4. =The Slopes and velocities of the Nile in its different reaches.=--Table 2 of Appendix B and Plate II comprise all the information available under this head which I have been able to collect. For the slopes I have adopted the following data:
R. L. of Lake Victoria 1129 metres above mean sea „ Fowera 1060 „ „ „ Lake Albert 680 „ „ „ Khartoum (flood) 389 „ „
From Khartoum to Wady Halfa I have adopted the generally accepted levels of the original Soudan railway survey. From Wady Halfa to the sea I have levelled myself. Upstream and downstream from the adopted levels I have carried the levels by the aid of slopes calculated from velocity and hydraulic mean depth data. It seems to me absurd to adopt a level for Lake Choga 50 metres above that for Fowera, and then to add, that in the 140 kilometres between the two places the Victoria Nile has a gentle slope, wide bed and gentle velocity. By a strange fatality, this very error has crept into the figures under Lake Choga on Plate II. The error is noted in the corrigenda attached to the Plate. The Section is drawn correctly but these wrong figures have been interpolated by an oversight.
The Victoria Nile Falls 450 metres in 400 kilometres, but has four reaches; the first ¹⁄₁₂₀₀, the second ¹⁄₂₀₀₀₀, the third ¹⁄₁₈₀ past the Murchison Falls, and the fourth ¹⁄₁₀₀₀₀.
The Albert Nile falls 277 metres in 1290 kilometres. The first reach past Wadelai has a slope of ¹⁄₂₅₀₀₀, the second over the Fola and following cataracts has a slope of ¹⁄₇₀₀, the third ¹⁄₁₂₀₀₀, the fourth ¹⁄₂₀₀₀₀, the fifth ¹⁄₂₅₀₀₀, and the last below lake No of ¹⁄₇₅₀₀₀ in flood.
Erase the two figures under Lake Choga in the “Height” column.
THE BLUE NILE, THE WHITE NILE, THE ALBERT NILE, THE VICTORIA NILE & THE SEMLIKI RIVER
The White Nile falls 14 metres in 840 kilometres and has two slopes in flood; ¹⁄₅₀₀₀₀ in its upper reach, and then ¹⁄₁₀₀₀₀₀.
The Blue Nile falls 1370 metres in 1370 kilometres, which may roughly be divided into three reaches. The first from Lake Tsana to Rosaires on a length of 750 kilometres ¹⁄₆₀₀, the second Rosaires to Sennaar ¹⁄₄₅₀₀, and the third ¹⁄₇₀₀₀. These are very approximate indeed.
The Atbara falls 1640 metres in 880 kilometres. In the first 300 kilometres the slope is ¹⁄₂₀₀; in the next 300 kilometres the slope is ¹⁄₂₅₀₀, and in the last reach of 280 kilometres it is ¹⁄₆₀₀₀. These are approximate.