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OPEN CHANNEL MEASUREMENTS. 



The flow of water in open channels may be measured by four 

 different methods : 



Velocity-Area Method. 



The velocity-area method involves the measurement of the 

 cross-sectional area at any point in the channel and the mean 

 velocity at this point with a current meter. This method involves 

 a fairly permanent cross section, a record of surface fluctuations 

 and sufficient measurements at varying stages of the flow to 

 determine a rating curve and table. 



This method is used exclusively by the U. S. Geological Sur- 

 vey and the Division of Hydrography in rating streams and 

 ditches where prohibitive cost, lack of head room, or other rea- 

 sons, prevents the installation of weirs. The rating of a stream 

 or ditch by this method also requires considerable period of time 

 in order that measurements may be made at sufficient varying 

 stages of from extreme low water to high floods to definitely fix 

 the rating. 



Under favorable conditions results are obtained under the 

 velocity-area methods which have a limiting error of five per cent 

 or less. When conditions are poor the possibility of error may 

 reach as high as 20 or 25 per cent. This method is explained in 

 detail in the water supply reports of the U. S. Geological Survey. 



Slope Measurements. 



The flow in an open channel, where the longitudinal slopes of 

 the bottom and water surfaces, the dimensions of the channel and 

 the condition of roughness., vegetation, etc., of the channel are 

 known, can also be obtained by using one of several formulas, 

 the most popular of which is that derived by Kutter and Ganguil- 

 let, commonly known as Kutter's formula. These formulas, 

 which may be obtained from any hydraulic text book, are used 

 exclusively in designing canals and ditches, and have been trans- 

 formed into handy diagrams and tables from which results may 

 be obtained without the necessity of working out the formula. 



Weirs. 



Unquestionably a weir properly constructed and of a type for 

 which accurate coefficients have been determined, is one of the 

 most convenient and reliable means of measuring water in ditches 

 and small streams. In practice, however, weirs rarely conform 

 to the requirements imposed by the experimenters who derived 

 the coefficient. This is especially true in Hawaii, and observa- 

 tions made on various types of weirs in Hawaii show that many 

 weirs are not giving accurate results and that the erroneous rec- 



