1) Debris flows occur when high intensity rainfall generates abundant runoff that entrains large quantities of debris material in channels, forming solid-liquid flows. 2) Debris flows can damage infrastructure and be deadly, as evidenced by photographs of destroyed villages, flooded roads, and debris deposits from past events. 3) Proper modeling of debris flows requires considering the hydrologic, triggering, and hydraulic processes involved - from runoff generation to downstream routing of the solid-liquid mass.

1. Carlo Gregoretti23/5/2017 Mekelle University - Debris Flows
Debris Flow: the phenomenon

2. Carlo Gregoretti23/5/2017 Mekelle University - Debris Flows
Debris flow is a phenomenon occurring when a
water stream entrains a large quantity of debris
material, so that a solid-liquid current can form
and route downstream
It is halfway between a landslide and a flood
French: Lave torrentielle
German: Murgang or Mure
Italian: Colata detritica

3. Carlo Gregoretti23/5/2017 Mekelle University - Debris Flows
Landslide

4. Carlo Gregoretti23/5/2017 Mekelle University - Debris Flows
Afghanistan, 2007
Debris flow and flood

5. Carlo Gregoretti23/5/2017 Mekelle University - Debris Flows
Rio Acquabona (Cortina d’Ampezzo, km98 SS51, 12 June 1997)
Debris flow–basin head area - 1

6. Carlo Gregoretti23/5/2017 Mekelle University - Debris Flows
Rovina di Cancia (Borca di Cadore, km80 SS51, 23 July 2015)
Debris flow–basin head area - 2

7. Carlo Gregoretti23/5/2017 Mekelle University - Debris Flows
Rovina di Cancia (Borca di Cadore, km80 SS51, 4 August 2015)
Debris flow–basin head area - 3

8. Carlo Gregoretti23/5/2017 Mekelle University - Debris Flows
Debris flow–basin head area - 4
Punta Nera (Cortina d’Ampezzo, km97 SS51, 21 July 2016)

9. Carlo Gregoretti23/5/2017 Mekelle University - Debris Flows
Debris flow–basin head area - 5
Punta Nera (Cortina d’Ampezzo, km97 SS51, 5 August 2016)

10. Carlo Gregoretti23/5/2017 Mekelle University - Debris Flows
Mount Yakedake (Japan, 1994)
Debris flow–intermediate part of the
basin -1

11. Carlo Gregoretti23/5/2017 Mekelle University - Debris Flows
Illgraben (Svizzera), 28 June 2000
Debris flow–intermediate part of the
basin - 2

12. Carlo Gregoretti23/5/2017 Mekelle University - Debris Flows
Monitoring station in the Gadria basin (Prov. of Bolzano), 18 July 2013
Debris flow–intermediate part of the basin -
3

13. Carlo Gregoretti23/5/2017 Mekelle University - Debris Flows
Debris flow–intermediate part of the
basin - 3
Monitoring station in the Gadria basin (Prov. of Bolzano), 14 July 2014

14. Carlo Gregoretti23/5/2017 Mekelle University - Debris Flows
https://youtu.be/0ENe7wDKP6I
Debris flow–intermediate part of the
basin - 4
Illgraben 22 July 2016

15. Carlo Gregoretti23/5/2017 Mekelle University - Debris Flows
Debris flow–intermediate part of the
basin - 5 Aconguaga (Chile) 26 December 2015

16. Carlo Gregoretti23/5/2017 Mekelle University - Debris Flows
Debris flow–intermediate part of the
basin - 6 (Chile) 2017
https://youtu.be/8kdIsQxXUss

17. Carlo Gregoretti23/5/2017 Mekelle University - Debris Flows
Strengen (Arlberg), 13 July 2010
Debris flow- lower part of the basin -
1

18. Carlo Gregoretti23/5/2017 Mekelle University - Debris Flows
Debris flow- lower part of the basin -
2 Taibon Agordino (BL) , 11 January 2016

19. Carlo Gregoretti23/5/2017 Mekelle University - Debris Flows
Debris flow- lower part of the basin -
3 Taibon Agordino (BL) , 11 January 2016

20. Carlo Gregoretti23/5/2017 Mekelle University - Debris Flows
Ortles, 1 August 2010 (Geosciencie Dept. - Pauda University video)
Debris flow – glacial till area

21. Carlo Gregoretti23/5/2017 Mekelle University - Debris Flows
Occurrence of debris flows? Where, how and when
they initiate?
Runoff descending from channels incised on rocky cliffs
entrain debris material laying at their feet, initiating a solid-
liquid wave.

22. Carlo Gregoretti23/5/2017 Mekelle University - Debris Flows
Runoff descending from the channel incised on cliffs impact
sediments laying at their feet generating a solid-liquid flow
(images of a channel incised on the Mount Cristallo walls).

