Flat sewer regarded to steep sewer and upsized
Flat sewer regarded to steep sewer and upsized
(OP)
Hi All,
Please can you comment on the following design solution-
A small village serviced by an exisiting 225mm sewer which drains to a wet well of the inlet of the treatment works. The final 600m section of the main is laid very flat~ 1 in 450 slope. Multiple incident of sewage flooding occured in some private properties due to backing up of sewage. The properties are located 600m upstream of the well. These properties drains to the flat section of the sewer.
Based on spreadsheet calculations and surveys (CCTV, Manhole, Flow monitor), it was confirmed that the existing 225mm mm sewer at the downstream section of flooding properties was undersized resulting in surcharging during storm condition. As a result the sewage backs up from the main sewer into the existing 150mm lateral sewer serving the properties and causing flooding. It was concluded that the main sewer was laid at a slack gradient ~1 in 450; as a result the sewer was susceptible to frequent deposition of debris and grease in dry condition which exacerbated the flooding situation. The existing sewer was laid relatively flat and hence did not comply with engineering standards requirements of 0.75m/s self cleaning velocity.
Due to the hydraulic incapacity problems of the existing system and because of operational problems due blockages in the existing sewers, it was recommended re-grading 600 m length of the main sewer and upsizing from 225mm to 300mm up to the wet well.
The sewer at the wet well was lower 1.72 meters, provided just 0.5m above the pump start level. There is a overflow in the wet well at a high level. In its original condition the exisiting sewer from the village is slightly higher than the strom overflow. WIth the change in the design the new sewer is about 1.75m below the overflwo pipe.
My concern is why the exisiting sewer was laid at such a flat gradient-I can not say that they were lazy! must be reasons behind this.
By regrading the sewer and upsizing the sewer, we are inviting more flow and at a faster rate. During storm conditions when the inflow is greater than the pumped flow, there will be more spill from the overflow unless adequate storage has been provided in the 300mm sewer.
Members, please advice in anyway can be justify the above design. Any thoughts about the above design.
Also, can anyone advice any spreadsheet to calculate storage volume of sewer considering the gradient of the sewer.
Regard
Please can you comment on the following design solution-
A small village serviced by an exisiting 225mm sewer which drains to a wet well of the inlet of the treatment works. The final 600m section of the main is laid very flat~ 1 in 450 slope. Multiple incident of sewage flooding occured in some private properties due to backing up of sewage. The properties are located 600m upstream of the well. These properties drains to the flat section of the sewer.
Based on spreadsheet calculations and surveys (CCTV, Manhole, Flow monitor), it was confirmed that the existing 225mm mm sewer at the downstream section of flooding properties was undersized resulting in surcharging during storm condition. As a result the sewage backs up from the main sewer into the existing 150mm lateral sewer serving the properties and causing flooding. It was concluded that the main sewer was laid at a slack gradient ~1 in 450; as a result the sewer was susceptible to frequent deposition of debris and grease in dry condition which exacerbated the flooding situation. The existing sewer was laid relatively flat and hence did not comply with engineering standards requirements of 0.75m/s self cleaning velocity.
Due to the hydraulic incapacity problems of the existing system and because of operational problems due blockages in the existing sewers, it was recommended re-grading 600 m length of the main sewer and upsizing from 225mm to 300mm up to the wet well.
The sewer at the wet well was lower 1.72 meters, provided just 0.5m above the pump start level. There is a overflow in the wet well at a high level. In its original condition the exisiting sewer from the village is slightly higher than the strom overflow. WIth the change in the design the new sewer is about 1.75m below the overflwo pipe.
My concern is why the exisiting sewer was laid at such a flat gradient-I can not say that they were lazy! must be reasons behind this.
By regrading the sewer and upsizing the sewer, we are inviting more flow and at a faster rate. During storm conditions when the inflow is greater than the pumped flow, there will be more spill from the overflow unless adequate storage has been provided in the 300mm sewer.
