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Volume 33 (1) - 2010

TURITTO O., BALDO M., AUDISIO C. & LOLLINO G.

A LiDAR application to assess long-term bed-level changes in a cobble-bed river: the case of the Orco River (North-Western Italy)

Pages 61-76

Abstract

The technique of terrestrial or airborne LiDAR (Light Detection And Ranging) has recently provided many topographic data at high accuracy and speed in various kinds of applications as urban planning, forest inventory, coastline protection, flood hazard forecast, glacier, avalanche and landslide monitoring. In the topic of fluvial investigations the LiDAR technique has provided useful information on river morphology. In particular, starting from a first Digital Terrain Model (DTM) generation, the following LiDAR surveys have allowed every smallest morphological adjustment to be assessed. They also allowed an accurate quantification of erosion and deposition processes which occurred in the river bed, deduced by an assessment of altimetric and volumetric changes, on a time scale of some days, some months or some years (short-term investigations). The great potential of the data acquired by LiDAR suggested a different and particular use: to test a method which can provide an assessment of the incision occurred in a cobble-bed river on a time scale of the last 100 years (long-term investigation). The study case refers to the Orco River (906 km 2 of drainage basin), an Alpine tributary of the Po River in North-Western Italy. Its channel morphology underwent a severe transformation above all in the second half of the 20 th century, mainly due to human interventions. In a 25 km reach the channel width decreased down to 44% on average from 1954 to 1989 and the multi-thread original pattern turned towards the singlethread pattern. Moreover, the stereoscopic analysis of aerial photographs also showed vertical changes, induced by the river bed incision, but the lack of previous topographic data did not allow to quantify its extent. In 2003 a detailed DTM of the Orco River was generated by an airborne LiDAR acquisition. In addition to the present river course this DTM covered the surrounding floodplain with relic forms of the older channel positions. These forms are identified and dated thanks to historical maps and aerial photographs. This particular circumstance suggested the use of this DTM to test the method for the assessment of the occurred channel incision. In a selected river reach (3.5 km long) six topographic cross-sections were drawn so as to cross the present and the older river positions. The river incision was obtained as a difference between the recent and the previous mean levels of its bed. A bed level incision of 2.7 m on average came out; the maximum (more than 3 m) was found near a narrow bridge and the minimum (2 m) was found where the river had a wider space for its planimetric mobility.

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