How does sediment supply influence refugia availability in river widenings?

Abstract

Habitats that mitigate the effects of a disturbance event (e.g. flood) are referred to as refugia. Their occurrence in heavily impacted river systems is often limited, and their restoration rarely pursued. This paper presents the results of a combined laboratory and numerical modeling study to assess flood refugia availability to mobile aquatic organisms in the context of river restoration and dynamic river widening. We used a calibrated 2D hydrodynamic model based on eight topographies obtained in laboratory experiments to assess refugia availability by analyzing the hydro-morphological conditions under varying sediment supply. Overall, sediment equilibrium sustains more complex hydro-morphological conditions with low bed shear stress zones being maintained during elevated discharges. Furthermore, our results suggest that the floodplain is an important potential refuge that becomes accessible for discharges with a return period of approximately one year. Conversely, sediment deficit results in a homogeneous flow field with steadily increasing hydraulic forces for high flows and impaired lateral connectivity except for very large flood events of a 30- to 100-year return period. Dynamic river widening implemented in a channel with sediment equilibrium conditions as opposed to a sediment deficit is thus more likely to provide flood refugia.

Keywords:

• River restoration
• dynamic river widening
• hydrodynamic modeling
• habitat availability
• flood refugia

Acknowledgements

This study is part of the interdisciplinary research program Hydraulic engineering and ecology in its fourth project phase Riverscapes - Sediment Dynamics and Lateral Connectivity. The research program is funded by the Swiss Federal Office for the Environment (FOEN), the Swiss Federal Institute of Aquatic Science and Technology (Eawag), the Swiss Federal Institute for Forest, Snow and Landscape Research (WSL), the Platform of Hydraulic Constructions (PL-LCH) at EPFL Lausanne, and the Laboratory of Hydraulics, Hydrology and Glaciology (VAW) at ETH Zurich. The authors would like to thank Barbara Stocker (master student) and the technical staff at VAW for their valuable assistance with the experimental study. We wish to thank two anonymous reviewers for their helpful comments on the manuscript.

Disclosure statement

No potential conflict of interest was reported by the authors.

Additional information

Funding

The first and second author are financially supported by the Swiss Federal Office for the Environment (FOEN), Grant 16.0113.PJ/P501-1050.