The intermittent nature of most renewable energy sources requires their coupling with an energy storage system, with pumped storage hydropower (PSH) being one popular option. However, PSH cannot always be constructed due to topographic, environmental, and societal constraints, among others. Underground pumped storage hydropower (UPSH) has recently gained popularity as a viable alternative and may utilize abandoned mines for the construction of the lower reservoir in the underground. Such underground mines may have complex geometries and the injection/pumping of large volumes of water with high discharge could lead to uneven water level distribution over the underground reservoir subparts. This can temporarily influence the head difference between the upper and lower reservoirs of the UPSH, thus affecting the efficiency of the plant or inducing structural stability problems. The present study considers an abandoned slate mine in Martelange in Southeast Belgium as the lower, underground, reservoir of an UPSH plant and analyzes its hydraulic behavior. The abandoned slate mine consists of nine large chambers with a total volume of about 550,000 m³, whereas the maximum pumping and turbining discharges are 22.2 m³/s. The chambers have different size and they are interconnected with small galleries with limited discharge capacity that may hinder the flow exchange between adjacent chambers. The objective of this study is to quantify the effect of the connecting galleries cross-section and the chambers adequate aeration on the water level variations in the underground reservoir, considering a possible operation scenario build upon current electricity prices and using an original hydraulic modelling approach. The results highlight the importance of adequate ventilation of the chambers in order to reach the same equilibrium water level across all communicating chambers. For fully aerated chambers, the connecting galleries should have a total cross-sectional area of at least 15 m² to allow water flow through them without significant restrictions and maintain similar water level at all times. Partially aerated chambers do not attain the same water level because of the entrapped air; however, the maximum water level differences between adjacent chambers remain relatively invariant when the total cross-sectional area of the connecting galleries is greater than 8 m². The variation of hydraulic roughness of the connecting galleries affects the water exchange through small connecting galleries but is not very influential on water moving through galleries with large cross-sections.

Underground pumped-storage hydropower (UPSH) is a promising technology to manage the electricity production in flat regions. UPSH plants consist of an underground and surface reservoirs. The energy is stored by pumping water from the underground to the surface reservoir and is produced by discharging water from the surface to the underground reservoir. The underground reservoir can be drilled, but a more efficient alternative, considered here, consists in using an abandoned mine. Given that mines are rarely waterproofed, there are concerns about the consequences (on the efficiency and the environment) of water exchanges between the underground reservoir and the surrounding medium. This work investigates numerically such water exchanges and their consequences. Numerical models are based on a real abandoned mine located in Belgium (Martelange slate mine) that is considered as a potential site to construct an UPSH plant. The model integrates the geometrical complexity of the mine, adopts an operation scenario based on actual electricity prices, simulates the behavior of the system during one year and considers two realistic scenarios of initial conditions with the underground reservoir being either completely full or totally drained. The results show that (1) water exchanges may have important consequences in terms of efficiency and environmental impacts, (2) the influence of the initial conditions is only relevant during early times, and (3), an important factor controlling the water exchanges and their consequences may be the relative location of the natural piezometric head with respect the underground reservoir.

The fragmentation of rivers caused by the construction of dams and weirs disturbs fish migration and poses a threat to fish populations and aquatic biodiversity. Fish passages around hydraulic structures aim to restore river connectivity; however, the effective design of fish passages is a challenging problem that depends on several processes. The present experimental study investigated how the characteristics of a trash rack at the entrance of a fish passage for downstream migration affects fish behavior and subsequently the effectiveness of the fish passage. A series of experiments was carried out to systematically analyze the behavior of Atlantic salmon smolts in a flume with two outlets featuring the same 1:1 physical model of the entrance of a downstream passage with or without a trash rack. The parameters that were tested were the spacing of the vertical round bars of the trash rack, the location of the trash rack at the fish passage, and the velocity gradient at the entrance of the passage. Aggregated results showed that only 34% of the fish selected the outlet with a trash rack to exit the flume while 66% preferred the unobstructed outlet. More fish swam through the outlet with the trash rack when the spacing of the vertical bars increased from 10 cm to 20 cm and when the rack was placed in the higher velocity region compared to the lower velocity one. These results show that a trash rack acts as an obstacle to Atlantic salmon smolts passing through a downstream passage. When possible, trash racks should be avoided at the entrance of downstream fish passages.

