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 m
Inset: map of the study area; the Manival catchment is in solid red and the impressive debris fan is hatched. Main: aerial view of the Manival catchment, draped over a topographic model; sediment supply is concentrated in the headwater (production zone) as erosion activity from the middle and lower catchment is not connected to the torrent (zone of transfer) (image: Aerodata International Surveys; DEM: Irstea UR ETNA).
In steep mountain catchments, 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). In many cases, the properties of the channel reach which determine the amount of debris that can be entrained can be often more important than the mechanisms of initiation induced by the hydrological or meteorological conditions prior to the event (Hungr, 2011; Theule et al., 2015). 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 (e.g. Benda and Dunne, 1997; Bovis and Jakob, 1999; Berti et al., 2000). This implies efficient hillslope–channel coupling (Hooke, 2003; Schlunegger et al., 2009; Johnson et al., 2010). Therefore, the rate of sediment supply needs to be considered for predicting debris flow hazards (Rickenmann, 1999; Jakob et al., 2005). However, the difficulty results in quantifying sediment processes and rates and volumes from hillslopes and in-channel debris storage (Peiry, 1990; Zimmermann et al., 1997).
The quantification of the overall sediment production and transfer rate has increasingly relied upon multi-temporal digital stereophotogrammetry (Coe et al., 1993; Chandler and Brunsden, 1995; Veyrat-Chavillon and Memier, 2006) and elevation difference from high-resolution digital elevation models (HRDEMs) (Smith et al., 2000; Wu and Cheng, 2005; Roering et al., 2009; Theule et al., 2012). In terrain dominated by steep slopes, traditional aerial-derived digital elevation models (DEMs) are typically inappropriate to study geomorphic processes. Limitations include the poor rendering of small topographic changes (Perroy et al., 2010), the poor representation of steep terrain with small curvature radii and data gaps in vertically oriented and overhanging topography. Even on gentler gradients, the sharp breaks in slope, encountered in erosion scars for instance, are often insufficiently modelled by airborne HRDEMs, leading to erroneous volume estimations (Bremer and Sass, 2011). This represents a serious drawback in estimating the sediment budget of steep terrain, where sediment activity comes mostly from rock walls and rugged gullies. Because of these issues, many hillslope and rock slope process studies have used terrestrial laser scanner (TLS) data to build the topographic model (Jaboyedoff et al., 2012). The recent development of long-range TLS devices provides an effective means of acquiring high-resolution topographic information that can adequately reflect the morphology of steep bedrock-dominated areas. The practical disadvantages in data acquisition inevitably related to ground surveys can be compensated for by flexibility in transport, ensuring a full coverage with minimal zones of topographic shadowing.
Geological map of the catchment headwater (production zone) after Gidon (1991) and location of first-order debris flow channels (thick blue line) and their respective watersheds (white lines). For the ease of analysis, the Roche Ravine and Col du Baure subcatchments in the east side were further subdivided according to their gully complex (dotted white lines).
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 TLS. This is illustrated on the Manival (French Alps), a torrent that experiences runoff-generated debris flow almost every year (Péteuil et al., 2008). The surveys captured hillslope processes and sediment dynamics occurring throughout the system including the tributary channels down to the main torrent and were performed periodically over 16 months. The spatio-temporal variability of debris production and subsequent transport and storage of sediment are analysed on a seasonal timescale, in order to discuss the debris supply dynamics and the implications in debris flow initiation. This study also complements a parallel investigation regarding the controls on debris flow erosion and bedload transport in the Manival torrent (Theule et al., 2015).
Maximum rainfall intensity over the monitoring period measured by a rain gauge located at the top of the torrent (see Fig. 4) and calculated for a 5 min time interval. The mean annual precipitation is about 1500 mm in the headwater of the Manival (modified from Loye, 2013).
Geomorphic process map (contour interval: 20 m) illustrating the spatial pattern of sediment sources and transfer in the first-order channel complex. Note the impressive rock collapse deposits now crossed by four first-order debris channels. Their bed incision is strongly constrained by a series of check dams (marked as black “T” on the map), but erosion scars all along the deposit suggest that the reaches are still subject to lateral erosion.
