24. The pressure distribution is assumed to be hydrostatic. This 

 assumption is valid if the surface curvature is small and is true of tidal 

 flows. The pressure force at Section 1 acts toward the right, and the 

 pressure force at Section 2 acts toward the left. The net pressure force acts 

 toward the left and is equal to pgA(dh/dy)hy. 



25. The gravity force is equal to the weight of the fluid inside the 

 element and is equal to pgAAy . The component of the gravity force in the 

 direction of motion is -pgAAy(Az^/Ay) . 



26. The possible shear forces consist of bottom stress due to friction 

 and eddy viscosity, and the surface stress. The shear produced by the eddy 

 viscosity is believed to be small and can, in concept, be assumed to be 

 incorporated with the term describing the bottom friction stress, which 

 requires specification of an empirical friction coefficient. The main source 

 of the surface stress is the wind. The bottom stress opposes the motion 

 (directed to the left in Figure 1) . The direction of the surface stress 

 produced by the wind depends on the wind direction with respect to the channel 

 alignment. If the bottom shear stress is designated by tj,, the shear force 

 becomes T^PAy. If the surface shear stress is designated by t^, then the 

 surface shear force would be T^BAy. 



27. Form drag results from abrupt changes in the flow area and can be 

 represented in the same manner as a shear stress . The effect of the form drag 

 is conveniently expressed as a transition head loss or an expansion- 

 contraction head loss. It is computed as the product of an empirical coeffi- 

 cient of drag and velocity head difference at the cross section where the 

 abrupt change is located. Values of the coefficient of drag are determined 

 empirically. The basic theory and procedure for describing expansion and 

 contraction losses are discussed under the heading "Minor Losses" in fluid 

 mechanics books. The transition head loss is expressed as gAS^Ay, where Sg is 

 the rate of head loss with longitudinal distance y and will be discussed in 

 more detail in the following paragraphs. 



28. The momentum inside the volvime element is pAv or pQ . The momentum 

 inflow rate to the volume element is pQv. The rate of change of convective 

 (spatial) momentum is given as, 



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