Suspended sediment transfer during a flood along the dam-regulated Segre river(NE Iberian Peninsula)

Abstract

Suspended sediment transport has crucial functions along the river ecosystem and the disruption of its transport has morphological, ecological and economical effects. In this study we have designed a near basin scale sampling and monitoring approach in order to study the effects of the Oliana-Rialb dam complex on the suspended sediment dynamics and transport in the River Segre (13,000 km2 basin located in NE Iberian Peninsula) during a five-year return period flood event (peak flow of 776 m3•s-1) occurred in May 2008. The approach presented here allowed estimate the sediment load passing the lowermost part of the basin and ultimately, being supplied to the main River Ebro upstream the Riba-roja dam. Results show that the two large reservoirs in the Segre River retained almost all the suspended sediment carried by the flood (between 75% and 97%). Downstream from the dams, in-channel and bank sediments were mobilized thus progressively increasing the sediment load in the downstream direction and mitigating the deficit of sediment caused by the dams. The five-year-return-period analysed flood transported the 26% of the annual suspended sediment load at the lowermost River Ebro, one of the largest rivers in the Iberian Peninsula. This study shows that Oliana and Rialb reservoirs have a great influence in the transfer of suspended sediment in the Segre River and that this influence could be modified in some extent through dams’ management, maximizing the transfer of sediment downstream and reducing the effects of the sediment deficit caused by the impoundment.