0) 

 c ^ 



c a; 

 a3 >. 



100- 

 90- 



80- 

 70- 

 60 

 50 

 40 

 30 

 20 

 10 

 



Dcn= Median Riprap Diameter 



■'50 



0.1 D. 



50 



0.5D, 

 Sieve Size 



'50 



-"50 



2C^ 



Figure C-3. Suggested gradation for riprap (after Karaki et al. 1974) 



Velocity and Deptti of Flow. A primary factor influencing erosion is 

 ttie local velocity of ttie flow. Direct flow measurements are recommended, 

 but these may be difficult to obtain during flood events. In the absence 

 of measured data, Neil I (1973) reconmends the local velocity against a slope 

 be taken as: 



• Two thirds of the average velocity in straight reaches. 



• Four thirds of the average velocity in severe bends. 



The shear stress on the rock riprap is proportional to the depth of flow 

 above the riprap. Thus the rock size should increase with increasing depth. 



Steepness of Slope. The stability of riprap revetment decreases with 

 increasing steepness of slope. The steepest slope on which riprap will rest 

 without flow forces is the angle of repose of the material, which is gen- 

 erally between 35 and 45 degrees. Flow against the rock will decrease the 

 angle of stability. It is recommended that slopes of 2:1 (2 horizontal to I 

 vertical) be used. Slopes steeper than 1,5:1 generally should not be used. 



121 



