T Notches Weirs + Coefficients 1

[mbherlihy]

I am designing a pond outlet control structures (OCS) in georgia. I am getting confused on what coefficient to use when modeling (we use pond pack) a T notch weir. Specifically, the 'notch' part to the "T" (said another way, the 'stem' to the letter T). The weir will be submerged at the higher end storms 50 yr, 100 yr. I think i should use something like a 0.6 coefficient. (this happens to be what we use for circular orifice flows)Then for my emergency spillway, a traditional longer length weir (located at the top of the T Notch, I would use something like 3.1.

 

[Highway27]

It can generally work either way, but you may not be able to just change the coefficient without changing the modeling method. You can either keep it at the 3.1 coefficient for weir flow, and take that width out of the larger weir, or you can make it an orifice with 0.6 and the weir is the entire length.

Example: the 'stem' is 1' wide and 2'below the main part of the 'T', the top is 5' wide. You could do two weirs, 1' and 4', with different elevations. Or you can do a 5' weir at the 'T' elevation, and a 1'x2' orifice at the lower elevation.

 

[gbam]

I am not sure but does pond pack model weir submergence? If it does, it should account for the back water. I wrote a spreadsheet that accounts for submergence in the computation and keeps the coefficient constant. The submergence approach that I use adjusts the Qfreeflow by a percentage based on the submergence depth.

 

[mbherlihy]

Highway 27, ok I think we're on the same page...i assume you use pond pack. For the Notch, (in your example ...the 1 x 2 foot stem), I am using Pond Pack's orifice selection, choosing AREA, entering the square footage (your example 1 x 2 = 2 SF), number of notches, and setting this area to the invert of the notch. When I select orifice (area), pond pack allows me to enter a coefficient. That is why i was asking the original question.

As for my Main Part to the "T", I am using a Weir with (your example, 5' length)set at the invert elevation of the Main Part (or top horizontal segment to the "T").

 

[Highway27]

Heh, no, we don't use any package software, just straight up TR-20's.

But from what you're describing, it sounds like you're on the right path. If you're using orifice flow, 0.6 is probably the correct coefficient to use, and 3.1 for the weir computation. The length and elevation would be as you say.

I've done comparisons of both ways (dual weirs vs. orifice and weir) and they really come out with almost exactly the same results. So your model should work either way.

Make sure your software knows the shape of your orifice. A 1'wide by 2' high orifice will have different flow characteristics than a 2'wide by 1'high one.