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Italian Journal of Engineering Geology and Environment - Book www.ijege.uniroma1.it © 2011 Casa Editrice Università La Sapienza
779
DOI: 10.4408/IJEGE.2011-03.B-085
SEDIMENT BUDGET MONITORING OF A DEBRIS-FLOW
TORRENT (FRENCH PREALPS)
J
osHua
THEULE
(*)
, f
RedeRiC
LIEBAULT
(*)
, a
lexandRe
LOYE
(**)
,
d
ominiQue
LAIGLE
(*)
& m
iCHel
JABOYEDOFF
(**)
(*)
Unité de Recherche Erosion Torrentielle Neige et Avalanches (ETNA), Cemagref Grenoble, 2 rue de la Papeterie, BP76,
38402 Saint-Martin-d’Hères, France - Email: joshua.theule@cemagref.fr
(**)
Institute of Geomatics and Risk Analysis, University of Lausanne, UNIL-Sorge, Amphipôle, CH-1015 Lausanne, Switzerland
K
ey
words
: debris-flow, bedload transport, sediment budget,
terrestrial LiDAR, Manival, French Prealps
INTRODUCTION
Volumes of debris-flows are highly influenced by
channel scouring making sediment availability in tal-
wegs a critical factor for predicting debris-flow magni-
tude (H
unGR
et alii, 1984; m
aRCHi
& d'a
Gostino
, 2004;
H
unGR
et alii, 2005; R
emaitRe
et alii, 2005; C
oe
et alii,
2008). Therefore, the recharge rate, defined as the rate
at which sediments accumulate in the channel during
time intervals between debris-flows, controls the fre-
quency of debrisflows and has been used to differentiate
supply and transport-limited debris-flow basins (b
ovis
& J
akob
, 1999; J
akob
et alii, 2005). Although erosion
and deposition in steep channels have been recognized
as key factors for understanding debris-flow dynamics,
few studies have tried to measure it in the field. Another
problem is the coexistence of various sediment trans-
port mechanisms in torrent channels. These are induced
by both bedload transport and debrisflow and there is
a paucity of work trying to compare the geomorphic
impacts of both (m
ao
et alii, 2009).
Catchment-scale sediment budgeting can be used
to identify main sediment sources, to evaluate the
respective contribution of these sources in the sedi-
ment yield of the catchment and to better understand
sediment transfers in complex geomorphic systems
(d
ietRiCH
& d
unne
, 1978; J
oHnson
& w
aRbuRton
,
2002; G
omez
et alii, 2003; s
CHueRCH
et alii, 2006).
ABSTRACT
The Manival near Grenoble (French Prealps) is a
very active debris-flow torrent equipped with a large
sediment trap (25 000 m
3
) protecting an urbanized al-
luvial fan from debris-flows. We began monitoring the
sediment budget of the catchment controlled by the
trap in Spring 2009. Terrestrial laser scanner is used
for monitoring topographic changes in a small gully,
the main channel, and the sediment trap. In the main
channel, 39 cross-sections are surveyed after every
event. Three periods of intense geomorphic activ-
ity are documented here. The first was induced by a
convective storm in August 2009 which triggered a
debris-flow that deposited ~1,800 m
3
of sediment in
the trap. The debris-flow originated in the upper reach
of the main channel and our observations showed that
sediment outputs were entirely supplied by channel
scouring. Hillslope debris-flows were initiated on
talus slopes, as revealed by terrestrial LiDAR resur-
veys; however they were disconnected to the main
channel. The second and third periods of geomorphic
activity were induced by long duration and low inten-
sity rainfall events in September and October 2009
which generate small flow events with intense bed-
load transport. These events contribute to recharge
the debris-flow channel with sediments by depositing
important gravel dunes propagating from headwaters.
The total recharge in the torrent subsequent to bedload
transport events was estimated at 34% of the sediment
erosion induced by the August debris-flow.
