Sediment budgeting techniques in gravel-bed rivers
The difficulties in obtaining reliable sediment transfer data from direct field measurement or from sediment transport formulae are widely recognized by geomorphologists and river engineers. Quantifying morphological changes (erosion and deposition) in rivers by the analysis of archive data or by field survey, however, can overcome many of these difficulties and provide a mechanism by which sediment budgets can be calculated over a variety of spatial and temporal scales. This paper applies three sediment budgeting methods based on morphological changes in a hypothetical braided reach. These methods range from a simple two-dimensional planform budget to more sophisticated three-dimensional cross-profile and morphological budgets. The development of each budget technique is described and the limitations and applicability of each identified. The three methods are then used to calculated sediment transfer rates in a multi-thread reach on the River Severn in mid-Wales, UK. Results show that across four budget periods spanning 2.5 years the reach was a net exporter of sediment. Application of the planform budget to eight time periods since 1836 shows a similar pattern of net sediment export in the nineteenth century, but during the majority of the twentieth century the reach was a net sediment sink. Finally, the applicability of applying budgeting techniques to extended centennial and millennial timescales is discussed and an assessment made of the role they might play in advancing our understanding of Holocene river dynamics and informing sustainable river management practices.