Land Degeneration Due to Water Infiltration and Sub-Erosion

sustainability Article Land Degeneration due to Water Infiltration and Sub-Erosion: A Case Study of Soil Slope Failure at the National Geological Park of Qian-an Mud Forest, China Xiangjian Rui, Lei Nie, Yan Xu * and Hong Wang Construction Engineering College, Jilin University, Changchun 130026, China * Correspondence: [email protected] Received: 7 August 2019; Accepted: 26 August 2019; Published: 29 August 2019 Abstract: Sustainable development of the natural landscape has received an increasing attention worldwide. Identifying the causes of land degradation is the primary condition for adopting appropriate methods to preserve degraded landscapes. The National Geological Park of Qian-an mud forest in China is facing widespread land degradation, which not only threatens landscape development but also endangers many households and farmlands. Using the park as a research object, we identified the types of slope failure and the factors that contribute to their occurrence. During June 2017, a detailed field survey conducted in a representative area of the studied region found two main types of slope failure: soil cave piping and vertical collapse. Physicochemical properties of the soil samples were measured in the laboratory. Results show that soil slope failure is controlled by three factors: (1) the typical geological structure of the mud forest area represented by an upper layer of thick loess sub-sandy soil and the near-vertical slope morphology; (2) particular soil properties, especially soil dispersibility; and (3) special climate conditions with distinct wet and dry seasons. Keywords: landscape; mud forest; land degeneration; water infiltration; sub-erosion; soil slope failure 1. Introduction Recently, the sustainable development of natural landscapes has received an increasing attention worldwide. Sustainability studies of landscapes have evolved into a vibrant research field, especially since 2004–2006 [1]. The landscapes of a territory are the consequence of its history [2] and the result of biodiversity distribution in accordance with physiographic and structural traits [3,4]. It is important to explore the nature of landscape change [5]. Aiming for landscape sustainability science to move forward, it needs to integrate landscape sustainability into other disciplines. Emphasizing both “linking knowledge to action” and “understanding human-environment interactions,” is the essence of sustainability science [1]. Land degradation occurs in all types of landscapes over the world [6]. Concerning the face of mounting changes and threats to landscapes [7], we should be keenly aware of this challenge [8]. The threat to sustainable development caused by land degradation was explicitly recognized at the 1992 Earth Summit and 2002 World Summit on Sustainable Development [9]. Land degradation involves deterioration in soil properties related to crop production, infrastructure maintenance, and natural resource quality [10,11]. It also is associated with the decline in the productivity of ecosystems over time [12]. Approximately 60% of the world’s land area is regarded as degraded and land degradation, including soil erosion, is one of the greatest challenges for land managers [11,13]. Soil erosion is not only a geomorphological but also a land degradation process that may cause environmental damage affecting people’s lives [14,15]. Ebabu et al. [16] regarded soil erosion as a major Sustainability 2019, 11, 4709; doi:10.3390/su11174709 www.mdpi.com/journal/sustainability Sustainability 2019, 11, 4709 2 of 17 cause of land degradation in different regions of the world. Chalise et al. [11] pointed out that land degradation, particularly soil erosion, is currently a major challenge for Nepal, and rainfall infiltration is thought to accelerate land degradation. Soil erosion by water is also one of the major threats to soils in the European Union, with a negative impact on ecosystem services, crop production, and drinking water [17]. Similarly, land degradation due to soil erosion is a major challenge in Africa [18]. Although land degradation occurs in all kinds of landscapes over the world, the drivers of land degradation vary from region to region [6]. Generally, soil erosion results in the loss of nutrients and fine particles as well as desertification in most semi-arid areas [19–21]. The loess region of China also has suffered from serious soil erosion for years [19]. Found in loess areas, mass movement is very typical and characteristic. The occurrence of mass movement is controlled by a series of internal and external factors, such as rainfall [22–24]. Dry loess can sustain near-vertical slopes; however, a loess area can rapidly disaggregate when locally saturated by rainfall [25] and, thus, a loess slope is highly prone