Abstract

Conceptual models of first-order controls governing river channel dynamics in mountain streams have been rarely tested in the field. In this Ph.D. thesis we examine the effects of hydro-meteorological forcing and sediment supply on the bedload transport dynamics of mountain streams. To this purpose we select three step-pool mountain streams that share identical granitic lithology, but exhibit contrasting sediment supply and hydro-climatic conditions. The three study sites, which are located in Trentino, Eastern Italian Alps, include the Ussaia Creek (2.3 km2) in Val di Sole, and the Grigno and Tolvà Creeks (7 km2) in Valsugana. The former is characterized by high, sand-rich sediment supply delivered by some 20 m-thick glacigenic deposits. The latter two, which flow through glacially carved bedrock terrain, are disconnected from colluvial sediment inputs so that sediment sources are limited to channel banks and bars during high flows. Mean annual precipitation is respectively 844 mm in Ussaia Creek and 1511 mm in Grigno and Tolvà Creeks. All study streams experience, to variable extents, snowmelt and rainfall-induced bedload transporting flows. To estimate quantitatively the effects and the interactions associated with sediment supply and hydro-meteorological forcing, we monitor precipitation and atmospheric temperature. Hydrological levels at instrumented sections are recorded via pressure transducers. Bedload transport is monitored by tagging and tracking 632 stones (b-axis: 30 to 131 mm; weight 88-4004 g). The tracking of these PIT-tagged tracers was conducted from December 2013 to December 2015 by means of an RFID portable pole antenna. Cumulatively, a total of 16, 11 and 19 bedload events were monitored respectively at Grigno, Tolvà and Ussaia Creek. We measured displacement lengths occurred during inter-survey periods, induced by peak flows associated to snowmelt, rainfall or a combination of the two (mixed-type). Active channel depth was evaluated via direct digging tests at the three study sites finding a median burial depth of 0.1 m at Grigno and Tolvà Creeks, and 0.25 m at Ussaia Creek; recent findings (Schneider et al., 2014), show that ordinary bedload events of boulder-bed streams in the Alps, active layer thickness is comprised within 0.01 and 0.22 m; Houbrechts et al. (2012) demonstrated that in mountain streams the active layer thickness is lower than D50. The active layer width was evaluated via orthophoto maps obtained through Structure-from-Motion. To characterize the streambed roughness and the channel slope we conducted topographic and morphologic surveys. To evaluate event-based bedload sediment volumes we applied the virtual velocity approach (Haschenburger and Church, 1998). In particular, to assess the minimum discharge able to entrain clasts, determining the virtual transport duration of each tracer weight class, we used the competence flow method. In order to evaluate the uncertainty associated with methodology that has been customarily applied in the literature, we performed a sensitivity analysis of the evaluation of bedload transfer proposing three scenarios varying the assumptions that (i) virtual velocities are normally distributed and therefore justifying the adoption of median virtual velocities instead of the average virtual velocity, (ii) that active channel width is constant in time, hence replacing bankfull width with site-specific active widths, evaluated on the base of PIT-tagged particles displacements. The monitoring year 2014 was characterized by a total annual precipitation two times larger than the historical mean, associated to a prolonged snowmelt and to heavy storm front events. By contrast, in 2015 we observed no snowmelt and no precipitation occurred in November and December. At Grigno and Tolvà Creeks, the majority of sediment is transported during autumn storm fronts (median travel distance: 30 m) and secondarily by summer convective storms (median travel distance: 4.5 m). At Ussaia Creek, snowmelt-related events induced 17% of the observed displacement lengths (median travel distance: 2 m), but the primary source of sediment transport is associated chiefly with prolonged storm fronts (median travel distance: 200 m). The mass of tracers does not affect virtual velocities, that are instead affected by seasonal distribution of hydro-meteorological events. In fact, at Grigno and Tolvà Creeks we observe a stratification of velocities according to hydro-meteorological forcing, with the largest values observed during rainfall season, commonly associated to highest values of peak discharge. Variability of virtual velocities at Ussaia Creek does not depend on seasonal hydro-climatic forcing and peak discharge values, with distributions of virtual velocities partly overlapping among snowmelt- and rainfall-related events. The seasonal pattern is translated to bedload transport volumes, with Ussaia Creek transporting by the end of the snowmelt period in 2014, three times more sediment than Grigno Creek. This is testified by a prolonged autumn rainfall that hit simultaneously the study sites, caused a debris-flow that transported 1084 m3 at Ussaia Creek, a much larger quantity compared to the 32.2 m3 evaluated at Grigno and the 62.5 m3 at Tolvà Creek. The definition of rainfall intensity-duration thresholds of precipitation events triggering bedload at our sites shows that transport-limited systems (i.e., Ussaia Creek) result sensitive to precipitation inputs characterized by low rainfall intensity (below 5 mm hr-1) and large duration. Conversely, supply-limited systems (i.e., Grigno and Tolvà Creeks) exhibit armoured beds and interlocked, resilient structures that limit entrainment processes, mobilizing bedload only in response to short-duration/high-intensity rainfall (10 mm hr-1). These channels preserve their morphological structure even under high flow events, triggering sediment transport processes limitedly to peaked storm hydrographs. In the present study we show that the variability in bedload transport among different study reaches is linked to sediment supply conditions and to peculiar hydro-climatic settings. An additional study site characterized by dry conditions, Strimm Creek (Alto Adige, Italy), allows us to obtain a latitudinal transect from dry to wet conditions across the Eastern Italian Alps. By monitoring tracer displacements from 2011 to 2015 at this formerly-glaciated, high-elevation mountain basin, we observe that limited sediment-supply conditions exert a strong control on bedload, chiefly triggered by snowmelt events that account for 73% of the overall travel distances. At Grigno and Tolvà Creeks, also characterized by limited sediment supply, transport is dominated by rainfall events, responsible of driving 95% of the overall travel distances. Sediment availability at Ussaia Creek is responsible for triggering the largest observed bedload events, associated to prolonged autumn precipitation and secondarily to snowmelt events.