The response of braided planform configuration to flow variations, bed reworking and vegetation: the case of the Tagliamento River, Italy (original) (raw)
Related papers
Geomorphology, 2010
Large gravel-bed braided rivers show a highly dynamic evolution, where even moderate flow pulses can produce significant morphological adjustment. An estimation of network variations is needed by river authorities as well as by ecologists for better managing river ecosystems. However, few field investigations have been performed to measure the effect of unsteady discharges in braided rivers. In this paper we report on the morphological changes induced by six different floods in a 1-km-long reach of the Tagliamento River, Italy. Topographic surveys were performed after each significant flood, and an automatic digital camera continuously monitored the planimetric configuration. Repeated measurements of cross section profiles allowed the quantification of the transported sediments volume and, in particular, provided a basis to estimate the percentage of the floodplain subject to morphological evolution. Data analysis showed a good relationship between peak flow level, active width, and mean bed level changes. Similarly, lateral channel shifting is likely to increase for larger floods. Monitoring of a range of discharges up to bankfull conditions revealed a fundamental difference between flow pulses (events on the average occurring one or more times per year) and flood pulses (with a return period larger than two years). The morphological evolution in the former case is mainly limited to a few active branches and is locally driven by bank erosion at the bend apex. On the other hand, larger floods induce a complete reworking of the network configuration, where bifurcations and confluences play a crucial role. Single morphological units (like branches, nodes, bars) are no longer recognizable after this type of event. Braided rivers are therefore characterized by the occurrence of two different spatial and temporal scales that force the ecosystem dynamics.
Vegetation turnover in a braided river: frequency and effectiveness of floods of different magnitude
Earth Surface Processes and Landforms, 2014
This work addresses the temporal dynamics of riparian vegetation in large braided rivers, exploring the relationship between vegetation erosion and flood magnitude. In particular, it investigates the existence of a threshold discharge, or a range of discharges, above which erosion of vegetated patches within the channel occurs. The research was conducted on a 14 km long reach of the Tagliamento River, a braided river in north-eastern Italy. Ten sets of aerial photographs were used to investigate vegetation dynamics in the period 1954-2011. By using different geographic information system (GIS) procedures, three aspects of geomorphic-vegetation dynamics and interactions were addressed: (i) long-term (1954-2011) channel evolution and vegetation dynamics; (ii) the relationship between vegetation erosion/establishment and flow regime; (iii) vegetation turnover, in the period 1986-2011. Results show that vegetation turnover is remarkably rapid in the study reach with 50% of in-channel vegetation persisting for less than 5-6 years and only 10% of vegetation persisting for more than 18-19 years. The analysis shows that significant vegetation erosion is determined by relatively frequent floods, i.e. floods with a recurrence interval of c. 1-2.5 years, although some differences exist between sub-reaches with different densities of vegetation cover. These findings suggest that the erosion of riparian vegetation in braided rivers may not be controlled solely by very large floods, as is the case for lower energy gravel-bed rivers. Besides flow regime, other factors seem to play a significant role for in-channel vegetation cover over long time spans. In particular, erosion of marginal vegetation, which supplies large wood elements to the channel, increased notably over the study period and was an important factor for in-channel vegetation trends. Figure 2. The flow record at the Venzone gauging station in the period 1986-2011. The dashed vertical lines indicate the acquisition dates for the aerial photographs. The horizontal lines indicate the four discharge thresholds considered in the analysis of vegetation erosion and establishment: the two lower discharges have a recurrence interval (RI) < 1 year, while RI of the other two is 1.2 years and 2.5 years respectively.
Your article is protected by copyright and all rights are held exclusively by Springer Basel. This e-offprint is for personal use only and shall not be self-archived in electronic repositories. If you wish to self-archive your article, please use the accepted manuscript version for posting on your own website. You may further deposit the accepted manuscript version in any repository, provided it is only made publicly available 12 months after official publication or later and provided acknowledgement is given to the original source of publication and a link is inserted to the published article on Springer's website. The link must be accompanied by the following text: "The final publication is available at link.springer.com".
The timing and extent of the morphological changes that occurred in the last 30 years in a gravelbed river (the Brenta River, eastern Italian Alps) have been analysed using eight sets of aerial photos, repeated topographic measurements and morphological-vegetational surveys. Human activities have produced modifications in the natural sediment regime and the cessation of gravel extraction in the late 1990s seems to have caused vegetation erosion and channel widening. Alteration of sediment regime has played a major role in the medium and short-term channel evolution. However, only relevant flood events (RI > 10 years) appear to determine substantial islands erosion. The analysis at smaller scale (sub-reach level) proved to be more effective in describing morphological responses and its relationships with the sediment dynamics within the study reach (20 km). The understanding of sediment transfer at the sub-reach level will provide helpful guidelines for the discussion of channel recovery potential.
