1. Chengdu University of Technology (CDUT), P. R. China
Rainfall thresholds for debris flow initiation in the Wenchuan earthquake-stricken area, southwestern China
Wei Zhou
State Key Laboratory of Geo-Hazard Prevention and Geo-Environment Protection (SKLGP)
Chengdu University of Technology (CDUT), P. R. China
Jan. 10, 2014

2. Outline Introduction Study area Rainfall data
Rainfall patterns for debris flows in the Wenchuan earthquake area
Rainfall thresholds analysis
Comparison of rainfall thresholds

3. Giant debris flow in Beichuan (Tang, 2011)
1. Introduction
Local intense or prolonged rainfall events often trigger debris flows in southwestern China after the Wenchuan earthquake, causing serious casualties and economic losses.
Giant debris flow in Beichuan (Tang, 2011)

4. Photograph of Qingping Town (from internet)
1. Introduction
Wenjia watershed
Xingfu bridge
Old bridge
Qingping
Photograph of Qingping Town (from internet)

5. Photograph of Qingping Town after the Wenchuan earthquake (Xu, 2010)
1. Introduction
Wenjia watershed
Photograph of Qingping Town after the Wenchuan earthquake (Xu, 2010)

6. Photograph of Qingping Town after the giant debris flow (Xu, 2010)
1. Introduction
Wenjia watershed
Photograph of Qingping Town after the giant debris flow (Xu, 2010)

7. 1. Introduction 30×104m3 V=70×104m3 40×104m3 Hongchun watershed
Xu, 2010
30×104m3
V=70×104m3
40×104m3

8. 1. Introduction
These post-seismic disasters will still last for 5 to 10 years (Tang et al. 2009), or may last also longer -10 to 15 years-, even up to 30 years (Cui et al. 2008; Xie et al. 2009).
Post-earthquake debris flow events increased significantly due to the increased loose materials and intense rainfall.
To analyze the primary causes of debris flows, it is necessary to understand the relationship between rainfall thresholds and debris flow initiation.
A threshold is the minimum or maximum level of some quantity needed for a process to take place or a state to change (White et al., 1996).

9. 1. Introduction Thresholds can be grouped in four categories:
Rainfall intensity-duration (ID) thresholds
Thresholds based on the total event rainfall
Rainfall event-duration (ED) thresholds
Rainfall event-intensity (EI) thresholds
ID thresholds for triggering of debris flows are the most common type of thresholds proposed in the literature and have been widely identified in many different climates and geologic settings.
ID thresholds have the general form
I=c+αD-ß
where I is rainfall intensity, D is rainfall duration, and c ≥ 0, α > 0, and ß > 0 are parameters.

10. 1. Introduction The main purposes are:
obtaining the rainfall-triggering patterns in the Wenchuan earthquake area
determining empirical ID thresholds for debris flows in the Wenchuan earthquake-stricken area;
establishing empirical normalized IMAPD thresholds for debris flows in the Wenchuan earthquake-stricken area;
providing important information for local government to mitigate debris flow hazards.

11. 2. Study area
Location map of the study area, north Sichuan Province (SW China). The epicenter to Hongchun, Bayi, Wenjia, and Weijia watersheds are only 10, 13, 80 and 130 kilometers, respectively.

12. 2. Study area Dashui watershed Dashui watershed Catchment area (km2)
0.45
Stream length (km)
0.99
Mean channel gradient (‰)
525
Maximum/minimum elevation (m a.s.l.)
1609/713
Maximum relief (m)
896
Volume of effective loose materials (×104 m3) 91
Geology
sandstone, marlstone and mudstone

13. 2. Study area Dashui watershed Dashui watershedN N m m
Photograph of Dashui watershed (from Google map, 2001)
Photograph of Dashui watershed (from Google map, 2010)

14. 2. Study area Wenjia watershed Wenjia watershed Catchment area (km2)
7.65
Stream length (km)
4.33
Mean channel gradient (‰)
467
Maximum/minimum elevation (m a.s.l.)
2402/883
Maximum relief (m)
1519
Volume of effective loose materials (×104 m3)
7490
Geology
shale; sandstone and siltstone; limestone and dolomite

15. 2. Study area Hongchun watershed Hongchun watershed
Catchment area (km2)
5.35
Stream length (km)
3.6
Mean channel gradient (‰)
358
Maximum/minimum elevation (m a.s.l.)
2168.4/880
Maximum relief (m)
1288.4
Volume of effective loose materials (×104 m3)
384.3
Geology
granite and diorite

16. 2. Study area Bayi watershed Bayi watershed Catchment area (km2) 8.3
Stream length (km)
2.93
Mean channel gradient (‰)
376
Maximum/minimum elevation (m a.s.l.)
2456/850
Maximum relief (m)
1606
Volume of effective loose materials (×104 m3)
520
Geology
marlstone; sandstone and mudstone

