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Rainwater management is an important solution to climate change and flood risk reduction in the urban area. This study presents a nature-based solution to conserve rainwater in the urban area: a kind of permeable brick made of desert sand, of which compressive strength is intensive, water permeability is high, and fabrication cost is low. The desert-sand bricks presented here will be more in line with both the environmental and economic aims of developing countries. The compressive strength is one of the most important factors that impact the service life of permeable bricks, while the fabrication cost is one of the key reasons influencing the market share of permeable bricks. In this study, the minimum compressive strength of the desert-sand brick is 34.9 Mpa, which has exceeded the requirements of the standard JC/T 945-2005. Moreover, the mean and maximum compressive strengths of the desert-sand bricks are 69.6 and 102.8 Mpa, which are 2.3 and 3.4 times of the strength required by the standard JC/T 945-2005, respectively. The fabrication cost of the water-permeable brick designed by the authors is only 12 US dollars/m2, which is much lower than that of the sand-based permeable bricks that already exist in the market. Additionally, more ecological benefits could be gotten because the main aggregate in the sand-based permeable bricks is the desert sand rather than river sand. The results show that the kind of permeable brick can render substantial economic benefits and ecological improvements.Keywords: Urban area; Water conservation; Permeable brick; Desert sand

Reservoir-bank avalanche is a common hydrogeological problem whether in China or other countries, and analysis for the mechanism of collapse on reservoir bank is an important issue in the field of disaster prevention and mitigation. This paper reviews the methods which had been used to study the mechanism of reservoir-bank avalanche, including field monitoring, field investigation, model or field experiment, and numerical simulation. In virtue of many advantages, e.g., high efficiency and automation, field monitoring has become a powerful tool in evaluating reservoir-bank collapse. Nevertheless, field monitoring is also subject to some external factors related to remote sensing technology, such as weather conditions, environment, and other factors. Field investigation has the characteristics of flexibility and maneuverability. However, a field investigation is a sampling method based the observation in the representative area, and the hazard information cannot be comprehensively obtained via field investigation in the study area. A common merit for the field monitoring and investigation is that data observed with the methods mentioned above may be used to calibrate the results of the model experiment and numerical simulation, but field monitoring or investigation are not applicable to track the process of bank collapse. Different from the monitoring or investigating method, an experiment of reservoir-bank collapse under closely monitored or controlled experimental conditions focuses on the regularities of bank collapse from a micro perspective. The model experiment may be conducted in a laboratory far away from the study area, and the experimental scenario may be freely designed if needed. Yet the researchers should consider the similarity of the cumulative effect while designing the downscaled model experiment for reservoir bank failure. Maybe it is easy to simulate and observe the complicated topographical conditions of bank collapse in the field experiment, because no change exists in the scale of the underlying surface and properties of erosion material. In fact, usually to make accurate observations and simulations is relatively difficult in the field. Numerical simulations have been widely used to analyze and predict the reservoir-bank avalanche from a macro perspective all over the world, whereas the result of the numerical simulation has to be verified with that obtained from monitoring, investigation or experiment. In summary, each research method presents its own characteristic set of advantages and limitations. Scientists may use an appropriate route for analysis according to objectives and contents of a specific project.Key words: Reservoir-bank avalanche; Field monitoring; Field investigation; Model experiment; Numerical simulation