River Research and Applications, 2009
The Tagliamento River, Northeast Italy, represents an important Alpine to Mediterranean braided system, where interactions between river flows, sediment dynamics and vegetated landforms can be investigated within a relatively unconfined setting.We analysed data from contemporary and historical sources, including stage records, photographs and topographic surveys. From these we identified river stages at which thresholds in surface hydrological connectivity and biogeomorphological adjustment appeared to occur, contributing to a shifting habitat mosaic.Significant adjustments in landscape elements within the active tract commence at river stages well below bankfull with return periods of a few months. Flow pulse events with return periods from a few months to 2 years support a dynamic inundation pattern, ranging from a patchwork of isolated water bodies within a predominantly terrestrial landscape at low river stages to isolated vegetated islands within a fully connected aquatic landscape as the river approaches bankfull. Across this range, interactions between flow, sediment and vegetation lead to gradual and abrupt transitions in persistence, form and connectedness of different landscape elements. Bankfull flows (return period over 2.5 years) topple and disperse significant numbers of large trees, seeding the next generation of vegetated patches, and larger floods (return period around 10 years) induce significant turnover of established islands and floodplain surfaces.The results reported in this paper illustrate how extensive interdisciplinary research on a single river system can provide useful insights concerning the time scales and thresholds that characterize water–sediment–vegetation interactions in piedmont reaches of Alpine to Mediterranean braided systems. Anthropogenic effects on river systems are ubiquitous throughout Europe. However, systems such as the Tagliamento River that retain significant process dynamism and morphological integrity, provide a laboratory within which reference processes and process–form interactions can be investigated, understood and then incorporated into innovative restoration design on more impacted systems. Copyright © 2009 John Wiley & Sons, Ltd.
Morphological response to river engineering and management in alluvial channels in Italy
Geomorphology, 2003
In response to various types of human disturbance, most Italian rivers have experienced considerable channel adjustment during the last centuries and in particular in the last decades. This paper reviews all existing published studies and available data, and aims to reconstruct a general outline of the main channel adjustments that have occurred in Italian rivers during the past 100 years.
2007
Riverine floodplains are pulsing ecosystems that expand and contract with changing flow. In this study we quantified large-scale expansion and contraction dynamics of surface waters along a 41•5 km braided section of the last remaining semi-natural large Alpine gravel-bed river (Tagliamento River; NE Italy). To assess surface-subsurface exchange patterns we measured discharge and vertical hydraulic gradients at multiple locations along the corridor. We identified two river sections delineated by distinct geomorphic knickpoints. In the upper 29 km, Section I (losing zone), surface flow decreased on average by 2•5 ± ± ± ± ± 0•8 m 3 s − − − − −1 per river-km. In the downstream 12•5 km, Section II (gaining zone), surface flow increased on average by 0.3 ± ± ± ± ± 0.1 m 3 s − − − − −1 per river-km. The losing zone experienced frequent and extensive drying and rewetting cycles. The length of the dry river section was measured over a 1•5 year period using differential GPS. Up to 23 km of Section I fell dry at the surface. Frequent and irregular flow pulses led to rapid expansions of the wetted channel at velocities of up to 3 km h − − − − −1 , while the subsequent contraction velocities were less than 0•5 km h − − − − −1. Water level was linearly regressed against the total length of the dry river section (r 2 = = = = = 0•74; p < < < < < 0•0001). This relationship, in combination with a continuous stage record, was used to evaluate expansion and contraction dynamics over a 4 year period. Timing, frequency, magnitude (spatial extent) and duration of expansion and contraction dynamics reflected the flashy flow regime of the Tagliamento River, including a high intra-and inter-annual variability of surface drying and rewetting. Our study emphasizes that even small changes in flow can cause major increases or decreases of ecosystem size, thereby creating a highly dynamic and harsh environment for both terrestrial and aquatic organisms.
Geomorphological Approaches for River Management and Restoration in Italian and French Rivers
Geophysical Monograph Series, 2011
River management and restoration in Italy and France are increasingly considering physical processes and trends of channel adjustment as a basic knowledge for enhancing river conditions and promoting channel recovery. Italian and French rivers are characterized by a long history of human disturbances and land use changes. As a consequence, trends of channel adjustments and related management problems are similar, with a historical phase of aggradation followed by a period (last century) of intense channel incision and narrowing. A general overview on recent progress in using geomorphic approaches to river management in Italy and France is presented here by illustrating a series of examples of studies and management applications. A synthetic state of the art on the recent morphological changes of Italian and French rivers is first reported. Some examples of quantification of bed load are also illustrated, providing a necessary quantitative knowledge for possible interventions or strategies for promoting bed load recovery.
Journal of Hydrology, 2015
The quantitative analysis of the planform changes of the unconfined reach of the Taro River, in the Italian Northern Apennines, has allowed the channel evolution in the last 200 years to be outlined. Nine sets of maps and orthophotos, ranging from 1828 to 2011, have been used to evaluate the medium-term changes in channel morphology along the entire time interval, as well as the short-term changes in the most recent decade. Starting from the digitized channel limits and bars, a number of shell scripts based on GIS commands has been used for fast and automatic calculation of the main morphological parameters (channel length, width, braiding, sinuosity, centerline shifting) and for the drawing of graphs showing in detail their continuous variations along the entire study reach. The analysis of the differences in parameters on subsequent dates has revealed that, at least until the end of the 20th century, continuous reduction in channel width (up to a total of 75%) and braiding (43%), as well as continuous increase in channel length (13%) and sinuosity (29%), took place. This is in agreement with most of the previous studies on other rivers, both in Italy and abroad. In contrast with the results of other studies, the most recent evolutionary trend of the Taro River shows substantial morphological stability with possible slight narrowing. The identification of the variations along the channel, facilitated by the analysis of the parameter curves and supported by the historical documentation, reveals that these variations can be substantially attributed to human activities. In particular, the continuous narrowing is largely due to the recurrent subtraction of riverbed areas to be used for agricultural and industrial purposes, as well as to the construction of 10 bridges with the relevant bank protections. The intense mining between 1950s and 1980s seems to have caused a sharp incision and partial narrowing only. The morphological changes due to the reduction in the flow regime, which seem to emerge from the sporadic and discontinuous hydrological data, are expected to be negligible and, in any case, are immaterial when compared with the remarkable changes due to human activities.