Physical Geography
PhD / DPhil
| Status | Complete |
|---|---|
| Part Time | No |
| Years | 2002 - 2006 |
| Project Title | Flow Processes and Channel Change in Sand-Bedded Braided Rivers |
| Project Description | This thesis is concerned with the study of sand-bedded braided rivers in general and a well-studied example, the South Saskatchewan River, Canada in particular. Studies of sand-bedded rivers are far rarer in the literature than studies on their gravel-bed counterparts and hence our understanding of the morphological patterns and patterns of morphological change is relatively limited. Until very recently, studies of both sand- and gravel-bedded rivers have been of limited spatial extent, caused by an inability to undertake high accuracy measurements of form over a large spatial scale. Recent developments in our ability to measure the three-dimensional topography of braided rivers have now made it feasible to increase the spatial area of study from short reaches hundreds of metres long to the kilometre scale and larger. This study presents the first attempt to apply digital photogrammetric measurements to a large sand-bedded braided system. It has been necessary to refine and develop methods for reliably determining subaqueous topography over a large spatial extent. The end products are high resolution digital elevation models and orthophotographs (0.5 m and 0.25 m planform resolution, respectively), yielding a total of 71.2 million data points over a 15.2 km study reach of the South Saskatchewan River. These new data have been been combined with existing flow and sediment transport datasets to explore the dynamics of the system over the past 60 years. The construction of Gardiner Dam on the South Saskatchewan River in 1967 has played a crucial role in moderating flood flows and reducing suspended sediment concentrations. Numerical modelling of the semi-coupled two-dimensional flow and sediment transport equations suggests that reducing suspended sediment concentration may also have implications for braid bar dimensions and dynamics and the braiding mechanism. In the post-dam era, unit (free) bars are the key component of the river system in providing topographic and morphological variation. This is in contrast to previous studies that have focussed on the confluence-diffluence unit. It is hypothesised that this focus is a result of the scale of studied systems: in situations where unit bars are too small or migration rates too fast (e.g. in coarse-grained systems), the confluence-diffluence unit is more likely to have been studied. Comparison of topographic and topographic change data from the South Saskatchewan to similar data from other braided rivers has led to the speculation that the behaviour of the South Saskatchewan is akin to the truly large river systems, with a scale sufficient to form substantial features at multiple elevations and with individual anabranches that have a width: depth ratio sufficient to promote the formation of unit (free) bars. The thesis concludes with a number points for further discussion and investigation. |
| Awarding Institution | University of Leeds |