As a large-scale check dam, Yudiba Dam on the Loess Plateau of China, is a kind of check dam with the height more than 5 m which is used to retain soil and water, produce grain and control flood. The Yudiba dam plays an important role for vegetation restoration in the process of returning cropland to forest in the Loess Hill Ravine Region of northern Shaanxi. This study explores the telecoupling relationship between the check dam in Yulin, and the vegetation reforestation in the local and adjacent areas based on the statistical yearbook data and remote sensing data of the northern Shaanxi from 2000 to 2020. The results show that: (1) The normalized vegetation index (NDVI) of Yulin, Shaanxi Province, shows an increasing trend during 2001-2015. The NDVI is higher in the east of northern Shaanxi, Yulin, where the check dams are more concentrated, and shows a decreasing pattern in the west of northern Shaanxi where the check dams are more dispersed. The NDVIs of Mizhi, Suide and Qingjian counties in the east are 0.510, 0.511 and 0.520, respectively. The NDVI of Dingbian County in the west is only 0.354, while the NDVIs of Zizhou and Hengshan counties which are closer to the east, are 0.414 and 0.390, respectively. This indicates that the construction of Yudiba dam in Yulin significantly contributed to the growth of vegetation cover in local and other surrounding regions of Yulin. (2) Driven by the policy of returning cropland to forest in Shaanxi Province, the construction of Yudiba dam in the west of northern Shaanxi, e.g. Dingbian county, may be also strengthened because of the need of vegetation restoration. The construction of Yudiba dam will promote the infertile slope farmlands to be replaced with high-productivity check-dam farmlands in the west of northern Shaanxi. Hence the construction of Yudiba dam in Yulin may be helpful in realizing sustainable development of resource and environment in northern Shaanxi Province.Keywords: Telecoupling, Yudiba dam, sloping farmland, vegetation restoration, returning cropland to forest

：The reservoir-bank collapse has caused soil erosion and bank expansion in the lower Yellow River, which seriously affects the ecological environment and agricultural development. Understanding the processes of mass movement occurred on reservoir-bank is important to predict and control bank expansion. However, little research exists concerning how to accurately quantify the process of bank collapse and trace the source of sediment. In this study, a reservoir-bank model with the gentle slope of 3°, steep slope of 40° and abrupt slope of 70°, was constructed according to geomorphological characteristics of the soil reservoir-bank in Xiaolangdi Reservoir, in which rare earth elements were used to trace the provenance of sediment originated from mass movement on reservoir-bank under different rainfall conditions, and quantify the soil loss from the bank contributed to sediment deposition in reservoir. The results show that the sediment in reservoir mainly comes from steep slope, and the percentage contribution of abrupt slope to the total soil loss increases significantly in rainstorms with the precipitation larger than 60 mm. Under the rainstorms, the contributions of the gentle slope, steep slope and abrupt slope to soil loss were 10%, 55% and 35%, respectively. Without rainstorm, the contributions of the gentle slope, steep slope, and abrupt slope to soil loss were 4%, 72% and 24%, respectively. Meanwhile, sediment deposition in reservoir also mainly derived from steep slope and abrupt slope. The contribution of steep slope and abrupt slope to sediment deposition were 49% and 40% under the rainstorms, and the contribution of steep slope and abrupt slope to sediment deposition without rainfall were 67% and 28%, respectively. In addition, most of the sediment generated from the lower abrupt slope accumulates near the reservoir-bank, while the sediment generated from the steep slope accumulates at a distance from the reservoir-bank. Under the rainstorms, the contribution of upper steep slope to sediment deposition was 54% at 240 cm from the reservoir-bank, while the contribution of lower steep slope to sediment deposition without rainfall was 70% at 180 cm from the reservoir-bank. Whether with or without rainfall, the contribution of lower abrupt slope to sediment deposition was all about 54% at 40 cm from the reservoir-bank. Thus, in the near future, engineering measures such as grid protected slope may be used in the reservoir area to protect the steep slope of reservoir-bank, which can effectively reduce soil erosion and bank expansion in the reservoir area.Keywords: Bank collapse; Mass movement; Xiaolangdi Reservoir; REEs; Rainstorm

Gully sidewall expansion is an important geomorphic natural hazard, and the expansion destroys a large extent of agricultural land in the loess regions every year. The main aim of this study was to identify the mechanisms behind gully sidewall expansion through a series of simulated rainfall experiments. The results show that land loss on the gentle slope was the result caused by the water and gravity erosions, and gravity erosion was the primary driving force. The correlation coefficient between the area of land loss on gentle slope and volume of gravity erosion on the gully sidewall was 0.93, and the correlation coefficient between the area of land loss on gentle slope and volume of water erosion was 0.71. The gravity erosion was the dominant impetus driving the change in slope gradient of the gully sidewall. The amount of gravity erosion in 17 of the 19 rainfall events causing a change greater than 5° in the slope gradient of the gully sidewall accounted for more than 50% of the total amount of sidewall erosion. Furthermore, the dynamic variation of the retreat rates for the gully shoulder line showed a similar trend to that of the total volume of sidewall erosion and exhibited an increase–decrease–increase tendency. The most significant factors affecting the change in slope gradient of the gully sidewall and retreat rate of the gully shoulder line were the rainfall duration and intensity, of which the sensitivity coefficients were 2.2 and 4.0, respectively. As a result, a combination of vegetation measures on the gentle slope, structural and ecological practices on the sidewall and powerful structural practices, e.g., check dams, on the gully floor, is preferred for sidewalls vulnerable to expansion.

