Maximum slope on outfall pipe
Maximum slope on outfall pipe
(OP)
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?
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?





RE: Maximum slope on outfall pipe
RE: Maximum slope on outfall pipe
RE: Maximum slope on outfall pipe
RE: Maximum slope on outfall pipe
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.
RE: Maximum slope on outfall pipe
ftp://ftp.odot.state.or.us/techserv/roadway/std_ar...
RE: Maximum slope on outfall pipe
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.
RE: Maximum slope on outfall pipe
Hydrology, Drainage Analysis, Flood Studies, and Complex Stormwater Litigation for Atlanta and the South East - http://www.campbellcivil.com
RE: Maximum slope on outfall pipe
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.