BACK WATER. 



235 



126. Back Water. When a dam is built across a stream 

 so as to raise the water and form a pond, the surface of the 

 water in the pond will not be horizontal. Let AB represent 



Fig. 66 



a dam, and C the surface of the water directly over the dam. 

 If the horizontal line CD be drawn from the surface at C 

 to the point D, where it intersects the natural surface of 

 the stream, the surface of the water in the pond will be 

 everywhere above this line, except at C, its height increasing 

 as the distance from the dam increases, and this elevation 

 may extend for quite a distance up the stream above the 

 point D. 



The elevation CDFE above the horizontal CD is called 

 back water. As the stream approaches the horizontal sur- 

 face DC, its velocity is diminished, because the slope on 

 which the velocity depends is very small, and as the velocity 

 is diminished the water is heaped up above DC, even ex- 

 tending up the stream, until the slope is sufficient for the 

 water to flow off. When this slope is established, the stream 

 FEC flows smoothly along its liquid channel. 



Fig. 66 shows a longitudinal section of the river Weser, 

 in Germany, where a dam was built. The mean depth of 

 the stream was about 2.5 feet, the surface was raised 7.5 

 feet, the slope of the stream was quite uniform for a distance 



