Nature-based solutions in freshwater pearl mussel rivers – A novel approach for self-dynamic desanding developed in a physical model test

Abstract

In rivers of the Bohemian Massif, an unbalanced sediment regime in the form of a sediment surplus constitutes a growing problem. The dominant weathering grain size fractions (1–10 mm) are unsuitable habitats for, among others, freshwater pearl mussels (Margaritifera margaritifera), which occur in remaining populations in the Austrian part of the Bohemian Massif, the so-called Gneiss and Granite Plateau. In addition to ecological degradation, the above sediment surplus can also increase the flood risk in certain river stretches, which in turn often leads to frequent riverbed interventions (e.g., riverbed dredging). To transition from purely technical mitigation measures to near-natural solutions, the concept of nature-based solutions (NbS) represents an opportunity to mitigate ecological and flood risk issues. However, a comprehensive process understanding is necessary for effective measure planning. This study addressed the deficits in process understanding via physical model tests, in which a technically optimized NbS was developed for the natural process of self-dynamic desanding (SDD). SDD describes the deposition of sand and fine gravel on riverbanks or floodplains under high-flow conditions. The model tests demonstrated that in-stream measures could promote SDD by increasing the sediment output discharged from the river onto the floodplain. In addition, a set of measures was developed on a lowered floodplain, which resulted in optimized floodplain sedimentation under one-year flood conditions. Up to 14.7 % of the total transported sediments could be discharged.