Mario Aristide Lenzi

Suspended sediment load during floods in a small stream of the Dolomites (northeastern Italy)

Abstract Suspended sediment load was analyzed in a small, high-gradient stream of the Eastern Italian Alps which was instrumented to measure discharge and sediment transport. The ratio of suspended to total sediment yield and the relations between sediment concentration and water discharge were analyzed for seven floods which occurred from 1991 to 1996 in summer and autumn (from June to October). Different patterns of hysteresis in the relation between suspended sediment and discharge were related to types and locations of active sediment sources. The within-storm variation of particle size of suspended sediment during a major flood indicates a coarsening of transported material for increasing discharge. An analysis of grain size has shown that erosion areas on hillslopes were the main sources of suspended load.

Interannual variation of suspended sediment load and sediment yield in an alpine catchment

Abstract The results are described of 16 years operation of a measuring station for the automatic recording of water discharge, bed load and suspended sediment transport in the Rio Cordon catchment, a small alpine basin (5 km2) located in northeastern Italy. Hillslope erosion processes were investigated by surveying individual sediment sources repeatedly. Annual and seasonal variations of suspended sediment load during the period 1986–2001 are analysed along with their contribution to the total sediment yield. The results show that suspended load accounted for 76% of total load and that most of the suspended sediment transport occurred during two flood events: an extreme summer flash flood in September 1994 (27% of the 16-years total suspended load) and a snowmelt-induced event in May 2001 accompanied by a mud flow which fed the stream with sediments. The role of active sediment source areas is discussed in relation to the changes in flood peak—suspended load trends which became apparent after both the 1994 and the 2001 events.

Local scouring and morphological adjustments in steep channels with check-dam sequences

This paper describes bed profile and grain size distribution adjustments in a mountain river (Maso di Spinelle River, Italian Alps) stabilized by a sequence of boulder check-dams. The control works were originally designed to simulate the geometry of natural step-pool channels, where tumbling flow is the dominant hydraulic regime. Local scouring downstream of 29 drop structures is analysed through the use of nondimensional parameters where maximum scour depth and scour length are normalised to the drop height. Prior laboratory data reveal a pattern similar to field scours, where complex interactions occur between drop height, critical flow depth, and step spacing. The linkage between scour length and depth is also discussed. There seems to be a maximum step height for impinging jets that is approximately twice the drop height; this maximum may explain the upper limit of the steepness factor found in high-gradient step-pool streams. If such a maximum upper limit is confirmed by further studies, this may aid designs of foundation heights for transverse control works in steep channels.

Stream bed stabilization using boulder check dams that mimic step-pool morphology features in Northern Italy

During the last 10 years, the check dams made of boulders have been fully employed for torrent management, particularly for high-gradient stream stabilization. The analysis of this typology is shown in relation to three aspects: building features, field of use and design according to geomorphologic principles. The “Maso di Spinelle” torrent located in the Province of Trento, Italy (North Italian Alps) is an example where a sequence of low-check dams made of boulders have been used for bed stabilization. The design criteria are taken from the step–pool morphology features and the results are encouraging for further applications. The artificial step–pool grade-control structures in the Maso di Spinelle torrent have been successfully tested by floods events with return periods of about 7–10 and 20–25 years.

Local scouring in low and high gradient streams at bed sills

The main characteristics of local scouring downstream of bed sills, forming a staircase-like system in high-gradient streams with non-uniform alluvium, have been investigated through 13 clear-water laboratory runs. Three initial longitudinal slopes and different flow rates were considered, keeping the same distance between the baffles. The grain size distribution of the sediment is that of a real alpine torrent scaled to the model dimensions. The measured scour depth, length and shape are compared to previous results concerning low gradient and uniform sediment gradings. A dimensional analysis approach appears to remain valid; nevertheless some simplifications cannot be made, since the jet regime plays an important role both for the depth and the length of the scour, and consequently affects the scour shape. Two equations are proposed for the estimation of the maximum scour depth and length. The equations are from previous data sets on low-gradient tests and a new one of experimental results on high-gradient cases.

An experimental station for the automatic recording of water and sediment discharge in a small alpine watershed

Abstract Rating sediment transport contributes greatly to an improved knowledge of the relationships between hydrological and sediment transport processes. In alpine watersheds the importance of these processes is not matched by a corresponding amount of available field data on sediment transport and particularly on bed load. Detailed measurements made in experimental watersheds can help collect data on sediment transport in small mountain streams. In this connection, a small watershed (5 km2) in the Italian Alps strongly affected by erosion has been equipped with an experimental station for recording water and sediment discharge data. The experimental station operates chiefly on the separation of bedload from suspended load. The two fractions are measured by different devices. The experimental equipment has been devised with different objectives: to record sediment transport data under various different hydrological conditions including flood events; to provide reliable input data for deterministic erosi...

