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Italian Journal of Engineering Geology and Environment - Book www.ijege.uniroma1.it © 2011 Casa Editrice Università La Sapienza
135
DOI: 10.4408/IJEGE.2011-03.B-016
RECONSTRUCTION OF DEBRIS-FLOW DYNAMICS AND TRIGGERS
SINCE AD 1570 - LOOKING BACK TO A DECADE OF TREE-RING
RESEARCH AT RITIGRABEN (VALAIS, SWITZERLAND)
m
aRkus
STOFFEL
(*, **, ***)
(*)
Laboratory of Dendrogeomorphology (dendrolab.ch), Institute of Geological Sciences,
University of Bern, Baltzerstrasse 1+3, CH-3012 Bern, Switzerland
(**)
Chair for Climatic Change and Climate Impact Research, Institute for Environmental Sciences,
University of Geneva, route de Drize 7, CH-1227 Carouge-Geneva, Switzerland
(***)
Avalanches, Debris Flows and Rockfalls, Swiss Federal Research Institute WSL,
Zürcherstrasse 111, CH-8903 Birmensdorf, Switzerland
parture zone (i.e. a rock glacier) allowed estimation
of frequency-magnitude relationships. We also related
debris-flow events reconstructed for the Ritigraben
torrent with flooding records of neighboring rivers.
K
ey
words
: debris flow, tree-ring analysis, dendrogeomor-
phology, deposits, sediment, triggering events, permafrost,
climate change, Swiss Alps
INTRODUCTION
Records derived from trees provide annually re-
solved data on past mass movements spanning several
centuries, thus allowing identification and accurate
dating of events prior to instrumental records (s
tof
-
fel
et alii, 2010). As a result, dendrogeomorphic ap-
proaches have been applied repeatedly in the past to
determine time series of past debris flows in mountain
regions (b
ollsCHweileR
& s
toffel
, 2010b). Pioneering
work on tree-ring damage resulting from debris-flow
impacts was performed by Hupp (1984; H
uPP
et alii,
1987) on the slopes of Mount Shasta (California). A
few years later, s
tRunk
(1989, 1997) was the first to
use the approach in Europe, reconstructing time series
of debris flows in the Italian Dolomites. More recently,
dendrogeomorphic techniques have been improved sig-
nificantly through the inclusion of wound-induced tan-
gential rows of traumatic resin ducts (TRD) in tree-ring
studies (e.g., b
ollsCHweileR
et alii, 2008b; s
CHneuwly
et alii, 2009; s
toffel
& H
itz
, 2008). As a result, repli-
cation and identification of past events in conifer trees
ABSTRACT
Records derived from trees growing in temper-
ate regions can provide annually resolved data on past
debris-flow activity that span several centuries. They
therefore allow identification and accurate dating of
events prior to instrumental records or missing in his-
torical archives. As a result, dendrogeomorphic methods
have repeatedly been used over the last decades to re-
construct debris-flow frequencies in mountain regions of
Europe or North America. While these studies furnished
valuable data on the minimum frequency of events that
would have occurred in the channels and cones chosen
for analysis, they did, however, not explicitly take ac-
count of e.g., depositional processes on cones, volumes,
periods of activity in currently abandoned channels or
on the age of individual lobes and levees.
Over the past eleven years, dendrogeomorphic
investigations on past debris-flow activity in the
Ritigraben catchment (Valais, Swiss Alps) and its
intermediate cone went several steps beyond the as-
sessment and pure dating of debris-flow “signatures”.
Through the analysis of some 2450 tree-ring sequenc-
es obtained from more than 1200 trees growing on the
cone and along the current flow path, we were able to
identify evidence of 123 events since 1570. The large
amount of data also allowed analysis of the spread and
deposition of material on the cone as well as deter-
mination of activity in currently abandoned channels.
A combination of tree-ring with ground survey data,
channel recharge rates and debris delivery in the de-
background image
m. Stoffel
136
5th International Conference on Debris-Flow Hazards Mitigation: Mechanics, Prediction and Assessment Padua, Italy - 14-17 June 2011
Following an active-layer detachment at the rock
glacier front in 1993, massive ice lenses were exposed
at the front of the rock glacier. In summer 1994, deg-
radation of the exposed ice lenses caused intense ret-
rogressive erosion at a monthly rate of ca. 1 m at the
rock-glacier front (l
uGon
& m
onbaRon
, 1998). The
accumulated material was partly removed during a
rain-on-snow event on September 24, 1994, result-
ing in a debris flow with a volume of roughly 5,000
m
3
(z
immeRmann
et alii, 1997). Retrogressive erosion
continued in summer 1995, leading to the formation
of subsidence close to the front of the rock glacier.
