Francesco Comiti

Magnitude-frequency analysis of bed load data in an Alpine boulder bed stream

[1]xa0The purpose of this study is to investigate the connection between bed load, channel processes, and sediment sources in mountain basins using data from the Rio Cordon basin (northeastern Italian Alps). The main channel is a steep, boulder bed, step pool stream, and bed load volumes are measured at a special facility where particles >20 mm are trapped. Results from a combined frequency analysis of peak water discharges and total bed load volumes based on 17 years of field data are presented, focusing on discrepancies between recurrence intervals of peak discharge and bed load volume for each event. A cause of major disturbance is a high-magnitude, low-recurrence event that occurred in 1994. Dimensional and nondimensional bed load intensity-duration curves are also reported, which emphasize differences between short- and long-duration events. The relationship flow–bed load rates (investigated using a total of 180 data) shows no breakpoints, and two different curves are evident, representing pre-1994 and post-1994 floods, respectively. The analysis demonstrates both the control exerted by sediment availability on bed load transport rates and the persisting long-term impact of major floods on mountain streams.

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.

Dimensions of standing waves at steps in mountain rivers

[1]xa0Oscillations of the water surface are common features of flows around critical conditions (Fr ∼ 1) and of both natural and structure-induced undular hydraulic jumps. The paper describes experimental tests and field data on the dimensions of standing waves created by artificial drops in steep gravel bed rivers. The purpose of the research is to analyze what parameters affect the phenomenon, to provide semiempirical formulations for wave dimensions, and to assess whether previous models for undular hydraulic jumps and antidunes are reasonably predictive. The dimensionless wavelength and wave amplitude were correlated to the Froude number at the sill. The first wavelength appears linearly related to the Froude parameter, whereas the amplitude of the first wave shows a less defined behavior, possibly indicating that a maximum value occurs for sill Froude numbers around 1.6–1.7. The corresponding wave steepness (i.e., wave amplitude/length) results between 0.15–0.16, and this range also characterizes the point at which the dimensionless trough depth below the sill achieves a local maximum, probably related to hydrodynamic conditions just before wave breaking. The transition between roller and undular jumps at drops is then argued to likely represent a critical stage in step pool streams, and the hydraulic and morphological implications of this finding are also discussed, with particular emphasis on step stability and step wavelength.

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.

Local scouring at grade-control structures in alluvial mountain rivers

[1]xa0Scour holes below 73 grade-control structures (check dams and bed sills) in six mountain rivers located in the eastern Italian Alps have been surveyed. The most likely formative water discharge is used to evaluate jet thickness at each structure, which along with drop height, appears to determine scour hole dimensions, as shown by the consistent trends observed for nondimensional plots of both maximum scour depth and length versus the respective drop ratio. Sediment differences regarding size and lithology apparently play a minor role in determining scour hole dimension. Measured maximum scour depths are well predicted by a semiempirical equation developed through laboratory results, showing an average relative error of 0.13. Scour hole geometry is described by several ratios that are thought to represent approximately invariant characteristics. A new energy-based normalization for scour hole dimensions is proposed as the most suited to evaluate the role of jet geometry and aeration upon scouring efficiency. It is noteworthy that the ratio between the maximum scour depth and the total energy available at a drop tends toward an asymptotic value around unity for increasing drop heights.

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.

The effects of large wood elements during an extreme flood in a small tropical basin of Costa Rica

In-channel large wood elements (LW) enter the river system as a consequence of natural processes independent of channel activity (i.e. windthrow, large landslides, extended wildfire, disease outbreaks), or as a result of river processes (bank erosion, avulsions). LW exert a substantial beneficial influence on river morphology and freshwater biodiversity, but on the other hand may increase flood hazards in densely inhabited floodplains. This is due to a variety of processes, such as flow surges following collapse of temporary wood dams, strengthening of debris flows, local bed scour and local bank erosion, and clogging of culverts and bridges. However, despite a general consensus among populations and river managers about the hazards related to floating wood during floods, few studies have addressed the real contribution of LW to flood damages, as well as its sources and effects on flood dynamics. This paper presents a post-event analysis in the Rio Portalon, a small tropical basin in the Pacific coast of Costa Rica, which experienced an extreme flood in September 2005. On that occasion, floating wood apparently had a considerable role in flood dynamics and in causing damages to the nearby village. Interviews with local people and a simple field survey were used to shed light on event dynamics, and in particular on role played by LW. Finally, a range of options for the management of riparian vegetation in tropical areas are discussed.

Residuos leñosos de gran tamaño en un torrente de la Cordillera de Los Andes, Chile: su funcionalidad e importancia

Resumen es: Se registraron la cantidad, caracteristicas e impacto morfologico de los residuos lenosos de gran tamano (large woody debris o LWD) en el torrente Tres A...