Technical analysis on the emergency handling of Tangjiashan barrier lake

Technical analysis on the emergency handling of Tangjiashanbarrier lakeLiu Ning，Yang Qigui2(1. The Ministry of Water Resources of China, Beijing 100053，China;2. Design Institute of Changjiang W ater Resources Commission, Wuhan 430010, China)Abstract:This paper gives a brief introduction to the emergency handling of Tangiashan barrier lake. Some technologiesfor the application of geological and topographical data are summarized and the mechanism of formnation of a barrier lakeis analyzed. Based on the safety status evaluation, the dam breach flood point is calculated. The paper concludes withdiscussion of the practical effects of emergency handling scenarios and diferent drainage channel designs.Key words:earthquake; Tangjiashan barrier lake; landslide; dam breach1 Introduction2 Technologies for the data acquisition andapplicationTangjiashan barrier lake is located in the Kuzhureservoir, upstream of Tongkou River, 3. 2 km away2.1 Topographical data acquisition and applicationfrom Beichuan County of Sichuan Province. It is theTangjiashan landslide was an abrupt event. Theremost dangerous landslide dam triggered by the Wen-is no systematic topographical data with high accuracychuan earthquake, presenting a great peril to over 1.3of the lake itself or the downstream affected area. Themillion people and threatening strategic assets such asfirst stage of the emergency handling was to get accu-the Ianzhou- -Chengdu oil pipeline and Baoji- Chengdurate data such as the total storage capacity , the storagerailway.variation, and affected area etc. For that purpose siteTangjiashan section of the Tongkou River runssurveys were carried out including collection othrough a steep and entrenched valley where the watertopographical and image data utilzing remote sensinglevel was about 665 m before the earthquake occurred.technology.The barrier lake runs approximately 803 m along the2.1.1 Handling of multi-sourced datariver channel with width of about 611 m resulting in aAll of the collected data were used for the emer-total area of the dam of about 0.3 million m'. The damgency handling after a digital processing appropriate toheight is 82 ~ 124 m with a volume of 20. 37 million m'. .different sources including: verification of the elevationThe dam top elevation varies greatly with the highestsystem，cloud deletion for the remote sensing data ，point at 793.9 m on the left side and 775 m on theand re-construction with interpolated data.right side. There is an arcuate channel through the2.1.2 Storage curve of the barrier lakedam on the right side with a bed width of 20 ~40 mThe storage curve of a barrier lake is importantand the highest point at 752.2 m in the middle. Thedata for dam breach analysis, flood mitigation, andelevation at the bottom of the barrier lake upstream theplanning of technical measures for emergency manage-dam is 663 m and the lake has the maximum storagement. Based on maps with scale of 1: 50000，digitalcapacity of 316 million m' .elevation model (DEM) data with scale of 1:50 000,On May 14, 2008 ，Tangiashan barrier lake wasthe storage curve of Xuanping power station, the topo-found out on an aerial map taken after the earthquake.graphical data with scale of 1:2 000，a number ofOn May 21 , with the help of the helicopters of People'sresearch institutes offered different storage curves foLiberation Army ( PLA), technical experts were sent toTangjiashan barrier lake. After a comprehensive analy-Tangjiashan dam site. After a field survey they identifiedsis, a storage curve based on the data from Xuanpingthat the barrier lake was highly dangerous, and it waspower station, and the tnnoranhical data with scale ofpossible to take technical measures to reduce the danger.1:2 000中国煤化工handling and floodYHCNMHGReeived 15 July 201138 Engineering Sciencesobservation and forecasting.Although these databroken stones. Some eluvia soil of the original moun-reflected the situation before the earthquake, which wastain is covered on the surface of the right side of theslightly changed due to the landslide influence， thedam. The dam material is composed of 14 % of eluviaaccuracy was still enough for emergency management.soil and 86 % of broken stone. It is believed, after2.1. 3 Topographical data acquisition for flood miti-analysis of the dam form, that the stability is good andgation calculationa sliding collapse of the entire dam is impossible.Accuracy priority was given to field survey data forSite investigation and geological analysis showedwhich identical elevation system ( datum) had been es-that the possibility of comprehensive seepage damagetablished. Where there was no available data from awas small, because the huge pushing force of landslidelarge scale mapping, the data from DEM was used.caused the deposit and heaping of fine sands with soil2.2 Elevation system standardizationfrom Kuzhu area at the upstream face of dam，perform-A local elevation datum was set on May 21 ,ing certain seepage-prevention function and stabilizing2008. Based on this system dam site topography wasthe seepage condition of the dam. The concem is thatsurveyed and hydrological data was observed. Butne phenomenon