FLOW OF WATER IN DREDGED DRAINAGE DITCHES. 15 



of flow changes. For low water conditions the loss in shock and eddies 

 is quite appreciable, and as the values of n were obtained by directly 

 measuring this total loss of head throughout the course these values 

 would be necessarily large. The table shows that for such conditions 

 the value of n decreases as the depth increases. Tins would be ex- 

 pected, for as the depth increases the second factor mentioned above 

 becomes comparatively smaller, and the total loss of head thus be- 

 comes to a greater extent due to roughness of wetted perimeter and 

 to a less extent due to the influence of eddies and shock. 



In measurement No. 9, Table 1, for 71=0.0475 the discharge is 60 

 second-feet; that for n = 0.030/ and the same hydraulic elements, is 



96 second-feet. — ™ — X 100= 60 per cent. That is, the capacity 



of the lower part of the channel for low water could be increased 60 

 per cent by eliminating irregularities in the bottom slope and abrupt 

 changes in cross section. As will be seen later, the bottom slope and 

 cross section of Mud Creek are practically free from irregularities, 

 and the values of n do not vary much from low to high-water flow. 



MUD CREEK. 



Measurements were made on this channel during the early part of 

 the year 1914. For the slope measurements a straight course 1,194 

 feet long was selected above the highway bridge about 1 mile east of 

 Tupelo. A suspension footbridge was constructed at a site where 

 conditions were ideal for making accurate current meter measure- 

 ment's. 



A good idea of the condition and regularity of the channel can be 

 obtained from Plate III, figure 1. Although in the same valley, the 

 soil is decidedly different from that found in Old Town Creek. This 

 part of the bottom land is made up of sediment carried from the east- 

 ern part of the Mud Creek watershed, where the soil contains consider- 

 able sand. The soil in the channel is a sandy, waxlike clay that 

 erodes very easily. This was a comparatively new channel at the 

 time of these experiments, the ditch having been finished in January, 

 1913; and, although it had eroded to some extent, it had retained its 

 original uniform slope and comparatively uniform cross-sectional 

 area (see figs. 1 B, and 2 B). 



In Table 1, measurements 22 to 34, are shown the various hydraulic 

 elements of Mud Creek as computed from field measurements, and 

 the values of n obtained. That for all stages of Mud Creek is about 

 0.025, while that obtained for the higher stages in Old Town Creek, 

 after clearing, is about 0.030. The lower value of n as obtained for 

 Mud Creek can readily be ascribed to the facts that this is a more 



1 See measurements 22-20, Table 1. 



