Fractional bedload transport during small floods in a regulated gravel-bed river

Fractional bedload transport during small floods (<Q2) was studied in the regulated gravel-bed Ebro River during the period 2003-2004. This paper analyses the relations between shear stress and particle entrainment and hiding, the relation between discharge and fractional bedload transport, and quantifies overall sediment export from the study reach. Analysis is based on bedload samples collected during small floods and bed grain-size distributions obtained at the beginning and at the end of the study period. Data indicate that, under the existing supply-limited conditions, partial transport controls the progressive coarsening of the bed-surface materials in the reach. This process affects sediment entrainment by increasing particle hiding and reducing dimensionless critical shear stress. In addition, a reduction of bed-load transport rates occurs as a consequence of the lower availability of gravel fractions on the bed surface, which are mobilised and not replaced from upstream. These sedimentary processes caused a progressive reduction (almost 50%) in the net export of sediment of all size-classes from the reach during the study period.