11- years of temporal evolution of the ‘Mare à Poule d’Eau’ landslide on La Reunion island using optical SPOT5 images correlation.

Friday, 18 December 2015
Poster Hall (Moscone South)
Rejanne LE Bivic1, Christophe Delacourt1, Pascal Allemand2, André Stumpf3, Amélie Quiquerez2, Bertrand Aunay4, Laurent Michon5 and Nicolas Villeneuve6, (1)IUEM Institut Universitaire Européen de la Mer, Plouzané, France, (2)Université Lyon 1, Lyon, France, (3)University of Strasbourg, Strasbourg Cedex, France, (4)BRGM, Saint-Denis-de- la Réunion, France, (5)Université de La Réunion, Laboratoire Géosciences-IPGP, 97744 Saint Denis, France, (6)Institut de Physique du Globe de Paris, Paris, France
In order to measure and understand the relationship between water and landslides in tropical environment we studied the correlation between annual rainfall and ‘Mare à Poule d’Eau‘landslide located in the Salazie erosion basin in the Island of La Reunion. This island is characterized by annual cumulative rainfall locally exceeding 7m. Among the three major techniques used to measure landslide displacement (GNSS measurements, InSAR techniques and image correlation), we chose correlation of SPOT5 images acquired between 2002 and 2014 which gives a synoptic view of landslide displacements on a period of several months to years combined with GNSS measurements. The combination of these two techniques avoids the smoothing of the temporal signal produced by image correlation only and preserves a synoptic view of the landslide displacement. We used 10 panchromatic SPOT 5 images acquired with similar incidence angles and ortho-rectified by CNES (French spatial agency) at a spatial resolution of 2.5m. The dates of these images are 02/2002, 07/2003, 05/2004, 06/2005, 06/2006, 05/2008, 02/2010, 08/2012, 01/2014, 05/2014. The correlation, realized with a precision of 0.1pixel, was realized using MicMac open source software (IGN–French geographical institute). Every pixel was correlated with a correlation window of 9 px. The GNSS data were acquired by the BRGM since 2003 at a daily period. The rainfall data have been acquired by Meteo France on 7 stations located among the Salazie erosion basin. The first results of this study shows that the maximum displacement of the landslide measured by image correlation is 1.70 m.y-1 and also that there is a large spatial variability among the landslide. These values match well with GNSS data. There is not a good correlation between averaged yearly rainfall values and landslide displacement; however there is a significant correlation between intense meteorological events and displacement. In 2007, the tropical cyclone GAMEDE (02/2007) produced intense rainfalls. The velocity of the landslide was the largest of this study (1.70m.y-1). When the rainfall was less intense (between 10/2002 and 07/2003, when no intense event happened) the maximum velocity decreased to 0.50m.y-1. This dataset shows a good correlation between punctual intense rainfalls and landslide displacement.