Regardless of the type of dam, it is necessary to de-water the site for final geological inspection, for foundation improvement and prepartation, and for the first stage of dam construction. The magnitude, method and cost of river diversion works will depend upon the cross-section of the valley, the bed material in the river, the type of dam, the expected hydrological conditions during the time required for this phase of the work, and finally upon the consequences of failure of any part of the temporary works.
At most sites it will be necessary to move the river whilst part of the dam is constructed; this part will incorporate either permanent or temporary openings through which the river will be diverted in the second stage. If the first diversion is not large enough the initial stages of construction will be inundated, if the second stage outlets are too small, the whole works will be flooded.
At some sites there is a distinct seasonal pattern of river flows and advantage can be taken of such conditions but noting that Nature is random.
Construction of the Hendrik Verwoerd Dam, South Africa required a sophisticated arrangement of cofferdams. An approach was developed based on the frequency and distribution of floods that could occur over a five year period of construction. The following is an extract of the original detailed specification:
First Stage (A) -
Second stage (B) & (C) -
- Construction from each bank of the river of groynes a short distance upstream of the dam, to alter the direction of flow and thus to move the low water channel towards the left bank of the river at the dam site.
- Construction of a semi circular concrete arch cofferdam on the right bank of the river.
- De-watering of this cofferdam and excavation within it for the main dam blocks, the proportion of the overspill apron and the sections of the mid channel cofferdams.
- Concreting of the dam blocks, numbers 14 to 28 up to a minimum level of 1200 meters, the portions of the overspill apron and of the mid channel cofferdams within this cofferdam. In blocks of the dam constructed on this stage, temporary openings were formed through which the river was later diverted.
Third stage (C) & (D) & (E) -
- Construction of a semi circular arch cofferdam on the left bank of the river.
- Construction of the flanking portions of each of the upstream and downstream mid channel concrete arch cofferdams which cross the river upstream and downstream of the central section of the dam.
- Excavation of a channel along the right bank, leading to the temporary openings through the dam,
demolition of portions of the right bank cofferdam to permit the diversion of the river through the temporary openings and such clearing out of the right bank diversion channel as may be necessary.
- The cutting of a channel through the portion of the right bank groyne adjacent to the bank to form an entrance to the diversion channel described above.
- The placing of rockfill to connect together the right and left bank groynes so as to divert the river flow into the right bank diversion channel, thereby cutting down the velocity of the water in the vicinity of the mid river cofferdams.
- Completion of the upstream mid river cofferdam
completion of the downstream mid river cofferdam.
- Placing of spoil, excavated from the works, in the flood channel on the left bank upstream of the dam to prevent the river flooding into the area to the protected by the mid river cofferdams.
Diversion can also be achieved by means of a tunnel, which depends on the nature of the rock and depth of weathering and should be far away from the dam itself to not interferre with the foundations. The tunnel also should be large enough to avoid the possibility of job jams.
(C) Thomas, Henry H. The Engineering of Large Dams
- De-watering of the left bank cofferdam and excavation for the dam blocks and the portion of the overspill apron within this cofferdam.
- Concreting of the dam blocks numbers 9 to 27 to a minimum level of 1206 meters.
- Demolition of the left bank cofferdam.
- Demolition of the remaining portion of the right bank cofferdam within the areas protected by the mid channel cofferdams.
- The de-watering of the mid channel cofferdam and excavation within it for dam block numbers 1 to 7 and 2 to 12 and portion of the overspill apron.
- Concreting of dam blocks 1 to 7 and 2 to 12 to such levels that the contraction joints in the lower part of the dam up to gallery can be grouted.
- Concreting within the mid channel cofferdams of the portion of the overspill apron downstream of blocks 1 to 7 and 2 to 12.
- Cooling of the concrete and grouting of the dam construction joints.