Spatial and Temporal Occurrence of Past Debris Flows in the Valais Alps - Results from Tree-Ring Analysis
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AUS DEM DEPARTEMENT FÜR GEOWISSENSCHAFTEN - GEOGRAPHIE UNIVERSITÄT FREIBURG (SCHWEIZ) Spatial and temporal occurrence of past debris flows in the Valais Alps - results from tree-ring analysis INAUGURAL - DISSERTATION zur Erlangung der Würde eines Doctor rerum naturalium der Mathematisch - Naturwissenschaftlichen Fakultät der Universität Freiburg in der Schweiz Michelle BOLLSCHWEILER aus Basel BS und Unterägeri ZG (Schweiz) Dissertation Nr. 1572 Multiprint, Fribourg, 2007 Die vorliegende Arbeit wurde an der Mathematisch - Naturwissenschaftlichen Fakultät der Universität Freiburg, Schweiz, im Juni 2005 aufgrund der Gutachten von Prof. Michel Monbaron, Prof. David Butler, Dr. Florian Rudolf-Miklau und Dr. Markus Stoffel angenommen. Als Jurypräsident waltete Prof. Bernard Grobéty. The Faculty of Science of the University of Fribourg, Switzerland, accepted the present work as dissertation in June 2005 based on the experts’ reports of Prof. Michel Monbaron, Prof. David Butler, Dr. Florian Rudolf-Miklau and Dr. Markus Stoffel. President of the Jury: Prof. Bernard Grobéty Fribourg, 12.07.2007 The PhD thesis’ director: The Dean: Prof. Michel Monbaron Prof. Titus Jenny Zitierung (Citation): Bollschweiler M., 2007: Spatial and temporal occurrence of past debris flows in the Valais Alps - results from tree-ring analysis. PhD thesis No. 1572, University of Fribourg. GeoFocus 20, 182 pp. GeoFocus is the publication series of the Dept. of Geosciences, University of Fribourg, Switzerland, and can be ordered at Department of Geosciences - Geology University of Fribourg 1700 Fribourg http://www.unifr.ch/geoscience/geology Table of contents - 1 TABLE OF CONTENTS TABLE OF CONTENTS 1 4 STUDY SITES 31 ABSTRACT 5 ZUSAMMENFASSUNG 7 ACKNOWLEDGEMENTS 9 CHAPTER B: TANGENTIAL ROWS OF TRAU- MATIC RESIN DUCTS AFTER WOUNDING BY DEBRIS FLOWS CHAPTER A: GENERAL INTRODUCTION ABSTRACT 35 1 INTRODUCTION 13 1 INTRODUCTION 36 1.1 BACKGROUND 13 1.2 AIMS OF THE STUDY 14 2 MATERIALS AND METHODS 37 1.3 STRUCTURE OF THE THESIS 14 2.1 STUDY SITE 37 2 DEBRIS-FLOW PROCESS 15 2.2 FIELD COLLECTION AND SAMPLE PREPARATION 37 2.3 ANALYTICAL PROCEDURES 38 2.1 DEFINITION AND CLASSIFICATION 15 2.2 PREDISPOSITION AND TRIGGERING OF DEBRIS 3 RESULTS 39 FLOWS 16 2.3 TRANSPORT AND FLOW BEHAVIOR 17 3.1 TRD POSITION WIHTIN ANNUAL GROWTH RING 39 2.4 DEPOSITION 18 3.2 AXIAL EXTENT OF TRD 39 3.3 TANGENTIAL EXTENT OF TRD 40 3 DEBRIS FLOWS AND TREE RINGS 21 4 DISCUSSION 42 3.1 PRINCIPLES OF DENDROCHRONOLOGY 21 3.2 INFLUENCE OF DEBRIS FLOWS ON TREES AND ACKNOWLEDGEMENTS 45 THEIR REACTIONS TO THE IMPACT 22 3.3 METHODS IN DENDROGEOMORPHOLOGY 25 3.3.1 Geomorphic mapping 25 CHAPTER C: RECONSTRUCTION OF DEBRIS- 3.3.2 Sampling methods and strategy 26 FLOW FREQUENCIES 3.3.3 Sample preparation, counting of tree rings, skeleton plots and ring-width ABSTRACT 49 measurements 27 3.3.4 Building a reference chronology 28 1 INTRODUCTION 50 3.3.5 Dating events in tree-ring series 28 3.3.6 Preparation of micro-cuts 29 2 STUDY SITES 50 3.4 RESEARCH ON DEBRIS FLOW USING TREE RINGS - A STATE OF