10th Asian Regional Conference of IAEG (2015)

A preliminary study on the comprehensive threshold for debris-flow early warning
Jian HUANG, Nengpan JU
State Key Laboratory of Geohazard Prevention and Geoenvironment Protection
Chengdu University of Technology, Chengdu, Sichuan 610059, China
E-mail: huangjian2010@gmail.com
Abstract
Debris-flows not only cause a great number of property loss, but also kill and injure
people every rainy season in mountainous region of China. In order to reduce hazard risk,
several methods of rainfall thresholds have been provided at present, frequently according
to the statistical models relying on rainfall exceedance. However, the limited rainfall datawith debris-flow occurrence or non-occurrence always caused threshold analysis to become
very difficult. This paper, therefore, presented a comprehensive threshold combined withthe factors of pore-water pressure and rainfall, which are the most important inducing
parameters during debris-flow occurrences. The three-level early warning criteria (Zero,
Attention, and Warning) has been adopted and the corresponding judgement conditions has
been defined based on real-time monitoring data. Finally, a case study, Wenjiagou gully is
introduced to show that the detail application of the provided method, and the results also
indicate that it's likely to be a useful approach in a warning system for safeguarding of
population in debris-flow prone areas.
Keywords: Debris-flow, comprehensive threshold, pore water pressure, Wenjiagou gully
1.

Introduction
Accompanied by fast population increase andhigh speed economic development in developing
countries, a great number of debris-flows occurred in
mountainous area every year in rainy season, which
always caused considerably catastrophic accidents
and socio-economic losses. On Aug. 7 2010, a debris
flow at Zhouqu County, Gansu Province, China, took
about 1765 people's lives living on the densely
urbanized fan (Tang et al. 2011). Additionally inSouthwestern China, the Wenchuan earthquake on
May12, 2008, Yushu earthquake on April 14, 2010,
Lushan earthquake on April 20, 2013 and Ludianearthquake on August, 3 2014 trigged thousands ofgeo-hazards, which also indicated that the humanvulnerability to natural hazards as well as the lack ofknowledge on natural disaster prevention andmitigation. People living in mountainous regions
have been deeply sensitized so that there is an urgent
demand for an effective method to reduce the hazard
risk. Therefore, researches have been working on
forecasting debris-flow occurrences and setting up
warning systems. Especially in regional scale, themethods for debris-flow early warning are frequently
based on statistical models relying on rainfall
exceedance, and which have already been proved
their importance in predicting debris-flow
occurrences several decades before (Baum and Godt
2009; Guzzetti et al. 2007b; Keefer et al. 1987;
Segoni et al. 2014; Shuin et al. 2012; Tropeano andTurconi 2004). Usually, there are several parameters
selected for the study of rainfall threshold, mainly
including rainfall intensity and duration (Cannon et al.

2008; Guzzetti et al. 2007a; Guzzetti et al. 2007b;
Keefer et al. 1987), antecedent precipitation (Glade etal. 2000), and cumulative rainfall(Guo et al. 2013) as
well. At the meantime, Baum and Godt (2009) used acomprehensive threshold with a combination of
cumulative
rainfallthreshold,rainfallintensity-duration threshold and antecedent water
index or soil wetness for shallow landslide
forecasting.
Although those approaches widely used in themountainous areas, they are currently affected bysome drawbacks which still restrain a fully
operational application to early warning systems. Oneof the main problems is lack of available data about
rainfallwithdebris-flow
occurrenceor
non-occurrence, e.g. most of mountainous area inSouthwestern China. Even though there are enoughdata for statistical model, parameters selected forforecasting debris-flow occurrences are commonly
limited to rainfall information, e.g. Yu et al. (2013)presented a normalized critical rainfall factor with aneffective cumulative precipitation and a maximum
hourly rainfall intensity, based on the existing
topographic and geological factor.
Consequently, this paper presents a recent study
on establishing one comprehensive threshold forpredicting debris-flow occurrence based on the twomain factors: rainfall records above the groundsurface and pore pressure under the ground surface.
The purposes of this paper are: (i) to propose amethod for establishing a comprehensive threshold by
the limited available data; (ii) to introduce theapplication and improvement of the comprehensive

