Numerical Study of Discharge Adjustment Effects on Reservoir Morphodynamics and Flushing Efficiency: An Outlook for the Unazuki Reservoir, Japan

The Unazuki Reservoir is located on the Kurobe River, which is influenced by a catchment with one of the highest sediment yields in Japan. Due to a sufficiently available discharge during flood events, annual sediment flushing with full water-level drawdown (i.e., free-flow sediment flushing) is conducted to preserve the effective storage capacity of the reservoir. Nevertheless, the upstream half of the reservoir (i.e., study segment) suffers from the excessive deposition of coarser sediments. Remobilization of these coarser materials and their transportation further downstream of the reservoir is a priority of reservoir owners for sustainable reservoir functions, such as flood-risk management and hydroelectric energy generation. In this paper, an already conducted sediment-flushing operation in the Unazuki Reservoir is simulated, and its effects on sediment scouring from the study segment of the reservoir together with changes in bed morphodynamics are presented. A fully 3D numerical model using the finite volume approach in combination with a wetting/drying algorithm was utilized to reproduce the hydrodynamics and bed changes using the available onsite data. Afterwards, the effects of discharge adjustment on the morphological bed changes and flushing efficiency were analysed in the study segment using an additional single-discharge pulse supplied from upstream reservoirs. Simulation results showed that an approximately 75% increase in the average discharge during the free-flow stage changed the dominant morphological process from deposition into an erosive mode in the study segment. If the increase in discharge reaches up to 100%, the flushed volume of sediments from the target segment can increase 2.9 times compared with the initiation of the erosive mode.

Freeze-Up Ice Jam Formation in the River Bend, a Case Study on the Inner Mongolia Reach of Yellow River

Concern has been expressed regarding the impacts of climate change on river ice and ice jam formation in cold regions. Ice jams are easily initiated in bends and narrow channels and cause disasters. In this study, observations and remote sensing monitoring are used to study the freeze-up ice jam formation of bends. Sediment transport and freezing process of the river interact, influencing bed changes profile and sedimentary budget. River ice processes, channel evolution, ice hydro-thermodynamics, and ice jam accumulation are explored. The results show that the channel topography determines the river thalweg, and that the channel elevation interacts with the river ice through sediment transport. The channel shrinkage increases the probability of ice jam, and the sharp bend is prone to ice jam formation. Under the effect of secondary circulation flow in the bend and in the outer bank, the juxtaposed freeze-up and the hummocky ice cover occur in the same location, and frazil ice accumulates under the junction of the main channel and the shoals. Affected by the increase of the hydraulic slope and the velocity downstream, open water reaches develops downstream of the ice accumulation. An open water section is emerged upstream of the bend, due to the ice deposition, and partly cut-off supply of the frazil.

Modeling shelter abundance for juvenile Atlantic salmon in a residual flow reach of a hydro power plant using SSIIM 2

<p>Nowadays, the aquatic biodiversity is highly under pressure due to anthropogenic changes of the rivers such hydraulic structures changing the diversity of flow and aquatic fauna as well as sediment continuity. This can have severe consequences on the fish population in the river reach. Fish are strongly depending on a certain substrate composition throughout all their life stages. Juveniles for example are depending on a certain availability of shelter in the substrate in order to survive this stage.</p><p>Therefore, we investigate the effects of changes in the sediment composition at a hydropower plant in Switzerland on the availability of potential shelter for juvenile fish. By utilizing the observed correlation between parameters describing the fine tail of a riverbed’s grain size distribution and shelter abundance for juvenile Atlantic salmon, we predict the available shelter in a river reach by using a 3D hydrodynamic numerical model directly coupled to a morphodynamic model. The initial substrate composition was assumed to be spatially uniform, its parameters based on a grain size distribution curve derived from collected sediment samples.</p><p>This model can now be used for habitat improvement scenario modeling. Based on the assumption that a specific mixture of sediment coming from upstream travelling through the river reach will positively influence the potential shelter availability, different scenarios can be investigated. The baseline for comparison was the simulation of the bed changes without any sediment supply from upstream. The baseline discharge was set to 100 m<sup>3</sup> /s and was applied for 24 hours. The resulting bed changes create a map of the potential shelter availability of this grain size mixture. Then, two scenarios with sediment inflow from the upstream boundary were simulated. One coarse and one fine mixture of sediment were chosen as inputs, with the goal of investigating their impact on shelter abundance. The former designed to have a positive effect while the latter expected to reduce interstitial voids in the substrate and have a negative effect on available shelter.</p><p>The investigation is conducted as part of the EU Horizon 2020 funded project FIThydro (funded under 727830)</p>

