dynamics of large wood during a flash flood in two mountain catchments

Abstract. Understanding and modelling the dynamics of large wood (LW) in rivers during flood events has spurred a great deal of research in recent years. However, few studies have documented the effect of high-magnitude flash floods on LW recruitment, transport and deposition. On 25 October 2011, the Magra river basin (north-western Italy) was hit by an intense rainstorm, with hourly rainfall rates up to 130 mm h−1 and event rain accumulations up to 540 mm in 8 h. Such large rainfall intensities originated flash floods in the main river channels and in several tributaries, causing severe damages and loss of lives. Numerous bridges were partly or fully clogged by LW jams. A post-flood survey was carried out along the channels of two catchments that were severely and similarly affected by this event, the Gravegnola (34.3 km2) and Pogliaschina (25.1 km2). The analysis highlighted a very relevant channel widening in many channel reaches, which was more marked in the Gravegnola basin due to highly erodible material forming the slopes adjacent to the fluvial corridor. Large wood recruitment rates were very high, up to 1270 m3 km−1, and most of it (70–80 %) was eroded from the floodplains as a consequence of channel-widening processes, while the rest came from hillslopes processes. Overall, drainage area and channel slope are the most relevant controlling variables in explaining the reach-scale variability of LW recruitment, whereas LW deposition appears to be more complex, as correlation analysis did not evidence any statistically significant relationship with the tested controlling variables. Indeed, in-channel LW displacement during the flood has been mostly limited by the presence of bridges, given the relatively large width attained by channels after the event

Calibration of an acoustic pipe sensor through bedload traps in a glacierized basin

Quantifying sediment transport in small mountain basins is of great relevance to assess the morphological and ecological dynamics of the entire channel network and to predict flood hazards. In high-elevation, glacierized basins, seasonal variability in sediment transport is dramatic, but despite the relevance of such basins in many regions worldwide, very few investigations have tried to quantify it. Since direct methods to assess bedload transport are time consuming and practically challenging at high flows, indirect surrogate methods, allowing continuous measurements over time, are highly desirable. Yet, these methods require calibration to provide reliable estimations. The present research is focused on the calibration of an acoustic pipe sensor in the recently established (Spring 2011) monitoring station in the Saldur basin, a high-elevation glacierized watershed in the Eastern Italian Alps. The acoustic pipe signal (which is amplified through 6 channels having different gains) was calibrated against samples collected over 26 sampling periods using "Bunte" bedload traps along a cross-section 12m upstream of the pipe. Samples were collected from June to August 2011 during daily discharge fluctuations (ranging from 1.40 to 3.63m3s-1) due to snow- and glacier-melt, featuring very different bedload rates (up to 0.14kgs-1m-1). In order to calibrate the pipe sensor signal, the average number of impulses was plotted against the corresponding unit bedload rates for the associated bedload sampling periods. As expected, the signal from the two most sensitive channels of the acoustic sensor resulted dampened even at low discharges, and thus could not be used for calibration and bedload assessment. Instead, power laws (R2 from 0.76 to 0.92) relating the number of impulses per minute to unit bedload rate were obtained using channels having intermediate and low sensitivities, with higher correlations associated with the less sensitive channel

