To calculate the flow of water through an orifice we need to know things: the velocity of the water and the cross-sectional area of water. To make this calculation we have to assume that the orifice is a perfect cylinder.
Measure the length of the orifice with a tape measure. Record the distance in meters (1 foot equals 0.304 meters) on a piece of paper.
Drop an object (a rubber ball will work) at the top of the orifice and record the number of seconds the object takes to reach the end of the orifice.
Calculate velocity by dividing the length of the orifice by the number of seconds the object took to reach the bottom of the orifice. So, for a length of 20 meters and a time of 100 seconds, the velocity should be 0.20 meters/seconds.
Find the cross-sectional area of the orifice, next. First, you need to calculate several variables. Measure the radius, which is the distance between the center point and edge of the orifice. Record this distance in meters, and label as "r" on a piece of paper. Next measure the distance from the top of the orifice to the top of the water (the area of empty space). Record in meters, and label as "x". Measure the width of the surface of the water in the orifice, and record as "z" in meters.
To calculate area you will need to you the following formula: ((pi · r2) + (r-x) (2r+z))/2) where pi=3.14
So, if r=100; x=60; z=185 (all measured in meters); then, the area equals 2.34 meters-squared (m^2)
Measure flow rate by multiplying velocity by area. In using the above examples, the flow would be 0.20m/s by 2.34m^2 to equal 0.47m^3/s