The megacities in China experienced increasing impacts from the urban floods due to the climate change in the past ten years. However, it is still unknown how to deal with the potential hazards under the impact of extreme precipitation. This paper briefly reviews the general characteristics and challenges of urban flooding in China with a case study of the extreme rainstorm in Zhengzhou on 20 July 2021. Results indicate that, to comprehensively protect a megacity with existing building standards and emergency plans is difficult under the extreme rainstorms, and the disruption of weak lifeline facilities, e.g. the road network, may further disrupt the provisions from neighbouring cities. This study offers a unique perspective reflecting a multi-scale approach from a single local city to regional area, an urban cluster or even a country on the urban floods control, urban infrastructure design and policy development. In more details, the measures are anticipated to increase the control rate of annual rainfall via the development of sponge city, improve monitoring and forecasting in the urban area, and enhance the risk-response strategies from the local government. The study strongly suggests expand the experiences and achievements of the flood management from a disaster area to adjacent or distant cities coupled with different pathways including government cooperation, water conservation practices, information exchange or human migration. The policymakers in a megacity are also encouraged to update the viewpoints for the policies and strategies to control floods. Hopefully a clue for relatively effective water management and land use for the sustainable urban development may be obtained from the study.

Vegetation has long been recognized as an efficient way of preventing soil erosion. However, the effects of plants on gravity erosion are still broadly unknown. This study conducted a plant-root modelling experiment with the Pinus tabuliformis and Amorpha fruticosa under heavy rainfall on a micro-plot with a slope of 4° in the Wangdong Village, Shaanxi Province. The results showed that: (1) As the depth of the soil increases, the soil becomes more compacted and the influence of tree and shrub roots on soil consolidation ability increases. In the shallow soil with the depth of 20 cm , the maximum soil shear strength was 17 kpa and the maximum soil hardness was 21 N/cm2 next to Pinus tabuliformis, while in the deep soil with the depth of 40 cm, the maximum soil shear strength was 22 kpa and the maximum soil hardness was 44 N/cm2. In the shallow soil, the maximum soil shear strength was 16 kpa and the maximum soil hardness was 20 N/cm2 next to Amorpha fruticosa, while in the deep soil the maximum soil shear strength was 19 kpa and the maximum soil hardness was 48 N/cm2. (2) The differences of the root types make the plants differ in their soil consolidation abilities, for the tree root have a stronger consolidation ability than shrub roots. In the shallow soils, the soil water content next to Pinus tabuliformis was slightly lower than that of Amorpha fruticosa, while in deeper soils they were close to each other, so the soil cohesion next to Pinus tabuliformis was slightly higher than that of soil next to Amorpha fruticose. The maximum soil cohesion next to Pinus tabuliformis was 22 kpa and that of soil next to Amorpha fruticosa was 19 kpa. The maximum soil hardness next to Pinus tabuliformis was 44 N/cm2 and that of soil next to Amorpha fruticosa was 48 N/cm2.

