Slotted Gate/Narrow Channel?

[schurman]

I'm trying to develop a rating curve for an existing control structure that has a slotted gate like opening and I'm looking for advice on what flow equation to use.

Here is what the control structure entails; it is 17-feet across the front as the stream flows through the trash rack. The trash rack consists of ten 5-inch diameter pipes. Immediately on the downstream side of the trash rack there is a 1-foot thick wall perpendicular to the stream with a 3-foot wide opening and it extends approx. 17-feet high. From here it discharges into a 17-foot wide by 10-foot long (in the downstream direction) and 20-feet high. Then it enters a 7-foot wide by 6-foot high concrete box culvert.

If I lost anyone, here is another way to picture the control structure is it is a box (17-feet wide by 10-feet long by 20-feet high) with 3-foot door extending 17-feet towards the top.

Anyways does anyone have advice on what equation should be used for the 3-foot opening? I was thinking of using Manning's equation. Is there any other equations I should consider?

If you have any other questions or need additional info let me know.

Thanks,
Rob

 

[RWF7437]

You may be able to approximate a rating curve for this structure using a combination of weir and orifice equations from King's Hanbook or similar sources. But would'nt it be easier and more accurate to calibrate the structure in place using known flows, heads, etc.

Also, since this appears to act as a trash rack, isn't it possible it may be frequently clogged and that any calibration or rating curve for it would not be valid under those conditions ?

One wonders why you want to get a rating curve for it ?

Good luck

 

[RWF7437]

Another approach would be to analyze the strean, grate, weir and box culvert using HEC-RAS. Do you have that program and know how to use it ? It is free and very useful.

Good luck

 

[schurman]

Reply to: RWF7437

Thanks for your reply. There was a rating curve developed for this control structure in the 80's, but the city doesn't think it is valid. I believe the city is hoping that there has been some research with something similar and we can take that equation and apply it to this structure.

I do have and know RAS. I think maybe this weekend I'll drop this configuration into RAS and see what it gives me.

However I'm still up for ideas and/or possible equations or papers related to something similar.

Rob

 

[RWF7437]

I wonder what makes "the City" think Newton's laws of energy and momentum have changed since 1980 ? I wonder how they think the rating curve can change unless the geometry of the structure has changed ? Unless velocities in the stream are approaching the speed of light, I believe these principles remain valid. Assuming the original rating curve was calculated correctly, of course.

The HEC-RAS approach is probably the easiest for your case and should be convincing to anyone who knows anything about open channel hydraulics.

Good luck with "the City"

 

[cvg]

the first response is correct, use weir and orifice equations to evaluate the inlet capacity. Calibration using known flows would be great if you have the data. If your velocities are over about 2 or 3 feet per second you will also need to calculate the headloss through the trash rack to get the actual water surface elevation upstream of the rack. You should assume that the trash rack is partially clogged with trash. In this area, we are required to assume that any grate or rack is 50% clogged. That is, the flow area is reduced by 50%.