HydroCAD: Modeling a catchbasin as an outlet..
HydroCAD: Modeling a catchbasin as an outlet..
(OP)
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..





RE: HydroCAD: Modeling a catchbasin as an outlet..
Please note: You'll get a better reply to HydroCAD-related questions in the HydroCAD forum.
forum789: HydroCAD Stormwater Modeling
Peter Smart
HydroCAD Software
www.hydrocad.net
RE: HydroCAD: Modeling a catchbasin as an outlet..
RE: HydroCAD: Modeling a catchbasin as an outlet..
RE: HydroCAD: Modeling a catchbasin as an outlet..
However, weir flow often controls at low heads, and this will NOT be the same due to the different perimeter.
Peter Smart
HydroCAD Software
www.hydrocad.net
RE: HydroCAD: Modeling a catchbasin as an outlet..
I disagree to the following statement
The vena contracta will result in a smaller total area when there are more obstacles in the way
Hydrae
RE: HydroCAD: Modeling a catchbasin as an outlet..
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
www.hydrocad.net
RE: HydroCAD: Modeling a catchbasin as an outlet..
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.
RE: HydroCAD: Modeling a catchbasin as an outlet..
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
www.hydrocad.net
RE: HydroCAD: Modeling a catchbasin as an outlet..
Handbook of Hydraulics, Brater and King, 6th Edition, Chapter 4, especially page 4-16,
RE: HydroCAD: Modeling a catchbasin as an outlet..
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
RE: HydroCAD: Modeling a catchbasin as an outlet..
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?
RE: HydroCAD: Modeling a catchbasin as an outlet..
Peter Smart
HydroCAD Software
www.hydrocad.net
RE: HydroCAD: Modeling a catchbasin as an outlet..
Here's their online calculator for a specific grate:
http://
RE: HydroCAD: Modeling a catchbasin as an outlet..
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
www.hydrocad.net