Hume and Dartmouth Dams Operations Review IssuesRelease in 1996/97In October and November 1996, a flood occurred in the River Murray due to widespread heavy rainfall. Coincidentally, at the same time as the flood, significant structural defects were detected in Hume Dam. A release was made from Hume Reservoir after the peak of the flood to reduce the risks to the structure associated with the defects. The total volume of this release was 900 GL, compared to the volume of natural floodwaters passing Hume Dam of 1 900 GL. This release caused prolonged inundation of some land and caused some additional damage to pasture and crops. Following the release of the water from storage the Murray Darling Basin Ministerial Council agreed to ex gratia payments to affected parties for the incremental damage caused by the release. The payments were determined on the basis of losses additional to those caused by the natural floods. Examples where payments were made are for pasture death where inundation due to the natural flood was for several days (insufficient to kill the pasture, but sufficient to reduce productivity) but with the additional release the inundation was for sufficient time to kill the pasture. No payment would be made where the natural flood would have inundated the land for sufficient time to kill the pasture Following community concern about these events the Murray Darling Ministerial Council initiated a review of operating procedures for Hume and Dartmouth dams. The review will bring into focus the trade offs required between the competing demands of water supply, salinity, stream ecology, flood mitigation, recreation, and hydro electric power. Consideration of environmental needs will receive closer attention. The Commission's Ministerial Council can then make informed and open trade offs.
Remedial worksHume Dam is subject to the Australian National Committee on Large Dams (ANCOLD) guidelines. These specify how the dam is to be checked for safety at regular intervals and the level of surveillance monitoring that is to occur at the dam. The dam has been subjected to major remedial works several times in the past. These included the installation of post tensioned cables in 1961. With advances in technology and concern regarding the earlier works (which were considered state of the art when installed) a further set of post tensioned cables were added in 1985. The new cables are like a large bolt and have their tension checked at regular intervals. This was not possible with the first set of anchors. Checking the tension allows the dam safety engineers to be sure that the anchors are working and to monitor any changes in tension. As part of the regular on going surveillance some movement in embankment Number 1 was noticed. Investigations showed that the dam was still safe but that the movement would continue and would eventually result in the dam becoming unsafe. As a result of this work was carried out near the bend in embankment number 1 in 1997 to place additional earth at the toe of the bank to stop the movement. This work was termed phase 1 of the remedial works. Phase 2 of the remedial works involved investigation of the junction of the concrete corewall in embankment number 1 and the concrete spillway section. This investigation indicated that there may be some problems with the original bitumen seal placed in the junction. The first stage of the remedial works involved grouting the upstream area of the junction to provide a secondary seal. It was during these grouting works that the corewall moved 5 millimetres resulting in the need to reduce the water level in the dam by releasing water. Detailed investigation and design work has been carried out and remedial works at the junction have commenced. The works involve: The construction of a weighting berm on the downstream side. This will provide additional strength for the embankment. The construction of additional drainage downstream of the junction. This will intercept any water that leaks through the junction and release it at a point where it can be monitored. The installation of a flexible concrete wall immediately upstream of the corewall to provide a secondary seal. The flexible concrete wall will be approximately 40 metres deep. In addition to these works additional works are being carried out at the southern end of embankment number 1 and at embankment number 2. These works involve the construction of downstream weighting berms to provide additional strength that has been identified as necessary for the revised estimate of the maximum credible earthquake. It should be noted that the original design complied with the earthquake loads at the time of construction but with additional information, in particular the Newcastle earthquake details, and better analysis techniques, the loads have been increased requiring the walls to be strengthened to meet international safety standards. |
RIVER MURRAY WATER