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



recently constructed channel and that the bottom slope and cross- 

 sectional area are more uniform throughout the course than for Old 

 Town Creek. 



TWENTY-MILE CREEK. 



The slope course on Twenty-Mile Creek was rather short, being 324 

 feet in length, and was located below the highway bridge 1 mile east 

 of Baldwyn, Miss, (see PI. Ill, fig. 2; and figs. 1 C, and 2 0). The 

 lower part of the channel was quite smooth and regular, but the upper 

 part and edge of bank were irregular. The channel is eroding rapidly, 

 and many stumps along the banks have been undermined and fallen 

 into the channel. Table 1 shows the hydraulic elements and the 

 values of n obtained for Twenty-Mile Creek. The value of n increases 

 as the stage increases, due no doubt to the irregularities in the wetted 

 perimeter for the higher stages. The soil in the channel is a waxy 

 clay loam, and in some parts of the channel contains considerable 

 sand. 



CHAWAPPAH CREEK. 



Slope measurements for Chawappah Creek were made on a rather 

 short course (320 feet long) between the highway and railroad bridge, 

 one-half mile south of Shannon, Miss. The gauging section was located 

 at the highway bridge. The view of Chawappah Creek for low water 

 (PI. IV, fig. 1), shows very well the condition of the channel. The 

 soil varies from a sandy loam to a waxy clay, and the sides and bottom 

 were quite hard. The channel is eroding very rapidly, which is 

 partly the cause of the turbulent water surface and the resulting 

 comparatively high values of n. The average value obtained for n 

 was about 0.035 (see figs. 1 D, and 2 D). 



COONEWAH CREEK. 



Only one observation for slope was made on Coonewah Creek, 

 The value computed for n was 0.0430. Plate IV, figure 2, shows the 

 course on which the slope was measured and the condition of the chan- 

 nel. The soil is quite similar to that found on Chawappah Creek, 

 but erosion has not been as active as on the latter (see figs. 1 E, and 

 2 E). 



DISCUSSION OF LEE COUNTY EXPERIMENTS. 



Of the experiments made in Lee County, Miss., the results obtained 

 from those on Old Town and Mud Creeks are the most reliable. It 

 appears that for conditions of flow the value of n for channels similar 

 to Mud Creek is 0.025, which means that the bottom and sides should 

 be fairly regular and free from any form of obstruction to flow, and 

 the slope and cross-sectional area practically uniform. These con- 

 ditions general!} 7- obtain only in new ditches, and it would therefore 

 not be wise to use a coefficient as low as 0.025 in the design of chan- 

 nels, since the efficiency of a channel generally decreases with age, as 



