FLOW OF WATER IX DREDGED DRAINAGE DITCHES. 57 



downstream from the gauging station. At the time that the measure- 

 ments were made during 1916 the cross section of the channel was 

 fairly uniform, except for the lowest stage (Plate XIX, fig. 2 ; and 

 figs. 11 C and 12 E). The side slopes were rather irregular. The 

 soil in the bottom of the channel was largely clay, and on the side 

 slopes a very hard clay. There were some water weeds in the bottom 

 of the channel and a growth of grass and lilies near the water edge at 

 low stage. As would be expected, a high value of n was obtained 

 (Table 6) for the lowest stage, due to the vegetation and to the rough 

 condition and abrupt variations in cross section. The other values 

 were also rather high, due no doubt to the irregularities and vege- 

 tation in the channel. 



The channel was cleaned out and deepened with a drag-line exca- 

 vator after the 1916 measurements had been made. After this work 

 had been done, values of n were again determined for the channel 

 during 1917. The upstream end of the 1917 course was located 160 

 feet farther upstream than that of the 1916 course. The course was 

 straight and was 900 feet long. The cross section of the channel was 

 fairly uniform, there being no very abrupt variations (fig. 12 F), 

 and was in rather good condition except for irregularities in the 

 bottoms and some weeds that the excavating machine did not remove. 

 Comparing the values of n obtained during 1917 with those for 1916, 

 it is seen that much lower values were obtained after the channel was 

 cleaned out and deepened. 



LATERAL DREDGED CHANNEL NO. 2. NEAR FORT PIERCE. 



Values of n were determined for two courses of this lateral channel, 

 course A being in good condition and course B in very poor condition. 

 The gaugings were made from suspension foot bridges. A good idea 

 as to the shape, condition, and uniformity of these channels can be 

 obtained from Plate XX, and figures 11 D, 11 E, 12 G, and 12 M. 



Course A was straight and was 503 feet long; it was located at a 

 place where the channel passed through a ridge of hard clay. The 

 side slopes of the channel were probably about as they were left by 

 the excavating machine and showed no evidence of erosion or slough- 

 ing off. As may be seen from Plate XX, figure 1, there was some 

 vegetation in the channel. The rather high values of n obtained for 

 this channel were probably due to the presence of vegetation and to 

 the abrupt variations in cross section. 



Course B was straight and was 500 feet in length. This course 

 was located in one of the so-called sand ponds. The soil is sandy and 

 easily eroded. The banks have been washed in, and considerable 

 material had been deposited: hi the channel. The cross section was 

 very broad and shallow as compared with that for course A (fig. 11 

 E). There was considerable vegetation, covering the entire perim- 