Gravity erosion (mass movement) generates an enormous volume of sediments on steep slopes throughout the world, yet its effect on the particle size distribution of suspended sediments (PSDSS) remains poorly understood. The objective of this study is to quantify the effects of gravity erosion on soil loss, especially the changes of particle size distribution of the suspended sediment. In this study, experiments were conducted in a field mobile laboratory in which mass movements were triggered on steep slopes under simulated rainfall. During the experiments, water-sediment samples were collected in polyethylene bottles directly from the gully and channel flows before and during the mass movements. The volumes of each mass failure during and 20 min after the rainfalls were measured by a topography meter. The particle fractions of the samples were analyzed by combining the sieving and photoelectric sedimentometer techniques. A suite of indexes such as median sediment size (d50), sediment heterogeneity (H), fractal dimension (D), and enrichment/dilution ratio (Red) were then used to evaluate the effect of mass movement on PSDSS. (1) Gravity erosion had a significant influence on PSDSS. After the mass movements occurred, the proportion of sand-sized particles was decreased from 71 to 51%, whereas the proportions of clay and silt were increased from 1 to 7% and 28 to 42%, respectively. (2) The d50, H, and D were significantly correlated with slope failures. The d50 was decreased from 0.084 to 0.051 mm, whereas the H and D were increased from 5.6 to 26.8 and from 2.60 to 2.78, respectively. This implies that mass movements make the PSDSS more nonuniform and irregular. (3) The suspended sediment tended to be enriched in the silt and clay fractions and diluted in the sand fractions after mass movements. Reds for clay, slit, and sand fractions were 13.9, 1.4, and 0.7 respectively. The changes of PSDSS after mass movements reflected a combined complex effect of soil sources, erosion types, selective detachment, and deposition processes. Mass movements led to a drastic increase of sediment concentration and the enrichment of fine particles, which developed into hyperconcentrated flows.

Check dams are widespread and effective soil and water conservation structures throughout the world. This review paper presents an overview of the use of check dams for soil and water management and runoff control with examples from the literature based on field measurements from four continents. More than 150years of research has reported that check dams are civil engineering landmark structures used all over the world. Among all civil engineering structures, check dams are probably the most emblematic of torrent control works. They were used for centuries, and are located all around the world. Over the past several hundred years, people have increasingly realized the envisioned advantages of check dams such as land development, environmental improvement, agricultural production, enhancement of gully stabilities, and mitigation of intensive flood. The optimum size, location and type had great influences on the efficiency of check dams under watershed management. Moreover, in both the theoretical and practical realms, check dams have been proved to be a useful tool for controlling soil erosion and flooding at a catchment scale. This paper will be helpful for policy makers to extend check dam projects in the whole erosion-prone areas.

The previous inventory datasets of mass movement typically include large‐sized mass movements (e.g., landslides), whereas small ones are often missing due to either being too small to be interpreted from remote sensing images or too time‐consuming to measure them in the field. Thus, the relative significance of different‐sized mass movement in sediment production remains unknown. In this study, first, a total of 547 recent mass movements, with a volume ranging from 0.3 to 19,000 m³, are surveyed in a small complex‐terrain watershed on the Loess Plateau of China; then, sediment contributions by mass movement of different magnitude and frequency are evaluated; furthermore, the effects of causative factors on both mass movement frequency and magnitude are examined and compared; finally, Geographic Information Systems (GIS) and statistical methods are employed to assess mass movement susceptibility and erosion risk. The results showed that majority of the mass movements are very small (

Riverbank collapses frequently occur in the lower reaches of the Yellow River, China, which cause environmental changes around the riverbanks and result in a great loss of farmland. An analysis was carried out to understand the riverbanks of the Jiyang Reach via Google Earth. The results show that the three representative segments in the Jiyang Reach, the maximum annual-lateral displacement and average retreat area were 26.0 m/a and 1083.8 m2 during the period 3/31/2016–5/10/2018, respectively. Several factors such as the soil properties, upstream river-control works, bridge piers, and channel bends may change the river flow direction and the scour intensity, thereby increasing the probability of downstream riverbank collapse, which are all causes of riverbanks collapse in the lower Yellow River. Field investigation and research data show that the lower reaches of the Yellow have serious bank-collapse disasters and their riverbanks are still in an unstable state.keywords: Riverbank collapse; Yellow River; Channel evolution; Riverbank protection

