Medium term fluvial island evolution in relation with flood events in the Piave River

Abstract

River islands could be defined as discrete areas of woodland vegetation surrounded by either water-filled channels or exposed gravel which exhibits some stability, and remain exposed during bunk-full flows. Islands are important from morphological, functional, and ecological points of view. The aim of the present study is to analyze the dynamics of stable, young and pioneer islands in the Piave River, which suffered intense and multiple human impacts especially due to dam building and in-channel gravel mining. 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. The results suggest that stable islands lie at higher elevation than young and pioneer islands, and are characterized by a thicker layer of fine sediments deposited on their surfaces after big floods. There was a progressive decrease in the active corridor area from 1960 to 2006. After the 1966 exceptional flood (RI> 200 years) there was a moderate increase of island extend and numbers, followed by a further increase from 1991, due to a succession of flood events, in 1993 and 2002 with RI> 10 years, as well as a change in the anthropic management characterized by a control relating to mining activities. The narrowing trend (1960-1999) of the morphological plan-form, certainly enhanced the chance of the island to become stable and this explains the reduction of active channel, the increase of stable island and the reduction of pioneer island. Even though a general channe

Characteristics of fluvial islands along three gravel bed-rivers of North-Eastern Italy

“…On the other hand, along the Tagliamento river there is a higher difference between the sizes of the stable islands and the other two kinds, this is probably due to the fact that only the more established islands are able to resist to the very common flood events that could happen different times per year moving huge volumes of sediments . These observations matched with those made by different authors (Bertoldi et al, 2009;Comiti et al, 2011;Picco et al, 2012a) wherein the island dynamics were found to be strictly associated to the occurrence of major floods (RI > 10-15 years), which are the only ones able to generate substantial island erosions.…”

“…As expected, the established ones lie at higher elevation. This is due to the fact that older vegetation growing on established islands allows the deposition of fine sediments which, in turns, enhances the conditions for vegetation growth, providing even better circumstances for the vegetation to establish on stable islands (Picco et al, 2012a). The flood control (by dams) and the sediment recruitment control (by embankments), can be the factors that trigger the higher difference in the ground elevation between the three island types along the strongly affected Brenta river.…”

“…The median grain size varies between 20 and 50 mm. The Piave River features a complex pattern of vegetation distribution along the analysed study reach (Picco et al 2012a(Picco et al , 2012b(Picco et al , 2014, which is far more composite than those identified in less disturbed river systems (Hupp and Osterkamp, 1996;Gurnell and Petts, 2002;Tabacchi et al, 1998;Bendix and Hupp, 2000). …”

Riparian forest structure, vegetation cover and flood events in the Piave River

“…The ceasing of gravel extraction in the late 1990s seems to have determined a reversal in the evolutionary trend, with evidence of vegetation erosion/channel widening (Comiti et al [15]). This complex series of planimetric and vertical adjustments, and especially the recent active channel widening tendency may explain why "mature" patches lie at elevations similar to areas covered with the most pioneer plant communities (Picco et al [24]). …”

Medium- and short-term channel and island evolution in a disturbed gravel bed river (Brenta River, Italy)

“…Comiti et al (2011) estimated that ten years represents the return interval of the peak discharge able to considerably modify the fluvial planimetric shape, especially of floodplains and islands. This is confirmed also by island reduction processes that take place due to flood events of considerable magnitudes (Comiti et al, 2011;Picco et al, 2012aPicco et al, , 2012bMao et al, 2012). Because of the importance of highmagnitude floods (RI>5 years) in shaping the river, a question arises as to the actual effectiveness of near-bankfull flows (RI~1.5 years) in determining the morphology of the Brenta River.…”