Suchergebnisse

In 2005 the settlement of Matatā on Awatarariki fan, Bay of Plenty, North Island, New Zealand was devastated by a >3 x 105m3 debris flow; several dwellings were destroyed and many damaged, but no fatalities occurred. In the 7 years following the event, design options for a debris-flow containment structure in the catchment were developed. Following a formal determination by the Government's building control agency in 2006, building consents were granted for a number of replacement dwellings on the fan. In 2012, the previously chosen containment structure project was cancelled due to effectiveness and cost concerns. Subsequent investigations confirmed there were no viable engineering solutions to manage debris-flow risk from this catchment, and risk analyses have demonstrated that no debris-flow management systems, warning and evacuation systems, or individual dwelling protection mitigation measures, independently or in combination, could deliver a residential environment with tolerable risk-to-life levels. Since 2013, Whakatāne District Council (the Council) has been working towards a non-legislated managed voluntary retreat from the area where the risk to life is greater than about 10-4a-1, which is also the area of boulder deposition in the 2005 event. This has involved many meetings with affected landowners, including legal counsel and experts, but the currently 13-year delay in resolving uncertainty about landowners' futures has generated considerable stress and even hostility. A parallel legislation-based workstream the Council has undertaken to fulfil its statutory responsibilities has exacerbated tension between the Council and some property owners. From a technical perspective, this study emphasises the danger of lay officials and consultants placing too much confidence in immature technologies to reliably modify debris-flow occurrence. From a public management perspective, it highlights the immaturity of New Zealand's natural hazard management policy framework, in particular the significant disconnect between policy intent and policy implementation and its polarising effects on a small provincial community.

Abstract. Many debris-flow catchments pose an underappreciated hazard, especially where there are dwellings on debris-flow fans and other depositional areas. There is a need to make communities and those involved in community governance aware of situations where there may be a credible risk to life from debris flows. This needs to be simple and cheap to do, since funding is often not available to study unrecognised natural hazards. Here, we use published models to (1) estimate the threshold annual recurrence interval (ARI) for debris flows in a catchment, below which there is an unacceptable annual risk to life for the occupants of any dwellings, and (2) identify the "window of non-recognition" where debris flows are sufficiently infrequent within a catchment that it is not recognised as susceptible yet frequent enough that the risk to life exceeds the acceptability threshold. Using four Aotearoa New Zealand (NZ) studies, we estimate a 95 % credible interval range for the ARIs of life-threatening debris flows of between 100 and 500 years. We show that given these credible intervals and precautionary but realistic assumptions about debris-flow behaviour and the vulnerability of dwellings and their occupants, catchments with no history of debris-flow activity can pose an unrecognised and unacceptable annual risk to life (P=0.256 that the annual risk-to-life threshold of 1 in 1000 is exceeded).

Abstract. In quantitative risk assessment, risk is expressed as a function of the hazard, the elements at risk and the vulnerability. From a natural sciences perspective, vulnerability is defined as the expected degree of loss for an element at risk as a consequence of a certain event. The resulting value is dependent on the impacting process intensity and the susceptibility of the elements at risk, and ranges from 0 (no damage) to 1 (complete destruction). With respect to debris flows, the concept of vulnerability – though widely acknowledged – did not result in any sound quantitative relationship between process intensities and vulnerability values so far, even if considerable loss occurred during recent years. To close this gap and establish this relationship, data from a well-documented debris flow event in the Austrian Alps was used to derive a quantitative vulnerability function applicable to buildings located on the fan of the torrent. The results suggest a second order polynomial function to fit best to the observed damage pattern. Vulnerability is highly dependent on the construction material used for exposed elements at risk. The buildings studied within the test site were constructed by using brick masonry and concrete, a typical design in post-1950s building craft in alpine countries. Consequently, the presented intensity-vulnerability relationship is applicable to this construction type within European mountains. However, a wider application of the presented method to additional test sites would allow for further improvement of the results and would support an enhanced standardisation of the vulnerability function.

Introduction -- Debris-flow Watersheds and Fans: Morphology, Sedimentology and Dynamics -- Fatalities from Debris Flows: Worldwide Distribution and Trends -- Flume Modeling of Debris Flows.-Numerical Modeling of Debris Flows: A Conceptual Assessment -- Sediment Entrainment and Deposition -- Causes and Triggers -- Debris-flow Dating and Magnitude Reconstruction -- Statistical Techniques for Debris-flow Frequency-Magnitude Analyses -- Climate Change Effects on Debris Flows -- Post-wildfire Debris Flows.-Lahars: Origins, Behavior and Hazards -- Regional Debris-flow Hazard Assessments -- Debris-flow Risk Assessment -- Functional Design of Mitigation Measures: From Design Event Definition to Targeted Process Modifications -- Advances in Design of Barriers for Debris-flow Impact -- Check Dam Failures -- Warning Systems and Instrumentation -- Land Use Planning on Debris-flow Fans.

