FLOW OF WATEK IK DREDGED DRAINAGE DITCHES. 17 



is shown by the experiments on Old Town Creek. It is believed that 

 if the ditches are kept in good order the value of n can be maintained 

 at approximately 0.030 for this section of country. If this value of 

 n is to be maintained systematic maintenance work from the time 

 the ditch is first constructed is necessary. 



EXPERIMENTS IN BOLIYAU COUNTY, MISS. 



The experiments in Bolivar County, Miss., were made on the fol- 

 lowing streams: Bogue Phalia, Bogue Hasty, Pecan Bayou, West 

 Bogue Hasty, and East Bogue Hasty. 



BOGUE PHALIA. 



The conditions for the accurate discharge and slope measurements 

 on this channel were ideal. A straight course of practically uniform 

 cross section was selected for slope measurements, about half a mile 

 above the Yazoo & Mississippi Valley Railroad bridge, 2 miles from 

 Helm. The length of the course was 1,003 feet. The gauging station 

 was located near the lower end of the course, and the velocity meas- 

 urements were made from a movable car suspended from a steel cable 

 which was stretched across the stream at right angles to the direction 

 of flow and supported by upright poles on either bank (Plate I, fig. 2). 



The left side slope of Bogue Phalia, along the course, was quite 

 regular while the right side was subject to caving and was only fairly 

 regular. The channel was very smooth for low stages and of uniform 

 section for all stages (fig. 4 A). Very little vegetation of any sort 

 was found in the channel (Plate V, fig. 1 ; and fig. 3 A) . The soil in 

 the lower part of the channel is sandy. The soil in the upper part is 

 a clay loam of close texture and is quite susceptible to caving, 

 particularly when wet. 



The slope was indeed an extremely variable quantity. No two 

 measurements of slope were found to be the same, due to the back- 

 water conditions. The greatest slope was found for the lowest stage, 

 and the others varied according to the effect of backwater at the time 

 that measurements were made. 



The principal hydraulic elements of the channel and the values of 

 G and n obtained therefrom are shown in Table 2. Little variation 

 was found in the values of G, and the values of n were found to vary 

 from 0.0223 for the lowest stage to 0.0313 for the next to the highest 

 stage. As would be expected, the values of n were found to be low 

 .for the lower stages, where the channel was quite smooth and uniform 

 in section, and higher for the higher stages, where a greater resistance 

 was offered to flow, by the rougher and more irregular sides of the 

 channel. On the whole, it can be said that this channel was in excel- 

 lent shape, a fact which is substantiated by the comparatively low 

 values of n obtained. 



146908°— 20— Bull. 832 2 



