1. A FMCW-Doppler-Radar detects triggered
debris flows in the upstream of Spreitgraben
by measuring movement velocities.0 2 4 6 8 10
0
10
20
30
40
Velocity (m/s)
Intensity(a.u.)
Event 02.12.2009 08:08
300m
330m
360m
(a)
1
GEOTEST AG, Birkenstrasse 15, Zollikofen, Switzerland, +41 31 910 01 01, email: isabelle.kull@geotest.ch
1
GEOTEST AG, Birkenstrasse 15, Zollikofen, Switzerland, +41 31 910 01 01, email: daniel.tobler@geotest.ch
2
Oberingenieurkreis I, Schlossberg 20, 3601 Thun, Switzerland, +41 33 225 10 60, email: nils.haehlen@bve.be.ch
Guttannen, Bernese Oberland, Switzerland
I. 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
INTRODUCTION
Since 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.
EVENTS
Small 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 with
the 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 Modelling
deposition height [m]
2009 plus 500’000 m3
DEPOSITION AREA / DAMAGE POTENTIAL
Within 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 WARNING
Due 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 - 2011
height [m]
Legend
international gas pipeline
main road
electric power supply
abandoned houses
residential areas
wastewater treatment plant
affected 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 - 2011
height [m]
Legend
international gas pipeline
main road
electric power supply
abandoned houses
residential areas
wastewater treatment facility
affected 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
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, Zollikofen
Tobler, 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, Thun
Hählen, N. 2012b: Murgänge Spreitgraben und Rotlaui – Folgen für Infrastrukturanlagen. presentation document OcCC Workshop, 21th
September 2012