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J. THEULE, F. LIEBAULT, A. LOYE, D. LAIGLE & M. JABOYEDOFF
780
5th International Conference on Debris-Flow Hazards Mitigation: Mechanics, Prediction and Assessment Padua, Italy - 14-17 June 2011
which are mobilized by shallow landslides, hillslope
debris-flows and snow avalanches. Limestone rock
faces are prone to active rockfalls which supplies de-
bris to talus slopes. During the snowmelt season, gul-
lies located under rock faces can experience one rock-
fall every 5 to 10 minutes (according to the authors’
field experiences). Debris-flows and colluvial slope
failures are often initiated in the talus slopes influenced
by a “firehose effect” (G
odt
& C
oe
, 2007).
Archive analysis of the Manival flood history
during the last two centuries showed that the torrent
can produce large debris-flows ranging from 10 000
to 60 000 m
3
(P
eteuil
et alii, 2008). Since 2008, the
Manival has produced one debris-flow each year.
METHODS
In Spring 2009, we started to monitor the sedi-
ment budget of the catchment above the sediment trap
by implementing a detailed topographic survey of the
channels and hillslopes. Two different techniques are
used: cross-section resurveys along the main chan-
nel and terrestrial LiDAR resurveys of representative
hillslopes and gullies (Tab. 1). The sediment trap is
used to characterize sediment outputs by measuring
sediment deposition from terrestrial laser scanning.
The monitoring of channel morphology with cross-
sections resurveys have shown to be very effective for
assessing bedload transport in gravel-bed rivers (m
aR
-
tin
& C
HuRCH
, 1995; R
eid
et alii, 2007). This mor-
phological approach has rarely been applied on steep
headwaters for characterizing the volume of debris-
flows (v
eyRatCHaRvillon
& m
emieR
, 2006). With the
recent development of airborne and terrestrial LiDAR,
it is now possible to implement topographic surveys
with very high resolution and frequency. This technol-
ogy has been increasingly used for capturing morpho-
logical changes in rugged terrain (m
ilan
et alii, 2007;
s
CHeidl
et alii, 2008; C
onway
et alii, 2010).
This paper presents results from a sediment budg-
et monitoring program of a very active debris-flow
torrent located near Grenoble (Manival Torrent) com-
bining cross-section and terrestrial LiDAR resurveys.
The reconstruction of event-based sediment budgets
by integrating monitoring techniques gave interesting
observations of erosion and deposition patterns along
the torrent channel and allowed us to compare mor-
phological and sedimentary responses associated with
debris-flow and bedload transport processes.
STUDY AREA
The Manival is a very active debris-flow torrent
located near Grenoble in the Northern French Prealps
(Fig. 1). The catchment is located in the Chartreuse
Mountains where the torrent flows into the Isère
River. The close proximity to Grenoble, easy access
throughout the main channel and presence of a large
sediment trap (25 000 m
3
) in the channel to protect the
urbanized fan against debris-flows make the Manival
a practical site for implementing a monitoring pro-
gram of sediment transfer in steep slope torrents.
Above the sediment trap, the mean channel slope
is 16% with a drainage area of 3.6 km
2
for a total re-
lief of 1 200 m. Approximately 180 check-dams con-
structed since the 1890s throughout the main channel
and small gullies are managed by the French Forest
and Torrent-Control Service (ONF-RTM service).
The geology of the catchment is typical of the
sedimentary prealpine ranges. Bedrock is composed
of highly fractured, alternating sequences of Jurassic
marls and limestones. A long reverse fault runs through
the axis of the catchment with secondary faults found
regularly on the head and east side of the catchment.
The bedrock is covered by thick colluvial deposits
Fig. 1 - View of the Manival Torrent channel and production
zone of the upper catchment (photo: J
oShuA
t
heule
)
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SEDIMENT BUDGET MONITORING OF A DEBRIS-FLOW TORRENT (FRENCH PREALPS)
Italian Journal of Engineering Geology and Environment - Book www.ijege.uniroma1.it © 2011 Casa Editrice Università La Sapienza
781
sediment trap and the sediment balance within the
channel, the sediment supply from the production
zone can be determined.
Uncertainties of volume estimates for each sub-
reach between two cross-sections,
σ
δV
, were calculat-
ed according to the propagation of uncertainty’s law
of Taylor (R
eid
et alii, 2007):
The rainfall has been measured since October 2008
using a tipping bucket rain gauge installed in the up-
per basin (resolution of 0,14 mm).
