Runoff-sediment Coupling Mechanism Of Different Geomorphic Unit In The Loess Hilly-gully Region

The scale problem exists in the whole process of soil erosion,and the scale of soil erosion is considerably important in soil erosion process simulation.Diversity stratum,various erosion type and nonlinear energy consumption process are the unique erosion environment in the hilly and gully region of the Loess Plateau.So,in the ?slope-gully-watershed? system,the effect on soil erosion,sediment transport and deposition mechanism is not completely clear,geomorphic units of different spatial scales water and sediment collocation differences is not clear.Therefore,to study the runoff-sediment coupling mechanism of different geomorphic unit in different spatial scales is urgently-needed.Based on this,in this paper,the methods of field location monitoring,indoor simulated rainfall experiment,hydrological analysis and mathematical analysis are used to carry out this research.In terms of field prototype observation,a typical small watershed(Peijiamaogou watershed)in the Loess hilly-gully region was taken as an prototype observation area and makes use of the observation facilities therein including four gauging stations,namely the first branch of Qiaogou,the second branch of Qiaogou,the Qiaogou and the Peijiamao,and five large-scaled field runoff plots,namely the top-hillyslope,the down-hillyslope,the hillyslope,the gullyslope and the entire slope which located in different topographical units.Observation data including rainfall,runoff and sediment during 1986-2008 of the four gauging stations and five large-scale field runoff plots were collected.Based on these observation data,the characteristics of sediment transport and flow-sediment relationship of different spatial and temporal scales in the system of ?slope-gully-watershed? system were systematically analyzed.And the difference of flow-sediment relationship of different spatial and temporal scales was expounded.In terms of indoor rainfall simulation,Qiaogou was taken as the prototype of laboratory-simulation experiment.According to the level scale(1:60)and vertical scale(1:50),the generalized small watershed model was built with bilayer-ditch network.Based on the rainfall simulation experiment,the process of the runoff and sediment yield and the runoff-sediment relationship comprehensively in different topographical units such as slope,channel and watershed was analyzed.Furthermore,the flow-sediment coupling mechanism was analyzed from hydrodynamic and geomorphic evolution.So,to research on further understanding the process of soil erosion in hilly area of the Loess Plateau and its mechanism has the important scientific significance and application value for ecological environment management of Loess plateau.The main conclusions are as follows:(1)At different spatio-temporal scales,there are different characteristics of erosion and sediment transportation in the different geomorphic units.At the annual scale,for the entire slope,the distribution of soil erosion modulus in the different micro-geomorphology unit of the entire slope showed: the top-hilly slope < the down-hilly slope < hilly slope < gully slope < entire slope.The ratio of erosion modulus of different geomorphic units is 1: 1.2: 1.7: 2.4:2.9,which illustrate that all sediment produced by erosion was transported by runoff in the different geomorphic units of the whole slope.For the ?slope-gully-watershed" system,the sediment transport modulus of slope was greater than each watershed's and sediment transport modulus trended to be decrease with the watershed scale increasing.For the single rainfall event,the crest value of sediment transport modulus may occur in someone of the geomorphic unit of the whole basin,so the distribution patterns of erosion and sediment yield were nonunique.(2)There were different dynamic factors of soil erosion and sediment transport in different topographical units of different spatial scales.At the slope scale,sediment yield was mainly related to depth of runoff.The relationship could be well described by runoff shear stress and there are obvious linear correlation between runoff shear stress and sediment transport rate.At the watershed scale,sediment yield was related not only to runoff depth but also to peak flood flow.Sediment delivery rate was a power function of the runoff erosion power at the watershed scale,the decisive factor of the power function fitting relationship reached 0.80 above.(3)For the ?slope-gully-watershed" system,sediment transport capacity are significantly different in the different topographical units of different spatial scales.With the increasing in spatial scale,the sediment transport capacity decreased dramatically.The sediment concentration carried by unit flow power of the entire slope runoff plot,the first-branch of Qiaogou and the Qiaogou was 186 times,77 times and 58 times that of entire Peijiamao watershed respectively.It is further confirmed that slope scale triggered tremendous intensity of soil erosion,and the probability of hyper-concentration flow decreased when upscaling.For the slope scales,it's prone to arise landslides,avalanches and other gravity erosion in the gully slope,therefore the materials separation rate is large so that sediment concentration carried by unit flow power is the greatest in the entire slope.For the watershed scale,there is close relationship between the sediment transport capacity of channel and hyper-concentration flow.The riverbed produced by hyper-concentrated flows mostly is narrow and deep usually,so the hyper-concentrated flows will become unstable in wide and shallow section and lots of sediment deposit.The smaller the ratio of width to depth the bigger the probability of hyper-concentrated flow.In this study,the ratios of width to depth of the watershed gullies were 8.15,15.86 and 38.09,respectively at the three watershed scales from first branch of Qiaogou to the whole Peijiamao watershed With the spatial scale increasing,the sectional aspect ratio of watershed increases and the probability of hyper-concentration flow turning into the non-hyper-concentrated flows becomes large,so that the sediment transport capacity continues to decrease.