Replenishment of sediment downstream of dams

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.

Geomorphic work by gravity currents with varying initial conditions

Jessica Zordan

Gravity currents are density-driven flows that are able to transport high amounts of sediment and are responsible of great geomorphic changes. Moreover they can have severe repercussions on the environment since they are conveyors of substances, e.g. pollutants, for long distances depending on their flow dynamics. The latter are determined by the release conditions and by the exchange at the upper and lower boundaries. The aim of this research study is to characterize the turbulent structure of gravity currents, as a consequence of the initial release conditions, to relate with their transport capacity. For this purpose, saline gravity currents are experimentally created in a laboratory channel through the lock-exchange technique. Different initial conditions, representing different configurations that can be found in reality, are considered during the experiments. The tested parameters are: initial buoyancy of the current in the lock, initial volume of release (i.e. lock-length), lock slope and grain sizes of the sediment that form an erodible bed over which the current flows. High-resolution velocity measurements, performed with an Acoustic Doppler Velocity Profiler (ADVP), allowed to describe both horizontal and vertical structures of the gravity currents. A universal criterion is established to isolate head and body of the current which are characterized by different dynamics and their extensions vary in relation with the conditions of release. A new parametrization, based on the computed temporal evolution of shear stress, allows to quantify water entrainment from the upper interface and sediment erosion capacity at the bottom. The effect of the increment of gravitational forces on its erosion capacity is tested by introducing a slope in the lock reach. The range of considered slopes goes from horizontal to S=8%. This latter is identified as a transient case in which two mechanisms compete i.e. on one side, the current entrains more water from the upper interface due to the increment of friction, the current expands, dilutes and therefore slows down; on the other side, the head is fed by the faster rear steady current thus inducing an acceleration. At the bottom, high shear stress associated with intense ejection and burst events influence erosion and bed load transport. The coupling of hydrodynamic mechanisms and processes of erosion, transport and deposition of sediment are investigated. It is shown that the upward motion, caused by mean and turbulent velocity components, promote vertical mixing of sediment fromthe channel bed. The feedback between the hydrodynamics of a gravity current and the geomorphic changes of a mobile bed are analysed. The shape of the front changes due to sediment entrainment and the deposition of sediment downstream creates characteristic patterns whose geometry reflect the coherent turbulent structure of the current. The scientific contributions elaborated in this research project allow to ameliorate the modelling of these flows, describing their inherent complex mechanisms under various initiation conditions and the interaction with suspended material. This helps to formulate adapted mitigation measures for the retention of these phenomena which frequently have a negative effect and e.g. induce reservoir sedimentation, subaqueous structure damages and scour processes in the vicinity of submerged pipelines or exacerbate pollutants dispersion.

EPFL2018

Sediment Transport and Flow Conditions in Steep Rivers with Large Immobile Boulders

Tamara Ghilardi

Recent flood events in Switzerland and across Europe pointed out several deficiencies in hazard assessment, planning, and prediction methods for flood risk mitigation. A good understanding of the dynamics of mountain rivers, grounded on a sound physical-based theoretical framework, is primordial both from an environmental and a safety point of view. Although mountain rivers control sediment supply to lowland rivers, relatively few studies have been carried out on steep mountain channels, mainly during the last two decades. While these studies provide a multitude of sediment transport equations, generally with similar forms, most of them does not take into account the extreme conditions characterizing alpine torrents. For these latter, the presence of macro-roughness elements, such as large relatively immobile boulders, disrupts the flow and alters channel roughness. Moreover, bedload fluctuations have been observed over time in steep rivers and flumes with wide grain size distributions, even under constant sediment feeding and water discharge. This research project investigates the impact of randomly distributed boulders (cascade morphology) on the sediment transport capacity and bedload fluctuations in steep channels. This is done by means of 41 laboratory experiments, carried out on a tilting flume at the Laboratory of Hydraulic Constructions (LCH) at Ecole Polytechnique Fédérale de Lausanne (EPFL). The influence of several boulder sizes and distance between roughness elements is investigated for three flume slopes (S=6.7%, 9.9% and 13%). Sediment transport, bulk mean flow velocities and variables describing the morphology were assessed regularly during the experiments. Firstly, the detailed analysis of a 13 hours laboratory experiment is presented. Periodical bedload pulses are clearly visible on this long duration experiment, along with correlated flow velocity and bed morphology fluctuations. The relation among bulk velocity, morphology variables time evolution and bedload transport is investigated by correlation analysis, showing that fluctuations are strongly linked. Visual observations indicate that the detected periodical fluctuations correspond to different bed states. Furthermore, the grain size distribution through the channel, varying in time and space, clearly influences these bedload pulses. For all the experiments, bedload pulses were then characterized by their amplitude and period. It is shown that for higher stream power the fluctuations decrease, both in duration of a cycle and in amplitude. The presence of boulders increases the stream power needed to transport a given amount of sediments, thus decreasing the fluctuations. The impact of increasing channel slopes on sediment transport is well known. The present research shows that it is also indispensable to take into account the presence of boulders in the estimation of the sediment transport capacity, since it is strongly decreasing with dimensionless boulder distance. Sediment transport capacity is better estimated when taking the liquid discharge as basis parameter instead of bed shear stress. The critical discharge for incipient motion is shown to be dependent not only on the channel slope but also on the dimensionless distance between boulders. A sediment transport formula based on excess discharge and taking into the presence of boulders is herein developed.

EPFL2013

Sediment replenishment: Influence of the geometrical configuration on the morphological evolution of channel-bed

Elena Battisacco, Mário Jorge Rodrigues Pereira Da Franca, Anton Schleiss

Dams trap sediment in the upstream reservoir, which may lead to river bed armoring, streambank erosion and failure, channel incision and reduction of the morphological diversity in the downstream river reaches. The replenishment of sediment is a mitigation measure for this problem to be applied in river reaches downstream of dams. Previously performed field experiments always used one single volume of sediment replenishment. To explore different alternatives, the replenished volume was here divided in four deposits with the motivation to influence also the morphological evolution downstream. Six different geometrical configurations together with three submergence conditions of sediment replenishment were tested for the first time in a laboratory experiment and are herein discussed. The results of the sediment replenishment mitigation technique are described in terms of occupied surface of the flume bed and the temporal evolution of erosion and transport of the introduced sediments. It is shown that, under our experimental conditions, complete submersion of the replenishment volume results in complete erosion of the placed sediment, with a high persistence of the added material along the channel length. The geometrical configuration of the replenishment volume plays a key role for the evolution of bed-forms downstream. Parallel configurations lead to a wider spread of material across the channel. Alternated configurations are suitable to produce sediment clustering and high persistence of placed material in the channel. Observed periodic mounds, considered as the initiating condition for alternate bars, follow a wavelength related to the length of the replenishment when the replenishment volumes are alternating.