Study On Calculation Method Of Effective Rainfall In Early Stage Of Debris Flow Based On Soil Water Content

Debris flow is a kind of natural disaster widely distributed in mountainous areas in western China.It has a short formation time and strong outbreak force.It has caused great harm to mountain residents and infrastructure facilities such as highways and bridges.Among them,the ditch bed-initiated debris flow is mainly distributed in the mountainous areas of southwestern my country.The early warning of this type of debris flow is mainly realized by monitoring rainfall,mud level,infrasound,ground sound and video.It is limited by the influence of the stability of monitoring equipment.Early warning methods such as infrasound cannot be widely promoted and applied,while monitoring methods based on rainfall are more direct,economical and more widely used.Rainfall is mainly divided into early effective rainfall and stimulated rainfall.The effect of the effective rainfall in the early stage is to increase the water content of the surface soil.At this time,the infiltration capacity of the soil decreases.At this time,as long as the rainfall is greater than the surface permeability coefficient of the soil,super-osmotic runoff will occur.Slope runoff converges to the channel to form a powerful mountain torrent,which strongly scours the solid matter in the channel and then lifts the bottom and mixes with water to form a debris flow.In the current fine-grained prediction model for the initiation of the gully bed,the preliminary rainfall calculation method does not take into account the problem of evaporation attenuation,and the preliminary rainfall calculated under the special rainfall type is too small,which is likely to cause underreporting.Based on the forecast model,this paper improves the previous rainfall algorithm in its rainfall factor R,analyzes and fits a large number of meteorological data,and obtains the calculation method of the effective rainfall of the debris flow based on the soil water content in different regions,and respectively proposes it with Yu Bin and Chen Jingwu The comparative analysis of the previous rainfall calculation methods,combined with the debris flow data in other regions,analyzes the contribution weight of the previous rainfall in the occurrence of debris flow,and determines the new forecast critical value.Now draw the following conclusions:（1）Using multi-year average rainfall and multi-year average sunshine hours,China is divided into humid zone,transition zone and arid zone.（2）Through years of meteorological data analysis,the surface wetness index M_i calculated by rainfall P and temperature T,combined with soil water content data for fitting analysis,the soil water content algorithm for different regions and different periods of time is obtained:humid area:S_m=α（0.0168 ln（M_i）+0.3452）,α=1 when daily rainfall ≥1mm,α=1.15 when daily rainfall<1mm;transition zone:S_m=0.0168 ln（M_i）+0.3452;arid zone:S_m=β（0.0058lnM_i+0.2475）,β=1 when the daily rainfall is ≥1mm,β=0.8 when the daily rainfall is<1mm.（3）Collect field monitoring data and meteorological observation data of soil water content in rainy season in many regions,and fit the relative relationship between previous rainfall B and soil water content S_m:B=3× 10^-8e^59.4Sm（humid zone）;B=1 × 10^-8e^56.7Sm（transition zone）;B=1×10^-17e^161.6Sm（arid area）.（4）In the case of different rainfall types,the previous rainfall results calculated by the algorithm in this paper are compared with the results of the previous rainfall algorithm and the rainfall day attenuation method of the original model.The algorithm in this paper takes into account the evaporation effect,in the case of continuous rainfall and short-duration heavy rainfall.The previous rainfall is generally 10%to 20%smaller than the original model algorithm result,which is closer to the real situation;in the case of two rainfall pauses for more than 6 hours,the algorithm in this paper takes into account the impact of the previous rainfall on the soil,and the calculation result is larger than the original model.Compared with the original model,the previous rainfall algorithm has a better effect.（5）In this paper,the refined prediction model of the original gully bed start-up debris flow:P=RT^0.2/G^0.38=（B+KI）/R₀C_v[JA/L²（A/W²）^0.2]^0.2/（D/D₀）^0.38≥C_r,using the previous rainfall algorithm proposed in this paper to replace the previous rainfall B in the original model rainfall factor R,and based on the occurrence of debris flows in different regions Analysis of the contribution rate of rainfall B in the early and mid-term,the new 1h early warning model critical value C_r is obtained.（6）The improved debris flow warning model was applied to the Wangmo debris flow in Guizhou Province and Dechang debris flow in Sichuan to verify the model.The verification results also confirmed that the improved model has better performance than the original model after considering the influence of soil moisture characteristics.Better science and reliability.