Analysis of linears, landforms and geology relationships and anthropic effects in the period 1976-1994, northern Asso River basin (Siena – Italy)

Abstract

The knowledge of how and how much a landscape changed in a specific time period is of paramount importance to predict scenarios and trends for the future. Landscape analysis can help in understanding state factors influence on environmental changes and effects. Among the effects of the different state factors, linears, as an expression of local and regional tectonic activity, and landforms, also as consequence of anthropic actions, are to be considered. Changes of landform features (e.g type, number, etc) are to be analysed as change effects. In this view, it is important to control how anthropic pressures can lead to an increase in land degradation as a consequence of the influences of over-stressed landforms on hillslopes and watershed regime. Aim of this paper is the analysis of linears and landforms present in the landscape, their relationships and relational typologies, and the variations of the persistence of the landforms in time as a consequence of anthropic action. In a study area, located in the northern Asso River basin (Siena, Central Italy), about 173 km², characterised mainly by marine pliocenic sediments, representative of the Central Italian neogenic basins, were analysed linear features (linears) and landforms, i.e. calanco (C), biancana (B), levelled biancana (Bl), shallow seated gravitational deformations (SSGD), levelled ones (SSGDl), calanco potential areas (CPA), mudflows (Fco), rockfalls (Fcr) and balza (Bz), their relationships, typology and temporal persistence, by aerial photo interpretation and ground control. Geology was used as first discriminant in the analysis of linears and landforms. Data were reported on a 1:10000 scale base map and analysed on 1 km² base grid. Several linears were found to define directional trends and affecting a large part of the study area, showing also the main regional tectonic directions: N37°, N162°, N145°, N90°, N180°; these trends are called «main trends». Main trends linears are mainly related with Pliocene clays (pa). Other linears show directions different from the «main trends»: the linears are called «local linears», displaying minor extension than the main trends. Different «landscapes of linears» were found as a function of the frequency of the «local linears» (n of linears/ km²), statistics of the directions of the linears and geology. Local «landscapes of linears» are related to local geology, mostly to pa. Relationships between landforms and linears were analysed. Variations in landform presence in the period 1976-1994 were also considered (see also the maps included). In the examined time period, all landform types, with the exception of increased CPA, were subjected to a strong decrease, either on absolute value basis, or on a relative percentage basis, referred to their presence in the initial year, 1976. Landforms levelled before 1976 (SSGDl and Bl) seem mostly subjected to continuous, yearly, slope reshaping. C, SSGD and B appear to be, at present, at major risk by new slope reshapings. These variations must be considered as effects of anthropic pressures on the landscape due to the present, mainly agricultural, land use.