of partial overhanging of broken rocksthese data based on a local elevation system cannot becannot be excluded. When water level rises to certaindirectly used for flood forecasting and flood impact cal-elevation, it is possible to have concentrated seepage,culations. After a comprehensive analysis of the riverwhich may cause partial collapse of downstream face ofslope and water surface, it was recognized that the wa-the dam. .ter level of Zhicheng hydrological station upstream ofThe right section of the dam has a relatively lowTangjiashan barrier lake is about 0.4 m different to theelevation. The top of right section is composed of bro-level at the dam. Based on that fact a relationship be-ken rocks and soils from the original mountain slope.tween the elevation of Tangjashan barrier lake and theUnder natural conditions, when the water of the lakenational elevation system was established by which ob-rises to the level of the lowest part of the dam, which isserved topographical map data were verified. It wason the right side, broken rocks and soils as well as se-demonstrated that this was a practical method in emer-riously weathered rocks on the top of dam would bgency management situations.easily washed away and the water flow channel woul2.3 Rapid assessment of hydrological data at thebe quickly deepened. If a drainage channel is excava-dam siteted on the right section of the dam, the volume of exca-Tanjiashan barrier lake was located in the Long-vation is small and the geological condition of brokenmeng rainstorm area. The catchment area is aboutstones and soils is favourable for excavation. In addi-3550 km2. The annual average precipitation andtion, the distribution of broken and cracked rocks atdischarge stand at 1 355.4 mm and 92.3 m’/s respec-the mouth of the drainage channel will reduce thetively. Based on the historical data from nationaldownward erosion speed of water flood and mitigate thehydrological stations， a rainfall-runoff relation atimpact of the flood discharge to the downstream area.Tangjiashan dam section was obtained. Design floods3 Analysis on the mechanism of theand their processes were deduced with a hydrologicalformation of a barrier lakeanalogy method which was based on the similar catch-ment area of Tangjiashan and Xuanping station. It is3.1 Forming conditions of a landslide damestimated that the peak flood discharges for 100-year3.1.1 Topographical and morphological conditionsretur period and 200-year retum period are 6 040 m'/sThe shape and the slope of the valley are two im-and 6 970 m'/s respectively.portant factors for allowing landslide blockage of 82.4 Rapid evaluation of the geological datariver. The depth of the valley cut determines the criti-Some experts reached Tangjiashan barrier dam bycal volume for the stability and energy. The slope ofa helicopter on May 21, 2008, and field investigationthe landslide body determines the stability. Usually ,and geological surveying were carried out. Based onthe critical slope of valley sides is 30。to 45。, whichthe observations of these experts technical parametersallows a landslide blockage to occur. Where the slopeof the dam material were analysed.is larger than 45。，the possibility of a landslide block-With reference to similar geological conditions itage to a river is even higher. Tangjiashan section owas concluded， according to the experts ，thatTongkou Riv中国煤化工h a steep rightTangjiashan barrier dam was formed by the sliding,bank and a s:YHCNMH(Cat the landslidecrushing and breaking of the bedrock on the right riverrisk is very. angjiashan sec-bank. The dam material is mainly composed of crushedtion is cut down deeply and the valley topography isVol. 10 No. 1,Feb.201239narrow. All these conditions make Tangjiashan valleyken bedrock material piled up on the left bank. Theunstable and at high risk of blockage in the event of aupper part and some broken material piled up on thelandslide.right bank, fomming a lanslide dam with higher part3.1.2 Earth layer conditionson left and lower part on right, as shown in Fig. 1.Landslide damming is related to the existence ofsliding-prone stratum in the region. In general strata100180 }21 200composed of clay, mudstone, shale, marl and meta-1 100morphic rock, soft and hard inter-bedded rock or easily量1 001000E 900900weathered rock are liable to landslide. The investiga-800年tion results show that the bed rock of the two banks of700-2)Tangjiashan dam consists of thin chert， sandstone,;00marl and shale of Qingping group of lower CambrianWidth of the river bed/mseries, soft and hard inter-bedded, and the atitude isNote: the original groundline is eut from the map of 1/50 000N60 °E/NW C60 °. The bedrock fssure of the right8- Rordcr.of landslide body,(2)- Helicopter Pbank of the valley is more developed. Therefore, thestrata slope and rock joint fssure pattermn provide physi-cal infrastructure for mass sliding when encountering。 