HR: 0800h
AN: H31B-03    [Abstracts]
TI: Morphological Changes in a Braided River of the Italian Alps During the Last Two Centuries and the Related Dynamics of Riparian Vegetation
AU: Comiti, F
EM: francesco.comiti@unipd.it
AF: Dept. Land and Agroforest Environements, University of Padova, Viale dell'Università 16, Legnaro (PD), 35020, Italy
AU: Da Canal, M
EM:
AF: Dept. Land and Agroforest Environements, University of Padova, Viale dell'Università 16, Legnaro (PD), 35020, Italy
AU: Surian, N
EM: nicola.surian@unipd.it
AF: University of Padova, Department of Geography, Via del Santo, 26 , Padova, 35100, Italy
AU: Mao, L
EM: luca.mao@unipd.it
AF: Dept. Land and Agroforest Environements, University of Padova, Viale dell'Università 16, Legnaro (PD), 35020, Italy
AU: * Lenzi, M A
EM: marioaristide.lenzi@unipd.it
AF: Dept. Land and Agroforest Environements, University of Padova, Viale dell'Università 16, Legnaro (PD), 35020, Italy
AB: This study aims to determine the timing and the extent of the morphological changes occurred in a 30-km long reach of a gravel bed river of the Eastern Italian Alps (the Piave river, in the Val Belluna synclinal), now covered with abundant riparian vegetation and forested islands that may represent important source of wood during floods. The Piave river (drainage area 3,300 km2 at the exit of the montane district) was once characterised by a wide, braided active channel confined by climatic terraces formed during the last glacial retreat (Holocene). It underwent a strong narrowing during the last century that led to a wandering/single-thread morphology, leaving large areas available to the establishment of riparian forests (genus Populus, Salix, Robinia). It is notable that some of these woodlands are now most valuable under an ecological perspective, in particular they represent important nesting sites for avifauna (EC BIRDS Directive). Nine sets of historical maps and aerial photos (from 1805 to 2003) were used to determine changes in the Piave river corridor (active channel width, vegetation cover, island areas and number), georeferencing and editing maps by a GIS software. Two series of cross-sections (1935-2003) allowed to get insights into the vertical evolution of the streambed. Discharge data (from 1926) permitted to assess the possible effect of flow regulation on formative discharge. Results indicate that average channel width (i.e. channel area/channel length) dropped from about 600 m during the XIX century to 200 m in the 1991, and increased thereafter up to 300 m in the 2003. The dynamics of riparian vegetation follows a clear complementary trend. Such a pattern reflects the different human impacts on the river system, particularly the gravel mining activity intensely carried out in the '70-'80. Large forested islands were already present at the early XIX century, but their extension relative to channel area was much smaller (3 times) than in 1991. However, present island areas are back to early XX century values. Also, island erosion is clearly linked to flood events having recurrence intervals R.I. > 10 yr. Magnitude-frequency analysis of discharge data allowed to rule out the effect of flow regulation on channel forming discharges (R.I. 2-3 yr), thus suggesting that sediment starvation - due to lower basin supply earlier and in-channel mining later - is the primary reason for the observed channel adjustments. Finally, a map showing the location of likely wood sources in case of intense flood events was derived by intersecting the channel migration corridor with present riparian forest areas. These areas may then be subjected to specific management in order to reduce flood hazards for downstream sections.
DE: 1825 Geomorphology: fluvial (1625)
DE: 1834 Human impacts
DE: 1856 River channels (0483, 0744)
DE: 1862 Sediment transport (4558)
SC: Hydrology [H]
MN: 2007 Joint Assembly