Monitoring, Stimulation, Prevention and Remediation of Dense and Debris Flows IV, 2012
ABSTRACT Until very recently, rivers have been considered as the result of the interaction betwee... more ABSTRACT 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
2) Pontificia Universidad Catòlica de Chile, Santiago, Chile (lmao@uc.cl) The bedload transport p... more 2) Pontificia Universidad Catòlica de Chile, Santiago, Chile (lmao@uc.cl) The bedload transport plays a fundamental role in the mountain streams affecting several aspects as morphology and ecological status of the fluvial system, influencing fields as engineering, natural hazard and river management. The monitoring, assessment and quantification of this type of transport is not simple due to impulsive nature that distinguishes it. Moreover, in mountain streams the particle motion is strongly affected by the bed structures as well as by the highly heterogeneous bed material, acting especially through the hiding/exposure, consolidation and embedding effects. Thanks to the assumption that bedload transport may be understood as the result of random individual particle displacements, the sediment tracing method was widely employed in the last years. Such method allow to obtain precious data concerning sediment dynamic in mountain streams, improving the results obtainable by samplers, tra...
Monitoring, Stimulation, Prevention and Remediation of Dense and Debris Flows IV, 2012
... Plan-form changes of river features from 1960 were analyzed using aerial photos, and a LiDAR ... more ... Plan-form changes of river features from 1960 were analyzed using aerial photos, and a LiDAR was used to derive the maximum, minimum and mean elevation of island surfaces and the maximum and mean height of the island vegetation. ...
A modern approach to problems of non-point source water pollution and erosion in a watershed cons... more A modern approach to problems of non-point source water pollution and erosion in a watershed consists in linking distributed models to Geographic Information Systems (GIS). Water quality and quantity are affected by the inherent spatial and time variability of the hydrological attributes within a watershed. Spatially distributed models and data are more suitable than conventional lumped ones. They act directly
Suspended sediment transport in high mountain streams display a grater time-space variability and... more Suspended sediment transport in high mountain streams display a grater time-space variability and a shorter duration (normally concentrated during the snowmelt period and the duration time of single floods) than in larger lowland rivers. Suspended sedi- ment load and sediment yield were analysed in a small, high-gradient stream of East- ern Italian Alps which was instrumented to measure in continuous water discharge and sediment transport. The research was conducted in the Rio Cordon, a 5 Km2 small catchment of the Dolomites. The ratio of suspended to total sediment yield and the re- lations between sediment concentration and water discharge were analysed for eleven floods which occurred from 1991 to 2001. Different patterns of hysteresis in the re- lation 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 mayor flood (September 1994, 30 years<Tr<50 years) indicates a coarsening of transported material for increasing discharge. An analysis of grain size has shown that erosion areas on hill-slopes were the main source of suspended load. The relation between water discharge and S.S.C. for both floods and snowmelt runoff shows larger scatter for both series of data, with snowmelt data less scattered than rain- fall induced floods. This is accounted for by the variable effectiveness of erosion pro- cesses and sediment supply mechanisms during snowmelt and rainfall-induced floods. During snowmelt, erosion processes essentially consist in the removal of loose, fine- grained sediment from slopes by surface runoff; as a consequence, suspended sedi- ment transport takes place also with rather low discharges. Abundant suspended sedi- ment transport was recorded during the snowmelt period of May 2001, that followed a winter characterized by a huge snow cover and late snowfalls. Different sources of sed- iment contribute to suspended load during the May11, 2001, snowmelt induced-small- flood. Therefore, the most important erosion processes was a landslide that originates a debris-mud-flow, occur only as a consequence of infiltration-saturation processes and hypodermic runoff (without associated rainfall and/or snow fall). This situation caused the instability of a large part of a sub-catchment hill-slope, the movement and the transport of a big quantity of fine-medium sediment (4176 m3) as mud flow, part of which deposited on the fan tributary (4100 m3) and afterwards reached the main 1 collector and subsequent the instrumented station (400 m3). Total sediment yield of this snowmelt flood measured at the Rio Cordon station is three orders of magnitude greater than previous one. The main processes by which sediments are transferred from hillslope to the channel outlet is also shown. 2
