Research Of Lahar Dynamics And Hazards

As the most critical secondary disaster of volcanic eruptions, lahar is characterized by theacutest activities，fastest spread, and broadest range. The lahars of the Changbaishan Tianchimillennium eruption are so poudage that they are always the focused subject of domestic andoversea lahar research. The impoundagc of Tianchi reaches20.4X1083m, and the Tainchi volcanois covered with thick snows and ices for a long time every year. Once Changbaishan Tianchivolcano eruptes suddenly again, the lahar disaster is easy to be triggered. Erdaobaihe town withover50,000populations is just located50km away from the Tianchi crater and the potentiallahars may put the life and porperty of people at risk. The previous studies show that the lahars ofChifeng phase eruption are mainly distributed not only in the upper reachcs of the Songhua Riverwhich is close to the crater, but also along both sides of the Erdaobai River, Tumen River andthe west side of the Yalu River. The previous researches focus on the distirbution, stratigraphicsequences and depositional environment, but the facics of lahars and the hazards zonation oflahars have never been established due to lack of research.To address these Issues, a series of lahars fades diagrams have been plotted in this study byanalyzing the distribution characteristics, grain size characteristics, and microscopic morphology,chronology and accumulation characteirstics of Tianchi volcanic lahars. Based on these workresults, the Tianchi volcanic lahars hazard zoning map has been completed and a comprehensivestudy on lahars dynamics mechanism and disaster from both macro and micro aspects has beenconducted. Finally the same method is applied to study the genetic mechanism and disaster of themud volcano at Ramree Island in south of Myanmar.1. Research contents(1)This thesis discusses the potential lahar danger of the Tianchi volcano through studyingthe tectonic setting, environment, geological background and climatic conditionscomprehensively.(2)The outcrop sequences of lahar are set up synthetically through studying the stratigraphy,sedimentology and hydrodynamics of historical lahar deposits. In the same time, characteirstics ofthese sequences are studied. (3) The facies diagrams of the Tianchi volcano lahars are built by statistically analysis of thehorizontal and vertical particle size, microscopic morphology, component characteristics,formation age and accumulation mechanism etc.(4)The possible future laliar disaster range of the Tianchi volcano is simulated numerically,the different scales of the eruption of volcanic debris flows distribution are calculated, and thehazard zoning map of Tianchi volcano is compiled by means of parameter simulation summarizedin outcrop sequence, in view of TIN and DBM diagram of the Tianchi volcano area.(5)The same research method is applied to make particle size analysis and study themicroscopic morphology of the mud volcano in Ramree Island. Then I analyze its geneticmcchanism and discuss the disaster of the mud volcano.2. Research methodology(1)Thelahar distirbution characteristics of the millennium eruption in the ChangbaishanTianchi volcano along the Erdaobai River’ Tumeti River, Yalu River and Toudaobai River arestudied by surveying the Changbaishan Tianchi volcanic lahar field distribution comprehensively.The main study range of this research is the Erdaobai River, and the outcrop sequence of theBrdaobai River and Tumen River are established.(2)Thespatial distribution characteirstics, strata structural characteristics, depositioncharacteristics, fluid mechanics, and other features are used to study the accumulationcharacteirstics system of outcrop sequence according to the different facies of debris flaw,transition flow and hyperconcentrated lfow.(3)The samples from the filed are used for particle size analysis, microscopic morphologyanalysis, age of formation analysis and accumulation mechanism analysis, and the lahar faciesdiagrams are established from macro and micro perspectives.