Hydrological and sediment transport dynamics of flushing flows: implications for river management in large Mediterranean Rivers

Flushing flows have been implemented in the lower Ebro River since 2003, with the principle objective of removing excess macrophytes. This paper describes the hydrology and sediment transport of these flushing flows in comparison with natural floods and discusses their effects in terms of riverbed adjustments. During flushing flows suspended sediment, concentration is more constant and double that of natural floods, although discharge is lower. Flashiness, measured as the rate of discharge increase per unit time, is an order of magnitude higher during flushing flows than during natural events. Consequently, flushing flows exhibit higher transport capacity than their natural counterparts despite their considerably lower magnitude and duration. Flushing flows remove up to 95% of macrophytes close to the dam, but their effectiveness reduces with distance downstream. Despite several constraints, flushing flows have significant potential to entrain and transport sediment and careful management of these releases may, therefore, play an important role in enhancing physical habitat in the river and supply sediment to the delta. The River Ebro case study shows that flushing flows are not incompatible with HEP production, and may actually result in a positive trade-off due to reduced clogging of water intakes. Nevertheless, it remains important to reassess their effectiveness regularly and monitor adverse geomorphic effects such as riverbed degradation. In summary, flushing flows are an important instrument of river management, but one which must be employed as part of a spectrum of approaches to enhance physical habitat conditions.