Software Status Bulletin 100

HydroCAD^® Stormwater Modeling - Since 1986

Software Status Bulletin # 100

Subject: Excess outflow from pipe reach

Applies to: HydroCAD 6.0, 6.1, 6.2

Description

When water is detained by a pipe reach using the Sim-Route procedure, the outflow reported after the peak may be higher than intended. This situation can occur only if all of the following conditions are met:

1) The Sim-Route procedure is selected for reach routing

2) A hydrograph is being routed through a pipe reach

3) The inflow is greater than the capacity of the pipe, causing detention at the inlet. (Warning message [76] is issued to bring attention to this situation.)

Under these conditions, the outflow hydrograph is limited to the Manning's capacity of the pipe, as intended for a pipe reach. However, after the detained volume is passed through the pipe, the outflow may fail to decrease. This can be confirmed by the presence of an outflow volume that is significantly greater than the inflow volume. (The inflow and outflow volumes are displayed on the reach summary report.)

Comments

1) The peak outflow is not altered by this situation. To the extent the trailing volume is too high, this should result in an overly conservative design. There is no known situation under which the volume will be too low.

2) This problem is always accompanied by warning message [76], which cites the inherent limitations in pipe reach detention, and recommends alternate solutions. By heeding this advice the problem is avoided.

3) Warning message [76] may occur in other situations that are not effected by this issue. The occurrence of this message alone does not indicate that this problem has occurred.

4) This problem was revealed during ongoing product development and testing. It has not been reported by any HydroCAD customers.