How to Model Channel Controlled by Outlet Structure

Micco,

Just to expand a bit - 

The water surface elevation in the pond will not impact the upstream channel (conduit element). Meaning, the elevation will not be communicated upstream as a tailwater acting on the channel. You can right click on the channel and choose "channel rating table" to see the normal depth-based elevation vs. flow curve.

So, as Scott mentioned, you would need to model this situation as a pond. You can check out the "ICPM channel" example model included with PondPack (Samples folder within the installation folder). In this example, the pond named "Junction" (set as a tiny volume, but could also be set as no volume) represents the channel - the downstream outlet has a rating curve based on the channel characteristics and the upstream outlet is used to communicate the channel water surface elevation to the upstream pond via the interconnected ponds (ICPM) option. I documented more explanation in the project properties, which you can find under File > project Properties. You can also read more about this here: How to model a channel between two ponds

Another option would be to use SewerGEMS or CivilStorm. The dynamic solvers available in these products can account for the pond tailwater natively. That is, you can lay out a channel connected to a pond (and a pond outlet downstream), and the pond elevation will impact the hydraulics of the upstream channel automatically. 

However, depending on your modeling needs and what you ultimately need the model to tell you, PondPack's more simplified assumptions may be acceptable. (why do you need this rating table?)

Thanks again Jesse, 

Regarding the approach of modelling the channel as a pond, the pond element doesn't allow a longitudinally sloped bottom, so I'm concerned there would be some inaccuracies there. Perhaps there's a way to account for that though?

I'm working on a project where discharge to existing ditches must be limited to 0.2 cfs per acre of contributing area. Based upon the rainfall in the area, and the land use, the developed runoff rates are ~1 cfs/acre.

We've got 25 foot wide channels on either side of the roadway with check dams. We're considering using the check dams for weir overflow, or adding a trickle tube (orifice discharge) through the check dams to control the discharge rates. 

Minimum slopes in the channel are 0.5%, so we'll have 5 feet of drop for every 1,000 feet of channel.

So, I'm concerned the flat pond bottom would neglect the travel time in the channel, and overaccount for storage, since if we've got 1' of depth at the channel outlet, that water will really occupy a triangular shape (in profile view of the channel) that extends only 200' up the channel, rather than 1' depth for the entire 1,000 feet. 

What do you think is a good approach?

Also, I don't have access to SewerGEMS or CivilStorm, but I do have access to both InRoads and Civil3D storm and Sanitary. Would those softwares be capable of modelling this scenario?

Micco Emeson said:

Perhaps there's a way to account for that though?

This can be accounted for in PondPack by using the channel's rating curve (which accounts for the channel slope, etc) as the downstream outlet structure user defined rating table. Thus for a given flow that enters the "fake" pond, it looks up the corresponding elevation, which is then communicated upstream via ICPM. If you are trying to model a series of channels with weirs and orifices between them, you could try modeling it as a series of ponds in this way, but the elevation from the channel can only communicate via ICPM to the next-upstream pond since that next-upstream pond would have a "real" outlet (weir and/or orifice) and be able to generate an EQTW curve to account for tailwater. If you have another channel upstream of that, it would be using another user defined rating curve in its outlet structure (representing the channel rating table), which cannot be used with ICPM since it is not an EQTW table (just a single elev vs. flow for all tailwaters).

If I understand your situation correctly, CivilStorm or SewerGEMS are likely a much better tool as they would be able to handle this situation without the extra complexity, due to to their dynamic solvers. You could model it as a series of conduit elements with inline weir conduit control structures as seen in the below screenshot. Or, a series of channels and ponds where the pond is used to model the outlet structure with the weir and orifice.

Micco Emeson said:

Also, I don't have access to SewerGEMS or CivilStorm, but I do have access to both InRoads and Civil3D storm and Sanitary. Would those softwares be capable of modelling this scenario?

From a brief check, I believe your organization may have access to CivilStorm, though we may need to connect offline about that. I do not know about the capabilities of Civil3D Storm and Sanitary and from the small amount of experience I have with Inroads, I am fairly confident that it does not include the ability to model inline control structures and ponds like shown above, and does not have a dynamic solver. If you need to pursue this route I can look into this further but note that Inroads has been replaced with OpenRoads Designer. OpenRoads Designer has the StormCAD solver built in and can also use the SewerGEMS/CivilStorm solvers/functionality, but will require a SewerGEMS/CivilStorm license to do so.

Thanks again Jesse,

I reached a satisfactory resolution by using a pond element defined by a stage-volume table that accounts for the channel slope.

For now, I don't really have time to dive into looking at another solution, but I really appreciate the insight. Down the road I may look into access for CivilStorm.

Micco

Glad to hear it. Modeling the channel segments' volume as a pond will likely be fine if the channels are relatively short.

If there is significant infiltration, you can use the infiltration option in the pond properties.

Glad to hear it. Modeling the channel segments' volume as a pond will likely be fine if the channels are relatively short.

If there is significant infiltration, you can use the infiltration option in the pond properties.