Effect of vegetation on Mixing and Dispersion Processes at the apex section of a meander bend

Aquatic vegetation exerts a strong influence on the fluvial ecosystem. Understanding flow characteristics and turbulent structure in the presence of vegetation is especially important with respect to environmental processes as sediment transport and mixing of transported quantities. In the present paper attention is focused on the kinematic and mixing processes in the presence of flexible submerged vegetation on the bed of a curved channel. In particular, the effect of vegetation on the flux of mass distribution and on the transport process at the apex section of a meandering bend is investigated by comparing the distributions of the dispersion coefficient estimated in vegetated areas and in no-vegetated ones

Flow Velocity and Turbulence Structure in gravel-bed rivers: experimental investigation in a straight laboratory flume

Gravel-bed flows are characterized by complex fluid/bed interactions. The presence of gravels on the bed determines spatial heterogeneities of flow velocity and turbulence structure. In this paper, experimental data collected in a gravel-bed straight laboratory channel are analyzed in order to evaluate the effect of the bed roughness on the velocity and the turbulence intensity profiles. Results show that gravel inflects the velocity distribution near the bed creating an S-shaped profile. The experimental velocity distributions are positively compared with the hyperbolic tangent profile of a pure mixing layer previously tested in vegetated flows. It has been verified that the averaged turbulence intensity profiles have a maximum at a relative water depth which approximately corresponds to that of the inflection point of the averaged velocity profiles

Evaluating the Hydrological stressors by Monitoring the Mussel’s Behaviours

Freshwater mussels (FM) are suitable biological indicators to assess environmental stressors to detect disturbances on ecosystems from 1950 (Hiscock, 1950), and Kramer has started to use the monitoring of the mussels as a BEWS since 2001 (Kramer et al., 2001) but the suitability of FM for monitoring the impact of hydraulic stressors is still lacking. During the last two decades, these methodologies have been used to measure the presence of pollutants in water bodies. To reach the aim, the Valvometric method has been used, based on the use of Hall sensors (real-time remote monitoring tool) to get the data. The behavioral responses of mussels are characterized by the valve opening amplitudes and opening-closure frequencies. We relate these behaviors to hydrological conditions and sediment transport mimicking the onset of floods. The experiments conducted in a laboratory flume (Fig. 1) were carried out by starting with a stage of constant discharge (without sediment transport) followed by an abruptly increased value of discharge, which in most cases is accompanied by sediment transport. Hall sensors and magnets were fixed on the shells of mussels (Fig. 2) and connected to an Arduino system. The opening and closing of the valve were continuously monitored, along with the hydro-morphological conditions. FMs maintained a constant valve gaping frequency that characterizes their normal behavior (feeding and movement). FMs promptly reacted to extreme discharge conditions with sediment transport by increasing valve gaping frequencies, shifting from normal to transition behavior. We checked that a minimum number of animals is necessary to reach some degree of accuracy in the statistical treatment of the data and overally, unambiguous sentences. Most mussels (87 to 97%) reacted promptly to increased discharge with sediment transport, showing a transition from their normal behavior to a significantly higher valve gaping frequency and the intensity of their reaction significantly increased from the lowest to the highest stress levels. Fig. 3 shows the frequency of mussels’ gapping during the experiments, we can observe that there is a threshold between the experiments without and with sediment transport on the bed (bedload) which is 0.025 Hz. Therefore, if the responses of mussels are higher than 0.025 Hz, the condition is with high variation which meant sediment transport has started. In the end, FMs response to hydro-morphological was fast and accurate, showing that they can be used as a reliable BEWS, under general flow conditions

