HydroCAD: Modeling a catchbasin as an outlet..

[hydroponder]

What is the best way to model a catchbasin as an outlet for an infiltration system. I usually just use a horizontal orifice, but it doesn't take into accout the grate. I suppose I should use an array of orifices, but was hoping there was an accurate, more simple way..

 

[psmart]

Yes, you can use an orifice array to model a grate. If this feeds into a culvert that might control under some conditions, you can set up a compound outlet with the grate routed to the culvert.

Please note: You'll get a better reply to HydroCAD-related questions in the HydroCAD forum.
forum789



Peter Smart
HydroCAD Software

http://www.hydrocad.net

 

[kmatte]

Do you size your oriface based on the square foot opening of the grate? You should be able to get this from the supplier. This will reduce the outflow considerably.

 

[hydroponder]

I have the free open area from the Neenah catalog, but does a circular (or square) orifice size to meet the free open area act the same (allowable flow) as an array of smaller orifices that more accurately reflect the grate used?

 

[psmart]

Orifice flow is proportional to the open area, so a single opening will have the same discharge as several smaller opening with the same total area.

However, weir flow often controls at low heads, and this will NOT be the same due to the different perimeter.



Peter Smart
HydroCAD Software

http://www.hydrocad.net

 

[hydrae]

Peter
I disagree to the following statement

Orifice flow is proportional to the open area, so a single opening will have the same discharge as several smaller opening with the same total area.

The vena contracta will result in a smaller total area when there are more obstacles in the way

Hydrae

 

[psmart]

hydrae: The standard orifice equation is strictly proportional to the flow area:

Q = Cd a sqrt(2hg)

The vena contracta is accomodated by the efficiency constant Cd, which is generally .59 or .60, regardless of the orifice size.

It's true that grate flow will often be lower than predicted by a multiple-orifice equation alone. Under low-head conditions it may be better estimated with weir flow around the grate perimeter. In this case, each opening is not flowing full, so the standard orifice equation will tend to overestiate the flow.

Peter Smart
HydroCAD Software

http://www.hydrocad.net

 

[RWF7437]

"The standard orifice equation is strictly proportional to the flow area:"

According to the Handbook of Hydraulics, Brater and King, 6th Edition, Chapter 4, especially page 4-16,

The discharge coefficient is not a constant but varies with Reynolds Number. Although this variation is often ignored for many engineering applications it does exist and can be measured.

 

[psmart]

Any variations in Cd do not not preclude the fact that the orifice discharge IS proportional to the orifice area. And blockage of a grate or other opening is commonly modeled by an appropriate reduction of the flow area. (I believe that was the basic issue we were addressing.)

Yes, Cd can vary based on roughness of the opening. But I believe the variation is small in comparison to the effects of flow area. Hence the common use of a standard Cd value. And I see no evidence that Cd is directly related to the flow area.

Of course, different Cd values can certainly be used as required. Does anyone have a practical source for variations in Cd?


Peter Smart
HydroCAD Software

http://www.hydrocad.net

 

[RWF7437]

"Of course, different Cd values can certainly be used as required. Does anyone have a practical source for variations in Cd?"

Handbook of Hydraulics, Brater and King, 6th Edition, Chapter 4, especially page 4-16,

 

[hydrae]

I still stand by my argument:
The standard orifice equations assumes a round orifice and thus is adjusted for the vena contracta in the constant. This was part of the experimental method that derived the equation.

Other shapes of orifii will need a different constant either derived from N-S equations or by experimental methods

The amount of effective area reduction due to vena contrata would be a function of the linear wetted length of the grate's bars. This modification of the equation would follow the same procedure used to model flows in non circular pipes such as inside heat exchangers or open channel flows.

To go back to the original question, is there an easy and accurate way to model an orifice with a grate. Sorry but NO

Hydrae

 

[hydroponder]

Thanks to all for the posts. After reviewing all of the posts, I have refined my question to this:

Are the variations in modeling a catchbasin as a single orifice with the diameter proportional to the free open area versus an array of orifices negligible when said modeling is being used for pre and post development runoff rates?

I am guessing that the potential variations in the variables used to predict runoff (CN, TC, area) are going to have a much greater affect on the rates. Thoughts?

 

[psmart]

hydroponder: I agree with your conclusions. Any variations in the discharge coefficient are small in relation to the other uncertainties in the model. If in doubt, do a quick sensititivity anaylsis to determine the effect of different values. In 99% of the cases I've seen, modest changes in the discharge coefficient have a negligible effect on the overall hydrograph routing and peak elevations. If in doubt, test it yourself. As a last resort, you could enter an actual stage-discharge curve for the grate (using a special outlet device) but that data is rarely available.

Peter Smart
HydroCAD Software

http://www.hydrocad.net

 

[eea]

hydroponder, Neenah should also have published lab test data showing your specific inlet's characteristics under different heads and when the control changes from weir to orifice. I understand they have a wet lab and a good amount of empirical test data.

Here's their online calculator for a specific grate:

http://www.nfco.com/content/Inlet Grate Capacities/Weir and Orifice Equation/

 

[psmart]

eea: Good link. For orifice flow conditions, it's worth noting that Neenah bases the grate capacity on the total free area, and uses a discharge coefficient of 0.60 for all grate configurations.

For low-head conditions, Neenah transitions to weir flow when this is less than the orifice flow that would be predicted at a given head. You can replicate this behavior in HydroCAD by using a "compound outlet" with an orifice preceded by a weir, and the upcoming HydroCAD-9 release will handle this automatically using the orifice screen alone.

Just a reminder, this discussion has been about submerged grate flow, with the grate acting as an outlet control on a pond. Gutter flow and bypass calculations are an entirely different matter...


Peter Smart
HydroCAD Software

http://www.hydrocad.net