Steep Storm Drain 2

[Sowega]

I've got a 20 foot drop from an existing 42" oulet pipe that needs to drain to a creek 78 feet away. I was initially thinking about a concrete lined open channel with step-downs to slow the velocity. I believe an open channel would cause a liability for the client. (That could be a heck of a water slide.) I think drop structures would work but I'm still going to have steep slopes on the pipe or major drops in the structures. I read some of the threads on this but I'm still not sure if a structure could handle a 10' drop over time. Any other ideas?

 

[beej67]

Gabions?

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

http://www.campbellcivil.com

 

[Steeleguy]

We can fabricate a drop structure to your specifications. This option drops water about 15' and has been inplace for 10 years. Antiseep diaphrams are installed along with headwall on the inlet end.

see attachment

 

[gbam]

We have spec'd that configuration a few times in the past. However, we used HDPE rather than metal. The only issue we had was joints separating and erosion of the slope as a result. If this configuration is used double check outlet velocities as you may encounter high velocities requiring more than a riprap plunge at the outlet. We used a concrete impact basin in our latest design.

 

[Steeleguy]

Strengths of Corrugated Steel Pipe (CSP)are positive joint connections and long lengths. Both were used to minimize the joint separation and erosion concerns. 2 connections per run were designed for the 8' diameter double drop structure. It was designed with a concrete toe and head wall system, and large riprap in plunge pool.

 

[cvg]

weaknesses of CSP are corrosion and erosion. With these high velocities and a little bit of sand in the water, you may erode the invert of the pipe. HDPE might handle the erosive forces better than either metal or concrete. Joint connections and pipe lengths are essentially the same for either CSP or HDPE. A headwall and energy dissipater is recommended regardless of which method is used. A concrete baffled chute could also be used if you want to avoid pipe and skateboarders.

 

[Steeleguy]

Aluminized Type II and polymeric coatings are available for the corrosive soils if any(clay). The thickness of the steel can also be increased if a heavy bedload is encountered. It is not uncommon to install 50' to 80' lenghts of (single pipe) in these application to reduce joint separation.

See attachment: 8' diameter - 80' long

Compaction is always important for all pipe installations.

 

[gbam]

cvg mentioned a great alternative that we have used as well, A reinforce concrete baffled chute. Of course, your final design may depend on cost.

 

[Sowega]

Thanks everyone. @gbam Do you happen to have an image or detail of your concrete impact basin?

 

[msquared48]

I have also used a system of catch basins, or similar structures, where the outlet is higher by several feet than the inlet, letting the energy gradually dissipate as the water moves through the system.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[CarlB]

For impact basin. search for energy dissipators and stilling basins

Design procedure and figures (HEC 14) at

http://www.fhwa.dot.gov/engineering/hydraulics/pubs/06086/

 

[gbam]

Here is a photo of one for a box culvert. The design was based on HEC-14.

 

[Ryb01]

As you had mentioned, I believe there has been some previous threads that may also provide some helpful insight and guidance regarding storm sewer design on steep slopes.

A couple additional things to consider, to expand on what the other forum members have suggested, is aquiring the expertise of a geotechnical engineer for the overall slope stability, pipe installation and possibly using pipe anchors and ensuring the outlet(if using a engergy dissipator) will not overturn to name a few.

Also, contatct a contractor in your area, that you trust or vaule their opinion that has done similar types of work on slopes. Can you actually construct what you are proposing? What are the complications? How do you construct the project? etc

Hope this helps.

 

[SMIAH]

I guess that you don't have any concerns about fish passage here? heh