Sediment–water flows in mountain streams: Recognition and classification based on field evidence

Sediment transport in mountain streams occurs through different flow types that are categorized in accordance with hydraulics and physical properties of a flowing material, and considering the dominant transport mechanism as debris flow, debris flood, hyperconcentrated flow, and water flow. Given that directly monitoring transport processes is often infeasible, a sound alternative to process recognition is to consider the morphological and sedimentological features of related deposits—attributes that chiefly depend on flow type. Accordingly, this work developed a post-flood survey protocol for distinguishing various flow types on the basis of the geomorphological and sedimentological features of deposits and their effects on vegetation. The case study selected for developing and applying the protocol is the Tegnas catchment (Dolomites, Italy), a mountain basin affected by an intense storm in October 2018. We conducted a literature review to identify diagnostic evidence for identifying different flow types and developed a survey form to ease data collection and interpretation. Field surveys were integrated with grain size analyses, measurements of the inclination angle (δ) of imbricated clasts, and estimations of organic content (OMLOI) in deposits. Field criteria allowed us to classify each channel sub-reach in accordance with the characteristics of flood deposits. The main stem of the catchment and its steep tributaries were characterized mainly by water and debris flow processes, respectively. Nevertheless, our survey also showed that debris floods occurred in several sub-reaches of the main stem and tributaries. The comparison of δ and OMLOI under different flow type deposits uncovered significant differences in δ and slight differences in OMLOI. The findings led us to conclude that a combination of field diagnostic criteria and quantitative measurements of additional parameters in a post-flood survey enables a reliable recognition of flow types.