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

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

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)

Photograph of Qingping Town (from internet) 1. Introduction Wenjia watershed Xingfu bridge Old bridge Qingping 2010.8.15 Photograph of Qingping Town (from internet)

Photograph of Qingping Town after the Wenchuan earthquake (Xu, 2010) 1. Introduction Wenjia watershed 2008.12.23 Photograph of Qingping Town after the Wenchuan earthquake (Xu, 2010)

Photograph of Qingping Town after the giant debris flow (Xu, 2010) 1. Introduction Wenjia watershed 2010.08.22 Photograph of Qingping Town after the giant debris flow (Xu, 2010)

1. Introduction 30×104m3 V=70×104m3 40×104m3 Hongchun watershed Xu, 2010 30×104m3 V=70×104m3 40×104m3 2010-8-14

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).

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.

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.

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.

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

2. Study area Dashui watershed Dashui watershed N N 0 250 500 m 0 250 500 m Photograph of Dashui watershed (from Google map, 2001) Photograph of Dashui watershed (from Google map, 2010)

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

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

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

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

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

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

3. Rainfall Data Hyetograph Dashui watershed Debris flow occurrence 2008-9-23 Dashui watershed Debris flow occurrence 2008-9-24

3. Rainfall Data Hyetograph Wenjia watershed Debris flow occurrence 2010-7-31 Debris flow occurrence 2010-8-13 Wenjia watershed

3.Rainfall Data Hyetograph Debris flow occurrence 2010-8-19 2012-8-17 Debris flow occurrence Debris flow occurrence 2012-8-14 2012-8-17

3.Rainfall Data Hyetograph Hongchun watershed Debris flow occurrence 2010-8-14 2011-8-21 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

3.Rainfall Data Hyetograph Bayi watershed Debris flow occurrence 2010-8-13 Debris flow occurrence 2010-8-19 Bayi watershed

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

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

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

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

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

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

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 50- 60 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。

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.

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.

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 0.0069-0.0236 (h−1) of MAP. The rescaling slightly reduced the variation of rainfall intensity.

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 = 0.0069-0.0236 of MAP <2 h, IMAP =0.0236 h−1 (i.e., 2.36% of MAP) >15h, IMAP =0.0069 h−1 (i.e., 0.69% of MAP)

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)

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 (1134.8 mm), Hongchun (1253.1mm) and Dashui (1399.1 mm). For DFBD1 event, the rainfall intensity is more important than the cumulative rainfall in discriminating rainfall conditions triggering debris flow.

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)

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)

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.

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.

Thank you for your attention!