HydroCAD Check Dam Modeling

[Joni_DeWolf]

I was wondering if my HydroCAD model for stone check basins within a swale is correct.

I have a series of pond-reach-pond-reach-pond-reach... to model 16 check dams. However I do not know if I am doing the storage and outfall routing correct. For my ponds I have detention pond with custom stage data as storage and rock fill as primary outlet and sharp crested trap weir as secondary outlet.

For my storage im using surface area with 0 sqr ft at the base elevation of the check dam and 23 sqr ft at an elevation 1 foot higher. Is that my total storage since my check dam is 1 foot high?

For my outlets I have Rock Fill with its invert at the base elevation and then a length of 6, breadth of 1, and height of 1. (Rock dia=6 , SD=0.1) THEN, for my secondary outlet I have a trap weir with a crest length of 6 (to match the check dam) and a rise of 1 (so that there is an open gap where the check dam is). I am trying to use the weir to model the water that flows over top of the check dam. Is this the correct way to model this?

Here are my stage discharge graph and hydrograph

 

[psmart]

Looks reasonable. But what is the objective for your HydroCAD model? As you can see from the hydrograph, the check-dams have very little effect on the inflow hydrograph. You have minimal storage, so the water than doesn't pass through the check dam simply passes over the top.

Also see

http://www.hydrocad.net/checkdams.htm

Peter Smart
HydroCAD Software

http://www.hydrocad.net

 

[Joni_DeWolf]

Thank you for the response.

The objective was to see the feasibility of using check dams in the swale.
Using the check dam spacing equation the actual requirement be 5 ft spacing and 170 check dams to effectively slow down the flow, pool water, and limit erosion.
Eventually that may need to be done but for now we're just going to suggest using 16 check dams placed at 6 foot elevation drops

I am still a little uncertain about my outlets.

Does this profile view look right to you?

And, under advanced tab of my ponds, would it be correct to set automatic overflow to secondary for my weir

 

[psmart]

Since you already have a weir to handle the overflow, you should NOT use automatic overflow. These are mutually exclusive.

A weir is more accurate, but make sure you define storage ABOVE the crest in order to handle the maximum WSE. Check the warning messages...


Peter Smart
HydroCAD Software

http://www.hydrocad.net

 

[Joni_DeWolf]

Okay, that makes sense. When I define storage above the crest, am I on the right track to do it as so:
The outlet elevation of my weir is 1175 and my storage goes up to 1175.1

I think if I do this correctly then the storage should span in elevation up until the next check dam.

 

[psmart]

You need to define the storage up to the maximum WSE, whatever that may be. I suspect it's more than 1/10 foot above the crest.

I assume you have also defined the storage impounded by the check dam? You can generally just add another stage to that storage definition.


Peter Smart
HydroCAD Software

http://www.hydrocad.net

 

[Engimental]

Why not just use EPA SWMM? IMO it is better suited for this detailed level of modeling (I do love HydroCAD though).

 

[Joni_DeWolf]

The project im working on is 4 different sections and im using hydrocad for the rest of them so I just want to keep the report consistent. I will have a look into EPA SWMM for future, thanks.

So, should I be accounting for storage all the way up to the elevation of the previous check dam base? Would that be my max WSE

The schematic of my swale and two check dams. The check dam drops to 9" in the center and slopes up to 1.5' at the edges of the swale

 

[psmart]

When modeling any pond you need to provide complete storage data in order to get an accurate routing. Otherwise you will get get a "storage exceeded" warning and the pond behaves like a closed vault - not what you want with a check-dam.

I would go a foot or so above the crest. It depends how high the water gets. Extra storage data never hurts.

Peter Smart
HydroCAD Software

http://www.hydrocad.net

 

[Joni_DeWolf]

Okay, my storages are set up good and there are no exceedance warnings however my flows arent slowing down at all even during a 1 year event.

 

[psmart]

I'm not surprised. There isn't much storage behind a check dam. They will cause a slight flow reduction based on increasing the overall "roughness" of the channel, but don't expect a big effect.

From

http://www.hydrocad.net/checkdams.htm

As always, remember to use the simplest model that will address your specific objectives. Although check-dams can be modeled in great detail, this may or may not have a significant effect on the overall results of your model. If in doubt, do a trial study to determine the effects and plan your model accordingly. Although properly constructed check-dams can be useful in controlling erosion, their effect on hydrograph routing may be minimal for all but the smallest rainfall events. For further details please see the general modeling tips.

Peter Smart
HydroCAD Software

http://www.hydrocad.net