(4)At different spatio-temporal scales,there are different characteristics of flowsediment relationship in different topographical units of different spatial scales.In the annual average scale,the trend of the rainfall-runoff and runoff-sediment relationship in different geomorphic units is consistent.According the relationship between the total of average runoff for many years and the total amount of sediment,the annual average runoff and sediment transport showed a close linear relationship.In addition,the relationship between flow and sediment don't show significant difference different spatial scales and the sediment load was controlled by runoff more and more.For the single flood event,all flow-sediment relationship of runoff and sediment transport modulus in the different topographic units of different spatial scales showed some linear relationship.It is also showed that curve slope decreased with watershed area increased,with highest slope of 1.10 ? 106 at whole slope runoff plot and 121 at entire Peijiamao watershed indicating that the relationship between runoff and sediment is more complex in the slope scale.This phenomenon illustrated that the complexity of flood-runoff processes from the slope scale to watershed scale will become low with the scale increasing and the main drivers of sediment output trend to be single.Moreover,the variable flow-sediment relationship also trend to be stable.At the inter-event,affected by the flood rising and fading,the relationship between flow and sediment concentration are different at the rising and fading stages in different topographic unit of different spatial scales.At the situation of the same flow,sediment concentration in the fading stage is significantly higher than sediment concentration in the rising stage.Moreover,with the spatial scale increasing,the sediment concentration gap is greater more and more.(5)The simulated rainfall experiment results showed that when the rainfall intensity is 1.0mm/min,only the slope surface water flow is laminar flow,and the flow pattern of each geomorphic unit is turbulent flow after 1.0mm/min.With the increase of rainfall intensity,runoff flow in different geomorphic units changed from tranquil flow to rapid flow.When the geomorphic unit increased from slope scale to basin scale,the number of Froude number of different geomorphic unit showed an increasing trend.The resistance coefficient of different geomorphic unit on the slope scale is always larger than that of the watershed scale.On slope scale,resistance coefficient is greater than that of gully-slope.At the watershed scale,the average resistance coefficient of different channel section different spatial scale showed the fluctuation trend.(6)Comparing hydraulic dynamic parameters of different geomorphic units,runoff shear stress in different geomorphic positions on slope scale is bigger than that of the different channel section on watershed scaler.On the slope scale,the runoff shear stress has a significant linear function relationship with the simulated rainfall intensity.In the basin scale,the channel average runoff shear stress in different sections and simulated rainfall intensity is a power function.Runoff power at hilly slope and simulated rainfall intensity showed a significant linear relationship.However,runoff power at the gully slope and the different section of watershed scale has a significant power function with simulated rainfall intensity.In terms of unit runoff power,the unit runoff power of different topographic sites on the surface of the slope is more severe than that of the basin scale channel.When the geomorphic unit increases from the scale of the slope to the basin scale,the runoff power of the unit is decreased.(7)The combination of fractal theory and comentropy to study the relationship between the different geomorphic units of gully network fractal structure,geomorphic information comentropy and geomorphic evolution is a preliminary attempt.Geomorphology fractal dimension can reflect the plane shape of ditch network in different topographic unit,and the change of comentropy can reflect the ditch network development in different geomorphic units.(8)With the increase of runoff shear stress,inflow sediment coefficient increased.And the runoff shear stress has a significant power function relationship with inflow sediment coefficient(P < 0.01).With the increase of the fractal dimension and the geomorphologic comentropy,the inflow sediment coefficient of different geomorphic units is increasing,and the exponential function can be used to fit the relationship between inflow sediment coefficient and fractal dimension.