SLiX: A GIS Toolbox to Support Along-Stream Knickzones Detection through the Computation and Mapping of the Stream Length-Gradient (SL) Index

Abstract

The stream length-gradient (SL) index is widely used in geomorphological studies aimed at detecting knickzones, which are extensive along-stream deviations from the typical concave-up shape assumed for stream longitudinal profiles at steady-state conditions. In particular, SL was practical for identifying anomalous gradients along bedrock stream channels in mountainous catchments. This work presents the GIS toolbox SLiX designed to extract values of the SL index, starting from Digital Elevation Models (DEMs). SLiX is also suitable for the spatial analysis of the SL values, allowing for the identification of landscape portions where anomalous high values of SL occur and, consequently, those catchment sectors where stream channels show peaks in the erosional dynamic. The SLiX main outputs are (i) point shapefiles containing, among stream channels attributes, the extracted values of SL along the stream network analyzed, and (ii) SL anomaly maps in GeoTIFF format, computed through the Hotspot and Cluster Analysis (HCA), that permit the detection of the catchment sectors where the major SL anomalies occur and consequently the principal knickzones. The application of the proposed tool within an experimental catchment located in the Northern Apennines of Italy demonstrated the proper functionality and the potential of its use for different geomorphological and environmental studies. The accurate and cost-effective detection of anomalous changes in stream gradient ensured by SLiX is of great interest and can be useful for studies aimed at unravelling the Earth processes responsible of their formation (e.g., active hillslope processes, such as landslides directly interacting with the streambed, presence of geological structures, and meander cut-off). The applications of SLiX have clear implications for preliminary analyses, at a regional scale in different morpho-climatic contexts, for the hydrological management of river basins and/or to prevent geological hazards related to the fluvial erosional dynamics.

Applicability of Geomorphic Index for the Potential Slope Instability in the Three River Region, Eastern Tibetan Plateau

“…It is calculated using the following equation at the reach scale: SL = (ΔH/ΔL)∗L, where L is the stream length measured from the headwaters to the reach midpoint, and ΔH/ΔL is the local stream gradient. The SL index is typically used to determine the gradient changes along the stream’s longitudinal profile associated with tectonic uplift, rock type, and/or surface processes [ 46 , 47 , 48 ]. However, the interpretation of the traditional SL index values is limited by the subjective choice of thresholds for defining the anomalous values along the channel profile [ 13 , 14 ].…”

“…Recently, hotspot and cluster analysis of the SL index (SL-HCA) was proposed to improve the visualization of the anomalous values in the identification of tectonic structures and large landslides [ 14 ]. The SLix toolbox can be used to obtain the SL index map via the SL-HCA method [ 48 ]. Indeed, the SL anomaly zones are associated with rapid changes in the channel gradient (e.g., knickpoint) along the longitudinal profile.…”

Morphoneotectonics of the Abruzzo Periadriatic Area (Central Italy): Morphometric Analysis and Morphological Evidence of Tectonics Features

“…By applying the above relation to a river long profile, it is possible to highlight knickpoints that, in the absence of lithostructural discontinuities or natural/man-made disturbances, may be related to tectonically induced ground deformations, such as geological structures (i.e., faults) [64,137,138]. In this work, the SL index was extracted using the GIS toolbox SLiX; it works in the ArcGIS software and is designed to derive SL index' values from DEM data [65]. The SL index calculation was performed along the whole drainage network, considering a fixed stream segmentation (∆L equal to 50 m, based on the DEM resolution), and the extension of the whole study area.…”

“…Several morphometric indexes have been involved and computed to support this kind of analysis, among which the Irta, SL index, and k sn seem to be the most helpful to identify tectonically-induced drainage systems' unsteadiness/disequilibrium and tectonic deformation processes [7,[62][63][64][65][66][67][68]. In detail, this work is based on the correlation of morphometric analysis of relief and drainage networks, investigation of morphological field evidence of tectonics, and analysis of the temporal and spatial arrangement of both erosional (e.g., knickpoint) and depositional (e.g., fluvial terraces) features within the lower reaches of the main drainage basins.…”

A Normal Faults System in the Monte Nerone area and its significance in the recent seismo-tectonic setting of the Northern Umbria-Marche Apennines (Italy)

“…This crops out a few kilometers upstream. This deposit is an anomaly in the river profile (Smacchia, 2017;Piacentini et al, 2020) and is difficult to explain with the present-day conditions of the riverbed. Not being an accumulation of landslides both for the lithology and for the rounded shape of the blocks, we must assume that some sort of physical barrier blocked the transport of the sandstones boulders, a slowdown that led to a decrease in the solid flow of the river itself.…”