Most Italian braided rivers have experienced considerable channel adjustments during the past 100 yr, particularly in recent decades. Channel adjustments have taken place mainly in response to sediment mining and the construction of dams—two interventions that have substantially altered sediment supply. This study examines recent and contemporary channel change along such braided rivers with a view to supporting and improving maintenance and management strategies. Three rivers in northeastern Italy, the Tagliamento, the Piave and the Brenta, represent different examples of channel adjustment to a decrease in sediment supply. Sediment mining has been very intense along all three rivers, but only the Piave and the Brenta have been strongly affected by the construction of dams. A historical analysis was performed on the three rivers using maps, aerial photographs, longitudinal profiles and cross sections. Morphological features that were examined included channel width, braiding index, bed elevation and channel configuration. Two types of channel adjustment have been dominant in these braided rivers: narrowing and incision. Channel-width reduction has been in the order of 50% (Tagliamento and Brenta rivers) or more (Piave River), whereas bed-level lowering has been up to 3–4 m in the Tagliamento and Piave rivers, and up to 5 m along the Brenta. Additionally, braiding intensity has decreased significantly in all of the rivers. Changes in channel pattern, from braided to wandering, have occurred mainly in the Brenta and the Piave rivers, but an upstream migration of the transition between braided and single-thread morphology has occurred along all three rivers. Temporal trends of channel adjustment show that the main phase of narrowing and incision has taken place since the 1950s or 1960s, and that now it may have finished, since other kinds of processes, such as widening and (local) aggradation, have occurred in the past 10–15 yr. For river management it is necessary to recognize the effects of channel adjustments on structures and the environment, but it is also necessary to understand the causes of these morphological changes, and in particular to recognize that sediment supply is a key issue in braided rivers.

Effects of human impact on braided river morphology: examples from Northern Italy

SURIAN, NICOLA
2006

Abstract

Most Italian braided rivers have experienced considerable channel adjustments during the past 100 yr, particularly in recent decades. Channel adjustments have taken place mainly in response to sediment mining and the construction of dams—two interventions that have substantially altered sediment supply. This study examines recent and contemporary channel change along such braided rivers with a view to supporting and improving maintenance and management strategies. Three rivers in northeastern Italy, the Tagliamento, the Piave and the Brenta, represent different examples of channel adjustment to a decrease in sediment supply. Sediment mining has been very intense along all three rivers, but only the Piave and the Brenta have been strongly affected by the construction of dams. A historical analysis was performed on the three rivers using maps, aerial photographs, longitudinal profiles and cross sections. Morphological features that were examined included channel width, braiding index, bed elevation and channel configuration. Two types of channel adjustment have been dominant in these braided rivers: narrowing and incision. Channel-width reduction has been in the order of 50% (Tagliamento and Brenta rivers) or more (Piave River), whereas bed-level lowering has been up to 3–4 m in the Tagliamento and Piave rivers, and up to 5 m along the Brenta. Additionally, braiding intensity has decreased significantly in all of the rivers. Changes in channel pattern, from braided to wandering, have occurred mainly in the Brenta and the Piave rivers, but an upstream migration of the transition between braided and single-thread morphology has occurred along all three rivers. Temporal trends of channel adjustment show that the main phase of narrowing and incision has taken place since the 1950s or 1960s, and that now it may have finished, since other kinds of processes, such as widening and (local) aggradation, have occurred in the past 10–15 yr. For river management it is necessary to recognize the effects of channel adjustments on structures and the environment, but it is also necessary to understand the causes of these morphological changes, and in particular to recognize that sediment supply is a key issue in braided rivers.
2006
Braided Rivers
9781405151214
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Utilizza questo identificativo per citare o creare un link a questo documento: https://hdl.handle.net/11577/1559896
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