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

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.

Replenishment of sediment downstream of dams

Elena Battisacco

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.

EPFL2016

Physical modelling of artificial river replenishment techniques to restore morphological conditions downstream of dams

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

First applied in the 1980’s, the artificial replenishment of sediments is one technique proposed to solve the problem of sediment deficit downstream dams. The present experimental research aims at improving the technique of river replenishment and at providing an engineering answer and framework for its application. A systematic series of laboratory tests are run to understand the hydrodynamics of the river flow when the replenishment technique is applied. Erodible volumes, with different lengths (occupancy of the replenishment volume is 1/3 of the channel width) and submergence conditions (100% and 130% submergence of the replenished volume), reproducing sediment replenishments volumes, are placed along a channel bank. Different geometrical combinations (single volume, double aligned volumes, double aligned half-volumes, double alternated half-volumes) of erodible sediment volumes are tested. The influence of discharge, the distance travelled by the eroded sediments and the time evolution of the erosion process are presented and discussed.

2015

Efficiency of Brushwood fences in shore protection against wind-wave induced erosion

Selim Sayah

Shore protection in confined water bodies is of major importance in Switzerland where many lakes suffer severe shore erosion. It occurs mainly in shallow zones due to several reasons such as: the fluctuations of the water level , the wind-wave impact during major wind events, and the increasing effect of incident waves due to solid reenforcement by non-adapted protection measures. In order mitigate this increased erosion and enhance the shore stability by sustainable coastal structures, pioneer and soft measures have been built during the past decades. Although such solutions proved their efficiency, they raised a new challenge for engineers: how to build soft and porous media that take into consideration the hydraulic and hydrodynamic conditions of the project site, and enhance its efficiency in sand trapping and shoreline accretion. By means of experimental and numerical modelling, the performance of a typical soft measure, called brushwood fence, was investigated in hydrodynamic conditions similar to those found in reality. A downscaled model of 1 : 10 was built on a fixed bed, and then tested in a wave tank of 10 m length and 6 m width under incoming perpendicular and regular waves. The response of the structure regarding the transmission coefficient has been analyzed for five dimensionless variables: 1) the relative freeboard of the structure Rc/Hi, 2) its relative height h/Hi, 3) the relative wave number kd, 4) the wave steepness Hi /gT2, and 5) the porosity of the structure p. The analysis of their effect on the performance of the protection structure allowed the establishment of an empirical relationship for the transmission coefficient. It takes into account the geometrical characteristics of the structure as well as the local hydrodynamic conditions. The response of the structure has to be treated separately in relation to its immersion condition. When the structure is submerged, every dimensionless variable has a differentiated effect on the transmission coefficient in comparison to the condition when the structure is emerging. The relative freeboard, wave steepness and the porosity are the three key variables in the response of the structure regarding wave damping. The efficiency of detached brushwood fences built on a movable bed was tested under the action of waves with fixed characteristics. The bed material was made of a granular, fine sand with a diameter d50 = 0.18 mm. The evolution of the shoreline and the transmission coefficient of the structure were determined using a first series of tests. It was demonstrated that time has negative impact on wave damping, at least during the period where the bed in the vicinity of the structure evolves to its equilibrium state. A second series of tests established the effect of the distance S of the detached structure (length B) from the shoreline on its evolution and on the deposited and eroded sand volumes at the leeside of the structure. It was proven that the wave motion through the porous media hinders the formation of significant salients and prevents the formation of tombolos when the structure was very close to the shoreline. Thus, for high ratio B/S = 1.48 no tombolo was formed in line with non porous breakwaters. The deposited and eroded volumes were also evaluated in a third series of tests with the presence of a single gap in the brushwood fences. The effect of the width of the gap on these volumes was analyzed. The results proved that the main deposition area is located between the gap and the shoreline where sediments are transported by the diffracted waves at the edges of the brushwood fences. The highest deposition rate at this location corresponds to the ratio B/S = 0.25. Using the solver of the elliptic mild slope equation developed under Mike 21, the wave field in an enclosed area surrounded by a porous structure was numerically investigated. The calibration of some major variables in the numerical model such as wave breaking and bottom friction parameters, based on the experimental results, proved the adequacy of the selected numerical scheme. The effect of a single and double gap in a linear infinite porous media was afterwards evaluated and diffraction diagrams were built for the enclosed wave field. A rule was proposed to use a relative gap width G/Wmax less then 0.12. The wave field for two gaps is different and significantly influenced by the spacing between the gaps. For low spacing values (Es/G = 2), waves in the middle of the protected area are high along the structure and very low close to the shore. For high spacing values (Es/G = 5), the wave field is also significantly deformed. However, values of Es/G comprised between 3 and 4 seem to be most appropriate since the corresponding wave field is less deformed. The two gaps configuration does not significantly increase the residual total energy behind the structure. It is relatively constant with spacing variations between two gaps and increased slowly with the increase of a single gap width. The experimental observations and the numerical results were successfully applied in Mörigen bordering Lake Biel. The wave fields were calculated numerically behind a series of segmented brushwood fences. Several wind-wave regimes and varying water levels were analyzed to optimize the performance of the porous protection structures.