Simulation Experimental Study On Hydrological Process Of Karst Matrix-Conduit System

Karst aquifers have well-developed fracture and conduit,and the underground hydrological processe is often very complex.The hydrological law of karst aquifer under the condition of complex fracture and conduit system had always been a hot issue in academic circles.To solve this problem,this study quantitatively studed the functional relationship between the parameters such as permeability coefficient,fracture rate,conduit rate,rainfall intensity,conduit wall permeability coefficient,the number of branch conduits and hydrological process characteristic parameters of karst fracture-conduit system through laboratory experiment and numerical simulation.In addition,the distribution law of attenuation coefficient under different fracture-conduit rate conditions was calculated,and the hydrological law of karst aquifer under different fracture-conduit development conditions was tried to be explored.（1）The experimental device of karst fracture-conduit system was designed by self-made bricks with holes.The variation law of spring flow characteristic parameters under the conditions of variable supply,variable branch conduit and variable conduit rate was obtained by using the device.The results showed that the confluence time is inversely proportional to the supply flow,while the rising time,steady flow,retreating time,attenuation coefficientsα₁ and α₂ are proportional to the supply flow,and α₃ basically did not change with the supply flow.The confluence time and retreating time were inversely proportional to the number of branch conduits,the steady flow and attenuation coefficients α₁ and α₂ were directly proportional to the number of branch conduits,and the rising time and α₃ basically did not change with the increase of the number of branch conduits.The confluence time and attenuation coefficient α₁ were inversely proportional to the conduit rate,while the rising time and steady flow were basically unchanged.Under the strong supply flow,the retreating time was inversely proportional to the conduit rate,and the low supply flow rate was opposite.No obvious quantitative relationship between α₂ and α₃ and the conduit rate was obtained.（2）The mathematical model corresponding to the physical model was established based on the CFP program,and the mathematical model was calibrated and verified by using the measured water yield and permeability coefficient.The results showed that the CFP model can well simulate the physical model.The measurement range of water yield in laboratory experiment was[0.015,0.026],and the permeability coefficient was 0.02058 cm/s;The permeability coefficient of CFP model was determined as 0.012 cm/s and the water yield was 0.025.Both permeability coefficient and water yield were consistent with the range of physical experimental parameters.The Nash coefficient of the model calibration was 97.76%,and the Nash coefficient of the model verification was above 86.58%.（3）Based on the ideal model,the regression curve was fitted with the three-segment exponential attenuation function,and the quantitative functional relationships between different parameters and peak flow and attenuation coefficient were obtained.With the increase of permeability coefficient,the peak flow rate increased shar_ply at first and then decreased slowly,and K=0.025 m/min was the segment point.The peak flow decreases with the increase of water supply,but when the water supply was μ≤0.001 or μ≥0.5,the peak flow was basically unchanged.The peak flow and attenuation coefficient were binomial exponential increasing function relationship with pipeline rate.There was a two-stage linear increasing function relationship between the peak flow and rainfall intensity,and the segmented node was i=0.015 m/10min.However,when i<0.015 m/10min,the peak flow increased with the increase of rainfall intensity.The relationship between peak flow rate,attenuation coefficient and permeability coefficient of pipe wall was a trinomial exponential increasing function.The peak flow rate and attenuation coefficient were positively correlated with the number of branch pipes in the range of 0-4.The relationship between late attenuation coefficient and permeability coefficient was a trinomial exponential increasing function.The late attenuation function was negatively correlated with the feedwater degree.When μ≥0.01,the late attenuation coefficient was basically unchanged.The attenuation coefficient had two opposite linear functional relationships with rainfall intensity.When i<0.015 m/10min,the attenuation coefficient increased with the increase of rainfall intensity,while when i≥0.015 m/10min,the attenuation coefficient was inversely proportional to rainfall intensity.（4）The Origin software was used to nonlinear fit the regression curve,and the applicable conditions of the three-segment exponential attenuation function were obtained.The accuracy of the attenuation coefficient distribution under different fracture-conduit rates was verified by using the hydrogeological parameters of S31 spring in Yagi test site.R=2 m was a critical value of attenuation coefficient varying with pipe rate.When r_p>0.00156%（R>2 m）,the same attenuation coefficient of different pipe diameters was equal.When r_p≤0.00156%（R≤2 m）,attenuation coefficients 1,2 and 3 decreased with the decrease of pipe rate.K/μ=0.5 m/min was a quantitative indicator for fitting the water regression curve with a three-segment exponential decay function.When K/μ≤0.5 m/min,a three-segment exponential decay function shall be used to fit the water regression curve.The attenuation coefficient simulated by the model is 0.00324 min^-1,while the actual attenuation coefficient in the field was 0.00286 min^-1,with a difference of 0.00038 min^-1,but in the same order of magnitude.The corresponding relationship between fracture rate,conduit rate,attenuation coefficient obtained by the model can provide a reference range for the determination of field hydrogeological parameters.