Temporal and spatial variations of bed material in the lower Ebro River

Abstract

This paper reports on the temporal and spatial variations of the riverbed grain-size distribution and armouring at-a-section and downstream in the strongly regulated lower Ebro River. Fieldwork was performed in 8 bars downstream from the Flix Dam between summer 2002 and summer 2004. River-bed grain-size distributions observed indicated that surface material was coarser (D50-s= 34-43 mm) and better sorted than subsurface sediments (D50-ss= 16-18 mm). The grain-size distribution varies considerably between years of sampling and sections. This fact can be primarily attributed to the high sedimentary activity within the Ebro riverchannel and to the role of floods remobilizing and transporting riverbed particles to downstream reaches. Armouring has been observed in several sections of the river, being the highest values located immediately downstream from the Flix Dam and the confluence with the Siurana River. The degree of armouring varies in time (mean values of 2.3 in 2002, 1.9 in 2003 and 2.3 in 2004) a phenomenon that can be directly related to the magnitude of the floods. Observations indicate that armouring is a fundamental hidrosedimentary processes in regulated rivers that remains active even decades after dams closure.