Disaster Advances Vo l . 6 (5) May 2013

Revelation from disastrous 6 July 2011 flood and debris flow on a gas well site, Tongjiang County, Southwestern Feng Wen-Kai*1, Huang Run-Qiu1, Lin Ze-Jun2 1. State Key Laboratory of Geohazard Prevention and Geoenvironment Protection ( University of Technology, Chengdu, 610059, CHINA 2. SINOPEC Southwest Oil and Gas Branch Company, Chengdu 610016, CHINA *[email protected]

Abstract three essential factors: sufficient loose material, surface On 6 July 2011, a disastrous flood and debris flow runoff and steepness of drainage channel on slopes.14 was triggered by a heavy rainfall in Yanfang gully, Qingyu Town, Tongjiang County, Southwestern China. In most mountainous and hilly regions of the world, debris The debris flow burst following three-hour rainstorm flows usually are major threats to human life, properties, with a maximum intensity of 62.6 mm per hour and an facilities, restructures and natural environment. Debris flow average intensity of 41.73 mm per hour. The gully has hazards and fatalities in wet seasons are often reported in two branches divided in the place with elevation of the mountainous regions in southwest of China.11 In 640m. The upper branch is in V-shape on the whole, engineering constructions, improper site selection or poor with large gully gradient and steep slope. The branch, precautionary consciousness would cause more serious as the main source zone and catchment zone, has good destructions in the case of debris flow occurrence.4,12,19,21 hydrodynamic condition and large catchment area. The lower branch, gentle and broad, takes on a A gas-well site of Sinopec Southwest Oil and Gas Branch U-shape on the whole. The branch is the main zone of is located in Qingyu Town, Tongjiang County, debris transport and deposit. The source material is Province of China, 60 kilometers away from the city of mainly derived from transportable gully floor Tongjiang. The design depth of the well is 2700m. The well alluvial-pluvial deposits, colluvium and landslide site is situated in a gully of Yangjiahe Tributary River in deposits. The volume of the source material is XiaoTongjiang river region and the gully is called Yanfang 16.77×104m3 and that of the dynamic volume is gully. 5.17×104m3. On 6 July 2011, a heavy rain hit Tongjiang County and its The gas well site is just situated in the surrounding area. At 11:23 am, a disastrous flood and gully-branch-division area, in the mouth of debris debris flow burst in Yanfang gully which immediately initiation area in the upper gully reach. In the gas well overwhelmed the whole gas well drilling platform and site construction, channel deflection in the gully caused serious damage to some machinery equipment. The lowered flood-discharge capacity. That, together with flood totally destructed downstream living quarters and improperly designed overflow bridge, unreasonable caused 6 lives and property loss (Fig. 1). site selection and well site lower elevation, resulted in serious gas well site destruction on 6 July disaster. The giant flood and debris flow was out-strikingly Incision and lateral erosion of the disastrous flood characterized by its large discharge volume, strong and debris flow produced intense bottom tearing down-cutting and flood carrying capacity. That is rarely scouring in the gully which caused the slope deposits seen in the region for years and fully shows that potential on the two sides of gully bed to undergo obvious destructive power of debris flow is closely related to disturbance deformation. That greatly increased rainfall erosion. Our job in the study is to document the debris flow risk happening again in rainy seasons. catastrophic event through careful field investigation, to Thereafter, it can be concluded that debris flow will analyze the features of debris flow source areas, transport burst frequently in Yanfang gully in the future. paths and deposits, so as to explore the mechanism and characteristics of the debris flow triggered by heavy rainfall Keywords: Debris flow, Rainfall intensity, Hydrodynamic and to analyze the underlying cause for sever destruction condition, Gas well site, Burst frequency, After-disaster on gas well site and after-disaster effect. The result can be effect. used for debris flow hazard assessment and protective measures design. Introduction Debris flows are rapid, gravity induced mass movements Description of the Yanfang gully consisting of a body of granular solids, water and air.18 The giant flood and debris flow occurred in Yanfang gully Because of their high velocity, large volumes and frequent of Yangjiahe Tributary River in XiaoTongjiang river region. recurrence in a wide spectrum of morphological settings, The gully is 32km north of Tongjiang County. The debris flows form serious natural hazards in mountainous mainstream is 5.6km long and the drainage area is 11.4km2. regions.10 They usually are triggered by a combination of Most of the gully slope is steep, with elevation varying