sands containing soils;(i2) -Lower Cambrian seriesinducing factors.3.1.3 Hydrodynamic conditionsFig.1 The project geological transverse sectionA landslide can not necessarily formn a lake unlessbrief figure of Tangjiashan barrier damsome hydrodynamic conditions are met. Firstly thelandslide material must be able to stay in the river bedAccording to the field survey it was a high speedwith river flow (i.e. not too much flow that washeslandslide body with an average speed of over 10 m/s.away the landslide material immediately) ; secondly theFor an objective understanding of the formation processriver has enough water flow so that it can form a lake inof the landslide dam and the verification of the field as-a short time. Based on the statistical data, annual av-sessment, a mathematical simulation was carried outerage daily water volumes are 6.5 million m' and 6.92with a DDA ( discontinuous deformation analysis )million m' in the middle ten days and the late ten daysmethod after the emergency handling had been comple-of May respectively. While the average daily runoff isted. The results show that the slide lasts about 40 s7.12 millin, 7.5 million m' and 6. 65 million m' inand the maximum slide speed reaches 29 m/s with anthe first ten days, the middle ten days, and the last tenaverage speed of 15 ~ 16.5 m/s. Fig.2 and Fig. 3days of June respectively. These water flows offer theshow the variation of slide speed and distance withnecessary hydrodynamic conditions for the formation oftime.Tangjiashan landslide dam and barrier lake.3.1.4 Triggering conditions. 30: -431Precipitation and earthquake are the main reasons士4384for the triggering of Tangjiashan barrier lake.Especially the high-strength earthquake can triggermany landslide dams easily. Based on the literature“Iso intensity Line of Wenchuan Earthquake" edited byEarthquake Bureau of China, the earthquake intensity5101520253035 4045Time/sgrade was over 10 at the dam site of Tangjiashan bari-Fig.2 The slide speed curve along with timeer lake. When the earthquake occurred, the functionof horizontal and longitudinal waves induced the high-600厂speed slide of the right bank of Tangjiashan section and00 +4431士8964a barrier lake was formed.400 ←43843.2 Forming process of the landslide dam300 tTangiashan barrier lake was formned by a land-slide. The mountain where the slide occurred is 634 m00 thigh and the slide base is a bedrock layer. The slide中国煤化工一多面$body moved across to the left bank at a high speed andreached 140 m high. The original lower part of the bro-Fig.3 The slide distance curve along with time几CNMHG40 Engineering SciencesThe main stress distribution in the dam body islong-term stability. Based on the estimation, its DBIshown in Fig. 4. It is illustrated that an extruding stressranged from 4.16 to 4.26, larger than 3. 08, thereforeidentical with the slide direction was produced in thelying in the unstable domain, so there existed high po-lower and middle part of the landslide body, owing totential for a dam breach.the extruding function. The direction of the main stress4.2 Scour/ erosion resistance analysis of theon the river bed is nearly parallel with the bed surface.landslide damThe value of the main stress is 2 ~4 MPa with the max-Based on the materials with which the landslideimum value 6 ~ 7 MPa. In certain ranges of the frontdam was formed, the following empirical formulae wereand back edges of the landslide body, the' landslideused to preliminarily analyse the scouring resistance ofbody was broken and so the stress was smaller.the landslide dam.Sediment incipient velocity- -Shamov Formula :U。=1.14 -gd(宁)古(2)Sediment incipient velocity- -Tang Cunben For-mula or Tang's Formula:U。=1.53./3; -gd(宁)古(3)Sediment incipient velocity- -Zhang Ruijin Formu-la or Zhang's Formula:Fig. 4 The primary stress distribution after theformation of a barrier lake as a result of landslideU。= 1.34、.2 -gd(岛)(4)Scouring resistance velocity for rock foundation:4 Safety assessment of the landslide barrierV= (5 ~ 7)、d(5)lakeBlock Stone incipient velocity- - Izbash Formula:A landslide dam is a natural damming of a river∪。=K、2_Y2gd(6)by the mass of the landslide. Unlike the properly de-signed and constructed hydraulic facilities, it is madeIn Eqs. (2) ~ (6), K is the block stone eff-up of heterogeneous，unconsolidated or poorly consoli-cient; r。is the bulk density of the sediment particledated earth and rock masses and liable to various prob-(or rock block); γ is the density of water; d is thelems, such as piping failure，overtopping failure, sub-grain size of the sediment or ( dimension of the rocksidence and stability failure, which directly influenceblock); g is the acceleration of gravity; h is the waterand determine the potential dam failure scenario and itsdepth.evolution processes.The estimated results are shown in Table 1 and4.1 Long-term safety assessment of the landslideTable 2. It can be seen from these preliminary analysisdamresults that significant scouring/ erosion would occur forBecause of the complexity of the landslide dam,loose soil or sand (d≤20 mm) if the flow velocitythere currently exists no authoritative formula for asses-reached around 2 m/s, for strongly broken rock fragsing its long-term safety. So an expedient and empiri-ments (d≤200 mm) if the flow velocity reachedcal method, i. e. the Dimensionless Blockage Index, isaround 4 m/s, and for rock blocks ( boulders, d=1 ~usually used for that purpose:2 m) if the flow velocity reached around 8 m/s.DBI=I1A.xHs)(1)4.3 Failure analysis of the landslide damIn general, landslide dam failures fall into threeIn Eq. (1), Ab is the catchment area; Hjis thescenarios : slope stability failure with subsequent damheight of the dam and Vg is the volume of the dam.breach, seepage destruction failure with subsequentBased on the statistics of 84 landslide dams worldwide,dam breach and overtopping failure with subsequentthose with DBI <2. 