Debris-flow protection in recurrent areas of the Pyrenees. Experience of the VX systems from output results collected in the pioneer monitoring station in Spain

Abstract

The south eastern part of the Pyrenees is currently affected by debris flow phenomena. The combination of Mediterranean and Continental climate, the orog- raphy (up to altitudes of 3300 m), the glacial materi- als on the slope’s surface, the lack of arboreal cover- age compared with the rest of the Pyrenees, the high seismicity and the increasing human occupation in the valleys, put altogether is a very dangerous combination and makes the debris flow management extremely dif- ficult. The first monitoring system for debris flow phe- nomena was installed in Spain in 2005. The aim was to monitor the debris flow phenomenon’s behaviour in the Erill basin, in the north of the province of Lerida. The Erill location in the south of the Pyrenees is one of the places where the debris flow phenomenon is common and where, apart from the possible magnitude of any event, the likely affected area includes the urban area of the Erill village. Remote controlled autonomous moni- toring equipment was installed in this location. It was composed of an automatic meteorological station, a set of geophones to activate the measuring and recording systems, a VX160 barrier protection system, monitored with load-cells, a camera and digital recording equip- ment, all connected to a data logger with a GSM mo- dem. The information provided by the detection system was completed during 2009 by a topographic schema, created by the LIDAR system, of the basin that gener- ates the debris flow. The objective was to try to detect, before the event, the specific deformations that cause the debris flow. We have collected information related with different events of small volume (< 100m3) that had a direct correlation with the intensive rainfall in the basin. During 2009, based on the knowledge gained from the behaviour of the Erill basin, the first applica- tions and designs of VX160 systems were created for other sites. The work done in the Portainé gully is an example of this. In 2008, this zone was affected by a debris flow of more than 20.000 m3. The protection sys- tem that was installed was made up of 9 VX160 trans- versal protection lines, with a total retention capacity of approx. 25.000 m3. Three months after they were installed, the fences were completely full as a result of two events caused by summer storms. The installed solution costs 40% less than the traditional check dam solution. This paper shows the importance of the re- search, of these specific phenomena in the Pyrenees, to the development of protection technology. The autono- mous measurement equipment, together with the tested protection system can be applied, with the correspond- ing reduction in costs, to civil protection and hydrologi- cal correction situations in urban and suburban zones where debris flow is a recurring phenomenon.