Morphological and Fish Habitat Responses to Managed Floods

Recent years have seen an increase in the frequency of the release of managed floods in rivers, particularly in the alpine region. The objective for flood releases can be broad, for example, experimental floods or ecological floods are aimed at restoring ecological functions and improving the integrity of strongly regulated rivers. On the other hand, sediment flushing events are also managed floods but with the purpose of recovering reservoir storage capacity and preventing potential hazards related to sedimentation. The relatively large releases of water during these events are capable of modifying, for instance, the morphological structure of rivers. Although both of these examples are defined as managed floods, their motivation, and thus the effect they have in downstream ecosystems is antagonistic. Reservoir flushing was designed as an effective low-cost strategy to coup with sedimentation behind dams, and studies have shown that sediment-laden flows coming from reservoir flushing operations can cause severe impacts on riverine ecosystems. In contrast, experimental floods arose from the need to reestablish natural-like conditions in regulated rivers, and research has evidence of a variety of positive effects after their long implementation. This work aims to contribute to the understanding of the effects that different types of managed floods exert on the river corridor. Two main arguments are established within this work: sediment dynamics and fish habitat; and two study cases: an experimental flood in the Spol River, Switzerland, and sediment flushing operations in the Avisio River, Italy. This document begins with a preface (Chapter 1), where an introduction and motivation lead to the fundamental research questions and objectives of this thesis. The scientific state-of-art, main concepts, tools and methods that converged to fulfill this work are discussed in detail in Chapter 2. The first case study (Chapter 3) takes place in in the Sp¨ol river, Switzerland, where since year 2000 experimental floods are released annually for ecological purposes. Downstream of the Ova Spin dam a very particular scene is observed: an unregulated tributary stream supplies the Spol with large amounts of sediments that substantially modify the morphological structure of the river. We analyzed and quantified the morphological and physical habitat variations associated with flooding with ecological purposes and the role of sediment supply as a driver of these changes. This investigation was carried out for 2021’s experimental flood. The second case study (Chapter 4) befalls in NE Italy, in the Avisio, an alpine river that is exposed every 3 years to sediment flushing operations. A multi-scale monitoring program was designed to study the suspended sediment dynamics linked to flushing operations along a 30 km river segment. Field and laboratory methods aided the quantification of the effects of 2019’s flushing operations and their resilience one year later. In addition, a comprehensive analysis and comparison of flow-sediment dynamics in storm events and flushing operations compliments this chapter. In Chapter 5, as a by-product of the previous chapter, we proposed a novel approach for the incorporation of turbidity as a factor responsible for the temporal impairments of fish habitats into a habitat prediction model at the meso-scale. Moreover, a literature-based methodology aimed to minimize fish mortality during sediment flushing operations was also projected. Finally, a synthesis of the key findings of the thesis and future research perspectives are presented in Chapter 6.