| China is one of the countries the most worst affected by natural disasters in the world, Wenzhou City is located in southeast China where frequently hited by typhoon landing in history. Slope geo-hazards such as collapses, rockslides or soilslides and debris flow often be triggered by typhoon landing which cause great losses in both economy and casualty, seriously affect the economy development in this region. Based on the collection, analysis of the data and database-building related to geo-hazards triggered by typhoon in Wenzhou City, this research, starting from distribution characteristics and influencing factors of geo-hazards, methods such as mathematical statistics, spatial analysis, grey incidence analysis, theoretical analysis and so on were adopted to study, reveal the distribution rules and influencing factors (especially the typhoon gale and rainstorm effects on the slope) of geo-hazards in Wenzhou City. In addition, the calculations of gale load on the slope were investigated. These study achievements can be used to do research on the mechanism of slope geo-hazards induced by typhoon and the slope stability in these areas can provide an important guidance and an effective method for future work on the disaster prevention and mitigation in southeast coastal region of China.Taking eleven typhoon processes in 2004-2009 as examples, and analysing 286 geo-hazards caused by typhoon, this study discuss the characteristics of typhoon gale, rainstorm and tracks, then give the following conclusions:This kind of geological hazard is mainly soil landslides(collapse), which components is mainly residual slope wash, which scale usually less than 1000m3, which failure surface is mainly the contact interface of quaternary overburden and underlying bedrock or the weak plane in soil layer. They distribute universally at low mountains and hills in Wenzhou, so are uneven distribution; the hazards caused by single typhoon have the obvious characters of regional, and some of which are centralized distribution. They distribute mainly in hilly area and low mountain area, between the elevation of 250~650m, the gradient of 20~40°, the aspect of SE-SW direction, the slope vegetation of tree, and in the Jurassic formation.Their distribution related closely to the typhoon tracks and the scale of typhoon gale and typhoon rainstorm, they are mostly limited within 300km vertical distance away from the center of typhoon tracks, most happened in the range which the maximal wind speed is above 7, some happened in the strong wind range over 10; and basically happened in the area which process rainfall is above 200mm or period rainfall is above100mm or the county(city) surface average rainfall is above 300mm. However, the amount and density of geological hazard are not simply positive correlation with the speed of gale, the intensity of rainfall and process rainfall, mainly because they are also influenced by geological environment conditions and some other factors.They are approximately synchronized with the process of landing-departure typhoon, gale and rainstorm, with no obvious hysteresis. They happened mainly during typhoon landing the day before to the day after, before the departure of the landing province, and happened mainly after the landing typhoon. It is that the gales keep long time to affect Wenzhou during the landing typhoon, which is helpful to cause geological hazard; and the instability time usually located in during the main typhoon rainstorm, many located in during the peak of intense rainfall.We adopted the gray correlation degree method to analyse the function degree of each factor that may induce geohazard, and to analyse the relationship among those factors. Their fuction degrees rank as following: wind speed, natural gradient, direction, petrofabric structure, rainfall, vegetation, elevation, the free surface slope angle and the duration of gale; Among them, the rank of arbor-slope as following: wind speed, natural gradient, petrofabric structure, direction and rainfall; the rank of bamboo-slope as following: wind speed, rainfall, petrofabric structure, direction, and natural gradient; the rank of shrub-slope as following: petrofabric structure, wind speed, rainfall, direction and natural gradient; the rank of grass-slope as following: natural gradient, petrofabric structure, direction, wind speed and rainfall.The speed of typhoon has a best relevance with arbor-slope, the higher speed the wind, the easier the instability of this slope. Bamboo-slope take second place, the frequentness of instability may reduce when the wind has a higher speed. And shrub and grass-slope has a lower relevance with it. The area precipitation of typhoon has a good relevance with bamboo-slope, the instability proportion of this kind of slope approximately amplify by the increase of area precipitation. And arbor-slope, shrub-slope, grass-slope take second place. The process rainfall has a good relevance with arbor-slope, and bamboo-slope shrub-slope grass-slope take second place. In a word, tree-slope has a higher sensitivity to the speed of typhoon than the rainfall and its intensity of typhoon.Typhoon gale induce slope instability mainly by act the rock in slope directly or vegetation on the surface of slope indirectly. Nevertheless, there were similarities and differences in mechanism and effect ralate to different types of vegetation lived in different types of slope affected by wind loads. For taproot-plants-slope, the soil underneath the plants in windward-slope under sliding force which can induce the instability in front edge; the soil underneath the plants in leeward-slope under resistance which also can induce the instability. For fibrous-root-plants-slope, the plants in windward-slope can increase the resistance; on the contrary, the plants in leeward-slope can increase the sliding force. Taking the plants in slope for biological bar, the tall trees and bamboos was regarded as high-building, the slope was regarded as dual-slope house, under this circumstance, and the wind loads can be calculated. Moreover, the wind vibration of tall trees in windward-slope, leeward-slope and lateral-slope can be obviously seen which play a key role in slope stability.The normal rainstorm and typhoon rainstorm have similar effect and mechanism influence to slope, without essential difference. But as for the vegetation slope, typhoon rainstorm has an enhanced effect. As for the vegetation slope, the influence caused by typhoon rainstorm could be divided into two parts. One is the influence to naked slope cause by typhoon rainstorm; another is the extra effect caused by the vegetation covered on it. The extra effect may reinforce or weaken the effect of typhoon rainstorm, and different types of vegetation slope have different results. What's more, the different rainfall intensity of windward slope, leeward slope and side slope also enhance or weaken the effect of typhoon rainstorm to some degrees. |
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