On its downward course to the Mattervispa river,
the Ritigraben torrent passes a large forested cone (32
ha; ca. 4.3 x 10
6
m
3
) on a structural terrace (1500-
1800 m asl), where debris-flow material affects trees
within an old-growth stand composed of European
larch (Larix decidua Mill.), Norway spruce (Picea
abies
(L.) Karst.) and Swiss stone pine (Pinus cembra
L.). Figure 1b illustrates the intermediate debris-flow
cone, which is of Holocene age.
MATERIAL AND METHODS
Analysis of past debris-flow activity included a
detailed mapping of all features associated with past
events, such as lobes, levees or abandoned flow paths on
a scale of 1:1000. In addition, mean sizes of blocks were
measured for each lobe and levee (< 0.5 m, 0.5-1 m, 1-2
m) and the vegetation cover present on the features was
qualitatively assessed (light, medium, dense).
On the debris-flow cone, most of the century-
old conifers show growth disturbances (GD) related
to past debris-flow activity (i.e. tilted stems, partial
trunk burial, denudation of roots, scars). Based on
the geomorphic map and an outer inspection of the
trunk surface, we sampled trees obviously disturbed
by past debris flows. We also selected undisturbed
was greatly facilitated (s
toffel
, 2008), as signs of past
damage are more easily identified in the form of TRD
and as these remain recognizable in the tree-ring record
even if the wound becomes completely overgrown.
In recent work, TRD have been used extensively for
the reconstruction of event histories of debris flows
(b
ollsCHweileR
& s
toffel
, 2010c; b
ollsCHweileR
et
alii, 2008a), hyperconcentrated flows (b
ollsCHweileR
et alii, 2007), debris floods (m
ayeR
et alii, 2010), and
lahars (b
ollsCHweileR
et alii, 2010).
While these studies provided invaluable data
on minimum frequencies, they did not explicitly
take account of depositional processes and patterns,
magnitudes, age of deposits, debris-flow triggers or
rainfall thresholds. In 2000, dendrogeomorphic re-
search on the cone of the Ritigraben torrent (Valais,
Swiss Alps) started with the reconstruction of event
frequencies of debris flows (s
toffel
et alii, 2005),
but has since evolved into a very detailed and prob-
ably the most complete database on past debris-flow
activity in mountain regions (s
toffel
et alii, 2008).
This contribution aims at summarizing the key find-
ings of more than a decade of tree-ring based debris-
flow research at Ritigraben and at providing possible
tracks for future research.
STUDY SITE
The Ritigraben torrent is located on the west-
facing slope of the Mattertal valley (Valais, 46º11’N,
7º49’E). The system spans a vertical range of 2000 m
from the summit of the Seetalhorn (3100 m asl) to the
torrent’s confluence with the Vispa River at 1080 m
asl. A rock glacier occupies a large part of the head-
water basin (1.4 km
2
) between 2500 and 2800 m asl
(Figure 1a), representing the principal source of loose
material in the upper part of the catchment area and
constituting the main starting zone of debris flows.
DC resistivity soundings detect low resistivity values
inside the rock-glacier body (10 to 110 kΩm, l
uGon
& m
onbaRon
, 1998), being characteristic of temper-
ate permafrost with temperatures close to the melt-
ing point. Five boreholes located near the front of the
rock-glacier body confirm this interpretation (l
uGon
& s
toffel
, 2010). Temperature profiles indicate a
mean annual temperature varying between -0.3 and
-0.6 ºC in the permafrost body for the period 2002-
2005 and a depth of the zero annual amplitude (ZAA)
at -13 m with a mean annual temperature of -0.3 ºC.
