If you use the conduit lengthening option in SWMM 5 then your short conduits will be lengthened based on the CFL or explicit time step criterion. Any conduits in which the Length Factor or the courant time step link length over the original length is greater than 1 will be lengthened and will have its roughness lowered so that the conduit is hydraulically the same at full conduit depth. The full area, full width and full hydraulic radius stay the same in the modified link – only the length, slope and roughness are altered.
Length Factor = (Wave Celerity + Full Depth Velocity) * Time Step / Conduit Length, and for those links in which the Length Factor is greater than 1
New Roughness = Old Roughness / (Length Factor) ^1/2
New Slope = Old Slope / Length Factor
A few metric's for showing how this option has altered the network are shown in the figure below:
1. The most important is the increase in Network full volume as you never want to drastically alter the volume of your network,
2. The number of conduits modified along with the new mean slope (lower) and the new total conduit length are important indicators,
3. The mean wave celerity, full flow velocity and courant time step mean give the user some idea of the optimal time step for the simulation.