The 3.9 km
The contemporary geomorphic activity contributing to the torrent's recharge with debris is concentrated exclusively in the headwater, where no remnant glacial deposits are found (Gruffaz, 1997). In the upper catchment, large old rock deposits flooring the west side hillslope (Fig. 4) have dramatically influenced the bottom topography, and thus the channel network, resulting in a conjunction of four first-order debris flow channels deeply incised down to the bedrock in several reaches. The upper catchment can therefore be subdivided into five subcatchments in terms of sediment recharge (Fig. 2). Bed entrenchment is now constrained by check dams. However, lateral erosion still occurs episodically by flooding and debris flow scouring.
The style of sediment production and delivery is somewhat different throughout the headwater, according to the local morphology and the lithologic and structural setting. The major geomorphic processes, identified preliminarily by observations from aerial photographs and field investigations, were initially characterized in a map (Fig. 4) describing the spatial distribution of geomorphic features and sediment transfer processes contributing to debris recharge in the first-order channels. The west and upper sides are dominated by rockfall. Large rock collapses delimited by persistent joints occur due to the progressive degradation of the slope underneath (Loye et al., 2011). Where the slope gradient allows scree and soil development, erosion scars can be observed; sediment sources are remobilized from discrete shallow landslides. Depending on the location and size, rockfall can reach the channels directly or accumulate on slopes or in ravines, before being subsequently routed to high-order segments by a combination of gravitational and hydrological processes. Towards the east, the erosion seems to be more progressive through the formation of gullies (Loye et al., 2012). Near the ridge, the slopes display mostly talus and scree deposits lightly covered with vegetation, whereas the hillside below exposes steepened rock slopes. Many active erosion scars can be observed. They contribute debris into gullies and talus slope deposits that are subsequently entrained in channels downslope.
Historical records of debris flows since the 18th century show a frequency
of 0.3 events per year that reached the apex of the fan (Brochot et al.,
2000). The largest event deposited approximately 60 000 m
Dates of TLS acquisitions. Note that for the analysis, the second survey was merged with the first one (see text for details).
TLS data and surface coverage characteristics of the five subcatchments from the first monitoring period (MP1). As the view points and parameters of acquiring remained similar, the values are essentially the same for all surveys.
The terrain was surveyed with an ILRIS-3D laser scanner (Optech Inc.). This
device provides a range of up to 1.2 km for 80 % reflectivity surface, and the
instrumental precision is about 7 mm/100 m range for both distance and
position (Optech Inc.). The overall coverage of the upper catchment with TLS
point clouds required 50 scans using a 20 % surface overlap. These scans
were collected over a 5-day period from nine individual viewpoints to ensure a
full 3-D rendering of the topography. Particular attention was given to
irregular regions and major breaks in slope, such as rock couloirs and
deep-cut gullies. Using multiple scanning locations allowed us to limit
shadow zones and increase the point cloud density of the scanned area. A
series of four surveys was performed for each season during 2009, and one extra
survey was performed in July 2010 to analyse the effect of the preceding
winter period (Table 1). The monitoring setup remained similar for all
surveys. Post-processing of the TLS raw data was done using Polyworks
(InnovMetric). Erroneous points and vegetation were filtered manually,
ensuring a total control of the removed data to preserve a high density of
points in topographic features with small radii curvature. Although this
procedure is time consuming, box (semi-)automatic approaches to filter
vegetation accurately still remain in a stage of development for dissected
mountain morphology (Brodu and Lague, 2012). Each of the multiple scans of a
survey was merged with another one using common tie points of permanent
topographic features and the dataset was processed as 12 standalone
sub-datasets, rather than all processed together. Given the size of the
monitored area, dividing the point cloud into smaller datasets avoids the propagation of inaccuracy through large co-registered scan series. ICP
(iterative closest point) algorithms (Besl and McKay, 1992), which minimize
the distance between two point clouds, were used to determine the best
alignment of subsets surveyed at different times in order to obtain the best
co-registration within a time series. The same procedure was applied between
subset point clouds and a point cloud derived from a commercial airborne laser scanner (mean density: 6.9 pts m
The active geomorphic features within two successive datasets were
identified on a point-by-point basis using the short-distance neighbouring
point search algorithm (Bitelli et al., 2004) that computes, in 3-D, the
shortest difference vectors between the points of two datasets. The vector
sign indicates the net change direction of topography, i.e. surface of
erosion or deposition. A set of points (cluster) was considered active if
at least eight adjacent points of similar sign displayed an absolute difference
above the limit of detection (LoD, see Sect. 3.4). Each active feature was
outlined visually using the point cloud of difference (Fig. 5a). The point
clusters of both survey datasets, which correspond to the topography of the
active features, were extracted according to their spatial extend
coordinates and each detected geomorphic feature was labelled as follows:
rock slope erosion, characterized by rockfall or rockslides; hillslope erosion, specifically the reworking of loose or compacted debris
on slope, in gullies, and in channels; deposition, including material aggradation initiated by both rock slope
failure (new production) and remobilization of debris.