(31) Disaster Advances Vo l . 6 (5) May 2013 from 500m to 1500m. There developed 6 tributary gullies and the whole plane form is approximate circular around Characteristics of debris flow in Yanfang gully the rear gully edge of the well site with an elevation of Initiation process and features of “7.6”debris flow: On 6 640m. Yanfang gully has two branches. The branch with July 2011, a heavy rain hit the drainage area of Yanfang elevation above 790m is narrow and in a “V” shape most of Gully in Qingyu Town. According to precipitation statistics the slope is steep and the drainage channel on slope has an provided by Meteorological Administration of average longitudinal gradient of 4 per cent. The branch City, in Banqiaokou rain-gauge station and Chenhe Station, with elevation below 790m is in a “U” shape and the near Qingyu Town, the maximum daily precipitation was drainage channel on slope is relatively gentle and has an 229.4mm; the maximum precipitation per hour was average longitudinal gradient of 1-1.5 per cent. Most of the 62.6mm (In Chenhe Station); the maximum precipitation slope is steep varying from 30°-45° (Fig.2). per two hour was 92.6mm (10:00am and 11:00am In Banqiaokou Station). Triggered by heavy rainfall, at 11:23 The gully catchment area is mainly underlain by a.m., flood and debris flows burst simultaneously in several quaternary-holocene cultivated soil, colluviums and tributary gullies in the gully drainage area which rapidly landslide deposits, talus-pluvial granular soil, packed into the mainstream. alluvial-pluvial boulder soil and inter-bedded mudstone and sandrock of Upper Jurassic Penglai town group (J3p). In That resulted in abrupt increase of discharge volume in the terms of geological structure, the gully drainage area mainstream and carried lots of such solid materials as mud, belongs to northern Yangtze Platform, the transitional boulders downstream. As the flood and debris flows region from Sichuan Basin to Qinling-Mountain reached the reinforced concrete bridge near the upstream geo-synclinal fold belt. gas well site, boulders carried by debris flows were stopped and deposited by the structure, due to large strength of Influenced by factors as sea-level elevation, topographic bridge structure and small flow cross-section. The bridge incision and human activity, vegetations in the gully openings were blocked and formed backwater height, drainage area take on obvious zoning characteristics and which caused that some bolder crossed the simple vegetation coverage is quite high on the whole. The region, flood-control rock block, flushed and overwhelmed the with elevation between 700m to watershed elevation, is well site (Fig.1 to Fig.3). That killed 6 persons and covered mainly with economic forest, few with shrubs, destructed some facilities and equipments. Judged from bamboos and coniferous forest. Thus vegetation coverage field survey and video taken in occurrence, the debris flow reaches 95％. The region, with elevation lower than 700m, belongs to diluted debris flow (Fig.4 and Fig.5). is covered mainly with cash crops and economic forest, few with coniferous forest. Vegetation coverage reaches 70％. History-documented debris flows and their scale: The region with elevation between 600m and 700m, According to field survey and interview, there occurred few vegetation coverage on left bank slope is higher than that in debris flow in the gully but there would be flood in wet right bank. season. “7.6”giant flood and debris flow is actually a Table I once-in-a-century one which has never been seen since Statistics of daily precipitation higher than 200mm in 60-70 years. As the local person Liu Mingrong and other Tongjiang county history aged people recall, in history, cases of flood occurrence Date Precipitation Date Precipitation (turbulent debris flow) in the gully are as follows: (mm) (mm) 1987.7.18 207.3 2007.7.3 245.5 (1) There occurred a flood and debris flow during wet season each year in the 1964, 1973 and 1984 but none on 1989.6.7 211.1 2011.7.6 229.4 the same scale as“7.6”giant flood and debris flow in the 1992.7.13 206.5 year 2011.

