IlPadrino
Well-known member
Does anyone have information (flow calculations, etc.) on Ogee spillways (aka ogee-crested weirs)?
I tried googling but didn't get much.
I tried googling but didn't get much.
Ogee Spillways
- Thread starter IlPadrino
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G
Guest
I have an ISCO Handbook that provides a tabluated information for ogee spillways. It's in my office - I can update my post on Monday.
What kinda information are you looking for?
JR
What kinda information are you looking for?
JR
IlPadrino
Well-known member
I was under the impression these spillways were a type of weir, but I couldn't find any relevant equations.
G
Guest
^^^ I happened to be near my office today and stopped in to check my ISCO text. There are numerous conditions covered in that book but it did not specifically call out Ogee Spillways.
However, I also have another reference ACOE Manual EM 1110-2-1603, that does provide some dicussion and formulae for Ogee Spillways.
Take a look specifically at,
Chapter 2 - Hydraulic Theory
Chapter 3 - Spillway Crest
Essentially, you will find that the chapters indicate that the ogee crest spillway is basically a sharp-crested weir with some additional considerations/manipulations.
It isn't NAVFAC but it should still be helpful
JR
However, I also have another reference ACOE Manual EM 1110-2-1603, that does provide some dicussion and formulae for Ogee Spillways.
Take a look specifically at,
Chapter 2 - Hydraulic Theory
Chapter 3 - Spillway Crest
Essentially, you will find that the chapters indicate that the ogee crest spillway is basically a sharp-crested weir with some additional considerations/manipulations.
It isn't NAVFAC but it should still be helpful
JR
IlPadrino
Well-known member
Chapter two may be what I was looking for... I think it has an equation that should be useful for calculating flow if you knew C.
Thanks.
Thanks.
jeb6294
Well-known member
Are you just looking for the equation to figure discharge? It's in the CERM in Chapter 19, Section 17, Broad-Crested Weirs and Spillways.
Discharge equation is pretty much the same as any other weir:
Q=2/3*C[SIZE=8pt]1[/SIZE]*b*(2g)^0.5*H^1.5
The big difference is the value for C[SIZE=8pt]1[/SIZE], for an ogee spillway it is between 0.60 and 0.75.
The equation you found in the Chelapati Workbook:
Q=CbH^1.5
is also in there...the Horton equation. It's basically the same equation with a different C-value (if you do the math C-[SIZE=8pt]Horton[/SIZE] is very close to C[SIZE=8pt]1[/SIZE]*(2/3)*(2g)^0.5). While the CERM does not have the graph to get a more specific C-[SIZE=8pt]Horton[/SIZE], it does say a value of 3.97 is a good approximation.
Discharge equation is pretty much the same as any other weir:
Q=2/3*C[SIZE=8pt]1[/SIZE]*b*(2g)^0.5*H^1.5
The big difference is the value for C[SIZE=8pt]1[/SIZE], for an ogee spillway it is between 0.60 and 0.75.
The equation you found in the Chelapati Workbook:
Q=CbH^1.5
is also in there...the Horton equation. It's basically the same equation with a different C-value (if you do the math C-[SIZE=8pt]Horton[/SIZE] is very close to C[SIZE=8pt]1[/SIZE]*(2/3)*(2g)^0.5). While the CERM does not have the graph to get a more specific C-[SIZE=8pt]Horton[/SIZE], it does say a value of 3.97 is a good approximation.
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IlPadrino
Well-known member
jeb,
Thanks for the reference. Can you help me understand the difference between a weir and a spillway? I also get a little confused about contractions.
Thanks for the reference. Can you help me understand the difference between a weir and a spillway? I also get a little confused about contractions.
You are not asking me but I throw my thought, just in case.
Whenever I hear weir what comes to mind is a flow measurement device than energy releasing spillway, the later is usually constructed to get rid of excess water from a dam to avoid overtopping and flooding of the entire structure. For example, part of operation of big reservoir would include operation of spillways during rainy season when you have a lot of water coming in. Therfore it is a part of the overall sturcture that releases this extra energy (through syphon, or any type of spillway) . Also almost always you will find an energy dessipation mechanism with spillway (followed by stilling basin), almost always there is hydraulic jump. With a weir you are not going to find this big energy dessipation mechanism. It provides empounding for example for small scale irrigations and it is more of discharge control structure than energy dessipation.
Hope this helps.
IlPadrino
Well-known member
Thanks, jeb!
OK... let's see if you agree with this summary.
Weirs are structures consisting of an obstruction across an open channel usually with a specially shaped opening or notch. The weir results in an increase in the water level which is measured upstream of the structure. This increase in water level can be the desired effect, the weir can be used to calculate the flow rate as a function of the head on the weir, or the weir can be used to control the release of water. Most weirs are plates (exceptions include the broad-crested weir.)
Spillways, a subset of weirs, are structures consisting of an obstruction across an open channel or body of water that are designed to control the release of water. Spillways are usually concrete and attempt to reduce water separation by taking a form that matches the underside of the nappe.
Broad-crested weirs can function as spillways except the the form isn't designed to match the underside of the nappe (like ogee spillways).
OK... let's see if you agree with this summary.
Weirs are structures consisting of an obstruction across an open channel usually with a specially shaped opening or notch. The weir results in an increase in the water level which is measured upstream of the structure. This increase in water level can be the desired effect, the weir can be used to calculate the flow rate as a function of the head on the weir, or the weir can be used to control the release of water. Most weirs are plates (exceptions include the broad-crested weir.)
Spillways, a subset of weirs, are structures consisting of an obstruction across an open channel or body of water that are designed to control the release of water. Spillways are usually concrete and attempt to reduce water separation by taking a form that matches the underside of the nappe.
Broad-crested weirs can function as spillways except the the form isn't designed to match the underside of the nappe (like ogee spillways).
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jeb6294
Well-known member
The way we usually do these in this area:
a weir is basically a wall with a notch cut in it that is used to restrict flow to a given release rate (when we use these it is most likely as a release structure in a detention basin).
A spillway is essentially a dam (these are used around here a lot as low-head dams).
It's been a while since I took the test, but if I remember right the difference between the two shouldn't really be a factor. They both use the same equations and either way they'll have to a width of some kind. One variation you may want to be prepared for is solving this in reverse, i.e. how wide does this weir need to be to limit the discharge from this pond to XX cfs in the event of a 2-inch rain event.
a weir is basically a wall with a notch cut in it that is used to restrict flow to a given release rate (when we use these it is most likely as a release structure in a detention basin).
A spillway is essentially a dam (these are used around here a lot as low-head dams).
It's been a while since I took the test, but if I remember right the difference between the two shouldn't really be a factor. They both use the same equations and either way they'll have to a width of some kind. One variation you may want to be prepared for is solving this in reverse, i.e. how wide does this weir need to be to limit the discharge from this pond to XX cfs in the event of a 2-inch rain event.