Bedload yield may have large seasonal or interannual variations as a result of changes in sedimen... more Bedload yield may have large seasonal or interannual variations as a result of changes in sediment supply or sediment storage. Sediment supply along mountain rivers is strongly influenced by both drainage basin and channel processes. Drainage basin sediment inputs include hillslope, valley bottom and tributary channel sources that may be gradual (e.g. slope wash, average tributary inflows, soil creep) or abrupt (e.g. debris flow, rockfalls, tributary flash floods). These inputs may be seasonally driven (e.g. Wetzel, 1994) or aperiodic. The paper illustrates the results of 16 years research conducted on the Rio Cordon catchment (5 km2), where an experimental sta- tion has been operating for the integral and continuous automatic recording of water discharge and sediment transport (Lenzi et al., 1999). The device operates by separat- ing bedload from water discharge and fine sediment and subsequently measuring the two solid components. The 15 recorded floods (1986-2001) are grouped in two dis- tinct categories according to the bedload transport rates: SordinaryT and exceptionalT & cedil;floods (with return times exceeding 30-50 years). Supply-limited conditions of sed- iment transport prevail in the period 1986-1994, before the exceptional flash flood of September 14, 1994. During this cycle, bed armouring is the dominant sediment transport response: the amount of material of a given size range in transport is lim- ited by its availability and not by the competence of the flow. During the subsequent cycle 1995-2001, conditions of bedload transport change a lot given an increases of the sediment budget. An abrupt and aperiodic input of fine-medium grain size material occured to the main Rio Cordon stream during the snowmelt-induced flood of May 11, 2001, by means of a tributary mud flow contribution. This input influenced the bed- load transport by altering the proportion of the bed material which can be moved by the competent flow. The injection of these relatively fine-medium material increased that proportion, thereby enabling the bed load transport to increase. The main processes by which this input of sediments are transferred from the hillslope (landslide) to the Rio Cordon channel and to the outlet (recording station situated 1100 m. downstream) is also shown. References Wetzel, K., 1994. The significance of fluvial erosion, channel storage and gravitational processes in sediment production in a small mountainous catchment area. In: Dynamics and geomorphology of mountain rivers, edited by P. Ergenzinger and 1 K.H. Schmidt, pp. 141-160, Springer-Verlag, Berlin. Lenzi, M.A., D'Agostino, V., Billi, P., 1999. Bedload transport in the instrumented catchment of the Rio Cordon, Part I: Analysis of bedload records, conditions and threshold of bedload entrainment. Catena 36, 171-190. 2
Monitoring, Stimulation, Prevention and Remediation of Dense and Debris Flows IV, 2012
ABSTRACT Until very recently, rivers have been considered as the result of the interaction betwee... more ABSTRACT 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
2) Pontificia Universidad Catòlica de Chile, Santiago, Chile (lmao@uc.cl) The bedload transport p... more 2) Pontificia Universidad Catòlica de Chile, Santiago, Chile (lmao@uc.cl) The bedload transport plays a fundamental role in the mountain streams affecting several aspects as morphology and ecological status of the fluvial system, influencing fields as engineering, natural hazard and river management. The monitoring, assessment and quantification of this type of transport is not simple due to impulsive nature that distinguishes it. Moreover, in mountain streams the particle motion is strongly affected by the bed structures as well as by the highly heterogeneous bed material, acting especially through the hiding/exposure, consolidation and embedding effects. Thanks to the assumption that bedload transport may be understood as the result of random individual particle displacements, the sediment tracing method was widely employed in the last years. Such method allow to obtain precious data concerning sediment dynamic in mountain streams, improving the results obtainable by samplers, tra...
Monitoring, Stimulation, Prevention and Remediation of Dense and Debris Flows IV, 2012
... Plan-form changes of river features from 1960 were analyzed using aerial photos, and a LiDAR ... more ... Plan-form changes of river features from 1960 were analyzed using aerial photos, and a LiDAR was used to derive the maximum, minimum and mean elevation of island surfaces and the maximum and mean height of the island vegetation. ...
