<ir{kj 



d.-l^y'V^Y" (7-127) 



w = armor unit weight for uniform stone 



W = Wp_ . for riprap 



50 mtn ^ '^ 



The maximum velocity of tidal currents in midchannel through a navigation 

 opening as given by Sverdrup, Johnson, and Fleming (1942) can be approximated 

 by 



V = 3^ (7-128) 



where 



V = maximum velocity at center of opening 



T = period of tide 



A = surface area of harbor 



S = cross section area of openings 



h = tidal range 



The current velocity at the sides of the channel is about two-thirds the 

 velocity at midchannel; therefore, the velocity against the revetments at the 

 sides can be approximated by 



V =-^^ (7-129) 



9 3TS ^ ^ 



If no prototype or model current velocities are available, this velocity 

 can be used as an approximation of V and to calculate the local boundary 

 shear. 



If the channel has a uniform cross section with identical bed and bank 

 armor materials, on a constant bottom slope over a sufficient distance to 

 produce uniform channel flow at normal depth and velocity, velocity can be 

 calculated using the procedures described in Appendix IV of EM 1110-2-1601 

 (Office, Chief of Engineers, U.S. Army, 1970), or Hydraulic Design Charts 

 available from the U.S. Army Engineer Waterways Experiment Station, Vicksburg, 

 Miss.). In tidal channels, different water surface elevations at the ends of 

 the channel are used to find the water surface elevation difference that gives 

 the maximum flow volume and flow velocity. If the conditions described above 

 hold, such that the flow if fully rough and the vertical velocity distribution 

 is logarithmic, the local boundary shear j-, is 



(7-130) 



5.75 log, 12.1 d 



'10 d 



9 



7-250 



