Determining environmental flow requirements for substrate maintenance in cobble and boulder bed rivers in South Africa

To ensure a healthy and biodiverse ecosystem in cobble and boulder bed rivers, it is imperative that the channel bed is maintained. In pristine catchments this is achieved by the range of flows characteristic of the natural flow regime, which typically incorporates “maintenance” flood flows of different timing, duration and frequency. The construction of dams however, leads to a change in the flow regime, the flooding magnitude and frequency, and the sediment transport capacity. In order to maintain a healthy and productive substrate environment it is necessary that the environmental flow requirement for these rivers accommodate a “substrate maintenance flow component”. This paper presents the findings of a project to develop guidelines for determining substrate maintenance flows in cobble and boulder bed rivers in South Africa. The study developed a model for predicting incipient motion based on unit stream-power theory and used this model to investigate the impact of particular flood events on invertebrate population densities. 1. Aims and Objectives The objective of this research was to develop a mathematical model for incipient motion in cobble and boulder bed rivers based on the theory of unit stream power. This mathematical model would then be used to support the development of guidelines for substrata maintenance flows in cobble and boulder bed rivers. The study was funded by the Water Research Commission and has resulted in the publication of two research reports. The first (Cullis et al, 2007) addresses the development of the hydraulic model, while the second research report (Ratcliffe, et al, 2007) discusses the ecological impact on the aquatic invertebrates. 2. Data Collection The research was conducted in two relatively undisturbed mountain streams in the Western Cape region of South Africa: the Molenaars River and the Berg River. Close to 800 stones ranging in size from 25 mm to 1100 mm were marked, 345 in the Molenaars and 435 in the Berg, just prior to the start of the flood season. The movement or nonmovement of the marked stones was recorded after each of thirteen flood events at both study sites. The floods ranged from small freshettes that resulted in negligible disturbance to bank full floods with a average return period of over two years. The largest observed flood resulted in the movement of 44% of the marked stones at one of the study sites. 1 Department of Civil, Environmental, and Architectural Engineering University of Colorado at Boulder Engineering Center ECOT 441, UCB 428 Tel: (720) 2994647 e-mail: [email protected]