75 stayed in the safety domain;dam breach, among which the last one， namely thethose with 2. 75 < DBI <3. 08 fell into the uncertainovertopping failure scenario， is the most commonlydomain, and those with DBI > 3. 08 fell into the unsta-seen.中国煤化工ble domain.The bredam consistingShorly after the formation of Tangjiashan land-mainly of larMHCNMHGouldbeanab-slide dam, its DBI was estimated and used to assess itsrupt collapse or a progressive failure ，dependingVol. 10 No. 1,Feb.2012 41mainly on its shape and materials as well as its breachOverflow water,jetWater flowcause.Impinging area of the water jet1) Overtopping erosion failure was the primarycause leading to the breach of Tangjiashan landslideHeadcut facedam. The dam consisted mainly of soil and rock frag-BackwardTailwater poolswirlingments in its upper right zone and broken or crackedrocks in all other zones. The failure of the upper rightStagnation pointzone was similar to that of an embankment dam.Fig.5 Sketch of the head-cutThe overtopping and erosion failure process of anembankment dam can be demonstrated with a head-cutThe overtopping flow causes first rill and micro-rillerosion model ( Fig. 5). A head-cut means a suddenerosion on the downstream slope of the embankmentdrop in elevation on the ground ( on the flow-bed) , re-dam. The erosion eventually develops into a network ofsembling a small waterfall or a short cliff. W hen waterrills that gradually evolves into a master rill or gully.flows over a head-cut, the overflow jet impinges thThis gully intially consists of multiple cascading head-flow bed downstream of the head-cut and generates acuts which simultaneously migrate upstream andbackward swirling eddy which imposes shear erosion towiden, until only a single large head-cutting channelthe vertical or near vertical face of the head-cut andremains. The head-cut eventually migrates to the up-undercuts its foundation, resulting in the collapse ofstream end of the embankment dam crest; from then onthe head-cut face and the upstream migration of theany further upstream migration of the head-cut resultshead-cut.in a crest lowering and rapid increase of the dischargeTable 1 Scouring resistance analysis of the soil/rockat the breach. This progression finally leads to a fullmass forming the landslide dam (d =20 ~ 200 mm)breach of the dam. The deepening and widening of theFormulad/mm .h/mIncipient velocitygully and the upstream migration of the head-cuts are/(m.s')thought to be mainly caused by the turbulence and hy-Shamov Formula201.24draulic shear stresses of the flows within the jet impin-Tang' s Formula1.6ging area downstream of the head-cut. The turbulentZhang' s Formula1.33Shamov Fornula1.40swirling erodes the base of the head-cut, causes the1.87collapse of the head-cut face and eventually results in1.47the breach widening and head-cut upstream migration.2(1.49The relevant experts met at the site of TangjiashanTang’s Formula2.01barrier lake and consulted for several times to analyzeZhang' S Formula1.562002. 68the possibility of an abrupt failure or a progressive fail-3.60ure of Tangjiashan landslide dam and came to the con-3.03clusions: unlike those dams formed by soil landslides ,3.014.04Tangjiashan landslide dam consisted mainly of cracked3.35and broken rocks, so soil flow failure would be very3. 22unlikely; the middle and lower part of its upstreamTang' s Fommula4.32slope was covered by the muddy silt clay deposited inZhang's Formula3. ssthe Kuzhu reservoir and therefore had relatively goodTable 2 Scouring resistance analysis of the soil/rockanti seepage property , plus the fact that the rapid land-mass forming the landslide dam (d=1.0~ 2.0 m)slide had strong compaction and the lower part of thedam therefore had a relatively dense structure, so pip-d/ming failure would not easily occur; because of its mas-/(m. s')sive volume , gentle upstream slope and dense lower part ,Scouring resistance velocity1.07.00an abrupt failure of the whole dam could be basicallyfor rock foundation- -Tang' s FommulaIncipient velocity- -lzbash Formula 1.07.51ruled out ; therefore, the most probable failure scenarioof Tangiasha landslide dam would be the erosion/ scou-for rock foundation- -Tang' 8 Formula1.5ring failure by flow of large discharge , which was con-Inceipient velocity- -lzbash Formula..59.20fimmed by ho otol hannenad _At the initial stage，2.09.90only sev中国煤化Ierved at the toe offor rock foundation- -Tang' s FormulaIncipient velocity--Lzbash Fommula 2.010. 63the downsYHCNMHGofaround669m).With the increase of the reservoir level，another outflow42 Engineering Scienceswith a stable discharge of around 1 ~2 m'/s was foundcal formulae.at the elevation of 700 m on the downstream slope. The1) Analysis of the breach width. The followingoutflow water was clear , which showed no indication ofempirical formulae were adopted to calculate the poten-piping failure in the dam.tial breach width.2) The material composition of Tangjiashan land-Formula developed by China Academy of Railwayslide dam was favourable for preventing it from a fullSciences :breach. Field estimation showed that upper right partb。