Reserve Capacity and Reserve Flow in a Link in InfoSWMM and SWMM 5

Subject:  Reserve Capacity and Flow in a Link

The Reserve flow and Reserve Capacity are modeling guidelines and do not actually influence the computed flows in a link.  If you have a positive Reserve flow or capacity then you MAY get more flow in the link based on the current flow being less than the Qfull for the link but not if the link is under surcharge, has backwater conditions or has large entrance and  exit losses. You cannot always assume that because the Reserve flow is positive the link can  handle more upstream flow.

Here are few graphs that show the relationship between Qfull, the actual Q in the link and the Reserve Flow or Reserve Capacity.  The Qfull is a reference flow and is not used during the computation in InfoSWMM and SWMM5.

Condition 1:   Positive Reserve Flow – the flow is always less than Qfull and the Reserve flow and  Reserve Capacity are Positive.

Condition 2:   Negative Reserve Flow – the flow is sometimes greater than Qfull and the Reserve flow and  Reserve Capacity are negative when this occurs.

Reserve Capacity – the Reserve in the link * the current link volume.