Abstract

Many gravel bed rivers in the European Alpine area suffered different ranges and types of human pressure that modified their morphology and altered their processes. This work presents the case of the middle portion of the Brenta River, historically impacted by human activities such as floodplain occupations, bank protection, gravel mining, hydropower schemes and water diversion. Dam operation and gravel mining have produced considerable modifications in the natural sediment regime generating important morphological channel responses (narrowing and incision). Large areas of the former active channel have been colonized by riparian vegetation, both as islands and as marginal woodlands. Overall, the river changed its morphological pattern from braided to wandering. The present study analyses the timing and extent of the planform morphological changes that occurred over the last 30years along the middle portion of the river (20km long) through the examination of aerial photos, repeated topographic measurements and hydrological data. A series of recent aerial photos (1981, 1990, 1994, 1999, 2003, 2006, 2008, 2010 and 2011) have been used to assess the medium and short-term morphological changes of the floodplains and the active channel area. As to the medium-term modification, the recent changes in in-channel gravel mining have determined a new trend of active channel widening through erosion of vegetated areas. The analysis has also allowed to assess the morphological effect of single flood events. Only floods with recurrence interval higher than 8-10years appear to be able to determine substantial erosion of floodplain and island margins.