If I remember correctly, it has to do with the way the minor losses were modelled (State of the Art, way back). With the updated HEC22 it does not matter now. You can match inverts centers or soffits. Anyone else?
1. To compensate for headlosses within the structure.
2. To maintain a uniform HGL and reduce the opportunity for a hydraulic jump within the structure.
3. Just because - its the way we have done it for years.
Often in sewer design the objective is to achieve minimum self cleansing gradient with minimum trench depth. Matching crown to crown (or 0.8 as corrected by civil person) is hydraulically the most efficient. if you match invert to invert with a larger downstream sewer then you are creating a backwater in the upstream sewer and you have not designed to minimum hydraulic gradient/velocity.
Set the criteria and then design to it. If you match invert to invert then you are not meeting the design criteria. .
In sewer design, the original reason for designing 0.8 depth (branch) to 0.8 depth (main pipe, was that we tried to match the working flow level of the branch with the main pipe.
One reason was to match the HGL but the main reason was to reduce the turbulence of combining flows to reduce the production of H2S & consequence of acid attack on the system below the junction.
I assume that in drainage systems, the requirement is to match the HGL.
A bit of a problem of mixing theory and practice here.
If inverts are matched there will be less turbulence.
The HGL in the larger downstream pipe will be higher than the HGL in the upstream pipe creating a backwater in the upstream pipe.
The velocity in the upstream pipe will be less than design.
The upstream pipe may in fact flow full
Because the velocity in the upstream pipe is less there is less turbulence at the junction. However, for pipers laid to minimum grade the self cleansing velocity will not be achieved and theoretically there will be settlement in the upstream.
setting crown to crown (HGL to HGL) gives flow through the junction without a drop or without a backwater. That is why it is done.
When a smaller sewer joins a larger one, the invert of the larger sewer should be sufficiently lower to maintain the energy gradient. An approximate method for securing this result is to place the 0.8 depth point of both sewers at the same elevation.
Flow Channel:
The flow channel through manholes should be made to conform in shape and slope to that of the sewers. A bench should be provided that should have a minimum slope of 2 inches per foot.
