# Civil engineering formula

• Jul 27th 2006, 06:08 AM
pashah
Civil engineering formula
Civil engineers use the formula Q = 3.32LH^3/2

to find the maximum discharge that the dam (a broad-crested weir) shown in
the figure can pass before the water breaches its abutments (Standard
Handbook for Civil Engineers, 1968). In the formula Q is the discharge in
cubic feet per second, L is the length of the spillway in feet, and H is
the depth of the spillway. Find Q given that L=60 feet and H=5 feet. Find H
given that Q=3000 cubic feet per second and 70 feet.

This is what I have come up with thus far.

Q = 3.32*60*5^3/2

Q = 12450

3000 = 3.32*70H^3/2

L = 2.95555
• Jul 27th 2006, 08:52 AM
topsquark
Quote:

Originally Posted by pashah
Civil engineers use the formula Q = 3.32LH^3/2

to find the maximum discharge that the dam (a broad-crested weir) shown in
the figure can pass before the water breaches its abutments (Standard
Handbook for Civil Engineers, 1968). In the formula Q is the discharge in
cubic feet per second, L is the length of the spillway in feet, and H is
the depth of the spillway. Find Q given that L=60 feet and H=5 feet. Find H
given that Q=3000 cubic feet per second and 70 feet.

This is what I have come up with thus far.

Q = 3.32*60*5^3/2

Q = 12450

3000 = 3.32*70H^3/2

L = 2.95555

Is your formula $\displaystyle Q = 3.32LH^{3/2}$ or $\displaystyle Q = \frac{3.32LH^3}{2}$?

-Dan
• Jul 27th 2006, 01:09 PM
ThePerfectHacker
Quote:

Originally Posted by topsquark
Is your formula $\displaystyle Q = 3.32LH^{3/2}$ or $\displaystyle Q = \frac{3.32LH^3}{2}$?

-Dan

Not both of them do not make no sense to not me.

See, Q is supposed to be in cubic feet per second.
But L and H are simplfy units of length. So how can you ever get 'per second' unit? Further the left hand side is a cubic measurement of legth while the right is not. See the problem?
• Jul 27th 2006, 01:14 PM
CaptainBlack
Quote:

Originally Posted by ThePerfectHacker
Not both of them do not make no sense to not me.

See, Q is supposed to be in cubic feet per second.
But L and H are simplfy units of length. So how can you ever get 'per second' unit? Further the left hand side is a cubic measurement of legth while the right is not. See the problem?

The constant is not dimensionless.

RonL
• Jul 27th 2006, 08:50 PM
pashah
Formula
This is the formula I am using

Q = 3.32LH^3/2
• Jul 27th 2006, 09:14 PM
CaptainBlack
Quote:

Originally Posted by pashah
This is the formula I am using

Q = 3.32LH^3/2

Topsquark's question is about whether you mean:

Q = 3.32LH^(3/2)

or:

Q = 3.32L(H^3)/2.

Given the numerical example you give I guess its is the second of these.

If this is true your answers look OK to me (except for the second that should
be H=2.95555)

RonL