Downstream wier intended to protect gravity sewer upstream - Experiences?

[oldestguy]

A 10" plastic gravity sewer was installed about 40 years ago and it then was 4 feet below stream bed at the crossing. Recent flooding conditions have exposed it so there is perhaps 3 feet of water depth beneath it and it is barely under the water surface now. One fix planned is construct a rip-rap weir downstream with the intent to cause sediment to deposit behind the weir and eventually cover the sewer. I am not too sure about this as working as intended. Currently the regulating agency seems to go with this idea, but no placement of riprap at the pipe itself allowed. This "fix" would supposedly restore the stream bed up to its earlier position. That probably would extend back upstream a few hundred yards. However, this location may be a gouged out hole. I have yet to see it. Sizing the weir and its materials is not the question, but will sediment gradually stay and keep cover at the pipe? Stream is about 20 to 30 feet wide and in a flood plain that enters still water a mile downstream, that is a lake. Grades are very flat and stream in slack times is barely moving. In flood times a guess of one to two ft./sec. velocity since this is at just upstream from a highway bridge. Experiences with this or similar please. Soil is silt and fine sand, occasional gravel.

 

[cvg]

you are describing a grade control structure which will raise and flatten the stream bed invert. but depending on your stream, it could take a long time to fill in. generally sewerlines are required to be fully encased concrete or steel casing below stream bed. this would never fly around here.

 

[gbam]

cvg nailed it as a grade control structure. and yes over time the bed would aggrade. I would think a more significant fix would be warranted since it has already scoured around the sewer pipe. One could use a two peice sleeve with skids/insulators to ecompass the sewer line and then encase it in concrete.

 

[LittleInch]

The problem is that the deposited material, by its very nature, will be uncompacted fine material and at every flood condition will tend to re-scour under the pipe. Whilst "plastic" is a bit vague it is probably better than a more solid material in resisting load, but sooner or later a bigger boulder will impact it. Why can't you make the weir over the pipe and solve both issues together??

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[gbam]

It would depend on the desired outcome for the design. Are you looking to fix a degrading system or a local repair? How far will this design/analysis go?

You could construct a sleeve as I mentioned above and then construct the control structure at the same location; but, if a drop is created there will be additional scour downstream and that should be mitigated as well.

 

[oldestguy]

Saw the job yesterday. Already a log jam is starting at the pipe. I had wrong dope on the pipe. It is either ductile Iron or steel, 8"ID. Earlier a contractor had proposed a barrier of reject concrete blocks 2 x 2 x 6' in a row upstream and one row downstream. State authority (22 year old new engineer) rejected that and suggested the weir. I still think some temporary barrier is needed. I don't trust an encasement since it is unlikely to get firm support permanently. However there is a chance to displace the mud under a rip-rap weir site and also some at the pipe by the placement method of rip rap. Still looking for actual experience with this idea. Thanks all. Doe to low sewage flows an inverted siphon won't work. Nearby bridge authority does not like pipes hung from their bridge. Grinder pump and force main very expensive alternative. Sewer serves only 6 homes.

 

[cvg]

modifying the invert of the river by constructing a grade control structure requires a bit of thought as you will be also modifying 1)hydraulics (both upstream and downstream, 2)floodplains 3)normal high water levels 4)downstream bed elevation and scour 5)environmental conditions

you may need to mitigate flooding or higher ponded water upstream due to your structure. alluvial deposits are not considered to be good protection for a pipeline, concrete or grouted riprap is much better. you may impact nearby properties when you raise the water level due to the weir. and it is entirely possible that you will have a significant environmental effect that may or may not be appreciated by the agencies that regulate

 

[bimr]

If this is correctly evaluated, you will probably determine that a 3-4-INCH force main and lift station is the most cost effective and timely.

You will need permits to fix the stream. As you noted, it has been eroding for years and will only get worse.

 

[oldestguy]

State agency ok's looking at a temporary barrier upstream now, at the elevation of top of pipe. The current situation catches floating trees. Barrier would be rip-rap "wind row" just upstream. The initial posted idea has been scrapped. Pipe is 8 inch ductile cast iron 37 feet unsupported, but under water, making it weigh about 16# per ft. Definition of temporary is something that may match what I recall when going to Cornell U after the big war. The temporary barracks type thrown up WWII buildings were still there well into the 50's only to be removed to make room for new structures. Maybe here that definition needs clarification. Village is looking at a grinder pump and force main, but needs money for that. So I suspect once in not much change for a while. Stream is a trout stream and somehow the trout guys are not involved.