Downstream wier intended to protect gravity sewer upstream - Experiences?
Downstream wier intended to protect gravity sewer upstream - Experiences?
(OP)
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.





RE: Downstream wier intended to protect gravity sewer upstream - Experiences?
RE: Downstream wier intended to protect gravity sewer upstream - Experiences?
RE: Downstream wier intended to protect gravity sewer upstream - Experiences?
My motto: Learn something new every day
Also: There's usually a good reason why everyone does it that way
RE: Downstream wier intended to protect gravity sewer upstream - Experiences?
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.
RE: Downstream wier intended to protect gravity sewer upstream - Experiences?
RE: Downstream wier intended to protect gravity sewer upstream - Experiences?
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
RE: Downstream wier intended to protect gravity sewer upstream - Experiences?
You will need permits to fix the stream. As you noted, it has been eroding for years and will only get worse.
RE: Downstream wier intended to protect gravity sewer upstream - Experiences?