Peter Hill, David Stephens, Kelly Maslin, Rachel Brown, Simon Lang, and Chriselyn Meneses
There has been a growing awareness of the potential dam safety risks associated with hydraulic structures in urban environments such as retarding basins, water quality detention basins and recreational lakes. This has required estimates of rare and extreme floods for urban catchments and there are a number of important characteristics of urban catchments which distinguish them from rural catchments such as impervious areas, lack of streamflow data, blockage of structures and complex hydraulics. This paper describes the key considerations for flood estimation in urban catchments and draws examples from a number of current flood studies for urban catchments in Canberra.
Marius Jonker and Dr Radin Espandar
This paper provides a summary of the current state of practice for arch dam design criteria that have been adopted by some international dam organizations, and where relevant, compares that with the criteria provided in the updated ANCOLD Guidelines on Design Criteria for Concrete Gravity Dams, with the view to provide a basis for consistent and unified design criteria for arch dams in Australia.
The paper draws on the authors’ experience with arch dams, including recent experience with a number of arch dam safety reviews in Australia, their past experience with arch dams over 200 m height, as well as their involvement with the development of the mentioned updated ANCOLD Guidelines.
Since the last arch dam was constructed in Australia, a number of international publications have been released on arch dam design practices, providing general information and guidance for the design of new dams and evaluation of the safety and structural integrity of existing arch dams. This paper compares these publications and proposes criteria that are aligned with the ANCOLD gravity dam guidelines.
Robert Kingsland, Andy Noble and Dr Eric Lam
Engineering design is necessarily context specific. However, engineering design produced in industrialised nations often comes encumbered with design methods, standards and construction process familiarities that can result in inappropriate design solutions for developing nations. This is no more apparent than with the design of small hydropower projects where budgets are small and the implications of poor decisions can easily threaten the viability of schemes.
In this paper we explore the challenges and opportunities for the scheme’s developer and designer, in striking an appropriate balance on engineering solutions that remain appropriate for the local construction practices. In most cases, based on our experiences from small, run-of-river developments, the available methods for feasibility study data collection, including geotechnical investigations and hydrology assessments, are in themselves a challenge. Consequently, the designer needs to work with what is readily available and often has to reset the established thinking to incorporate practical constructability into the designs, while giving special attention to the operation and maintenance aspects. More labour-intensive methods are not uncommon.
The stakeholders in small hydropower schemes are many: the community, the approval agencies, the lenders, the developers, the local construction industry, the government. Design decisions cannot be made in a vacuum. However, designers are often distant from the social, political, environmental and commercial context of their project. This separation can present significant challenges which, without due attention, can result in poor design outcomes.
This paper will, with reference to examples of good and poor design, discuss various facets of small hydropower development from a civil engineering perspective including, the scale of development, design methods, stakeholder engagement, local content involvement, constructability and financing. The paper concludes with suggestions for improving design outcomes for small hydropower projects.
William Ziegler and Heather Middleton
This paper presents the collation of over 20 years of data on vertical and horizontal movements around Cataract Dam in the Southern Coalfield of New South Wales, reporting subsidence that continues 25 years after extraction in the area ceased. The occurrence of increased vertical movement over old goaf areas as the result of extraction in the same seam at greater than 1km distance has been observed. Together with a change in the behaviour of measured head of water 6 years after extraction ceased in the area. These points raise the question, how long should subsidence monitoring continue after extraction has ceased in areas of important infrastructure?
JN Rossouw, AHM Görgens and PC Blersch
Shallow lakes or reservoirs generally exist in either of two stable states; a clear water state dominated by rooted water plants, or a turbid state dominated by free floating algae. A dramatic event can switch a shallow reservoir from one state to another. Voëlvlei Dam, a relatively shallow off-channel storage reservoir in the Berg River catchment, South Africa, switched from a stable, clear water system to a turbid, algal dominated system when it was severely drawn down during a drought in the mid-2000s.
It appears that there is tipping point beyond which a shallow reservoir can switch from one stable state to another and that there are buffers that maintain it in a specific state. Voëlvlei Dam is a good example of what such a switch might be (low water levels and high wind mixing) and what buffers (change to bottom-feeding fish species) may maintain it in the new state. It is only by understanding the hydrodynamic behaviour of a shallow reservoir that one can predict what these switches and buffers could be. Complex hydrodynamic modelling and comprehensive fish monitoring will facilitate more informed decision making and better management of reservoirs.
This paper describes the mechanisms that lead to the switch and how it can be prevented by developing an understanding of the hydrodynamic behaviour of shallow reservoirs through hydrodynamic water quality modelling.
Steven Slarke, Dr Martin Mallen-Cooper and Marcos Guirguis
Keepit Fishway Offsets
Fish passage structures are being provided by State Water Corporation as part of a strategic program to address fish passage barriers that triggered S218 of the Fisheries Management Act 1994 at Mollee Weir, Gunidgera Weir and Weeta Weir in the Namoi River. These sites are an offset for dam safety upgrade works on Keepit and Split Rock dams in the headwaters of the Namoi River. Rather than applying high-level fish lifts at the dams, the three lowland sites represent the top three ecological priorities in the Namoi River for fish passage facilities – a case of less cost for greater ecological outcomes. The objective of the fish passage facilities at these sites is to restore upstream and downstream fish passage for about fifteen native fish species. The key biological objectives are to pass adult and juvenile fish upstream and adult fish and larvae (which drift with the current) downstream.
Mollee Weir was constructed in 1973 on the Namoi River downstream of Keepit Dam, near Narrabri in northern NSW. The nine-metre high weir is used for irrigation and comprises a reinforced concrete structure featuring three bays with undershot gates and two piers. The upstream and downstream water levels are highly variable, with a maximum differential head of about six metres. Fish are unable to pass the weir during regulated and unregulated flows; even when the undershot gates are fully raised in high flows, due to high velocities in the opened weir. The weir’s large undershot gates are also a barrier to safe downstream fish passage during regulated flows. High water pressures and velocities beneath the partially raised gates create a high mortality rate for fish and larvae moving downstream.
Fish Passage and Regulator Structure
Designed for State Water NSW by URS Australia Pty Ltd in cooperation with Dr Martin Mallen-Cooper of Fishway Consulting services, Mollee Weir features a new fish lock for upstream-migrating fish and a dedicated overshot gate with dissipating pools for downstream-migrating fish, and was constructed during 2013 to 2014.
It is the tallest fish lock in Australia that is filled from the top.
The innovative design features two separate downstream fish holding bays and two fish lock entrance gates, to provide optimal entrance conditions at varying river flows and water levels.
To provide safe downstream fish passage at low to moderate river flows, a 4 m wide ‘downstream multi-function migration gate’ has been integrated beside the fish lock structure. This overshot gate also provides an attraction flow to the fish lock entrances, and tracks the upstream water level at high river flows to provide a high discharge pool and weir fishway as a bypass around the weir structure.
The Mollee Weir fish lock provides upstream fish passage for the full range of upstream and downstream water levels.