Magnitude-frequency characteristics of effective discharge for suspended sediment transport, Fraser River, British Columbia, Canada

Abstract

This study analyses archival discharge and sediment concentration data (1965–1988), monitored by Water Survey of Canada, to examine suspended sediment transport rates and their relationship to effective discharge (Qeff) based on daily discharge duration curves. Effective discharge was determined as the mid-point of the discharge class transporting the greatest portion of the suspended sediment load (hence class-based Qeff). Results showed that the concept of effective discharge was applicable to the Fraser River basin where the average class-based Qeff occurred during 8·4% of the study period with individual values ranging from 0·03% to 16·1%. The durations of effective discharge classes ranged from 0·02% to 19·6% while the transport of 50% of total sediment loads ranged from 3% to 22% with an average of 14% of the time. Equations for predicting the class-based Qeff in the Fraser River basin from bankfull discharge and drainage area are presented. The observed variations among stations in sediment-discharge regimes based on subjectively selected 20 discharge classes, seem to reflect the influence of sediment controlling factors such as geology, physiography, catchment size and land use practice in the basin. Future directions of research on applications of the effective discharge concept are explored. As a solution to the problem of lack of an objective method for determining the effective discharge, the effective discharge should be determined from event based assessments of sediment transport (event-based Qeff), avoiding any subjectivity in the selection of number of discharge classes used for its determination. In conclusion, it is proposed that continued use of the conventional method of determining Qeff should cease. Copyright © 1999 John Wiley & Sons, Ltd.