Partial Area Effects
The rational method typically assumes that peak flow occurs at the time when the whole upstream catchment area contributes to the flow. However it is possible that a 'partial area' can deliver higher peak flows than the total upstream area - for example when a large impervious area with a small time of concentration is connected to a network downstream of a small pervious area with a large time of concentration. Bentley StormCAD has an option to correct for 'Partial Area' effects (see Calculation Options)) as described below.
For any downstream node, the final combined system rational flow should not be smaller than the single maximum rational flow from contributing sources; these contributing sources include:
- Incoming and tributary pipes
- Captured surface inflow from catchments and gutters
- The inflows (rational portion) from diversions
- External rational flows
If the calculation option is enabled and there are multiple rational flow sources entering a node, the model will check all incoming flows to find out the largest rational flow from the sources. This identified maximum flow is called the "partial area flow". The combined rational flow is calculated by the normal procedure and then is compared with the partial area flow. If the partial area flow is smaller than the combined rational flow, the combined rational flow will be used. If the combined rational flow is smaller than the partial area flow, the partial area flow is used as the new system rational flow at this point.
When a partial area flow is used it also serves as a minimum rational flow for the downstream elements. The downstream elements still follow the normal calculation procedure and the calculated rational flow will be compared with any upstream partial area flows. If the normal calculation procedure results in a flow that is smaller than the partial area flow, the system flow is replaced by the larger partial area flow.
The following examples illustrate the "Correct for Partial Area Effects" calculation option.
Tributary inflows
"Correct for Partial Area Effects" = TRUE:
"Correct for Partial Area Effects" = FALSE:
Catchments
"Correct for Partial Area Effects" = TRUE:
"Correct for Partial Area Effects" = FALSE:
External flow
"Correct for Partial Area Effects" = TRUE:
(The large flow used is the external rational flow)
"Correct for Partial Area Effects" = FALSE:
Gutter
"Correct for Partial Area Effects" = TRUE:
"Correct for Partial Area Effects" = FALSE:
Diversion
"Correct for Partial Area Effects" = TRUE:
"Correct for Partial Area Effects" = FALSE:
