Peter Hill, Kristen Sih, Rory Nathan, Phillip Jordan
This paper presents a number of innovative hydrologic investigations undertaken for the recent detailed design of upgrades for Ross River Dam in North Queensland. A key issue for estimating extreme floods in the tropics is the estimation of flood events of long critical durations. The implication is that there is an increased focus on estimating the correct volume (not only the peakflow). This paper describes the regional analysis of flow volumes that was used to validate the estimated flood volumes.
Another issue of considerable importance is the assumed relationship between inflows and initial reservoir level. The analyses described in this paper showed that inflows are independent of reservoir levels for the more frequent events but for more extreme events they are correlated. This has important implication on how the initial reservoir level is incorporated in the hydrologic analysis. The final aspect covered by the paper is the derivation of seasonal flood frequency curves. This is particularly important given the highly seasonal nature of rainfalls in the tropics and the results are important for assessing risks during construction and scheduling the upgrade works
Eric J Lesleighter and Peter F Foster
The Ross River Dam was constructed in 1974 following design by the State Government, including hydraulic model testing, by SMEC. The maximum spillway discharge at that time was 1100 m3/s.Latterly, the dam and spillway have come up for a comprehensive review given that the dam is in an extreme hazard category because of its location only a short distance upstream of the city ofTownsville. The revised hydrology has produced outflow hydrographs peaking at over 4 000 m3/s –more than three and a half times the original – to be passed through the 130 ft (39.62 m) widespillway.
The paper describes the hydraulic modelling planned and carried out to determine changes needed to handle such high discharges. The modelling was to provide for the installation of radial gates and piers, and study of the water level, pressure and dissipation conditions in the dissipator for several key discharges through the range to PMF. Pressure measurements included transients, consideration of the potential for uplift of the basin floor slabs, the integrity of the walls to handle the differential loads, and, as a major consideration, the energy conditions in the flow exiting the dissipator and the integrity of the rock downstream to avoid erosion.Each of these aspects will be addressed in the paperboth from the modelling and interpretation standpoint and from the civil structural analysis standpoint, together with a description of the strengthening works required to achieve a satisfactory outcome.
Paul Hurst, Michael Smith
Wellington Dam is an extreme hazard concrete gravity dam located on the Collie River approximately 170km south of Perth. Originally constructed to a height of 19m in 1933, the dam was raised to its present height of 34m in 1960 by placing significant additional concrete against the downstream face of the original dam. To ensure a lasting bond along the interface between the original and secondary concrete, an open slot was formed and later grouted once the temperature of the secondary concrete was similar to that of the original dam.
A recently completed stability analysis identified that Wellington Dam falls well short of contemporary dam engineering standards for flood loading. Several assumptions were made during the preliminary analysis relating to concrete shear strength parameters, bonding between the original and secondary concrete and drain effectiveness that generated a significant range of results. On this basis, further investigation was carried out to define the concrete parameters and drain condition at Wellington Dam.
Exploratory drilling found that Wellington Dam is cracked from the upper gallery through to the downstream face. The drilling programme also confirmed that the interface between the original and secondary concrete has become unbonded and that the gravity dam is behaving like an unbonded short composite beam. The mechanism causing the observed behaviour of Wellington Dam can largely be explained by external temperature effects and Alkali Aggregate Reaction, (AAR).
This paper explores the techniques used to investigate the condition of the concrete and illustrates the relationship between concrete behaviour and temperature and AAR effects within a composite concrete gravity dam
Blowering Dam was constructed in 1968 by the Snowy Mountains Hydro-Electric Authority, on behalf of the Water Conservation and Irrigation Commission. It is a large earth and rockfill embankment dam, approximately 112m high and 808m long, with a concrete chute spillway at the right abutment. The reservoir holds about 1,628GL of water that is mainly for irrigation and supplying an 80MW hydro-electric power station. The dam is owned and operated by State Water Corporation, NSW.
Revisions to the design flood estimate have highlighted the dam requiring an upgrade to cope with increased discharge rates. The NSW Department of Commerce has carried out feasibility studies of different upgrade options. The need to evaluate the hydraulic performance of the existing un-gated spillway was identified. Flow overtopping the chute walls can potentially erode the backfill behind the walls, and, the rockfill on the downstream toe of the embankment. Consequently, this may lead to significant damage of the spillway and may risk the safety of the dam.
Hydraulic analysis of the spillway using a 3-D computational fluid dynamics model was performed for various flood levels to determine the discharge coefficients and the discharge rating curve. It was also required to identify whether the chute walls need raising to contain the increased discharges. These results were compared with those calculated by other “standard” methods. Such verification provided a level of onfidence in the analysis results which were then used in the studies to assess available upgrade options.
In order to have further confidence in the analysis, the computed results were validated against physical test data and some limited information from an actual discharge. Further verification against established theory was conducted by modelling a supercritical flow through a contraction in an open-channel in order to see if the computation could predict the shock wave effect that was observed in physical models as well as full scale channels. A reasonably good correlation was obtained from all validating tests.
This paper presents some background of the proposed dam upgrade, potential upgrade options considered and details of the hydraulic modelling of the spillway. Some interesting flow behaviour caused by the shock wave will be highlighted.
Mike Taylor, Jonathan Jensen and Greg Branson
Pykes Creek Dam is a 33 m high, 22,120 ML embankment dam, 72 km west of Melbourne owned and operated by Southern Rural Water.
The outlet works include a 30 m high “wet” outlet tower near the upstream toe of the dam on the right
abutment with its lower half comprising a concrete lined shaft excavated in rock. A 1.5 m diameter
concrete lined tunnel extends 30 m upstream from the base of the tower to a reinforced concrete inlet structure.
The only controls upstream of the downstream toe of the dam comprised 2 guard gates located on the downstream side of the tower, operated manually by means of handwheels from the top of the tower.
Major deficiencies with the outlet works included:
A major constraint in addressing these deficiencies was that any remedial works needed to be
undertaken without draining the reservoir or interfering with the releases required for downstream
consumers, including irrigators in Werribee and Bacchus Marsh.The paper describes how all of the deficiencies have been addressed with no interruption to supply, by means of a collaborative effort between the dam owner, the consulting engineer, and 5 separate contractors, with the dam owner playing a leading role.
Andrew Evans, Michael Cawood, Jonathon Reid
Eildon Dam, Goulburn Weir and Waranga Basin in Victoria are owned and managed by Goulburn-Murray Water (G-MW). Eildon Dam and Goulburn Weir are situated on the Goulburn River, while Waranga Basin is an offstream storage supplied from Goulburn Weir.
In November 2004 a dam safety emergency exercise involving the establishment of a central Emergency Coordination Centre at Tatura as well as Emergency Operations Centres at each of these three dam sites was conducted. The exercise presented a variety of emergency situations in stepped time increments, including earthquake, mechanical failure, a hazardous material spill and a terrorism related incident. External agencies were not involved.
The exercise was part of an ongoing G-MW program designed to test and improve dam safety emergency planning and response systems for all of G-MW’s dams and highlighted areas where procedures, situational management and communications can be enhanced.
Outcomes aimed for in G-MW’s program are improvement in Dam Safety Emergency Plans and internal communications, together with clarification of roles, responsibilities and capabilities.
The valuable experiences learned from this dam safety emergency exercise and plans for a larger scale exercise involving other emergency management agencies will be shared with others through this paper.