Abstract. The assessment of flow velocity has a central role in quantitative analysis of debris flows, both for the characterization of the phenomenology of these processes and for the assessment of related hazards. Large-scale particle image velocimetry (LSPIV) can contribute to the assessment of surface velocity of debris flows, provided that the specific features of these processes (e.g. fast stage variations and particles up to boulder size on the flow surface) are taken into account. Three debris-flow events, each of them consisting of several surges featuring different sediment concentrations, flow stages, and velocities, have been analysed at the inlet of a sediment trap in a stream in the eastern Italian Alps (Gadria Creek). Free software has been employed for preliminary treatment (orthorectification and format conversion) of video-recorded images as well as for LSPIV application. Results show that LSPIV velocities are consistent with manual measurements of the orthorectified imagery and with front velocity measured from the hydrographs in a channel recorded approximately 70 m upstream of the sediment trap. Horizontal turbulence, computed as the standard deviation of the flow directions at a given cross section for a given surge, proved to be correlated with surface velocity and with visually estimated sediment concentration. The study demonstrates the effectiveness of LSPIV in the assessment of surface velocity of debris flows and permit the most crucial aspects to be identified in order to improve the accuracy of debris-flow velocity measurements.

Abstract. The present study describes entrainment characteristics of bed material into debris flow, based on flume tests, numerical and dimensional analyses. Flume tests are conducted to investigate influences of bed sediment size on erosion rate by supplying debris flows having unsaturated sediment concentration over erodible beds. Experimental results show that the erosion rate decreases monotonically with increase of sediment size, although erosion rate changes with sediment concentration of debris flow body. In order to evaluate critical condition of bed sediment entrainment, a length scale which measures an effective bed shear stress is introduced. The effective bed shear stress is defined as total shear stress minus yield stress on the bed surface. The results show that critical entrainment conditions can be evaluated well in terms of Shields curve using the effective bed shear stress instead of a usual bed shear stress.

Dense urban development on a hilly terrain, coupled with intense seasonal rainfall and heterogeneous weathered profiles, gives rise to acute debris flow problems in Hong Kong. The Geotechnical Engineering Office (GEO) of the Hong Kong SAR Government has launched a holistic R&D programme and collaborated with various tertiary institutes and professional bodies to support the development of a comprehensive technical framework for managing landslide risk and designing debris flow mitigation measures. The scope of the technical development work includes compilation of landslide inventories, field studies of debris flows, development and calibration of tools for landslide runout modelling, back analysis of notable debris flows, physical and numerical modelling of the interaction of debris flow and mitigation measures, formulation of a technical framework for evaluating debris flow hazards, and development of pragmatic mitigation strategies and design methodologies for debris flow countermeasures. The work has advanced the technical understanding of debris flow hazards and transformed the natural terrain landslide risk management practice in Hong Kong. New analytical tools and improved design methodologies are being applied in routine geotechnical engineering practice.

Abstract. Quantitative risk assessments of debris flows and other hydrogeological hazards require the analyst to predict damage potentials. A common way to do so is by use of proportional loss functions. In this paper, we analyze a uniquely rich dataset of 132 buildings that were damaged in one of five large debris flow events in Switzerland. Using the double generalized linear model, we estimate proportional loss functions that may be used for various prediction purposes including hazard mapping, landscape planning, and insurance pricing. Unlike earlier analyses, we control for confounding effects of building characteristics, site specifics, and process intensities as well as for overdispersion in the data. Our results suggest that process intensity parameters are the most meaningful predictors of proportional loss sizes. Cross-validation tests suggest that the mean absolute prediction errors of our models are in the range of 11%, underpinning the accurateness of the approach.

Abstract. Reliable forecasts of building damage due to debris flows may provide situational awareness and guide land and emergency management decisions. Application of debris-flow runout models to generate such forecasts requires combining hazard intensity predictions with fragility functions that link hazard intensity with building damage. In this study, we evaluated the performance of building damage forecasts for the 9 January 2018 Montecito postfire debris-flow runout event, in which over 500 buildings were damaged. We constructed forecasts using either peak debris-flow depth or momentum flux as the hazard intensity measure and applied each approach using three debris-flow runout models (RAMMS, FLO-2D, and D-Claw). Generated forecasts were based on averaging multiple simulations that sampled a range of debris-flow volume and mobility, reflecting typical sources and magnitude of pre-event uncertainty. We found that only forecasts made with momentum flux and the D-Claw model could correctly predict the observed number of damaged buildings and the spatial patterns of building damage. However, the best forecast only predicted 50 % of the observed damaged buildings correctly and had coherent spatial patterns of incorrectly predicted building damage (i.e., false positives and false negatives). These results indicate that forecasts made at the building level reliably reflect the spatial pattern of damage but do not support interpretation at the individual building level. We found the event size strongly influences the number of damaged buildings and the spatial pattern of debris-flow depth and velocity. Consequently, future research on the link between precipitation and the volume of sediment mobilized may have the greatest effect on reducing uncertainty in building damage forecasts. Finally, because we found that both depth and velocity are needed to predict building damage, comparing debris-flow models against spatially distributed observations of building damage is a more stringent test for model fidelity than comparison against the extent of debris-flow runout.