| The storm runoff characteristics,erosion mechanism and hydrodynamic characteristics are not only the basis of dynamic study of slope erosion,but also important content of erosion and energy sources.Based on the severe rainstorm in 2013 in Yan'an area of Loess Plateau,this paper studied the characteristics of rainstorm,terraced erosion and its causes.In order to further discover the hydrodynamic characteristics of runoff on the slope under this kind of rainstorm condition and solve the problem of small water depth and difficult measurement of hydraulic parameters,based on simulation experiment of hydraulic erosion control of small watershed on the Loess Plateau,the unconventional scale of slope on Loess Plateau was studied and validated.The study could provide a new method for the study of hydraulic characteristics of hillslope.The main results are as follows:1.The characteristics of rainstorm,erosion and causes of the Loess Plateau were analyzed.Taking the Yanan region of the Loess Plateau as an example,the extreme rainfall characteristics,terrace erosion characteristics and the causes of rainfall were analyzed.The results showed: the total rainfall of Yan'an in 2013 was 959 mm,which was 448.8 mm higher than the average annual rainfall(510.2 mm)during 1951-2012.Rainfall from June to September reached 761 mm,accounting for 79.4% of the annual rainfall.Among them,the maximum monthly rainfall in July was 568 mm,reaching 10000 years standard.The erosive rainfall of Yan'an region in 2013 occurred 20 times,and the total amount was 737.1 mm.The total number of rainstorm was 6 times and the total amount was 409.3 mm.The number of erosion rainfall and rainstorm occurred more frequently,and the total amount was large.Under the extreme rainfall conditions of Yan'an region in 2013,the contiguous terraces eroded violently.There was significant power function relationship between the erosion modulus and the relative elevation and catchment area.In the middle and lower terraces,erosion was severe and the maximum soil erosion modulus reached 54 049.50 t/km2.Maximum precipitation occurred mainly at the peak sunspot number or 2a near it.According to the wavelet analysis results,a 41-year period of precipitation was the most obvious within the 64-year scale.Similarly,sunspot number showed a 16-year periodic variability.Correlation analyses of the 16-year and 41-year scales demonstrated that the relationships between precipitation and sunspot number were close.In addition,extreme precipitation often occurred in the year following El Nino events.According to 10-year moving average curves of precipitation and SOI,precipitation generally showed a downward trend when SOI was negative.2.Based on the basic principle of similarity and hydraulics,a set of un-conventional model relationship of rainstorm on the Loess Plateau was deduced.This study was based on the analysis of the characteristics of flow erosion and the simulation of similar characteristics on the Loess plateau.Based on the basic principle of similarity theory,a similarity transformation was applied to the differential equation of time for turbulent flow.According to the similarity criteria of gravity and resistance,the unconventional scale of erosion runoff of normal slope was designed,including geometric scale,velocity scale,slope scale and roughness scale.The geometric scale was only required to meet the requirements of depth scale.The main features were similar starting,similar sediment transport,similar bed deformation and similar motion of soil water.Finally,a set of unconventional metric formula of erosive runoff on the Loess Plateau was presented,which included similar geometric similarity,similar flow movement,similar sediment yield movement,similar bed deformation and similar soil water movement.3.The roughing effects of three roughing methods was analyzed.The changes of Froude number and resistance among smooth aluminum plate,sticky sand roughness and grass mat roughness under variable slope and variable flow conditions was studied.The results showed that as the resistance of the underlying surface increased,the Froude number of different underlying surfaces became smaller and smaller.Under the same slope condition,Froude number of smooth aluminum plate and sticky sand roughness increased as the flow rate increased and Froude number of grass mat roughness decreased gradually with the increase of flow rate and then tended to be stable.Under the same flow condition,the Froude number of three kinds of underlying surface increased as the slope increased.With the roughness of the underlying surface increasing,the resistance coefficients of different underlying surfaces became larger and larger.Under the same slope condition,the resistance coefficient of smooth aluminum plate and sticky sand roughness decreased with the increase of the flow rate.The resistance coefficient of grass mat roughness didn't decrease obviously with the increase of flow rate.Under the same flow rate,the resistance coefficients of three kinds of roughening methods,including smooth aluminum plate,sticky sand resistance and grass mat resistance increased with the increase of slope.4.Preliminary validation of runoff erosion runoff unconventional scale.On the basis of unconventional scale of eroded runoff on hillslope,a prototype and model slope with a geometric scale of 0.5 and a slope of 0.017 were constructed in this study.The prototype water depth was 1.5cm,1.8cm,2.0cm and 2.2cm,respectively,and the corresponding water depths were 3.0cm,3.6cm,4.0cm and 4.4cm respectively.In normal conditions,the flow continuity and inertia force and inertia force of gravity than the resistance ratio,flow similarity,surface velocity similarity,average velocity,roughness and other similar similar process of prototype and model were verified.The deviation degree was mostly within the allowable range of error,and preliminary verification was made to the unconventional scale model of erosive runoff on hillslope.The method could provide a new approach for the study of the hydraulic characteristics of hillslope. |
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