FLOW OF WATER IN DREDGED DRAINAGE DITCHES. 5 
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 O 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 
ef 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 #, 12 G, and 12 H. 
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 in the channel. The cross section was 
very broad and shallow as compared with that for course A (fig. 11 
EH). There was considerable vegetation, covering the entire perim- 
