[Table 37] gives the discharges of the river for gauges referred to the mean low water level. Between Esna and Kena the table is in excess of the truth, and between Assiout and Beni-Suef it is slightly under. Taken all round the table is reliable, calculated from the means of hundreds of discharges and carefully prepared.
[Table 45] gives the slope of the water surface of the Nile in flood and in summer between Assuân and Cairo. Owing to the more winding track of the low supply than of the flood waters, the former is 948 kilometres and the flood 900. The slope in summer is 1⁄13000 and in high flood 1⁄12200.
The other tables need no explanation.
23. The Rosetta and Damietta Branches.
—[Plates XVII] and [XVIII] give longitudinal sections of the two branches of the Nile and their cross sections are given on [Plate XI].
During winter, summer, and low floods, regulation at the Barrage interferes with the natural discharges of the two branches. The Damietta branch is gradually silting up and decreasing in size, while the Rosetta branch scours in high floods. The mean width of the Rosetta branch is 500 metres, and the mean area of the section in flood is 4000 square metres. The mean width of the Damietta branch is 270 metres and the mean section 2700 square metres. The mean velocity of the floods range from 1.00 metre to 1.60 metres per second. In summer the branches are hermetically closed at their heads and receive only the water which filters into them from the subsoil. This in the Rosetta branch amounts to 20 cubic metres per second, and less in the Damietta branch. It may be noted here that at Cairo the girder bridge at Kasr-el-Nil is 403 metres between the abutments and the smaller bridge is 178 metres, making a total width of 581 metres. The width of the Kafr Zayat bridge on the Rosetta branch is 530 metres, while the old Benha bridge on the Damietta branch is 285 metres. The average depth of water in flood in the two branches may be taken as 7 metres.
The barrage at the head of the Rosetta branch has 61 openings of 5 metres each and one lock 15 metres wide and the other 12 metres. They are all open in high flood. The Damietta barrage has 61 openings of 5 metres and one lock of 12 metres. The depth of water in a high flood is 9 metres. The Rosetta barrage has 10 openings too few, and the Damietta barrage 15 openings too many.
Before the construction of the Barrage in the middle of the 19th century, the maximum discharges of the two branches at the head of the Delta were nearly the same. A little lower down, however, the Rosetta branch had considerably more water than the Damietta. About 2 kilometres below the Barrage there was a branch called the Shalakan branch which flowed from the Damietta into the Rosetta branch. About 20 kilometres below the Barrage, the Bahr Ferounieh took about 1⁄3 the total discharge of the Damietta branch and led it into the Rosetta branch. Both these were closed by Mehemet Ali, while at the same time the Bahrs Sirsawiah, Baguria, Shebin, Khadrawiah, Moes, Um-Salama, Bohia and Sogair were also completely closed or provided with regulating heads, which very considerably diminished their discharge. During the time that they had been open the Damietta branch had lost water at every kilometre as it approached the sea, and though 400 metres wide at the head it had a channel only 200 metres wide in its lower reaches. The Rosetta branch on the other hand received the tail waters of many Bahrs and had only one escape, the Bahr Saidi near its tail.
The closing of so many escapes on the Damietta branch has caused this branch in its upper reaches to carry so much water that its tail reaches can not carry it without having the surface of the water raised inordinately and dangerously above the level of the country. An examination of the longitudinal sections will show that while the Rosetta branch in its middle reaches is from 1.50 to 2.00 metres above the level of the country in a high flood, the Damietta branch is from 2.50 to 3.00 metres. They will also show how the slope in the early reaches of the Damietta branch is considerably less than that in the early reaches of the Rosetta branch, which results in the gradual silting up of the former as already noted. The Karanain regulator at the head of the old Bahr Shebin, taking from the Damietta branch below the Bahr Ferouniah, was built in 1842 by Linant Pasha, with its wing wall 60 centimetres higher than any previous flood. By 1870 the Damietta branch had risen 70 centimetres above the wing wall as measured by Linant Pasha. In 1878, though the Damietta branch was relieved by the Gizeh breach in the left bank of the Main Nile which drained into the Rosetta branch, the flood water surface of the Damietta branch at Karanain was 1.50 metres above the wing wall.
PLATE XVII.