Note：On 6 July, 2011 precipitation is measured in Qingyu (2) On 24 July, 2010, it rained heavily in Tongjiang County Town and in other time, amount of precipitation was and disastrous flood followed. As is shown in documents recorded in other neighboring regions. collected by the neighboring meteorological stations, the

maximum rainfall recorded in Banqiaokou station is 22.7m The study area is located in typical humid, subtropical, (at 07:00a.m.that day), in Yanxi station 76mm (at monsoon climate zone where the rainfall is quite 06:00a.m.that day), in Chenhe station 17.4mm (at concentrated in summer and autumn and there is even flood 10:00a.m.that day). Location of each station is shown in in fall. The average temperature for many years is 16.7 oC, fig.1 and the Banqiaokou station in the northwest of with the highest 38.4 oC and lowest -1.1oC. The total annual Qingyu Town is closest to Yanfanf gully. According to precipitation is 1399.2mm. According to newly released calculation manual of rainstorm-triggered flood in statistical data provided by Meteorological Administration small-and-medium river in Sichuan Province, one hour of Bazhong City, there are mainly 5 floods with 200mm rainfall intensity recorded in Yanxi Station was once-in a daily precipitation recorded in Tongjiang history, century one. During the raining process, flood debris flow concentrated in June and July (Table I). also occurred in Yanfang area. But, the debris flow scale

(32) Disaster Advances Vo l . 6 (5) May 2013 was not smaller than“7.6”giant flood and debris flow in Further, from the observed rainfall intensity data from 2011, debris flow did not reach the well site and cross the Banqiaokou and Chenhe meteorological station on July 5-6, dam of the living area. 2011, hourly and cumulative rainfall distribution map are shown in fig.6. Giant flood and debris flow in Yanfang (3) On 21 June, 2011, it rained heavily once more in gully occurred at 11:23 a.m. July 6, 2011. At that time Tongjiang County and disastrous flood followed too. Data rainfall intensity there is 47.3-62.6mm/hour and before that from neighboring meteorological station showed that one rainfall intensity there is 41.73-42.37mm. The average hour rainfall intensity recorded in Yanxi Station is rainfall intensity is 41.73-42.37mm consecutively from 22.7mm(at 3:00 p.m.), in Yanxi Station 26.4mm (at 4:00 9:00a.m.-11:00a.m. Thus by reasoning, consecutive conce- p.m.) and in Banqiaokou Station 18.9mm (at 4:00 p.m.). ntrated short-time heavy rainfall is the triggering factor of Flood debris flow occurred in the gully area near the well “7.6”giant flood and debris flow occurrence.5,8,15,16,17,20 site but on smaller scale than “7.6”giant flood and debris flow in 2011. Hydrodynamic condition: The elevation of initiation area in Yanfang gully is 1500m and that in the gully mouth is From above survey and analysis,“7.6”giant flood and 500m. The gully is 5.6km long, with drainage area of debris flow in 2011 is really a once-in a century one. 11.4km2.The gully can be divided, from the gas well site According to the material composition, mud depth, area with an elevation of 640m, into two branches. investigation and test and at the same time, combining with the video capture of flow velocity, flux and flow state (1) The gully branch above the well site is the main investigation (Fig.4 and Fig.5), it can be determined catchment and material source area. The mainstream of the comprehensively that the bulk density of debris flow is gully is 3.05km long, with 860m elevation difference and 1.41t/m3, the flow velocity is 3.7m/s and the flux is global vertical ration of slope 282.0‰, quite a large one. 239.2m3/s. The gully is rounded and its drainage area is 8.38km2, accounting for 73.5％ of the total gully area, namely, with Triggering mechanism of “7.6”flood and debris a large catchment area. There developed 6 gully tributaries flow (Fig.7）and their ration of slope are larger than that of the “7.6”giant flood and debris flow falls into the type of mainstream. The gully branch with elevation between rainfall-triggered flood debris flow in gully, mainly with 640-790m is in U-shape and relatively quite gentle. Its turbulent debris flow or flood. The debris occurrence was width varies from 15m to 30m and has a slope ratio mainly triggered by heavy rainfall and favorite 100-150‰. The gully branch with elevation higher than hydrodynamic conditions, together with loose material 790m is in V-shape and quite narrow. The width of the source1,2. Concerning analysis was made from the branch varies from 5 to 15m, averaging 8m. It has a vertical following three aspects: slope ration 400‰ and has quite good hydrodynamic condition. Rainfall condition analysis: Although there are no rain-gauge stations in Yanfang gully, three are distributed (2) The gully branch below the well site is the main 20km around Yanfang gully, respectively Chenhe station, discharge deposits area. The gully is 2.55km long with Yanxi station and Banqiaokou station. The nearest one is elevation differrence140m and vertical slope ratio 54.90％. only 8 km away. Meteorological documents from Chenhe The branch on the whole is wide and gentle with varied and Banqiaokou stations showed that on that day the width from 100m to 140m. Field survey shows that most maximum rainfall intensity is 229.40 mm; the maximum solid material carried by Yanfang gully debris flow 1-hour rainfall intensity was 62.60 mm (at 11:00 a.m. in deposited in the branch, few flushed out of gully mouth. Chenhe station) and the maximum 2-hour rainfall intensity was 92.60 mm (at 10:00 a.m. and 11 a.m. in Banqiaokou According to the above analysis, the catchment area in station) (Fig. 6). Yanfang gully is large and its tailing-edge slope is steep which provides favorite dynamic condition for rapid According to calculation manual of rainstorm-triggered rainwater concentration and flow. flood in small and medium rivers in Sichuan Province, 1-hour and 24-hour rainfall intensity in 50-year return Material source condition: Generally, loose debris period was 69.65mm and 266.50mm respectively; 1-hour material can be derived from collapse or landslide sediment, and 24-hour rainfall intensity in 100-year return period was channel bed sediment and rill erosion.7 In Yanfang gully, 83.30mm and 309.40mm respectively. Comparing the with well-developed vegetations and thin layer of eluvial observed data in terms of 1-hour and 24-hour rainfall coverage, there are few erosion material sources on slope intensity, rainfall intensity in“7.6”giant flood and debris surface. Except for the first left gully tributary opposite to flow in 2011 is a once-in 50 year one. However, taking into the well site, there are few collapse landslide sediments in consideration concentrated local rainfall and field survey of the gully area. Therefore, the debris material source is historical disaster, “7.6”giant flood and debris flow can be mainly derived from channel sediment reinitiating.3 determined as a once-in a century one. According to field survey and prospecting result, present