Fig. 1 - Ritigraben torrent: (A) View of the debris-flow sys-
tem (catchment area: 1.36 km
2
, channel lengths:
3.5 km) (B) Detail of the intermediate debris-flow
cone (32 ha) and its conifer stand
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RECONSTRUCTION OF DEBRIS-FLOW DYNAMICS AND TRIGGERS SINCE AD 1570 – LOOKING BACK TO A DECADE
OF TREE-RING RESEARCH AT RITIGRABEN (VALAIS, SWITZERLAND)
Italian Journal of Engineering Geology and Environment - Book www.ijege.uniroma1.it © 2011 Casa Editrice Università La Sapienza
137
cent event could be dated. While much of the field
evidence typically resides on cones (H
aebeRli
et alii,
1991), subsequent incidences may overprint or remove
geomorphic and botanical evidence of previous events
(H
uPP
, 1984). An assessment of debris-flow magni-
tude considering exclusively deposits visible on the
present-day surface will run the risk of underestimat-
ing the size of individual events. To account for this
problem, the assessment of event magnitude and mag-
nitude-frequency (M-F) relations was not only based
on volumetric data of lobate deposits, and mean/maxi-
mum grain sizes of boulders, but also included analysis
of a series of second-order surrogate variables, includ-
ing (i) snout elevations of lobes as a proxy for stream
power; (ii) damage caused to the forest along the tracks
and to tree survival inside lobes as indicators of im-
pact energies involved; as well as (iii) the distribution
of lobes and levees belonging to the same event as a
proxy for the lateral spread of flows. In addition, we
used (iv) eyewitness reports and (v) volumetric data
of the most recent events (1993, 1994, 2002, 2008) to
further increase magnitude control. Based on the above
criteria, four different size classes of debris flows are
presented and given as S, M, L, and XL, reflecting a
magnitude estimate of event sizes at the cone apex
(s
toffel
, 2010). The temporal frequency of events
was analyzed per size class with the FHX2 software
(G
Rissino
-m
ayeR
, 2001). Parameters assessed include
debris-flow frequency, mean debris-flow intervals,
Weibull median intervals, minimum and maximum
debris-flow intervals as well as lower (0.125) and up-
per (0.875) exceedance intervals.
In the source area of debris flows, variations in
horizontal rock-glacier movement rates were assessed
with high-resolution geodetic surveys (1995-) and
low-resolution photogrammetric analyses covering
the last 50 years. The rate of sediment delivery to the
foot of the rock-glacier snout was determined through
reference trees from a forest stand located next to the
cone. In total, 1204 trees were sampled (2450 incre-
ment cores): 539 L. decidua, 429 P. abies and 134 P.
cembra
trees (2246 cores) from the debris-flow cone
as well as 102 trees (204 cores) of the same species
from undisturbed reference sites.
Samples were analyzed and data processed fol-
lowing the standard procedures described in s
toffel
& b
ollsCHweileR
(2008, 2009). Single steps of sample
analysis included surface preparation, skeleton plots
as well as ring-width measurements using digital posi-
tioning tables, a stereo microscope and TSAP 3.0 (Time
Series Analysis and Presentation) software (R
innteCH
,
2010). Growth curves of the disturbed samples were
crossdated with the reference chronology constructed
from undisturbed trees to separate insect attacks or cli-
matically driven fluctuations in tree growth from GD
caused by debris flows (C
ook
& k
aiRiukstis
, 1990).
Growth curves were then used to determine the
initiation of abrupt growth reduction or recovery
(m
C
a
uliffe
et alii, 2006). In the case of tilted stems,
both the appearance of cells (i.e. structure of the reac-
tion wood cells) and the growth curve data were ana-
lyzed (e.g., f
antuCCi
& s
oRRiso
-v
alvo
, 1999; Figure
2). Finally, the cores were visually inspected so as to
identify further signs of past debris-flow activity in the
form of callus tissue overgrowing abrasion scars or
TRD formed following cambium damage (s
CHneuwly
et alii, 2009; and references therein).
As conifer trees react immediately to damage
with the formation of callus tissue or TRD, the po-
sition of GD within the tree ring was used to assess
the timing of debris-flow activity in particular years,
rendering dating of events possible with monthly pre-
cision (k
aCzka
et alii, 2010). The results obtained on
the intra-seasonal timing of debris flows were then
compared with rainfall records from a local meteoro-
logical station, operational since AD 1863, and with
archival records on floods in rivers of the Valais Alps
(l
ütsCHG
-l
ötsCHeR
, 1926; R
ötHlisbeRGeR
, 1991).
The age of lobes was assessed by attributing se-
vere GD in the tree-ring series to deposits in the field.