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10th Asian Regional Conference of IAEG (2015)

threshold for debris-flow early warning with a casestudy.
2. Study area
Wenjiagou gully located at the north of Qingping
town, Mianzhu city, Sichuan province, Southwest
China, has a catchment area of 7.8 km2 and a 5.2 kmlong main channel, as shown in Fig. 1. The elevationof this study area ranges from 300 m to 1,600 m
above sea level, and the valley between mountains is

deeply cut by the Mianyuan river with an inclinationof hill slope from 30° to 70°. The average yearlytemperature of about 15.7 ℃, and general climate ismild semi-tropical moist climate with abundant
rainfall and four distinguishable seasons. Most of therainfall are concentrated in the three months from
July to September about 80 % of the annual
precipitation every year.

Fig. 1. Location of Wenjiagou gully modified from Huang et al. (2013). The inset photograph of Wenjiagou gullyat the left bottom was taken from the other side of Mianyuan River on August 10, 2008.
Before Wenchuan earthquake on May12, 2008,
debris-flow triggered by a heavy rainfall on August
this area was covered by rich vegetation, and the13, 2010, the peak discharge of which is up to 1,530
channel was smooth and stable. At that time, there is
m3/s and the total volume of which is up to 3.1 × 106
m3, which caused 7 persons dead, 5 missing, 39
few geological disasters occurred in this region. Thus,
injured and 479 houses buried, and the mostso many farmers settled down at the foothills along
downstream dam in the catchment as well (Yu et al.
the Mianyuan River. However after earthquake, there2012).
was a giant landslide occurred at the top of mountain,
3. Methodology
which caused abundant co-seismic rock falls and
The presented method for forecasting debris-flow
landslides deposited in the gully, particularly at theoccurrence mainly consisted of two main
platform with an elevation of 1,300 m above sea levelcomponents: (i) collect history events about rainfall(Fig. 1, the photograph at left bottom of the mainwith debris-flow occurrences and non-occurrences,map). These deposits are very likely to provide a(ii) establish a comprehensive threshold based onlarge amount of loose solid materials that were easily
events analysis and real-time monitoring data.
eroded and developed into debris-flow in rainy season
3.1 Data analysis(SHIEH et al. 2009). Unfortunately, the catastrophic

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10th Asian Regional Conference of IAEG (2015)

Usually, data set is a solid foundation to establish
there are too many differences before and after thestatistical model of rainfall threshold for debris-flow
earthquake in the study region, so the integrated data
early warning system. Therefore, several methods
are mainly about rainfall events with debris-flowwere provided to collect more available data for theoccurrences and non-occurrences, which occurredfollowing study, including debris-flow inventory
after the Wenchuan earthquake in Wenjiagou gully,
maps, technical reports and documents presented byas shown in Table 1.
government agency. Moreover, for the reason that
Table 1. Primary rainfall events in Wenjiagou gully (2008-2011), from Xu (2010) & Yu et al. (2012)
Maximum hourly

Accumulated

rainfall intensity

precipitation(Ih: mm/h)(Rdt: mm)

Sep. 24, 2008

30.5

Jul. 18, 2009

Debris-flow

Volume of debris

occurrence or not

flow (m3)

