Assessing hotspots of sediment sources and related sediment dynamics through the integration of geomorphological data, sediment connectivity and sediment transport modelling – The HOTSED model

Sediment connectivity is a pivotal concept in geomorphological studies aimed at assessing watershed sediment dynamics. It is expressed through the spatial arrangement and physical linkages of system components (Structural Connectivity, SC) and the actual transfer of water and sediments facilitated by dynamic processes (Functional Connectivity, FC). However, a limited number of studies have simultaneously assessed SC and FC. Moreover, traditionally sediment connectivity studies primarily rely on comparing independent results from GIS modelling, field-based assessments, and mapping. Thus, it remains a common practice to treat geomorphic processes and connectivity as separate variables, often without joining them into an integrated modelling approach. Accordingly, this research aims to introduce a novel methodology that integrates geomorphological data derived from a detailed mapping approach with SC and FC. In particular, we developed a new GIS-based integrated model named HOTSED, designed to assess potential hotspots of sediment sources and related sediment dynamics at the watershed scale. We tested our approach in a geomorphologically highly active Mediterranean watershed in the Northern Apennines (Italy), starting with the elaboration of an Inventory Map (IM) of sediment sources through fieldwork, photointerpretation, terrain analysis, and digital mapping. Furthermore, we used IM-derived data to estimate the geomorphic Potential of Sediment Sources (PSS) adopting a relative scoring system. Moreover, we computed Structural Sediment Connectivity (STC) and the Potential for Sediment Transport (PST) by combining terrain and hydrological parameters, land use data, and rainfall erosivity. Subsequently, the integration of PSS, STC, and PST was achieved through a raster-based calculation method, yielding the HOTSED model. The application of the model in the study area provided a single and intuitive output depicting the location of hotspots of sediment sources. It allowed the derivation of “relative hazard” classes for sediment production and delivery using the fluvial system as target feature. The results show that HOTSED successfully highlighted hotspots associated with active complex and polygenetic geomorphic systems located in areas close to the main channels, as well as linear hotspots corresponding to tributary drainages acting as stream corridor sources. Furthermore, it successfully identified areas prone to store sediments in depositional landforms with low hazard, considering both low geomorphic potential and sediment connectivity. Thus, this study proves that our conceptual model is particularly effective in geomorphologically complex areas such as the Northern Apennines.