= K[w亦B+店](7)of the dam, which had a volume of about 7 million ~Formula developed by Yellow River Conservancy7.5 million m' and consisted mainly of alluvial depositsCommission:and strongly weathered rocks, would be prone to ero-b。= K[w*BzHJ]$(8)sion. The other parts of the dam, with a total volumeFormula developed by Xie Renzhi:of 13 million m' ，consisted mainly of broken orbm = KWHg/[3A]cracked rocks. So the dam was formed mainly by thebroken and cracked rocks. Unless it was overtopped byIn Eqs.(7) ~(9), b. is the width of breach offlows of high head and large discharge, an abrupt fullthe dam, m; K is the coefficient related to the dambreach would be very unlikely to occur.material; W is the storage of the reservoir, 10'm' ; B is .3) The broken and cracked rocks observed at twothe width of the river valley at dam site, m; Hois thelocations along the route of the drainage channel wouldmaximum water depth of the reservoir in the vicinity ofbe favourable for slowing down the breaching speed.the dam, m; A is the cross-section area of the valleyOn-site geological investigation showed that there were(below the crest elevation) at the dam site, m2. Bytwo locations, one at the outlet section of the drainageusing these threeformulae，the breach depth ofchannel and the other in the middle of the drainageTangjiashan landslide dam was estimated at 200 m,channel route, where broken and cracked rocks domi-400 m and 120 m respectively.nated. The excavation of the drainage channel con-Based on the composition and distribution of thefirmed the findings of the geological investigation. AI-materials in the dam, it was believed that a relativelythough the rocks at these two locations were highly bro-stable slope , from the middle of the dam crest down token or cracked, their formation structure was still ob-the right side，would be formed by erosion during dis-servable. They were thought to have relatively goodcharging of the impounded water in Tangjiashan bariererosion/ scouring resistance and might slow down thelake , and the maximum breach width would be 340 m.breaching of the landslide dam.Comprehensive analysis on the site concluded that4) Quick progressive failure would occur at cer-the breach width would be 340 m at the elevation otain elevations. The upper part of Tangjiashan land-752.5 m. It was predicted that left side of the breachslide dam consisted mainly of soil and strongly weath-would be relatively steeply sloped and the right sideered rock fragments. Once overflowing occurred, bothgently sloped, in a trapezoid shape.vertical or lateral erosion and/or scouring would be un-2) Analysis of the breach bottom elevation. Ac-avoidable. As mentioned earlier， the broken andcording to the field investigation ，geological investiga-cracked rocks at the outlet section of the drainagetion data of surrounding areas and the weathering statuschannel and at the lower part of the dam had relativelyof similar rock strata, it was estimated that there mightgood erosion/ scouring resistance, so it could be con-be a deluvial deposit cover ( flood deposited overbur-cluded that breach of Tangjiashan landslide dam wouldden) of about 20 m thick on the right bank and a 10 mbe a progressive failure. Induced by the drainage chan-zone of strongly weathered rock below it. Consideringnel, the erosion would develop along the channel andthe erosion/scouring resistance of the broken andthe channel would be deepened and widened by down-cracked rocks which formed the lower part of the dam,ward and head-ward erosions. Because the upper partthe lower limit of the strongly weathered rock zone,of the dam consisted mainly of loose materials， thisi.e.720 m, might be probably the lowest invert eleva-progressive breaching process developed very fast, last-tion. The corresponding breach height would be abouting for only several hours.one third of the dam height.4.4 Analysis of the breach parametersIn case of extreme conditions such as strong after-The shape and size of the breach were thought toshock, he中国煤化工wave caused by abe the key parameters to determine the dam-breakpotentialM L ran Landslide withflood. They were estimated based on the on-site geo-an estimatYHCNMH Gn m', high headlogical investigation and analysis and by using empiri- and large discharge overtopping might occur and resultVol. 