88.0

Yes

5.0×105

20.5

70.5

No

-

Aug. 25, 2009

28.9

86.7

No

-

Sep. 13, 2009

15.4

84.6

No

-

May 27, 2010

10.5

34.9

No

-

Jun. 13, 2010

5.5

95.1

No

-

Jul. 25, 2010

11.6

89.6

No

-

Jul. 31, 2010

51.7

60.2

Yes

1.0~2.0×105

Aug. 13, 2010

70.6

227.0

Yes

4.5×106

Aug. 19, 2010

31.9

72.6

Yes

3.0×105

Sep. 18, 2010

29.0

52.0

Yes

1.7×105

Sep. 22, 2010

24.5

81.2

No

-

May 2, 2011

5.6

35.8

No

-

Time

Jul. 5, 2011

12.5

61.3

No

-

Jul. 21, 2011

23.5

63.2

No

-

Jul. 30, 2011

18.2

78.3

No

-

Aug. 16, 2011

10.5

44.3

No

-

Aug. 21, 2011

13.6

76.6

No

-

Sep. 7, 2011

15.2

51.3

No

-

Oct. 27, 2011

8.5

36.9

No

-

debris-flow occurrence was going down as atendency. Two years later after the earthquake,several giant debris-flows happened again, whichcaused catastrophic losses and shook the public andgovernment for its huge destructive power andlong-term impact. Particularly on Aug. 13, 2010, agreat rainstorm lasting for 2 hours in the midnight
triggered a giant debris flow in the gully, whichburied the Qingping town at the mouth of gully and
Mianyuan River. According to the inventory report,
the maximum deposition height was up to 6 m, andmost of the check dams located at the downstream ofgully collapsed and lost their effectiveness after thepassing of debris-flow, which eroded the channelbottom over a depth of about 13 m (Yu et al. 2012).
3.2 Comprehensive threshold for debris flow
Debris-flow occurrence is a complex process,which is not only triggered by rainfall but also linked
to other factors (Itakura et al. 2005; Osanai et al.
2010; Yu et al. 2012). Therefore, in order to improve
the warning thresholds which not just represent a
simple relationship between rainfall and debris-flow
occurrence, the pore-water pressure of landslide
deposit was selected into comprehensive thresholdconsideration for it is an important geotechnicalparameter in physical model study of debris-flow.

Fig. 2. Layout map of the monitoring sensors inWenjiagou gully, the meaning of pink tags are shownin Table. 2. The insert column maps are monitoringdata at 18:00 on Aug. 14, 2012 ( The base map isfrom Google Earth, Dec. 18, 2010, and the elevationof sight is 5.01 km).
It can be seen from the Table 1, the number of3

10th Asian Regional Conference of IAEG (2015)

Table 2. List of monitoring sensors in Wenjiagou gully
No.

Longitude

LatitudeElevation(m)YL01

E104°8'21"

N31°33'32"

1652

Purposes

YL02

E104°7'55"

N31°33'11"

1390

YL03

E104°8'39"

N31°33'14"

1671

YL04

E104°8'16"

N31°32'47"

1490

YL05

E104°7'47"

N31°32'39"

1433

YL06

E104°7'46"

N31°33'29"

1166

YL07

E104°7'9"

N31°32'59"

1025

SY01

E104°8'12"

N31°33'9"1210

To gain the pore-water

SY02

E104°8'11"

N31°33'9"

1212

pressure, and analyze the

SY03

E104°8'11"

N31°33'8"

1208

relationship between

SY04

E104°7'49"

N31°32'55"

1092

pore-water pressure and

SY05

E104°7'48"

N31°32'56"1081

debris-flow occurrence.

To gain the rainfall in the
study area, and analyze therelationship between rainfall
and debris-flow occurrence.

Fig. 3. The rainfall in Wenjiagou gully on Aug. 14, 2012 (the column maps are hourly rainfall and the single line
maps are cumulative rainfall)Fig. 4. The rainfall and pore-water pressure in Wenjiagou gully on Aug. 14, 2012 (the column maps are hourly
rainfall and the single line maps are pore-water pressure)
Therefore, pore-water pressure and rainfall
debris-flow occurrence. During the rainstorm,
monitoring sensors have been selected to complementmonitoring sensors (YL05, YL07 and SY02, SY05)
the data set and prepared for the relationship analysis
were lost connection from the local site to theamong rainfall, pore-water pressure and debris-flow
monitoring center. Except the four sensors, otheroccurrence. The real-time monitoring system inmonitoring sensors worked well and received the
Wenjiagou gully include 7 automatic rain gauges andmonitoring data, as shown in Fig. 2, Fig. 3 and Fig. 4.
5 pore-water pressure monitoring instruments, as
It can be seen in Fig. 2 and Fig. 3, the rainfall was
shown in Fig. 2 and Table 2. During the rainy season
almost concentered in two hours from 17:00 to 18:00,
in 2012, there was a heavy rainfall event which
and the amount of precipitation were very different
happened on August 14 and caused a small
along the channel of Wengjiagou gully. The

