Particular Issues for Storm Pipe, 100' drop/1300 ft

[HC1975]

Project where a detention pond is approximatley 1300 feet horizontally and 100' vertically from the outfall. Our current concept is to use a series of drop manholes throughout the run to reach the outfall elevation (see attachment)

Some of the manholes will be 15' deep with the inflow and outflow inverts being 6' apart vertically. The majority of the pipe will be 36" RCP.

We are trying to keep the velocity within the guidelines of the RCP pipe specs.

Are there any other factors we should be taking into consideration other than velocity for a vertical drop that large?

Will the turbulence within the manhole be an issue that needs addressing?

Most of our work is in Florida where we never see these types of challenges but have taken a project in NY, so we just want to be safe by asking.

Any insight is appreciated.

-HC1975

 

[CarlB]

Issue of steep slopes was discussed in past few weeks on Eng-Tips, try a search. Offhand I don't see any major concern with slopes or drops that would be harmful to pipe or manhole.

Wow, 11 manholes to go 1300 feet, are these all top inlets or for laterals, if not use more spacing, usually 300' or even 400' is allowable.

 

[jgailla]

On storm inlets with large drops, a sump can be used.
The sump will hold water, mitigating the erosion effect of water discharging directly onto the concrete manhole bottom.

 

[HC1975]

Thanks everyone. So as a review here are the suggestions I received/gleaned:

•Use oversized manholes and have the inflow pipe be offset from center to allow the stormwater to swirl the inside of the structure

•Size your pipe so that it flows full thus dissipating the greatest amount of energy

•use a material with a high manning's "n" value, even use a corrugated pipe to dissipate energy

•use a manhole with a deep sump so that the inflow water hits sitting water as opposed to the structure

•use a manhole with deep sump and have the inflow invert lower than the outflow invert, thus the inflow pipe will be submerged

•check to verify you do not create a hydraulic jump anywhere in the system

•robust energy dissipation at the end of the system

•can any portion of the flow be open channel or a series of pools?

•look into anchoring the pipe at the steepest sections.

 

[RWF7437]

Add one to your gleanings. Get licensed in New York.

 

[puzzler77]

I would target a maximum velocity of 9 ft/s to prevent excessive turbulence in the downstream MH's. The cross sectional area of a 36" RCP is 7.069 SF, Q=vA=63.6 CFS. Derived from Mannings formula, the c value (c=(1.486/n)AR^2/3)for 36" RCP is 666 for n=0.013 so the slope that gives this flow rate (S=(Q/c)^2) is 0.91%. At this slope, nearly 12 ft of fall is obtained over the 1300 ft leaving 18 ft of fall at the MH's. At 6 ft drop per MH, you would need three drops. 1300 ft/3=433.33 ft, an acceptable distance for this size pipe. Of course if the flow will be considerably less than 64 CFS, a smaller pipe should be evaluated. Hope this helps!

 

[chicopee]

Isn't there a chance of water scouring the inners of the manholes as it cascades between manholes? Are deflector shields,if they are made, advisable?

 

[cvg]

deflector shields are a good way to catch debris and plug the manhole.