As exemplified in Fig. 2, dating of a lobe was possible
if (i) a survivor tree was injured through the deposition
of material; (ii) its stem base buried by debris; or (iii)
if it was tilted (Fig. 2). Special attention was addressed
to multiple GD identified in the tree-ring series. Here,
only the geomorphic features left during the most re-
Fig. 2 - Tree-ring “signatures” used to determine the age
of debris-flow deposits
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m. Stoffel
138
5th International Conference on Debris-Flow Hazards Mitigation: Mechanics, Prediction and Assessment Padua, Italy - 14-17 June 2011
ing to the release of debris flows at Ritigraben, before
(iv) the role of meteorology on debris-flow magnitude
and (v) the implications that climatic change might have
on the occurrence of debris flows are discussed.
RESULTS AND DISCUSSION
The study of the 2246 tree-ring sequences sampled
from 1102 trees allowed reconstruction of 124 events
since AD 1570 (Fig. 3). Geomorphic mapping permit-
ted identification of 769 features related to past debris-
flow activity on the intermediate cone. The features
inventoried in the study area included 291 lobes, 465
levees and 13 well-conserved paleochannels. Based on
tree-ring records of disturbed trees growing in or next
to the deposits, ~86% of the lobes identified on the
present-day surface could be dated. Figure 4 illustrates
that a majority of the dated material was deposited over
an assessment of creep velocity [cm yr
-1
], rock-glacier
width [m], and depth of the shear zone [m]. In addition
to sediment yield, we analyzed changes in the posi-
tion of the upper limit of the rock-glacier snout (i) to
identify visible signs of retrogressive erosion and (ii)
to derive volumes eroded at the front. Tree-ring data
on M-F relations of events recorded at the cone were
then compared with (iii) volumes transported to the
source area of debris flows through rock-glacier creep
and (iv) changes in the position of the upper limit of
the rock-glacier snout (l
uGon
& s
toffel
, 2010).
In a last analytical step, data on former debris-flow
events were coupled with meteorological records (AD
1863-2008; s
toffel
et alii, in review) to study (i) precip-
itation totals recorded during events, (ii) the role of an-
tecedent rain or snowmelt (i.e. “rain-on-snow” events),
and (iii) storm type (i.e. advective or convective) lead-
Fig. 3 - Tree-ring based reconstruction of debris flow activity at Ritigraben between AD 1566 and 2005 containing 123
events. The sample depth (dotted line) shows the number of cores available for analysis at specific years in the past
Fig. 4 - Deposition of debris-flow material on the intermediate cone during past events: Material deposited between
1935 and 1993. Only events that are associated with >600 m
3
on the present-day surface of the cone are
indicated on the map. Deposits shown in black are dated, but are older than the time segment illustrated
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RECONSTRUCTION OF DEBRIS-FLOW DYNAMICS AND TRIGGERS SINCE AD 1570 – LOOKING BACK TO A DECADE
OF TREE-RING RESEARCH AT RITIGRABEN (VALAIS, SWITZERLAND)
Italian Journal of Engineering Geology and Environment - Book www.ijege.uniroma1.it © 2011 Casa Editrice Università La Sapienza
139
vals of 5.4 (SD: 3.2) and 7.4 years (SD: 6.7), respec-
tively (Table 1). Class XL events (10
4
-5×10
4
m
3
) are,
in contrast, only identified three times over the past
150 years, and major erosional activity on the cone
was restricted to two of these events in 1948 and 1993
(Fig. 6). A comparison of results with hydro-meteor-
ological records shows that class L and XL events are
typically triggered by advective storms (with rainfall
totals >50 mm) in August and September, when the
active layer of the permafrost body in the source area
of debris flows is largest. It also becomes obvious that
over the past ~150 years, climate has exerted control
on material released from the source area and prevent-
ed triggering of class XL events before 1922.