17. 3. Rainfall Data Flow chart DATA ANALYSIS STEPS APPLICATION Cumulative
Rainfall duration
Hourly rainfall intensity
Mean rainfall intensity
Peak rainfall intensity
DATA
Hourly rainfall intensity
Redefinition
Cumulative
rainfall
Mean rainfall intensity
Rainfall duration
ANALYSIS STEPS
ID thresholds
Comparison
Normalization
Comparison
Debris-Flow Warning Model
APPLICATION

18. 3. Rainfall Data Rainfall data
Rainfall duration? Cumulative rainfall? Mean rainfall intensity？
One continuous rainfall ?

19. Sketch map for one continuous rainfall
3. Rainfall Data
Rainfall data
One continuous rainfall
Start
End
＞4mm
<4mm in 6 consecutive hours
Knowledge of the rainfall characteristics in a particular area
requires well-recorded debris flows and corresponding rainfallduration
data. In the Wenchuan earthquake-stricken area, however,
systematically recorded debris flow data are rarely available
because few giant debris flows occurred. Besides, precipitation
gauging stations are not installed in all the debris flow gullies. 4
Sketch map for one continuous rainfall

20. 3. Rainfall Data Hyetograph Dashui watershed Debris flow occurrence
Dashui watershed
Debris flow occurrence

21. 3. Rainfall Data Hyetograph Wenjia watershed Debris flow occurrence
Debris flow occurrence
Wenjia watershed

22. 3.Rainfall Data Hyetograph Debris flow occurrence
Debris flow occurrence
Debris flow occurrence

23. 3.Rainfall Data Hyetograph Hongchun watershed Debris flow occurrence
Hongchun watershed
映秀镇被淹没
岷江改道
堰塞堆积体
红椿沟
原岷江河道

24. 3.Rainfall Data Hyetograph Bayi watershed Debris flow occurrence
Debris flow occurrence
Bayi watershed

25. 3.Rainfall Data Rainfall data
Rainfall data for post-seismic debris flows in Wenchuan earthquake-stricken area

26. 4. Rainfall patterns for debris flows in the Wenchuan earthquake area
The rainfall-triggering patterns in the Wenchuan earthquake area can be divided into three categories, namely a rapid triggering response pattern (RTRP), an intermediate triggering response pattern (ITRP), and a slow triggering response pattern(STRP).
Rapid triggering response pattern
Intermediate triggering response pattern
Slow triggering response pattern

27. Rapid triggering response pattern
4. Rainfall patterns for debris flows in the Wenchuan earthquake area
Rapid triggering response pattern
Short rainfall duration
A small amount of antecedent rainfall
The rainfall intensity usually increases rapidly or nearly instantaneously to a maximum relative high value and then drops quickly to 0.
The debris flow will start prior to or after the maximum rainfall intensity.
The rapid triggering response pattern is characterized by a small amount of antecedent
rainfall before triggering of the debris flow. The rainfall intensity usually increases rapidly
or nearly instantaneously to a maximum relative high value and then drops quickly to 0.
Rapid triggering response pattern

28. Intermediate triggering response pattern
4. Rainfall patterns for debris flows in the Wenchuan earthquake area
Intermediate triggering response pattern
The rainfall duration before the triggering of the debris flow is longer and the cumulative rainfall is larger.
The debris flows were triggered when the rainfall intensity reached its maximum.
The rainfall intensity increases slowly from a small value to a maximum value and then decreases rapidly from the maximum value to 0.
Intermediate triggering response pattern

29. Slow triggering response pattern
4. Rainfall patterns for debris flows in the Wenchuan earthquake area
Slow triggering response pattern
Long rainfall duration
The rainfall intensity shows 2–3 peaks
Debris flows were triggered when the rainfall intensity reached the second or third peak.
The cumulative rainfall is large and debris flows will occur after a long rain period.
Slow triggering response pattern

30. Cumulative rainfall (mm) Triggering rainfall intensity(mm/h)
4. Rainfall patterns for debris flows in the Wenchuan earthquake area
Cumulative rainfall and triggering rainfall intensities for debris flows in the Wenchuan earthquake area
Location
Time
Rainfall pattern
Cumulative rainfall (mm)
Triggering rainfall intensity(mm/h)
Qingping
7/31/2010
RTRP
26.0
39.1
8/13/2010
ITRP
83.4
37.4
8/19/2010
122.5
33.0
Tangjiashan
9/13/2008
96.5
27.4
9/24/2008
STRP
177.6
41.0
Yingxiu 
8/14/2010
179.8
16.6
8/21/2011
125.8
56.5
Longchi
17.7
15.8
8/18/2010
195
34.3