THE ART 29 3 MATERIAL AND METHODS 52 2 - Table of contents 3.1 FIELD METHODS 52 ACKNOWLEDGEMENTS 79 3.2 LABORATORY METHODS 54 4 RESULTS 54 CHAPTER E: COUPLING OF EVENT RECON- STRUCTION WITH MINIMUM AGE 4.1 AGE STRUCTURE OF THE STAND 54 DATING 4.2 DATING OF PAST DEBRIS-FLOW EVENTS 55 4.3 RECONSTRUCTED DEBRIS-FLOW FREQUENCIES AT ABSTRACT 83 REUSE DE SALEINAZ AND TORRENT DE LA FOULY 56 1 INTRODUCTION 84 4.4 SPATIAL DISTRIBUTION OF TREES AFFECTED BY DEBRIS-FLOW EVENTS 57 2 STUDY SITE 85 4.5 COMPARISON OF RECONSTRUCTED EVENTS WITH DATA ON FLOODING IN NEIGHBORING TORRENTS 3 METHODS 86 IN VAL FERRET 57 3.1 GEOMOPRHIC MAPPING 86 5 DISCUSSION 58 3.2 SAMPLING STRATEGY 86 3.3 DATING OF DEBRIS-FLOW EVENTS AND 6 CONCLUSION 61 RECONSTRUCTION OF THEIR SPATIAL EXTENT 87 3.4 MINIMUM AGE DATING 87 ACKNOWLEDGEMENTS 61 3.5 DETERMINATION OF LAST MOMENT OF ACTIVITY 88 4 RESULTS 88 CHAPTER D: RECONSTRUCTION OF SPATIAL PATTERNS OF PAST DEBRIS-FLOW 4.1 GEOMOPRHIC MAPPING 88 EVENTS 4.2 AGE STRUCTURE OF THE STAND 88 4.3 GROWTH DISTURBANCES AND DEBRIS-FLOW ABSTRACT 65 FREQUENCY 88 4.4 SPATIAL EXTENT OF PAST EVENTS 90 1 INTRODUCTION 66 4.5 APPROXIMATION OF LAST MOMENT OF PAST ACTIVITY 91 2 STUDY SITE 66 5 DISCUSSION 95 3 MATERIAL AND METHODS 67 6 CONCLUSION 97 3.1 GEOMOPRHIC MAPPING 67 3.2 SAMPLING STRATEGY AND DATING OF PAST ACKNOWLEDGEMENTS 97 EVENTS 67 3.3 RECONSTRUCTION OF PAST CHANNEL ACTIVITY AND SPATIAL PATTERNS OF FORMER EVENTS 69 CHAPTER F: SYNTHESIS 4 RESULTS 69 1 OVERALL DISCUSSION AND CONCLUSIONS 101 4.1 GEOMOPRHIC MAPPING 69 1.1 MAIN RESULTS 101 4.2 AGE STRUCTURE OF THE STAND 69 1.2 COMPARISON OF DEBRIS-FLOW FREQUENCIES 102 4.3 GROWTH DISTURBANCES AND MINIMUM 1.3 INFLUENCE OF CLIMATIC CHANGES ON DEBRIS-FLOW FREQUENCY OF PAST DEBRIS-FLOW EVENTS 70 ACTIVITY 106 4.4 RECONSTRUCTION OF FORMERLY ACTIVE 1.4 LIMITATIONS OF THE METHODS 106 CHANNELS 72 1.5 IMPLICATIONS OF TREE-RING STUDIES FOR HAZARD 4.5 SPATIAL PATTERNS OF FORMER EVENTS 73 ASSESSMENT 107 5 DISCUSSION 76 2 FURTHER RESEARCH SUGGESTED 111 6 CONCLUSION 78 2.1 METHODOLOGY 111 Table of contents - 3 2.2 DEBRIS-FLOW RESEARCH 112 1.4.4 Spatial distribution of trees disturbed during past debris-flow events 125 1.4.5 Spatial distribution of past avalanches CHAPTER G: APPENDICES and identification of source areas 126 1 DIFFERENTIATING EVENTS ON A CONE INFLU- 1.5 DISCUSSION AND CONCLUSION 128 ENCED BY DEBRIS-FLOW AND SNOW AVA- LANCHE ACTIVITY - A DENDROGEOMORPHO- ACKNOWLEDGEMENTS 129 LOGICAL APPROACH 117 2 APPENDIX OF CHAPTER B: TRAUMATIC RESIN ABSTRACT 117 DUCTS IN LARIX DECIDUA STEMS IMPACTED BY DEBRIS FLOWS 131 1.1 INTRODUCTION 118 1.2 STUDY AREA 118 2.1 STUDY SITE 132 1.3 MATERIAL AND METHODS 119 2.2 ILLUSTRATIONS OF THE LARIX DECIDUA MILL. 1.3.1 Geomorphic mapping and sampling of TREES SELECTED FOR ANALYSIS 136 increment cores 119 2.3 ILLUSTRATIONS OF CHARACTERISTIC STEM DISCS 140 1.3.2 Tree-ring analyses 121 1.3.3 Age structure of the stand 121 3 STUDY SITES - ILLUSTRATIONS 143 1.3.4 Growth disturbances in trees and their seasonal timing 121 3.1 REUSE DE SALEINAZ (PRAZ-DE-FORT, 1.3.5 Spatial distribution of trees disturbed ORSIÈRES) 144 during past debris-flow and snow 3.2 TORRENT DE LA FOULY (LA FOULY, ORSIÈRES) 150 avalanche