Llena M., Cavalli M., Vericat D., Crema S., Smith M., 2017, Assessing the effects of changes on land cover on sediment connectivity by means of historical aerial photographs and digital photogrammetry, 12 Convegno GIT - Geology and Information Technology, Gavorrano (GR), 12-14/06/2017,
URL: http://www.cnr.it/prodotto/i/373404

A significant number of mountain catchments have suffered important land use changes due to agricultural abandonment during the second half of the 20th century. These changes do not only induced an increase in the vegetation cover but also a series of natural and anthropic morphological changes (e.g. terraces) that influence the water and sediment flux through catchments. In the present study the effect of these changes (i.e. land cover and topography) on sediment connectivity is analyzed in different scenarios of change in the Upper Cinca Basin (Southern Pyrenees). Potential sediment connectivity can be assessed by a morphometric indicator named Index of Connectivity (IC) developed by Cavalli et al (2013) based upon the original approach by Borselli et al (2008). The input variables of IC are determined by the land use or landscape roughness, and topography. In this work we present a methodological workflow to obtain such variables from historical aerial photos using digital photogrammetry through Structure from Motion and Multi View Stereo algorithms (SfM-MVS). A prerequisite of the analysis is to reconstruct the land use and the landscape properties at the period in which the IC is estimated. The analysis consists of three interrelated steps_ (a) extraction of historical ortophotomaps and point clouds from historical photos (aerial photos from 1957 and 1977) using SfM-MVS, (b) derivation of land use maps and topographic models for those periods to parametrize IC, and (c) assessment and comparison of historical sediment connectivity in each period. The workflow was tested in different contrasted sub-catchments representative of three main scenarios_ (1) Changes on land cover; (2) Topographic changes on agricultural fields (terracing); (3) Topographic changes associated to infrastructures (road construction). From the methodological point of view, results highlight as the selection of the suitable resolution of the Digital Elevation Models used in each scenario is crucial for the correct analysis of IC changes. Terraces affect connectivity in a major degree than land use changes, following a more spatially concentrated pattern, while the morphological variation due to road construction led to an increase of erosional activities uphill the road. In the light of these results, the developed workflow to assess sediment connectivity at multiple temporal scales has proved to be a promising and useful methodology to plan and asses potential risks associated to morphological and land use changes.

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