Using the images captured by the TLS integrated camera, clusters of points not corresponding to geomorphic process activity, such as snowmelt, were ignored.
3-D detection
Distribution of the distance between two survey point clouds after the process of georeferencing using the ICP procedure. The distance approaches normal distribution with a zero mean, showing that errors generated by multiple scan registration and point cloud survey georeferencing are Gaussian, random, and independent. Data are given in metres.
Registration and georeferencing standard deviations (in centimetres) of the position uncertainty on a point by point basis that was used to derive the LoD at 95 % confidence interval and subsequently to detect topographic changes down to a certain minimum volume of geomorphic features.
As the volume of active features cannot be directly computed by differencing
TLS point datasets, the active features of two successive point clouds must
be interpolated into continuous surfaces (DEM). Gridded model (or raster) is
regarded as being the most effective type of model to use for irregularly
distributed datasets, which sometimes contain few or no points (El-Sheimy et
al., 2005), as can be the case for rockfall and erosion scars. The algorithm
chosen for the interpolation of the DEM has little influence on the final
result, as TLS data provide an extremely dense coverage of the detected
objects (Anderson et al., 2005). Therefore, they were interpolated using linear
inverse distance weighting (Burrough and McDonnell, 1998) and generated in a
regular grid separately. The grid spacing and direction of interpolation were
designed in a specific way for each feature: the coordinate system of
reference was replaced by a local orthogonal system where the
A reliable identification of erosion and deposition features requires the
definition of a LoD, where the change in elevation between successive point
clouds can be considered real as opposed to noise. Each TLS data point theoretically has a unique precision depending on the range and laser
incidence angle (Buckley et al., 2008). In practice, the individual point
precision of a scan can be assumed to model a surface with a global uniform
uncertainty, considering the very high point density (Abellàn et al., 2009).
Given the homogeneity of surface error and considering that the distance
between sequential points at a position (
In the case of volume computation, information on elevation uncertainty
associated with each point cloud survey needs to be extended to the DEM on a
cell-by-cell basis. For any grid cell (
Monitoring of the coarse-sediment transfer has been performed all along the main torrent channel to the sediment trap located downstream on the alluvial fan. The in-channel storage change was established after every noticeable flow event, using the morphological approach based on cross-section survey techniques (Ashore and Church, 1998), and the volume of sediment deposited in the sediment trap was measured by TLS survey differencing. Sequential volumes of recharge enable us to study the influence of debris supply from the production zone through the seasons. The characteristics and observational analysis of this event-based monitoring were documented in detail in Theule et al. (2012, 2015) and are therefore not described any further.
A rate of debris production for the study period is obtained from the total
volume of rock slope erosion. An objective estimation can be deduced by
characterizing the cumulative distribution of rockfall volumes with a power
law as follows (Gardner, 1970):
The topographic changes recorded from July to August 2009 did not show any relevant geomorphic activity (only a few small rockfalls). These results were therefore merged with the preceding monitoring period.
Rock slope activity is dominated by individual small rockfalls distributed
throughout the upper catchment. Only few events exceed 1 m
Geomorphic activity revealed by comparing the topographic differences of the two successive TLS surveys operated in April and August 2009. The sediment budgets for each subcatchment are detailed in Fig. 13.
