Keywords

1 Introduction

Shoreline refers to the banded area where water and land intersect on both sides of the river or within a certain range around the lake. It is an important component of the natural ecological space of rivers and lakes. River shoreline not only has the natural attributes about flood discharge, regulating water flow and maintaining river ecological balance, but also has the resource attributes about development and utilization value and providing services for social and economic development.

Carrying capacity originally refers to the strength of the foundation and the load-bearing capacity of the building. Nowadays it has evolved into the most commonly used term to describe the limitation of development. At the end of the 19th century, the concept of carrying capacity was clearly put forward in the field of Applied Ecology, for example, the livestock carrying capacity management (Malthus 1970). The concept of carrying capacity has been widely used in the field of ecology since the 1950s (Park and Burgess 2019). After the 1960s, with the advancement of industrialization and urbanization, the contradiction between resource supply and human demand has become increasingly prominent. Many scholars have widely applied the carrying capacity to the research in the field of resources and environment, and successively put forward the concepts of resource and environment carrying capacity, such as land resource carrying capacity (Sun et al. 2020), water resource carrying capacity (Song et al. 2011), forest resources carrying capacity (Martire et al. 2015), mineral resources carrying capacity (Wang et al. 2016), regional environmental carrying capacity (Lane 2010), atmospheric environmental carrying capacity (Su and Yu 2020), urban environmental carrying capacity (Lu et al. 2017), coastal environmental carrying capacity (Wei et al. 2014), and etc. The internal essences of the concepts about different carrying capacities in all time stages are the same. It is the supporting capacity of resources and environment to population and economic development scale and the internal and external pressure on resources and environment. Carrying capacity can be described as the interactive coupling relationship between a material foundation and its receiving carrier, and its final manifestation is the quantitative characteristics of the receiving carrier that the material foundation can maintain (Price 1999).

Generally speaking, as long as the consumption speed of shoreline resources does not exceed its regeneration speed, the shoreline will always be able to recover itself and there will be no crisis. However, if the consumption speed of shoreline resources exceeds its regeneration speed, the regeneration basis of shoreline resources will be destroyed, making the recovery process of shoreline extremely slow and even impossible in serious cases. Therefore, although the shoreline is rich in resources, its carrying capacity for human economic activities is also limited. This degree is called the shoreline carrying capacity.

The carrying capacity of shoreline actually includes five aspects, namely, the stability of shoreline, resource supply capacity, environmental service capacity, ecological service capacity and social service capacity. Among them, the stability of the shoreline is the basic condition to ensure that the shoreline can exercise other capabilities. The resource supply capacity refers to the limit of human economic activities that shoreline resources can sustainably support. The environmental service capacity refers to the maximum pollution assimilative capacity of the shoreline. The ecological service capacity refers to the limit of ecological regulation, ecological support and ecological culture services that the shoreline system can provide. The social service capacity refers to the maximum degree of social services that shoreline can provide to human society.

In this study, the evaluation index system of shoreline carrying capacity was systematically established, and was use to evaluate the shoreline carrying capacity of the Nanjing reach of the Yangtze River. The research on shoreline carrying capacity is of great practical significance to promote the sustainable development of shoreline. As long as we get the carrying capacity threshold of a certain section of shoreline, we can correspondingly regulate human activities, so as not to cause damage to the shoreline ecology.

2 Study Area, Data and Methods

2.1 Study Area

Shoreline boundary line refers to the boundary line delimited along the riverbank or around the lake bank, which is divided into waterfront boundary line and outer boundary line. The waterfront boundary line is the boundary line in the waterfront zone defined along the flow direction on the waterfront side along the river or on the waterfront side around the lake (reservoir). The outer boundary line is the outer boundary line of the shoreline zone defined on the land side along the river or around the lake (reservoir) according to river and lake shoreline management, protection and maintenance requirement.

The Nanjing reach of the Yangtze River is a plain river, and its frontage boundary line is generally the intersection line between the water level corresponding to bed forming discharge or flat discharge with the land. In this study, the water land intersection line is used as the frontage boundary line. The outer boundary line can be the demarcated embankment line, as shown in Fig. 1.

