August 9, 2008
How Has the Ecological Reserve Influenced the Design and Operation of the Berg River Dam?
By Rossouw, Nigel Grobler, Dana
THE BERG WATER PROJECT (BWP) is the culmination of a 14-year strategic integrated planning process by the Department of Water Affairs and Forestry (DWAF) to identify appropriate measures to address the water shortage in the Western Cape. The planning process was initiated in 1989 and was called the Western Cape Systems Analysis (WCSA). The purpose of the WCSA was to reconcile the water demand and water supply for the Western Cape region. The WCSA was a democratic public strategy process and was supported by technical and scientific assessment to aid decisionmaking. The water situation assessment conducted as part of the WCSA determined that the City of Cape Town (CCT) would be the first metropolitan area in South Africa where water demand would exceed the available water supply. The WCSA provided a list of projects that could be implemented to increase the water supply.
In April 2002 Cabinet approved the construction of the BWP on condition that the CCT reduces the demand for water by 20 % by the year 2020. The BWP is the first bulk water resource development project that is directly linked to water demand management. In May 2002 the Minister of Water Affairs and Forestry directed the Trans- Caledon Tunnel Authority (TCTA) in terms of section 103(2) of the National Water Act (No 86 of 1998) to fund and implement the BWP as the agent for DWAF.
The BWP will increase the yield of the Western Cape Water Supply System by 81 million m^sup 3^ or 18 % from 2008.
The BWP was the first large water resources infrastructure development project in South Africa to be designed, constructed and operated within the framework of the National Water Act and in accordance with the guidelines of the World Commission on Dams. The dam was therefore required to be able to make releases to satisfy all aspects of the ecological reserve as prescribed by the National Water Act.
WHAT IS THE ECOLOGICAL RESERVE?
In the National Water Act (No 36 of 1998, chapter 3, part 3, section 16) the Ecological Reserve is defined as:
... the basic human needs reserve and the ecological reserve. The basic human needs reserve provides for the essential needs of individuals served by the water resource in question and includes water for drinking, for food preparation and for personal hygiene. The Ecological Reserve relates to the water required to protect the aquatic ecosystems of the water resource. The Reserve refers to both the quantity and quality of the water in the resource, and will vary depending on the class of the resource.
In South Africa the terminology used for the provision of water of a suitable quality to protect the water resource is the 'ecological reserve'. The term instream flow requirement (IFR) is also in common usage in South Africa and refers specifically to the flow requirement (both flow volumes and variability) to maintain a desired level of ecosystem functioning. Internationally other terms such as 'environmental flow' and 'ecological flow' are also used. Globally accepted definitions are based on the following two aspects:
* The quality, quantity and timing of water flows required to maintain the components, functions, processes and resilience of aquatic ecosystems which provide goods and services to people
* The foundation from which socially valued resources are derived and supported, and without which no sustainable use of the resource possible
A sustainable water resource and catchment management plan must be built upon a foundation of detailed scientific knowledge about the river flows (in terms of quantity and variability) and water quality needed to sustain ecosystem health and functioning. When the water needs of aquatic ecosystems (for example rivers, wetlands, estuaries and groundwater) are clearly denned by scientists, engineers and other professionals, water managers will be able to find ways of meeting human needs for water while maintaining adequate river flows for the ecosystem to ensure longterm sustainable use of the Berg River. In South Africa, an aquatic ecosystem's water needs are determined during an ecological reserve determination study. This is an environmental water requirement prescription which describes the necessary seasonal and inter- annual variation needed in low flows, high flows and floods, as well as the water quality requirement to support critically important ecological functions and for the continued provision of valued services.
The ecological reserve provides for the maintenance of critically important aquatic attributes, goods and services (such biodiversity, dilution capacity, habitat integrity, prevention of sedimentation etc.) and associated social services (such as fishing and water for river-dependent users and communities).
WHAT IS THE ECOLOGICAL RESERVE FOR THE UPPER BERG RIVER CATCHMENT?