(33) Disaster Advances Vo l . 6 (5) May 2013 solid material source mainly comes from the gully mouth of the well site. As the too high overflow bridge, after flood of four gully tributaries from G1 to G4 and from the branch is blocked, uplifts water level and deactivates the originally of mainstream bed with elevation from 640m to 730m. The designed low-level flood-proof function of the well site on total elevation is lower than 800m in the gully area; the upstream face. Flood directly flushes into the site (Fig.3 whole volume of debris material source is 16.77×104m3 and and Fig.4). What is more, on the right of the site the the dynamic volume participating debris flow activity is newly-expanded gully mainstream channel is quite narrow, 5.17×104m3. Material source6 is distributed in the gully with maximum width of 13m, minimum width of 7m and area as in fig.7. The way in which solid sediment depth of 4m. It can not satisfy the need to discharge flood participates in debris flow is bottom tearing scouring and at all. side erosion of gully bed13 (Fig.8 and Fig.9). (3) Compared to upper stream of the gully mainstream on Revelation from the gas well site disaster upper-stream face and the mouth of G1 gully tributary, the As mentioned above, the gas well site was seriously well site is, on the whole, in low-lying area and becomes damaged in“7.6”giant flood debris flow disaster. Part of the the object which will be possibly swallowed by the debris site facilities and living quarter were destructed and 6 flow. persons were killed. Through field investigation and analysis, main causes of the well site destruction are After-disaster effect analysis of “7.6” debris flow concluded as follows: Impact Analysis of debris flow reoccurrence liability: Grading standard of debris flow gully quantified scoring Unreasonable well site selection: The gas well site is and the reoccurrence liability quantified assessment, in located in the floodplain area on the left of the gully stream. Norms of debris-flow-disaster control engineering The 50m-wide well site blocks most flood discharge cannel prospecting (DZ/T0220-2006) (National standard of China), and weakens flood-discharge capacity of the gully. the debris flow gully scored 82 points on natural condition Elevation of the well site is lower than that of first terrace and the gully is comprehensively assessed as one with and the site is directly opposite to upper-stream straight slight reoccurrence degree on the basis of analyzing the gully branch which make the site become the direct target comprehensive characteristics of the debris flow gully. of debris flow flush. Further, the well site is just located in the gully area with an elevation around 640m and the After “7.6”debris flow, the gully especially the gully upper-stream gully branch is quite narrow with steep gully tributary experienced deep scouring dissection; deposits on bed and large catchment area. The branch is the main debris both gully sides were disturbed greatly and deformed flow initiation area and also the debris flow outlet after obviously on free surface. That formed new potential concentration. The well site is just situated in the mouth of unstable loose deposits (Fig.7 to Fig.9). There were broken G1 gully tributary and also the direct flush object of debris tree trunks in some gully tributaries which caused a certain flows from the tributary. Thus the site selection was degree of gully block and increased block degree index. extremely unreasonable and there exists great potential Therefore, after “7.6”debris flow occurrence, influenced by hazard of debris flow disasters, which show bitter lack of the disturbance, the degree of debris flow reoccurrence disaster prevention and reduction awareness. liability in the gully tributaries went up largely and that increased debris flow reoccurrence possibility in the gully Shortcomings in well site flood-control design: The well mainstream. site blocks straight and wide channel branch. Although a channel is expanded on the right of the site out of Impact analysis of debris flow occurrence frequency expectation to discharge flood, insufficient consideration and development trend: According to above mentioned still is made in design, shown as follows: investigation analysis of debris flow disaster history in the area and the vegetation development on the gully bank, (1) The designed flood-proof height of the well site in before “7.6”debris flow occurrence, the gully is a upstream-face direction is not enough. And the flood-proof low-frequency debris flow gully and its frequency is larger facilities are just alluvium simply piled and have poor than once in 50 years. But after the occurrence, the deposits flood-prevention capacity. on both sides of gully tributaries deformed obviously, the gully bed suffered great scouring disturbance, unstable (2) A simple multi-span overflow bridge is constructed in material source increased a lot and there remained some upper-stream channel near the well site, with bridge deck broken tree trunks blocking the gully. All of these, together 3.5m higher than channel bed. The bridge is just located in with the factor that the gully has a large slope gradient, the turning connections of the straight gully mainstream formed block- and-break –type debris flow with great branch with the newly-expanded discharge channel hazard. That, to some degree, would increase material (Broken Bridge Fig.10). The oblique crossing of the bridge source amount, debris flow occurrence possibility and its and gully mainstream on upstream face functions similarly frequency in the gully mainstream. as an inclined T-shaped water-proof wall on the right of gully mainstream. Flood after blocked can not turn right, Therefore after “7.6”occurrence, debris flow occurrence then flushes against the left bank and increases the hazards frequency would get higher during a specific period of time

