Hydraulics for Determination of the Ecological Reserve for Rivers
Report No 1174/1/04

Dec 2004



Water resources in South Africa are limited and their management and protection is critically important for the sustainable economic and social development of the country. Over the last 10 years an effort related to developing policies, structures and methodologies for the management and protection of South African water resources has been made. The Department of Water Affairs and Forestry and other agencies (including DEAT and the Department of Agriculture) responsible for natural resource management have been developing approaches for making decisions on resource protection and management (DWAF, 1999). A number of institutions and organizations, such as the Water Research Commission (WRC), Institute for Water Research (IWR), Southern Waters, Centre for Water in the Environment (CWE), Freshwater Research Unit, South African National Parks, Council for Scientific Industrial Research (CSIR), have contributed to research and development of methods and approaches related to protection of South African rivers.

As a result, a variety of new policies, tools, approaches and procedures have been developed including:

The National Water Act (No.36 of 1998) (NWA) has been developed to fundamentally reform the law relating to water resources. A major innovation of the Act is the incorporation of the concept of the Reserve. The Reserve consisted of two parts: “Basic Human Needs Reserve” and “Ecological Reserve”. Implementation of the NWA requires that an ecological Reserve be determined for all significant resources, with those for which development is planned receiving priority attention. Ecological Reserve determination is an estimation of the environmental flow requirements (EFR) of different components of a river. It focuses on the amount of water required to maintain the system in a particular ecological condition. The estimation of flow for different aquatic components has required development of suitable methodologies and approaches.

In South Africa, activities addressing the development of holistic methods for assessing EFR of rivers were initiated in 1989 (King and Tharme, 1994). Since 1989, two comprehensive holistic methods namely the Building Block Methodology (BBM) and Downstream Response to Imposed Flow Transformations (DRIFT) as well as a more recent method, the Flow Stressor-Response (FS-R) (designed to be used on its own or as part of holistic methods such as the BBM and DRIFT), have been developed. These have to incorporate hydraulic analyses to provide the critical link between the hydrology and the other factors necessary to set the EFR.

Current methods of river hydraulics are, however, based to a large degree on principles that are more appropriate to high flows and flood analysis than the low flows characteristic of EFR. Furthermore, hydraulic characterization is generally limited to transects or cross-sections, and does not adequately describe conditions of river sites and/or river reaches. Most hydraulic analysis techniques currently used in EFR assessment were developed originally for engineering applications and many underlying concepts and procedures are inappropriate for environmental applications. There is consequently an urgent need to define the role of hydraulics within the context of EFR for rivers. This requires a fairly radical assessment of what hydraulic characterizations are important for assessing ecological functioning, and the development of appropriate methods for measuring, describing and predicting them.


The statement of objectives as specified in the contract is as follows:

  1. To provide hydraulics methods to link hydrologic river characteristics and biotic requirements necessary for setting the ecological Reserve.
  2. To provide hydraulic methods for the setting the Reserve when hydraulic data are limited.
  3. To develop three-dimensional habitat modelling to assist in the determination of the ecological Reserve for rivers.
  4. To develop an index of hydraulic characteristics for quantifying habitat availability.