12 BULLETIN 832, U. S. DEPARTMENT OF AGRICULTURE. 



OLD TOWN CREEK. 



Measurements were made on this channel early in 1913 and in 1914. 

 A straight course of 1,224 feet below the steel highway bridge, one- 

 half mile east of Tupelo, Miss., was selected for making the measure- 

 ments. The discharge measurements for 1913 were made from the 

 highway bridge and those for 1914 from a suspension footbridge 

 constructed near the upper end of the course. The latter gaging 

 section was much more desirable for accurate measurements than the 

 former one. 



An experiment for determining the effect upon the flow in the 

 channel, due to the clearing of brush and other resistances to flow, 

 was conducted on Old Town Creek. No clearing was done for the 

 measurements during 1913. One side slope of the channel and part 

 of the bottom were practically covered with small saplings, brush, 

 and cane, and were quite irregular. The other side slope was com- 

 paratively smooth and uniform. For the measurements during 1914 

 all brush, logs, and other obstructions were cleared from the course 

 of the channel and for 500 feet above the upper end and for the same 

 distance below the lower end of the slope course, so that a comparison 

 could be made as to the relative values of n before and after clearing. 

 The slope of the left bank was quite regular while that of the righ 

 bank was extremely irregular, a condition due to the growth of brush 

 causing the bank to erode unevenly. The soil in the bottom and sides 

 of the ditch was quite hard. The views shown in Plate II give a good 

 idea of the conditions existing in the channel before and after clearing. 

 (See also fig. 1 A, for average cross section of the channel.) 



In Table 1, measurements 1 to 21, are given the values obtained for 

 n, together with the various observed hydraulic elements. It is 

 obvious, from the values of n obtained before and after clearing, that 

 the efficiency of a channel is greatly decreased by permitting the 

 growth of vegetation in it. 



During low water the uniform fall of the surface of the water 

 was interrupted by irregularities in the bottom and sides of the chan- 

 nel. For such conditions the hydraulic grade throughout the section 

 consisted of a series of comparatively steep slopes followed by flatter 

 ones, due to the depth of water being so small that the irregularities 

 in the channel gave rise to quite appreciable variations in area of 

 cross section from point to point, thus causing variations in velocity 

 and in the hydraulic grade (see fig. 2 A). As a result the values of n 

 obtained for low water conditions are high. The total loss of head 

 throughout the course under such conditions is the sum of the loss 

 due to the roughness of the wetted perimeter and the quite appreciable 

 losses in shock and eddies that occur where the hydraulic gradient 

 changes from a steeper to a flatter slope and where the cross section 