Geomorphic activity revealed by comparing the topographic differences of the two successive TLS surveys operated in August and November 2009. The sediment budgets for each subcatchment are detailed in Fig. 14.
Geomorphic activity revealed by comparing the topographic differences of the two successive TLS surveys operated in November 2009 and July 2010. The sediment budgets for each subcatchment are detailed in Fig. 15.
Rock slope activity remains similar in spatial extent and volumes to the
previous survey period, but rockfall frequency is higher (Fig. 8). Hillslope
process activity was more widespread on the east side but more localized on
the western valley walls, while the rock couloirs showed no geomorphic
activity. In the upper headwater, material reworking was concentrated almost
exclusively in the steep tributary gullies. They displayed scouring of a
relatively large volume (
Cumulative volume distribution of the rockfall observed
during the first
This period showed an important increase in rock slope erosion, both in
frequency and magnitude, resulting from the occurrence of large slope
failures and enhanced localized rockfall activity, for instance in rock
walls made of calcareous marl situated directly above the Manival (2035 m
The upper channel reaches were clearly depositional, as a consequence of
large slope failures. The Manival channel showed a continuous zone of
remnant accumulation of 948 m
Sediment budget (in cubic metres) of the Manival torrent established after noticeable events using the morphological approach after Theule et al. (2012). The torrent recharge (sediment input) is estimated from in-storage changes in channels and volumes deposited in the sediment trap (output).
Over the 16 months, 1866 rockfalls with volumes ranging from 10
Torrent in-channel storage changes per unit length and sediment budgets of cumulative volumes transported in the torrent from the headwater outlet to the sediment trap downstream for each monitoring period (MP). The torrent recharge (sediment input) was estimated given the in-storage change and the volume deposited in the sediment trap (see Table 4 for details on values) (modified from Theule et al., 2012).
Overall sediment budget
Overall headwater sediment budget recorded during the three survey periods and net sediment balance of the 16 months of monitoring. Sediment budgets for each catchment subsystem are detailed in the Supplement.
Overall headwater sediment budget observed during the first monitoring period revealing the sediment dynamics through the spring–summer season and the net balance of sediment recharge in the downstream torrent for several months preceding the August 2009 debris flow.
Overall headwater sediment budget observed during the second monitoring period revealing the sediment dynamics and the net balance of sediment recharge in the downstream torrent during the autumn.
Overall headwater sediment budget observed during the third monitoring period revealing the sediment dynamics through the winter–spring and the net balance of sediment recharge in the downstream torrent for the period preceding the June 2010 debris flow.
Continuous lines: erosion rate as function of size of
events for a certain volume of production (potential maximum volume
V
Rock slope debris production rate estimated from the inventory analysis using power law distribution of volume for potential rockfall (Fig. 10).
Two debris flows with multiple surges and several remarkable bedload
transport events were observed in the main torrent during the survey period
(Theule et al., 2012). A debris flow occurred on the 25 August 2009, caused
by a short-duration rainstorm. The volume of sediment eroded in the torrent
(5232 m
The overall transfer dynamics, from debris source zone to the apex of the
fan, are illustrated in Fig. 12. The volumes detected during the 16-month
study period reveal a net export of 3378 m
In the spring–midsummer period, the hillside sediment budget yields a total
rock slope production of 99 m
During the late summer–autumn season, the total volume of hillside erosion
is
During winter–spring 2010, a total deposition volume of
Debris production from rock walls shows a strong seasonal pattern. The great majority of recorded rock instabilities in both magnitude (95 %) and frequency (75 %) occurred during the cold period. Previous studies of the calcareous cliffs near Grenoble, which have a similar morphotectonic context, revealed that freeze–thaw cycles are the main triggering factor of rockfall (Frayssines and Hantz, 2006). Ice jacking can cause microcrack propagation, leading to failure (Matsuoka and Sakai, 1999). Along the eastern ridge, the bedrock surface is often highly fractured, suggesting frost shattering. The spatial pattern of rockfall also strongly suggests a tectonic-lithological influence that can be explained by differential erosion between the successive limestone and marl beds. In the rock wall series on the west side, the monoclinal configuration of the bedding, combined with a strong difference of competency between stratigraphic sequences, gives rise to an overhanging formation highly susceptible to failure. On the east side, the bedding is mostly cataclinal and approaches dip slope, depending on the slope. Rock failures initiated by planar sliding on bedding planes were observed.
