Detecting and preventing invalid hydraulic solutions in SWMM 5

David Ksyniak, Khalid K. Khan, Robert E. Dickinson and James T. Smullen.


Three key dynamic equation checks were added in SWMM 5 to specifically prevent the problem of constant unusual node depth or constant unusual link flow predictions for extended simulation periods in SWMM 4. The depths or flows are termed “unusual” because the boundary conditions surrounding the node or link would indicate that the depth or flow should change over time.

The node continuity equation or the link flow equation became “stuck” at one setting and not able to respond correctly to changing boundary

The three additions made in SWMM 5 to prevent a solution sticking are:

  1. The flow equation calculations are symmetrical in SWMM 5 compared to SWMM 4. The same two flow equations (St. Venant and the Normal Flow) are used for both positive and adverse sloped conduits in SWMM 5. In SWMM 4 only the St. Venant equation was used for adverse sloped conduits. The normal flow equation is the most important check for the validity of the St. Venant equation flow prediction.
  2. The flow in a link is set to 0.0 if the upstream node “dries” up. This condition mainly applies to outfall conduits.
  3. A check is made about the reasonableness of the components of St. Venant equation in SWMM 5. The major component of the St. Venant equation (HGL term) g * A * dh / dx is not allowed to change more than the value of Qfull between successive iterations without checking the St. Venant equation flow compared to the Normal Flow equation flow. The HGL term is the dominant term of the St. Venant equation.

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