UNCORRECTED PROOF Debris flow impact estimation for breakers Dirk Proske a *, Ju¨rgen Suda b and Johannes Hu¨bl b a Department of Civil Engineering and Natural Hazards, University of Natural Resources and Applied Life Sciences, Vienna, Austria; b Institute of Mountain Risk Engineering, University of Natural Resources and Applied Life Sciences, Vienna, Austria (Received 30 April 2009; final version received 13 August 2010) The paper gives an overview about certain steps during the development of an Austrian code of practice for the design of debris flow barriers. In the beginning the authors discuss different debris flow impact pressure models. Results of these models are related to miniaturised and full-scale debris flow impact pressure measurements. In a further step, the consequences of uncertain input data for the different models are investigated by the application of First-Order Reliability Method and the Monte Carlo Simulation. These probabilistic methods are used either to estimate the coefficient of variation of the debris flow impact force or the weighting factors of the single input variables depending on the uncertainty. Additionally, the relevance of process modelling is discussed in short as well as the pressure distribution, the statistical consideration of the debris flow event and the choice of a partial safety factor for the load. Finally, recommendations for applications are given and remaining questions are listed such as impact forces with an angle. Keywords: debris flows; impact force; pressure; barriers; breakers; code calibration; impact pressure; uncertainty 1. Introduction, problem and terms Alpine regions are exposed to several gravitational hazards, including, but not limited to, rock falls, flash floods, landslides, avalanches and debris flows. In this paper, the authors focus on debris flows. Peak discharges of debris flows may be tens of times greater than for floods occurring in the same catch- ment area (cf. Hungr et al. 1984, Pierson 1986), reaching several hundred thousands of cubic metres of sediment which move from a mountain torrent catchment area to a valley area. The front of debris flows can reach velocities up to 30 m/s (cf. Costa 1984, Rickenmann 1999). The debris flow material itself is inherently complex, varying from clay-sized solids to boulders of several metres in diameter; therefore, the density of a debris flow often exceeds that of water by more than a factor of two. To protect humans and human settlements in Alpine regions against such hazards, different mitiga- tion measures are used. These measures can be divided into structural and non-structural mitigation measures. The most important structural measures against debris flows in torrents are debris flow breakers. A debris flow breaker works together with a retention basin (Figure 1). The debris flow enters the retention basin and interacts with the dissipation structure. A part of the debris flow is deposited in the basin. Due to the lower inclination of the basin’s level and the flow resistance of the breaker, the kinetic energy of the process will be reduced. The surplus of solids produced by a debris flow would be stored, and later released if the tail water is deficient of area sediments. These structures are mainly built of structural concrete or structural steel. An introduc- tion to the general design can be found in Kettl (1984), Gotthalmseder (1998) and Bergmeister et al. (2008). Debris flow events occur very irregularly and locally, making the observation and modelling of such events difficult. As such, simplified methods for debris flow impact force estimation were required for design purposes of debris flow breakers. In this paper we discuss certain topics, which were of major importance during the development of an Austrian code of practice for engineers to design such struc- tures against debris flows. The discussion includes some methods and their background as well as uncertainty related. 2. Current knowledge on debris flow impact models 2.1. General A debris flow impact model contains the estimation of the impact force F Mu or maximum impact pressure p max and the distribution of the pressure on the structure. In order to determine these parameters, two different types of debris flow models are necessary 5 10 15 20 25 30 35 40 45 *Corresponding author. Email: dirk.proske@boku.ac.at AQ1 AQ2 AQ3 Georisk Vol. 0000, No. 0, Month 2010, 113 ISSN 1749-9518 print/ISSN 1749-9526 online # 2010 Taylor & Francis DOI: 10.1080/17499518.2010.516227 http://www.informaworld.com C:/3B2WIN/temp files/NGRK516227_S100.3d[x] Tuesday, 31st August 2010 19:33:29