The Study Of The Mid-and-long Term Mechanics Of Gully Debris Flows After Earthquake In Wenchuan District

After “512” Wenchuan Earthquake, the debris flows in research area hassignificantly enhanced. Many no-debris flow gullies turned to debris flow gullies andlow frequency debris flow gullies turned to medium-high frequency debris flowgullies. During the five years after earthquake, three super-huge scale debris flows,“814”Yinxiu debris flow,“703”Luoquan gully debris flow and“710”debrisflow,occurred. Such super-huge scale debris flows caused fearful casualties andproperty losses and has hindered the reconstruction in disaster area. So the study ofmechanism of mid-and-long term gully-type debris flow is necessary for predictionand prevention in disaster area.On the basis of area’s geological environmental conditions and formationcondition of debris flow, this paper focused on the study of mechanism ofmid-and-long term gully-type debris flow by multi-temporal remote sensing andinvestigation about landform, loose materials source, valley blocking, migration andsupply of source and critical rate of rainfall. The main conclusions are as followed：（1）The advantage shape of gullies and massive channels form plenty waterresource to initiate debris flow and passageway for provenance migration. Averagelongitudinal grade is200‰～400%, which provide potential energy for debris flowinitiation and Slope gradient is25°～75°,which is easy for collapses and landslides.Such advantage condition increased slope flow velocity and reduced concentrationtime.（2）There were a massive of collapses, landslides and surface debris triggeredby “512” Wenchuan earthquake. They had looseness of structure, in critical stableor unstable state, and were stored in or near gullies, which offer abundant provenancefor debris flow.（3）The channel in study area were changed by “512” Wenchuan earthquakeobviously. A abundant of solid sources were heaped up in the channel or totallyblocked the channel, which seriously affected the runoff. Now mid-and-long termdebris flows are mainly affected by three ways:①Debris flow occurred by existingdam-collapse directly；②The debris flow scale were enlarged by Block-collapse situation；③Channel blockage coefficient increased and the debris flow’s abilities ofwashing and carrying were enhanced.（4）The main solid sources of debris flows in study area are from collapses andlandslides triggered by earthquake. The mega volume of sources couldn’t totallycarried out from valleys by single debris flow so that there are enough sources fornext debris flow, for example, though Qipan gully debris flow took away78.2×104m3solid sources,558.05×104m3are still left. The solid sources supply for debrisflow is an accumulation process that lasted mid-and-long term, the main supplypatterns are as followed：①Gully edge collapses；②Migration of Collapses andlandslides；③Tributary debris flows；④Cataclastic rock avalanches；⑤. Deposit ofold debris flows.（5）Most of the debris flows in study area are from medium to super-huge scale,mainly inducing by rainfall. And the critical rate of rainfall are reduced by earthquake.So this paper speculated that the critical rate of rainfall is20～40mm/h through theanalysis of rainfall data and referred to existing materials.（6）On the synthetic condition of study area’s landform, valley blocking,massive provenance and mid-and-long term supply, debris flows will be in active stateduring mid-and-long term. It may last15a～25a and more until the potentialprovenance and critical rate of rainfall come to stable state as that before earthquake.