Development of a Debris-Flow Monitoring System in the Réal Torrent, Southern French Alps O. Navratil (1), F. Liébault (1), E. Travaglini (1), H. Bellot (1), J. Theule (1), X. Ravanat (1), F. Ousset (1), D. Laigle (1), V. Segel (2), M. Fiquet (3)
The general aim of this study is to improve our understanding of (1) sediment responses during flash floods and debris flows in small Alpine torrents, (2) their propagation and (3) their interaction with the torrent channel morphology.
Rainfall Triggering Conditions During Spring/Summer 2011
Monitoring Sites and Topographical Surveys Three debris-flow monitoring stations were developed in 2011 in the Réal Torrent (2.4 km²; 1218-2069 m asl). These stations are based on a combination of different techniques involving rain gauges, geophones, US/radar elevation sensors and imagery acquisition. Intensive field surveys (i.e. Lidar, cross-sections) were also implemented to capture the morphological responses of the main channel and to study channel erosion by debris-flows and floods with bed load.
O
Rainfal events Floods with bed load transport Debris-Flow occurrence at Réal_S1 Debris-Flow occurrence at Réal_S1 and Réal_S2 Debris-Flow propagation from Réal_S1 to Réal_S3
Volume = 4 400 m3
Volume = 6 100
Monitoring stations
Volume (m3)
Debris Flows Interaction with the Torrent Channel Morphology
15 Ib > 13 mm/hr
10 5 0 0
50 100 Maximum intensity (mm/hr)
Topographical surveys Station volumes: May-June debris flows Topographical survey Station volumes: 29 June Debris-flow Topographical surveys Station volume: 17 September debris-flow
10000 Réal_S3 DA = 2.0km² S = 9.5%
20
vsurge = 2.7 m/s
12000
Upstream station: Réal_S1
25
vsurge = 3.5 m/s
m3
Réal_S2 DA = 1.7km² S = 12.3%
30
29, June 2011
vsurge = 4.2 m/s
Volume = 8 600 m3
Réal_S1 DA = 1.3km² S = 19.5%
35 Imax>26mm/hr Db>20min
Research Aims
Mean Surge Intensity (mm/hr)
(1) Cemagref-IRSTEA, ETNA; (2) RTM-ONF; (3) Conseil Général des Alpes Maritimes (06); Contacts:
[email protected];
[email protected]; Web site: http://www.irstea.fr/navratil
Large Debris-Flow Propagation on 29 June, 2011
Channel Erosion Channel Deposition
8000 6000 4000 2000 0 0
500 1000 1500 2000 Distance From Apex of Torrent Valley (m)
Conclusions Background figure: signals of the three geophones and the US sensor. 29, June debris flow at Réal_S3
During this first monitoring period, we put into evidence the significant sediment production from hillslope sources in the Réal Torrent. The first observations confirmed that rainfalls are the main debris-flows triggering factor. Significant interactions between debris flows and the channel morphology (erosion/deposition) were also measured. These field observations outline the important role of antecedent conditions for the propagation of debris-flows in the Réal Torrent.
