The breadth of the Gódávari at the point selected for the dam is rather over 3¾ miles; but of this more than a third is occupied by three islands and the head of the central delta, which separate the river into four channels. About a mile from the Dowlaishweram (or eastern) bank of the river is the island known as the Pichika-lanka, nearly 800 yards broad, the branches flowing on either side of which are known as the Dowlaishweram and Ráli branches respectively. Next beyond the Ráli branch comes the head of the central delta, known as the Bobbarlanka, which is about 470 yards wide. Then comes a narrow channel called the Maddúr branch; next the Maddúr lanka, about 630 yards broad; and, lastly, the fourth, or Vijésvaram, branch of the river. The lengths of the sections of the dam over each of the four branches, exclusive of under-sluices and wings, were as given in the margin. It will be seen that the total length of the work was about 4,000 yards. It was intended to be 12 feet high and connected with embankments on the different islands 2,455 yards in length.
| Yards. | |
|---|---|
| Dowlaishweram branch | 1,646 |
| Ráli branch | 953 |
| Maddúr branch | 516 |
| Vijésvaram branch | 866 |
The river bed was of pure sand and the islands were thin alluvial deposits thereon, while floods upwards of 25 feet in depth swept one and a half millions of cubic feet of water past the place every second. The problem how to bring the river under the necessary control at such a site was thus no easy one.
The actual design of the dam was modified more than once; and none of the sections across the various branches is precisely similar to any other. The original plans provided for a narrow crest with a vertical drop for the water on to a cut-stone floor behind, the section being very similar to that of the Upper Anicut on the Coleroon. Before work began, however, Sir Arthur adopted a very different design with a broad crest and a long sloping apron behind it of rubble masonry covered with cut-stone. The great advantage of this was that it required much less cut-stone work, for skilled masons were exceedingly scarce. It was not adhered to universally, different modifications being introduced in each of the four sections, but the general principle of a long rough-stone apron was retained in all. This had a very serious drawback, the full effect of which its designer did not first appreciate. Water rushing down such a sloping apron sets up reverse under-currents which tend to scour holes in the river bed and so undermine the foundation of the work. It was soon found that a further extension of the apron by a long