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10th Asian Regional Conference of IAEG (2015)

maximum hourly rainfall intensity is 73.5 mm(YL01), and the standard deviation of which is 12.76.
The cumulative rainfall is 118 mm (YL03), and thestandard deviation of which is 14.75. It means thatthe maximum of hourly rainfall and cumulative
rainfall are not the one on the top of mountain, andthe difference of cumulative rainfall is much larger
than the maximum hourly rainfall intensity. The Fig.
4 is to show the relation between hourly rainfall andpore-water pressure, it can be seen that small rain
won't trigger any change in pore-water pressure from2:00 to 5:00 with the maximum hourly rainfall of
12.5 mm. However, the pore-water pressure has
increased fast when the concentrated rain occurredfrom 17:00 to 18:00, and triggering a debris-flow
occurrence. The values of sudden change are 1.97
kPa (SY01), 4.4 kPa (SY03) and 2.27 kPa (SY04)
with an average of 3.19 kPa, which indicated that asudden rise of pore-water pressure played a very
important role in debris-flow occurrence.
According to the above-mentioned introduction,
several factors have been used to research on rainfall
threshold for debris-flow prediction. But the limited
available data still make it's very difficult to establish

an effective threshold, particularly in a short time.
Therefore, considering the acquired available data,
the maximum hourly rainfall (Ih: mm) and cumulative
rainfall (Rt: mm) are selected as the basic triggeringrainfall parameters for the comprehensive threshold,and the change value of pore-water pressure (Rc) has
been defined as an independently required factor in
forecasting debris-flow occurrence.
Based on the collected rainfall events withdebris-flow occurrence and non-occurrence, Rt and Ih,
two related rainfall parameters triggering debris-flow
occurrence, then can be plotted on a single graph withx and y axes, e.g. a heavy rainfall with debris-flow
occurrence on Aug. 13, 2010 as shown in Fig. 5 (TagA). Hereafter, combined with the approach proposed
by Jan et al. (2002) and modified by Zhuang et al.
(2014), the rainfall threshold for predicting
debris-flow occurrence can be defined and calculated,
as shown in Equation 1 and Fig. 5 (Tag C).

Rt  2.4I h  120(1)

Where Rt is the cumulative rainfall (mm), Ih is the
maximum hourly rainfall (mm).

Fig. 5. Rainfall threshold based on maximum hourly rainfall and cumulative rainfallof debris-flow occurrence. Therefore, the combined
It can be seen in Fig. 5, there are thirteen points
comprehensive threshold can be used to forecastof rainfall events with debris-flow non-occurrence
debris-flow occurrence in the near future. Along with
under the rainfall threshold, and two error points
more available data collected, the supplement andabove the rainfall threshold. The ratio of right points
improvement will make it become more reliable andis 86.67%, which is much suitable for the emergency
accurate for debris-flow prediction.
and preliminary application under the initial stage.
4. Example of application
However, as mentioned above, only rainfall threshold
Presently, it is known that trying to forecast
is not enough to predict debris-flow occurrence.
debris-flow occurrence and alert people is a very
Pore-water pressure, therefore, has been selected tocomplex work in practice. Therefore, some
supplement the warning threshold. According to theimplications has to be done for a better use. For the
pore-water pressure monitoring data, the change
preliminary stage, a three-level early warning criteria
value of pore-water pressure (Rc) is much more
important to trigger debris-flow occurrence, as shown
to response after each warning level has been adopted,
in Fig. 4. Thus, the judgement condition of Rc is
as shown in Table 3.
defined as Equation 2.
Level one (Blue) represent that there is no risk
(2)of debris-flow to occur. Level two (Orange) represent
R c  3.5that there is a chance of debris-flow occurrence in the
While both of the two equations (Eq. 1 and Eq.
near future, and warning messages need to be sent to2) are satisfied, there must be a very high possibility

