Calibration and validation of rainfall thresholds for shallow landslide forecasting in Sicily, southern Italy.

Abstract Empirical rainfall thresholds are tools to forecast the possible occurrence of rainfall-induced shallow landslides. Accurate prediction of landslide occurrence requires reliable thresholds, which need to be properly validated before their use in operational warning systems. We exploited a catalogue of 200 rainfall conditions that have resulted in at least 223 shallow landslides in Sicily, southern Italy, in the 11-year period 2002–2011, to determine regional event duration–cumulated event rainfall ( ED ) thresholds for shallow landslide occurrence. We computed ED thresholds for different exceedance probability levels and determined the uncertainty associated to the thresholds using a consolidated bootstrap nonparametric technique. We further determined subregional thresholds, and we studied the role of lithology and seasonal periods in the initiation of shallow landslides in Sicily. Next, we validated the regional rainfall thresholds using 29 rainfall conditions that have resulted in 42 shallow landslides in Sicily in 2012. We based the validation on contingency tables, skill scores, and a receiver operating characteristic (ROC) analysis for thresholds at different exceedance probability levels, from 1% to 50%. Validation of rainfall thresholds is hampered by lack of information on landslide occurrence. Therefore, we considered the effects of variations in the contingencies and the skill scores caused by lack of information. Based on the results obtained, we propose a general methodology for the objective identification of a threshold that provides an optimal balance between maximization of correct predictions and minimization of incorrect predictions, including missed and false alarms. We expect that the methodology will increase the reliability of rainfall thresholds, fostering the operational use of validated rainfall thresholds in operational early warning system for regional shallow landslide forecasting.

[1]  J. E. Gartner,et al.  Storm rainfall conditions for floods and debris flows from recently burned areas in southwestern Colorado and southern California , 2008 .

[2]  M. Martina,et al.  Probabilistic rainfall thresholds for landslide occurrence using a Bayesian approach , 2012 .

[3]  N. Caine,et al.  The Rainfall Intensity - Duration Control of Shallow Landslides and Debris Flows , 1980 .

[4]  Fausto Guzzetti,et al.  Rainfall thresholds for the possible occurrence of landslides in Italy , 2010 .

[5]  A. Speranza,et al.  Sensitivity of Precipitation Forecast Skill Scores to Bilinear Interpolation and a Simple Nearest-Neighbor Average Method on High-Resolution Verification Grids , 2003 .

[6]  Pietro Aleotti,et al.  A warning system for rainfall-induced shallow failures , 2004 .

[7]  W. Briggs.  Statistical Methods in the Atmospheric Sciences , 2007 .

[8]  M. Arattano,et al.  Ten years of debris-flow monitoring in the Moscardo Torrent (Italian Alps) , 2002 .

[9]  B. Efron.  Bootstrap Methods: Another Look at the Jackknife , 1979 .

[10]  Fausto Guzzetti,et al.  Lithological and seasonal control on rainfall thresholds for the possible initiation of landslides in central Italy , 2012 .

[11]  Giovanni B. Crosta,et al.  Regionalization of rainfall thresholds: an aid to landslide hazard evaluation , 1998 .

[12]  S. L. Gariano,et al.  An algorithm for the objective reconstruction of rainfall events responsible for landslides , 2015, Landslides.

[13]  G. Pedrozzi.  Triggering of landslides in Canton Ticino (Switzerland) and prediction by the rainfall intensity and duration method , 2004 .

[14]  S. Leroueil,et al.  The Varnes classification of landslide types, an update , 2014, Landslides.

[15]  S. T. Buckland,et al.  An Introduction to the Bootstrap. , 1994 .

[16]  A. K. Turner,et al.  Landslides : investigation and mitigation , 1996 .

[17]  A. Kiureghian,et al.  Aleatory or epistemic? Does it matter? , 2009 .

[18]  S. L. Gariano,et al.  Rainfall thresholds for shallow landslide occurrence in Calabria, southern Italy , 2013 .

[19]  J. Godt,et al.  Early warning of rainfall-induced shallow landslides and debris flows in the USA , 2010 .

[20]  D. Hosmer,et al.  Applied Logistic Regression , 1991 .

[21]  M. Rossi,et al.  Rainfall thresholds for the initiation of landslides in central and southern Europe , 2007 .

[22]  I. Jolliffe,et al.  Forecast verification : a practitioner's guide in atmospheric science , 2011 .

[23]  Samuele Segoni,et al.  Rainfall thresholds for the forecasting of landslide occurrence at regional scale , 2012, Landslides.

[24]  Samuele Segoni,et al.  Landslides triggered by rainfall: A semi-automated procedure to define consistent intensity-duration thresholds , 2014, Comput. Geosci..

[25]  G. Wieczorek,et al.  Effect of rainfall intensity and duration on debris flows in central Santa Cruz Mountains, California , 1987 .

[26]  J. Corominas,et al.  Reconstructing recent landslide activity in relation to rainfall in the Llobregat River basin, Eastern Pyrenees, Spain , 1999 .

[27]  Roberto Giannecchini,et al.  Critical rainfall thresholds for triggering shallow landslides in the Serchio River Valley (Tuscany, Italy). , 2012 .

[28]  Roberto Giannecchini,et al.  Rainfall triggering soil slips in the southern Apuan Alps (Tuscany, Italy) , 2005 .

[29]  Tom Fawcett,et al.  An introduction to ROC analysis , 2006, Pattern Recognit. Lett..

[30]  S. L. Gariano,et al.  Populating a catalogue of rainfall events that triggered shallow landslides in Italy , 2012 .

[31]  David M. Schultz,et al.  CORRIGENDUM: False Alarm Rate or False Alarm Ratio? , 2009 .

[32]  Jason W. Kean,et al.  Objective definition of rainfall intensity–duration thresholds for the initiation of post-fire debris flows in southern California , 2013, Landslides.

[33]  P. Reichenbach,et al.  Regional hydrological thresholds for landslides and floods in the Tiber River Basin (central Italy) , 1998 .

[34]  Michele Brunetti,et al.  SANF: National warning system for rainfall-induced landslides in Italy , 2012 .

[35]  M. Rossi,et al.  The rainfall intensity–duration control of shallow landslides and debris flows: an update , 2008 .