Mean flow and turbulence measurements collected in a shallow Halodule wrightii shoal grass fringe highlighted significant heterogeneity in hydrodynamic effects over relatively small spatial scales. Experiments were conducted within the vegetation canopy (~4 cm above bottom) for relatively sparse (40% cover) and dense (70% cover) vegetation, with reference measurements collected near the bed above bare sediment. Significant benthic velocity shear was observed at all sample locations, with canopy shear layers that penetrated nearly to the bed at both vegetated sites. Turbulent shear production ($P$) was balanced by turbulent kinetic energy dissipation ($ϵ$) at all sample locations ($P/ϵ\approx 1$), suggesting that stem-generated turbulence played a minor role in the overall turbulence budget. While the more sparsely vegetated sample site was associated with enhanced channel-to-shore velocity attenuation $($71.4 $\pm$ 1.0%) relative to flows above bare sediment $($51.7 $\pm$ 2.2%), unexpectedly strong cross-shore currents were observed nearshore in the dense canopy ($V_{NS}$), with magnitudes that were nearly twice as large as those measured in the main channel ($V_{CH}; \overline{V_{NS}/V_{CH}}$ = 1.81$\pm$0.08). These results highlight the importance of flow steering and acceleration for within- and across-canopy transport, especially at the scale of individual vegetation patches, with important implications for nutrient and sediment fluxes. Importantly, this work represents one of the first hydrodynamic studies of shoal grass fringes in shallow coastal estuaries, as well as one of the only reports of turbulent mixing within H. wrightii canopies.

Salt intrusion occurring in estuarine environments can be aggravated by the presence of scour holes in the bed. Saltwater has a higher density than freshwater. Therefore saltwater accumulates in scour holes which may exacerbate the salinization of freshwater once it is entrained by wind or other forcings. Wind can contribute to estuarine circulation and stratification through three main mechanisms: direct wind mixing, wind straining and wind-driven lateral circulation. Previous studies (Csanady, 1982; Winant, 2004), suggest that horizontal wind circulation mostly comes into play in estuaries with laterally varying bathymetry, which is the case for our study site. The Haringvliet estuary in the Rhine-Meuse delta is a former tidal basin in the western part of the Netherlands; it varies in bathymetry and has been closed off by floodgates. The gates are only opened during ebb tide to discharge river into the sea, and also for a short period of time during flood tide for ecological purposes. The complex geomorphology of the estuary is composed of shoals and deep scour holes. An extensive field campaign was carried out for over six months in the Haringvliet, at the locations of the scour holes, in which we measured flow velocity, salinity, discharge, and wind speed and direction. Results indicate that, under an axial wind over the estuary, a horizontal circulation forms by downwind flow over shoals and upwind flow in the deep channels. Based on the collected dataset, a change from a down-estuary to an up-estuary wind direction occurred while the floodgates were closed. As a result of the wind influence, the flow direction in the stratified deep channel changed quickly, which provided sufficient shear and turbulence in the whole water column for vertical mixing. The sharp drop in the salinity concentration corresponding to the mixing and flushing in the scour hole occurred due to the wind-induced lateral circulation without having high river discharge. This research shows that, in a semi-closed estuary like the Haringvliet, lateral currents and the momentum transfer corresponded to that can exert a predominant control on estuarine circulation and stratification.Acknowledgments: This research was funded by the Netherlands Organisation for Scientific Research (NWO), research program SALTISolutions with project number P18-32. Rijkswaterstaat, the Dutch Ministry of Infrastructure and Water Management, is thanked for providing extensive field data for this research. References: Csanady, G.T., 1981. Circulation in the coastal ocean. In Advances in geophysics (Vol. 23, pp. 101-183). Elsevier.Winant, C.D., 2004. Three-dimensional wind-driven flow in an elongated, rotating basin. Journal of Physical Oceanography, 34(2), pp.462-476.