Surface flows patterns and their correlation with faults geometry using SRTM data in the basin-central Italy.

Topography is basic to many earth surface processes. It is used in analyses in hydrology and geomorphology, and many others, as a means both of explaining processes and of predicting them through modelling. Our capacity to understand and model these processes depends on the quality of the topographic data that are available. In that context morphometry is defined as quantitative measurement of landscape shape. At the simplest level, landforms can be characterized in terms of their size, elevation (maximum, minimum or average), and slope. Quantitative measurements allow to compare different landforms and to calculate less straightforward parameters (geomorphic indices) that may be useful for identifying a particular characteristic of an area (i.e. its level of tectonic activity). Empirical observations from fluvial systems in recent work in the northern Apennines reveal a consistent power-law scaling between channel slope and contributing drainage area. Theoretical arguments for both detachment and transport limited erosion regimes suggest that rock uplift rate should exert first order control on this scaling. Here we present a preliminary geomorphologic analysis based on the Shuttle Radar Topography Mission (SRTM) Digital Elevation Model (DEM) in order to find evidences for surface expression of tectonic activity in the Terni basin. The Terni basin is located in the central–southern part of the Umbria region (central Italy). This area, which forms the middle part of a much vaster intermontane depression that extends northward to the borders of Tuscany and southward to the Lazio region, is bordered in the north by the southern side of Martani ridge, and in the west it reaches as far as the northernmost end of Narnese–Amerina ridge. Its morphologic shape is the result of different tectonic phases. The recent tectonic activity, characterized by low rates makes the localization and characterization of deformation difficult when using geodetic and seismologic data alone while drainage morphology shows to be more sensitive to vertical tectonic and tilting. In this analysis we have used the SRTM DEM with a resolution of 3-arc-seconds. Assemblage and local interpolation of the SRTM was performed using an Arc-Macro Language procedure. We prepared the SRTM to carry out hydrological analysis on a regional scale, like the definition of surface drainage pattern including diffuse flow (i.e., alluvial fans) and braided channels. The original SRTM DEM and the derivative data sets were processed in a way to optionally delineate drainage networks, overland paths, watersheds for user-specified locations, sub-watersheds for the major tributaries of a drainage networks, and pour point linkages between watersheds. While watershed and overland flow paths are closely related to slope, aspect, and inflection information, they also present non-neighbourhood problems such as determining direction of flow in the interior of a large flat-area. The shape of a surface determines how water will flow across it and using a DEM as input it is possible to delineate a drainage system and quantify the characteristics of that system very easily. Delineation of watersheds from a grid DEM data has become standardized on the ‘eight-direction pour point model’ in which each cell is connected to one of its eight neighbour cells (four on the principal axes and four on the diagonals) according to the direction of steepest descent. Given an elevation grid, a grid of flow directions is constructed and from this is derived a grid of flow accumulation counting the number of cells upstream of a given cell. Then streams are identified as lines of cells whose flow accumulation exceeds a given number of cells (thresholds) and thus a specified upstream drainage area is also identified. So watersheds (river drainage basins) are identified as the set of cells draining through a given cell identified as outlet point (“pour” point). Once the watersheds were defined the boundaries were used to perform a “Zonalstat” analysis. The “Zonalstat” function, was used to compute mean slope and mean elevation using the watersheds as polygon masks. Based on that on a reach of any basin was then calculated a gradient-index that allows meaningful comparison of channel slope on stream-basin of different size. The index reflects the product of the channel slope at a point. In an adjusted topography, changes in gradient index values along a stream could correspond to an introduced load. This because in any landscape the gradient index of a stream-basin is related to total relief and basin area. The area-slope index is calculated based on the Leopold’s et al. law (1964): S = CA- where C is a constant and the exponent  ranges from 0.37 to 0.83 with a mean of 0.6 that represents the stead state. Given that the pattern of rock uplift is best defined on the northern-east limb of the Terni basin we utilize the watershed groups (north) to calibrate a power law area\slope model for this landscape. The model allowed us to determine the slope exponent directly from the convexity of the channel profile. We assume that when the channels are in steady state, the ratio is 0.46 (the mean of strike-parallel basins) and we utilized the Hack exponent derived for each river basin to highlight the different behaviour in the north side. We estimate that the slope/area index for the total watershed area ranges from 0.37 to 0.9. The north side is characterized by a cluster with a mean value of 0.37. This study illustrates the potential of stream-gradient based on the area/slope index analysis as a quantitative method for extracting information on the spatial distribution of rock uplift from digital topographic data. Systematic variations in channel gradient and, particularly, concavity (nick points) can highlight regions of underlying variation in rock-uplift rate and place direct constraints on the geometry and distribution of active structures. Such analysis is subject to complications introduced by variations in lithology or by transient conditions.