Hydraulic-morphological habitat assessment in rivers considering residual flow, sediment replenishment and artificial flood events

Dams store water and trap sediment in their reservoirs. Downstream river segments, hence, are affected by a limited sediment dynamics. This becomes obvious by river bed incision, limited geomorphological variability and depletion of hydraulic habitats for in-stream ecological organisms. Replenishment is a measure to counteract the lack of sediment. In segments dams such a sediment replenishment can be combined with the release of an artificial flood, by activating spillways and bottom outlets. Recent laboratory studies revealed the potential of multi-deposit replenishment configuration towards an increase of geomorphological richness in the river. However, this has not yet been tested in the field.

The hydro-morphological index of diversity (HMID), is based on the spatial variability flow depth and velocity in a river segment, allowing the quantification of the change in morphology such a measure. Its ease in computation and its objectivity makes the index a useful tool for a large variety of users. However, knowledge concerning data sufficiency and a standardized sampling procedure does not exist. Further, the HMID does not account for temporal variability.

This research aims at closing these gaps of knowledge for a sustainable management of complex floodplains downstream of dams.

Flow depth and velocity data from 19 river segments were analyzed with the HMID. Structured sub-sampling and analysis of the data revealed different data sufficiency depending on the complexity of the river geomorphology. A representative HMID value is computed from at least 100 datapoints distributed over eight cross-sections, in braided rivers more datapoints are required.

The geomorphological changes by an artificial flood event in the Sarine were analyzed with the HMID. In the impact perimeter of the multi-deposit sediment replenishment, the HMID increased by 36 % in the Sarine, in the rest of the river 18 %. A similar study was investigated at the Spöl, pointing out that sediment availability is crucial for the hydraulic habitat diversity.

With the help of 489 tracer stones, the movement of the replenished sediment was investigated in detail. The deposition pattern results in repeating sediment clusters on the riverbed, as it was observed in the laboratory experiments. For the two characteristic diameters, the tracers allowed the estimation of transport velocities of 19 cm/min (dm = 57 mm) and 28 cm/min (d90 = 113 mm) during the flood event and consequently the erosion efficiency of the different parts of the flood hydrograph.

Macroinvertebrates were less affected by the artificial flood event in the hydropeaking river segment of the Sarine compared with the residual flow segment. The species diversity indices decreased significantly due to the artificial flood event. A direct link between flow regime and EPT richness was observed. A river with a natural flow regime hosts the highest richness of macroinvertebrates followed the residual and finally the hydropeaking flow regime. For other ecological indicators, additional physical river parameters for example the frequency duration curve or the grain size distribution need to be considered as well.

This research aims at closing these gaps of knowledge for a sustainable management of complex floodplains downstream of dams.

Experiments with sediment replenishment in a residual flow reach: Comparison of field datawith laboratory experiments

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

Hydropower is an important pillar of the electricity production in Switzerland and its extension is planned in the next decades. Therefore, a more sustainable management of hydropower plants is needed. Sediment replenishment combined with artificially-triggered flood pulses are gaining increasing interest to restore residual flow reaches. In the Sarine river in Switzerland, such a measure was conducted. Mimicking laboratory experiments for the first time, four deposits, totaling 1000 m3 of sediment, were added to the river downstream of Rossens dam. Some 489 pebbles were equipped with RFID PIT tags and distributed among them. After the flood pulse passed, 57% of the tags were re-located. The maximum detected travel distance of a pebble was 284 m. Some deposits were eroded, while others resisted. Due to a limited submergence, mainly lateral erosion was observed. Nevertheless, erosion of deposits as well as deposition of eroded material in clusters resulted in the same scheme as observed in laboratory experiments which could be validated with this field experiment.

2018

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

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.