XU Yandong,GAO Huiwang, WEI Xiao, and ZHU Jinlong

The Effects of Reclamation Activity and Yellow River Runoff on Coastline and Area of the Laizhou Bay, China

XU Yandong1), 2),GAO Huiwang1),3), *, WEI Xiao2), and ZHU Jinlong2)

1)Key Laboratory of Marine Environment and Ecology, Ministry of Education, Ocean University of China, Qingdao 266100, China 2) Shandong Provincial Key Laboratory of Restoration for Marine Ecology, Shandong Marine Resource and Environment Research Institute, Yantai 264006, China 3) Laboratory for Marine Ecology and Environmental Science, Pilot National Laboratory for Marine Science and Technology (Qingdao), Qingdao 266237, China

Study on morphological changes of a bay can help to identify the effects of anthropogenic activities on coastal environment and guide the exploration of marine resources. In this paper, morphological data including coastline and water areas in five discrete years between 1968 and 2015 were selected and extracted from the remote sensing images and historical marine charts to study the morphological changes in Laizhou Bay (LZB), one of the bays in the southwest of the Bohai Sea. A systematic analysis on spatial variations of the coastline and the surface areas of different types of waters in LZB was conducted. The results showed that the surface area of LZB was decreased by 1253.2km2in the last half century, which is 17.4% of the total in the 1970s. The areas of the natural wetland and the intertidal zone were decreased by 17.2% and 56.1%, respectively, and the average water depth varied from 9.05m to 8.16m at low tide level from 1968 to 2015. The coastline and shape variations of the bay turned to be complex after the 1980s, and the shape index of LZB showed an increasing trend in more recent years. The centroid of the bay generally migrated to the northeast direction,.., the direction of the center of the Bohai Sea, and the shrinking direction of the bay was consistent with the migration direction of the coastline. The reclamation area during 1968–2015 in LZB was 1201.7km2, and 94.1% was in the intertidal zone. The overall morphological change of the bay during the last half century was mainly controlled by the coastal reclamation activities, and the Yellow River runoff including the river course change and sediment load variation was also an important controlling factor.

morphological change; coastal reclamation; Yellow River runoff; Laizhou Bay

1 Introduction

The bay located along coastline, because of its abun- dant natural resources and geographical advantage, has become the ocean-land transportation hub, coastal industrial base and economical hinterland (Hou, 2016a; Huang, 2016). However, as the intensive anthropogenic activity and climate change, many bays have experienced various environmental issues of functional degradation, such as decreases of surface water area, deteriorating morphological changes and the corresponding degenerations of the ecosystem services (Halpern, 2010; Strokal, 2014; Tan, 2016). These environmental issues have seriously affected the coastal resi- lience and impeded the sustainable development of the coastal economy and society. Under such circumstances,the long-term (over decades) environmental change study on bays is highly appreciated.

The fast-growing technologies of remote sensing (RS) and geographic information system (GIS) have greatly improved the accessibility of the data, which make it possible to study the long-term environmental change of a bay. Previous studies mainly focused on the long-term changes of bays from the perspectives of coastline (Peng, 2013; Zhu, 2014; Ding, 2019), shape (Hou, 2016a; Song, 2018; Li, 2020b), morphodynamics (Deng, 2016; Sallaye, 2018; Li, 2019b), ecosystem service (Lee, 2014; Shen, 2015), landscape (Zhu, 2016; Tian, 2019) as well as hydrodynamics (Pelling, 2013; Liu, 2016; Liu, 2017; Xu, 2018). For example, Hou(2016a) analyzed the shape changes of 85 bays in China from early 1940s to 2014 using topographic maps and remote sensing data; Song(2018) and Ding(2019) studied the shape change of the Bohai Sea in different periods; while Li(2020b) studied the spatiotemporal evolutions of the major bays in the East China Sea. These studies drew some similar conclusions: the general geometric shapes of these bays became increasingly complicated (Hou, 2016a; Ding, 2019; Li, 2020b), and the human activity such as coastal reclamation was the main and dominant driving force in the changing process (Hou, 2016a; Song, 2018), with its intensity being correlated positively to the coastline length and shape index of the bay yet negatively to the surface area of the bay (Li, 2020b). Yang(2018) analyzed the tidal zone dynamics of the Jiaozhou Bay in the recent 30 years (1984–2018) and found out that the reduction of the tidal zone area and the deviation of its centroid were mainly induced by the large-scale aquaculture, salt pan activities and small-scale sea reclamation from 1984 to 2001, and by the large-scale sea reclamation afterwards.

Many studies (Hou, 2016a; Song, 2018) have been done on the analysis of the morphological change of the major bays in China, but most of these studies either cover a large spatial scale (.., a nation- wide scale or a sea scale) or focus only on one or several characteristics of a bay within a specific research. Taking Laizhou Bay (LZB) in the Bohai Sea as an example, a long-term systematic study of the shape changes of the bay, the changes of the bathymetry and area, as well as the cause of these changes are still relatively rare and unclear. Besides, the effects of the Yellow River runoff on the coastline and area changes of the Laizhou Bay have also yet been studied. To address these questions and fill the corresponding gaps in both data and understanding, in this paper, we compiled the Laizhou Bay morphological database from various remote sensing images including Landsat, KH-4B, SPOT5, ALOS and GF-1, as well as the data from historical marine charts during 1968 and 2015, and extracted the spatial characteristics and surface area of different types of water in LZB in these years. The Landsat MSS remote data with an 80-m resolution, coupled with the KH-4B remote sensing data with 1.8m reso- lution were utilized to improve the accuracy of the study. A systematic analysis of LZB spatial changes and the main driving forces during the last half century (47 years) was conducted by the surface area, water depth, shape, centroid of the bay, as well as the surface area of different types of water. This study provides a typical case and reference for the bay evolution research, especially for the national projects, such as the Comprehensive Treatment of Pollution in the Bohai Sea and the Blue Bay Remediation Action in China.