(4) The lahar possible range is simulated based on field parameter measurements on the1:250000DEM of the Tianchi Volcano. The lahar disaster zoning map is prepared based on thedifferent threshold volumes to forecast the possible hazard range,(5) The mud volcano hazard of Rainree Island in Myanmar is studied by using the samemethod, the samples are dealt with particle size and microscopic morphology analysis to ascetrainthe genetic mechanism and the hazard. In summary, the lahars of Tianchi volcano are studied comprehensively from macro and micro perspectives by dynamic analysis, fluid dynamic analysis,accumulation characteristics analysis and geological characteristics analysi. The dynamicmechanism and the hazard zonation are determined，then using this method I discuss the mudvolcano hazard.3. Main ConclusionThis thesis discusses the hazard of Tianchi volcano lahar mainly from distribution patterns,bulk and debulk characteristics, particle size analysis，microscopic morphology analysis,formation age and accumulation characteristics, the Tianchi volcano lahar facies diagram isestablished based on these analysis, and the Tianchi volcano lahar disaster zoning map is preparedby from the numerical simulation combined with field parameters, as well as the same method isused to study the mud volcano hazard of Ramree Island in Myanmar. The conclusions of thisresearch are as follows:The source of the material composition of the vulcanic laliar mainly consists of three parts,the fragments generated by the Changbaishan Tianchi volcano millennium explosive eruption,abundance ashes and punnice produced in the eruption column collapse, deposits in thepaleochannel surround the craterThe accumulation mechanism of lahars is mainly considered from two aspects of time andspace considerations. In time aspect, it was easy to form large-scale volcanic debris avalanche toconvert into lahar in the earlier stage of eruption because of the plenty rain and provenance. In the’meantime, the lahars transport action ability was strong, the debris lfow move rapidly carryinghuge boulder, messy accumulate, poorly sorted, and the deposits were rounded because of the longdistance of transportation, but the deposits of this period weer washed and changed many times bysubsequent lahars. Residual less, then accumulated at a special location such as a irver bend,because of the rapid change of hydrodynamic conditions, there would farm accumulation withthick layers and weak bedding. With the transportation distance increasing, potential energy andkinetic energy decreasing, the sediment carrying in lahar accumulated slowly. As the reduction oferuption scale and the passage of time, most of the cjecta were transported by lahar. In thecondition of lacking material source and rainfall, th’e lahars scale became smaller, ability oftransportation and accumulation became weaker, the main material of transportation becamc ashes and pumice.On the spatial distribution, th，e lahars carrying capacity gradually weakened with thegrowth of the transportation distance, the grain size of accumulation changed from coarse to fine,the thickness of the deposits became smaller gradually, lahar flooded the main irver channel andexpanded outward to make the channel wider the at the same time. The granularity of depositionbecame ifne, sorting became fine and the bedding became obvious. In the main channel, the mainaccumulation was coarse debris lfow, and the bedding was parallelly formed in the condition ofstrong hydrodynamic conditions.The lahar hazard in the millennium eruption of the Changbaishan Tianchi volcano isdistributed along the Erdaobai river, Songhua river, Tumen river and Yalu river. And because theErdaobai irver is formed directly from Tianchi and outflows through an U-shaped valley, the laharhazard in Erdaobai is particularly serious. The lahar in the Erdaobai river is originated fromnearby Erdaobai he town，40km away from the Tianchi crater. The area from Erdaobaihe town toQianjiang, Erdaojiang and Shuitian village is subject to debris flow hazard area where thecharacteristic is stratum of mingle accumulation without obvious particle grade and bedding. Thefragments are mainly rock fragments containing some gray-white and gray-black pumice particles.The shapes of the fragments are angular and subangular, rounded poorly, the chemicalcomposition of fragments are mainly Si and Al. The accumulation is mingling because of thestrong hydrodynamic conditions and the high velocity of ground water. The hazard area of theShuitian village to Liangjiang town is a transformation hazard zone, where the characteristic isstratum of near horizontal accumulation, appreciably reverse particle graded because of thetransformation faceies，appreciably bedding. The fragments are mainly rock fragments, and thecontent of pumice increases, some show lens-face and pinchout reproduction. The shape of thefragments are mostly subangular, subrounded, roundness better, the ingredients are mainly Si, Aland Na. The accumulation is nearly hoirzontal because