HydroCAD Stormwater Modeling - Since 1986

Inflow Area, Inflow Depth, & Runoff Depth

Starting with HydroCAD 8.0, the inflow area and inflow depth are calculated for each node and shown at the top of the node summary.  For a subcatchment, the runoff depth is displayed.

HydroCAD 8.5 (and later) can also display the fraction of the inflow area that is impervious.

What is the Inflow Area?

The inflow area is the total runoff area contributing to a given point in the watershed.  This is typically the sum of all subcatchment areas that lie upstream of a specific node.

  1. The inflow area is normally conveyed by the primary outflow from upstream nodes.  (If desired, the area can be apportioned between the primary and secondary outflows by use of the Area Multiplier added in HydroCAD 8.5)
  2. If there is no runoff area contributing to a given node, the inflow area is not reported.
  3. If the routing diagram includes a "flow loop", the inflow area is undefined and is not reported.
  4. When linking projects with a file link, the inflow area is automatically passed between projects along with the outflow hydrograph.
  5. When using a manual link, the inflow area may be entered manually.  This allows accurate determination of the inflow area and depth for downstream nodes.

What is the Inflow Depth?

The inflow depth is determined by dividing the node's inflow volume by the inflow area.  This yields an average runoff depth from the upstream watershed, adjusted by any routing operations.

Inflow depths are subject to the same considerations that apply to hydrograph volumes:  The volume is calculated based on the flow that occurs within the specified time span.  A reduced time span will decrease the reported volume and inflow depth.  To report the entire volume and depth, set the time span to cover the entire period of flow.

Note: HydroCAD 7.1 and later will indicate partial depths by displaying "Depth>x.xx" instead of "Depth=x.xx".  A special hint will also appear in the message window if any hydrographs appear to be truncated.

What is the Runoff Depth?

The runoff depth is determined by dividing the subcatchment's runoff volume by the subcatchment area.

In most cases, the runoff depth will be somewhat less than the rainfall depth, due to the effects of the applicable runoff calculation.  Runoff depths are also subject to the time span issue discussed above.

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