STANDARD STEP METHOD 99 



ones, but caution should be used to select values corresponding to the 

 conditions which will exist when the flow takes place, for artificial 

 channels may deteriorate rapidly, and in either natural or artificial 

 channels the capacity may vary with the season of the year because of 

 plant growth. The roughness may not be the same throughout the 

 channel, and different values of the coefficient may have to be used in 

 the different reaches, or at different depths of flow in the same reach. 



When the choice of coefficient is uncertain, that value should be 

 selected which will give results on the " safe " side. For example, if 

 the investigations are being made to find the minimum draft for naviga- 

 tion, then the value should be used which will give the lowest surface 

 curve. For the common Ml curve, this will be the smallest possible 

 value of the roughness coefficient. If, on the other hand, possible 

 damages from flooding are being investigated, a large value of the 

 roughness coefficient, at the other end of the range of uncertainty, should 

 be used. 



The best way of preparing the elevation, area, hydraulic radius, and 

 roughness data for use in the actual step computations depends upon 

 whether the roughness varies with the elevation, and upon the number 

 of backwater curves (for different discharges or starting elevations) to 

 be obtained. If the roughness does not change with the elevation, and 

 only one or two curves are to be computed, the area-elevation data and 

 hydraulic radius data for each section are plotted to an undistorted 

 scale, and the step computations started without further preliminaries. 

 If the Manning formula is to be used, the step computations may be 

 arranged in tabular form, with column headings as follows: 



(1) Section identification, such as " station 35 + 00," " River 

 Street." 



(2) Water-surface elevation at the section. A tentative value is 

 entered in this column, to be verified or rejected on the basis of the 

 computations made in the remaining columns of the table. For the 

 first line of the table, this elevation must be known or assumed. When 

 the trial value in the second line has been verified, it becomes the basis 

 for the verification of tentative values in the third line, and so on. 



(3) Cross-sectional area corresponding to the water-surface elevation 

 in column (2). 



(4) Velocity head corresponding to the given discharge and the area 

 in column (3). (Divide Qhy A so that F will be on the D scale opposite 

 the index of the C scale. Then move the indicator to 1.55 on the B 

 scale and read the velocity head on the A scale.) 



(5) Elevation of total head line. Obtained by adding the velocity 

 head entered in column (4) to the water-surface elevation of column (2). 



