Measure the width of the stream at the "high water mark," which is the highest level achieved by flowing water. The high water mark may be significantly higher than the current water level. This measurement should be made in units of feet.
Measure the width in feet across the bottom of the stream.
Measure the height of the stream in feet from the bottom to the high-level mark.
Repeat all the previous measurements 100 feet upstream and downstream and then average the results. The average would be taken by adding the three measurements and dividing by three. As an example, if you measured the heights as 2 feet, 1.5 feet and 2.5 feet, you would add those figures and divide by three, which produces an average of 2 feet.
Add the average high-water width to the average bottom width and multiply this figure by the average height to calculate required culvert area. In the example, if the average high-water width was 3 feet and the average bottom width was 1 foot, you would add 3 feet plus 1 foot, which gives you 4 feet. You would them multiply this figure by the average height of 2 feet, which gives you a required culvert area of 8 square feet.