Changes in soil properties and erodibility of gully heads induced by vegetation restoration on the Loess Plateau, China

Soil erosion on the Loess Plateau of China is effectively controlled due to the implementation of several ecological restoration projects that improve soil properties and reduce soil erodibility. However, few studies have examined the effects of vegetation restoration on soil properties and erodibility of gully head in the gully regions of the Loess Plateau. The objectives of this study were to quantify the effects of vegetation restoration on soil properties and erodibility in this region. Specifically, a control site in a slope cropland and 9 sites in 3 restored land-use types （5 sites in grassland, 3 in woodland and 1 in shrubland） in the Nanxiaohegou watershed of a typical gully region on the Loess Plateau were selected, and soil and root samples were collected to assess soil properties and root characteristics. Soil erodibility factor was calculated by the Erosion Productivity Impact Calculator method. Our results revealed that vegetation restoration increased soil sand content, soil saturated hydraulic conductivity, organic matter content and mean weight diameter of water-stable aggregate but decreased soil silt and clay contents and soil disintegration rate. A significant difference in soil erodibility was observed among different vegetation restoration patterns or land-use types. Compared with cropland, soil erodibility decreased in the restored lands by 3.99% to 21.43%. The restoration patterns of Cleistogenes caespitosa K. and Artemisia sacrorum L. in the grassland showed the lowest soil erodibility and can be considered as the optimal vegetation restoration pattern for improving soil anti-erodibility of the gully heads. Additionally, the negative linear change in soil erodibility for grassland with restoration time was faster than those of woodland and shrubland. Soil erodibility was significantly correlated with soil particle size distribution, soil disintegration rate, soil saturated hydraulic conductivity, water-stable aggregate stability, organic matter content and root characteristics （including root average diameter, root length density, root surface density and root biomass density）, but it showed no association with soil bulk density and soil total porosity. These findings indicate that although vegetation destruction is a short-term process, returning the soil erodibility of cropland to the level of grassland, woodland and shrubland is a long-term process （8-50 years）.

Numerical model for homogeneous cohesive dam breaching due to overtopping failure

Based on the large-scale model tests, a simplified dam breach model for homogeneous cohesive dam due to overtopping failure is put forward. The model considers headcut erosion as one of the key homogeneous cohesive dam breaching mechanisms and we calculate the time-averaged headcut migration rate using an energy-based empirical formula. A numerical method is adopted to determine the initial scour position at the downstream slope in terms of the water head and dam height, and the broad-crested weir equation is utilized to simulate the breach flow. The limit equilibrium method is used to analyze the stability of breach slope during the breach process. An iterative method is developed to simulate the coupling process of soil and water at each time step. The calculated results of three dam breach cases testify the reasonability of the model, and the sensitivity studies of soil erodibility show that sensitivity is dependent on each test case's soil conditions. In addition, three typical dam breach models, NWS BREACH, WinDAM B, and HR BREACH, are also chosen to compare with the proposed model. It is found that NWS BREACH may have large errors for cohesive dams, since it uses a noncohesive sediment transport model and does notconsider headcut erosion, WinDAM B and HR BREACH consider headcut erosion as the breaching mechanism and handle well homogeneous cohesive dam overtopping failure, but overall, the proposed model has the best performance.

Experimental study on scour and erosion of blocked dam

This paper presents new experimental data of the erosion rate and sediment transport rate during the processes of dam break caused by overtopping. In order to study the headcut migration, the erosion coefficient was calculated and its peak value was determined near the downstream edge of the dam crest. Then the characteristics of vertical erosion during dam break processes were analyzed by dividing the dam into three regions： the upstream region, middle region, and downstream region. The three regions show different features during headcut migration, but all are exposed to the most intense erosion at the third stage of the dam break process. Finally, three relevant parameters affecting sediment transport were discussed： the length of the dam crest, the inner slope, and the dam composition. The results show that a longer dam crest and flatter inner slope reduce the peak sediment transport rate and prolong the arrival time of peak sediment transport rate; and with the increase of the non-uniformity coefficient S, the peak sediment transport rate initially increases, and then decreases.

Simulation of dam breach development for emergency treatment of the Tangjiashan Quake Lake in China

The Tangjiashan Quake Lake is the largest quake lake triggered by the 5.12 Wenchuan Earthquake that happened on May 12,2008 in China,posing high risk of catastrophic flash flood hazards to downstream human life and properties.A physics-based numerical simulation approach is proposed for real-time prediction of dam breach development of the Tangjiashan Quake Lake in the case of emergency treatment.Bed erosion and lateral development of the dam breach are represented through accounting for the underlying physics including selective sediment transport and gravitational collapse.Conceptualized breach erosion model that involves few parameters enables quick calibration based on the monitored hydrological data in emergency analysis where fully geotechnical information about the barrier dam is not available.The process of dam breach development is found to be nonlinear in cascades due to the combined effects of headcutting and bank collapse.The agreement between the simulation results and the observed data shows the applicability of the present approach for emergency analysis of quake lakes.Limitations will arise in the situation where the soil composition of barrier dam is significantly inhomogeneous.Incorporation of circular arc failure for cohesive soil and lateral seepage in bank slope will also enhance its applicability to complex situations.

Overtopping breaching of cohesive homogeneous earth dam with different cohesive strength

In consideration of the range of clay content of Chinese earth dams, the world's highest prototype tests have been made to research on the effects of cohesive strength of filling of cohesive homogeneous earth dam on breach formation. Three breach mechanisms were presented, they were the source-tracing erosion of dam body with the form of "multilevel headcut", "two-helix flow" erosion of dam crest and collapse of breach sidewalls due to instability. It can be concluded that the cohesive strength of filling of earth dam has great effect on breach formation. When the cohesive strength is bigger, the breach process becomes slower, and the peak outflow and the final width and depth of breach become smaller. The main character of the breach formation is head cutting and dumping collapse. When the cohesive strength is smaller, the breach process becomes faster, and the peak outflow, the final width and depth of breach become bigger. The main character of the breach formation is single level head cutting and shearing collapse.

Experimental investigation on breaching of embankments

Breaching of embankments has recently drawn more and more attention due to its importance in the development of early warning systems for embankment failures,in the evacuation plans of people at risk,in the design method of embankments based on a risk-approach,etc. The erosion process observed during embankment breaching tests in the laboratory and the analysis of the results are described in this paper. Five embankments,one constructed with pure sand,four with different sand-silt-clay mixtures were tested. The height of the embankments was 75 cm and the width at the crest was 60 cm. Examination of the data from these tests indicated that headcut erosion played an important role in the process of breach growth in the embankments made of cohesive soil mixtures. Flow shear erosion,fluidization of the headcut slope surface,undermining of the headcut due to impinging jet scour and discrete soil mechanical slope mass failure from the headcut were all observed during these tests. For the embankment constructed with pure sand,the breach erosion process was dominated by shear erosion,which led to a gradual and relatively uniform retreat of the downstream slope. The cohesive proportion in the sand-silt-clay mixtures strongly slowed down the erosion process.