to mass movement processes [26]. Mass movements are common and typical and in loess areas, especially following intense rainfall or prolonged rainfall [27]. Mass movement involves multifarious movement types, and several classification systems for mass movements have been developed [28]. Varnes [29] developed a mass movement classification based on the material (mud, soil, earth, rock, and debris) and movement type (falls, topples, slides, lateral spreads, and flows). Varnes [29] also proposed a further movement type, which he named complex, this type is a combination of two or more principal types of movements [30]. Based on geometry and movement mechanisms, mass movement also can be divided into four categories: bedrock contact landslides, palaeosol contact landslides, mixed landslides, and slides within loess [31]. Xu et al. [32] suggested a systemic classification of loess landslides, including slides, flows, and combined loess and bedrock landslides. Abramson et al. [33] recognized five types of mass movement: falling, toppling, sliding, spreading, and flowing, however, there are few researches on the type of mass movement caused by sub- erosion. Sub-erosion generally refers to various forms of erosion caused by groundwater below the surface [34,35]. Li et al. [35] pointed out that under the action of sub-erosion, the loess soil slope failure with caves mainly is divided into three types: the formation of a cave or cave system and its subsequent deformation and failure; the whole or partial deformation of slope due to the existence of caves; and the subsequent transformation of newly created caves on the slope failure due to rainfall after the overall slope failure. The National Geological Park of Qian-an mud forest in China is used as the research object in this study. Qian-an mud forest was officially approved as a national geological park in 2009. Due to its unique landform (Figure1) caused by sub-erosion, this landscape has become the only protected “mud forest” site in China with these geological features, giving the area a high aesthetic and scientific investigation value. Soil slope failure is a direct and main form of land degradation in the mud forest area. Recently, soil slope failure has become increasingly serious, and the destruction trend is evident. Observed in this area, sub-erosion is the main process of soil slope failure, causing damage to its unique and beautiful landscape and leading to soil mass failure. The destruction scope of the soil slope reaches over 5 km. Soil mass failure threatens numerous households and farmlands in the vicinity (Figure2), as well as the development of the mud forest geological park. This limits the sustainability of the mud forest landscape and local agriculture. The mud forest landscape has distinctive features. It is rare at home and abroad. It is a geological landscape of great ornamental and research value [34]. To make it provide landscape services in a long and stable way, we must understand the causes of landscape degradation. Agricultural income is the main source of income for local residents. Accompanying the expansion of the destruction range of the soil slope, the cultivated land area decreases. Timely suppression of land degradation promotes local sustainability. Several studies have been conducted on the mud forest area, most of which focused on the formation of the mud forest landscape. Take, for example, Zhou et al. [36] who considered the area a peculiar geological landscape formed by the combined action of various geological factors. They proposed the view of protecting the landscape. Chi et al. [37] presented that the mud forest formation is not only related to internal and external dynamic geological processes, but also closely related to the soil composition. Zhu et al. [38] divided Sustainability 2019, 11, x FOR PEER REVIEW 3 of 18 Sustainability 2019, 11, x FOR PEER REVIEW 3 of 18 Sustainability 2019, 11, 4709 3 of 17 related to internal and external dynamic geological processes,processes, butbut alsoalso clclosely related to the soil composition. Zhu et al. [38] divided the formation of mud forest landforms into four periods. Few studies,the formation however, of mud have forest focused landforms on the slope into four failure periods. of the Fewmud studies, forest at however, its current have condition. focused During on the 2013,slope failurecertain ofcontrol the mud measures forest atwere its current implemented condition. in locations During 2013, that certainposed controlthe highest measures threats were to residentialimplemented area, in such locations as cutting that posed slopes, the spraying highest concrete, threats to and residential anchorage, area, however, such as these cutting measures slopes, ultimatelyspraying concrete, failed

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