FIELD MEASUREMENT OF IMPACT FORCE OF DEBRIS FLOW

The impacting manner of debris flow is the most destructive way for artificial constructions in the alluvial fan.Because of difficulties under the field condition,it is hard to directly obtain reliable field data of impact force.Model experiment and investigation of structural damage are the most usual approaches to acquire this kind of data.On August 25,2004,the channel change of Jiangjia Gully(Dongchuan,Southern China) brought an opportunity to measure the impact force in virtue of new pile foundation work and three sensors with gauge 10 kN.It is the first time to achieve a long-duration hydrograph of impact force with perfect wave profile at different flow depths.The sampling frequency of data acquisition system is 2 kHz,which is enough to capture the impulse caused by large-sized grain in the debris flows.After the raw data are denoised by low-pass filter designed using the combination of Fourier and wavelet transforms,it is found that the continuous flows in the first stage show greater impact than the subsequent surge flows at the similar velocity.As far as the surge flows are concerned,the velocity is not the predominant element which determines extreme impact pressure.It is argued that the collisions of big boulders suspended in the debris flows are responsible for the extreme values.The maximum impact pressure is equal to 2 628.4 kPa.Preliminary analysis of three pressure hydrographs of the same surge at different flow depths indicates that most of moderate-sized grains in the debris flows concentrate on the front and the upper surface of the fluid.This is of special significance for debris-flow protecting engineering design.