Assesment of bedload equations using data obtained with tracers in two coarse-bed mountain streams (River Narcea basin, NW Spain)

Abstract

Fluvial channels placed in mountain basins show an important decrease in slope when compared with the immediate upstream torrential reaches, and this fact happens very close to the sediment producing areas. Consequently, coarse sediment deposits into these reaches. Then, channel bed sediment typically falls in coarse ranges of grain sizes, being bedload transport very sporadic and occurring with flow conditions close to the threshold ones. Rivers from NW Spain are defined by this situation, dropping by 2,000 m in a short path (50 km). In this region, drainage networks with relatively steep slopes, and where channel bed sediment is typically coarse, develop. Tagged stones were used to trace bed sediment movement during flood events in River Narcea basin. By this way, bedload transport rates between 0.2 and 4.0 kg/s were estimated for six flood episodes. After doing that, nine different transport equations were applied. The bedload discharges estimated with these formulas were compared with the bedload rates obtained with the tracer experiment. None of the chosen equations performed adequately: all of them tend to overestimate when compared with the tracer results, being the Wilcock-Crowe (2003) equation the only exception. We think that the overestimation is related to the particular geomorphology of coarse bed mountain rivers in humid environments. These equations assume how the channel will carry the entire load which is able to transport, without taking into account potential limitations in the supply of sediment. But in these rivers, armoring and structural arrangements in the bed are ubiquitous; these features, together with a low sediment supply coming from upstream forested reaches, define a supply-limited condition for these channels limiting the potential use of bedload equations. Wilcock-Crowe (2003) equation introduces complex corrections into the ‘hiding fuction’, and considers the effect of sand content on entrainment; this could explain why it performs better.