Flow into submerged Grate Inlet 3

[nutbutter]

Hello,

I was wondering if anyone had any advice on calculating the flow into a submerged grate inlet. I'll be contacting the vendor shortly because I know the flowrate will be highly dependent on the grate geometry.

Basically, we're using a submerged grate inlet to act as a flow control for a detention basin. The initial detention basin outfall was constructed incorrectly and instead of tearing it out and constructing a new one, we're just going to add a grate inlet where the detention basin outfalls.

This grate will be submerged by about 1"-2" of water. I don't have the exact WSE yet. My goal is to limit the flow to 5 cfs for the water surface elevation that is generated by the 10 year - 72 hour storm. I'm assuming that flow into the grate would be orifice flow since it's entirely submerged. I don't really know where it transitions from weir flow to orifice flow. I'm hoping that the vendor will have some design information that will help me.

Does anyone have any advice on how one would go about designing this grate inlet to limit flow to 5 cfs with a given head (WSE)? I was just going to use the orifice flow equation:

Q = Cd*A*(2*g*d)^0.5, where

Cd = orifice coefficient
A = clear opening of grate
g = g (duh)
d = depth of flow or WSE elevation above grate (head)

Would this be the right way to go about it? Any help would be greatly appreciated.

Peace,
Stoddardvilla

 

[cvg]

don't be so quick to assume orifice flow, 1 - 2 inches is quite shallow and could be weir flow

 

[nutbutter]

How can one determine what type of flow it is? There are different equations governing each. Would it be prudent to solve for both equations and choose the more conservative approach?

It seems like there's no established method of determining whether the flow into a grate inlet is orifice or weir flow.

Peace,
Stoddardvilla

 

[nutbutter]

Well, I've determined that the water surface elevation will be 2.9" above the grate inlet. I'm having trouble determining whether the flow into the inlet will be governed by the weir flow equation or the orifice flow equation. Anybody have any ideas?

Peace,
Stoddard

 

[cvg]

Without more information, can't answer your question. FHWA HEC22 URBAN DRAINAGE DESIGN MANUAL gives some guidance on this.

 

[gbam]

I always evaluate grates for both weir flow and orifice flow. If one graphs the two equations for the same inlet you can see the depth where Weir flow begins to control.

 

[nutbutter]

Thanks cvg.

I found an awesome reference for Neenah grate inlets....exactly what I was looking for. Check it out:

http://www.nfco.com/literature/brochures/gratecapacities.html

Click on the link to the right for "Sag or Ponding Conditions". It looks like it calculates using a combination of the two equations AND it does it for specific actual products. I haven't messed around with it yet, but it looks like it does the trick. If it works right, it's going to be a lifesaver for me. Check it out, let me know what you think or how it compares to FHWA HEC22 Urban Drainage Design Manual.

Peace,
Stoddard

 

[nutbutter]

thanks gbam as well.

 

[ccor]

UDFCD is a multi-jurisdictional drainage agency in the metro Denver area and I find their manuals are quite good. Check out Volume 1, Chapter 6, Section 3 which covers this subject quite thoroughly. You can also find design spreadsheets on the website which may be of help.

http://www.udfcd.org/downloads/down_critmanual.htm

 

[GeoPaveTraffic]

Don't forget to look at the possiblity that the grate will clogged by debris. I assume that there is a high water outlet that would prevent the structure from being overtopped.

 

[bltseattle]

I agree with GeoPaveTraffic about the clogging. You will need to carefully plan to avoid debris clogging which will throw off all the hydraulic calcs (changes the open area, orifice coefficient and effective weir length). Grates are very unreliable and perform inconsistently, I don't see how you can assure a specific flow rate restriction unless you can guarantee that debris will not get caught in it. But maybe it is not that critical of an application..... (the pond has to have an emergency spillway, right?).

 

[LHA]

Generally, weir controls until head is approx. 1.4 times perimeter (or diameter, for round opening), then orifice control kicks in. Pages 62 and 63 of FHWA HEC12 show this graphically and with equations.

But, as the graph indicates, if you calc Q for both, the lower Q (or the higher H) always controls. That automatically determines your control.

Engineering is the practice of the art of science - Steve

 

[LHA]

Oops, I meant 0.4 times diameter, not 1.4, which would definitely be under orifice control. But, again, if you ran both, you would get orifice as the max H/min Q, so you woudl have the correct result.

Engineering is the practice of the art of science - Steve