Combined sewer - Clarification

[SMIAH]

Does sewer network separation increase peak flow rate? e.g. Installing a new stormwater sewer next to an existing sanitary (combined) sewer.
More precisely, if and only if the existing combined sewer is not able to carry a certain peak flow, installing a stormwater sewer that has the capacity to do so next to it and connecting it togueter somewhere downstream will increase the flow rate at this particular point (connection)?

This seems trivial to me... But I got mixed interpretation.

 

[cvg]

your question is not very clear. but if I understand - adding another parallel pipe upstream may not solve your capacity problem. The flow capacity downstream is controlled by the downstream pipe and that may also limit the capacity upstream.

 

[SMIAH]

But can it increase a flow capacity problem downstream? Increase the peak flow as the new stormwater has more capacity than the existing combined sewer.

 

[cvg]

assuming the sewers are not running supercritical, if you have restricted flow downstream, than adding more pipes upstream will have limited impact on the downstream capacity

But if you have excess capacity downstream and a restriction upstream, than adding another pipe upstream could increase the flow rate throughout both reaches.