In this chapter, multiple sets of data collected from both closed (with artificial boundaries) and open plots (without artificial boundaries) are used to investigate soil and water loss caused by sheet and rill erosion. In particular, the effects of slope gradients (ranging from 5° to 30°), land use, and soil crusting on soil and water loss are examined. The results indicate that the difference in both runoff generation and soil shear strength are limited among plots of different slope gradients. However, there is a drastic increase in soil loss with slope gradient. This mainly results from the increasing erosivity of flows with an increase of slope gradient. All types of soil conservation measures can significantly reduce soil loss but not necessarily reduce runoff. Breaking up the pre-existing soil crust does not significantly change soil moisture nor runoff but increases soil loss in large storms. The major problem associated with closed plots is the plot boundaries, which makes it difficult to extrapolate soil loss from plot to watershed scale. In addition, we should be cautious when using plot data to evaluate the effectiveness of soil conservation at the catchment or regional levels.

This chapter first describes the major categories of soil erosion models, and then presents Topog and Erode that are examples of simulating hydrological and erosion processes in the Yangdaogou (YDG) catchment. Finally, the challenges and problems of modeling erosion processes in watersheds with complex erosion processes are discussed. By using the Topog model, stormflows were reasonably well simulated in large events with recurrence intervals greater than 2 years but were poorly simulated in small and medium events. The Erode model captures major types of erosion processes, with process-based splash, rill and interrill submodels, and empirically based gully erosion and tunnel erosion submodels. This requires a large amount of data to calibrate the model and thereby may limit its applications.

This chapter presents a retrospective study on soil erosion and control in the representative small watersheds of the Loess Plateau, the Jiuyuangou Catchment and Nanxiaohegou Catchment. No close relationship was found between the runoff and precipitation; instead, the runoff may be sensitive to other factors, e.g. temperature and land-use. The seasonal distribution of soil loss is mainly influenced by the changes of the climatic factors. Particularly, the soil loss from the mass movements on the gully sidewalls plays important roles in causing soil loss in the small watersheds of the Loess Plateau. Fortunately, soil and water loss has been greatly mitigated through various conservation practices, and check dams are the most appropriate conservation practice on the Loess Plateau.

This chapter first describes the spatial distributions of mass movements, gullies, and tunnels within the Wangjiagou (WJG) watershed; then, the spatial relations between gully development and mass movement, tunnel formation and gully development, and mass movement and tunnel formation are analyzed. Furthermore, the effect of runoff generation from the intergully area on sediment production in the gully area is evaluated. The upper intergully area is dominated by sheet and rill erosions, and the lower gully area is active in mass movement, tunnel erosion, and gully erosion. Runoff generated from the intergully areas not only causes erosion there but also causes significant erosion in the gully area. In the gully area, tunnel formation is closely associated with gully development in the headwater and hillside gullies but is largely absent in the valleyside gullies and ephemeral stream channels. Mass movement is also active in gully development and is dominated by earth falls in headwater gullies, slides and falls in hillside gullies, and soil creeps in ephemeral stream channels. The spatial interactions of erosion processes should be taken into consideration in erosion control and planning in the watershed.

This chapter examines soil and water loss and its controls at different spatial scales by using the data collected by the Shanxi Institute of Soil and Water Conservation (SISWC). First, soil and water loss as well as its temporal changes are examined in the WJG watershed; second, flow discharges, sediment concentrations, and sediment yields are compared between the YDG (unmanaged) and CZG (managed) subwatersheds on a yearly and event basis; third, the effectiveness of different conservation measures in reducing soil and water loss is assessed at the plot scale; finally, sediment budgets are calculated for the YDG and CZG subwatersheds as well as the WJG watershed. The mean annual soil loss in the WJG watershed is 4057.4 t/km2, ranging from 0 to 40,106 t/km2. Soil loss shows a decrease over time, resulting from the decrease of annual rainfall after 1970s, and the cumulative effectiveness of the soil and water conservation measures implemented in the watershed. The mean annual soil annual loss in YDG, an unmanaged subwatershed, is 20,811 t/km2 ranging from 136 t/km2 to 62,812 t/km2, in comparison of 8503 t/km2, ranging from 1.6 t/km2 to 29,423 t/km2 in CZG, a managed subwatershed. This represents a reduction of 59% soil loss through conservation measures in CZG. This study demonstrates that calculating sediment budgets by using soil loss data from closed plots significantly underestimate the sediment yields of the subwatersheds/watersheds.

