Künstliches Hochwasser an der Saane : eine Massnahme zum nachhaltigen Auenmanagement

Durch Wasserkraft genutzte Flüsse und Auenlandschaften sind in ihrem Abfluss- und Geschieberegime und damit in ihrer Morphologie und Ökologie oft stark beeinträchtigt. In der Saane unterhalb der Staumauer Rossens führten eine jahrzehntelange Restwasserbewirtschaftung und eine starke Reduktion der Geschiebefracht zu einer mangelnden Abfluss- und Geschiebedynamik und damit zu einer Reihe von Defiziten wie einer fehlenden, für Auen typischen Habitatdynamik oder einer an stabile Verhältnisse angepasste Makrozoobenthosgemeinschaft. Künstliche Hochwasser und Geschiebeschüttungen sind mögliche Massnahmen zur Verbesserung solcher morphologischen und ökologischen Defizite. Ein solches künstliches Hochwasser in Verbindung mit einer Geschiebeschüttung im September 2016 führte in der Restwasserstrecke der Saane zu einer Reihe von ökologischen und morphologischen Veränderungen. Dazu gehörten unter anderem Habitat- und Geschiebeumlagerungen, Entfernung von Kolmation und Kiesbank überwuchernder Vegetation oder auch die Reduktion von sehr hohen Individuendichten in der Makrozoobenthosgemeinschaft. Allerdings konnte auch gezeigt werden, dass die positiven Auswirkungen, insbesondere auf das Makrozoobenthos, nur kurzfristig anhielten, was eine regelmässige Durchführung solcher Massnahmen nahelegt, um dauerhaft und nachhaltig wirksam zu sein. Das ist insbesondere für komplexe Auenlandschaften von Bedeutung, die unter natürlichen Bedingungen eine hohe Dynamik aufweisen. Insgesamt konnten mit dieser Studie die Auswirkungen dieses künstlichen Hochwassers umfassend quantifiziert, bewertet sowie der praktische Nutzen gezeigt werden. Die Resultate können dazu beitragen, solche Massnahmen aus ökologischer und ökonomischer Sicht (Wasserverlust zur Energieproduktion) in Zukunft angemessen zu dimensionieren und Modellvorhersagen als Planungsgrundlagen zu verbessern, um somit zur verbesserten ökologischen Verträglichkeit der Grosswasserkraft als wichtiger Energieträger beizutragen

Seven decades of hydrogeomorphological changes in a near‐natural (Sense River) and a hydropower‐regulated (Sarine River) pre‐Alpine river floodplain in Western Switzerland

This is the peer reviewed version which has been published in final form at https://doi.org/10.1002/esp.5017 . This article may be used for non-commercial purposes in accordance with Wiley Terms and Conditions for Use of Self-Archived Versions.Hydropower alteration of the natural flow and sediment regime can severely degrade hydromorphology, thereby threatening biodiversity and overall ecosystem processes of rivers and their floodplains. Using sequences of aerial images, we quantified seven decades (1938/1942–2013) of spatiotemporal changes in channel and floodplain morphology, as well as changes in the physical habitats, of three floodplain river reaches of the Swiss pre‐Alps, two hydropower‐regulated and one near‐natural. In the Sarine River floodplain, within the first decades of hydropower impairment, the magnitude and frequency of flood events (Q2, Q10, Q30) decreased substantially. As a result, the area of pioneer floodplain habitats that depend on flood activity and sediment dynamic, such as bare sediments, decreased dramatically by approximately 95%. However, by 2013 vegetated areas had generally increased in comparison to the pre‐regulation period in 1943, indicating general vegetative colonization. Between 1943 and 2013, the active channel underwent essential narrowing (up to 62% width reduction in the residual flow reach) and habitat turnover rates were very low (5% of the total floodplain area changed habitat type five to six times). In contrast, from the 1950s onwards, the near‐natural floodplain of the Sense River experienced recurrent narrowing and widening, and frequent changes between bare and vegetated areas, reflecting the shifting habitat mosaic concept typical for natural floodplains. In the three reaches investigated, we found that the active floodplain width and erosion of vegetated areas were primarily controlled by medium to large floods (Q10, Q30), which combined with reduced time intervals between ordinary floods ≥ Q2 most likely mobilized streambed sediments and limited the ability of vegetation to establish itself on bare gravel bars within the parafluvial zone. These findings can contribute to restoration action plans such as controlled flooding and sediment replenishments in the Sarine and other floodplain rivers of the Alps

