Ultrasonic Doppler flow velocity measurements as a co-indicator for the eco-morphological assessment in a residual flow reach

It is a complex task to evaluate the effects of a river rehabilitation measure on the riverscape’s habitat mosaic. This study investigated the medium term effects of a rehabilitation measure in a residual flow reach downstream of a hydroelectric dam. The rehabilitation measure consisted of an artificial flood coupled with a sediment augmentation measure. The evaluation was based on the indicator set of habitat diversity, published by the Swiss Federal Office for the Environment (FOEN) for the outcome evaluation of restoration projects. It is composed of six ecomorphological indicators, among which 1.3 and 1.4 assess the variability of water depth and flow velocity, respectively. In each study reach, the measurements were taken along hydro-morphologically representative cross sections for at least eleven points per cross section. A handheld Acoustic Doppler Velocimeter (ADV) was used to measure flow velocity. Digital, GNSS-supported surveying with a mobile GIS application significantly enhanced the assessment workflow. The study results suggest that neither the single artificial flood nor its coupling with the 2016 sediment augmentation were sufficient to restore a functional habitat mosaic in the medium or long term.