debris flow assessment, monitoring and early warning in the spreitgraben
TRANSCRIPT
A FMCW-Doppler-Radar detects triggered debris flows in the upstream of Spreitgraben by measuring movement velocities. 0 2 4 6 8 10
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Velocity (m/s)
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Event 02.12.2009 08:08
300m330m360m
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1 GEOTEST AG, Birkenstrasse 15, Zollikofen, Switzerland, +41 31 910 01 01, email: [email protected] GEOTEST AG, Birkenstrasse 15, Zollikofen, Switzerland, +41 31 910 01 01, email: [email protected] 2 Oberingenieurkreis I, Schlossberg 20, 3601 Thun, Switzerland, +41 33 225 10 60, email: [email protected]
Guttannen, Bernese Oberland, SwitzerlandI. Kull1, D. Tobler1, N. Hählen2
Scoured and collapsed snowpack due to debris flows (2010, left / 2011, middle). Evolution of the channel cross-section on the cone (right).
Large debris flow surge (July 2010, left) and the occured scour underneath the overflow section of the pass road (avalanche gallery, right).
Location of the torrent system Spreitgraben between the Ritzlihorn (3,263 m AMSL) and the Aare river (910 m AMSL). (Google Earth)
Rockfall Ritzlihorn
Cone erosion
Deposition in river
Location
Ritzlihorn
Guttannen
INTRODUCTIONSince 2009 several extremely erosive debris flows occurred in the Spreitgraben near Guttannen (central Switzerland). Initially triggered by rockfall events, harmless flows were observed first. Within three years they became huge, destructive events with an enormous hazard potential. Strong erosion along the debris flow channel caused a total amount of 650,000 m3 bedload depositions in the receiving stream Aare.
EVENTSSmall debris flows were triggered in rockfall depositions, transported bedload underneath the snowpack and blocked the drainage channels under the snowpack. The release of the dammed-up water produced very large debris flow surges peaking at 500 - 600 m3/s. From 2009 to 2011 the carried bedload increased from 100,000 m3/year to nearly 300,000 m3/year. Within this time periode the channel erosion reached 15 - 20 m. The analyses of the events show that - apart from the less relevant precipitation intensity - the most important parameters for triggering debris flows were progression of air temperature and antecedent precipitation.
References:
Debris Flow Assessment, Monitoring and Early Warning in the Spreitgraben
Bedload deposition in the Aare river 2009 and 2011 illustrated in an orthophoto (top) and in a profile (below). Debris flow modelliong withthe simulation program RAMMS, WSL (forecast 2009 plus 500,000 m3 simulation, right).
Webcams
Interferometric Radar
Red Lights
Pluviometer and Temperature Measurements
Trigger Line
Gauge Radar
Geophone
Increasing damage potential affected by the debris flows depositions and erosions in the receiving stream Aare.
The comprehensive, reliable, and redundant monitoring, early-warning and alarm system in the Spreitgraben.
FMCW-Doppler-Radar
Two Laser scanners measure the dimension and velocity of debris flow surges in the channel (100 measurments / sec).
Laser Scanner
2009 2011
RAMMS Modellingdeposition height [m]2009 plus 500’000 m3
DEPOSITION AREA / DAMAGE POTENTIALWithin only three years the total amount of bedload deposition in the receiving water Aare has reached 650,000 m3 and the depositioin height locally attained more than 20 m. Scenario-based debris flows have been simulated for the near future with the program RAMMS (WSL) in order to estimate the progression and the extension of depositions and the affected area.
MONITORING / EARLY WARNINGDue to the enormous damage potential a comprehensive, high-tech monitoring, early warning and alarm system has been established for road traffic and the building sites in the lower reaches. The system has to satisfy tough requirements: reliability, redundancy, low-maintenance, and maximum triggering safety. A combined record of events by trigger lines, gauge radars and geophones activates the alarm. A profound knowledge of the on-going processes is the precondition for a reliable alarm system. Therefore webcams, precipitation, temperature and further data enable an online monitoring of the events via the internet. In addition a interferometric radar detects the rockfall activity of the Ritzlihorn.
Damage Potential
Deposition 2009 - 2011height [m]
Legend
international gas pipeline
main road
electric power supply
abandoned houses
residential areas
wastewater treatment plant
a�ected area
< 5.0
5.0 - 7.5
7.5 - 10.0
10.0 - 12.5
12.5 - 15.0
15.0 - 17.5
17.5 - 20.0
20.0 - 22.5
Damage Potential
Deposition 2009 - 2011height [m]
Legend
international gas pipeline
main roadelectric power supply
abandoned houses
residential areas
wastewater treatment facility
a�ected area
< 5.05.0 - 7.5
7.5 - 10.010.0 - 12.512.5 - 15.0
15.0 - 17.517.5 - 20.0
20.0 - 22.5
Important infrastructures are increasingly affected: the main road, the international gas pipeline between Germany and Italy, and numerous inhabited houses. The gas pipeline was partially re-laid on the opposite side of the valley. Two houses close to the Aare depositions have been abandoned and eliminated in 2011. Assuming debris flow events of similar magnitude to those in 2011, the expected damage potential additionally extends to certain parts of the settlement (Flesch, Leen, Boden) and to the wastewater treatment facility. The main road is endangered by erosion in several places, and there might be additional locations affecting the gas pipeline.
Currently new high-tech instruments are tested in the Spreitgraben. A radar is intended to detecte debris flows in the upper part of the cone in order to extend the alert time. Furthermore, laser scanners have been installed to measure the dimesion and velocity of debris flow surges.
GEOTEST AG 2012 / 2010: Guttannen, Murgang Spreitloui – Analyse und Prognose. Bericht Nr. 1409237.3 and Nr. 09237.8, ZollikofenTobler, D., Kull, I., Hählen, N. 2012: Hazard management in a Debris Flow Affected Area – Spreitgraben. Extended Abstract, Global Risk Forum, Davos. Hählen, N. 2012a: Murgänge Spreitgraben 2011 – Ereignisse und Erkenntnisse, unpublished. Oberingenieurkreis I, ThunHählen, N. 2012b: Murgänge Spreitgraben und Rotlaui – Folgen für Infrastrukturanlagen. presentation document OcCC Workshop, 21th September 2012