INTERACTION OF REEDS, HYDRAULICS AND RIVER MORPHOLOGY
1 BACKGROUND AND MOTIVATION
Environmental management of rivers requires understanding and prediction of the processes linking management actions to biological response. This involves firstly relating the discharge (or flow rate) in a river to management decisions concerning upstream land use and water resources development, and secondly relating this managed discharge to biological response. The relationship between discharge and biological response necessarily involves the local hydraulic conditions, manifest by flow depth, velocity and boundary shear stress. The ability to describe the hydraulic conditions in a river is therefore crucial to effective management.
In a river, the local hydraulics, channel form (or morphology), and instream vegetation constitute a mutually dependent trinity, and no one entity can be meaningfully considered independently of the other two. The project aimed at developing a better understanding and description of this mutual dependence for the purpose of increasing the effectiveness of river management.
Reeds are the vegetation type focussed on in this study because of their widespread presence in South African rivers, the relative simplicity presented by their common occurrence in monospecific stands, and the existing evidence of their important influence on morphological change and ecological functioning. Heritage et al (1997) have shown that the interaction between reeds and sediment is an important process in morphological change in rivers of the Kruger National Park. The occurrence of reeds is highly dynamic, ecologically important (Carter and Rogers, 1989) and a major contributor to transpiration loss (Birkhead et al, 1997). The influence of reeds on hydraulics also has relevance to engineering applications, including flood analysis and channel stabilization.
The statement of objectives as specified in the contract is as follows:
For budgetary reasons it was agreed at the first Steering Committee meeting to modify the specific objectives, and particularly to reduce the scope of the biological and modelling objectives. No new research would be undertaken into the phenology and propagation modes of reeds and the project would rely on existing biological knowledge. The modelling objectives were reduced to the first two items listed above, and the intention would be to produce a conceptual framework rather than a complete model.