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Ditch plugs and overland flow

Ditch plugs and overland flow

Ditch plugs and overland flow

(OP)
I am trying to model a ditch plug that doesn't reach the top of the bank.  I want to check a weir vs. channel flow for the plug to see which is more likely to control.  Since a reach node won't give me the elevation that the plug will back water to, I took a reach node, entered my plug info, took the resulting discharge table, and used that as a user-defined discharge in a pond node.  My first question is if that is an accurate approach or not?

Assuming that the first part is OK, my second question is that if I try to model a weir and overland flow (primary and secondary) using the above approach, I will end up double counting the water above the plug during out-of-bank events.  Any suggestions on resolving that?   

RE: Ditch plugs and overland flow

You can sometimes use a series of ponds to model check-dams in a channel.  This approach assumes there is a level-pool being created behind each check dam, so that a pond routing is applicable.  

But if this is predominantly channel flow (as opposed to weir flow), then a reach routing is more appropriate.  In this scenario, you can make allowance for the check-dams by using a higher Manning's value.

It's quite possible that the first scenario will occur for smaller rainfall events, and the second scenario for larger events.

Peter Smart
HydroCAD Software
www.hydrocad.net

RE: Ditch plugs and overland flow

(OP)
If I have a broad-crested weir where the breadth is 15' or more, the weir coefficients are almost constant at 2.63.  The broad-crested weir device does not have the rise function to "turn it off" once the elevation of the next flow control has been met.  

So, since the sharp-crested vee/trap outlet allows me to assign a weir coefficient and set a rise, would it be accurate to set the weir coefficient at 2.63, set the rise equal to the point where overland flow becomes dominant, and model the broad-crested weir with a sharp-crested outlet?

RE: Ditch plugs and overland flow

Only a few situations benefit from the use of a broad-crestd weir over a sharp-crested.  In many cases there is so little variation in the coefficients that the difference in the final results is often very small.  So yes, I would go with a sharp-crested weir.

The weir "rise" parameter is used to limit the depth of flow over a weir, effectively turning it into an orifice at higher water levels.  This is used primarily with "stacked" weirs, such as a v-notch cut into a rectangular spillway.  The rise would be used to set the height of the notch, so that the flow isn't double-counted when the water reaches the rectangular spillway.

Perhaps I'm missing something, but I don't see how this would apply to your situation...

Peter Smart
HydroCAD Software
www.hydrocad.net

RE: Ditch plugs and overland flow

(OP)
The project is an attempt to raise the groundwater level in a given area.  I need to show that the surface water levels will not be any different pre-project vs. post-project.  Permanent pool levels are easy enough to show, but I also have to prove the flows during storms aren't going to be a problem.  Modeling the flows using a reach won't let me show a backwater profile, so I need a pond node.

Since the overland flow characteristics are not changed by the project, I can create a reach for it, take the stage-discharge chart, and use it to create a user-defined stage-flow in my pond node.  The problem is when I model the flow over the plug/weir, flows are being double-counted if I can't "turn off" the weir portion when the overland kicks in.

So, it looks like by modifying the vee/trap weir for my plug, I can model the flows pre- and post-project and show the maximum water level occurring during a flood.  Now to prove it to the permit folks...

Thanks for the help.

RE: Ditch plugs and overland flow

I think you'll need to choose one flow regime:  Weir flow, or the custom outlet to represent channel flow.  Using both at once will generate excess flow, as you suggest.

But you might be able to use the device routing to get around this.  By default, your two outlets are routed to primary, so the flows are added.  But if you route one device through the other, it will limit the flow to the lesser of the two flows.  This will allow the weir to control at low flows, and transition to channel flow as the depth increases.

Peter Smart
HydroCAD Software
www.hydrocad.net

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