| Product(s): | StormCAD, CivilStorm, SewerCAD, SewerGEMS |
| Version(s): | 08.11.XX.XX and higher |
| Area: | Modeling |
Problem
Is there a way to direct subsurface flow from a conduit, or channel flow, back to the surface in the Storm and Sewer products? For example, the subsurface flow will spill into a gutter to be directed toward another inlet.
Background
Subsurface flow is typically not directed to surface flow, like a channel. In addition, if the channel flow approaches an inlet and some of the flow may bypass into the downstream gutter (while some may be captured) the surface flow would need be in the form of a gutter, and subsurface flow cannot be directed straight to a gutter.
Solution - Version 2024 and higher
When using the GVF-Rational solver (StormCAD) starting with version 2024, an option is available to accept a single incoming channel as surface (gutter) flow which approaches the inlet opening. Open the properties grid and set the Gutter Type field to "Use Upstream Channel."
Note: this is only currently available for in-sag inlets.
But what if the inlet is on-grade?
As mentioned above, the "Use Upstream Channel" option is currently only for in-sag inlets, as of version 26.00.00.532 (July 2026). If the first first inlet in the ditch is on-grade (possibility of bypass flow to the next inlet along the ditch), then you need to use gutters. So, if there is also upstream pipe flow going into the ditch (upstream of the on-grade inlets inside the ditch), you can use the approach mentioned in the next section below, which involves modeling them as separate networks - observe the flow at the outfall then enter it as an additional carryover flow in the downstream on-grade inlet.
Solution - Old Versions or On-grade Inlets
For other solvers and older versions, or if the first inlet in the ditch is on-grade, try splitting the system up into two parts, similar to the following illustration:
First, model the subsurface system ending in an outfall. The results at the outfall can be used as inflow into the catch basin in the second subnetwork. The second subnetwork will include the gutter used for the surface flow. For example with StormCAD or the GVF-Rational solver, this can be done by setting the flow in the catchbasin's "Flow (Additional Carryover)" field, which will be added at the catch basin as surface flow that can be bypassed into the gutter. With the Implicit or Explicit solvers (in SewerGEMS and CivilStorm), you can copy/paste the outfall hydrograph into a Hydrograph Load in the catchbasin's "Inflow Collection".