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Report No 678-F-001
METSI CONSULTANTS: SUMMARY OF MAIN FINDINGS FOR PHASE 1 DEVELOPMENT
backwaters, or small floods to stimulate fish spawning, then the IFR would stipulate the magnitude,
duration and frequency of the required floods.
By defining water quality, physical habitat and biotic communities that characterize specific river
conditions - in this manner, the IFR is linked to measurable goals that can be used to assess whether
the desired river condition is being achieved.
Even the most successful IFR would only partially mitigate the effects of a water-resource development as the
presence of a dam would, in itself, inevitably result in downstream impacts. Application of an IFR moreover
cannot guarantee a desired condition in a river since other activities in the catchment also affect river condition.
For instance, even if the IFR is implemented correctly, pollution from industry or agriculture could result in
changes in water quality in the river. Thus, an IFR should be established and implemented as part of a catchment
management plan that has as part of its objectives the maintenance of the desired condition of the river.
2.3. METHOD DEVELOPMENT
Methods for determining an IFR for regulated rivers and streams have been in use in North America for the past
50 years and have been used increasingly in South Africa for the past two decades. Earlier methods used
elsewhere, applied statistical analyses of historic hydrological data to determine minimum flows for fisheries or
other specified ecological features. Subsequent methods placed emphasis on hydraulic rating assessments.
Habitat simulation as a way to establish IFRs is in widespread use in some countries, has been used to a limited
extent in South Africa, and is best represented by the instream flow incremental method (IFIM) and its many
derivatives.
More holistic approaches to determining IFRs have been advocated in Australia and South Africa, typified by the
development of the Holistic Approach in the former country and the Building Block Methodology (BBM) in the
latter. These methods consider various types of flow and relate them to biophysical conditions in the river under
study. Full descriptions of these methodologies are provided in the LHDA 648 reports (e.g., Report No. 648-02).
In consultation with the LHDA and the Panel of Environmental Experts, the Consultant decided that an holistic
approach to the LHWP IFR was justified, and the method applied should:
allow the flow requirements to be assessed for all major components of the riverine ecosystem (e.g.,
riparian vegetation; channel form);
assess the flow requirements for several rivers;
address both water quality and quantity requirements for the rivers;
allow several potential flow regimes to be described, each with its predicted consequences (i.e., a
scenario-based approach);
incorporate a comprehensive and structured socio-economic component.
The initial approach used in the study is based on the BBM. However, to meet the specific requirements of the
study area and the biophysical conditions and constraints, a new approach was developed during the study and
is termed DRIFT (Downstream Response to Imposed Flow Transformations). Using the present-day flow regime
of the river as a starting point, this is used to:
carry out a preliminary characterization of the rivers and select eight representative IFR sites on the
basis of geomorphology, proximity to gauging stations and general accessibility;
describe the various biophysical consequences for the river of further reducing (or, if relevant, of
increasing) the flow in a number of different ways;
create a database of these biophysical consequences, each linked to its flow reduction details;
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