Don Macfarlane; Nick Eldred; Sigi Keis
Project Aqua was planned to be a major hydropower development along the lower Waitaki Valley, New Zealand. Geotechnical investigations for the project were conducted in two main stages – from the late 1970’s to mid-1980’s, and again in the period from 2002 to 2004.
Community consultation was an important part of the 2002-2004 investigations, and was a key risk management issue for Meridian Energy. The proposed scope of the work included 512 drillholes and 734 test pits spread along the 60km project corridor. All proposed drillholes and test pits were subject to the Resource Management Act 1991 and needed Resource Consent applications, which required consultation with landowners, territorial authorities, and community and cultural groups including three Maori tribes.
A number of proposed investigations could not be undertaken because the landowner would not allow land access, but over 70% of the proposed work was completed with community support.
R.A. Ayre and T. L. McGrath
SunWater as an owner of 25 major dams in Queensland has completed a programme to update the design flood hydrology of all of its referable structures in accordance with the latest methodology for estimating extreme design floods. This programme ensures the adequacy of existing spillways is included in an overall dam safety portfolio risk assessment in a consistent fashion.
This paper describes the methodology adopted in the re-assessment of the design flood hydrology of the storages. Principally this has meant the use of a design hydrograph approach utilising runoff-routing methods as described in Australian Rainfall and Runoff (1999). Design rainfall inputs have been based on generalised techniques derived by the Bureau of Meteorology such as the Revised Generalised Tropical Storm Method and the Generalised Short Duration Method for the estimation of Probable Maximum Precipitation. These estimates, coupled with the use of a regional design rainfall estimation technique known as CRC-Forge that is used for determining large to rare design rainfall estimates, have been used to derive a complete estimate of the inflow/outflow flood frequency curve for each dam.
The paper also provides an insight into the significant factors and relationships that are involved in the changes resulting from this process. Overall, there has been an increase in design rainfall depth estimates for the extreme events, and a general reduction to neutral change in the large to rare rainfall range. These changes plus the influence of temporal effects and the assignment of Annual Exceedance Probability (AEP) has led to substantial changes from previous estimates of design floods. The implication of these changes is profound for
an organisation such as SunWater.
Peter D Amos, Pip Nicolson, M Grant Webby, Murray D Gillon
To obtain a resource consent to build and operate any new water resource or hydro-electric development in New Zealand, the developer is required by the Resource Management Act (RMA) to consult with the community over the effects that the development could have, including describing how public safety risks will be avoided, remedied or mitigated. The community has the opportunity to respond to the authorities issuing the resource consent and influence the conditions attached to the consent.
The proposed Project Aqua Scheme in the South Island, New Zealand, comprised a 60 km long canal system to convey 340 cumecs flow from the Waitaki River across alluvial river terraces and through a chain of six hydro-power stations before returning the water back to the river. Each section of canal between stations would have contained between 4 and 6 million m3 of water within embankments up to 20m high. A breach of any one of these canals had the potential to flood farmland, residential buildings, highways, and other infrastructure, thereby posing a safety risk to local residents together with the potential for significant economic loss.
The paper describes the methodologies that were developed and used to assess the impacts, the measures proposed to avoid, remedy or mitigate safety risks and the public reaction to the associated report that was provided for public consultation prior to abandonment of the project. The methodologies used required adaptation of dam safety and consequence assessment practices usually applied to in-river dams, and applied here to the 60 km long length of canal embankment.
Ian Cordery, Peter S. Cloke
Scientists advocate more hydrological monitoring but in most regions publicly funded monitoring is in
steady decline. The lack of measured data at dam sites means there are many designs for new dams and remedial work that are insufficiently supported by factual information. Unfortunately data –free modelling exercises will usually produce favourable results – favourable to the modeller’s purposes, but not necessarily favourable to the determination of physical reality or truth. In these days of the popularity of modelling it is common to find decisions being made based on model studies for which little or no local data were available for model calibration or verification. How can the ‘large dam’ fraternity encourage (ensure) more data use? Causes of lack of data are many. For example governments fund data collection but others need the data, and data collection is a long-term activity that produces few benefits in the short term. Some years ago it was shown that hydrological data collection and archiving provided benefits to the community of at least nine times the costs of the data.
The real costs of comprehensive data collection are not large but examples will be given of the huge
costs, mainly due to the need to allow for uncertainty, that result from unavailability of data. Those
who understand this problem need to explain it to their communities, politicians and CEOs in a clear,
unmistakably persuasive manner, and to demand an increase in data collection. If we do not, no one
P Maisano, M Taylor , M Barker and A Parsons
South Para Dam, completed in 1958, is located on the South Para River, 38 km north of Adelaide. The embankment is 45 m high and comprises compacted crushed phyllite with rockfill toes. The 13 m high rock fill toes are protected with three-stage filters but the remaining 32 m of embankment height has no downstream filter protection.
The South Australian Water Corporation (SA Water), the owner and operator of the dam, is considering modifications to the dam, to augment its flood mitigation role. The proposed works, while not affecting the full supply level, involve a modification to the spillway crest and raising of the embankment crest to accommodate increased flood levels. SA Water therefore commissioned a dam safety review to assess the need for any piping or overtopping protection that may be required. This was followed by concept designs to ensure that flood mitigation work is compatible with any required dam safety upgrade work.
The results of a detailed dam failure risk analysis using event trees showed that the Societal Risk for the existing dam needed to be reduced, and that the proposed spillway modifications increased the Societal Risk due to the increased risk of piping failure with higher flood levels.
The risk analysis showed that eliminating the overtopping modes of failure by raising the dam crest is not sufficient in itself to achieve the required reduction in risk. The provision of filter protection to reduce the risk of piping failure is required, but it was shown that it is not necessary to provide full height filters as the provision of filters only above full supply level would be sufficient to achieve the required reduction in risk.
The recommended upgrade works, in addition to the proposed spillway modification for flood mitigation purposes, consist of filter protection and a weighting fill above the top berm (4.4 m below FSL) to facilitate connection to a possible full height filter in the future, and a parapet wall to provide overtopping protection.The resulting cost saving compared with the installation of full height filters is in excess of $2 Million.
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
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.