Response to some E-Mail Questions I Received

[Guest]

I received an e-mail asking some general types of WR questions. I thought I would post here for everyone's benefit.

1. Water Hammer problem, there is two equations to calculate time. T=L/a and T=2L/a what is the difference? Which one applies to time before water hammer happens ??

Immediately following valve closure (e.g. sudden closure) the fluid in the valve is brough to a rest causing a local pressure. The compression of the liquid and the expansion of the pipe wall incrementally work themselves upstream in the form of a pressure wave. The time taken by the wave front to reach a point of pressure relief (typically denoted as a resevoir) is T = L/a.
Once the wave front reaches the pressure relief point (resevoir), the liquid in the pipe decompresses and the pipe material contracts similuating a reflected wave back towards the closed valve. The time for the REFLECTED wave is also T = L/A.

However, if you look at the TOTAL time it takes for the pressure wave to propagate in the pipe (e.g. wave towards the resevoir and reflected back towards the closed valve) this time is denoted as the water-hammer period and is expressed as T = 2L/c.

2.There are two forumlas for Spillway Q values. One is like this CERM pp 19-13, eq 19.58 Q=3/2 * C1*b * SQRT(2g) *H ^3/2 and the other one is on the same page and eq 19.60 Q = Cs * b* H ^ 3/2. I have seen these two forumals used for Spillway. What is the difference?

The primary difference I can see is the coefficient, C --> Cs appears to incorporate the SQRT(2g) somehow. The best way to approach this problem is to stick with the first equation UNLESS you are asked to use the HORTON EQUATION with an insignificant velocity approach. If you are asked a question on the PE Exam, typically you will be given enough information to determine which weir/spillway equation will be needed as well as the coefficient that will be needed.

I hope this helps.

JR