Tabular statement of falls and shoals in Dan River, Virginia, &c.—Continued. In the two estimates submitted below, the same method of improve ment is assumed as the basis of calculation. Where the river is divided by islands all but one of the channels will be closed by dams. Where it is necessary to back the water, spur dams will be built, projecting from opposite sides of the river, and at other points the same arrangement of dams will be employed to concentrate the current for the purpose of scouring out a channel through the bars composed of gravel and sand. The dams employed to back the water will be arranged in pairs, with a channel-way between them of about 60 feet, which will be increased or diminished so as to give 3 inches of rise at each dam. The interval between successive pairs of dams will be about 300 feet, and, as the rise at each dam will be 3 inches, the resulting slope will be 4.4 feet per mile. The velocity where the depth of water is 3 feet will be 3.22 miles per hour. The excavation will generally be made with a width of 35 feet and a slope of 10 feet to the mile, giving a resulting velocity of 4.6 miles per hour where the river is 3 feet deep. These velocities will increase with the rise of the water until the rapids begin to lose themselves in the average slope of the river. From an examination of the profile it would appear that the maximum velocity would be attained when the flood has reached a height of about 5 or 6 feet above low-water, making the depth from 7 to 8 feet. The maximum velocity in the channel between the dams would then be 6.17 miles per hour, and in the excavated channel it would be 7.46 miles per hour. As these velocities are obtained by the application of formulæ, it would be safe to assume that in some localities the velocities would be greater and might even reach 8 or 9 miles per hour. It is proposed to overcome this resistance by means of a chain anchored at the head of the rapids and buoyed at the foot. Its use is simple. At the foot of the rapid the chain is taken up, passed two or three times around a steam capstan on the bow of the boat, and is paid off as the boat advances upstream. The longest slope to be overcome in this way will be 3,500 feet. Estimate No. 1 proposes to give 24 feet at low-water. |