A geomorphological framework for the assessment of instream flow requirements

Abstract

As South African ecologists become increasingly aware of the possible long term impacts of river regulation schemes on channel structure and associated aquatic habitats, fluvial geomorphologists are being called upon to assist in recommending instream flow requirements for future river regulation schemes. In South Africa, the standard procedure for the assessment of Instream Flow Requirements (IFR) is the Building Block Methodology (BBM). Central to the BBM is the assumption that the flow regime can be divided into three groups of flow: the low flows, freshes and floods. Each group has a particular ecological (and geomorphological) significance. The IFR assessments are applied at selected sites, chosen to represent the river reaches downstream of the dam site. This paper presents a geomorphological framework developed as a component of the BBM. This framework recognises five important geomorphological issues which are addressed within an IFR exercise: (1) A general assessment is made of the catchment condition to determine the potential for morphological change within the river. (2) An evaluation of the geomorphological characteristics of the river network is carried out to aid the selection of IFR sites within representative reaches. (3) At each IFR site an assessment is made of the relationship between hydraulic diversity and discharge for the different morphological units present at the site. This assessment relates in particular to the low flow component of the BBM. (4) The freshes and floods required to maintain channel form and bed condition are determined at each IFR site. An assessment is also made of the likely pattern and direction of morphological change that are likely to result from the recommended flow regime. (5) In the case of interbasin transfers, an assessment should be made of the potential impact of transferred water on the receiving channel. To illustrate the practical application of the geomorphological framework to an IFR procedure, an example is given from a recently completed study carried out in the Tugela River in KwaZulu-Natal, commissioned by the Department of Water Affairs and Forestry. The underlying theoretical principles upon which decisions were made are outlined in this paper.