Erosion processes of a novel configuration of sediment replenishment during an artificial flood

Floodplains downstream of dams with constant residual flow discharge often lack sediment supply and periodic inundation due to the absence of flood events. At the Sarine floodplain in Fribourg, western Switzerland, an artificial flood was released from Rossens Dam in 2016. This flood was combined with a novel configuration of sediment replenishment. The replenishment design was tested previously in laboratory experiments but not in the field. To investigate the erosion processes of the replenishment design, 489 stones with an average b-axis of 5.7 or 11.3 cm were equipped with RFID PIT tags and mixed in the replenished sediment. Downstream of the replenishment area, a pass-over loop fix-antenna was installed in the river to record passing PIT tags. With a mobile antenna, PIT tags were relocated after the flood. Peak discharge of the flood corresponded to a flood with a return period of two years; bankful discharge was observed during the flood peak. With the information of the fix-antenna, an average transport velocity for both PIT-tagged sediment sizes was calculated. Applying this measured transport velocity on all transported and post-flood detected stones allowed calculation of the erosion time for each stone and sediment deposit. The results show that the rising limb of the hydrograph is significantly more effective in mobilizing replenishment material than the decreasing limb. These findings are in line with observations made in the laboratory experiments.

Erosion, transport and deposition of a sediment replenishment under flood conditions

Christopher Robinson, Mário Jorge Rodrigues Pereira Da Franca, Anton Schleiss

River reaches downstream of dams with constant residual discharge often lack sediment supply and periodic high flows due to dam sediment retention and flow regulation, respectively. To test a novel multi-deposit methodology for defining environmental flows for activating the dynamics of the river morphology downstream of dams, a flood was released from Rossens Dam in Switzerland. This event was combined for the first time with a multi-deposit configuration of sediment replenishment consisting of four artificial deposits allocated as alternate bars along the riverbanks as a restoration measure. To validate the sediment transport behaviour observed in laboratory tests, stones were equipped with radiofrequency identification (RFID) passive integrated transponder (PIT) tags, a fixed antenna was installed at the river bed and a mobile antenna was used to enable the investigation of the erosion, transport and deposition of replenished sediments. The duration of the erosion period was determined for the tracked stones, and average transport velocities were found to be on the order of 10(-3)m/s. To estimate the erosion efficiency of the flood, defined as the eroded tagged stones compared with the released water volume, the hydrograph was divided into different periods: rising limb, constant peak discharge, decreasing limb. During the rising limb of the flood, which lasted for 20% of the total flood duration, more than 40% of the PIT tags were transported. The definederosion efficiencyis a measure to support the hydrographic design of artificial flood releases from dams. The deposition of tagged stones resulted in a repeating cluster formation, as expected from previous laboratory experiments, creating an increase in hydraulic habitat diversity. Comparison of the results obtained in the field and from laboratory experiments confirmed the robustness of the multi-deposit sediment replenishment method. Combined with the knowledge gained on the erosion efficiency, these results could motivate further applications and research into multi-deposit sediment replenishment techniques as a habitat-oriented river restoration measure. (c) 2020 John Wiley & Sons, Ltd.

2020

Flow-induced uprooting of young vegetation on river bedforms

Benoît Crouzy, Katharina Maria Edmaier, Paolo Perona

Riparian vegetation stabilizes sediment by its roots and henceforth impacts riparian morphodynamics. After germination or vegetative reproduction on river bars or islands, juvenile plants are exposed to a high risk of mortality due to uprooting by floods. We distinguish two main types of root erosion by flow. As Type I root erosion, we defined a flow induced drag mechanism, which causes a nearly instantaneous uprooting of mainly very young vegetation with not fully developed root system by pullout drag exceeding root resistance. Type II root erosion arises as a combination of bedform erosion resulting in a decreased anchoring resistance of the roots and subsequent Type I uprooting. This second type applies to later stages of root development and is a delayed process induced by sediment erosion of morphodynamic origin. In laboratory experiments we tested the validity of both mechanisms. We investigated the first Type of root erosion mechanism with static uprooting experiments with 1550 seedlings of Avena sativa and Medicago sativa grown in low-cohesive sediment in order to quantify the distribution of their anchorage forces for different sediment size and moisture conditions as well as for varying root structure. Furthermore, we measured root strength of Avena sativa seedlings and compared pullout and breaking force of young vegetation with identical root structure. Type II root erosion mechanism, which is driven by the reduction of root anchorage due to sediment erosion, was investigated in laboratory flume experiments. The intensity of sediment erosion that was required before uprooting occurred increased with increasing root length. The higher the flow, the less time was necessary to erode a seedling of certain root length. Thus, the duration that a given flow requiresto erode a certain root structure, can be associated to a certain vegetation maturity stage. Following, Type II root erosion results from the balance of timescales of both vegetation growth and flood occurrence and duration.

Crc Press-Taylor & Francis Group2014

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