(34) Disaster Advances Vo l . 6 (5) May 2013 in the gully mainstream which should be attached special attached. attention. Particular attention should be paid to the left G1 gully tributary directly opposite to the well site. There Acknowledgement would be higher frequency of debris flow occurrence The research project is supported by The National Natural recently and much more potential hazards in there. Science Foundation of China (Grant Number: 41172278), The Key Project of Education Department of Sichuan Conclusion Province (Grant Number: 12ZA010) and The Research Debris flow is among the common geo-hazards in Fund of State Key Laboratory of Geohazard Prevention and mountainous area of southwest of China which causes Geoenvironment Protection (Grant Number: serious influences on local residents’ life and important SKLGP2011Z004). Authors express gratitude to all people engineering facilities. On 6 July, 2011, a giant flood debris or authorities concerned for the help they have got in the flow occurred in Yanfang gully of Qingyu Town in research. High tribute is paid to Professor Tang chuan and Tongjiang County southwest of China. Before that, it rained Dr. Li Weile for their assistance in research. Last, thanks extremely heavily continuously for 3 hours with an average shall go to lecturer Qiao Qiuchun from Chengdu University rainfall intensity of 41.73-42.37mm per hour and maximum of Technology for his help with English in paper-writing. rainfall intensity of 62.6mm per hour. The three hour accumulative rainfall is the major cause of the debris flow. References Further the source zone and catchment zone, with large 1. Alemaw B.F., Flood Hazard Forecasting and Geospatial gully gradient and steep slope, have large catchment area Determinants of Hydromorphology in the Limpopo Basin, and good hydrodynamic condition. The source material is Southern Africa, Disaster Advances, 3(4), 573-581 (2010) mainly derived from transportable gully floor alluvial-pluvial deposits, colluvium and landslide deposits. 2. Bovis M.J. and Jakob M., The role of debris supply conditions The volume of material source is 16.77×104m3 and that of in predicting debris flow activity, Earth Surface Processes and the dynamic volume 5.17×104m3. Landforms, 24, 1039-1054 (1999)

