Modeling pipes directly underneath ditches?

I want to model a storm drain system. Ditches are proposed to be replaced alongside the road edge with intermediate ditch-inlets. The system ultimately outfalls to one headwall. In between every inlet there is a pipe connecting the two structures. So between every storm inlet there is both a ditch and a pipe (directly underneath the ditch). 

The way I have my storm system laid out is by having each ditch have a cross-section node at the upstream side, and an inlet node at the downstream node. The upstream cross-section node is directly next to the upstream inlet node. For the pipe underneath the ditch, both the upstream and downstream side have inlet-node connections. I have my catch basins connected to the cross-section nodes, so that the flow goes into the ditch and falls into the inlet. The water then flows to the downstream pipe until reaching ultimate outfall. So to recap, in between two inlets nodes, there is a pipe (underground), a ditch (on ground), and one cross-section node directly next to the upstream inlet node. The upstream inlet node is connected to the pipe, and the upstream cross-section node is connected to the ditch. The downstream inlet node has connections to both the ditch and pipe (at different elevations of course).

To do storm water comps, would this be the best way to model the flow? I have done some testing, but I am experience some Q losses at each inlet structure. In theory, at one intermediate inlet, flow should come from the upstream pipe and the runoff from the catch basin, and the same flow should exit via the downstream pipe. But I am seeing flow decrease- the flow that exit the inlet is less than what flows into the inlet. Which is why I am wondering if my model is the best way to structure the storm drain- or if there is a better solution.

Any insight would be appreciated. I am using OpenRoads Designer CE - 2021 Release 2 (10.10.21.04). Below is a picture of the general design. 

  • How much flow loss are you getting? Could it be that the system time in the network is coming into play here, which will reduce the flow in the system as you move downstream. In Rational Method hydraulics, the intensity reduces as the system time increases. This Wiki provides more information on this subject:

    How is flow balanced at junctions with the GVF Rational Solver (StormCAD)? - OpenFlows | Water Infrastructure Wiki - OpenFlows | Water Infrastructure - Bentley Communities

    Your question also reminds me of another recent post on this community, because you're modelling a ditch that discharges into a ditch inlet. You may have already noticed that the flow in the ditch (which is considered as a conduit by the software) doesn't cause surface flow, so you won't get a spread width calculated at the ditch inlet. One workaround is to model the ditch as a gutter element.

  • Within my system of 10 pipes, my greatest percentage difference was 14% (expected 2.095 vs openroad's 1.791) and the biggest Q difference was 1.843 (expected 34.724 vs openroads' 34.373). Thank you for providing that Rational Method hydraulics wiki; that link could prove insightful.

    I am also familiar with that recent ditch post, but in this specific case, spread is not a criteria. I was just mostly curious if there was a specific method to modeling pipe under ditch already known and utilized.

    Thanks

  • It may just be personal preference, but I would use gutter links between the inlets rather than adding cross section nodes and "ditch" links.  This allows correct calculation of bypass.  As Jon noted, the ditch links are modeled as being connected to the inlet vaults, not the surface grates.  The gutter links will give you all the hydraulic results in the roadside ditch for depth, velocity, etc.  The only potential downside to this approach is that flow to the gutter is not taken into account until it is added to the downstream inlet (which is what you would use for the catchment's outflow node).  I see that's not what you want.  Rather than adding cross section nodes and using ditch links, I would instead just add a dummy inlet immediately downstream of the real inlet (and using a short gutter link), with the dummy inlet set to zero capture (as Robert Garrett suggested in a previous post).  This allows you to add the catchment runoff to the top of the gutter (or anywhere you want to place the dummy inlet).  Either approach requires you to add some kind of node to direct the flow to the top of the roadside ditch rather than to the inlet at the downstream end, but I prefer using the gutter links rather than ditch conduit links.

    Karl Dauber, PE
    Advance Consulting
    Laurens County, SC
    karldauber@advconsult.net
    www.advconsult.net
    www.linkedin.com/in/karldauber