, would representing this by introducing two additional nodes
at each terminus of the sagged section and fitting those nodes with
bolt-down lids at the pipe obvert elevation closley represent the
hydraulics of the system? Are there any other suggestions how to
represent such a sag? Thanks very much.

Chris Kresin
chris@KRESINENGINEERING.CA

No problem, EXTRAN doesn't mind modeling the saggy parts of a sewer
reach. Do your best to match the sags with additional nodes & links.

I'm not sure about your "bolt-down lid" option. If the sewer is pretty
deep you can just set the node ground elevation to the actual ground
elev and let EXTRAN handle the pressure flow. If it's not very deep and
surface flooding is expected, you might need to represent the surface
overflow and storage properties.

Michael A. Gregory, P.E.
GregoryMA@CDM.COM

Chris - I would do as you mention and add one more node at the point of
max sag. The bolt-down option prevents flood losses through the
manholes; it will also cause higher pressure locally and in the
surrounding part of the system. So run it first without the "bolt-down"
option and see if you are getting flooding. If not, the HGL is good and
the "bolt down" option is inoperative. Note that these program "bolts"
only stop water. They do not stop air. If the real bolts and gaskets
stop air leakage, you might get higher internal semi-transient pressures
than the program indicates.

Ron Kilmartin
ronkilmartin@attbi.com