Replenishment of sediment downstream of dams: erosion and transport processes

Dams on rivers alter the sediment continuum, trapping water and sediment in the upstream reservoirs. River reaches downstream are affected by several negative effects, such as bed incision, reduction of the river morphological variability, development of an armoured layer and depletion of ecological habitat for fish. The replenishment of sediment technique has been used since the 1970s to mitigate the lack of sediment transport in the downstream reaches of dams. The method is mainly used to re-establish a sediment continuum, restore a natural bed morphology and to recover spawning grounds for fish. Even if both experimental and field tests have been performed in the past, there is still a lack of knowledge regarding the necessary flow released from the bottom outlet of the dam, the amount of sediment and the configuration of the replenished deposits. This research aims at filling these gaps by means of experimental tests at the Laboratory of Hydraulic Constructions at Ecole Polytechnique Fédérale de Lausanne (EPFL). The main characteristics of an alpine stream were reproduced in a channel facility in terms of grain size distribution, slope and hydraulic conditions. The sediment replenishment technique was investigated as an influence to the geometrical configurations and volume of deposits, as well as applied discharge. The experiments showed that the bed morphological pattern created by the eroded replenishment material was linked to the initial geometrical arrangement of the replenishment volumes. When applying parallel configurations of replenishment volumes, a general bed fining was obtained with material spread over the entire channel width. Alternating replenishment positioning lead, in turn, to the creation of bed morphological patterns. The wavelength of these bed forms was seen to be related to the replenishment length. Both constant discharge and transient flow were investigated. Three submergence conditions of the replenishment were tested: unsubmerged, completely submerged and over-submerged. A complete submergence was optimal to obtain a complete erosion of the volumes with transport of the material along the channel and persistence of the material on the channel bed. Four slopes for the increasing and decreasing limb of triangular-shaped hydrographs, having the same maximum discharge, were tested and the results compared with the constant discharge cases. A discharge having a triangular distribution in time is similar to the operational condition of dams when releasing an artificial flood. Transient flows with steep rising limbs lead to a reduction of 70% of water consumption compared to a constant flow, although achieving the erosion of the replenished material. The application of the complete hydrographs showed to be counter-productive in terms of local effect of the sediment replenishment. These hydrographs may be useful for reaching longer distance impacts in sediment replenishment. Lastly, the effect of consecutive replenishment of sediments was investigated. A second replenishment can increase the deposition heights and volumes downstream. As expected, a second replenishment affects a longer downstream channel reach. Thus, consecutive replenishments are useful in field applications to have effects at longer downstream distances.

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