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Sediment budget from morphology : Vedder River, British Columbia Martin, Yvonne Elizabeth


This study investigates the morphologic approach to sediment transport analysis and applies it to the Vedder River, British Columbia. The approach is based on the assumption that changes in channel morphology indicate sediment transport in the river. Despite the connection between these phenomena, only a few studies have examined this relation. The procedures, assumptions and limitations of the morphologic approach are discussed. It is more straightforward to construct a sediment budget for bed material than for wash material, as bed material travels relatively small distances. The Vedder River is a cobble gravel river with dyked banks. Therefore it is a good location for a study of the morphologic approach as bed material can be distinguished easily from wash material. The results of this study are important as aggradation in Vedder River has resulted in major flooding problems. Cross-section survey data were used to estimate volume changes along the Vedder River for incorporation into sediment budgets for several periods over the last decade. The construction of a sediment budget requires knowledge of at least one transport rate or transfer distance. Although the zero downstream transport assumption used in this study was found to be incorrect, it was retained as there are no transport rate measurements. Furthermore, the transport assumption is probably within the error ranges of the sediment budgets. Most of the errors in the sediment budgets were attributed to uncertainty in volume change estimates. When volume change estimates were calculated for different sets of cross-sections, the values varied significantly. This indicates that there is bias in the results. It is difficult to evaluate the degree of bias without a knowledge of actual channel changes. It. was found that the uncertainty in the transport estimates at Vedder Crossing ranged from ±8% to ±25%. These values compare favourably with error analysis results of direct measurements in the Fraser River (see McLean and Church, 1989). An analysis was performed to evaluate the cross-section density that is necessary to obtain a reasonable representation of actual channel changes. The average distance between cross-sections should be relatively smaller in reaches which have large variability in channel change patterns. It was suggested that cross-section spacing in the Vedder River should average between 250 and 300 m. The sediment budget results provide valuable information about the patterns of channel change and the magnitude of flows responsible for large amounts of deposition in the Vedder River. It was found that significant aggradation occurs during exceptional flood events. Most of the material is deposited in the several kilometers immedatiately upstream of the Vedder Canal. The morphologic approach provides a good method for evaluating the sediment transport regime of a river. The usual management time-scale ranges from several years to decades, which is coincident with the time-scale of this approach. Furthermore, the total field effort is less than that required for direct measurements.

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