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Water Reconciliation Strategy Study for the Large Bulk Water Supply Systems: Greater Bloemfontein Area 8
Table 3.1: Summary of Primary Surface Water Resources
Source Registered Water Use (Mm 3 ) Yield (Mm 3 )
Rustfontein Dam 9.74
Mockes Dam 14.8
8
Groothoek Dam 0.74
Welbedacht/Knellpoort System 37 92
Totals 62.28 100
In the current study, the yield analyses conducted in previous studies were first verified and, after that, the
yield model was modified to take account of more recent information on the operation of the system and
additional hydrological data.
The determination of the yield of the combined system is complex and is affected by the assumptions made
about losses in the system and the way in which the system is operated. The yield analysis is described in
detail in the Reconciliation Strategy Report, and only the more important results are presented here.
Table 3.2: Adjustment of the Yield to Incorporate Recent Operating Information and Hydrology
Impact
Model Adjustment
on Yield
Implement Tienfontein Pump operation described in "Extension of the capacity of the Novo
Transfer Scheme - Study (EB/2009/5) by V&V Consulting Engineers. According to this
-7
report the pumps transfer 1 m 3 /s when the inflow reaches 4 m 3 /s, 2 m 3 /s when the inflow
reaches 6 m 3 /s and 3 m 3 /s when the inflows reached 10 m 3 /s.
Replace Welbedacht's average WTW capacity of 124 Ml/d with the equivalent seasonal
-4 capacity that is lower in summer (100 Ml/d due to siltation problems) and higher in winter
(145 Ml/d).
-3 Introduce 10% conveyance loss on the pipeline from Welbedacht to Bloemfontein
-3 Reduce Welbedacht live storage from 11.7 to 6.6 million m 3
Replace Caledon System upstream of Welbedacht with a present day streamflow sequence
developed during the Orasecom Study - kindly provided by Bennie Haasbroek. The
-3 demands downstream of Welbedacht from the Orange River System Analysis were retained
while the Orasecom Study was checking why the new demands(14 million m 3 /a) were
significantly lower than the earlier demands (35 million m 3 /a)
Implement Knellpoort Pump operation described in "Extension of the capacity of the Novo
Transfer Scheme - Study (EB/2009/5) by V&V Consulting Engineers. According to this
3
report the transfer varies from 1.5 m 3 /s when the storage level in Knellpoort is above
RL1436 m and 1.67 m 3 /s at RL1452.1 m.
Model 35 million m
1
3 /a irrigation located d/s Welbedacht Dam d/s of the dam (as opposed to
u/s) and release all water up to 2 m 3 /s flowing into Welbedacht when Knellpoort +
Rustenberg Storage > 150 Mm 3 , reduce to 25% (0.5 m 3 /s) when less
-16 Total net impact
When various operational constraints and the estimated environmental water requirements downstream of
Welbedacht Dam, as well as existing and proposed agricultural water requirements, which include the
water requirements for resource poor farmers, are taken into account, the combined historical firm yield
shown in Table 3.1 is reduced by 16 million m 3 /a to 84 million m 3 /a. The factors contributing to this
reduction are shown in Table 3.2.
Interventions Report June 2012