The preliminary determination of the ecological reserve for the upper Berg River catchment for water quantity was set at 31,1 % (that is, 44,061 million cubic metres) of the mean annual runoff of 141,7 million cubic metres.
For the ecological reserve determination, detailed background studies and a comprehensive analysis were undertaken of the historical flows for the upper catchment. In the determination of the ecological reserve, the duration curves for the low flow releases to be made from the Berg River Dam were determined and the high flow releases required were established to be as follows:
* Daily average peak 65 m^sup 3^/s over 3 days = 10,11 million cubic metres (160 m^sup 3^/s instantaneous peak)
* Daily average peak 30 m^sup 3^/s over 3 days = 4,67 million cubic metres
* Daily average peak 5 m^sup 3^/s over 3 days = 0,78 million cubic metres
What this means is that the ecological reserve releases will be based on providing a portion of the natural flow contributions (or inflows to the dam) of the upper Berg River catchment for ensuring the continued functioning of the aquatic ecosystem below the dam.
According to the determination, the ecological category of the Berg River was set at C (see Box 1).
HOW HAS THE ECOLOGICAL RESERVE INFLUENCED THE DESIGN OF THE BERG RIVERDAM?
The requirement to implement the ecological reserve has dictated that the Berg River Dam be designed to cater for two distinct flow release systems, that is, small releases and large releases. The system for small releases occurs in the range 0,3 m^sup 3^/s to 12 m^sup 3^/s. These releases occur continually and are adjusted in magnitude as required by the ecological reserve and depending on the inflow into the Berg River Dam.
The radial arm gate system for large flows is able to make releases up to 200 m^sup 3^/s. These large flow releases will mimic naturally occurring flood events. The system for large flood releases is purely as a requirement of the ecological reserve and is unique. The system consists of a wet well in the intake tower, a concrete conduit through the dam wall and control gates.
HOW WILL RELEASES FROM THE DAM BE MADE TO MEET THE REQUIREMENTS OF THE ECOLOGICAL RESERVE?
* No base flow and high flow releases will occur out of phase with inflows into the Berg River Dam
* Environmental flow releases to meet the requirements of the Reserve will comprise the summer and winter base flows of 4 m^sup 3^/ s on average in June, July, August and September and the winter high flow releases of up to 160 m^sup 3^/s
* During the summer months inflows into the dam will be released to supply the ecological reserve
* Flood events of different magnitudes will be made each year to simulate the natural inflow patterns into the dam. This includes floods with an average daily peak of 65 m^sup 3^/s and a maximum instantaneous peak of 160 m^sup 3^/s. The outlet structure of the dam is designed to allow the instantaneous peak to be increased to 200 m^sup 3^/s. Dam releases will be operated in phase with the natural flood events
* High flow releases from the dam will be no greater than the inflows into the dam, in other words these would coincide with the magnitude of natural events
* During periods of drought the magnitudes of the reserve releases would be reduced
* In drought years high flows will be released unless no natural flood inflow into the dam occurs at the appropriate time of the year
* On average approximately 16 million cubic metres will be allocated for high flow releases each year and 27 million cubic metres for low flow releases
Compliance with the requirements of the ecological reserve is expected to achieve the following benefits: * Maintain the Berg River in Ecological Category C
* Maintain the river ecosystem to continue to provide users with acceptable water quality and an ecosystem that can support the living organisms in it
* Prevent increased sedimentation in specific areas downstream of the dam (for example at Paarl)
* Ensure that release patterns occurs as close to the natural flow variability as possible (for instance for inter- and intra- annual floods and wet and dryseason low flows)
The Berg River Dam is unique and is the first large in-stream dam in South Africa that is required to make both low and high flow (flood) ecological reserve releases. The dam will be operated to ensure that the releases of low flows and high flows coincide as closely as possible with natural inflows and natural flood events.
Text Nigel Rossouw
Cape Action for People and the Environment
Copyright The South African Institution of Civil Engineers Jun 2008
(c) 2008 Civil Engineering : Magazine of the South African Institution of Civil Engineering. Provided by ProQuest Information and Learning. All rights Reserved.