Sediment motion and velocity in a glacier-fed stream

Current understanding of coarse sediment transport (e.g. threshold for motion, travel length and virtual velocity) in mountain rivers is still quite limited, and even less is known about glacial streams. However, the hydrological characteristics of these systems (strong daily discharge fluctuations, high water turbidity) pose challenges to the use of tracers to monitor bed sediment dynamics, as tagged clasts are usually located after bedload events when flow stage has receded, e.g. by means of portable antennas in the case of Passive Integrated Transponders (PIT). The use of stationary antennas, still scarcely in use worldwide, to detect PIT-tagged particles has potential advantages in glacier-fed streams. If water discharge is monitored continuously, a stationary antenna provides real time data on the actual discharge at the moment of tracer particles passage. This study focuses on incipient motion and virtual velocity of bed particles measured by a stationary antennas system in a steep mountain channel (Saldur River, drainage area 18.6 km2, Italian Alps) where significant daily discharge fluctuations and bedload transport occur as a result of a nivo-glacial regime. Four stationary antennas were installed 50-m apart in the study reach. A total of 629 PIT-tagged clasts were inserted in the studied reach between 2011 and 2014, ranging in size from 35 mm to 580 mm, with an overall recovery rate of around 44%. Critical discharge for sediment entrainment was obtained by detecting the movement of tracers placed immediately upstream of antennas. Virtual velocity was derived by knowing distances between the antennas and travel time of tracers. Results on initiation of motion show that the relationship between the size of transported tracers and the discharge measured at the time clasts were passing the stationary antenna is very weak. The influence of antecedent flows on incipient motion was thus investigated by dividing the highest discharge recorded between each PIT deployment and the subsequent entrainment by the actual critical discharge at the time of movement (ratio Qmax/Qc). Results show that approximately 50% of tracers moved at Qmax/Qc ≤ 1.2, and that 73% of tracers moved at Qmax/Qc < 1.5. Therefore, about 30% of tracers had to previously experience a discharge substantially greater than the one that actually mobilized them. Also, coarser particles moved at higher Qmax/Qc ratios, suggesting that higher antecedent flows may be needed to destabilize bed clustering. Results on the virtual velocity of the PIT-tagged clasts employed in the field show that the virtual velocity turned out to be highly variable (ranging from 101 to 10− 5 m min− 1) and weakly related to either particle size or flow discharge. However, virtual velocity was well correlated with the highest flow discharge experienced by each tracer normalized by a percentile of the flow duration curve. This evidence further stresses the importance of flow history on sediment entrainment and transport. Finally, the pros and cons of the deployed monitoring technology are discussed

Physical modelling of the combined effect of vegetation and wood on river morphology

The work described in this publication was supported by the European Union Seventh Framework Programme through the grant to the budget of the Integrating Activity HYDRALAB IV, Contract no. 261520 (HyIV-HULL-01). The experiments have been performed thanks to the invaluable support of the Geography, Environment and Earth Sciences Department — University of Hull, in particular Stuart McLelland, Brendan Murphy, Rob Thomas, and Lucy Clarke. Diego Ravazzolo produced the wood dowels and helped in the executions of the experiments, along with Nana Osei and Sandra Zanella. The paper has benefitted from comments and suggestions by three anonymous referees

Brief Communication: A new testing field for debris flow warning systems

Abstract. A permanent field installation for the systematic test of debris flow warning systems and algorithms has been equipped on the eastern Italian Alps. The installation was also designed to produce didactic videos and it may host informative visits. The populace education is essential and should be envisaged in planning any research on hazard mitigation interventions: this new installation responds to this requirement and offers an example of integration between technical and informative needs. The occurrence of a debris flow in 2014 allowed the first tests of a new warning system under development and to record an informative video on its performances. This paper will provide a description of the installation and an account of the first technical and informative results obtained

Contrasting physical and chemical conditions of two rock glacier springs

9openInternationalBothRock glaciers are increasingly influencing the hydrology and water chemistry of Alpine catchments. During three consecutive summers (2017–2019), we monitored by recording probes and fortnightly/monthly field campaigns the physical and chemical conditions of two rock glacier springs (ZRG, SRG) in the Zay and Solda/Sulden catchments (Eastern Italian Alps). The springs have contrasting hydrological conditions with ZRG emerging with evident ponding (pond-like), and SRG being a typical high-elevation seep (stream-like). Water temperature was constantly low (mean 1.2°C, standard deviation 0.1°C) at both springs. Concentrations of major ions (dominated by SO42−, HCO3−, Ca2+ and Mg2+) and trace elements (As, Sr, Ba, U, Rb) increased, and water became more enriched in heavy stable isotopes (δ18O, δ2H) towards autumn. This solute and isotopic enrichment had an asymptotic trend at SRG, and a unimodal pattern at ZRG, where peaks occurred 60–80 days after the snowmelt end. Wavelet analysis of electrical conductivity (EC) and water temperature records revealed daily cycles only at SRG, and significant weekly/biweekly fluctuations at both springs attributable to oscillations of meteorological conditions. Several rainfall events triggered a transient (0.5–2 h) EC drop (of 5–240 μS cm−1) and water temperature rise (of 0.2–1.4°C) at SRG (dilution and warming), whereas only intense rainfall events occasionally increased EC (by 15–85 μS cm−1) at ZRG (solute enrichment and thermal buffering), with a long-lasting effect (6–48 h). Building on previous research, we suggest that rock glacier springs with differing flow conditions, that is, stream-like and pond-like, have contrasting fluctuations of water parameters at different timescales. Thus, for pond-like springs, peaks of EC/solute concentrations might indicate a seasonal window of major permafrost thaw. Our quantitative description of the hydrochemical seasonality in rock glacier outflows and the physical and chemical response to precipitation events provides relevant information for water management in mountain areas under climate change.openBrighenti, Stefano; Engel, Michael; Tolotti, Monica; Bruno, Maria Cristina; Wharton, Geraldene; Comiti, Francesco; Tirler, Werner; Cerasino, Leonardo; Bertoldi, WalterBrighenti, S.; Engel, M.; Tolotti, M.; Bruno, M.C.; Wharton, G.; Comiti, F.; Tirler, W.; Cerasino, L.; Bertoldi, W

