COMMUNICATION No. 5
A. D. LEWIS
UNION OF SOUTK.AFRKA
SECOND CONGRESS ON LARGE DAMS
WASHINGTON, D. C, 1936
SILTING OF FOUR LARGE RESEI
fOO ПУСГ.И 'ПАЯ НАучнО-ТЕХНИЧ[1.СКАЯ ^ЕЛИОТЕКА СССР
►IRS IN SOUTH
A. D. Lewis, M. A., M. Inst. С. E.,
Director of Irrigation Union oj South Africa
The four reservoirs are: Lake Arthur,...(More) Grassridge, Van Rynevelds Pass, and Lake Mentz. The first two are on separate tributaries of the Great Fish River, and the last two are on the Sundays River, Van Rynevelds Pass Reservoir being about 120 miles above Lake Mentz. These two rivers are the most silty rivers in South Africa. The catchment areas with which we are concerned lie wholly within the limits of 31°10' to 33°12' south and 24°0' to 26°21' east of Greenwich. The whole of this area is occupied by the Karroo geological formation" of about Permian age. This formation consists generally of an immense thickness of soft and easily erodible rocks, the materials of which were laid down in an inland sea. Shortly after deposition they were raised to a great height, though the strata still lie almost horizontally. They have thus been subject for a very long-period to erosion, which has resulted in th? formation of a pronounced ^ escarpment. This falls southward toward the seacoast, and has in the course of time retreated to an average distance of about 150 miles from it. The average elevation of the edge of the escarpment is about 5,500 feet, and it drops to 3,500 feet in an average distance of about 30 miles. The higher parts of the catchment occasionally hold snow for very short periods only. Three of the four reservoirs derive most of their water from the escarpment or from spurs left
colmaLu" ie quatre grands barrages dans VAfrique da Sud.
ndung \ 'or j
vier Gross-Staudammen in Sudafrika. 'e cuatro grandes depdsitos en el Africa del Sur.