Stormwater Piped Down Steep Slopes

[cher122976]

I have a 48" Stormline flowing 75% full at 5% that approaches the top of a 30' 1:1 slope. What is the best way to pipe down something like this taking velocities and energy dissipation into account.

The current solution is to make the 84" manhole at the top of slope 25' deep with a 48" outside drop for the pipe coming in. This will create a pipe with a slope of approx 5% to the manhole at bottom of slope.

Any suggestion on a good energy dissipation device in a pipe system?

 

[gbam]

A couple of options come to mind:

you can use a system of drop manholes to minimize the pipe slope and also as energy dissipation.

you can slope the pipe but then you need to anchor the pipe into the slope to mitigate sliding and consider the velocity/scour within the steep section. At the outlet you can design an impact basin for headloss.

 

[cvg]

concrete lined spillway with an energy dissipater at the bottom.

 

[MadeHand]

+1 for multiple manholes with drops. I wouldn't use drop connections (external or internal) unless specifically required by review authority.

The drop will allow for the velocity reduction and reduce your dissipator at the outfall.

 

[beej67]

cvg's idea is the safest.

Hydrology, Drainage Analysis, Flood Studies, and Complex Stormwater Litigation for Atlanta and the South East -

http://www.campbellcivil.com

 

[3feethighandrising]

US Bureau of Reclamation Type VI Dissipator- been around awhile and they are still using it. Good for up to 50 fps and 400 cfs.

I bet a Type VI structure would end up costing less than a 25' deep 7' dia. MH because of the cost of the very deep excavation through the slope for the pipe to get 5% grade. The Type VI would not need shoring to constuct, and the maintenance on the Type VI will be much easier since it is above ground.

If you end up with more than 20 fps on the pipe at capacity using a Type VI, I recommend HDPE with proper concrete anchor collars at intervals for movement. See the Design of Small Canal Structures, USBR, 1974 for information on Type VI.

You could also do a Type 1 Pipe drop, which would put the energy dissipator (hydraulic jump) underground in the pipe at the bottom of the hill. This would allow a relatively shallow trench depth for the pipe to the bottom of the hill without a deep Manhole.