xpswmm/xpstorm Resource Center xps

The significant differences between this finite difference solution and the finite difference solution in EXTRAN versions 3 and 4 are:

  1. The  term in the momentum equation has a different derivation in EXTRAN Version 3 and Version 4 and 5
  2. An additional  is factored out of equation 18.
  3. The finite difference approximation of the friction slope Sf is an average of the friction slope at the upstream and downstream end of the conduit, or the conduit center friction slope.
  4. An additional bound on the iterated values of junction head and conduit flows is used in EXTRAN version 5. The change in head is restricted <1 percent in any one iteration. The change in Q is restricted <10 percent in any one iteration.

The main consequence of the above differences occurs during the rising and falling portions of the hydrograph. During steady flows the momentum equation reduces to a balance of the hydraulic head slope and friction slope. However, as the flow is increasing or decreasing the convective acceleration term increases in importance.