RE: [UDFCD-support] Digest for udfcd-support@googlegroups.com - 3 Messages in 1 Topic
Andrew Earles
For a good derivation of the weir coefficient, see Chow’s Open Channel Hydraulics. The bottom line is that the equation is derived based on momentum principles, and the “C” term contains (2*g)^0.5, which gives it the units you noted since g has units of length per time squared.
Andrew
T. Andrew Earles. Ph.D., P.E., D.WRE
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- Is the weir coefficient really unitless? [3 Updates]
"Doug Trieste" <do...@flowtechnologies.biz> Dec 30 08:47AM -0700 ^
I was wondering if someone can provide me with a logical answer to the
following associated with weir coefficients? Here is my problem.
* The weir equation is Q = Cw x L x H^1.5, where Q is flow over a weir
in cubic feet per second, Cw is suppose to be unitless, L is in feet, and H
is in feet (of course SI units are also applicable).
* Taking the units associated with L x H^1.5, we get (feet) x
(feet)^1.5 = (feet)^2.5
* If Q has the units of cubic feet per second, doesn't this indicate
that Cw is NOT unitless? Instead, it appears that there are units
associated with Cw, which are (feet)^0.5 per second. Otherwise, it is not
feasible to get Q in cubic feet per second.
I, like many others in the profession, have always assumed that Cw is
unitless. However, I am wondering if there really are units associated
with Cw.
Any comments and supporting documentation will be greatly appreciated.
Thanks
Jerald S. Fifield, Ph.D., CPESC, CISEC
HydroDynamics Incorporated
P.O. Box 1327
Parker, CO 80134
Phone: 303-841-0377
Fax: 303-841-6386
E-Mail: h...@ecentral.com
www.hdi-consultants.com <http://www.hdi-consultants.com/>
Interested in:
* Designing for Effective Sediment and Erosion Control on Construction
Sites
* Field Manual on Sediment and Erosion Control Best Management
Practices for Contractors and Inspectors
Check out <http://www.foresterpress.com/>
www.foresterpress.com
"Chandan Das" <CD...@jfsato.com> Dec 30 09:03AM -0700 ^
Jerald,
You are right. Cw is not unitless but has a coefficient of ft^0.5/sec.
For a trapezoidal weir (Cipoletti weir) for example Cw is approximately
equal to 1.86 in SI units and 3.367 in U.S. units. I normally use the
CRM (Civil Engineering Reference Manual) by Lindeburg (Eleventh
Edition). See Page 19-13. Hope that helps.
Regards,
Chandan Das, Ph.D., P.E.
Water Resource Manager
J.F.Sato & Associates
5878 South Rapp Street
Littleton, Colorado 80120
Tel: 303.797.1200
Fax: 303.797.1187
________________________________
From: udfcd-...@googlegroups.com
[mailto:udfcd-...@googlegroups.com] On Behalf Of Doug Trieste
Sent: Wednesday, December 30, 2009 8:48 AM
To: udfcd-...@googlegroups.com
Cc: 'Jerald S. Fifield'
Subject: [UDFCD-support] Is the weir coefficient really unitless?
I was wondering if someone can provide me with a logical answer to the
following associated with weir coefficients? Here is my problem.
* The weir equation is Q = Cw x L x H^1.5, where Q is flow over a
weir in cubic feet per second, Cw is suppose to be unitless, L is in
feet, and H is in feet (of course SI units are also applicable).
* Taking the units associated with L x H^1.5, we get (feet) x
(feet)^1.5 = (feet)^2.5
* If Q has the units of cubic feet per second, doesn't this
indicate that Cw is NOT unitless? Instead, it appears that there are
units associated with Cw, which are (feet)^0.5 per second. Otherwise,
it is not feasible to get Q in cubic feet per second.
I, like many others in the profession, have always assumed that Cw is
unitless. However, I am wondering if there really are units associated
with Cw.
Any comments and supporting documentation will be greatly appreciated.
Thanks
Jerald S. Fifield, Ph.D., CPESC, CISEC
HydroDynamics Incorporated
P.O. Box 1327
Parker, CO 80134
Phone: 303-841-0377
Fax: 303-841-6386
E-Mail: h...@ecentral.com
www.hdi-consultants.com
<http://www.hdi-consultants.com/>
Interested in:
* Designing for Effective Sediment and Erosion Control on
Construction Sites
* Field Manual on Sediment and Erosion Control Best Management
Practices for Contractors and Inspectors
Check out www.foresterpress.com <http://www.foresterpress.com/>
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"Bill Ruzzo" <bill....@comcast.net> Dec 30 09:05AM -0700 ^
You probably can also find your answer in King & Brater "Handbook of
Hydraulics" (McGraw Hill 1963) which is one of the earliest an often cited
reference for weirs.
_____
From: udfcd-...@googlegroups.com [mailto:udfcd-...@googlegroups.com]
On Behalf Of Doug Trieste
Sent: Wednesday, December 30, 2009 8:48 AM
To: udfcd-...@googlegroups.com
Cc: 'Jerald S. Fifield'
Subject: [UDFCD-support] Is the weir coefficient really unitless?
I was wondering if someone can provide me with a logical answer to the
following associated with weir coefficients? Here is my problem.
* The weir equation is Q = Cw x L x H^1.5, where Q is flow over a weir
in cubic feet per second, Cw is suppose to be unitless, L is in feet, and H
is in feet (of course SI units are also applicable).
* Taking the units associated with L x H^1.5, we get (feet) x
(feet)^1.5 = (feet)^2.5
* If Q has the units of cubic feet per second, doesn't this indicate
that Cw is NOT unitless? Instead, it appears that there are units
associated with Cw, which are (feet)^0.5 per second. Otherwise, it is not
feasible to get Q in cubic feet per second.
I, like many others in the profession, have always assumed that Cw is
unitless. However, I am wondering if there really are units associated
with Cw.
Any comments and supporting documentation will be greatly appreciated.
Thanks
Jerald S. Fifield, Ph.D., CPESC, CISEC
HydroDynamics Incorporated
P.O. Box 1327
Parker, CO 80134
Phone: 303-841-0377
Fax: 303-841-6386
E-Mail: h...@ecentral.com
www.hdi-consultants.com <http://www.hdi-consultants.com/>
Interested in:
* Designing for Effective Sediment and Erosion Control on Construction
Sites
* Field Manual on Sediment and Erosion Control Best Management
Practices for Contractors and Inspectors
Check out <http://www.foresterpress.com/>
www.foresterpress.com
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