Modeling Weir with a Trash Rack 1

[MapEngr]

I am trying to model a weir with a trash rack. The trash rack consists of a series of bars. With that, I have a few questions:

1) Does anyone have guidance on discharge coefficents for a weir with a trash rack or structure similar to a trash rack such as a screen?

2) The weir equation incorporates the length of the weir to determine the flow area. As the bars on the trash rack will reduce the flow area, do we reflect this reduction in the length value of the weir? Or, does the discharge coeffcient compensate for the reduction in area? It appears that is could be either of the aforementioned, however, which is common practice?

Thanks!

 

[Drew08]

In this situation I think it more appropriate to adjust the H, not C. If you look at the derivation of the weir equation you'll see it determines the energy of flow with a given Froude (F), where C is a function of F and g only. In most circumstances F = 1, and C therefore = 3.08(US). Only if F is not = 1 should C be adjusted.

If I understand correctly, energy losses occur through the trash rack, then the water goes critical, F = 1, as it spills over the weir. In which case you need to account for entrance losses: Q = CL(H-DH)^1.5 where DH = Ke V^2/2g. Of course you still need to determine Ke, which is dependent on the bar thickness and spacing, but isn't any better understood in this situation. A quick sensitivity analysis for various Ke, might show a low sensitivity to reasonable values (likely 0.1 < Ke < 1) if V is low.

A rounded edge culvert might have a Ke = 0.04, assuming these are rounded bars, this might be a good starting point. It will increase considerably though as the rack becomes clogged.

Also, a quick google search for, “Head loss through trash rack” came up with the following providing alternate equations for DH:

http://www.greeleygov.com/Stormwater/Documents/Stormwater Design Standards/SECTION 9A-FINAL.pdf

http://www.infobarscreens.com/trash-rack.htm

http://www.knoxcounty.org/stormwater/pdfs/vol2/3-3-5 Trash Racks and Safety Grates.pdf

http://www.co.adams.co.us/devregs/9_06_Post_–_Construction_Runoff_Control_Criteria.htm

 

[cvg]

the rack will increase the h on upstream side of the rack or conversely, reduce it as it flows through and then over the weir. Practically speaking, the rack should be sized 2 - 3 times larger or more than the weir to account for clogging and head loss.

 

[beej67]

How big is the trash rack, and how big is the weir?

I'm not sure I'd go through all those hoops if it was an outlet control for a reasonably sized pond. (couple acres) Just design the trash rack to be far enough away from the weir so that it won't have a significant effect in your professional opinion, and move on with your life. Chances are much higher that it fails due to the rack clogging than due to the slight changes in the weir math from an unclogged rack. It is built to catch trash, after all.