STANDARD STEP METHOD 97 



from field notes of measurements of cross sections of the channel. In 

 many natural channels there are bays or inlets where the water is 

 quiescent or eddying. If these are included erroneous results will be 

 obtained because their area is not effective in conducting the flow. 

 They should not be included in the values of area and hydraulic radius 

 used in the computations. 



The reaches should be short if accuracy is desired, and the sections 

 marking the ends of the reaches should be located at points where the 

 cross-sectional area begins to increase or decrease, where the roughness 

 changes, or where changes of bottom slope occur. It is well to have a 

 man experienced in hydraulic work in charge of the field party; other- 

 wise the length of reach may be dictated by positions where cross- 

 sectioning or sounding is most convenient. 



The volume of data necessary to determine the variation of area and 

 hydraulic radius at all of the sections is so great that it becomes impor- 

 tant to plan the work of reduction of field notes carefully, and to repre- 

 sent the final results in the form most convenient for the step computa- 

 tions. The obvious method of reduction of the field notes is to plot the 

 data to an undistorted scale, planimeter the areas, and measure the 

 wetted perimeters. It is worth while, however, to introduce certain 

 simplifications developed by J. C. Stevens.^ The area at low water is 

 planimetered, and the increments of area to be added to obtain the area 

 at successively higher stages are computed from widths scaled off at 

 appropriate elevations. For the wetted perimeter at low stage, an 

 approximate formula is used. Let w represent the horizontal distance 

 between two adjacent soundings, Di and D2 the consecutive depths, 

 and p the corresponding distance along the bottom. If there is no 

 break in the slope of the bottom between the two soundings 



p^ -w^ = {D2 - Di? = {Wf [901] 



Factoring out ^ -f w and solving for p 



{bDf 



p = w -\- 



^ -\- w 



For the comparatively flat slopes of natural channels, p -\- w may be 

 replaced by 2w without introducing appreciable error. 



p = w + ^—^ [902] 



2w 



^ " Computing Backwater Curves for Surface Slopes in Streams," J. C. Stevens, 

 Engineering News-Record, Oct. 1, 1925, v. 95, p. 550. 