Mussel behaviour as a tool to measure the impact of hydrodynamic stressors

Supplementary Information: The online version contains supplementary material available at https://doi. org/10.1007/s10750-022-05126-x.Freshwater mussels (FMs) are useful bioindicators to detect environmental disturbances. However, studies that evaluate FMs suitability for monitoring the hydrodynamic stressors impact are lacking. Since future climatic scenarios predict an increase in frequency and intensity of extreme events, understanding how flood conditions affect freshwater organisms are crucial for their conservation. In this study, we performed experiments in an artificial flume to evaluate the eligibility for FMs behaviour for developing a tailored biological early warning system (BEWS). For this, we used the valvometric technique (Hall magnetic sensors) to measure the FMs valve gaping behaviour when subjected to increasing discharges/sediment transport mimicking the onset of floods. After analysing baseline behaviour in non-stressful conditions, we performed experiments in steady and transient conditions to verify the FMs' response and the threshold that prompted it. Under steady conditions, FMs maintained a constant gaping frequency that characterizes their normal behaviour. The FMs promptly reacted to discharge variations with sediment transport, showing a transition from their normal behaviour to higher valve gaping frequencies. We demonstrated that FM transition behaviour is a useful tool to measure hydrodynamic stressors. A future step will be the application of this BEWS on natural ecosystems to assess possible hydrodynamic changes in real-time.This study was conducted within the PRIN 2017 project ENTERPRISING—IntEractions between hydrodyNamics and bioTic communities in fuvial Ecosystems: advancement in the knowledge and undeRstanding of PRocesses and ecosystem sustainability by the development of novel technologies with fIeld monitoriNg and laboratory testinG supported by Ministero dell’ Universitá e della Ricerca (Prot. 2017SEB7Z8)

Real-time biological early-warning system based on freshwater mussels’ valvometry data

<jats:p>Abstract. Quantifying the effects of external climatic and anthropogenic stressors on aquatic ecosystems is an important task for scientific purposes and management progress in the field of water resources. In this study, we propose an innovative use of biotic communities as real-time indicators, which offers a promising solution to directly quantify the impact of these external stressors on the aquatic ecosystem health. Specifically, we investigated the influence of natural river floods on riverine biotic communities using freshwater mussels (FMs) as reliable biosensors. Using the valvometry technique, we monitored the valve gaping of FMs and analysed both the amplitude and frequency. The valve movement of the FMs was tracked by installing a magnet on one valve and a Hall effect sensor on the other valve. The magnetic field between the magnet and the sensor was recorded using an Arduino board, and its changes over time were normalised to give the opening percentage of the FMs (how open the mussels were). The recorded data were then analysed using continuous wavelet transform (CWT) analysis to study the time-dependent frequency of the signals. The experiments were carried out both in a laboratory flume and in the Paglia River (Italy). The laboratory experiments were conducted with FMs in two configurations: freely moving on the bed and immobilised on vertical rods. Testing of the immobilised configuration was necessary because the same configuration was used in the field in order to prevent FMs from packing against the downstream wall of the protection cage during floods or from breaking their connection wires. These experiments allowed us to verify that immobilised mussels show similar responses to abrupt changes in flow conditions as free mussels. Moreover, immobilised mussels produced more neat and interpretable signals than free-moving mussels due to the reduced number of features resulting from movement constraints. We then analysed the response of 13 immobilised mussels under real river conditions during a flood on 31 March 2022. The FMs in the field showed a rapid and significant change in valve gap frequency as the flood escalated, confirming the general behaviour observed in the laboratory in the presence of an abrupt increase in the flow. These results highlight the effectiveness of using FMs as biosensors for the timely detection of environmental stressors related to natural floods and emphasise the utility of CWT as a powerful signal-processing tool for the analysis of valvometry data. The study proposes the integration of FM valvometry and CWT for the development of operational real-time biological early-warning systems (BEWSs) with the aim of monitoring and protecting aquatic ecosystems. Future research should focus on extending the investigation of the responsiveness of FMs to specific stressors (e.g. turbidity, temperature, and chemicals) and on testing the applications of the proposed BEWSs to quantify the impact of both natural stressors (e.g. heat waves and droughts) and anthropogenic stressors (e.g. hydropeaking, reservoir flushing, and chemical contamination). </jats:p&gt