events 122 3.3 BRUCHJI (BLATTEN B. NATERS, NATERS) 156 1.4 RESULTS 122 3.4 GROSSE GRABE (MATTSAND, ST.NIKLAUS) 164 1.4.1 Geomorphic mapping and identification of forms 122 1.4.2 Age structure of the stand 123 BIBLIOGRAPHY 171 1.4.3 Growth disturbances in trees and their CURRICULUM VITAE 181 seasonal timing 124 ***** 4 - Table of contents Abstract - 5 ABSTRACT Debris flows are common mass-movement processes TRD could only be observed in the earlywood cell in most mountainous regions of the world, where layers of the new growth ring. The vertical extension their unpredictable and sudden occurrence represents of TRD averaged 74 cm, but was much greater a major threat to transportation corridors and above rather than below the injury. At the height settlements. Increased anthropogenic activity in of the wound, TRD were present in 18% of the regions exposed to debris-flow risk renders a detailed circumference remaining vital after the impact. In hazard assessment inevitable. However, archival addition, a certain delay in the onset of the reaction data on past events remains scarce and, most of the could be observed with distance from the centre time, fragmentary. Similarly, tree-ring analyses have of the impact. Therefore, increment cores should been used only exceptionally to investigate past be sampled close to the wound in future studies in debris-flow activity. It is therefore the aim of this order to avoid dating mistakes. PhD thesis to reconstruct debris-flow frequencies for different torrents within the Valais Alps (Switzerland) For the reconstruction of past debris-flow events in using dendroecological methods in order to (i) two catchments located in Val Ferret (Valais Alps, contribute to the systematic acquisition of data Switzerland), a total of 556 increment cores from on past events for hazard assessments, and (ii) 278 heavily affected European larches, Norway to reconstruct the spatial extent and behavior of spruces (Picea abies (L.) Karst.) and Scots pines previous events. Also, the extension of tangential (Pinus sylvestris L.) were sampled from the cones rows of resin ducts (TRD) was assessed in trees of Reuse de Saleinaz and Torrent de la Fouly. Tree- injured by debris-flow activity in order to improve ring analyses allowed reconstruction of 39 events for knowledge regarding growth reactions of impacted the period 1743 to 2003 at Reuse de Saleinaz. Along trees. the debris-flow channel of the Torrent de la Fouly, 30 events were reconstructed for between 1862 and In the first paper, 28 injuries from 8 European larches 2003. Although the catchments and channels of (Larix decidua Mill.) wounded during debris-flow the two torrents evince considerable differences in activity in the Feergraben (Simplon region, Valais geology and morphology, debris-flow frequencies Alps) were investigated. The aim of the study was are very similar with, on average, one event every to assess the onset of TRD after wounding, as eight years for the period reconstructed. In both well as their vertical and tangential extensions. catchments, material is apparently readily available Consequently, 182 stem discs were prepared for and the triggering and occurrence of events thus analysis.