Riverbank collapses frequently occur in the lower reaches of the Yellow River, China, which result in a great loss of farmland and significant hydro-morphological evolution in the channel. A combination of field investigation and remote sensing analysis was conducted to understand the current status of riverbank collapse in the Shandong Reaches of the lower Yellow River. The results show that the planar failure and upward-concave collapse were the main types of river failures in these reaches. Taking the Jiyang section as an example, the average lateral dynamic displacements in the Jiyang section were 2.8 and 11.4 m, the retreat areas were 248.8 and 835.0 m2 and the maximum lateral dynamic displacement were 7.4 and 26.0 m during the periods 3/31/2016-4/18/2017 and 04/18/2017-5/10/2018, respectively. Factors such as the soil properties, upstream river-control works, and channel bends may change the probability of downstream riverbank collapse. Building materials that are effective, low-cost and environmental friendly, and easy to use, are anticipated in the river management projects to protect the riverbanks and improve the ecological environment in the study area.

Gravity erosion is one of the most remarkable natural hazards in mountainous regions, especially on the Loess Plateau of China. Nevertheless, the measurement of failure mass is very difficult because gravity erosion usually occurs randomly and it combines with hydraulic erosion. Here we present a novel testing technique that could quantitatively measure time-variable gravity erosion on the steep loess slopes. A structured light 3D surface measuring apparatus, the Topography Meter, was designed and manufactured in our laboratory. Dynamic variation of the steep slope relief was monitored under rainfall simulation and the slope deforming process was recorded by a computer video technology. With the help of laser marking, plane figures were vectorially transformed into 3D graphs, thus the shape of target surface was accurately computed. By comparing the slope geometries in the moments before and after the erosion incident on the snapshot images at a particular time, we could obtain the volume of gravity erosion and many other erosion data, including the volume of slide mass, the amount of soil loss eroded by overland flow, etc. A series of calibration tests were conducted and the results showed that the accuracy of this technique was high and sufficient for exploring the mechanism of slope erosion. More than 120 rainfall simulation events were subsequently tested with the apparatus, further confirming its feasibility and reliability.

To study the mechanism of vegetation on gravity erosion may provide the theoretical basis for the design of soil and water conservation on the Loess Plateau, China. This study explores the effects of vegetation on the gravity erosion using the models of loess gully sidewall with the slope degree of 70 ° and height of 1.5 m under series of rainfall simulations, in which vegetational and bare lands were used, respectively. The experimental results are shown as follows: (1) The influence of the vegetation on the total amount of gravity erosion may be ignored. The average amount of gravity erosion for each event of rainfall on the vegetational sidewall was only decreased by 12% compared with that on the bare sidewall. (2) The plant had a positive effect on the mass failures after rainfalls. The ratio of the failure mass volume after rainfall to the total on the planted slope was almost 12%, while the ratio on the bare slope was only 1%. (3) The vegetation on the gully sidewall had different influences on the varied types of gravity erosion. Compared with those on the bare land, the average amounts of the avalanche and mudslide on the vegetational slope was decreased by 72% and 69%, respectively. In the meantime, the average amount of landslide in the vegetational slope was increased by 220% compared with that in bare slope. (4) The effects of vegetation on gravity erosion were caused by several factors. The plant roots may reinforce the soil and reduce the amount of gravity erosion, but the self-gravity of the plant and increase of the permeability on the planted slope might increase the gravity erosion. In addition, the soil permeability was increased by the plant roots, which could intensify the landslides. As a result, the plant practice was preferable for the sidewall vulnerable to mudslide. The results may be referred to analyze the mechanism of gravity erosion and design the control of mass failure.

