n 

 where V = mean flow velocity in culvert (m/s) 

 n = Manning roughness coefficient 

 R = hydraulic radius (m) 

 S = slope of culvert invert (m/m) 



Approximate values of roughness coefficient are listed below: 



smooth lined culverts n = 0.012 



corregated metal culverts n = 0.024 



culverts part iaily filledwith grave Is and cobbles n = 0.036 



Estimates of the hydraulic radius of culverts can be obtained from Figure 

 H-l. A nomograph for solving Manning's equation and an example problem are 

 given in Figure H-2. 



DISCHARGE MEASUREMENTS 



Standard Measurement Technique 



The U. S. Geological Survey has developed a technique for measuring 

 the discharge in a river (Buchanan and Somers, 1969). A relatively straight 

 and uniform reach of river should be selected for taking discharge measure- 

 ments. The width of the channel (s) should be divided into a number of sub- 

 sections (25 or more are recommended) that are often, but do not have to 

 be, the same width (Figure H-3 ) . Velocities are measured at each of the ob- 

 servation points at one or more depths depending on the flow depth, desired 

 accuracy, and rate of change of the flow. Generally speaking, the accuracy 

 of the mean velocity increases with increasing number of current measurements 

 at one observation point. An exception to this is when the flow is changing 

 rapidly, thus requiring that the discharge measurements be completed in a 

 short time span. Equations for calculating mean velocity are given in Figure 

 H-3 for three common measurement techniques. Discharge in each subsection 

 is the product of the mean velocity in the subsection and the cross-sectional 



149 



