Vegetation turnover in a braided river: frequency and effectiveness of floods of different magnitude

Abstract

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.

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

“…They also illustrate how the magnitude of that influence increases with the development of the riparian "forest" and the quantity of wood supplied to the river (Wohl, 2013;Surian et al, 2015).…”

Relationships between woody vegetation and geomorphological patterns in three gravel-bed rivers with different intensities of anthropogenic disturbance

“…However, this is not the case in the Brenta River where limitation of sediment supply by dams is higher than in the Piave. For the Tagliamento River, Surian et al [63] argue that an expansion of in-channel vegetated areas (i.e. islands) can be envisaged for the next years because of a marked increase in the availability of wood from the channel margins, which are now much more forested than in previous decades.…”

“…1) and there are torrent control works (check dams) only in some of the tributaries. The current morphological processes can be considered to take place under near natural conditions, with relatively frequent flows able to erode woody vegetation [63]. However, sediment mining was quite extensive in the river in the 1970s-1990s period (Tab.…”

“…In the last century, also the Tagliamento River was affected by channel narrowing in response to gravel mining, but in this case the morphological pattern remained braided, and the incision was estimated to be quite limited (0.5 1.0 m, [63] and [70]). Now that commercial gravel mining is banned, the current hydro-morphological processes in this river are much less altered than the Piave and Brenta, as already mentioned above (case C, …”

Bank erosion and large wood recruitment along a gravel bed river

“…The assessment of LW recruitment is based on direct and indirect techniques. Field measurements have been applied to estimate the importance of different processes (i.e., mortality, bank erosion, landslides) on the LW recruitment rates (Benda et al, 2002;May and Gresswell, 2003), to compare recruitment rates from different riparian landforms and during different flood conditions (Palik et al, 1998;Golladay et al, 2007;Surian et al, 2015). The main advances achieved by indirect techniques regard the development of LW recruitment models that can estimate the input derived from hillslope instabilities , predict LW input according to riparian characteristics, channel morphology and bank instability (Downs and Simon, 2001), and detect the LW supply during windthrow events (Bahuguna et al, 2010).…”