Members, please advice in anyway can be justify the above design. Any thoughts about the above design.
Also, can anyone advice any spreadsheet to calculate storage volume of sewer considering the gradient of the sewer.
Regard





RE: Flat sewer regarded to steep sewer and upsized
Hydrology, Drainage Analysis, Flood Studies, and Complex Stormwater Litigation for Atlanta and the South East - http://www.campbellcivil.com
RE: Flat sewer regarded to steep sewer and upsized
RE: Flat sewer regarded to steep sewer and upsized
Your problem now is the pump station. The pump station must be sized to handle maximum inflows. The wet well is provided to hold sufficient influent during normal flows so that the pump(s) do not rapidly cycle on and off. Neither the wet well or the upstream sewer pipe should be used as a method for storing storm flows until the pump can "catch up" with the heavy flows. Sewage that just sits in a pipe for even a short time will deposit solids that normal flows may not scour clean. Hence, more maintenance work.
Your wet well and sewer pipes should be used to transport waste water to the plant - not act as storage detention basins.
Your task is to increase the pump station capacity so it can handle storm inflows. This will require a top to bottom analysis. Maybe you need two pumps with second pump set to turn on when water in the wet well reaches a higher level than the first pump can handle. Then you must look to see if the pipe from the discharge side of the pumps can handle the extra flows. Then a hundred other details must be checked, like can the existing electrical capacity to run two pumps?
It sounds like you might benefit from a source control study of an old sewer system. Why are you getting such high inflows during storms? Manholes that leak? Cracked sewer pipe? Or residents who connect their outside drains into the sanitary sewer. A thorough CCTV of your upstream sewer collection system could reveal a lot of problems with storm flows entering the sanitary system. I suspect your treatment plant would be a lot more stable with fewer storm flows in the waste stream.
RE: Flat sewer regarded to steep sewer and upsized
RE: Flat sewer regarded to steep sewer and upsized
Richard A. Cornelius, P.E.
WWW.amlinereast.com
RE: Flat sewer regarded to steep sewer and upsized
RE: Flat sewer regarded to steep sewer and upsized
This actually sounds like a construction problem to me. A little over 20 years ago, I was responsible for oversight on a couple projects with similar size sewers at the same and even slightly less than slope than the slope on the subject sewer (as little as 0.16%). On those projects, the choice was either very small slopes, or pump stations, which the owner wanted to avoid. For a sewer to work properly at that kind of slope, the installation must be virtually PERFECT, meaning that the pipe invert elevation at every point must be exactly correct with NO measurable variance at any point along the pipe length. Similarly, the pipe horizontal alignment must be dead straight. This also means that the trench must be properly graded to allow consistent thickness of bedding, and the bedding must be properly compacted at every location to ensure there is never any settlement of the pipe. This is especially important if the type of pipe used is flexible, such as PVC. Extreme care must be taken to ensure that the perfectly laid pipe doesn't move during backfill and compaction. The pipe cushion on the sides of the pipe must be properly compacted as well. This is often a problem because the contractor does not excavate the trench wide enough to allow room for proper compaction equipment, or corners are cut on compaction. If the material on the sides of the pipe are not properly compacted, subsequent cushion and backfill will deform or even partially break and crush the pipe. Any construction defect creates a place that can trap debris and cause problems such as you describe. I remember at the time thinking I don't know how this could be accomplished with "old school" methods. Using trench and pipe lasers, it really wasn't very difficult, but it does take good methodology and meticulous attention to detail. Some contractors are very good at this kind of construction, usually ones that specialize in this kind of work. Those are the exceptions; most other contractors aren't used to working with the close tolerances required and may not take the required amount of care to produce an acceptable result.
I've paid attention to those sewers over the years because I doubted at the time if those design slopes would be adequate. They have performed problem free.
RE: Flat sewer regarded to steep sewer and upsized