A modern approach to problems of non-point source water pollution and erosion in a watershed cons... more A modern approach to problems of non-point source water pollution and erosion in a watershed consists in linking distributed models to Geographic Information Systems (GIS). Water quality and quantity are affected by the inherent spatial and time variability of the hydrological attributes within a watershed. Spatially distributed models and data are more suitable than conventional lumped ones. They act directly
Suspended sediment transport in high mountain streams display a grater time-space variability and... more Suspended sediment transport in high mountain streams display a grater time-space variability and a shorter duration (normally concentrated during the snowmelt period and the duration time of single floods) than in larger lowland rivers. Suspended sedi- ment load and sediment yield were analysed in a small, high-gradient stream of East- ern Italian Alps which was instrumented to measure in continuous water discharge and sediment transport. The research was conducted in the Rio Cordon, a 5 Km2 small catchment of the Dolomites. The ratio of suspended to total sediment yield and the re- lations between sediment concentration and water discharge were analysed for eleven floods which occurred from 1991 to 2001. Different patterns of hysteresis in the re- lation 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 mayor flood (September 1994, 30 years<Tr<50 years) indicates a coarsening of transported material for increasing discharge. An analysis of grain size has shown that erosion areas on hill-slopes were the main source of suspended load. The relation between water discharge and S.S.C. for both floods and snowmelt runoff shows larger scatter for both series of data, with snowmelt data less scattered than rain- fall induced floods. This is accounted for by the variable effectiveness of erosion pro- cesses and sediment supply mechanisms during snowmelt and rainfall-induced floods. During snowmelt, erosion processes essentially consist in the removal of loose, fine- grained sediment from slopes by surface runoff; as a consequence, suspended sedi- ment transport takes place also with rather low discharges. Abundant suspended sedi- ment transport was recorded during the snowmelt period of May 2001, that followed a winter characterized by a huge snow cover and late snowfalls. Different sources of sed- iment contribute to suspended load during the May11, 2001, snowmelt induced-small- flood. Therefore, the most important erosion processes was a landslide that originates a debris-mud-flow, occur only as a consequence of infiltration-saturation processes and hypodermic runoff (without associated rainfall and/or snow fall). This situation caused the instability of a large part of a sub-catchment hill-slope, the movement and the transport of a big quantity of fine-medium sediment (4176 m3) as mud flow, part of which deposited on the fan tributary (4100 m3) and afterwards reached the main 1 collector and subsequent the instrumented station (400 m3). Total sediment yield of this snowmelt flood measured at the Rio Cordon station is three orders of magnitude greater than previous one. The main processes by which sediments are transferred from hillslope to the channel outlet is also shown. 2
Bedload yield may have large seasonal or interannual variations as a result of changes in sedimen... more Bedload yield may have large seasonal or interannual variations as a result of changes in sediment supply or sediment storage. Sediment supply along mountain rivers is strongly influenced by both drainage basin and channel processes. Drainage basin sediment inputs include hillslope, valley bottom and tributary channel sources that may be gradual (e.g. slope wash, average tributary inflows, soil creep) or abrupt (e.g. debris flow, rockfalls, tributary flash floods). These inputs may be seasonally driven (e.g. Wetzel, 1994) or aperiodic. The paper illustrates the results of 16 years research conducted on the Rio Cordon catchment (5 km2), where an experimental sta- tion has been operating for the integral and continuous automatic recording of water discharge and sediment transport (Lenzi et al., 1999). The device operates by separat- ing bedload from water discharge and fine sediment and subsequently measuring the two solid components. The 15 recorded floods (1986-2001) are grouped in two dis- tinct categories according to the bedload transport rates: SordinaryT and exceptionalT & cedil;floods (with return times exceeding 30-50 years). Supply-limited conditions of sed- iment transport prevail in the period 1986-1994, before the exceptional flash flood of September 14, 1994. During this cycle, bed armouring is the dominant sediment transport response: the amount of material of a given size range in transport is lim- ited by its availability and not by the competence of the flow. During the subsequent cycle 1995-2001, conditions of bedload transport change a lot given an increases of the sediment budget. An abrupt and aperiodic input of fine-medium grain size material occured to the main Rio Cordon stream during the snowmelt-induced flood of May 11, 2001, by means of a tributary mud flow contribution. This input influenced the bed- load transport by altering the proportion of the bed material which can be moved by the competent flow. The injection of these relatively fine-medium material increased that proportion, thereby enabling the bed load transport to increase. The main processes by which this input of sediments are transferred from the hillslope (landslide) to the Rio Cordon channel and to the outlet (recording station situated 1100 m. downstream) is also shown. References Wetzel, K., 1994. The significance of fluvial erosion, channel storage and gravitational processes in sediment production in a small mountainous catchment area. In: Dynamics and geomorphology of mountain rivers, edited by P. Ergenzinger and 1 K.H. Schmidt, pp. 141-160, Springer-Verlag, Berlin. Lenzi, M.A., D'Agostino, V., Billi, P., 1999. Bedload transport in the instrumented catchment of the Rio Cordon, Part I: Analysis of bedload records, conditions and threshold of bedload entrainment. Catena 36, 171-190. 2
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