31. 4. Rainfall patterns for debris flows in the Wenchuan earthquake area
Maximum rainfall intensity of debris flow events after the earthquake
Mean rainfall intensity of debris flow events after the earthquake
In analyzing the rainfall data, one can find an interesting phenomenon.
The mean values of maximum rainfall intensity of debris flows that occurred between 2008
and 2012 are 39.0, (-), 50.0, 56.0, 68.0 mm/h, respectively (Fig. 5a). It means that the
maximum rainfall intensity increased with the time. A similar trend can be found for the
values of average rainfall intensity
According to the literature, triggering rainfall intensity before the Wenchuan earthquake is mm.
地震后该区域的激发雨量发生变化，累积雨量和临界雨量较地震前都有所降低。

32. 5.Rainfall thresholds analysis
ID thresholds
The threshold is expressed as
(2<D<15)
where I is the mean rainfall intensity (mm/h), D is the rainfall duration (h).
With an increase in rainfall duration, the intensity that is likely to initiate debris flow decreases.

33. 5.Rainfall thresholds analysis
ID thresholds
Rainfall events of shorter duration (e.g., <2 h), a mean rainfall intensity of 44.8 mm/h has the potential to initiate debris flow.
For a longer duration (e.g., >15 h), rainfall intensity of about 10.2 mm/h also has the potential to cause debris flow.

34. 5.Rainfall thresholds analysis
Normalization
The IMAPD thresholds with the lower boundary can be expressed as:
(2<D<15)
The range of rescaled rainfall intensity is (h−1) of MAP. The rescaling slightly reduced the variation of rainfall intensity.

35. 5.Rainfall thresholds analysis
Normalization
It is useful in estimating rainfall intensity for a debris flow event in the form of a percentage of MAP.
IMAP = of MAP
<2 h, IMAP = h−1 (i.e., 2.36% of MAP)
>15h, IMAP = h−1 (i.e., 0.69% of MAP)

36. 5.Rainfall thresholds analysis
Normalization
The EMAPI and the EMAPD thresholds are expressed by:
(10.2<I<44.8)
(2<D<15)
Where EMAP is the normalized event rainfall (in percent)

37. 5.Rainfall thresholds analysis
Normalization
DFBD1
EMAP decreases with increasing I and increases with increasing D.
The results depend on the MAP value. In fact, the MAP obtained for Wenjia (1086 mm) is lower than those for Bayi ( mm), Hongchun (1253.1mm) and Dashui ( mm).
For DFBD1 event, the rainfall intensity is more important than the cumulative rainfall in discriminating rainfall conditions triggering debris flow.

38. 6. Comparison of rainfall thresholds
The thresholds fall in the range of rainfall intensity and duration defined by other thresholds.
The thresholds are higher than other global, regional, and local thresholds.
Thick lines (black and gray): global thresholds (1-3)
Thin lines (black and gray): regional thresholds (7-14)
Blue lines: local thresholds (6, 16)

39. 6. Comparison of rainfall thresholds
Puerto Rico
The normalized ID threshold is similar to the regional threshold proposed by Jibson (1989) for Puerto Rico (7).
Compared to other places, the normalized ID threshold value is high, whereas it is less than the thresholds obtained for the Puerto Rico (Jibson 1989) and Nepal Himalaya (Dahal and Hasegawa 2008).
Nepal Himalaya
Japan
Hong Kong
World
Central and Southern Europe
Thick lines (black and gray): global thresholds (1-4)
Thin lines (black and gray): regional thresholds (5, 7-9)
Blue lines: local thresholds (6)

40. 7. Limitations of this research
Limitation of the rainfall data
Before the Wenchuan earthquake, few debris flows occurred in the study area. Therefore, precipitation gauging stations were not installed in potential dangerous debris flow gullies.
After the Wenchuan earthquake, stations were installed in the middle and lower parts of the watersheds with frequent debris flows. However, few precipitation gauging stations were installed in the source area of the large watersheds, and the data are of limited access.
In this work, for each debris flow, the corresponding effective rainfall data on the day of debris flow occurrence were obtained from the nearest rainfall station. The rainfall data in the source area at higher altitudes is different from that of the lower lying meteorological stations. Although the rainfall data is few, they can be used to analyze the relationship between ID thresholds and debris flow occurrence.

41. 7. Limitations of this research
Limitations of the rainfall ID analysis
ID plots depict only mean rainfall intensities and do not necessarily reflect high rainfall intensity or low rainfall intensity at the time of debris flow occurrence.
This study does not take into account the role of antecedent rainfall in triggering debris flows in the Wenchuan earthquake-stricken area.
Further studies are necessary to establish local ID thresholds considering the variation of rainfall intensity, antecedent rainfall, and other variables such as topography and geology.

42. Thank you
for your attention!