Affected by global climate change, water shortages and other extreme weather, water scarcity in world is alarming sign. China is suffering from severe water shortage in northern areas due to multiple reasons such as declining of runoff of main rivers to northern areas and serious rainfall shortage. This article provides the evidences regarding the feasibility and their technical, financial, political, socioeconomic, environmental from the aspects of two projects, Tunnel and Tianhe projects. The Tianhe water diversion project is expected to bring 2.5 billion cubic meters of precipitation in the Sanjiangyuan area, 200 million in the Qilian Mountains area and 120 million in the Qaidam area every year. On the other hand, the Tibet-Xinjiang Tunnel, which would be the longest water divert tunnel in the world, plan to divert the water from Tibet to Xinjiang using a 1000-kilometer tunnel. The project is expected to turn Xinjiang into California, which could carry 10 to 15 billion tons of water each year. These two projects are anticipated to solve the problem of water, food and drought in the country. In addition, the integrated telecoupling helps to promote ecosystem services research and governance, identify knowledge gaps, guide research design, and strengthen the interaction between researchers and stakeholders.

The effects of plants on gravity erosion are still controversial, although vegetation has been widely used in soil and water conservation. In order to evaluate the influence of vegetation on gravity erosion, a series of laboratory experiments was performed on a 1.5-m-high loess-gully sidewall with a 70° slope, involving two types of shrubland (SL model) and bare land (BL model). The results revealed only an 8–20% decrease in total gravity erosion in the SL model compared to the corresponding BL model, which indicated that the impact of vegetation on gravity erosion is not significant. Compared with the BL model, the average landslide volume was 42% greater in the SL model, while the average avalanche and mudslide volumes in the SL model were 50% and 36% less, respectively. In addition, vegetation can change the type of gravity erosion and improve the degree of soil fragmentation. The amount of erosion after rain in the SL model was 33.17×103 cm3 more than that in the BL model. Compared with the anchoring effect of vegetation, the change of soil water caused by vegetation on the slope has a greater effect on the gravity erosion under heavy rains. It is suggested that the shrub restoration measures can be used in the areas with frequent debris flow and hydraulic erosion, and structural measures may reduce the negative impact of plants on gravity erosion of the Loess Plateau.

Observation of the gravity erosion in the field with strong sunshine and wind poses a challenge. Here, a novel topography meter together with a movable tent addresses the challenge. With the topography meter, a 3D geometric shape of the target surface can be digitally reconstructed. Two methods can be used to obtain a relatively clear video, despite the extreme steepness of the slopes. One method is to rotate the laser source away from the slope to ensure that the camera sightline remains perpendicular to the laser plane. Another way is to move the camera farther away from the slope in which the measured volume of the slope needs to be corrected; this method will reduce the distortion of the image. In addition, the installation of tent poles with concrete columns helps to surmount the altitude difference on steep slopes. Results observed by the topography meter in real landslide experiments are rational and reliable.

Rainfall simulation is widely used for the experimental study on soil and water conservation. This chapter reviews the characteristics, application, and prospect of rainfall simulator. The rainfall simulators could be divided into two main groups: the non-pressurized rainfall simulators, including the thread droppers and the needle droppers, and the pressurized water rainfall simulators, including the spout and the sprayer. The former could be hardly found at present due to its drawback that the minimum size of drop produced is far larger and more even than most natural rainfall and the simulator could not easily generate rains with the similar energy to the natural one. Rainfall simulation halls with automatic operation and observation systems are booming in recent years, but the basic simulators will not be completely replaced in virtue of the cheap price and convenient manipulations.

Soil pipes and tunnels have been reported in a wide range of climatic conditions in the world, yet their hydrological and sediment processes have been much understudied in comparison with surface flows. In this chapter, storm flows and sediment processes were monitored at 6 tunnel systems in an experimental catchment in the Loess Plateau. The results indicate that all the tunnel flow was derived from overland flow entering via inlets. However, both flow discharges and sediment concentrations were highly erratic due to the instability of tunnel systems. Tunnels could be clogged by collapse inside, which reopened in subsequent events. An intensive storm could develop new inlet(s), which either joined the existing tunnel system or started a new one. Partial damming within the tunnel systems also frequently occurred. This chapter showed that at least 43% of water flows and 57% of sediments were routed via tunnels in the experimental catchment.