23. Carlo Gregoretti23/5/2017 Mekelle University - Debris Flows
Debris flow occurred the 12th of
June 1997 at Acquabona
Two examples of runoff descending along rocky cliffs filmed
at Acquabona (left) and Dimai (right) in the area of Cortina
d’Ampezzo that initiated debris flows.
Debris flow occurred the 18th of
August 2011 at Dimai
Runoff descending along rocky cliffs - 1

24. Carlo Gregoretti23/5/2017 Mekelle University - Debris Flows
On the fan at the base of channels incised on the rocky cliffs, runoff
action dig channel. On the bottom of these channel, debris deposits due to
previous erosive events (rock falls, bank failures, sediment transport) are
entrained by runoff corresponding to high intensity rainfalls
9 mm in 15 minutes (7/11/2015)
No debris flow occurrence
18 mm in 10 minutes (8/18/2011)
Debris flow occurrence
Fiames (Cortina d’Ampezzo) at the base of Dimai peak
Runoff at the base of rocky cliffs - 1

25. Carlo Gregoretti23/5/2017 Mekelle University - Debris Flows
Debris flow initiation
Entrainment “en
masse” of sediments
of the deposits laying
on the channel
bottom into a water
stream

26. Carlo Gregoretti23/5/2017 Mekelle University - Debris Flows
Schematic 2D view of a debris flow
event occurrence and development
Deposition
Debris flows commonly occur
in channels draining small
steep rock basins located in
the upper part of the slopes
where high intensity, short
duration rainfalls, rapidly
generate high runoff
discharges , that are able to
entrain large debris material
laying at the bottom of
channel, forming a solid-liquid
wave
RUNOFF
Firehose effect
debris flow triggering
Formation of an immature
debris flow
Debris flow increases its velocity
and erodes sediment s laying on
the bed
Formation of a mature debris flow
with sediment sorting along the
profile
Debris flow slows and
sediment deposition
starts
Rock
cliffs
Scree
sediment

27. Carlo Gregoretti23/5/2017 Mekelle University - Debris Flows
Schematic 3D view of a debris flow
event occurrence and development
Initiation
area
Transport
area
deposition
area
Debris
material
availability
Very high
slopes
Abundant
runoff

28. Carlo Gregoretti23/5/2017 Mekelle University - Debris Flows
In brief, a debris flow results from three different physical
processes
1. Abundant runoff production after high intensity
precipitation
2. Entrainment of debris material laying on the bottom of
channels with formation of a solid-liquid wave
3. Downstream routing of the solid-liquid wave
We need three different models for debris flow simulations:
1. Hydrologic model (Rainfall-Runoff) for rocky headwater
basins
2. Triggering model for determining the solid-liquid
hydrograph
3. Hydraulic model for downstream routing
Physical processes associated to debris flow

29. Carlo Gregoretti23/5/2017 Mekelle University - Debris Flows
The effect of a debris flow - 1
Val Pola

30. Carlo Gregoretti23/5/2017 Mekelle University - Debris Flows
Chieppena torrent (Trentino)
November 1966
Venzo e Largaiolli (1968)
The effect of a debris flow - 2

31. Carlo Gregoretti23/5/2017 Mekelle University - Debris Flows
The village of Chiapuzza (San Vito di Cadore, BL) after
the debris flow of the 4th of November 1966
The effect of a debris flow - 3

32. Carlo Gregoretti23/5/2017 Mekelle University - Debris Flows
The effect of a debris flow - 4
Debris flow deposits
after the event of 7
August 1996 in Borca
di Cadore (Cancia
debris flow)

33. Carlo Gregoretti23/5/2017 Mekelle University - Debris Flows
The effect of a debris flow - 5
Val Pola
Nusdorf Debant, Drava Valley, Lienz, Austria - After
the debris flow occurred on August 1997 (Fuchs et al., 2007)

34. Carlo Gregoretti23/5/2017 Mekelle University - Debris Flows
The effect of a debris flow - 6
Val Pola
Results from the spilling of mud water from the retaining
basin upstream Borca di Cadore (BL) the 18th of July 2009
Retaining wall

35. Carlo Gregoretti23/5/2017 Mekelle University - Debris Flows
The effect of a debris flow - 7
Val Pola
Hamlet of Campolongo di Baselga di Pinè (TN) after the debris
flow occurred the 15th of August 2010

36. Carlo Gregoretti23/5/2017 Mekelle University - Debris Flows
The effect of a debris flow - 8
Val Pola
The National Road SS. 51 (km 97.500) between Cortina
d’Ampezzo and San Vito di Cadore (BL) flooded by the debris
flow occurred the 8th of August 2015.

37. Carlo Gregoretti23/5/2017 Mekelle University - Debris Flows
The effect of a debris flow - 7
Val PolaBayi watershed (Longxi river), Wenchuan , China; after the
big earthquake of 2008 intese rainfall triggered several
debris flows the 13-14th of August 2010

38. Carlo Gregoretti23/5/2017 Mekelle University - Debris Flows
The effect of a debris flow - 8
Val Pola
Bayi watershed (Longxi river), Wenchuan , China; after the
big earthquake of 2008 intese rainfall triggered severa
debris flows the 13-14th of August 2010

39. Carlo Gregoretti23/5/2017 Mekelle University - Debris Flows
Reference
Fuchs, S., Heiss, K., Huebl J., 2007. Towards an empirical vulnerability function for use in
debris flow risk assessment. Nat. Hazards Earth Syst. Sci. 7, 495-506.

40. Carlo Gregoretti23/5/2017 Mekelle University - Debris Flows
The effect of a debris flow - 9
Val Pola
Bayi watershed (Longxi river), Wenchuan , China; after the
big earthquake of 2008 intese rainfall triggered several
debris flows the 13-14th of August 2010