12 GEOLOGICAL SURVEY OF ALABAMA. 



DISCHARGE MEASUREMENTS. 



These records show the date, the gage height at time of 

 measurement, and the amount of water in cubic feet per sec- 

 ond, or "second-feet," that is found by the measurement to 

 be flowing in the river. (Second-feet means the same as 

 cubic feet per second.) If we imagine a small stream filling 

 a rectangular flue I foot wide and i foot deep, we have a 

 stream whose sectional area is i square foot. The volume 

 of this stream will vary in proportion to the speed with 

 which the water flows through the flume. If the water i's 

 moving at a velocity of i foot per second, the flow! or volume 

 of water is i cubic foot per second, and would fill a vessel 

 5 feet wide, 5 feet long, and 4 feet deep in just 100 seconds, 

 as such a vessel would hold 100 cubic feet of water. If 

 the water in the flume i foot wide and i foot deep flows with 

 a velocity of 2 feet per second, the volume will be 2 cubic 

 feet per second, or 2 second-feet, and so on for any other ve- 

 locity. In the same way if the flume is 20 feet wide, and 5 

 feet deep, its sectional area will be 100 square feet, and if 

 the average velocity is 3 feet per second, the volume will be 

 300 cubic feet per second, or 300 second-feet. In each of 

 the discharge measurements here enumerated, a cross-sec- 

 tion of the stream is measured, and velocities taken with an 

 electric current-meter at many points of the cross-section. 

 Instead of multiplying the entire cross-section by an aver- 

 age velocity, the area was divided up into a large number of 

 small sections by soundings from 5 to 10 feet apart, and the 

 area of each of the small sections multiplied by the velocity 

 at the small section, thus giving the second-feet flowing in 

 each small section. The sum of the discharges of all the 

 small sections makes the total discharge of the stream. 



RATING TABLE. 



This is a table showing the discharges in second-feet 

 (cubic feet per second) for all stages of water on the gage. 

 Hence when the gage heights are known, the corresponding 

 discharges can be taken from the rating table and written 

 opposite each daily gage height, thus giving the flow in 

 cubic feet per second on each day in the entire year. 



