On the dynamics of debris flows : case study Fjærland, Western Norway - a debris flow triggered by a natural dam breach

• Main Author: Breien, Hedda
• Other Authors: Anders Elverhøi og Kaare Høeg
• Format: Master Thesis
• Language: English
• Published: 2005
• Subjects: geologi geohazard skreddynamikk jordskred flomskred; VDP::450; Bingham; Glacial Lake; Moraine Ridge; Norway; glacier
• Online Access: http://hdl.handle.net/10852/12374; http://urn.nb.no/URN:NBN:no-10804

Abstract Debris flows represent a major threat to human life and property. Due to their ability of entraining material and reaching long run-outs, they have a potential of causing massive damage. There are many studies on debris flow dynamics. Still, the topic reveals a number of challenges in understanding the forces involved and in representing them numerically. In this respect the recent debris flow in Fjærland, Western Norway, is looked upon as a unique full scale experiment with its 1000 m height drop and 3000 m long run-out. The Fjærland case includes the breach of a moraine ridge damming a glacial lake, the additional water coming from the glacier, and its resulting flood. The study of this case illustrates how a water flood can evolve into a full debris flow through bulking. Even though the event started out as a flood of water, the study has revealed that the deposited materials and the degree of erosion also result from a significant grain-to-grain contact. It is seen that treating such events as floods is not appropriate where the height drop is large and the bed material erodible. The primary objective of the thesis has been to provide a thorough documentation and description of the event. Through this work, several eyewitnesses have been interviewed, and detailed pre- and post flow terrain models have been developed through laser scanning and photogrammetry in the purpose of estimating the volume involved in the debris flow. The same terrain data have been employed in a numerical model (BING), trying to simulate the dynamics of the flow. The model uses a Bingham rheology, but is modified to include Coulomb friction and entrainment by Dr. Fabio DeBlasio of ICG. The expanded BING-model predicts a run-out, erosion depth and a volume encouragingly similar to what is seen in nature. However, many physical simplifications have had to be introduced, and challenges for further studies are several. The study reveals the necessity for a dynamical model which, depending on the contents and properties of the material involved, includes both viscous, plastic and frictional forces, allows forces and properties to vary in time and space, as well as taking material entrainment into account. A most interesting topic for further study is the factor of entrainment. Until now, this phenomenon is not fully understood. The thesis tries to illustrate the phenomenon with the recently accumulated data. One of the findings of the thesis is the recognition of a feedback mechanism, where volume growth increases the entrainment of bed material, which again increases the volume of the flow. For erosion, the volume of the mass flow seems more important than slope angle, at least in slopes steeper than a certain value. This was also recognised in the numerical modelling, shown by an exponential increase in volume. The ability of volume growth can be the explanation for debris flows with very far-reaching run-outs. There is historical evidence that similar debris flow events have happened in Fjærland twice during the last century. An event like the one that occurred 8 May 2004 is also likely to happen again - in Fjærland as well as any other place where large volumes of water are released at high altitude. This study is therefore important also in the evaluation of hazards related to other glaciers, lakes and dams as well as to landslide-triggered debris flows.