CROSS-SECTION SURVEYING
Thirty-nine monumented cross-sections were de-
ployed along the main channel between the sediment
trap and the confluence of the two largest gullies of
the production zone (Fig. 2). Cross-sections are sur-
veyed before and after flow events with a total station
(Leica FlexLine TS02, electronic distance measure-
ment precision of 1,5 mm +2ppm, angular resolution
of 7’’ or 0,34 cm of precision at a distance of 100 m)
for quantifying erosion and deposition volumes in the
channel and back-calculating sediment transport using
the morphological method (m
aRtin
& C
HuRCH
, 1995;
R
eid
et alii, 2007; R
aven
et alii, 2009).
Volumes of deposition V
D
and erosion V
E
between
crosssections are obtained by the following:
The estimated volumes of V
D
and V
E
cover the
length L between the two cross-sections n and n+1
with their mean cross-sectional area of erosion A
E
and deposition A
D
. The sediment balance δ
V
for the
channel reach between two cross-sections is deter-
mined by the difference of the two volumes V
D
and
V
E
. The principle of mass conservation is used to de-
termine the coarse sediment transport for each reach
with following equation:
V
out
= V
in
- δV
with V
out
the sediment output and V
in
the sediment in-
put. Through monitoring sediment outputs with the
Tab. 1 - Field measurements
performed between
events are listed
with their dates and
locations
Fig. 2 - Shaded relief map of the Manival catchment de-
rived from airborne LiDAR survey; main features
of the monitoring program are indicated in the map
(1)
(2)
(3)
(4)
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J. THEULE, F. LIEBAULT, A. LOYE, D. LAIGLE & M. JABOYEDOFF
782
5th International Conference on Debris-Flow Hazards Mitigation: Mechanics, Prediction and Assessment Padua, Italy - 14-17 June 2011
from raw LiDAR data for the active channel by cleaning
manually the sparse vegetation cover. The digital terrain
model was created with a linear drift kriging for smooth-
ing high density linear swaths of points in the airborne
scans which is required for using a 0,1 m grid resolution.
RESULTS
Three periods of intense geomorphic activity in the
channel occurred in 2009. The first was induced by a
high intensity convective storm in August which initi-
ates a debris-flow. The two others were associated with
long duration and low intensity rainfalls which generate
bedload transport flow events in September and October.
AUGUST 2009 DEBRIS-FLOw
The storm causing the debris-flow had a maximum
rainfall intensity of 48,9 mm h
-1
(Fig. 3a). The differ-
ence of the two LiDAR-derived DEM before and after
The terms
σ
d
and
σ
z
s refers respectively to er-
rors associated with distances and elevations of cross-
sections points. We attributed to both a value of 5 cm,
corresponding to the D
84
of the bed surface grain size
distribution, representing the roughness of the chan-
nel. The term
σ
L
, defined as the error in distance be-
tween two crosssections, is dependant on the interpre-
tation of flow path distances, and has been calculated
as the mean variation of determined distances by two
experts for all reaches equaling 97 cm.
LASER SCANNING
Laser scanning is a more precise technique for
quantifying erosion and deposition and is ideal for
surveying inaccessible slopes. The Manival catch-
ment provides ideal view points for maximum cover-
age limiting shadows for terrestrial scans. However,
there are still incised areas that or not feasible for
scanning as well as areas of very dense vegetation
providing limited ground points.
A terrestrial laser scanner (Optech Inc.) was
used for resurveying the sediment trap providing
detailed volumes of sediment outputs for the catch-
ment. Resurveys were also performed on an active
gully which experiences frequent rockfalls. The gully
is accessible on a trail and can be seen from a view
point with optimal angle and coverage. The majority
of the torrent channel in the production zone could
also be monitored providing information of sediment
transport upstream from the monitored cross-sections.
The coverage of this area was limited and difficult to
monitor which depended on high resolution airborne
LiDAR as its preliminary surface.