ANALYSIS OF THE PERFORMANCE OF DIFFERENT SEDIMENT TRANSPORT FORMULATIONS IN NON-HYDROSTATIC XBEACH

Process-based, wave-resolving models are essential tools to resolve the complex hydro-morphodynamics in the swash zone. The open-source Non-Hydrostatic XBeach model can solve the depth-averaged wave-by-wave flow in the nearshore region up to the shoreline and the intra-wave bed changes during time-varying storms. However, validation and testing of its morphological response are still limited in the context of sandy beaches. This work aims to assess the performance of the wave-resolving sediment dynamics modelling within Non-Hydrostatic XBeach for different sediment transport formulations. The sediment transport modelling approaches considered in this study were tested and compared to laboratory experiments involving wave trains over an intermediate beach. Numerical results show a better performance in the prediction of the intra-swash sediment dynamics when the newly implemented wave resolving transport equation is applied compared to the existing approach within the model.Recorded Presentation from the vICCE (YouTube Link): https://youtu.be/2sy-Dr8iJ1M

Response of specialized birds to reed-bed aging in a Mediterranean wetland: Significant changes in bird biomass after two decades

Traditional fish farming carried out in wetland is declining in many countries of Mediterranean Europe. This decline can lead to a lack of management of the reeds that tend to age progressively. In this work we compared, through a wide temporal range (2001-2019), the densities of four habitat-specialized birds (warblers), strictly linked to Phragmites australis reed-beds in a coastal wetland on the Tyrrhenian central Italy. In this wetland, following the abandonment of fish farming, the average density of reeds significantly decreased, and both the average reed diameter and habitat heterogeneity showed a significant increase. Comparing 2001 to 2019, we observed an increase in the total density of breeding warblers. The two species of Acrocephalus (scirpaceus and arundinaceus), and Cettia cetti showed a marked increase in density, while Cisticola juncidis showed a clear decrease. More particularly, a significant increase in Cettia cetti (p < 0.001) and a decrease in Cisticola juncidis (p < 0.05) emerged when comparing bird biomasses. Species diversity and evenness were more high in 2019 than in 2001. Our data suggest that: (i) these species could be considered indicators of long-term reed-bed changes and (ii) their biomass may be used as a more effective metric when compared to abundance.

A Two-Dimensional Depth-Averaged Sediment Transport Mobile-Bed Model with Polygonal Meshes

A polygonal-mesh based numerical method is developed to simulate sediment transport in mobile-bed streams with free surfaces. The flow and sediment transport governing equations are depth-averaged and solved in the two-dimensional (2D) horizontal space. The flow and sediment transport are further coupled to the stream bed changes so that erosion and deposition processes are simulated together with the mobile bed changes. Multiple subsurface bed layers are allowed so that bed stratigraphy may be taken into consideration. The proposed numerical discretization is valid for the most flexible polygonal mesh type which includes all existing meshes in use such as the quadrilateral-triangle hybrid mesh. The finite-volume method is adopted such that the mass conservations of both water and sediment are satisfied locally and globally. The sediment transport and stream bed processes are formulated in a general way so that the proposed numerical method may be applied to a wide range of streams and suitable for practical stream applications. The technical details of the numerical method are presented; model verification and validation studies are reported using selected cases having physical model or field measured data. The developed model is intended for general-purpose use available to the public.

Investigation of thermo-mechanical response of a geothermal pile through a small-scale physical modelling

Energy piles have been used around the world to harvest geothermal energy to heat and cool residential and commercial buildings. In order to design energy geo-structures, thermo-mechanical response of the geothermal pile must be carefully understood. In this paper, a small scale physical model is designed and a series of heating thermal cycles with various vertical mechanical loads are performed. The instrumented pile is installed inside a dry sand bed. Changes in pile head displacement, shaft strains and pile and sand temperatures are monitored using an LVDT, strain gauges and thermocouples, respectively. Prolonged heating cycles, which would continue until boundary temperature changes, would allow the investigation of excessive heat injection when service loads are active on the pile. The thermal response is discussed including confirmation of a temperature influence zone around the pile, the increase in soil temperature, and minimum vertical heat dispersion in the soil. The mechanical response includes plastic settlements when the vertical load passes 20% of ultimate capacity. Plastic settlements have been observed at the half of the capacity reported for the shorter thermal cycles in similar models. The decrease in the capacity indicates a reduction in elastic response of the soil during longer thermal cycles.