10 No. 1 ,Feb.201243in dam breach with one half of the dam height, or evenMany institutions participated in the dam-breakthe whole dam height.flood analysis work in order to confirm the evacuation4.5 Stability analysis of the landslide damarea and evaluate the risk to infrastructures in the eventThe stability of Tangjiashan landslide dam re-of dam-break flood. Nearly 200 schemes of dam-breakceived widespread attention. During risk elimination ,flood were completed, in which the flood propagationthe Ministry of Water Resources of China organizedfrom the dam location to Beichuan， Mianyang anmany institutions to conduct the stability analysis of theChongqing was analyzed; moreover flood detentionlandslide dam with different approaches. Some univer-effects due to the water conservancy facilities and thesities and scientifie research institutes also conductedflood wave reaching the Fujiang River and the Yangtzethe stability analysis of the dam voluntarily. AccordingRiver were also considered.to the comprehensive results, it was considered by theA dam-break event can be divided into 3 typicalmajority that the overall stability of the dam body waspatterns : full-dam-break， 1/2-dam -break, 1/3-dam-satisfied. Some results showed that partial failure mightbreak, and other secondary dam-break patterns in theoccur in case of high reservoir level and other adversedam-break flood analysis. In the typical patterms, theconditions. Some results concluded that if the watertrapezoidal cross section is used to generalize thelevel upstream reached 745 m，the downstream dambreach shape. Among the 3 patterns, the top width isslope would be in a critical state of sliding; if the water340 m in all; the bottom elevation is 663 m, 695 mlevel rose to 752 m, the dam body would be in a limitand 720 m respectively; the bottom width is 100 m,equilibrium state.35 m, 35 m respectively; the breach depth is 89 m,57 m and 32 m respectively; the initial water level is set5 Dam-break flood analysisto 752 m. The breaching times range from0.5 h to 24 h. .In general the analysis of dam-break flood is still .The downstream peak discharges and the arrivalin the exploration stage. For overtopping induced brea-times of 3 typical damn-break patterns are presented inches, the weir flow method and hydrodynamic methodTable 3. The peak discharge of breach is 117 200 m'/sare used usually. The methods based on sedimentand it diminishes to 26 800 m'/s in Mianyang, whichtransport equations and erosion equations are applied toalso exceeds the design standard of flood defence in thesimulate the erosion process of a dam breach flow. Toscenario that the dam-break pattern is full-dam-breaksimulate the development process of the breach, thereand the breach duration is 1 h. In another scenario thatare 4 kinds of methods : the mechanism method ( pre-the dam-break patterm is 1/3-dam-break and thedicting breach development and hydrograph based onbreach duration is 3 h, the peak discharge of breach ishydraulics，sediment transport mechanics and soil21 100 m'/s, which will diminish to 11 800 m'/s in .mechanism)，the parameters method ( experimental es-Mianyang.timation of the parameters for hydraulic equations )，The computed results show that the peak dis-the predictive equations method and the comparativecharge depends on the initial water level, breach dura-analysis method.tion, breach shape and the breach development processThe mechanism methods including hydraulics ，etc. The higher the initial water level, the shorter thesediment transport mechanics and soil mechanics arebreach duration, and the faster the development ofused to calculate the erosion process and outflow hydro-breach width and depth, the larger the peak dischargegraph of breach. The duration of the breach formation ,in the dam location. Therefore, excavating a spillwaythe final shape and size of the breach are estimatedcan lower the initial water level, reduce the water vol-first, and then the hydraulic equations are used to cal-ume in the barrier lake, lower the peak discharge inculate the outlow hydrograph, which is called the pa-the dam location and reduce the threat to downstreamrameters method. The predictive equations method in-areas caused by a dam-break flood.volves the development of an empirical formula to com-The computed results also show that the peak dis-pute the peak discharge， breach duration and thecharges all exceed the river safety discharge limit in thebreach width etc. ，and moreover an approximate rea-upstream reach of the Tongkou river and also approachsonable hypothesis about the breach developmentor exceed the river safety discharge limit in the down-proposed. The comparative analysis methods are usedstream reach for the schemes of short breach duration.according to the similar dimension and structure amongTherefore in additin tn the engineering meas-the recorded historical dam-break cases and then theures, we sho中国煤化工ake the evacua-comparative analysis methods are used to estimate theion plan inMYHCN MH G avoid the riskdam-break outflow and the other breach parameters.caused by dam-break. Furthermore, people living in the44 Engineering SciencesTable 3 The dam downstream flood peak runoff and arrival time in typical dam breach patternsBeichuanTongkou TownRailway bridgeMianyang CityDurationPeak food(4. 6 km downstream) ( 24.0 km downstream) ( 48.0 km downstream) ( 68. 9 km downstream )breachof damrunof/patterbreach/h(m' .s')Peak runof/ArrivalPeak runoft/Peak runofl/Peak runof/ Arival(m3●g') .time/h(m3 .s')(m'.s")(m'.s') time/h43 20038 7001.0828 40023 50012 8005.881/3 breach29 60028 9002.0824 80021 80012 9006.6721 10021 0502.9719 90018 30011 8007.4780 50075 3001.0353 5004350020 0005.001/2 breach47 20046 8002.0342 40038 10019 40032 60031 7002. 9032 20031 00018 0006.50117 200115 2001.0070 00048 70026 8004.38Full breach63 10062 0001. 