| Loess plateau is one of the most serious soil and water loss regions in the word, however, the soft rock region is the core of drossy rock yield sediment regions in the Loess plateau in china, and it is also an important part of much and coarse sediment regions located in Yellow River watershed. The region is called as "earth cancer" and "the most severe soil and water loss" due to severe soil erosion. The severe sediment deposition was engendered in the lower Yellow River, and a series of ecological and environmental problems was brought because of the severe soil and water loss and atrocious natural environment. The seabuckthorn flexible dam is a new biology engineering of using biology measure controlling soil and water loss and using nature reshaping nature, and is a radical measure of controlling sediment disaster in river. So, carrying out the research on effects of sediment retention and ecology and flow velocity vector field of seabuckthorn flexible dam, it can provide for the theoretic proof in spreading and planning plant of seabuckthorn flexible dam, and this is also very important and valuable in theory and practice.In this paper, seabuckthorn plant flexible dam was the core, soil and water conservation and ecology hydrology and soil environment effects of gully ecology system formed by seabuckthorn flexible dams were analyzed and studied. The presently advanced PIV(Particle Image Velocimetry) technology was exploringly and successfully employed, the impact of seabuckthorn flexible dam on flow behavior and velocity distribution was analyzed and discussed. Sediment retention mechanism of seabuckthorn flexible dam was theoretically analyzed and discussed by river dynamics and sediment transposition mechanics theory. Sediment retention effect of seabuckthorn flexible dam was effectively simulated by sediment mathematical model, and plan plant parameters of seabuckthorn flexible dam were effectively analyzed and discussed. Finally, the author provided some advice for controlling and harnessing soil and water loss of soft rock region. The main content and innovative achievements are as follows:1. On the spatio-temporal scale, sediment retention of seabuckthorn flexible dam was revealed from the qualitative and quantitative analysis. The idiographic results were:(1) seabuckthorn flexible dam had marked choice and prevention effect to sediment, sediment of gully bed was naturally chose to coarse and fine sand by joint using of flexible dam system and lower small rigid silt dam, this can make coarse sand deposit in gully and fine sand silt in lower small rigid silt dam; (2) the lower small rigid silt dam can also save clear water that has little fine sand, and form a small reservoir that can be used for local agriculture under the upper seabuckthorn flexible dam system filtration.2. By special measured ecological data, (1) ecological feature, soil improve effect, gully soil moisture characteristic and spatio-temporal dynamic variation of soil water were studied; (2) the method of geostatistics was employed to analyze the spatial variability of soil moisture in different directions in distinct vertical two-D areas in different spatial locations and the effect of plant flexible dam on the distribution of soil moisture in the vertical section of soil in gully with seabuckthom plant flexible dam; (3) the effect of excellent ecology, soil improvement and regulating gully runoff of seabuckthom flexible dam were showed, and the strikingly adjusting soil water resource through seabuckthorn flexible dam was also tested.3. The impact of seabuckthorn flexible dam on flow behavior was analyzed, and it was proved that flow velocity was slowed down in swell water area, and that sediment was deposited in the area because of very strong prevention, delay and swell water effect of seabuckthorn flexible dam on flow. These results are as follows:(1) it was revealed that dam length and plant density took the main role in flow velocity and water-depth distribution, and the role of dam length was stronger than plant density; (2) the maximum water table swelling occurred in the front of the flexible dam; (3) seabuckthorn yearly ecology investigation showed that the preventable effect of dam on flow was gradually stronger and stronger with growth of seabuckthom; (4) as the plant density is small, dam length is main reason for increasing upper water depth of seabuckthorn flexible dam, the next is plant density, and the last is gully bed slop; (5) as the plant density is big, dam length is yet main reason for increasing upper water depth of seabuckthorn flexible dam, the next is plant density, the third is growth density, and the last is gully bed slop.4. Based on field experiment, advanced velocity field measure technology-PIV was exploringly and