2 INTRODUCTION 



pletely the section. The only other measure that is generally used is 

 the hydraulic radius, which is equal to the quotient of the area divided 

 by the wetted perimeter. It has the dimension of a depth, and in very 

 wide channels becomes nearly equal to the average depth. The hydrau- 

 lic radius is not an entirely sa^^isfactory measure, as is illustrated by the 

 common procedure of dividing a flooded river valley into parts, and 

 computing the hydraulic radii of the main channel and overbank 

 portions separately. 



The water surface in a channel is level transversely, if oscillations due 

 to small waves are averaged out, except where there is a bend in the 

 channel, or where there are local disturbances caused by a change in 

 cross section. The water surface usually has a grade downward in the 

 direction of flow. 



Uniform Flow in a 

 Uniforni Channel 



Unsteady Flow 

 (Traveling Wave> 



Non- uniform Flow in 

 an Irregular Channel 



Non -uniform Flow in a 

 Uniform Channel (With Dam) 



Fig. 101. Examples of the Different Types of Flow. 



When the grade of the water surface in a uniform channel is the same 

 as that of the bottom of the channel, there is said to be uniform flow. 

 When the water surface is not parallel to the bottom, the flow is said to 

 be varied flow. If the water surface elevation at every section remains 

 the same, that is, does not change with respect to time, the flow is steady. 

 The flow is unsteady when stages rise or fall, as during the passage of a 

 flood wave. Varied flow may be steady or unsteady, but uniform flow is 

 necessarily steady, in an open channel. 



The vertical distance from the bottom of the channel to the water 

 surface is the depth of flow. If the channel is uniform, the grade line 

 of the bottom of the channel forms the most convenient base line from 

 which to refer vertical measurements. If the channel is not uniform, 

 and the profile of its bottom is not a straight line, it is usually preferable 

 to refer all vertical measurements to a horizontal datum. 



The volume of water passing a given section per unit of time is called 

 the discharge. If the flow is steady the discharge is the same at all 



