Changes in the hydrodynamics of a mountain river induced by dam reservoir backwater

Maciej Liro; Virginia Ruiz-Villanueva; Paweł Mikuś; Bartłomiej Wyżga; Ernest Bladé Castellet

doi:10.1016/j.scitotenv.2020.140555

Changes in the hydrodynamics of a mountain river induced by dam reservoir backwater

Sci Total Environ. 2020 Nov 20:744:140555. doi: 10.1016/j.scitotenv.2020.140555. Epub 2020 Jul 4.

Authors

Maciej Liro¹, Virginia Ruiz-Villanueva², Paweł Mikuś³, Bartłomiej Wyżga³, Ernest Bladé Castellet⁴

Affiliations

¹ Institute of Nature Conservation, Polish Academy of Sciences, al. Mickiewicza 33, 31-120 Kraków, Poland. Electronic address: liro@iop.krakow.pl.
² Institute of Earth Surface Dynamics, University of Lausanne, Geopolis, UNIL-Mouline, 1015 Lausanne, Switzerland.
³ Institute of Nature Conservation, Polish Academy of Sciences, al. Mickiewicza 33, 31-120 Kraków, Poland.
⁴ Flumen Institute, Universitat Politècnica de Catalunya - CIMNE, Jordi Girona 1-3, D-´1, 08034 Barcelona, Spain.

PMID: 32755769
DOI: 10.1016/j.scitotenv.2020.140555

Abstract

Upstream from a dam reservoir, river hydrodynamics may be directly changed by temporary inundation driven by the reservoir. This triggers morphological river changes which may additionally modify the initial hydrodynamics, even at the time when backwater inundation does not occur (indirect effects of backwater). We verified these hypotheses, applying two-dimensional hydraulic modelling of flood flows to a section of the mountainous Dunajec River upstream from the Czorsztyn Reservoir. The modelling was performed for small, medium and large floods, and hydraulic conditions were compared between the scenarios with lacking and maximum backwater inundation and between the river reaches subjected to backwater inundation and unaffected by backwater fluctuations. Direct effects of reservoir level fluctuations were limited to the reach subjected to backwater inundation during floods and comprised: significantly increased water depth and decreased flow velocity and bed shear stress in the channel and on the floodplain, as well as a re-established hydrological connectivity between the channel and floodplain during small and medium floods. Indirect effects of backwater inundation reflected channel widening and bed aggradation that triggered a positive feedback with changes in hydrodynamics, mostly by reducing the velocity of flood flows in the channel zone. These latter changes occurred on a longer distance upstream from the reservoir than the backwater reach itself, and they modified the river hydrodynamics even when backwater inundation did not occur. We propose a conceptual model which indicates that changes of mountain rivers upstream from dam reservoirs are driven by modified hydrodynamics and lead to different morphological adjustments than those induced by waters underloaded with sediment downstream from dams. Changes in hydrodynamics and the associated morphological and sedimentary adjustments of mountain rivers recorded upstream from dam reservoirs may locally mitigate impacts of channelization and channel incision on riverine and riparian ecosystems of these rivers.

Keywords: Dam reservoir backwater; Hydraulic modelling; Mountain river; River hydrodynamics.