Effects of loose deposits on debris flow processes in the Aizi Valley, southwest China

Loose deposits, rainfall and topography are three key factors that triggering debris flows.However, few studies have investigated the effects of loose deposits on the whole debris flow process.On June 28, 2012, a catastrophic debris flow occurred in the Aizi Valley, resulting in 40 deaths.The Aizi Valley is located in the Lower Jinsha River,southwestern Sichuan Province, China. The Aizi Valley debris flow has been selected as a case for addressing loose deposits effects on the whole debris flow process through remote sensing, field investigation and field experiments. Remote sensing interpretation and laboratory experiments were used to obtain the distribution and characteristics of the loose deposits, respectively. A field experiment was conducted to explore the mechanics of slope debris flows, and another field investigation was conducted to obtain the processes of debris flow formation, movement and amplification. The results showed that loose deposits preparation, slope debris flow initiation,gully debris flow confluence and valley debris flow amplification were dominated by the loose deposits.Antecedent droughts and earthquake activities may have increased the potential for loose soil sources in the Aizi Valley, which laid the foundation for debris flow formation. Slope debris flow initiated under rainfall, and the increase in the water content as well as the pore water pressure of the loose deposits were the key factors affecting slope failure. The nine gully debris flows converged in the valley, and the peak discharge was amplified 3.3 times due to a blockage and outburst caused by a large boulder. The results may help in predicting and assessing regional debris flows in dry-hot and seismic-prone areas based on loose deposits, especially considering large boulders.

Spatiotemporal variation of the loose deposits in Baisha River Basin after the 2008 Wenchuan earthquake

Extensive landslides were triggered by the Wenchuan earthquake(in China)on 12 May 2008,causing a tremendous amount of loose material suspended on the hillslopes,likely to be eroded and transported by rain.It is of great significance to study the long-term variation of the quantity and spatiotemporal distribution of loose deposits after a great earthquake in order to understand the restoration process,to assess the risk of future soil erosion,including geological hazards,and to further develop ecological governance strategy.In this study,information about the multi-temporal loose deposits on the ranges of the Baisha River Basin,an alpine valley near the epicenter,was extracted by objectoriented remote sensing interpretation,and analysis on their spatiotemporal variation showed that the earthquake in 2008 resulted in loose deposits covering an additional area of 81.09 km2,with a volume estimated at 0.357 billion m3.Within five years after the earthquake,the vegetation had recovered rapidly,and the extent of the deposits was significantly less.From 5 to 13 years after the earthquake,the vegetation continued to recover but at a slower rate and a decreasing trend in newly formed deposits was evident.The total area,volume and quantity of the loose deposits gradually stabilized.The geometric mean center of the loose deposits gradually moved back towards the upstream area after the earthquake and,by 2021,the mean center was about 4 km away from its pre-earthquake position.Taking the mean center of the loose deposits in 2008 as the center,the deposits in the downstream area shrank significantly after the earthquake,but collapses and landslides of deposits persisted in the upstream area to the west-southwest,west-northwest and northnorthwest from the mean center,indicating future source areas of new loose deposits in the basin.

Hydraulic mechanism and time-dependent characteristics of loose gully deposits failure induced by rainfall

Failure of loose gully deposits under the effect of rainfall contributes to the potential risk of debris flow.In the past decades, researches on hydraulic mechanism and time-dependent characteristics of loosedeposits failure are frequently reported, however adequate measures for reducing debris flow are notavailable practically. In this context, a time-dependent model was established to determine the changesof water table of loose deposits using hydraulic and topographic theories. In addition, the variation inwater table with elapsed time was analyzed. The formulas for calculating hydrodynamic and hydrostaticpressures on each strip and block unit of deposit were proposed, and the slope stability and failure risk ofthe loose deposits were assessed based on the time-dependent hydraulic characteristics of establishedmodel. Finally, the failure mechanism of deposits based on infinite slope theory was illustrated, with anexample, to calculate sliding force, anti-sliding force and residual sliding force applied to each slice. Theresults indicate that failure of gully deposits under the effect of rainfall is the result of continuouslyincreasing hydraulic pressure and water table. The time-dependent characteristics of loose depositfailure are determined by the factors of hydraulic properties, drainage area of interest, rainfall pattern,rainfall duration and intensity.