Eco-morphological evaluation of a sediment augmentation measure

In the residual flow reach of the Sarine river (Switzerland), downstream of the Rossens dam, the low discharge regime and deficient bed load dynamics impact the riverscape habitat mosaic and cause substrate degradation by colmation. In 2016, an artificial flood was released and coupled with a sediment augmentation measure to mitigate the ecological deficits. The present study evaluates the medium-term effects of this rehabilitation measure using the principal indicator set "habitat diversity" of a new evaluation guideline (WiKo), published by Switzerland's Federal Office for the Environment (FOEN) in 2019. For the assessment of substrate degradation, a complementary indicator is proposed, the Indicator of "Reproduction suitability based on Substrate degradation" (IRS). Although dedicated to a different category of rehabilitation projects, the WiKo indicator set proved to be a transferable and effective evaluation procedure for the sediment augmentation measure. The study's results suggest that neither the single artificial flood, nor its coupling with the sediment augmentation measure, were sufficient to restore a functional habitat mosaic after four years. The methodology of the proposed indicator IRS is supported by the good agreement of mapping results with external data of brown trout redds.