Dinámica y alternativas de manejo de material leñoso de gran tamaño en cuencas del sur de Los Andes

Los elementos lenosos tienen un efecto positivo en la morfodinamica de los canales y en la biodiversidad de los cauces de montana. Sin embargo, su presencia puede aumentar los riesgos asociados a las inundaciones extremas en las llanuras pobladas. Este estudio reporta acerca de la abundancia, la distribucion espacial, los mecanismos de incorporacion y deposicion de los elementos lenosos entre cuencas del sureste de los Andes (Tres Arroyos, Rio Toro, y Buena Esperanza). Los resultados muestran grandes diferencias entre las cantidades y los volumenes de elementos lenosos de las cuencas analizadas, debido a las diferentes perturbaciones historicas y a las distintas caracteristicas de cubierta del bosque. Para estimar la movilidad de las piezas lenosas, estas fueron marcadas en el Buena Esperanza y en el Tres Arroyos. Los eventos ordinarios (intervalo de recurrencia menores de dos anos) movieron solo elementos pequenos y aislados (diametro < 0,25 m, longitud < 3 m) y para distancias relativamente cortas. Las cuencas de estudio reportadas son tomadas como casos paradigmaticos para ilustrar una estrategia de gestion de elementos lenosos. La eliminacion de los elementos y los cortes de la vegetacion riberena, no son estrategias eficaces, porque los raros eventos de gran magnitud son capaces de movilizar los arboles de las laderas por remocion en masa. Areas propensas a inundaciones deben ser preservadas libres de asentamientos sensibles. Sin embargo, importantes infraestructuras presentes pueden ser protegidas con soluciones estructurales especificamente disenadas. Su potencial uso en el contexto de las tres cuencas andinas analizadas es presentado en este trabajo.

Medium-term fluvial island evolution in a disturbed gravel-bed river (Piave River, Northeastern Italian Alps)

Abstract River islands are defined as discrete areas of woodland vegetation surrounded by either water‐filled channels or exposed gravel. They exhibit some stability and are not submerged during bank‐full flows. The aim of the study is to analyze the dynamics of established, building, and pioneer islands in a 30‐km‐long reach of the gravel‐bed Piave River, which has suffered from intense and multiple human impacts. Plan‐form changes of river features since 1960 were analyzed using aerial photographs, and a was used to derive the maximum, minimum and mean elevation of island surfaces, and maximum and mean height of their vegetation. The results suggest that established islands lie at a higher elevation than building and pioneer islands, and have a thicker layer of fine sediments deposited on their surface after big floods. After the exceptional flood in 1966 ( > 200 years) there was a moderate increase in island numbers and extension, followed by a further increase from 1991, due to a succession of flood events in 1993 and 2002 with > 10 years, as well as a change in the human management relating to the control of gravel‐mining activities. The narrowing trend (1960–1999) of the morphological plan form certainly enhanced the chance of islands becoming established and this explains the reduction of the active channel, the increase in established islands and reduction of pioneer islands.

Acumulaciones de detritos leñosos en un cauce de montaña de Tierra del Fuego: análisis de la movilidad y de los efectos hidromorfológicos

SUMMARY This work reports on the geomorphic role of large wood pieces and jams in a third order mountain stream located in Southern Tierra del Fuego (Argentina), and draining an old-growth nothofagus forested basin not influenced by beaver damming activity. Even if the in-stream number of wood pieces (2,300) is comparable to that observed in other climatic areas, the slow growth of the nothofagus forest causes a lower wood abundance in terms of volumetric load (121 m 3 ha –1 ). Due to the relatively small dimensions of the large wood pieces located inside bankfull edges (83% of the total surveyed pieces), almost the 70% of them demonstrated to have been transported by runoff and 6% derived from bank erosion or landslides. Wood jams exert a significant influence on the channel morphology, being responsible for the creation of 30% of pools. The geomorphic influence of LW jams is also exerted by a considerable sediment storing capacity (about 1,750 m3). The LW-forced pool volume is strongly and positively correlated to the height of the LW jam. The results confirm that dead wood pieces, especially when organized in jams, play an important geomorphic role also in sub-Antarctic streams. The amount of large wood pieces quantified in the study site represents reference values for the assessment of the geomorphic effects of beaver activity in other Tierra del Fuego streams.

Riparian forest structure, vegetation cover and flood events in the Piave River

Until very recently, rivers have been considered as the result of the interaction between water and sediments, thus simplifying this very complex system. In doing so, one important component was missed. This is vegetation, namely trees growing on banks, floodplains, and bars/islands. The pattern of riparian vegetation in gravel bed rivers depends on the climate, hydrological regime, floods, sediment transport and the morphological settings of the river. Also, the marked spatial variability of density, height, species diversity, age, and rates of growth reflects the very complex nature of bed colonization, the strong influence of sequences and magnitude of floods, and the feedbacks between morphology, vegetation and hydraulics. Furthermore, a wide array of human impacts acting at either the basin or river network scales can influence substantially the morphodynamics and thus the characteristics, types and distribution of vegetation within the river corridor. The aim of the work is to analyze the relationship between the vegetation structure and the morphological characteristics of two sub-reaches of the Piave river which suffered both floods and intense and multiple human impacts especially due to dam building and inchannel gravel mining. Six cross sections were surveyed and vegetation, soil variables and sediment deposited were measured on 214 plots, (4 x 4 m each one and 10 m spaced). Each vegetation plot were recognized on a recent aerial photo (2009) and its ages were calculated performing both a multi-temporal analysis of older photos (1960, 70, 80, 91, 99, 2006 and 2009) and a dendro-chronological , Monitoring, Simulation, Prevention and Remediation of Dense and Debris Flows IV 137 www.witpress.com, ISSN 1743-3533 (on-line) WIT Transactions on Engineering Sciences, Vol 73,