Effects Of Underground Fissure On Runoff And Sediment Yield On Slope In The Karst Trough Alley Area

The sever soil erosion and exposed bedrock in Karst Trough valley in southwest China due to the special geological composition,climatic conditions and unreasonable human activities,which seriously restricts the economic and social development of the region.In addition,the extensive underground fissures on the slope in this region causes special and complicated dual loss soil loss in space by surface runoff and underground fissures.At present,it is very difficult to observe the underground process of soil and water loss in the Karst Trough Alley Area with the existing technology,and the research on the relationship between runoff and sediment in underground is rare at present.In this study,we conducted simulated indoors rainfall under the combinations of 5 fissure degrees(0%,1%,2%,3%,4%),3 rain intensity(30,60,90 mm/h),and 3 slope(10°,15°,20°)to study the variation characteristics of surface and underground runoff and sediment yield with rainfall duration under the combination of different fissure degrees,rainfall intensity and slope,and reveal the runoff-sediment relationships in surface and underground runoff,which provides theoretical support for the prevention and control of soil and water loss and the construction of soil erosion model in Karst Trough Valley.The main results were as follows:(1)Under different rainfall intensities and slope gradients,the surface runoff yield rate increased first and then tended to be stable with the rainfall duration,but there was a small fluctuation in the stable period in all fissure degrees.The law of underground flow yield rate became more complex under the interactions of rain intensity,slope and fissure degrees: Underground runoff yield rate increased first and then tended to be stable with rainfall duration at each fissure degree under 60 mm/h and 15° and 20°;Underground runoff yield rate first increased and then decreased with rainfall duration at each fissure degree under 60 mm/h and 10°,and 90 mm/h and 20°.Surface sediment yield rate was different under different conditions: Surface sediment yield rate increased first and then decreased to be stable with the rainfall duration under 30 mm/h and 15° and 20°,60 mm/h and 15°,and 90 mm/h and 15° and 20°;Surface sediment yield rate increased slowly with the rainfall duration under 60 mm/h and 10°.Underground sediment yield was not only affected by rainfall intensity and slope gradient,but also directly affected by underground fissures.Underground sediment yield rate increased first and then decreased slowly with rainfall duration and tended to be stable at 90 mm/h,20°,and 3% fissure degree.Underground sediment yield firstly increased and then tended to be stable with rainfall duration at 30 mm/h,15°,and 1% fissure degree.(2)The ratio of surface runoff and underground runoff to total runoff volume was 45%-79% and 21%-55% respectively under 30,and 60 mm/h across all slope gradients and fissures,indicating the water was lost by surface and underground runoff across,while surface runoff account for 60%-90% of total runoff volume under 90mm/h,suggesting the water was mainly lost by surface runoff.Fissure degree only showed significant linear negative correlation with surface runoff volume under 60mm/h and 15°(P<0.05),while showed significant linear positive correlation with underground runoff volume under 30 mm/h and 20°,60 mm/h and 10°,15°,and 20°,and 90 mm/h and 10° and 15°(P<0.05).The ratio of underground sediment yield to total sediment yield was highest up to 51% at 30 mm/h and 10°,and was 1%-28% at other conditions,indicating the soil mainly lost by surface.Fissure degrees only showed significant negative linear correlation with surface sediment yield at 30 mm/h and 15°(P<0.05),while only showed significant positive linear correlation with underground sediment yield at 90 mm/h and 20°.There was a linear positive correlation between surface runoff and surface sediment yield,and between underground runoff and underground sediment yield under different rainfall intensities and slopes(P<0.05).(3)Surface sediment concentration decreased first and then tended to be stable with the rainfall duration at each fissure degree under different rainfall intensities and slopes,and abrupt change occurred during the rainfall period due to the changes of water and surface microtopography during the rainfall duration.The variations of underground sediment concentration with rainfall duration were complicated:Underground sediment concentration of each fissure degree decreased first and then tended to be stable with the rainfall duration at 30 mm/h and 15°,while it increased first and then decreased and tended to be stable with the rainfall duration at 90 mm/h,15° and 2% fissure degree.Surface sediment concentration was significantly negatively linearly correlated with surface runoff volume(P<0.05).Underground sediment concentration was significantly negatively linearly correlated with underground runoff volume at 30 mm/h,15°,1% and 2% fissure,and 60 mm/h,20°,and 2% fissure,(P<0.05).Fissures showed no significant relationship with surface and underground sediment concentration(P>0.05).Surface total surface runoff volume only showed significant positive linear correlation with surface sediment concentration at 1% and 2% fissure degree(P<0.05),while there was no significant relationship between the total underground runoff volume and underground sediment concentration(P>0.05).