The gas well site is just situated in the mouth of debris 3. Caine N., The rainfall intensity-duration control of shallow initiation area in the upper Yanfang gully reach, with an landslides and debris flows, Geografiska Annaler, 62A, 23-27 elevation of 640m. In the well site construction, channel (1980) deflection in the gully lowered flood-discharge capacity. That, together with improperly designed overflow bridge, 4. Chen Xingzhang et al, “7.31” Debris Flow Hazards Occurred unreasonable site selection and well site lower elevation, at Diluo Water Power Construction Areas in Xixi River Basin, resulted in serious gas well site destruction 6 July disaster. Sichuan Province, Journal of Mountain Science, 128(11), 116-122 It shows that in the engineering construction, the threat of (2010) flood debris flow hazard in Yanfang gully was under-estimated and there was not enough precautionary 5. Costa J.E., Physical geomorphology of debris flows, In Costa awareness. After “7.6”debris flow occurrence, incision and J.E. and Fleischer P.J., eds., Developments and Applications of lateral erosion of the disastrous flood and debris flow Geomorphology, New York, Springer -Verlag, 269-317 (1984) produced intense bottom tearing scouring in the gully which caused the slope deposits on the two sides of gully 6. Dong Jia-Jyun et al, The role of the sediment budget in bed to undergo obvious disturbance deformation. That understanding debris flow susceptibility, Earth Surf Process greatly increased hazard of debris flow recurrence in wet Landforms, 34, 1612-1624 (2009) seasons. Thereafter, it can be concluded that debris flow will burst frequently in Yanfang gully in the future. 7. George N.J., Akpan A.E., Obot I.B. and Akpanetuk N.J., Attention should be paid to strengthen the disaster control. Geoelectrical Investigation of Erosion and Flooding using the Lithologic Compositions of Erosion and Flood- Stricken Road in Truly speaking, “7.6”giant flood debris flow disaster was Ukanafun Local Government Area, Akwa Ibom State, Southern triggered by gully bed deposits reinitiating and transports Nigeria, Disaster Advances, 1(4), 46-51 (2008) because of heavy rainfall concentration. There are bitter shortcomings in construction of the well site and the 8. Hu K.H. et al, Characteristic rainfall for warning of debris flood-proof project. There would be higher frequency of flows, Journal of Mountain Science, 7, 207-214 (2010) debris flow occurrence in Yanfang gully in near future. That shows there is great uncertainty in terms of debris 9. Hungr O., Morgan G.C. and Kellerhals R., Quantitative flow occurrence. Precautionary awareness should be analysis of debris torrent hazards for design of remedial measures, strengthened. So, there is great necessity to reinforce debris Canadian Geotechnical Journal, 21, 663-677 (1984) flow hazard control, to regulate land use in debris flow area, to transport people living in those area to safe place and to 10. Hürlimann M., Copons R. and Altimir J., Detailed debris flow install monitoring and warning system in potentially hazard assessment in Andorra, a multidisciplinary approach, -high-danger zone. Especially as for large and important Geomorphology, 78, 359-372 (2006) engineering construction, great attention should be

