August 1971 Gamtoos Valley Floods
Wednesday, 1st December 1971
Article by WJR Alexander published in "Die Siviele Ingeneur in Suid Afrika.
THE 'black south-easter' which swept across the Eastern Cape in late August 1971 was one of the most
widespread storms experienced in the area for several decades. About 40000 km2 extending from Steytlerville in the west to East London in the east and from Uitenhage in the south to Somersct East in the north received more than 100 mm of rain during the 36 hour period ending on the evening of Saturday 21 August.
The highest authenticated rainfall during this period was 273 mm recorded south of Somerset East. Grahamstown experienced 196 mm in this period, which was reported to be the highest since 1880.
Cradock had its first snowfall in 30 years and the heaviest since 1865 and exceptionally heavy snowfalls covered the whole Cradock Graaff Reinet-Beaufort West area.
Gale force winds swept along the coast disrupting power supplies and forcing harbours to close. Winds
of up to 143 kmJh were experienced at East London and a fishing vessel capsized and sank on the Wild Coast.
Run-off from the extremely intense rainfall caused the Gamtoos, Sundays and Zwartkops Rivers to overflow
their banks. More than 100 lives were lost in the floods which were responsible for about R4 million worth of damage. Damage was particularly severe in the Gamtoos Valley and to a lesser extent along the Zwartkops River and in the Sundays River Valley downstream of Lake Mentz.
Floods having recurrence intervals greater than 10 years were recorded at several stream gauging stations in the catchments of the Gamtoos, Sundays, Great Fish and Great Kei Rivers and also on some smaller coastal rivers. The flood in the Groot River (a tributary of the Gamtoos) at Steytlerville was the highest experienced during 43 years of record and that in the Sundays River at Jamenville was the third highest in 48 years.
Gamtoos Valley
Irrigation has been practised on the alluvial flood plain of the Gamtoos Valley since 1827. The first major irrigation structure built in the valley was the Hankey Tunnel which was completed in 1844. Three years later, in October 1847, the tunnel inlet was severely damaged by a high flood when the river rose some 10 to 14 m overnight and drowned 13 Hottentot labourers. In October 1867, almost exactly 20 years later, an even higher flood destroyed the village of Hankey when the river was reported to have risen more than 20m.
There have been five subsequent floods which inundated large areas of the alluvial plain. The first of lhese occurred in 1905 when two railway bridgcs were destroyed. The farmers were warned when the flood passed through Steytlerville and they were able to evacuate low-lying areas. However, there was no warning of the 1916 Aood which, like the most recent flood and the flood of 1847, rose rapidly during the night trapping several families on the flood plain. Twenty four persons were drowned and 14 per cent of the area irrigated in the valley was destroyed.
The severest flood in the Gamtoos Valley since the turn of the century occurred during December 1931, but thanks to timely warnings there was no loss of life.
More than 100 ha of irrigated land were destroyed in the upper end of the valley. Erosion was particularly severe and all diversion weirs on the Gamtoos River were destroyed or severdy damaged.
In March 1961 heavy rains in the Karroo brought the river down once more. Beervlei Dam on the Groot River upstream of Steytlerville effectively absorbed much of the flood and halved the flood peak, undoubtedly reducing the damage which would otherwise have been caused in the Gamtoos Valley. The flow at Steytlerville was nevertheless the highest recorded since 1929 when measurements commenced and has remained the maximum on record till August 1971. The recent flood The flood of August 1971 produced a higher death toll and greater damage to property than any previous flood. It is possible that the effectiveness of Beervlei Dam in absorbing floods in its catchment and the subsequent construction of the Paul Sauer Dam on the Khougha River lulled the inhabitants of the Gamtoos Valley into a false sense of security. However, the Paul Sauer Dam was full at the time of the flood. In addition, its flood absorption capacity is low because its basin is situated in a deep narrow gorge.
The major proportion of the flood in the Groot River was generated by rainfall on the catchment downstream of Beervlei Dam. It is significant that Beervlei Dam started spilling after the floods had reached the Gamtoos VaIley. Nevertheless, the flood in the Groot River at Steytlerville was the highest in 43 years of record and that in the Gamtoos Valley equalled if not exceeded the flood of 1932 which was certainly the highest in more than 100 years.
The flood arrived in the valley in the early hours of Sunday morning, less thaI) 40 hours after the start of the rain. The timing could .hardly have been W0rse. By daybreak the flood waters had traversed the whole length of the valley and covered most of the flood plain.
Farm labourers living in vulnerable areas were awakened by the waters rising in their homes. They took refuge in the nearest trees and on buildings from where 184 were later rescued by army helicopters, to by private helicopter and 22 by small boats. More than 100 persons were reported drowned, the highest loss of life from a single flood in the valley and possibly in South Africa.
When at their peak, floodwaters covered more than three quarters of the flood plain, damaging some 60 per cent of the area under irrigation in the valley. Preliminary estimates show that II per cent of the land was destroyed by erosion and 24 per cent severely damaged by heavy sediment deposition. The remaining 25 per cent was submerged without suffering permanent damage.
The areas which suffered most from erosion were those situated at the upper end of the valley where chute channels short-circuited bends in the river and where bank, erosion occurred on the outer banks of bends in the river. Damage due to sedimentation was dominant in the lower half of the valley. Some deposits were three metres and more deep.
Remedial measures
It would clearly be both impractical and uneconomical to construct levees and channel stabilization works to provide full protection against floods for the whole valley. However, clearing of lands close to the river banks and construction of dams within the catchment must have an effect on the river's equilibrium conditions and some control will be necessary to prevent an acceleration of the natural processes of erosion and deposition which are always at work in an alluvial flood plain.
The magnitudes and frequencies of occurrence of the floods experienced within the area are presently being determined by the Division of Hydrology of the Department of Water Affairs.
Acknowledgement
The permission of the Secretary for Water Affairs to publish this article is acknowledged with thanks.