of the less hydrodynamic conditions, lowerspeed, and the better depositional environments. The hyperconcentrated faceies of lahar startsfrom Liangjiang town with the characteristic of horizontal accumulation, obvious particle graded,obvious bedding, rich in gray-white pumice particle, the shapes of the fragments are mostly nearlyrounded, and the particles are fragmentary. The ingredients are mainly Si, Al, Na and K. Theaccumulation is nearly hoirzontal because of the weak hydrodynamic conditions, slow speed， better sorting and rich in pumice. Its reason maybe repeated transportation. There are a little laharhazards in the Tuinen river and Valu river because these two rivers are not rooting from Tianchicrater, but not as much as that in the Erdaobat river, The reason is that rainstorms washed the loosedeposits of volcanic pyroclastic flow, but not directly caused by the millennium eruption or waterfrom Tianchi, so the disaster degree is lower, but atso developed a certain scale of lahars.【nsummary, the area of the Erdaobai River is the severest hazard zone. Therefore, there should bemore effective preventive measures in the future iahar prevention.The different lahar hazard extents of the Tianchi volcano are simulated using LAHARZ andoptimal threshold selected by different gradient eruption volume values in the four rivers whichare prone to occur lahar. The simulation results are as follows: the lahars farthest distance of theErdaobai river is about100km, about to arrive the Yanjiang village; along the Songhua river thefarthest distance is about130km, about to arrive Yushu town; along the Yalu irver the farthestdistance is about80km, about to arrive Jinhua village; along the Tumen river the farthest distanceis about120km, about to arrive Nanping town. So, the possible future lahar disaster range isdivided into four hazard areas by different eruption volume thresholds, the radiuses are40km,70km,80km and100km, respectively and the highest lahar hazard level radius is40km，that meansthis area is the most urgent prevention region.The eruption type of the mud volcano in Ramree Island is not blast-type, and the outbreakintensity is week, the clotting time is short, and the dis^ter effect is not strong by analyzing thedisaster and genetic mechanism, the particle size and the microphotograph, but the existence of themuddy layer is likely to cause slumping. And there is a uniform layer of argillaceous to be theuniifed provenance of the Ramree Island mud volcano because the ejecta are from the sameprovenance.4. InnovationIn the course of this study, there are three main innovations;(1) This thesis studies the lahars of the Changbaishan Tianchi volcano comprehensivelybased on the previous studies, the wealthy information and new understanding. First of all, thefades diagram of lahar in the Tianchi volcano is established by analyzing the distribution range,particle size, microphotograph characters and the formation age, distinguishing the facies of debris fiow, the transition flow and the hyperconcentrated flaw in the aspects of distribution extents,formation conditions and the accumulation mechanism. In addition, the cause and the conditionsof formation, the accumulation mechanism are deduced by the judgment of comprehensivefeatures. The analysis are based on the multi-disciplinaiy research of hydrodynamic conditions,accumulation characteristics, composition characteristics, microscopic morphology characteristicsand particlc size characteirstics, of the Frdaobai river. Combining the conditions of others rivers,the conclusion can be deduced that there is higher incidence of lahar in the Brdaobai river than theothers. This work also studies the lahar in the Turiien river and Yalu river.The lahar ranges are simulated for in the Brdaobai river, Yatu river，Tumen river and the firstSonghua irver by setting different disaster volume values on the base of parameters from the ifeld.The disaster lever ranges are divided for the four rivers prepare to establish the ChangbaishanTianchi volcano hazard zoning map.The mud volcano formation mechanism and the potential disaster of the Ramree Island inMyanmar are ascertained by analysis of the particle size and the microscopic morphology of thecjecta combining with the background of regional geology. It can be deduced that there is auniform layer of argillaceous to be the unified provenance of the Ramree Island mud volcano,where the eruption is not blast-type, the outbreak intensity is week, the clotting time is shot, andthe disaster effect is not strong.