Preliminary Study On Effects Of Earthquake In Critical Rainfall Conditions Of Debris Flow

In mountain regions, strong earthquake result in loose and collapse of hillbody,forming a large mount of landslide of geologic disasters. It bring dangers to people’sliving and production, changed forming conditions of debris flow in mountain regionsto transform new debris flow and aggravate activity of old debris flow, leading moredangerous debris flow disasters.The forming condition of debris flow in earthquake region conclude these factors:soild sediment increased in valley, the sediment of landslides in gully increasedobviously; stability of hillbody has decreased and surface vegetation has degraded.Changes of these factors lead to lower rainfall of breaking debris flow. For example,Chenyoulan valley in Jiji earthquake region of Taiwan has precipitation decreased2/3to breakout debris flows, in Wenchuan earthquake region, prophase accumulateprecipitation to breakout debris flows has decreased14.8%-22.1%, hourly rainfalldepth has decreased25.4%-31.6%.This article compared critical rainfall before and after earthquake, showedrainfall condition in debris flow, avoid effect of geology and topography. Based onanalyzed critical rainfall mode of debris flow, ensure the research object of dailyrainfall （Id） and I-P mode of critical rainfall （rainfall intensity-prophase accumulateprecipitation）, analyzed space time characteristics and changes of critical rainfall inWenchuan, SonPGAn-Pingwu, Chi-Chi earthquake regions, studied the relationshipof critical rainfall variation and peak ground acceleration before and after earthquake,the relationship of critical rainfall variation and times after earthquake.At last come toa conclusion:（1）Start-up mode of debris flows in earthquake region mainly are gully, shallowlandslide, and barrier dam breaking starting. compared with condition beforeearthquake, the changes of solid sediments in gully, surface structure of shallow landslide and vegetation damage in surface cause easily break debris flow, and haslower critical rainfall.（2） analyzed formation mechanism of debris flow, I-P critical rainfall mode canbetter reflect critical starting status of debris flow, daily rainfall （Id） can also betterdistinguish critical rainfall parameter in breakout debris flow. The critical rainfallequation of I-P mode is I=KP+R, The coefficient of K changes small with vary areas,distributing at range of0.073to0.123, according to I60-P mode, The average value ofK are0.084.（3） In Wenchuan earthquake affected area, the power function would be used todescribe the relationship between varying parameter of critical daily rainfall andthe peak ground acceleration （PGA）’s relative value,the function:n=0.092G^-1.5,G=PGA/g,n≤1. the earthquake has a low-impact on the debris flow’scritical rainfall in the PGA range of0.2to0.3g, after the earthquake, the criticalrainfall of debris flow is only reduced to about80%of the original. From0.3to0.4g,the impact of earthquake increases rapidly on the critical rainfall of debrisflow,and the parameter dropped0.6to0.35. The parameter n dropped0.35to0.2inthe range of0.4-0.6g. the critical daily rainfall of debris flow is about10-20%of theoriginal in greater than0.6g range, and the more backward the n value changes,indicating that the local peak ground acceleration reaches a certain value, the impactof earthquake,which on critical rainfall of debris flow, to the extreme.（4） In the Pingwu-SonPGAn earthquake zone, the PGA in the region greaterthan200gal, the debris flow occurred after the earthquake, the critical rainfall changescan be divided into three stages, pre-slow recovery phase; a substantial recovery in themid-stage; later gradually approaches the pre-earthquake state. In different PGAregion, the critical rainfall characteristics change over time are not the same. Slowrecovery phase, owing to the amount of the material source of the earthquake zonewatershed is in a state of equilibrium, that is, on the one hand new landslides increasethe provenance, the other hand, landslides washed away the material source, thecritical rainfall slow recovery of the debris flow; substantial recovery stage due do notthe new material sources, the intensity of the shallow soil is gradually restored, andthe gradual recovery of the vegetation, making the mid-term recovery faster; late dueto the effects of earthquakes gradually being eliminated, the critical rainfall of debrisflow will gradually approach the pre-earthquake state.（5） Not only in the Wenchuan earthquake zone, in other earthquake zone after theearthquake verying parameter n of the critical rainfall change is also with the ground motion peak acceleration increases with increasing. The critical daily rainfall of theearthquake debris flow, the trend of the characteristic value R of the critical rainfallline of loose SonPGAn-Pingwu earthquake zone critical daily rainfall and the TaiwanChi-Chi earthquake zone is consistent with the overall trend in the form of a powerfunction. Critical rainfall line of the Chi-Chi earthquake in Taiwan eigenvalue Rtrends more than the critical day rainfall slow.（6） Overall, the impact of earthquakes on critical rainfall of debris flow reflectthe differences in space and time. In space, the critical rainfall in different regions inan instant reduced to varying degrees, reduce the degree of PGA related. In Time, thecritical rainfall and mudslides occurred suddenly affected by the earthquake andinstantly reduce the impact of this earthquake over time and gradually phaseddisappear. Finally the results,After the earthquake, the mudslides critical rainfallspatial and temporal variations of the functions: the first stage, when1≤t≤t₁, n=0.019（t-1）+0.092G^-1.5; the second stage, when t₁≤t,n=0.035t+0.092G^-1.5－0.016t₁－0.019; the third stage（conjecture）,t₂≤t≤t,n=0.016（t－t₁）+0.019（t₂－1）+0.092G^-1.5。t is the time after the earthquake; G is a relatively PGA values; t₁is the first stage ofthe debris flow duration after the earthquake; t₂is the sum of the first stage andsecond stage.