Resolution of terrestrial laser scans range from 2
to 10 cm which is highly dependant on the distance
from the scanning position (maximum ~800 m).
Scans were merged on Polyworks and georeferenced
to the airborne LiDAR scan creating regular maxi-
mum standard deviation of ± 0,08 m. Elevation mod-
els were developed through ordinary kriging which
gives the best interpretation for channel morphologic
features with terrestrial laser scans (H
eRitaGe
et alii,
2009). The digital terrain models are developed on a
0,1 m grid where differences are used for calculating
volumes of erosion and deposition.
The airborne LiDAR survey of the whole catchment
was flown in June 2009. The filtered point cloud has a
mean density of 6.9 points/m
2
. A 0.1 m grid was derived
Fig. 3 - Rainfall intensities (5 minutes interval) associated
with (a) the August debris-flow, (b) the September
and (c) October flow events
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SEDIMENT BUDGET MONITORING OF A DEBRIS-FLOW TORRENT (FRENCH PREALPS)
Italian Journal of Engineering Geology and Environment - Book www.ijege.uniroma1.it © 2011 Casa Editrice Università La Sapienza
783
slope with 274 m
3
deposited 40-80 m down the gully.
The gain of 38 m
3
of material most likely originated
above the immediate cliff. There was no change in
topography further then the extent of the deposition.
All of these observations show that the initiation
of the debrisflow occurred in the upper reach, just
downstream from the confluence between the two
main gullies of the production zone. The channel
slope here is 18% with a drainage area of 0,87 km
2
.
Upstream from the confluence, high water marks
could be recognized however no signs of debris-flow
features were visible. Levees begin to form ~100 m
downstream from the confluence. The debris-flow
grew in volume along a reach of 600 m length con-
tributing 3 000 m
3
(mean slope: 18%). This gives a
yield rate of about 5 m
3
/m. Most of these sediments
were transported without significant interactions
with the channel along a 730 m reach with a mean
slope of 15% (Fig. 4c). Deposition starts at a slope of
14% and terminates at the sediment trap.
the event in the sediment trap gives a sediment accu-
mulation of 1 873 ±62 m
3
. Topographic monitoring of
cross-sections along the main channel showed a net loss
of 2 034 ±559m
3
(deposition: 3 200 m
3
, erosion: 5 215
m
3
) (Fig. 4b). These results show that sediment outputs
from the production zone (upstream from the surveyed
reach) were insignificant despite the intensity of the
storm and that the volume of the debris-flow was en-
tirely supplied by channel scouring. This is confirmed
by laser scans made in the production zone. The up-
per channel experienced sediment deposition (Fig. 5a).
It trapped sediments coming from two active gullies
which are identified as active during the storm and gen-
erated a sediment discontinuity between the production
zone and the main channel. These observations show
that confined reaches with very steep channel slope
(32%) can store sediments during convective storms.
LiDAR resurvey of the gully showed that a talus
slope failure occurred at the upper end of the gully
(Fig. 5c). Erosion of 237 m
3
took place at the talus
Fig. 4 - Long profile (a), volume changes (b) and sediment transport (c) along the main channel for the 3 investigated periods
of geomorphic activity; data derived from cross-sections resurveys
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J. THEULE, F. LIEBAULT, A. LOYE, D. LAIGLE & M. JABOYEDOFF
784
5th International Conference on Debris-Flow Hazards Mitigation: Mechanics, Prediction and Assessment Padua, Italy - 14-17 June 2011
Fig. 5 - Morphological changes
captured in the produc-
tion zone with terrestrial/
airborne LiDAR resur-
veys: erosion and deposi-
tion patterns of the upper
main channel (a) after the
August debris-flow and
(b) autumn flow events;
(c) erosion and deposi-
tion pattern of a small
gully incised in talus
slope after the August
debris-flow; the shaded
relief map derived from
an airborne LiDAR sur-
vey is used in background
Fig. 6 - Morphological changes associated with the deposition of gravel dunes transported as bedload during autumn flow
events (photos: J
oShuA
t
heule
)
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SEDIMENT BUDGET MONITORING OF A DEBRIS-FLOW TORRENT (FRENCH PREALPS)
Italian Journal of Engineering Geology and Environment - Book www.ijege.uniroma1.it © 2011 Casa Editrice Università La Sapienza
785
CONCLUDING REMARKS
Recent studies reveal the significance of sediment
availability, channel recharge, and hillslope-channel
coupling for debris-flow occurrence (C
oe
et alii, 2008;
s
CHluneGGeR
et alii, 2009). Our observations indicate
that debris-flow is a scouring process for the channel
whereas bedload transport is a process that replenishes
the scoured reaches by deposition of gravel dunes (Tab.
