Re: How to accuratly measure the speed of water down a gutter

Hello,

The answer to your question is quite complex, and depends upon a number 
of factors. These factors include the viscosity of the fluid, the shape 
of the gutter, the "roughness" of the gutter material and the slope.  You 
must know either the flow rate of the water or the expected height in the 
gutter to determine the speed.

This falls into a type of engineering problem called "open channel 
flow".  It is more complex than determining the flow rate through a pipe, 
because the rate of flow in an open channel depends on the height of 
flowing volume in addition to other factors. (In pipe flow calculations, 
the pipe is considered "full", and therfore the speed and volume flow 
rate are in a fixed relationship with each other for a particular pipe 
diameter).

I have taken the easy way out by giving you the attached link.  This is 
an open channel flow calculator, and should give you reasonable results.  
Note that the "slope" requirement must be entered as the ratio of "rise" 
over "run".  If your gutter drops 2 feet in 100 feet of run, then the 
slope to enter is 2/100 = .02

This calculator will work for a trapezoidal shaped channel.  The "N" 
value that is required is a roughness factor.  A smooth gutter material 
will flow faster than a rough surface.  Click on the link shown to 
determine your "N" factor.  

The "Hydraulic Radius" is a value calculated and used in the internal 
formulas.  It is a ratio of the flowing area to the "wetted perimeter of 
the gutter.

Be careful of units that are inputed and outputted (inches, feet, cubic 
feet per second, gallons per minute, etc)and you should get your answer!

Good Luck,

Jay Shapiro



 http://www.geocities.com/Eureka/Concourse/3075/channelcalc.html