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

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.

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

Severin Stähly

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.

EPFL2019

Process-Based Precipitation-Driven Soil Erosion Modeling

Seifeddine Jomaa

Soil erosion due to rainfall is a complex phenomenon. Factors influencing erosion and subsequent transport include rainfall intensity, soil properties, topography, land cover and antecedent conditions. These factors and their interactions can be approximated in physically based models. This thesis aimed to develop new physical understanding of precipitation-driven soil erosion processes on the laboratory flume scale. Experiments were carried out using the 6-m × 2-m EPFL erosion flume. In all experiments, different surface treatments were employed, with sediment concentration data collected from the flume effluent. The experimental data were analyzed using a mechanistic process-based soil erosion model, known as the Hairsine-Rose (H-R) model, with the goal of assessing the model's ability to describe the data. The H-R model simulates an arbitrary range of sediment size classes, a feature that is crucial to describe flume-scale soil erosion in detail. A range of related phenomena was investigated, as outlined below. First, the effect of raindrop splash and transversal width of the experimental flume on soil erosion was examined. In addition, the parameter consistency of the H-R model under these conditions was tested. Results showed that the H-R model represented well the total sediment concentrations as well as those of the fine and medium sediment size classes only when the transverse width of the flume experiment was below/around a threshold value related to lateral splash length. It was found, however, that boundary condition-induced asymmetry markedly reduces the applicability of the H-R model (and likely any other erosion model). Furthermore, in common with some existing reports, it was confirmed that the H-R model does not predict well the breakthrough of larger sediment size classes, possibly because for those sediments the transport mechanism involves rolling on the bed, which is not accounted for in the model. Second, the proportionality between soil erosion and area-exposed to the raindrop splash was tested. Results revealed that under carefully controlled laboratory conditions, the cumulative eroded mass depends on the cumulative runoff, and that soil erosion is proportional to the area exposed to raindrops. However, this relationship was controlled to a smaller extent by the initial soil conditions such as moisture content, bulk density and soil surface characteristics. The sediment concentrations in the flume effluent during the initial phase of the erosion event were sensitive to the initial state of the soil surface. At steady state, it was observed that the concentration of eroded sediments were controlled mainly by the effective rainfall and area exposed to raindrops. In contrast, using published data it was shown that rain splash erosion in the field is, in general, not proportional to the area exposed. Unlike the controlled laboratory experiments, the field experiments were characterized by a non-uniform initial surface roughness, surface soil aging and heterogeneous rock fragment size and spatial distribution. The third part of this study was dedicated to investigation of the effect of rock fragments on flume-scale hydrological response and soil erosion delivery. An area-based modification of the H-R model that accounted for the rock fragment coverage was employed to analyze the experimental data. The results revealed that the rock fragments protected the soils from raindrop splash and retarded the overland flow, therefore decreasing its sediment transport capacity. Individual rock fragments prevented surface sealing beneath them, which in turn increased the overall infiltration rate and affected the development of water depth compared with bare soil conditions. At short times, rock fragments were found to affect selectively the different size classes. This is due to the delayed (compared to a bare soil) development of the overland flow depth at the commencement of the erosion event. At steady state, the rock fragments led to decreased concentrations of the individual size classes in proportion to effective rainfall and the area exposed to raindrops. It was found that the modified H-R model predictions agreed well with measured sediment concentrations when high rainfall intensity and low rock fragment coverage were used. On the other hand, the H-R model could not reproduce the details of measured sediment breakthrough curves for conditions of low rainfall intensity and high rock fragment coverage. Finally, the effects of initial soil conditions and rock fragment coverage on particle size distribution and eroded mass were investigated using multiple rainfall events. Results revealed that short-time soil erosion is sensitive to the erosion history, in particular to whether steady-state conditions were reached in the preceding erosion event. Because the largest sediment sizes take longer to reach steady state, this sensitivity was more pronounced for the larger size sediment classes. The results also showed that the presence of rock fragments on the topsoil surface reduced the time needed to reach steady state compared with bare soil. This reflects the reduction in rain splash erosion caused by rapid development of the overland flow depth (which results from the reduction of the flow cross-sectional area by rock fragments).

EPFL2012

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