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the Government, and has an embankment half a mile long, with sufficient space at one end for the escape of all ordinary floods, is in a catchment area that extends only some five miles in length above the embankment. The bank is about 20 feet high. The flood rose until it poured over the whole length of this embankment, and when I afterwards visited it there were several small logs and one large one stranded across its crest, left there by the water on their way over its top. Of course a deep breach was made at a place where this bank at last gave way.
All ordinary precautions against floods must be unavailing when such an outburst as this occurs. The design of the Vavunik-kulam scheme cannot be considered defective if it failed to meet such a contingency. The tank may have been in good working condition for many centuries before the former breaching of the bank took place, and many more may have passed before its final destruction.
The designer did, in reality, take quite unusual steps to ensure the safety of the reservoir. Though the crest of the bank is only ten or twelve feet wide in the better sections, the up-stream side slopes at the rate of 4-8 feet horizontal to I foot vertical, and the outer one at the rate of 4-6 feet horizontal to I foot vertical. It may be doubted if there are more than three or four other reservoirs in Ceylon with such flat slopes in their embankments. The whole bank is made of good material, and the side adjoining the water is protected up to the ordinary water-level by a layer of small boulders. Under ordinary conditions the work might have survived intact to the present day; but the person responsible for the design could not be aware-as, in fact, no one in the island knew twelve years ago-that this part of Ceylon is liable to experience such frightful rain-storms as that which I have just described-which was perhaps the heaviest that has visited the modern world.
The depth of water retained in the reservoir between the sills of the sluices and the level of the flood-escapes was about 18 feet, and the crest of the bank was 8 feet higher.
Only two sluices were found at this work, one being near each
side of the valley. They consisted, as usual, of a rectangular well, and a rectangular stone culvert which passed under the embankment. I did not see them; they were lost in the thorny jungle which enveloped the whole bank, and their sites were unknown when I visited the place. Their wells measured about 10 feet by 15 feet in plan, and were built of brickwork. According to the drawings, the northern one was 80 feet and the southern one 140 feet from the centre of the bank, these being distances that are far greater than those at other similar embankments, in which the well is usually placed near the point where the water-level meets the up-stream slope of the earthwork. This variation from later practice indicates the early date of the works.
On account of the dimensions of the bricks used in one of its sluices, another reservoir in the Northern Province, now called Pāvat-kulam, twenty-eight miles south of the last, also appears to be a work of either the third century B.C. or the following century. As its original name is unknown it cannot be traced in the histories, even if it is mentioned in them.
This reservoir was made at the junction of two streams which flow westward through the district to the south of Vavuniya, the total length of the catchment area being about 16 miles; the average rainfall amounts to a little more than 50 inches per annum.
Evidently the valley had been well explored before the position of the embankment was decided upon. Advantage was taken of the presence of a long and high rocky ridge which projected into the valley from the northern side, and the embankment was run in a south-south-westerly direction from its end to a continuation of it two miles away, on the opposite side of the valley, meeting on the way two high rocky detached portions of the ridge. There are thus three separate banks which fill up the gaps left in this rocky ridge. The total length from end to end is 9700 feet, or 15 miles, of which the artificial bank occupies about a mile and a half. The
122. Plan and Section of S Low level Sluice 1/480
FIGS. 120-125. Pāvat-kulam.
125. Part of Bridge over Waste-water Channel.
site was undoubtedly the best one in the whole valley for the formation of a storage reservoir.
The tank held a depth of 18 feet of water above the sill of the lowest sluice, up to the permanent level of the flood-escape; its area was 2029 acres and its capacity 779 million cubic feet. The scale of the work therefore resembled that of Vavunikkulam, but the quantity of earthwork in the bank was much less than at that reservoir. The quality of the soil in the irrigable tract is not so good as at the northern work, and as some difficulty was experienced in providing sufficient space for the passage of floods it is probable that Vavunik-kuļam would be the first to be selected for construction. The sole object of the work was the storage of water for the irrigation of rice fields.
The embankment has a total height of 28 feet in the deeper part of the reservoir; its crest was 8 feet above the permanent water-level. The top is usually about 10 feet wide, but on many sections (at which it may have been worn down) it is from 15 to 25 feet in width. The side-slope on the up-stream face is about 3-2 feet horizontal to I foot vertical, and on the outer face 2-6 feet to 1. The engineers were evidently beginning to recognise that it was unnecessary to give the outer face as flat a slope as that of the inner one. The inner slope was protected as usual by a layer of small boulders and wedged rubble stones, extending downwards from the water level. The top has been generally worn down three or four feet below its original level.
In order to allow the passage of floods three places were left open. Owing to the steep ends of the rocky ridges, the designer found it a difficult matter to provide sufficient space for this purpose, and as a matter of fact he must have underestimated the requisite extent; I calculated that with a probable flood of 11,500 cubic feet per second the water would rise within two feet of the crest of the embankment. In such a long bank the settlement or gradual wearing down of the top of the bank to this extent in some places might escape notice, and the result was that the embankment was breached two or three times. The sites of three repaired breaches are
visible, one of them being of large size, and at the present day there are two deep ones through which the rivers flow.
At the southern end of the bank there is a flood-escape 125 feet wide in the line of the bank. Its floor and the ends of the embankment at it are covered with large wedged slabs of stone, carefully laid, those on the floor occupying a transverse breadth of 60 feet. This may be a work of later date than the construction of the reservoir.
This 'waste-weir' is provided with a series of pairs of stone pillars, irregular in size and shape, a short one about two feet high being in front of a taller one five or six feet high, and a few inches distant from it. By the aid of these, a temporary dam of sticks and earth could be raised across the waste-weir after the floods had passed, so as to retain an extra depth of two feet of water. This would increase the area of the reservoir to 2400 acres and its capacity to 972 million cubic feet. A road-bridge of stone slabs laid on stone pillars enabled the stream from this flood-escape to be crossed when a considerable volume of water was passing down it (Fig. 125).
The other flood-escapes were simple overflow channels at rocky sites, one being 25 feet and the other 100 feet wide. At the former the ends of the bank are protected by squared stones laid in steps from the floor upwards.
The extent to which the reservoir was utilised may be gauged by its being provided with four sluices, in addition to a high level culvert under the floor of the southern wasteweir. One of these was a high-level sluice near the northern end of the embankment; the others were much lower, one being in the northern bank, another in the southern section, while the remains of the inlet of the third one can be seen near the middle of the work.
The southern sluice was the lowest, and was 18 feet 2 inches below the level of the waste-weir; the northern lowlevel sluice was I foot 8 inches higher; the northern highlevel sluice was 5 feet 1 inches above the lowest one; and the culvert at the waste-weir was 9 feet 9 inches above it. The sizes of the wells at these works were as follows, the longer dimension being the measurement parallel to the line