successfully employed, the law of flow velocity distribution and variation was systematically analysed and studied, and the velocity field characteristic of seabuckthorn flexible dam was discussed in detail. It was proved up that flow energy loss was mainly occurred in the upper swell water area of seabuckthom flexible dam, and it was pointed out that sediment was firstly deposited in the area. It was again tested that seabuckthorn flexible dam had striking resistance to flow and sediment deposit effect by above those conclusions, and this can provide theoretic proof for seabuckthom flexible dam controlling soil and water loss. Respective results were as follows:(1) flow velocity in seabuckthorn flexible dam was distinctly smaller than no seabuckthorn contrast area in front of flexible dam, this showed evidently reducing flow velocity in the seabuckthorn flexible dam; (2) cross section flow velocity distribution was reverse "flow tongue" shape, and cross section velocity distribution curve showed wave shape or zigzag shape in flexible dam; (3) it is completely feasible that the advanced PIV technology is used and spread in field flow velocity measure.5. Resistance of seabuckthorn flexible dam was analyzed and discussed by river sediment dynamics theory, estimation formula of flexible dam roughness was deduced, and value of flexible dam roughness was estimated, and estimated roughness was well tested and contrasted with field data; sediment transportation trait of east one branch gully was theoretically discussed and analyzed. The results showed:(1) that resistance of seabuckthorn flexible dam was comprised of seabuckthorn plant, gully bed, gully bed and riverside resistance, and bed formation resistance, the resistances were reflected by the roughness of plant flexible dam; (2) value of seabuckthorn flexible dam's Manning roughness is probably from 0.04 to 0.09, and it is concerned with average gully bed plant density, submersed volume scale, hydraulic radius, and plant growth, and the plant density and submersed volume scale also varied with water depth of flow; (3) sediment retention result was easily affected by using different sediment transportation formula.6. The effect of sediment retention of seabuckthorn flexible dam in east one branch gully was successfully simulated by using of one dimension flow and sediment mathematical model. The plan plant parameters of seabuckthorn flexible dam were optimized and discussed according to calculated result, and some advice was provided for planning and planting of seabuckthorn flexible dam by these results. The simulated results showed:(1) Manning's roughness of plant dam was bigger, sediment retention effect of seabuckthorn plant dam was better; (2) sediment retention rate of single plant dam would run to be some maximum stable value with dam length increase; (3) if the dam length was shorter, scope of sediment deposit would trend to the whole dam, if the dam was longer, the tail of dam was prone to be scoured, this indicated that it was not good that the dam length was longer, but the dam length was right range to avoid waste as planning plant dams; (4) as double plant dams were planned in gully, the gully bed was easily eroded, and comprehensive sediment retention rate of double dam was smaller and to be a stable value with increase of interspace between dams; (5) comprehensive sediment retention rate was not increased as two or more dams set in gully, and sediment retention effect of double dams of their zero gap was almost equal to single dam formed by two dams; (6) the lower plant flexible dam can effectively blocked interzone yielded sediment; (7) the sediment retention effect of plant dam was influenced by gully bed slop, and it get increased with gully bed slop's fall, and the sediment retention effect of plant dam on coarse sand was more than thin sand's; (8) because tail and lower gully bed of flexible dam was easily eroded, so some small engineering measures should be taken to confine scour of lower gully bed of plant dam at the exit of plant dam gully. 7. The research result showed that seabuckthorn is the leading small arbor for controlling soil and water loss of the soft rock region, and seabuckthorn plant flexible dam is very effective ecology engineering for control and harness of soil and water loss of it. Finally, in point of sedimeni transportation behavior of gully in the soft rock region, it was proposed that the comprehensive small watershed controll technology system should be insisted on using in gullys in the soft rock region, but the main body of the technology system had to be the seabuckthorn flexible dam, and sc soil and water loss in the soft rock region can essentially be controlled and restrained.
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