The observed debris production follows a power law distribution in a range
covering at least 3 orders of magnitude [10
Previous sediment budgets derived from topographic measurement using
stereophotogrammetry estimated the highest erosion rates over an average of
40 years to range from 10.8 to 17.8 mm yr
Upstream from the Manival channel, the scouring of debris slopes and scree hollows triggered by rock slope production accounted for about 40 % of the net erosion recorded during the autumn period and 25 % in the Baure Ravine over the entire study period. The spatial pattern of geomorphic work showed that hillslope process activity was observed principally in gullies and scree slopes situated directly below active rock walls. The dominant mode of debris supply in the Manival headwater is therefore highly episodic, implying a great spatial heterogeneity in sediment recharge rates.
As rock slope activity was very limited from spring to autumn, hillslope geomorphic activity dominated sediment recharge during this period. Until the end of August, hillside gullies and low-order channels remain almost inactive in terms of sediment delivery. Conversely, the autumn period was characterized by a general increase in the intensity of geomorphic activity. Continuous scouring and the relative paucity of deposition features from hillside gullies as well as clear incisions and micro debris flows in channel reaches indicate that mobilized material was almost entirely entrained downstream by runoff. For the entire area, the hillside contribution represents on average a volume 5 times larger than the volume that was observed in spring and summer, and channel bed reworking was of a much larger magnitude as well.
During winter–spring 2010, the total volume of deposition recorded on the hillside significantly exceeds the rate of deposition recorded so far, resulting from the huge increase in debris production that can be attributed to the winter according to observations carried out in the preceding spring. Hillslope and gully erosion remain on average comparable to the volumetric transfer of sediment observed in the preceding autumn, implying a clear connectivity.
These negative sediment balances in all sediment cascade components suggest a very high degree of connectivity between hillside and channels in autumn, and hillside fan deposits observed in early spring along low-order channel banks reflect an effective hillslope–channel coupling. This differs from effective sediment transfer occurring mostly during the summer (e.g. Berger et al., 2011; Cavalli et al., 2013).
The sediment input, back-calculated from the in-torrent storage changes, is
consistent with the net sediment output recorded from the headwater for the
first two survey periods. In the torrent, the morphological monitoring that
started in July revealed almost no sediment recharge (< 70 m
The Manival headwater experienced low geomorphic activity through the summer, and consequently low sediment recharge of the torrent, even though rainstorms were of sufficiently high intensity to trigger debris flows of significant magnitude in torrent. Considerations of the temporal pattern of sediment transfer and the analysis of erosion features, like alternating areas of scouring and infilling in gullies, suggest that runoff still has an important role in the headwater sediment dynamics. A clear relation between sediment transfer magnitude and precipitation remains complex, however (Fig. 3), as is often the case in mountainous catchments (Van Steijn, 1996; Bovis and Jakob, 1999; Pelfini and Santilli, 2008). The enhanced geomorphic activity observed in the hillside of several headwater subsystems, for instance during the autumn period, induced a simultaneous yet highly heterogeneous response in their channel reaches. A significant increase in bed incision and reworking similar to debris flow was observed in the upper reaches of the Manival subcatchment, implying an important sediment transfer. In contrast, the activity of other channel reaches was reduced by half, e.g. in Roche Ravine, or even remained geomorphically much less active, with only little sediment recharge.