80s5 00045 20026 6005. 082.174130038 00025 6005.75upstream area of Tongkou river estuary should also be7 Drainage channel excavation planevacuated.7.1 Plan concept6 The overall emergency planConsidering the surface of the right dam body,In view of the special complexity and high risk ofwhich has relatively low-level and low scour resistanceTangjiashan barrier lake and according to the risk elim-and consists mainly of gravelly soil, the use of head-cutination principle of “Safe, Scientific， Quick"， theerosion theory has been demonstrated in this paper foremergency engineering measure and emergency popula-planning the utilization of the full carrying capacity of thetion evacuation plan were simultaneously implemented ,stream. A new river channel may be formed by the scou-in order to ensure the target of zero casualties, mini-ing and expansion of the stream, so as to achieve themize the loss caused by the discharge of impoundedaim of quickly but safely discharging the barrier lake wa-water, and try to prevent sudden breach of the land-ter and eliminating the danger of sudden collapse.slide dam which might cause enormous disasters7.2 Water drainage channel layoutdownstream.The water drainage channel is on the right side ofThe emergency engineering measure was drainingthe dam body and presents a right lateral bending archthe barrier lake using an artificial drainage channel.in plane. Because the right side of the dammed body isThe emergency evacuation plan was formulated by locala low-lying construction requiring disposal of lttle ex-governments based on the flood routing processes duecavated volume, and the right side of the drainageto different dam breach patterns and the risk assess-channel is mainly composed of gravelly soil, the dis-ment results. According to the three-level early warn-charged stream will mainly scour and down-cut theing mechanism， yellow, orange and red respectively ，right bank. In the meanwhile, the foot of the hills ofthe population of 277 600 in Mianyang and Suiningthe right bank consists of solid and integrated rockdownstream of Tangjiashan barrier lake threatened by awhereas the left side of the channel is mainly1/3 dam breach pattern were transferred to a safecomposed of broken stones which are powerful in scou-zone，and evacuation plans were formulated for thering resistance; therefore, the whole slope will remainpeople threatened by 1/2 and total dam breach, to bestable.put into action if the risk of this eventuality increased.7.3 Water drainage channel structureIn order to guarantee the successful implementa-The drainage channel adopts a trapezoidal cross-tion of the emergency plan, emergency support systemssection, and the scale of the two slope sides is 1:1.5.were established ，including rainfall-flood forecastingIn order to adapt to possible different construction peri-system, remote real-time video monitoring of the land-ods caused by weather variations , three proposals withslide dam , dam safety zone monitoring, communicationidentical sid二ntical side slopesupport system in dam zone, and the expert consulta-ratio and中国煤化工ne1 boto weretion and decision-making mechanism for preventingdesigned.TYHCNMHGtheelevationofdam breach.the entrance of the channel is 742 m and the width ofVol. 10 No. 1 ,Feb.201245the bottom is 13 m. In the medium-level proposal, theelevation of the entrance of the channel is 745 m andthe width of the bottom is 22 m. In the low-level pro-posal, the elevation of the entrance of the channel is747 m and the width of the bottom is 28 m.7.4 Dynamic optimisation in construction processDynamic optimisation is a basic principle thatmust be followed by risk management projects. In theconstruction process of the drainage channel, accordingto the practical geological conditions and constructionability，the structure of the channel was optimised andadjusted many times to take account of the earth andFig.6 The river channel plane shape after therock fill structure of dammed body ，temporal require-draining of Tangjiashan barrier lakements of risk reduction , and the momentum and energyof released water. Towards the end of completion of theThe peak discharge equivalent to 200-year returnhigh-level proposal , the elevation of the entrance of theperiod design flood in this section of the river is onlychannel was reduced from 742 m to 741 m; then near6132 m'/s, less than the peak discharge on Junethe completion of that design the channel bottom was10th. Hence, the watercourse is able to give space forfurther reduced by 1 m to 740 m and the width was 7 ~200-year return period flood.10 m.About 5 million m' of the deposits of the barrierdam were removed during the water release process.8 Emergency management effectThe remaining barrier on the left was composed of hardThe emergency management scheme was fficiallybroken small stone， large stone and isolated stone ，put into practice on the morning of May 26th, 2008.which had strong resistance to infiltration, erosion andAnd the drainage channel was cut in advance in thesliding.early morning of June 1st, 2008，which decreased the-Due to the successful implementation of the emer-oretically the volume of the lake by 100 million m' andgency management of Tangjiashan barrier lake, thethe water head by 12 m in the barrier lake. On Junewater storage was reduced by 70 million m', and the6th，some additional work was carried out to reducewater level was reduced by 9. 