Artificial floods in residual flow reaches – How do they affect hydraulic habitats

Floodplains in hydropower-controlled rivers, where a constant residual flow discharge is imposed, often lack sediment supply and periodic inundation due to the absence of natural flood events. In this study we compare two residual flow systems. The first river is the Spöl, located in the Swiss National Park close to the boarder with Italy. Since the year 2000, artificial floods are released from the Ova Spin Dam on a yearly bases, with the aim to restore river dynamics and habitats. The second river, is the Sarine in Western Switzerland where an artificial flood was released from the Rossens Dam for the first time. At one location this flood event was combined with sediment replenishment. In this study, an analysis of the two different systems before and after the event using the Hydro-Morphological Index of Diversity (HMID) is presented. The index is based on the variability of flow depth and flow velocity and represents the habitat diversity on a reach scale. Results reveal that a flood pulse can significantly increase hydraulic habitat diversity in a river reach and that sediment availability is crucial. Nevertheless, the change in habitats of a single flood event has a minor effect if artificial floods are periodically released. Under such conditions, the river has already reached an equilibrium condition considering habitat diversity

Sediment replenishment combined with an artificial flood improves river habitats downstream of a dam

River reaches downstream dams where a constant residual fow discharge is imposed, often lack sediment supply and periodic inundation due to the absence of natural food events. In this study, a two-year return food was released from an upstream reservoir and combined with sediment replenishment to enhance instream habitat conditions downstream of Rossens hydropower dam on the Sarine River in western Switzerland. Sediment replenishment consisted of four sediment deposits distributed as alternate bars along the river banks, a solution which was previously tested in laboratory. The morphological evolution of the replenishment and of the downstream riverbed were surveyed including pre- and post-food topography. A hydro-morphological index to evaluate the quality of riverine habitats, based on the variability of fow depth and fow velocity in the analyzed reach, was investigated. The combination of the artifcial food with sediment replenishment proved to be a robust measure to supply a river with sediment and to enhance hydraulic habitat suitabilit

Triplet Excitons and Associated Efficiency-Limiting Pathways in Organic Solar Cell Blends Based on (Non-) Halogenated PBDB-T and Y-Series

The great progress in organic photovoltaics (OPV) over the past few years has been largely achieved by the development of non‐fullerene acceptors (NFAs), with power conversion efficiencies now approaching 20%. To further improve device performance, loss mechanisms must be identified and minimized. Triplet states are known to adversely affect device performance, since they can form energetically trapped excitons on low‐lying states that are responsible for non‐radiative losses or even device degradation. Halogenation of OPV materials has long been employed to tailor energy levels and to enhance open circuit voltage. Yet, the influence on recombination to triplet excitons has been largely unexplored. Using the complementary spin‐sensitive methods of photoluminescence detected magnetic resonance and transient electron paramagnetic resonance corroborated by transient absorption and quantum‐chemical calculations, exciton pathways in OPV blends are unravelled employing the polymer donors PBDB‐T, PM6, and PM7 together with NFAs Y6 and Y7. All blends reveal triplet excitons on the NFA populated via non‐geminate hole back transfer and, in blends with halogenated donors, also by spin‐orbit coupling driven intersystem crossing. Identifying these triplet formation pathways in all tested solar cell absorber films highlights the untapped potential for improved charge generation to further increase plateauing OPV efficiencies