Gravel transport in the braided Waimakariri River: Mechanisms, measurements and predictions

Abstract

Yields of grabel bed load in the braided lower Waimakariri River in New Zealand are investigated by several methods and for different timescales: short-term (several months), medium-term (several years) and long-term (1967–1983). Short-term estimates of gravel yield were made for five such periods in 1986–1987 by a morphological method. Bed morphology in a selected reach was mapped before and after flood events, the gross volume of scour in that time period measured, and a mean transport rate through the reach computed on the basis of the step distance of gravel movement from scour zones to areas of accretion. The findings indicate that a considerable fraction of the annual gravel load is derived from scour of braid banks, with the gravel being shunted downstream from one zone of temporary storage to another. Computation of bed load yields during the same period by the Meyer-Peter and Muller (MPM) equation revealed excellent agreement with the morphological estimate. Comparison of the MPM prediction, based on flow duration data, for 1967–1983 also showed excellent agreement with the long-term rate, assessed by stadia survey of cross-sections in the reach of accumulation downstream of the study reach. Predictions using the MPM formula for the medium-term however, while acceptable, showed greater differences from the surveyed rates. Evidence indicates that this is related to highly dynamic bed conditions in which the availability of more mobile, finer-grained gravel in the reach changes from year to year, as slugs of such sediment move intermittently downstream. Thus, while prediction of a short-term yield (based on bed conditions at the time of study) and a long-term yield (based on temporal mean conditions as inferred from bed conditions averaged along the river) are realistic goals, the accurate prediction of successive short-term or medium-term yields seems to be impossible, unless detailed analysis of bed conditions (especially grain size) is undertaken in each time period — a major task.