Photographic guidance for selecting flow resistance coefficients in high-gradient channels

Photographic guidance is presented to assist with the estimation of Manning’s n and Darcy-Weisbach f in high-gradient plane-bed, step-pool, and cascade channels. Reaches both with and without instream wood are included. These coefficients are necessary for the estimation of reachaverage velocity, energy loss, and discharge. Using data collected in 19 stream channels located in the State of Colorado and the Eastern Italian Alps, on slopes ranging from 2.4 to 21 percent, guidance is provided for low through bankfull flows. Guidance for low flow resistance estimation is additionally provided using data collected in 29 channels in the State of Washington, New Zealand, Chile, and Argentina. Bankfull n values range from 0.048 to 0.30 and low flow n values range from 0.057 to 0.96. Discussions of flow resistance mechanisms and quantitative prediction tools are also presented

Medium and short term riparian vegetation, island and channel evolution in response to human pressure in a regulated gravel bed river (Piave River, Italy)

During the last decades, several Italian gravel-bed rivers suffered from different types and levels of human pressures that changed their morphological and vegetation patterns. This study aims to analyze the medium- and short-term evolution of vegetation cover, fluvial islands and main channel characteristics along a gravel-bed reach of the Piave River (North-eastern Italy), with special emphasis on the changes in the vegetation cover type and structure. The Piave River experienced different types of human disturbance such as gravel mining, hydropower schemes, and land use changes. A sequence of aerial photographs (1960, 1970, 1982, 1991, 1999, 2006, 2010 and 2012) have been analyzed to detect the medium-term (about 50 yr) evolution of riparian vegetation along a reach about 30 km-long in the middle course of the Piave River. In addition, LiDAR data (2003) and seven repeated topographic surveys (done between 2007 and 2011) have been used to quantify changes in three cross sections on a sub-reach about 2 km-long, in order to analyze the channel evolution over a short period (1–8 yr). The medium-term analysis revealed that changes in the river evolutionary trend depended on the variations in human activities both in the main channel and at basin scale. In fact, during the last five decades there has been a consistent and continuous increase of riparian vegetation within the river corridor, from around 50% up to 68%, which corresponds to a continuous decrease from around 46% to 29% in the area of exposed gravel and low flow channels. Considering the different vegetation cover types, there was a predominant increase of stable and tall vegetation from around 34% up to a maximum of around 67% (1999), then a slight decrease to around 62% (2010 and 2012). After a slight recovery phase subsequent to the cessation of gravel mining in the late 1990s and associated to flood events in the early 2000s, the Piave River appears to have been in an equilibrium phase in terms of bed elevation, planform morphology and areas of vegetation, which is becoming taller and more mature, as a result of the combination of flow regulation, reduced bedload input from upstream and lack of relevant flood events. These results suggest that in highly regulated — in terms of flow and sediment fluxes — rivers the cessation of gravel mining alone is not sufficient to revert a degradation trend and thus restore prior morphological patterns