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Berg River Dam is 38% full. Read report below and check out the issue of water release. " No base flow and flood releases will occur out of phase with inflows into the Dam Reservoir."
THE BERG WATER PROJECT (BWP) The Berg Water Project (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 (No. 36, 1998) and in accordance with the guidelines of the World Commission on Dams (WCD, 2000). The Berg Water Project comprises a 65 m high Dam and a Supplement Scheme (which consists of a 1.6m high diversion weir which will divert a portion of the winter high flows into a 3ha off-channel balancing dam to be pumped via a 10km pipeline upstream into the Berg River Dam) situated approximately 10 km downstream. The outlet works of the Berg River Dam were designed to release both low flows and high flows with provision for a peak release of up to 200 m3/s. WHAT IS THE DEFINITION OF ECOLOGICAL RESERVE? In the National Water Act (Act No. 36 of 1998, Chapter 3, part 3, section 16) the 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 to protect the water resource is the ecological reserve, which has legal status. The term Instream Flow Requirement (IFR) is also in common usage in South Africa. Internationally other terms such as environmental flow and ecological flow are also used. All these terms are used synonymously and refers to the collective amount of water needed to sustain healthy, natural ecosystems based on scientific studies. 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 provides goods and services to people, and,
- The foundation from which socially-valued resources are derived and supported, and without which no sustainable uses are possible - not as a user of water in competition with other users
A sustainable water resource and catchment management plan must be built upon a foundation of detailed scientific knowledge about the river flows needed to sustain ecosystem health and functioning. When the water needs of a river ecosystem are clearly defined 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. In South Africa a river ecosystem's water needs are defined in an Ecological Reserve Determination. This is a flow prescription (for both water quantity and water quality) which describes the necessary seasonal and inter-annual variation needed in low flows, high flows and floods to support critically important ecological functions and services. The ecological reserve provides critically important environmental services (fisheries, trees and vegetation, biodiversity, dilution capacity, river morphological benefits and habitat integrity, etc.) and associated social services (such as water for farmers and river dependent 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% of the mean annual runoff of 141.7 million cubic metres. For the Ecological Reserve Determination, detailed background studies and a comprehensive analysis was undertaken of the historical hydrograph (i.e. water flow variability in the river) for the upper catchment. In the determination for the ecological reserve, the duration curves for the low flow releases and the high flow releases were established as follows: •· Daily average peak 65 m3/s: 3 days = 10.11 Mm3 (160 m3/s instantaneous peak) •· Daily average peak 30 m3/s: 3 days = 4.67 Mm3 •· Daily average peak 5 m3/s: 3 days = 0.78 Mm3 What this means is that the ecological reserve will be based on the natural flow contribution (or inflows to the Dam) of the Berg River Catchment. According to the Determination, the ecological class of the River was set at C (see Box 1). Box 1: Classification of water resources developed by DWAF as a river management tool. RIVER MANAGEMENT ECOLOGICAL CLASS Class | Description | A | Negligible modification from natural conditions. Negligible risk to sensitive species | B | Slight modification from natural conditions. Slight risk to intolerant biota | C | Moderate modification from natural conditions. Especially intolerant biota may be reduced in number and extent | D | High degree of modification from natural conditions. Intolerant biota unlikely to be present |
For some river systems, specific objectives have been set, for ecological, economic or social reasons. In such cases, ecological flows need to be defined to meet those objectives. The application of the objective-based approach necessitates first that the desired status of the river has been set. It then should be possible to define threshold flows above or below which a change in status will be evident. The Department of Water Affairs and Forestry sets objectives, according to different ecological management targets. The assessment of an ecological flow remains a practical river management tool. | HOW WILL RELEASES BE MADE TO MEET THE REQUIREMENTS OF THE ECOLOGICAL RESERVE (I.E. WHAT ARE THE OPERATING RULES)? - No base flow and flood releases will occur out of phase with inflows into the Dam Reservoir.
- Environmental flow releases to meet the requirements of the Reserve will comprise the summer and winter base flows of 1.6 m3/s average in July and August and the winter flood releases of up to 200 m3/s.
- During the summer months inflows into the dam will be released to supply the ecological reserve.
- On average two flood releases will occur each winter. One smaller flood release of up to 65 m3/s will occur early in winter and a larger release of up to 200 m3/s will occur later in the year. Dam releases will be operated in phase with the natural flood events. Flood peaks will be smaller and less frequent.
- Flood releases from the dam would 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.
- It is possible to operate the dam so that it never spills, however, it has been predicted that the Dam can spill on average 1 out of 4 years.
- The probable maximum flood which is equal to about 1300 m3/s is likely to be reduced to only 740 m3/s when it spills at the dam.
- The 1:100 year flood will be dampened by about 60% and the 1:20 year flood will be decreased from 280 m3/s to 115 m3/s at the dam spillway.
- In drought years no floods will be released.
- In any given year, 10 million m3 will be allocated for flood releases.
SUMMARY CONCLUSIONS The Berg River Dam is internationally unique and the first dam in South Africa which is required to make both low and high flow ecological reserve releases. The Dam will be operated to ensure that the release of high flows and floods coincides as closely as possible with inflows and natural flood events. |
