Submerged Storm Outlet

[CGAL]

I am working on a project that involves taping into an existing storm sewer that has a submerged outlet. In low water conditions, the outfall is only about a meter submerged. However, in flood conditions, the water level can raise 4 meters. What will this do to the flows coming down the line? As the flow continues the build up will the difference in head cause the water level to equalize or will the line continue to back up? What does this do to my flow and size of pipe?

 

[msquared48]

You will have to run the numbers, but generally, as the tailwater height increases, the outflow will decrease.

Also depends on if the head at the upstream end increases to match the tailwater increase too. If that happens, there will be no change due to the tailwater increase alone.

If the upstream head is greater than the tailwater increase, then the flow should increase.

Look at the total head in the system.

Mike McCann
MMC Engineering

 

[cvg]

under high flow conditions, this could raise the HGL in your pipe by nearly 4 meters in the lower part of your drain. that could cause backups through nearby catch basins or pop off manhole lids if they are not above this elevation. the effect on the flow in your storm drain is dependant on the geometry of your system.

 

[Ryb01]

Somethig to consider...... based on the maximum tailwater achieved(i.e 4m), if the catchbasins and mahnoles located at the downstream portion of you system surcharge, where will the water go? Is there basement connections to the existing storm sewer that will be affected by this tailwater condition?

A backwater analysis may be a good option to look at in this case. SWMM may be a good tool for this.