Mass balance analysis of ice sheets is a key component to understand the effects of global warming. A significant component of ice sheet and shelf mass balance is iceberg calving, which can generate large tsunamis endangering human beings and coastal infrastructure. Such iceberg-tsunamis have reached amplitudes of 50 m and destroyed harbours. Calving icebergs interact with the surrounding water through different mechanisms and we investigate five; A: capsizing, B: gravity-dominated fall, C: buoyancy-dominated fall, D: gravity-dominated overturning and E: buoyancy-dominated overturning. Gravity-dominated icebergs essentially fall into the water body whereas buoyancy-dominated icebergs rise to the water surface. We find with unique large-scale laboratory experiments that iceberg-tsunami heights from gravity-dominated mechanisms (B and D) are roughly an order of magnitude larger than from A, C and E. A theoretical model for released iceberg energy supports this finding and the measured wave periods upscaled to Greenlandic outlet glaciers agree with field observations. Whilst existing empirical equations for landslide-tsunamis establish estimates of an upper envelope of the maximum iceberg-tsunami heights, they fail to capture the physics of most iceberg-tsunami mechanisms.

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Horizontal bar racks have been used as trash racks at hydropower plants since the 1920s. With the installation of the first horizontal bar rack bypass system at a hydropower plant as a downstream fish passage facility in 2006, these racks rapidly gained importance as fish protection measures. Since then, they have been installed at more than 100 small-to medium-sized hydropower plants in Europe. Despite the large number of installed racks, systematic investigations of the head losses and velocity fields were missing. On the basis of detailed hydraulic experimentation with a large number of rack parameters and including up-to-date foil-shaped bars, the layout of horizontal bar racks and their hydraulic performance were assessed in the current study. This paper reports the results of the rack head loss investigation, whereas the accompanying paper entitled Velocity Fields at Horizontal Bar Racks as Fish Guidance Structures focuses on the up-and downstream velocity fields. By applying foil-shaped bars instead of rectangular bars, the loss coefficient was reduced by more than 40%, depending on the rack configuration. Bottom and top overlays are used to increase the guidance efficiency for fish, sediments, and floating debris. However, the altered flow field results in increased head losses. A new set of equations is proposed to predict head losses for current horizontal bar racks, including overlays for various hydropower plant layouts. The predictions are compared to literature data.

Horizontal bar racks used as fish protection measures at hydropower plants have rapidly gained importance in recent years. Despite the large number of installed racks in Europe, systematic investigations of the hydraulic losses and velocity fields were missing. To fill these research gaps, the hydraulic performance of horizontal bar racks was systematically investigated in a laboratory flume for a large number of rack parameters and different hydropower plant layouts. The results of the head loss assessment are published in a paper entitled Head Losses of Horizontal Bar Racks as Fish Guidance Structures, whereas the present paper focuses on the velocity fields. The measurements show that the bar shape, the horizontal approach flow angle, and the clear bar spacing have only a minor effect on the velocity fields. In contrast, bottom and top overlays might enhance the fish guidance efficiency for bottom and surface oriented fish, while the asymmetric downstream velocity field can reduce turbine efficiencies. The hydropower plant layout strongly affects the approach flow field to horizontal bar racks. For block-type hydropower plants, the installation of a dividing pier or partial opening of the spillways improves the flow field for better fish guidance.

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