The building of check-dams is one of the most effective measures for the conservation of soil and water on the Loess Plateau of China, and the hydro-sedimentologic balance is the most important factor influencing the relative stability of the check-dam systems. This means that soil and water in the small watersheds controlled by the check-dams will be absorbed internally, without the need of raising the height of the dams, if some given parameters have reached certain values. A runoff simulation experiment for a single check-dam and a rainfall simulation experiment for the whole check-dam system had been conducted to simulate the induced morphological changes affecting the stability of check dam systems. The results indicate that the main reasons causing the check-dam to show good relative stability are the enlargement of the dam-land area, the alleviation of erosion by the check-dam, and the auto-stabilizing mechanism of the gullies.

Gravity erosion is a dominant geomorphic process on the steep loess slopes. Here, we conducted rainfall simulation experiments to monitor occurrence and behavior of the mass failure on steep loess slopes. The results show that the quantity of soil loss caused by the avalanche and landslide was much more than that caused by the mudslide, and the avalanche was the most violent gravity erosion. As the slopes were eroded with five runs of rainfalls each at a duration of 48 mm, the total volume of avalanche, landslide, and mudslide were 150.9 × 103, 82.5 × 103, and 3.9 × 103 cm3/m, and accounted for 62, 36, and 2% of the total gravity erosion, respectively. Furthermore, the slope height and gradient had a remarkable positive correlation with the erosion amount. As a result, the avalanche and landslide, especially the former, played a crucial role in soil erosion on steep slope compacted by hand with loess.

The measurement of failure mass is very difficult because gravity erosion usually occurs randomly and it combines with hydraulic erosion. Here, we present a novel structured-light 3D surface measuring apparatus, the topography meter, which could quantitatively measure the time-variable gravity erosion on the steep loess slopes. With the topography meter, a 3D geometric shape of the target surface could be digitally reconstructed, and then, the slope parameters, including the volume, projected area, and gradient distribution, could be obtained. By comparing the slope geometries in the moments before and after the erosion incident on the snapshot images at a particular time, we could obtain the volume of gravity erosion and many other erosion data. A series of calibration tests were conducted and the results showed that the accuracy of this technique was high and sufficient for exploring the mechanism of slope erosion.

A successful experiment with a physical model requires necessary conditions of similarity. This chapter presents an experimental method with a semi-scale physical model. Four criteria are mentioned here: similarities of watershed geometry, grain size and bare land, Froude number (Fr) for rainfall event, and soil erosion in the downscaled models. The efficacy of the proposed method was confirmed using these criteria in two different downscaled model experiments. Experimental results show that while the amount of soil loss in the small scale models was converted by multiplying the scale number, the amount was very close to that of the large scale model. Obviously, with a semi-scale physical model, the experiments are available to verify and predict the soil loss in a catchment with the check dam system on the Loess Plateau, China.

The morphology of the failure scar has been a long-debated issue concerning the Loess Plateau of China, and the lack of normative data has hampered vital research in this area. In this chapter, a series of laboratory experiments were conducted to observe the failure geometries and volumes under rainfall simulations. The following six failure-scar types occurred: Tf, Cd, Cu, Ia, Ps, and Co. Tf, Cd, and Cu were the three major types of failure scars during the process of gravity erosion on a steep loess slope, and the scar Tf was the most significant. A relatively dangerous failure scar (Tf) might appear if the slope became steeper, or if the rainfall became more intensive. In addition, a long-duration storm could easily induce a large-volume failure with a Cu scar. The experimental results obtained here provide a morphogenic insight into the gravity-erosion control on a loess gully sidewall.

Gravity erosion is a dominant geomorphic process on the widespread steep loess slopes, yet it is not well understood due to the complexity of failure occurrence and behavior. This chapter conducted a series of laboratory experiments to test the stability of different slope geometries and rainfalls and then performed a sensitivity analysis. The following three types of gravity erosion were observed: landslide, avalanche, and mudslide. In an event of rainfall, various types of gravity erosion might emerge in the same period, and mass failures with the same mode and similar size often adjacently appeared. Climate-driven factors and topography triggers had prominent influences on gravity erosion. Whether for the total amount or the peak amount in an experiment, the largest sensitivity parameter on both landslides and mudslides was that of rainfall duration. The experimental results provide an insight into the pre-failure mechanisms and processes of steep loess slopes.