Fig. 1.
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Embankment in the Nanjing reach of the Yangtze River

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2.2 Data

The remote sensing images of shoreline are mainly from landsat4, landsat5, Landsat7 and landsat8 satellites (Hansen and Loveland 2012). The relevant parameters are shown in the table below. About 400 remote sensing images of Nanjing reach from Landsat satellite from 1984 to 2021 were collected, and four bands (red, green, blue and NIR) with a resolution of 30 m were used (Table 1).

2.3 Methods

The evaluation indexes of shoreline carrying capacity of Nanjing reach of the Yangtze River were listed in Table 2. The evaluation indexes are river bank stability, river bank width, human disturbance index, shoreline utilization rate, pollution blocking function index, vegetation coverage rate, wetland retention rate and water area width. All of these eight indicators can be calculated directly from remote sensing images (Ministry of Water Resources of China 2020).

Table 1. Relevant parameters of satellites (Landsat 4, Landsat 5, Landsat 7 and Landsat 8)
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Table 2. Evaluation index system of shoreline carrying capacity based on remote sensing images
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Table 3. Levels of shoreline carrying capacity
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The overall carrying capacity of shoreline is calculated as follows:

$$ C_{rl} = \sum\nolimits_{n = 1}^8 {w_n } X_n , $$
(1)

where Crl is the comprehensive index of shoreline carrying capacity; Xn is the evaluation index; wn is the weight of corresponding evaluation index.

In this paper, three levels are adopted for the classification of carrying capacity, namely, underloaded, critical loaded, and overloaded. The specific classification standards are shown in the table below (Table 3).

The land use classification can be used to calculate the index of shoreline carrying capacity. The river bank stability, river bank width and water area width can be calculated from the water body information; the human disturbance index and shoreline utilization index can be calculated from the farmland and construction land information; the pollutant blocking function index can be calculated form the natural vegetation, wetland, farmland and construction land information; the vegetation coverage index can be calculated from vegetation information (grassland, farmland and forest land); the wetland retention index can be calculated from wetland information. It is concluded that the above eight indexes can be calculated from six types of land use (water body, forest land, construction land, grassland, farmland and wetland) information.

The classification regression tree (CART) algorithm is used to classify land use types of remote sensing images (Shao and Lunetta 2012). Figure 2(a) shows the remote sensing image of Landsat 8 satellite in the Nanjing reach of the Yangtze River on October 31, 2019 with corresponding CART classification training sample regions. Figure 2(b) shows the CART classification results. It is obvious that the classification of forest land, water body and construction land in the figure is accurate.

In order to further evaluate the accuracy of CART classification results, Fig. 3 shows the classification accuracy of remote sensing images about the Nanjing reach. It can be seen that the CART classification accuracy in recent 40 years is more than 90%, indicating that the classification results have high accuracy.

Fig. 2.
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CART classification of land use types of remote sensing images of the Nanjing reach (2019-10-31)

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Fig. 3.
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CART classification accuracy of remote sensing images about the Nanjing reach

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3 Results

3.1 Shoreline Morphology Evolution

The Nanjing reach of the Yangtze River is located in Jiangsu Province, starting from Maozi Hill to Sanjiangkou, with a total length of 92.3 km (Fig. 4). It is one of the 14 key reaches of the middle and lower reaches of the Yangtze River. The upper reaches of Nanjing reach are connected with Ma’anshan reach, and its lower reaches flow into Zhenyang reach. There are five major sandbars distributed in the Nanjing reach, namely, Bagua Sandbar, Meizi Sandbar, Xinqian Sandbar, Xinsheng Sandbar, and Xinji Sandbar.

Fig. 4.
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Shoreline and sandbars in the Nanjing reach of the Yangtze River

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Figure 5 shows the shoreline evolution in the Nanjing reach of the Yangtze River from October 30th, 1984 to November 18th, 2020. It can be seen that in the past 40 years, only local shorelines have change. The shoreline change near the Yangtze River Village is the fiercest, while the overall pattern of shoreline in the Nanjing reach has not changed greatly. The change degree of the shoreline in the right bank is more softer than that in the left bank, and its shoreline change is not violent.