2). Despite erosion activity in the production zone dur-
ing high intensity convective rainfall, sediment was
stored in the gullies and upper channel and the sedi-
ment yield of the production zone was low. Volumes
entrained in gullies were small and travel distances
were short, creating a delay of delivery into the main
torrent channel. These sediments produced during con-
vective storm were delivered to the main channel dur-
ing autumn rainfall by bedload transport. The sediment
yield of the production zone was much more important
(3 orders of magnitude more), despite rainfall of less
intensity. This sequence of events is a typical annual
observation for the Manival catchment, the controlling
conditions for travel distances and connections of sedi-
ment transport still needs to be fully understood.
The first results for this project are very encour-
aging which motivates us to continue the monitoring
program on the Manival. Supplementary efforts are
also made to (1) quantify the contributions from the
catchment source with terrestrial laser scanning and
to (2) instrument the main channel for real time moni-
toring of flow events. These elements will allow us
to obtain necessary data for interpreting and predict-
ing responses of sediment transport in steep channels
and establishing comprehensive sediment budgets for
studying the hillslope-channel coupling.
ACKNOWLEDGEMENTS
This research was supported by the Interreg Alpine
Space Paramount Project and by the “Pôle Grenoblois
d’étude et de recherche pour la prevention des Risques
Naturels” (PGRN). Michael Deschatres, Hugo Jantzi,
Mathieu Cassel, Emilien Parisot, and Nicolas Talaska
are acknowledged for their assistance during field works.
SEPTEMBER AND OCTOBER 2009 EVENTS
The September event occurred after a long dura-
tion rainy period of moderate intensity (maximum
intensity of 16,9 mm h
-1
) (Fig. 3b). No traces of de-
bris-flows have been detected in the main channel and
no sediment deposition was observed in the sediment
trap. However, the cross-sections resurvey showed
substantial morphological changes in the main chan-
nel, reflecting a relatively large volume of sediment
transport by bedload, despite the moderate flows. The
sediment budget shows a net storage gain of 789 ±392
m
3
(deposit: 2 195 m
3
erosion: 1 410 m
3
) (Fig. 4b). We
also observed that deposition occurred preferentially
in subreaches that were scoured during the August
debris-flow (Fig. 6). We calculated that 39% of the
channel storage loss from the August debris-flow was
recharged by bedload transport of the September event.
Rainfalls in October have endured entire days
with maximum intensities of 1,6 mm hr
-1
(Fig. 3c).
Flow in the channel becomes more regular disappear-
ing and reappearing through the gravel/cobble stor-
ages. During peak intensities of some of these flows
bedload transport has been observed which constant-
ly changes the surficial flow direction in the channel.
The sediment budget of the main channel showed a
loss of 89 ±322 m
3
; however deposition of at least
233 m
3
in the upper part of the channel reveals more
recharge from the production zone (Fig. 6). The lower
half of the torrent towards the sediment trap experi-
enced incision of at least 321 m
3
due to the constant
flow channelizing through remnant deposits of the
debris-flow and flood. The sediment trap was dis-
turbed by operators extracting the material; therefore
the budget for this event could not be determined.
Further recharge however is being taken place in the
upper half of the monitored torrent.
During September and October events the chan-
nel storage in the production zone experienced ero-
sion (limited LiDAR coverage does not allow a full
estimate of volume). Figure 5b shows that the produc-
tion zone deposition during the debris-flow event was
removed by autumn flow events.
Tab. 2 - Sediment budget of the Manival catchment for 2009 events
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