INTERACTION OF REEDS, HYDRAULICS AND RIVER MORPHOLOGY
856/1/01
February 2002

EXECUTIVE SUMMARY

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.

2 OBJECTIVES

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

The overall aim of this project is to develop the knowledge and ability to model reedbed dynamics and the associated morphological change and hydraulic effects in semi-arid rivers.
 

This requires investigation of the characteristics of reedbed dynamics, hydraulics and sedimentation that influence their mutual interaction, and the formulation of a model to describe this interaction. The following specific objectives (with equal priority) are defined therefor:
 

Reedbed Dynamics
 

1. Document historical rates and extents of reedbed expansion and contraction in the Sabie and Letaba Rivers within the Kruger National Park, and correlate these with sedimentation patterns and flow regimes.
    
2. Describe the phenology and propogation modes of relevant reed species.
    
3. Determine the local hydraulic conditions and sedimentation states conducive to reedbed establishment, maintenance, expansion and contraction.
    
4. Describe the reed life history characteristics that influence hydraulics, and sedimentation, and how they might be affected by disturbance.
    

Hydraulics
 

1. Determine the flow resistance of a reedbed and how it is influenced by reed life history characteristics and water stage. Propose an appropriate method for quantifying reedbed resistance.
    
2. Determine the effect on overall resistance in a channel of the distribution pattern of reed cover, and propose a method for predicting overall resistance in a channel with a mixture of surface types.
    
3. Describe the variation of ecologically relevant and sediment-related hydraulic conditions in a partially reeded river reach, and propose methods for their prediction.
    

Sedimentation
 

1. Determine the effectiveness of a reedbed in trapping coarse and fine sediment, and the influence on trapping of reedbed distribution pattern, flow condition and reed life history characteristics.
    
2. Determine the stabilising effect of reeds on a sediment deposit and the conditions required for sediment remobilisation.
    

Modelling
 

1. Construct a rule-based model to describe the reedbed dynamics and associated morphological change.
    
2. Define modelling rules using the results obtained from the reedbed dynamics, hydraulic and sedimentation investigations.
    
3. Verify the model using observations from the Sabie and Letaba Rivers.
    
4. Apply the model to generate responses to different management scenarios.

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.