Fig. 5.
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Shoreline evolution in the Nanjing reach of the Yangtze River (1984–2020)

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Figure 6 shows the area variation of river water body and sandbars in the Nanjing reach in recent 40 years. It is worth noting that the water body area in Fig. 4(a) refers to the area surrounded by the shorelines in the Nanjing reach, and the sandbar area is also included. It can be seen that in the past 40 years, the water body area of the Nanjing reach has decreased slowly, from the largest 3.4 km2 to 3.2 km2 with a decrease of about 6%. This shows that human activities have been leading to the eroding of river shoreline in the past 40 years. Among the five large sandbars in the Nanjing reach, Bagua Sandbar, Meizi Sandbar and Xinji Sandbar are relatively stable and change little; The Xinsheng Sandbar changes greatly whose head degrades and tail grows, but the change degrees of the two evolutions are similar, resulting in a small change in its area. The deposition degree of the tail of the Xinqian Sandbar is serious, with an area increase of 200%. As there is almost no human activity there, the deposition of the tail of the Xinqian Sandbar is mainly caused by the change of water and sediment.

Fig. 6.
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Area variation of river water body and sandbars in the Nanjing reach in recent 40 years

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3.2 Shoreline Carrying Capacity

Figure 7 shows the comprehensive index of shoreline carrying capacity of the Nanjing reach of the Yangtze River in the last four decades. It can be seen that the shoreline carrying capacity in Nanjing reach has always been within the critical load zone, indicating that the development and utilization of the shoreline is still sustainable. The shoreline carrying capacity of the Nanjing reach was basically stable from 1984 to 2003. With the large-scale development and utilization of the shoreline since 2003, the carrying capacity of the Nanjing reach gradually decreased and approached the warning line. Due to the implementation of restrictive measures such as “action to clear the four chaos” and “the operation of responsibility system on river/lake leaders” by Chinese government after 2018, the carrying capacity of the Nanjing reach has rebounded rapidly.

Figure 8 shows the mutation analysis of the time series of the shoreline carrying capacity index detected by Mann-Kendall (MK) method. UF and UB are the statistical series calculated according to the order and reverse order of the time series respectively. If the value of UF or UB is greater than 0, it indicates that the series shows an increasing trend, and if it is less than 0, it indicates a decreasing trend. When they exceed the significance lines, it indicates that the increasing or decreasing trend is significant. If the UF and UB curves intersect and the intersection is between the significance lines, then the intersection is the mutation point.

In the past 40 years, the shoreline carrying capacity index of the Nanjing reach of the Yangtze River has shown an overall increasing trend with fluctuations, and the increasing trend exceeds the α = 0.05 significance line, which means the mutation is significant. UF and UB curves intersected in 1991, 2012 and 2018, and the intersection point was between the α =  ±0.05 significance line (±1.96), indicating that the shoreline carrying capacity of Nanjing reach mutated in 1991, 2012 and 2018.

Fig. 7.
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Shoreline carrying capacity of the Nanjing reach of the Yangtze River in the last four decades

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Fig. 8.
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MK mutation analysis of shoreline carrying capacity in the last four decades

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4 Conclusions

Taking into account five aspects of shoreline health, resource supply, environmental pollution, ecological service and social service, the evaluation index system of shoreline carrying capacity was systematically established, and the comprehensive index of shoreline carrying capacity is classified into three levels: underloaded, critical loaded, and overloaded.

Based on the remote sensing images over the past 40 years, the variation of shoreline carrying capacity in Nanjing reach of the Yangtze River are quantitatively calculated. The results show that the comprehensive index of shoreline carrying capacity in Nanjing reach is within the critical loaded zone, indicating that the development and utilization of this reach is within the bearing range. Due to the implementation of restrictive measures such as “action to clear the four chaos” and “the operation of responsibility system on river/lake leaders” by Chinese government in recent years, the value of shoreline carrying capacity has increased.

The research results can help to discover unsustainably social and economic activities, put forward the productivity layout adjustment, and guide corresponding management measures in the reach.