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Fig.1: Debris flow overwhelmed the whole gas well drilling platform and caused serious damage to some machinery equipment. The flood totally destructed living quarters 500m downstream and caused 6 persons death

Fig.2: Plan view of the catchment region（well site/gully name/Yangjiahe River）

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Fig.3: Simple reinforced concrete bridge upstream the well site, when its opening was blocked, the well site was severely destroyed by “7.6”debris flow. In the picture, the bridge was newly excavated after the disaster (picture taken from upstream to downstream)

Fig.4: The scene of “7.6”giant flood and debris flow flushing into well site. Blocked by upstream reinforced concrete bridge, debris flow forms backward height (picture taken on the spot from downstream to upstream)

Fig.5: The scene of “7.6”giant flood and debris flow bursting and overwhelming the well site (picture taken on the spot from downstream to upstream)

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Fig.6: Hourly and cumulative rainfall distribution according to data from Banqiaokou and Chenhe meteorological station on July 5-6, 2011. Giant flood and debris flow in Yanfang gully occurred at 11:23 a.m. July 6, 2011. At that time rainfall intensity was 47.3-62.6mm/hour and before that, rainfall intensity was 41.73-42.37mm. The average rainfall intensity was 41.73-42.37mm consecutively from 9:00a.m.-11:00a.m

Fig.7: Distribution map of drainage system and material source in the gully: material source is mainly distributed in the gully mouth of four gully tributaries from G1 to G4 and in the branch of mainstream bed with elevation from 640m to 730m; the whole volume of debris material source is 16.77×104m3 and the dynamic volume is 5.17×104m3; material source supply mainly comes from bottom tearing scouring and side erosion of gully bed

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Fig.8: General view of material source in G1 gully tributary opposite directly to the gas well site: the whole volume of debris material source is 1.62×104m3 and the dynamic volume is 0.97×104m3; material source supply mainly comes from bottom tearing scouring and side erosion of gully bed; local remaining tree trunk may cause block and form potential break-type debris flow

Fig.9: General view of material source in G1 gully tributary on right bank: the whole volume of debris material source is 1.17×104m3 and the dynamic volume is 0. 7×104m3; material source supply mainly comes from bottom tearing scouring and side erosion of gully bed; local remaining tree trunk may cause block and form potential break-type debris flow

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Fig.10: The well site with an elevation of 640m is located in the outlet of debris flow. Influenced by direct connection of gully mainstream and left G1 tributary, the site is in great danger. And owing to the shortcomings in its flood-proof design, the newly expanded channel has poor flood discharge capacity especially the shortcomings in site selection and design of overflow bridge there

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13. Pierson T.C., Erosion and deposition by debris flows at Mt. 19. Wang Wen-long et al, An analysis on forming condition of Thomas, North Canterbury, New Zealand, Earth Surface debris flows caused by human activities in Shenfu 2 Dongsheng Processes and Landforms, 5, 227-247 (1980) Coal Field, Research of Soil and Water Conservation, 10(2), 83-85 (2003) 14. Takahashi T., Estimation of potential debris flows and their hazardous zones, Journal of Natural Disaster Science, 3(1), 57-89 20. Wieczorek G.F., Effect of rainfall intensity and duration on (1981) debris flows in central Santa Cruz Mountains, California, In Costa J.E., Wieczorek G.F., eds., Debris Flows/Avalanches, Process, 15. Tang C., Zhu J. and Li W.L., Rainfall triggered debris flows Recognition and Mitigation, Geological Society of America, after Wenchuan earthquake, Bulletin of the International Reviews in Engineering Geology, 7, 93-104 (1987) Association of Engineering Geology, 68, 187-194 (2009) 21. Xie Hong, You Yong and Zhong Dun-Lun, A typical man 16. Tang Chuan et al, The Disastrous 23 July 2009 Debris Flow in made debris flow in the upper reaches of Changjiang River, Xiangshui Gully, County, Southwestern China, Journal Mountain Research, 12 (2), 126-128 (1994). of Mountain Science, 8(2), 131-139 (2011)

th th 17. Tecca P.R. and Genevois R., Field observations of the June 30, (Received 15 November 2012, accepted 20 February 2013)

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