Reach-scale suspended sediment balance downstream from dams in a large Mediterranean river

Abstract

This paper presents a reach-scale sediment balance of a large impounded Mediterranean river (the lower Ebro, 1998Ebro, -2008. Multi-temporal sediment storage and the influence of floods and tributaries on the sediment load were examined using continuous discharge and turbidity records. The mean annual suspended sediment load at the reach outlet (Xerta) is 0.12 × 10 6 t, corroborating previous results. Suspended sediment concentrations were low (SSC mean = 13 mg L -1 ), attaining a maximum of 274 mg L -1 . Erosion processes (channel-scour, bank erosion) are dominant, and net export of sediment occurs over the long term. Unexpectedly, ephemeral tributaries were found to contribute significantly: sediment delivered during torrential events attained 5% of the Ebro annual load, and was even larger than that in flushing flows. Overall, most of the suspended sediment load is transported by floods (up to 65% in some years). The results constitute basic information to underpin current management actions aiming to achieve the sustainability of the riverine and deltaic system. Key words sediment transport; dams; flow regulation; floods; sediment storage; erosion; sedimentation; River Ebro Bilan des sédiments en suspension dans un bief de grand fleuve méditerranéen en aval de barrages Résumé Cet article présente un bilan des sédiments en suspension à l'échelle d'un bief d'un grand fleuve méditer-ranéen harnaché (à l'aval de l 'Ebre, 1998'Ebre, -2008. Le stockage des sédiments à toutes les échelles de temps et le rôle des inondations et des affluents sur la charge sédimentaire ont été examinés en utilisant des enregistrements continus du débit et de la turbidité. La moyenne annuelle de la charge de sédiments en suspension à l'exutoire (Xerta) est de 0,12 × 10 6 t, ce qui corrobore des résultats précédents. Les concentrations de sédiments en suspension étaient faibles (CSSmoyen = 13 mg L -1 ), atteignant un maximum de 274 mg L -1 . Les processus d'érosion (affouillement du chenal, érosion des berges) sont dominants et, sur le long terme, on observe une exportation nette de sédiments. De façon inattendue, on constate que les affluents intermittents contribuent de manière significative: les sédiments exportés lors d'événements torrentiels représentent 5% de la charge annuelle de l'Ebre, et sont même plus importants que ceux des débits de chasse. Dans l'ensemble, l'essentiel de la charge de sédiments en suspension est transporté au cours des crues (jusqu'à 65% sur quelques années). Les résultats constituent une information de base permettant d'étayer la politique de gestion actuelle visant à assurer la viabilité du système fluvial et deltaïque.

Suspended sediment load at the lowermost Ebro River (Catalonia, Spain)

“…This value is the same as reported by Tena et al (2012) in Xerta, located 14.5 km upstream of our study section, (13 mg/l) and similar to estimates by Guill en and Palanques (1992) in Amposta (15 mg/l) (Fig. 1), but higher than the value obtained in Tortosa (9 mg/l) by N egrel et al (2007) for 1981e2004.…”

“…The main purpose of the dam system is hydropower production. Downstream reservoirs water and sediment inputs from tributaries are not significant, except during local heavy rain events (Tena et al, 2012). After dam construction, frequent floods (i.e.…”

“…As a result, the lower Ebro River and its delta are facing a severe sediment deficit leading to a progressive change of the river channel morphology and sediment transport dynamics (Guill en and Palanques, 1992;Tena et al, 2012), a degradation of the fluviodeltaic system (Ib añez et al, 2012a), and a dramatic reduction of fluvial sediment inputs to the delta (Jim enez and S anchez-Arcilla, 1993). In the long-term, a significant elevation loss of the delta plain due to subsidence and sea level rise is expected, with the prediction that 45% of the emerged delta will be under mean sea level at the end of this century (Ib añez et al, 2010).…”

Quantifying the Effects of Near-Bed Concentration on the Sediment Flux after the Operation of the Three Gorges Dam, Yangtze River

“…Floods are relevant for most of the suspended sediment load [34]. While sediment transport by floods correlated with flood regime, different flood regimes lead to different sediment transportation.…”

“…Measured Year Groups Zhicheng 1996, 1998, 200255 Shashi 1996, 1998, 2010, 2011, 2012130 Jianli 1986, 1998, 2002, 2010, 2011, 2012 Vertical profiles measured before and after 2010 are different. For data measured before dam operation, there are only five measuring points in each vertical line.…”