Debris production and volumetric changes at the
rock-glacier front are compared with debris-flow activ-
ity recorded on the cone and potential couplings and
feedbacks between debris sources, channel processes
and debris sinks. Acceleration in rock-glacier move-
ment rates is observed in the 1950s and 1960s, followed
by a decrease in flow rates by the 1970s, before move-
ments increase again after the early 1990s. At a decadal
scale, measured changes in rock-glacier movements at
Ritigraben are in concert with changes in atmospheric
temperatures in the Alps. Geodetic data indicates dis-
placement rates in the frontal part of the rock glacier of
up to 0.6-0.9 m yr
-1
since the beginning of systematic
measurements in 1995. While the Ritigraben rock gla-
cier has always formed a sediment reservoir for the as-
the past century. Signs of pre-20
th
century events are
often recognizable in the tree-ring record of survivor
trees, but the material that caused the growth anomaly
in trees has been completely overridden or eroded by
more recent debris-flow activity.
Based on the tree-ring records, we believe that cool
summers with frequent snowfalls in the headwater of
the Ritigraben torrent regularly prevented the release of
debris flows between the 1570s and 1860s; the warm-
ing trend combined with greater precipitation totals
in summer and autumn between 1864 and 1895 pro-
vided conditions that were increasingly favorable for
releasing events from the source zone. It can also be
seen from Figure 5 that enhanced debris-flow activity
continued well into the 20
th
century and reconstructions
show a clustering of events between 1916 and 1935
when warm-wet conditions prevailed during summer
in the Swiss Alps. In contrast, very low activity is ob-
served for the recent past (1996-2005) with only one
debris flow recorded on August 27, 2002. Since sedi-
ment availability is not a limiting factor, this temporal
absence of debris-flow activity is due to an absence of
triggering events, which shifted from June and July to
August and September over the 20
th
century.
Magnitude-frequency (M-F) relations were as-
sessed for 62 debris flows since A.D. 1863, i.e. for the
period where meteorological records are available.
Class S and M debris flows (<5×10
3
m
3
) encompass a
typical size of events and have mean recurrence inter-
Fig. 5. Reconstructed 10-yr frequencies of debris-flow events
between AD 1566 and 2005. Data are presented
as variations from the mean decadal frequency of
debris flows of the last 300 years (AD 1706-2005)
Fig. 6. Reconstructed time series of debris-flow magnitudes,
in four classes, for the period from 1858 to 2008.
Note the clustering of important events in the
early decades of the twentieth century and the ab-
sence of class XL debris flows before 1922
Tab. 1 - Statistics of temporal frequency of debris flows for different magnitude classes.
Thresholds for the lower and upper exceedances set at 0.125 and 0.875
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m. Stoffel
140
5th International Conference on Debris-Flow Hazards Mitigation: Mechanics, Prediction and Assessment Padua, Italy - 14-17 June 2011
sociated debris-flow system, annual horizontal displace-
ment rates of the rock-glacier body have remained quite
small and are in the order of decimeters under current
climatic conditions. Sediment delivery from the rock-
glacier front alone could not therefore be sufficient for
the alimentation of the 16 debris flows reconstructed on
the cone since 1958. On the contrary, debris accumulat-
ed at the foot of the rock glacier, landslide and rockfall
activity as well as the partial collapse of oversteepened
channel walls have to be seen as important sediment
sources of debris flows at Ritigraben and represent 65-
90% of the material arriving on the Ritigraben cone.
Noteworthy, as shown in Figure 7, there does not seem
to exist a direct coupling between displacement rates of
and sediment delivery by the rock-glacier body and the
frequency of class S and M debris flows. In contrast, a
direct link between source and sink processes clearly ex-
ists in the case of active-layer failures. In this case, fail-
ure processes at the rock-glacier snout and debris-flow
events in the channel occur simultaneously and are both
triggered by the rainfall event.
Based on observational meteorological data, we
then assessed changes in rainfall characteristics and
their impact on the triggering of debris flows over the
past ~150 years. No trends are visible in the frequency
of heavy rainfall events, but we observe a cluster of
the overall frequency of debris flows in the early dec-
ades of the 20
th
century and a concentration of advec-
tive storms in late summer and early autumn since the
late 1980s (Fig. 8). At the same time, dendrogeomor-
phic data point to a reduction of convective rainfall
capable of triggering events and a lowering of the
overall debris-flow activity since the mid-1990s.