510th Asian Regional Conference of IAEG (2015)

local authority and countermeasures need to beresidents need to be alerted and forbidden going todiscussed. Level three (Red) represent that there isthat place.
very likely to occur right now, therefore, local
Table 3. Recommended warning levels for Wenjiagou gully
Warning level
Ⅰ level

Trigger

Response

Default level, both of Eq. 1 and Eq. 2 are

Null: but data are checked daily. Weekly

not satisfied.

monitoring bulletin.
Watch: data are checked more frequently. Daily

Ⅱ levelAny one of Eq. 1 or Eq. 2 is satisfied.

monitoring bulletin. Authority and expert arealerted. Preparing for alarm.
Warning: data are checked even more

Ⅲ levelBoth of Eq. 1 and Eq. 2 are satisfied.

frequently. Two monitoring bulletins per day.
Local people are alerted.

one hour later at 8:00 am (Tag B), the real-time
rainfall has been exceeding the rainfall threshold, but
the Rc didn't met the Eq. 2 (R = 2.4 < Rc) whichindicated the warning level was in Orange, and there
was no need to send warning message at this level.
Finally, the rainfall went down after 8:00 am, andthere was no debris-flow occurrence during the rainytime on Jun. 19, 2013.

In order to show how this presented method usedin debris-flow predicting, a heavy rainfall event on
Jun. 19, 2013 has been introduced as a caseapplication, as shown in Fig. 6. The circle solid points
were the real-time rainfall information, withcumulative rainfall in X axis and hourly rainfall in Y
axis. E.g. the Tag A in Fig. 6 shows the monitoring
rainfall data at 7:00 am on Jun. 19, 2013, and thechange value of pore-water pressure (Rc) is 2.1. After

Fig. 6. Case application of the presented method in Wenjiagou gully (Jun. 19, 2013)
According to the case study, we can conclude that
only can deal with the insufficient of rainfall
the comprehensive threshold can make a great favor
threshold used before, but also meet the lack ofin predicting debris-flow occurrence in Wenjiagou
available rainfall events with debris-flow occurrence
gully. At least it’s very useful for debris-flow
or non-occurrence.
prevention and mitigation in mountainous area at a
Based on the two triggering factors of rainfall,
preliminary stage. Moreover, the comprehensive
maximum hourly rainfall and accumulative rainfall,
threshold can be improved and modified as long as
combined with the internal factor of pore-water
more available data collected during the subsequent
pressure frequently used in mechanism analysis, havestudy in the future.
been selected to establish a comprehensive threshold
5. Discussion and conclusion
for debris-flow occurrence prediction. Hence,
Debris-flow, usually triggered by rainstorm every
Wenjiagou gully was selected as the case study for ayear in the widely mountainous region in Southwest
detail explanation of the provided method, and the
China, which always cause significant harm both inresults show that it's indeed a well approach forhuman and property losses. Therefore, in order todebris-flow prevention and mitigation, especially at
prevent such natural disaster there is an emergency
the preliminary stage. However, there still are somerequirement for an effective method for debris-flow
limitations while using this method to another similar
occurrence prediction. The comprehensive thresholdregions. The basic restriction is this comprehensive
is provided and discussed in this paper, which notthreshold which has the usage limitation for the6

10th Asian Regional Conference of IAEG (2015)

special area (Wenjiagou gully). Another the mostcomplicated problem is the final determination
whether to alert local population, and any compulsory
actions need to be done at once, or a period of timelater. Debris-flow early warning is just regarded as apotential danger, and which is not an imminenthazard. In spite of the limitations, the method hasreached the goal of establish a comprehensive
threshold for debris-flow early warning based on thelimited available data, and provide a foundation forimprovement and modification during the subsequent
study in the future.
Acknowledgements
This study was financially supported by the StateKey Laboratory of Geo-hazard Prevention andGeo-environment Protection (Chengdu University ofTechnology) (Grant No. SKLGP2013Z007) andNational Natural Science Foundation of China (Grant
No. 41302242).
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