The demand for water from South Africa’s growing population is creating an ever-increasing pressure on the country’s rivers. The urgent need to provide more water services often conflicts with the desire to maintain or improve the ecological condition of the rivers. To provide guidance on the sustainable use of a river’s water-resources, the Building Block Methodology (BBM) has been developed for assessing the instream flow requirement for any river. Development has been done jointly over the last five years by the national Department of Water Affairs and Forestry (DWAF) and river scientists, and the accent is on identifying a complex of different magnitude flows for maintenance of entire river ecosystems. The BBM caters for the almost universal reality in South Africa of having rapidly to provide scientific guidance on such flows for a river in cases where biological data and understanding of the functioning of the river are limited. However, the methodology works equally well in data-rich situations. The BBM depends on available knowledge and expert opinion, gleaned from experienced river scientists in a structured workshop process. Limited new data of a specific nature are gathered to facilitate the process. Relevant data on the river are prepared in a way that workshop participants can easily understand and quickly begin to use. Scientists typically involved in the workshop, all with specific roles, are those with specialist knowledge of the river or similar rivers in terms of the fish, aquatic invertebrates, riparian vegetation, river importance, habitat integrity, fluvial geomorphology, local hydraulics, water chemistry and social dependence on the riverine ecosystem. Hydrological and hydraulic modelers provide data inputs and facilitate the workshop process by answering questions and producing additional data as requested. The workshop output, reached by consensus, is a quantitative description in space and time of a flow regime that should facilitate maintenance of the river ecosystem in some pre-determined desired future state. Information from a BBM workshop is used by DWAF in the Planning phase of a proposed water-resource development. Further development of the BBM, to extend it into the Design, Construction and Operation phases, has been initiated. This includes linking with a public participation process, input into design of the scheme, base-line studies of the river and subsequent monitoring to assess the efficacy of the recommended flow regime.

No abstract

No abstract

No abstract

An approach is presented for desktop-level environmental flow requirement (EFR) determination that is aligned with the Habitat Flow-Stressor Response (HFSR) method which evolved in South Africa over recent years. The HFSR method integrates hydrological, hydraulic and ecological habitat data, involves ecological and hydraulic specialists and is data-intensive and time-consuming. The revised desktop method integrates hydrological information with estimates of channel hydraulic cross-sectional characteristics to generate habitat-type frequencies under changing flow conditions. This information is used with the expected natural habitat requirements to determine acceptable habitat availability under different levels of ecological protection, which is then used with the hydraulic data to define flow regime characteristics that meet the ecological objectives. The paper describes the model components, discusses the assumptions, data requirements and limitations and presents some example results. The revised desktop approach uses approaches that are aligned with the more complex methods and generates results that are similar.Key words environmental flow requirements; hydrological regimes; channel hydraulics; habitat frequency Une nouvelle approche pour des évaluations rapides, sur un poste de travail, des débits environnementaux des rivières d'Afrique du Sud Résumé Nous présentons une approche de détermination, sur un poste de travail, des besoins en matière de débit environnemental. Cette méthode est alignée sur la méthode de réponse de l'habitat à un facteur de stress lié au débit (Habitat Flow-Stressor Response, HFSR), qui a évolué en Afrique du Sud au cours des dernières années. La méthode HFSR intègre des données hydrologiques, hydrauliques et écologiques sur l'habitat, implique des spécialistes en écologie et en hydraulique et est gourmande en données et en temps. La méthode révisée, applicable sur un poste de travail, intègre l'information hydrologique avec des estimations des caractéristiques hydrauliques de sections en travers du bief pour générer des fréquences de types d'habitat dans des conditions d'écoulement variables. Cette information est utilisée avec les besoins attendus en habitats naturels pour déterminer une disponibilité acceptable d'habitat pour différents niveaux de protection de l'environnement. Cette disponibilité est ensuite utilisée avec les données hydrauliques pour définir les caractéristiques des régimes d'écoulement qui répondent aux objectifs écologiques. L'article décrit les composants du modèle, discute ses hypothèses, les données qui lui sont nécessaires et ses limites, et présente quelques exemples de résultats. L'approche révisée, applicable sur un poste de travail, utilise des approches qui sont alignées sur des méthodes plus complexes et génère des résultats similaires.Mots clefs besoins en débits environnementaux ; régimes hydrologiques ; hydraulique en rivière ; fréquence d'habitat