Department of Land, Environment, Agriculture and Forestry, Università degli Studi di Padova; Viale dell'Università 16, 35020 Legnaro (PD), Italy; lorenzo.picco@unipd.it; diego.ravazzolo@studenti.unipd.it; alessia.tonon@studenti.unipd.it; marioaristide.lenzi@unipd.it.
* Corresponding author: lorenzo.picco@unipd.it, Department of Land, Environment, Agriculture and Forestry University of Padova, Viale dell'Università 16, 35020 Legnaro (PD), Italy; Phone: (+39) 0498272700, Fax: (+39) 0498272686.
Abstract
During the last years there was an increasing attention on Large Wood (LW), due to its role in influencing riverine processes and to its impact along fluvial systems. Particularly attention were devoted to the characterization of LW, while there is still a lack of knowledge about its recruitment. This study aims to analyze the recruitment of LW along a reach of a gravel bed river after an over-bankfull flood. The analysis were carried out along a reach of the middle course of the Piave River (North-East Italy). A buffer zone of 20 m–wide was established along floodplains and islands. Into this area all the standing trees with diameter ≥ 0.10 m were manually measured (Diameter Breast Height–DBH; Height), numbered tags were installed on each tree considered and its position were collected using a differential GPS. After an over-bankfull flood occurred in November 2014 (Q = 1329 m3s-1; R.I = 6 years), field survey were carried
out. During the flood event 428 and 247 were recruited from the floodplains and islands, respectively. A total amount of 80.92 m3 (0.19 m3/tree) and 39.88 m3 (0.16 m3/tree) was inserted into the active channel
from floodplains and islands, respectively. Field survey permitted to recover 155 and 32 trees coming from floodplains and island, respectively. For each tree, the displacement length was calculated, with maximum values of about 3642 m and 1024 m for trees coming from floodplains and islands, respectively. These results give useful indication for the management of the riparian vegetation.
Key words: Large Wood; Large Wood recruitment; Gravel bed river; floodplain; fluvial island; Piave River. Introduction
Large Wood (LW) in riverine environments exerts many different important functions on hydrology, morphology and ecology aspects (Piégay, 2003; Seo et al., 2010; Gurnell, 2013). On the other hand, it is necessary to underline that LW represent a potential hazard during flood events particularly interacting with sensitive structures as bridges (Mazzorana et al., 2011). Consequently more attention has devoted to LW, during the most recent years an increasing attention has been dedicated to define the wood budget (Schenk et al., 2013). Typically, the wood budget is defined as the sum of inputs (transport from upstream, recruitment from banks) and outputs (downstream transport, overbank deposition and decay) (Martin and Benda, 2001). However, there is still a lack of information about the lateral recruitment of LW from floodplains and fluvial islands in large gravel bed rivers environments. This study aims to better characterize the differences in recruitment from these two geomorphic units and increase the knowledge of LW transport after its recruitment, as to give more detail and information useful for a better management of the riparian area.
Study area
The Piave River flows south from the Alps (≈ 2000 m a.s.l.) for 220 km to the mouth in the Adriatic Sea, close to Venice. Its basin present a drainage area of about 3899 km2. The Piave River is a gravel bed river
historically affected by human activities (hydropower schemes, bank protections and gravel mining activities), over the basin and along the main channel (Comiti et al., 2011).The study has been carried out along a 3 km – long study reach located into the middle course of the Piave River. The reach has a width of about 400 m, mean slope of about 0.4 % and a transitional morphology between braided and wandering, and it is characterized by the presence of a big established island in the middle part of the reach.
Methods
A buffer zone of 20 m–wide was considered along the floodplains and islands. Into this stripe manual measurements were taken for every standing tree with diameter ≥ 0.10 m (Diameter Breast Height–DBH; Height). Moreover, for each tree the GPS position was recorded and a numbered tag was installed to simplify the post event recovery. The volume of each tree was calculated following Boivin et al. (2015). In November 2014 an over-bankfull flood (Q = 1329 m3s-1; R.I = 6 years) occurred. After this flood, field surveys
were carried out. Moving downstream from the upper part of the study area, every single LW has been checked to recognize tagged plants. For each tagged LW, GPS position and some qualitative details (morphological unit, orientation respect to the main flow direction, single or accumulation) (Ravazzolo et al., 2015) were registered.
Results
The over-bankfull flood eroded considerable portion of both islands and floodplain. The three erosional processes occurred along the floodplain appear similar, in fact these show extension of about 5514 m2, 4925
m2 and 4802 m2, respectively. On the other hand there is a considerable difference between the size of the
two erosional processes detected along the bigger fluvial island of the study reach (≈ 931 m2 and 12860 m2,
respectively). From the floodplain there was input of about 428 trees for a total volume of about 80.92 m3
(0.19 m3/tree), while from the islands there was recruitment of about 247 trees corresponding to a LW
volume of about 39.88 m3 (0.16 m3/tree). The trees recruited from the floodplain are characterized by
maximum and mean height of about 20.00 m and 8.88 m, respectively; while the maximum and mean diameter are 0.54 m and 0.14 m, respectively. On the other hand, the trees recruited by the island are characterized by maximum and mean height of about 15.00 m and 6.64 m, respectively. The maximum diameter is 0.44 m, while the mean one is 0.14 m.
During the post event field survey has been possible to find 155 trees recruited from floodplain (recovery rate = 36.21 %) and 32 trees coming from islands (recovery rate = 12.96 %). Thanks to these results has been possible detect also the travel distance for these recruited trees. Looking at the floodplain input, the minimum and maximum displacement distance ranges from 1 m to 3642 m, respectively. On the other hand, the recruited trees from islands were transported for a minimum and maximum distance of around 3 m and 1024, respectively. Considering the displacement length and the recruited tree diameter (Fig. 1) it is possible to underline as the highest travel distance correspond to the lower diameter, but it is not possible to define a clear correlation between these data. On the other hand, connecting the travel distance to the height (Fig.1)
it is possible to note as the highest displacement do not correspond to the smaller tree (1 m height) but to a tree of 6 m tall.
Figure 1: correlation between travel distance and recruited tree diameter (on the left), and between travel distance and recruited tree height (on the right)
Discussion and conclusion
These preliminary results suggest as the recruitment of LW due to bank erosion can be a considerable source of LW also during not extraordinary floods. A total amount of around 120 m3, can be considered as
considerable also because of the short length of the analysed reach. Interesting are the results relating to the subsequent transport of the LW after its recruitment. Though the Piave River is heavily impacted by hydropower schemes and water diversions, a not extraordinary flood can produce important displacement of logs. In this case the results suggest as the travel distance are higher than what already presented (Ravazzolo et al., this conference) though during higher flood events. These results can help planning the management of the riparian vegetation, particularly in the area prone to erosion and located just upstream of sensitive structures or river sections. Future challenges will be to better investigate the displacement trajectories after the recruitment and the mechanisms of resistance of the different vegetation cover types. Acknowledgments: This research is funded within, the University of Padova research Project CPDA149091- “WoodAlp: linking large Wood and morphological dynamics of gravel bed rivers of Eastern Italian Alps”- 2014-16 and the Project “SedAlp: sediment management in Alpine basins, integrating sediment continuum, risk mitigation and hydropower”, 83-4-3-AT, in the framework of the European Territorial Cooperation Program “Alpine Space” 2007-13.
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