Sediment mobility recovery experiences: dam removal and sediment transfer through dams

Dams, weirs and other transversal obstacles limit sediment transport. River restoration seeks the recovery of river processes and functions. Rivers need water, sediment and space. In this work we present a couple of examples of sediment transport recovery and monitoring. One of them was implemented through the removal of a dam and the second one through sediment transfers downstream a dam.

Dam removal allows the recovery of the longitudinal connectivity of rivers, facilitating water and sediment transfer. In Leitzaran River (Basque Country, Spain) bedload sediment transport was monitored from 2016 to 2022 in a reach of river affected by the Oioki dam, 7-meters high. The removal, funded through a LIFE project, started in 2018 with the first 3 meters of the dam, and was completed in 2019 with the remaining 4 meters. RFID technique was used for the monitoring. 300 tagged particles were introduced each year, 100 in each subreach (control, upstream the dam and downstream), 1800 in total. Each summer an intensive field work campaign searching these tags has been conducted. This long monitoring has given results of the sediment transport in different conditions, before removal, during partial removal and after complete removal. Once the dam is completely removed an important increase of the volume of mobilized sediment both in the upstream and downstream reaches is given. The mobilization of sediment is more a question of availability and change of river conditions, opening of a barrier and base level change, more than flows, as it was registered in the most discreet hydrological year of all the monitoring period.

In active hydroelectrical stations,, where removal is not an option, sediment transport recovery could be implemented differently. In Viar River, a tributary of Guadalquivir (Spain), is a completely clogged counter reservoir where the electrical company (ENDESA) needs to withdraw sediment to increase volume and pump it back to El Pintado reservoir for reversible hydroelectrical generation. Guadalquivir basin authorities require giving it back to the river instead of bringing to a dump. In 2021 autumn 700 m³ of sediment were withdraw from a deposit and located upstream the counter reservoir dam, terrain characteristics are difficult and do not allow downstream mechanical transfer. The clogged reservoir would allow the auto-transfer by the river. The goal is to propose an auto-transfer system, studying the critical flow and creating flow conditions from upstream reservoir to allow sediment movement and transfer, recovering environmental conditions and avoiding reservoir clogging.