Philip Styles, Brett Stephens, Stephen Perrett
The Wivenhoe Dam Spillway Augmentation Project involved the construction of an additional spill way on the right abutment of the main dam. The right abutment is located in massive sandstones and siltstones of Jurassic and Upper Triassic age.
Seismic refraction surveys and borehole drilling conducted at the design stage for the project indicated that part of the spillway area was likely to be marginally rippable to unrippable using a Caterpillar D9 bulldozer or equivalent. Further assessment and rock strength testing was conducted during the initial stages of excavation where D9 and D10 bulldozers were in operation. The results from this further work indicated that a section of the spillway extending from the proposed position of the ogee crest to approximately 100m further upstream were unlikely to be unrippable for a D9 dozer and marginally rippable for a D10.
Excavation options considered for this section included full scale blasting and load out, limited small scale ‘popping’ combined with ripping or the use of larger ripping equipment. Based on an assessment of cost-benefit, and given the availability of larger ripping equipment, it was decided touse a combination of D10 dozers and a Komatsu 475A bulldozer (D11 equivalent) equipped with single tine ripping tools. The use of this equipment proved successful with better than anticipated production rates being achieved. This resulted in significant cost and time savings for the project and reduced the likelihood of potential adverse impacts on the existing dam grout curtain, environment,travelling public and residents that may have occurred during blasting.
The Ross River Dam was first commissioned in 1974 and raised in 1976. The 8200 m long
embankment was not fitted with chimney filters and has suffered extensive desiccation cracking since it was raised. A significant component of the dam upgrade is the retrofitting of filter zones to ensure the embankment meets current dam safety guidelines.
This paper describes the process of investigation of the existing desiccation cracks and the use of Hole Erosion Tests (HET) and No Erosion Filter (NEF) tests to validate the design of the retrofitted filter. A significant challenge in the design is to provide a cost effective solution given the 7500 m length of embankment requiring treatment. Assessment of flow rates within cracks and expected piping erosion along the cracks was used to assess the required drainage capacity. This assessment of expected flow capacity allowed the deletion of the coarse filter in the design reducing the filter requirement from a triple filter to a single fine filter. Results of this assessment were incorporated into the Risk Assessment based design validation process.
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.
Robert Humphries, Caroline Minton, Andrew Baker and Mark Leathersich
There is a constant stream of criticism levelled at the perceived or actual adverse environmental
effects of large dams. These criticisms include prevention of fish migration, thermal and chemical and biological disturbance of downstream riverine habitats, silt trapping and drowning of terrestrial
habitats by flooding behind the dam wall. The beneficial effects of dams are rarely discussed, but
include aquatic habitat creation, catchment protection, flood mitigation, carbon sequestration and
protection of endangered species, amongst others.
Critics of large dams rarely present an analysis of the environmental costs and benefits of other water supply options, which include abstraction of shallow or deep groundwater, desalination of seawater, and reclamation of human or other wastewater.
In this paper we compare the environmental costs and benefits of water supply from large dams with
the common alternative options, and assess the relative sustainability of them all.
Joseph Thomas, Peter Thomson, John Grimston, Sally Marx
The Waimea Basin is located in the South Island of New Zealand. The area has an acute water shortage with recent studies showing the water resources to be over-allocated by 22% for a 1 in 10 year drought security. The current area irrigated is about 3,700 ha and there is additional productive land that could potentially be irrigated if more water were available. Water users have suffered severe restrictions on their water use over recent years through drought management measures imposed to meet critical environmental flow requirements and coastal salinity buffering. This has caused significant production cutbacks for irrigated crops resulting in regional economic loss, affected major urban water supplies resulting in water supply cut-backs affecting domestic and industrial users and also affecting the important environmental values of the Wairoa/Waimea Rivers and the coastal springs that are highly valued by the community and local iwi (Maori).
The principal objective of this project is to carry out a study into the feasibility of water storage in the upper parts of the catchment for enhancing water availability for both consumptive and environmental/community/ aesthetic benefits downstream. The outcome from this feasibility study will provide the community with the necessary information to make an informed decision on proceeding with potential storage options. The Waimea Water Augmentation Committee is overseeing this feasibility study. The study will be completed byJune 2007.
The Waimea Plains area is also quite unique as to the interest and values relating to the water resource as it has multi stakeholder interest. Being close to urban centres, the water resource not only caters for irrigation use but also public water supplies as well as recreational, community interest and cultural values.
This paper sets out the project’s aim, general methodology being followed, and summarises the progress to June 2005.
David S. Bowles, Loren R. Anderson, Terry F. Glover, Sanjay S. Chauhan, Ronn S. Rose
A risk assessment was performed for the Sacramento District of the U.S. Army Corps of Engineers to explore the justification for imposing an operating restriction on Lake Success to reduce the probability and consequences of an Earthquake-induced dam failure. The potential for both a sudden overtopping failure and a delayed “seepage erosion through cracks” failure were considered.
The risk assessment focused on the seismic performance of the dam, the potential life loss and economic consequences of Earthquake-induced dam failure, and the estimated residual risk and degree of risk-based justification for the Existing operating regime, a range of Potential Operating Restrictions, and an Indicative Improved Warning and Evacuation System. Risk assessment inputswere supported by seismic deformation analyses under various Earthquake loadings and pool elevations, dam break-inundation modelling, and reservoir simulation.
Evaluations against tolerable risk guidelines from the USBR, ANCOLD, and the UK HSE, together with insights into the relationship between pool elevation and dam failure risk, provided important inputs for the decision to implement an operating restriction.