0 m. This controlled thethe resistance and enlarge the flow capacity of theprocess of progressive dam breach. Moreover, the peakdrainage channel.discharge at the dam was reduced to 3 400 m'/sThe water in the barrier lake started to flowroughly and at the cross section of the Fujiang Bridgethrough the drainage channel at 7:08, on June 7th.to3 000 m'/s approximately. The peak discharges atThe retrogressive erosion became clear in the afternoonthe main cross sections in the downstream reaches fellof June 9th. And the draining peak discharge ofabout 33.6 % ~34. 6 %，compared with the analytical6 500 m'/s occurred at noon time of June 10th. Byresults of the 1/3-dam-breach scenario. All of this14:00, June 11th, the water level in the barrier lakecontributed to mitigating the flood threat to the peoplehad fallen to 714.13 m from its peak of 743. 10 m andand the properties in the lower reaches of Fujiangthe water storage declined to 86. 1 million m' fromRiver. .246.6 million m' . The dangerous circumstance of the9 Conclusionbarrier lake was sucessfully eliminated without anyloss of life during the draining process.It took one month to handle Tangjiashan barrierA new arc-shape watercourse convex to right banklake after it was formed by the strong earthquake onwith a trapezoid cross-section was formed ( Fig. 6).May 12, 2008. Successful risk elimination at TangjiashanThe opening of the section of the watercourse rangedbarrier lake was achieved by an unconventional but sci-between 145 m and 235 m, the bottom width betweenentific approach, during an unusual period and under80 m and 100 m, the elevation at the inlet bottom,unusual conditions. It was a succesful case of preven-commonly, between 715 m and 720 m. The elevationting possible secondary disasters resulting from a largeofa deep channel on the right side was 710 m. Thearrier lake._ The risk_ elimination concept，scientifiecross section remained stable. The drainage capacityplanning an中国煤化工ergency manage-during the process should not be afected despite thatment of TanYHCNMH Gserve as a valu-some local collapses and blockage may take place.able model to aeal wIth simuar asasiers in future.46 Enginering SciencesReferencesgtze River, 2008(22):9194.1] Liu Ning. Emergency handling of Tangjiashan barrier lake and dis-[8] Chen Jianci, Fan Kexu, Li Zhongpin. Analysis of regonal hydro-aster reduction management engineering [ J ].Engineering Sci-logical features of Tangjiashan barrier lake[J]. Yangtze River,ences, 2008, 10( 12) :67-72.2008(22) :26-28.[2] Yang Qigui, li Qinjun. Technical charateristics and findings of[9] Guo Haijin, Zhang Honggang, li Zhongpin. Hydrological analysisemergency handling of Tangjiashan barrier lake[J]. Yangtze Riv-computation for emergency hazards-removal at Tangjiashan barierer, 2008(22): 13.lake[J]. Yangue River, 2008(22) :29-31.[3 ] Yang Qigui. Key technologies of Tangjiashan barrier lake emergen-[ 10] Huang Minghai, Jin Feng, Yang Wenjun. Flood evolution analy-cy draining project[J]. China Water Resources, 2008( 16):8-sis of the breach of barrier lakes under diferent river channelconditions[J]. Yangize River, 2008(22 ) :66- 68.[4] Zhang Yi， Chen Pengxiao，Lu Yunfeng. Rapid acquisition of[11] Yangtze River Water Resources Comission, et al. Technicalcharacteristic pararneters of barier lakes and the role of regionalsummary of Tangjiashan barrier lake emergency hazards removalhydrological analysis in overall impact analysis and evaluation[J].project[R]. 200Yangtze River, 2008(22): 96-98.[5] Huang He, Zheng Jiaxiang, Shi Yubing, et al. Analysis of theTangjiashan barier lake draining and hazards removal project andformation mechanism of Tangjiashan barier lake and emergencydam breach flood evolution[J]. Science in China: Series E,handling engineering measures[J]. China Water Resources ，20082009 (4): 801- 809.(16): 12-16.[13] Zhang Xibin, Luu Jinyou, Fan Beilin, et al. Dam breach flood[6] Ma Guisheng, Luo Xiaojie. The formation mechanism of Tangjias-evolution and drainage process reproduction of Tangjiashan barrierhan landslide and engineering geological features of barrier lakeslake[J]. Yangte River, 2008(22) :76-79.[J]. Yangte River, 2008(22): 46- 48.[14] Liu Ning. Technical understandings of the handling of river dam-[7Wu Aiqing, Lin Shaozhong， Ma Guisheng. DDA simulation 比ming barrier lake induced by huge landslide [ J]. China Watersearch of Tangjiashan barrier lake formation mechanism[ J]. Yan-Resources, 2008 (16):1-7.AuthorLiu Ning, male, was born in 1962 and awarded a Doctor degree in Hydrology and W ater Resources by WuhanUniversity of Hydraulic and Electric Engineering in June 2000. He is a professor-level senior engineer and incum-bent vice minister at Ministry of Water Resources in China. He has been mainly engaged in the design , planning,research and management of water and hydropower projects for many years. He has published over 100 papers, edi-ted 12 monographs and obtained one national patent. He can be reached by E mail: liuning@ mwr. gov. cn中国煤化工YHCNMHGVol. 10 No. 1, Feb.2012 47