Maximum slope on outfall pipe

[Scooter3001]

I am working on a project to separate stormwater out of a combined system. The combined system is a 48" pipe that goes into a wooded right of way into a control structure for a CSO outfall. When not raining, the sanitary flow continues into a a smaller diameter pipe. When it rains, it overflows. The new stormwater pipe will be placed along side the existing CSO pipe and terminate prior to the CSO structure at an existing, but failing, outfall.

The new stormwater pipe will terminate at a new junction manhole in an existing outfall from a wetland area. The existing outfall is in very rough shape and will be replaced. Currently, the outfall drops 20 feet over 115 foot run for a slope of 17%. After modeling the proposed stormwater pipe, the velocity for a 25 year storm at the outfall is 25 fps. A plunge pool will be designed and channel lining downstream will be included. Also, as the attachment shows, the slope near the existing outfall will be rebuilt as it is suffering from erosion.

If this same slope is kept, what type of precautions should be taken to keep the pipe sections together, particularly at the outlet? Should I consider a not as steep outfall by designing a drop structure for the new junction manhole to try and reduce the velocity?

 

[rconner]

fyi

http://www.eng-tips.com/viewthread.cfm?qid=280847

(if you haven't seen this history)

 

[bimr]

One would not think that keeping the same slope is an option.

 

[Scooter3001]

Thanks rconner, I will check it out.

 

[rconner]

This is a good one. Sounds like it might be helpful to slow down the flow velocity, or to brake the water somehow to limit erosion. While there may be more than one way to do so, I know folks out West e.g. in the Rocky Mountain areas have to deal, perhaps more regularly than many others, with many such situations. You may thus be interested in e.g. the experience/approaches of large entities in that area e.g. reflected at

http://your.kingcounty.gov/dnrp/lib...er 4 Conveyance System Analysis & Design.pdf.

[It appears King County suggests the use of lesser pipe profile slope but with periodic drop manholes to reduce the flow velocity to no more than 10 fps, regardless of pipe material selected.]
As to what might be done to keep pipe joints from coming apart, many entities also employ restrained joint pipes and even transverse wall/step anchorage behind bells when slopes get very steep. I happened to notice while the outfall line itself is said to be at more than a 17% slope, by contour lines the ground between where the pipes reportedly separated and the outfall structure itself appears a WHOLE lot steeper (maybe even >50% slope!) If there is thus any chance the whole backfilled trench/w/ embedded piping there is as a mass in effect also sliding/moving towards the lower elevation outfall structure, anything you can due in the way of anchorage/structures that tend to stabilize same maybe wouldn't hurt. If you decide to put some concrete wall anchors behind bells in that lower area, make sure the piping is strong enough to take the local loading imposed.

 

[cvg]

slope anchors are some times recommended for slopes of 20% or greater. see typical details below


ftp://ftp.odot.state.or.us/techserv/roadway/std_archive/2012_12_01-2013_05_31/RD_Drawings/rd332.pdf

 

[Scooter3001]

Interesting enough, if you look at the attachment, a drop manhole towards the wetland area where massive erosion has already created a 10' hole from where the existing outfall pipe has separated may be utilized. A drop from the existing hole may only need to be 10' or less, with an equivalent height to the top of the new gravel access road to be installed. This will allow for space to install the drop manhole away from the sanitary sewer and reduce the amount of earthwork for the structure install. The long section from the road will be under 10 fps and if I get the structure right, the flow into the outfall and the pipe slope will be reduced. The depth of the drop structure outlet and the existing invert of the 48" rcp will have be balanced to minimize the load of the sanitary sewer during construction.

I will check out the link for King County. It is interesting to note that while the 10 fps max velocity is prevalent, I have found many municipalities that have upwards of 20 fps allowed. If the slope remains steep, anchors will be used as suggested.

Thanks again.

 

[beej67]

Can you end your 15% run about 20 feet before your discharge headwall, and put in another structure with a drop? You could use that structure as a kind of stilling basin and then run a 1% slope pipe for the final 20 feet or so, to keep discharge velocities down.

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

http://www.campbellcivil.com

 

[Scooter3001]

Hi Beej,

I thought about that but I am attempting to not have put a structure on the downsteam slope due to the surrounding slopes. Currently, I am working out the following:

There is another outfall about 60 feet away that is intact and has a less severe slope (16%). I will be routing the new stormwater pipe and the wetland outlet roughly 60 feet to this other outlet where a drop structure will be placed. This new outfall will have a series of plungepools/rip rap protection for the remaining drop to the final grade.