Abstract
Abstract. Debris flows have been recognized to be linked to the amounts of material temporarily stored in torrent channels. Hence, sediment supply and storage changes from low-order channels of the Manival catchment, a small tributary valley with an active torrent system located exclusively in sedimentary rocks of the Chartreuse Massif (French Alps), were surveyed periodically for 16 months using terrestrial laser scanning (TLS) to study the coupling between sediment dynamics and torrent responses in terms of debris flow events, which occurred twice during the monitoring period. Sediment transfer in the main torrent was monitored with cross-section surveys. Sediment budgets were generated seasonally using sequential TLS data differencing and morphological extrapolations. Debris production depends strongly on rockfall occurring during the winter–early spring season, following a power law distribution for volumes of rockfall events above 0.1 m3, while hillslope sediment reworking dominates debris recharge in spring and autumn, which shows effective hillslope–channel coupling. The occurrence of both debris flow events that occurred during the monitoring was linked to recharge from previous debris pulses coming from the hillside and from bedload transfer. Headwater debris sources display an ambiguous behaviour in sediment transfer: low geomorphic activity occurred in the production zone, despite rainstorms inducing debris flows in the torrent; still, a general reactivation of sediment transport in headwater channels was observed in autumn without new debris supply, suggesting that the stored debris was not exhausted. The seasonal cycle of sediment yield seems to depend not only on debris supply and runoff (flow capacity) but also on geomorphic conditions that destabilize remnant debris stocks. This study shows that monitoring the changes within a torrent's in-channel storage and its debris supply can improve knowledge on recharge thresholds leading to debris flow.
Highlights
In steep mountain catchments, rainfall intensity and duration are insufficient to predict debris flow occurrence, even though the initiation of runoff-generated debris flows requires significant water inflow (Van Dine, 1985; Decaulne and Saemundsson, 2007; Guzzetti, 2008)
This paper presents a quantitative study of sediment recharge and channel response leading to debris flow events, using 3-D digital terrain models acquired by terrestrial laser scanning (TLS)
They depend on the local geomorphological setting, such as slope gradient, local topographic hollow, and degree of convergence (Reneau et al, 1990; Stock and Dietrich, 2006; Mao et al, 2009), these observations tend to show that long-lasting rainfall reduces the stability of the coarse surface layer that armours the gullies and scree slopes. This in turn affects the amount of debris supply from the hillside, despite the flow capacity and sediment availability. This investigation of a yearly pattern of sediment dynamics underlines the fact that the seasonal cycle of sediment discharge from the headwater supplying the Manival torrent with debris consisted of two phases of recharge: one phase in early spring, linked to enhanced debris production and runoff conditions, and a second phase in autumn, during long periods of rainfall
Summary
Rainfall intensity and duration (including snowmelt) are insufficient to predict debris flow occurrence, even though the initiation of runoff-generated debris flows requires significant water inflow (Van Dine, 1985; Decaulne and Saemundsson, 2007; Guzzetti, 2008). The frequency and magnitude of debris flow have been recognized to be linked to the amount of material temporarily stored in channel reaches (Van Steijn et al, 1996; Cannon et al, 2003; Hungr et al, 2005), such that hillside sediment delivery, recharging those channels, represents a key factor for the occurrence of debris flows
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