These changes in triggering meteorological condi-
tions may be mirroring the observed changes in per-
sistent high-pressure systems over the Alps. In addi-
tion to the long-term changes in debris-flow systems,
we observe changing responses of the watershed to
the amount of rain at different moments within the
debris-flow season (i.e., early June through late Sep-
tember). Differences in system response clearly reflect
the state of the permafrost body in the source area of
debris flows, which allows for very small debris-flow
events after limited rainfall inputs (<20 mm) in June
and July. The same quantities of rain will result in non-
responses of the system in August or September, when
a large active layer of the permafrost body is capable of
absorbing water without producing debris flows. With
the projected amplitude of climatic change, changes
will probably occur in the seasonality, return intervals
and volumes of debris flows. Regional climate model
projections suggest a decrease in heavy summer rain-
fall events which will most likely result in a reduction
of the overall frequency of debris-flow events, leaving
more time for debris to accumulate in the channel. Such
increases of channel accumulation rates along with the
projected destabilization of the steep rock-glacier body
will ultimately exert control on the volumes of mate-
rial released from the source areas during future debris-
flow events. It is thus possible that extremely large
class XXL” events, beyond historical experience and
with volumes surpassing 5 × 10
4
m
3
at the level of the
debris-flow cone, may be observed in the future origi-
nating from this periglacial environment.
Fig. 7 - Sediment delivery of the rock glacier and M-F re-
lationships of debris flows at Ritigraben
Fig. 8 - Changes in seasonality of debris-flow events (top),
storm types triggering debris flows (centre) and rain-
fall totals recorded during convective and advective
storms with subsequent debris-flow releases (bottom)
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RECONSTRUCTION OF DEBRIS-FLOW DYNAMICS AND TRIGGERS SINCE AD 1570 – LOOKING BACK TO A DECADE
OF TREE-RING RESEARCH AT RITIGRABEN (VALAIS, SWITZERLAND)
Italian Journal of Engineering Geology and Environment - Book www.ijege.uniroma1.it © 2011 Casa Editrice Università La Sapienza
141
underline the growing potential to expand these studies,
possibly leading to the establishment of a range of tech-
niques and approaches that may become standard prac-
tice in the analysis of debris-flow hazards in the future.
Dendrogeomorphic studies, similar to the one out-
lined above, should be replicated on other sites and
in variety of contrasting geographic environments.
First data from the Valais Alps show that analysis of
neighboring catchments may further improve under-
standing of debris-flow dynamics at a regional level
(b
ollsCHweileR
& s
toffel
, 2007, 2010a) and help the
apprehension of rainfall events that are likely to lead
to debris-flow events. Dendrogeomorphic data should
also be used for the retrospective modeling of events
(G
Raf
et alii, 2009), as an accurate calibration and a
detailed accuracy assessment of debris-flow models
may greatly help to enhance parameter control in sce-
nario-defined runs for potential future events.
ACKNOWLEDGEMENTS
The authors offer sincere thanks to D. Conus, T.
Falco, M. A. Grichting, I. Lièvre and G. Maître for
assistance in the field and lab. Data from boreholes
B1-5 and theodolite measurement points were pro-
vided by Rovina&Partner AG, They are used with
permission of the Canton of Valais. Work has been
undertaken in the context of the EU-FP7 ACQwA (no.
GOCE-20290), and FOEN-SFP-SRCE RUFINE (no.
0931030100RA-0000008253) projects.
OUTLOOK
The significant contribution of dendrogeomorphol-
ogy (s
toffel
et alii, 2010) to the endeavors of mass-
movement research lies in their capacity to both pre-
serve evidence of past events and to provide critical
information on their dating with annual or sub-annual
resolution. Therefore, tree-ring records may represent
the most valuable and precise natural archive for the re-
construction and understanding of past events over the
last several centuries. The initial employment of tree
rings in earth-surface process studies was simply as a
dating tool (a
lestalo
, 1971) and rarely exploited other
environmental information that could be derived from
studies of ring-width variations and records of damage
contained within the tree itself. However, these unique,
annually resolved, tree-ring records usually preserve
potentially valuable archives of past geomorphic events
on timescales of a few decades to several centuries. As
many of the earth-surface processes are significant
natural hazards, documenting time series of events,
understanding their areal extent and controls provides
valuable information that can assist in the prediction,
mitigation and defense against these hazards and their
effects on society. This thesis will illustrate how tree-
ring analysis can be used to reconstruct natural hazards
and provide information that may be used to understand
the future occurrence of events. The different approach-
es illustrated above show the breadth and diverse ap-
plications of contemporary dendrogeomorphology and
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ollsCHweileR
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toffel
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ollsCHweileR
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