The quantity of silt present at various depths can be found by pumping specimens of water through pipes. At each change of depth the pipe, delivery hose, etc., should be cleaned. Allowance must be made for the velocity of ascent of the water up the pipe. Suppose this to be 1·4 feet per second. Then the velocity of sand of class (·2) would be 1·2 feet per second, and the quantity of sand actually found in the water would have to be increased by one-sixth.
5. Quantity and Distribution of Silt.—The quantity of silt present in water varies enormously. Fine mud, even though sufficient to discolour the water, may be so small in volume that it only deposits when the water is still, and even then deposits slowly. In the river Tay, near Perth, the silt was found to be ordinarily 1/10,000 of the volume of water, and at low water only 1/28,000. In the river Sutlej at Rupar, near where it issues from the Himalayas, the silt in the flood season is extremely heavy. Out of 360 observations, made at various depths, during the flood seasons of four successive years, in water whose depth ranged up to 12 feet, the silt was once found to be 2·1 per cent. by weight of that of the water. It was more than 1·2 per cent. on four occasions, and more than 0·3 per cent. (or 3 in 1000) on sixty-four occasions. Generally about one-half of the silt was clay and sand of classes finer than (·10), about one-third was sand of class ·1/·2, and the residue was sand of class ·2/·3. The sand of the river Chenab is generally coarser than that of the Sutlej. There are very great differences in the degree of coarseness of river sand. The sand in any river becomes finer and finer as the gradient flattens in approaching the sea. Sea sand has been found to be of class (·20). In the Sirhind Canal, which takes out from the Sutlej at Rupar, the maximum quantity of suspended silt observed in the four flood seasons was 0·7 per cent., on one occasion out of 270, and it exceeded 0·3 per cent. on twenty-five occasions. About 80 per cent. of the silt was clay.
In another part of the paper quoted, it is stated that the silt suspended in the canal water averaged, during the whole of one flood season, about 1/1700 of the volume of the water. This would be about 1/1200 by weight. The silt deposited in the bed of the canal, in a period of a few days, was sometimes as much as 1/1000 of the water which had passed along, and occasionally as much as 1/500. It was nearly all sand, only about 3 per cent. being clay. Silt of classes finer than (·1) gave no trouble, and were to be eliminated in future investigations. In a canal, as in a river, the sand on the bed becomes finer the further from the head.
Regarding the distribution of the silt at various depths, in water 5 to 17 feet deep, the quantity of silt near the bed may, when the charge is heavy and consists of mixed silt, be 1¼ to 3 times that at the surface. If the charge is fine mud, there is likely to be as much silt at the surface as near the bed, if sand, there may be none at the surface and little in the upper part of the stream.
In all cases single observations are likely to show extraordinarily discordant results; a number of observations must be made at each point and averaged.
6. Practical Formulæ and Figures.—A stream which carries silt generally rolls materials along its bed. The proportion between the quantities of material rolled and carried is never known, and this makes it impossible to frame an exact formula applicable to such cases, but Kennedy, from his observations on canals fully charged with the heavy silt and fine sand usually found in Indian rivers near the hills, arrived at the empirical formula for critical velocities V = ·84 D·64
The observations were made on the Bari Doab Canal and its branches, the widths of the channels varying from 8 feet to 91 feet, and the depths of water from 2·3 feet to 7·3 feet. The beds of these channels have, in the course of years, adjusted themselves by silting or scouring, so that there is a state of permanent régime, each stream carrying its full charge of silt, and the charges in all being about equal. From further observations referred to above (Art. 3, par. 2) it appears that this kind of silt forms about 1/3300 of the volume of the water, and that on the Sirhind Canal, sand coarser than the (·10) class, formed 1/35,000 of the volume of water.
The formula gives the following critical velocities for various depths:—
| D = | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 |
| V0 = | ·84 | 1·30 | 1·70 | 2·04 | 2·35 | 2·64 | 2·92 | 3·18 | 3·43 | 3·67. |
In Indian rivers not near the hills the silt carried is not so heavy, and the critical velocities are supposed to be about three-fourths of the above. Thrupp (Min. Proc. Inst. C.E., vol. clxxi.) gives the following ranges of velocities as those which will enable streams to carry different kinds of silt. It does not appear that the streams would be fully charged except at the higher figure given for each case.
| D = 1·0 | 10·0 |
| V = 1·5 to 2·3 | 3·5 to 4·5 (Coarse sand). |
| V = ·95 to 1·5 | 2·3 to 3·5 (Heavy silt and fine sand). |
| V = ·45 to ·95 | 1·2 to 2·3 (Fine silt). |