Considering that meteorological conditions were similar, this opposite behaviour may only be explained by a certain depletion of debris availability. This reduction in sediment yield can come not only within a supply-limited regime of the contributing area (Jakob et al., 2005; Glade, 2005) but also from the fact that check dams, like bedrock-dominated reaches, inhibit channel bed incision. Hence, the sediment storage has to be refilled either from the contributing hillside or from the upstream mass movement. A similar observation can be drawn from the Grosse Pierre Ravine sediment budget, whose gully downslope remained completely disconnected from the head of the subcatchment over the entire study period. Although this ravine is very steep and incises the large old rock deposits, no geomorphic work was observed, resulting most likely from the absence of debris supply from upstream. Hillside sediment delivery seems therefore to be clearly a limiting factor to sediment yield from low- to high-order channels and thus to the sediment recharge rate of the debris flow torrent downstream. As the occurrence of bedload transport and micro debris flows is controlled predominantly by the availability of sediment, even very intense rainstorm-derived runoff does not automatically lead to a significant transfer of sediment from the hillside to low-order channels in the case of material depletion.
Nevertheless, this behaviour is somehow equivocal, considering the fact that the transport capacity of ephemeral stream runoff and sheetwash related to high-intensity rainstorms is larger than the one generated by low-intensity long-duration rainfall, above all, when gully material (like in Manival) can be characterized as coarse and poorly sorted rockfall-fragment-derived debris. Lenzi et al. (2003) interpreted the annual fluctuation in sediment yield as the effect of sediment source destabilization or reactivation following a high-magnitude flow event, which facilitates material entrainment by subsequent runoff. Johnson and Warburton (2006) refer to the influence of sediment source characteristics in the control of hillslope sediment discharge. The explanation may be that the 25 August rainstorm dramatically altered the debris sources in a way that the autumn rainfalls – which, although they were of lower intensity, had a longer flood time – were able to transfer sediment downslope. Excess pore-fluid pressure in debris deposits can persist for days to weeks after sediment emplacement (Major and Iverson, 1999; Major, 2000), making debris deposits geotechnically less stable.
Although 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. Furthermore, the occurrence of the debris flow events was conditional on a net sediment delivery toward the torrent.
Overall, the torrent effectiveness seems to be controlled early in the year, from winter to spring, by sediment production and later in the year by the ability of hydrological effects to weaken the remnant debris sources, with debris availability being only one of the limiting factors at the Manival torrent. The rate of sediment delivery, directly recharging both hillside and low-order channels, is controlled by high-magnitude slope failure of moderate frequency which occurred mostly during winter time. Consequently, material re-entrainment concentrates locally in specific tributary gullies. The delivery of sediment to the torrent may be related to the hydrometeorological conditions since the last rainstorm rather than to flow capacity directly. Low-order reaches contribute significantly to the sediment delivery mechanism of the catchment headwater by controlling storage and routing processes. Hence, the recharge threshold required for a new debris flow to occur at the Manival depends primarily on the short-term debris supply, partly derived from the rate of rock slope sediment production and partly derived from mobilizing debris on the hillside. The rate of sediment recharge in the torrent is, however, greatly intermittent, since production and entrainment are both highly stochastic processes. This regime of headwater sediment delivery may have been identified in other nearby mountain environments, but very little literature exists (Alvarez and Garcia Ruiz, 2000; Veyrat-Charvillon, 2005; Berger et al., 2011) that has explored the timescale of sediment discharge in sufficient detail, e.g. on a seasonal basis.
Debris flow magnitudes have so far been mostly determined based on
volume estimates derived from past events, reducing the susceptibility
analysis to the known history. Monitoring of the in-storage changes within
the torrent linked to the debris supply can help to improve knowledge on the
recharge threshold leading to debris flow activity and therefore on their
prediction. According to the rock slope production observed in this study,
10 000 m
The authors would like to thank their colleagues at IGAR and IRSTEA Grenoble (ex. CEMAGREF), in particular A. Pedrazzini and M.-H. Derron, for their valuable comments during the preparation of this publication. This study was entirely supported by the University of Lausanne, except for the event-based cross-section surveys that were funded by the Pôle Grenoblois d'étude et de recherche pour la prévention des risques naturels. The ONF-RTM38 is acknowledged for making the access to the upper Manival catchment easier. This publication benefited from an interactive discussion with O. Sass and two other anonymous reviewers and from proofreading by S. Conway. Edited by: S. Conway