' ' l: R EN1 V!ETI t: s 



The vertical \> 1 rve method is valuable at a basis for 



the comparison ni .ill ,.th.-r meth,,, di-trnitiiiiiig the co- 



rth. i.-nt- to l,.- UM-d 111 ivdm nig the values obtained by other 

 mi thods to the true value, for use under new and unusual con- 

 ditions of HH\%, .ui.l f..r measurements under ice. 



i hi. In tin- t\%<> tenths-eight-tenths method, observaii -M- 

 of velocity are taken at t\N<> |...mt- located at depths of the sur- 

 face of 0.2 and 0.8 of the depth in tin- \.-riiral in whieh th.- 



Ilie.iMiremrill I" lll.lde. Tile IIHMII \e|... it\ i - t.k II .1- tin- III' .ill 



it ili.- \elneiiies at these h%<> i>"intH. This method is based on the 

 theory* that the vertical veltn it\ . urve is a parabola, as already 

 -i.ii.-.l. in \shi<h case the mean of the ordi nates at 0.2114 and 

 0.7886 depth IM-|..\N th<- surface gives the mean ordinal**. This 

 IN inathriiMtir.illx tnir 1'nr .ins p.ir.ilml.i and lir ans |M,iti..n 

 !' the thread of maximum \-ln ii\. \ -tudy of a large mini- 

 t \. itical velocity-curves shows that this holds tru<- m 

 Nature; and experience proves that this method gives more con- 

 -i-tf-nt i. -11 Us than any of the others except the vertical 

 rurvr mrthod. 



(c). In the sixth-tenth method, the observation of the 

 velocity is taken at a depth from the surface equal to 0.6 of the 

 depth of the stream. This method is also based on the theory** 

 that id.- \< itical velocity-cam is a parabola with the maximum 

 abscissa between zero and one-third of the depth, in which case 

 the- mean ordinate is between 0.58 and 0.67 of tin- drpih from 

 the surface. A study of a large number of vertical veUxit\- 

 < urve measurements shows that the mean depth of the mean 

 velocity is approximately 0.6 of the depth. This method has 

 the advantage of requiring a less number of velocity observa- 

 tions, and gives very satisfactory results, but not as good as 

 those obtained by the two-tenths-eight-tenths method. 



(d). In the sub-surface method, the mr.iMirement of 

 velocity is made at from 0.5 to 1 ft. below the surface, depend- 

 ing on the depth of the stream. The meter is held at sufficient 

 depth to be out of any surfao .ii-turbance. When this method 

 is used, the veltx-itv must be reduced by a coefficient to obtain 



See Rlrer Discharge." page M. 



See P. W. llamui. M. Am. 8oc. C. R. Enrfneerlnjc Newm January 

 11. 1008. 



