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Your Hydraulics Thinking Cap On?

Your Hydraulics Thinking Cap On?

Your Hydraulics Thinking Cap On?

(OP)
Assume an existing rectangular concrete flume,
slope varying from 0.020 to 0.040, width from
12' to 24', vertical wall height from 3' to 12',
overall length 2500'. ACE-style "channelization".

Plan to construct and install precast weirs at
average 25' oc, bevel-face, vertical back, with
a low-flow notch. Top of notch is +2', top of
weir is +2'-6", weir length is full flume width.
TOC weir elevations stair-step -0.7', weir:weir.

?How to model stormflow hydraulics? One approach
was used by applying Chezny-Manning formula, with
n=0.050 to simulate the "roughness" of the weirs,
Q=(1.49/n)(A^2/P)^0.67(S)^0.5.  Fairly simplistic.

Another approach used the Sharp-Crested Weir
formula, Q=CL(d^1.5), modified for velocity
head with relative velocities from Chezny.
Gave much, much lower throughput capacity.

Another approach used a "hacked" HEC-RAS, but
no way to QED independently verify the results.

Design storm flow is estimated at 1200cfs. <---

Depending on method used (above), either flow
mostly passes through, or jumps out of bank.

Anyone out there an expert flume-weir modeler?

EXISTING            Floor        Wall
Station        Width        Elev.        Height
290        17.0        107.4        4.7
354        17.8        108.6        5.3
390        12.2        109.3        2.8
520        13.3        112.6        6.2
570        14.1        114.1        9.4
660        12.0        116.8        6.3
700        13.0        118.0        8.0
759        12.9        119.8        6.4
913        13.0        123.9        6.0
1013        15.4        126.4        5.6
1113        13.8        130.4        5.7
1170        13.5        131.8        9.9
1313        19.3        136.6        6.4
1423        27.5        139.8        7.3
1553        22.9        143.2        8.5
1670        21.5        146.7        11.2
1913        13.3        153.6        6.7
2141        12.8        160.5        8.4
2293        19.4        165.1        7.5
2473        24.9        170.4        9.1
2571        23.5        173.3        8.8

RE: Your Hydraulics Thinking Cap On?

Go to your nearest University with a Hydro lab. Pay them to model it for you.

RE: Your Hydraulics Thinking Cap On?

You have highly supercritical flow, with velocities ranging from 29 to 35 fps and Froude no. up to 4 (not considering the "weirs".  Standard weir equations are not intended for this high velocity flow.  Using this equation without any research data to back you up would be un-wise!  Your velocity head is about 14 feet, which translates to a 14 feet high standing wave that will jump right out of the channel when (and if) you get a hydraulic jump over these weir blocks.  

Dicksewerrat is correct, have this modeled.  Or try another approach.

Recommend: Colorado State University, Engineering Research Center
Contact Dr. Steven Abt

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