2 Data and Methodology

2.1 Study Region

LZB is a semi-enclosed bay, which locates in the southwest of the Bohai Sea and on the northwest of the Shandong Peninsula, China. It is bounded on the north by the Yellow River Mouth–Qimu Island line and is one of the three major bays of the Bohai Sea (Fig.1a). LZB is a curved shallow bay, with the water depth mostly within 10m (Fig.1b). There is a wide tidal flat on its west due to the sediment delivery from the Yellow River (Compilation Committee of Chinese Bays, 1991; Chen, 2013; Shen, 2015). LZB isan important fishing and salt base in China, with a long-lasting history of reclamation.

Fig.1 The study region and the water depth.

2.2 Data Source

The data used for this study mainly consists of remote sensing data and historical marine charts. Remote sensing can be a useful tool to map changes of the bay in an effective way (Peng, 2013; Zhu, 2014). Historical marine charts give evidence on the topography in different time periods, and analysis of these data helps to quantify the long-term geomorphologic evolution (Wal and Pye, 2003; Li, 2019a).

2.2.1 Remote sensing data

The Landsat, KH-4B, SPOT5, ALOS and GF-1 multi- period remote sensing images with clear ocean-land bou- ndaries were compiled (Table 1). Image processing (Song, 2018; Kuenzer., 2019; Li., 2020a), such as geometric correction, band composite, false color com- posite, image registration and mosaic, were first carried out on these images using the ENVI5.2 software. The errors of the geometric precision correction were controlled within one pixel. Then the gridded data of the remotes sensing images were extracted respectively from 1968, 1984, 2002, 2009 and 2015. Another two years (2004 and 2010) high resolution remote sensing images (with the resolution of 2.5m) were selected as the calibration data to improve the resolution of the interpretation results.

Table 1 List of remote sensing data used in the study

2.2.2 Historical marine charts and other data

Six marine charts of LZB published between 1971 and 2016 were collected (Table 2). The water depth data and vector information of water were extracted based on the bathymetry contour lines in the marine charts. According to the measuring time of each chart, we finally determined the following years to study the bathymetry changes of LZB: 1968, 1984, 2002, 2009 and 2015.

Table 2 List of historical marine charts used in the study

2.3 Coastline and Shape Index of the Bay

2.3.1 Coastline data extraction

The coastline is defined as the boundary of mean high- water in spring tide (MHWST) (Hou, 2016b). Its location is highly influenced by tides in the southern and western parts of LZB with relatively wide intertidal zones. To account the tidal information in extracting the coastline data of LZB, the artificial visual interpretation was adopted from Sun(2011), which was developed based on the coastline characteristics of the remote sensing images and the corresponding geographic vector maps. The extracted data was well calibrated by the field measurements and literature data (Alesheikh, 2011; Zhu, 2014). In this way, the coastline data in the five periods was extracted. The area of the bay is defined as the area enclosed by the coastline and the northern boundary of the bay. The areas of the bay in the five periods were estimated by the coastline vector data obtained from the remote sensing image interpretations.

2.3.2 Shape index of the bay

The shape index of a Bay (SIB) is defined similarly with the lake shoreline development index of a lake, the most common morphological index that has been widely used for lake distinguishment and evaluations (Ding, 2019). SIB is defined as the ratio between the perimeter of the bay and that of a circle with the same area of the bay (Hou, 2016a) (Eq. (1)).

In whichis the perimeter of the bay (m) andis the area of the bay (m2).indicates the similarity between the shape of the bay and the circle: the smaller theis, the more similar the bay is to a circle (Hou, 2016a; Li, 2020b). It also quantitatively depicts the intensity of the internal and external-driving forces of the bay evolution, which implies the bay evolution process and the variations of the hydrodynamic characteristics.

The centroid of a bay is its geometric center in the two- dimensional plane space, whose variation is one of the quantitative indicators of the change of the bay. The geometric barycenter represents the spatial distribution of a geographic object, and its moving trajectory (speed, direction and distance) can be used to compute the moving speed of the bay and reflect the general characteristics of the shape changes (Hou, 2016a; Ding, 2019). The location of the centroid was computed by ArcGIS10.3 software. By analyzing the migration direction, pathway, and the distance of the centroid of the bay, one can describe the shape change characteristics of the bay at different study periods.

2.3.3 Extraction of areas of different types of waters

To further analyze the area changes of LZB, the waters have been divided into three different types: intertidal zone, area with water depth between 0–6m and area with water depth larger than 6m. The areas between the coastline and the waters shallower than 6m are defined as coastal wetlands (Davies and Claridge, 1993), which includes the intertidal zone and the area with water depth between 0–6m. The intertidal zone refers to the area enclosed between the coastline and the 0m bathymetry contour line at low tide. The marine charts were digitalized by the ArcGIS10.3, and the vector data of the water depth points, as well as the 0, 2, 5 and 10m bathymetry contour lines were extracted. Based on these data, the 6m bathymetry contour line was interpreted by the Surfer13.2 software. Using the spatial analysis function built in ArcGIS10.3, the area of the intertidal zone, the areas with water depth between 0–6m, deeper than 6m and deeper than 0m, as well as the area of the natural wetland were computed for the statistical analysis of the area variations of different types of waters.

2.3.4 Extraction of the coastal reclamation patch information

Coastal reclamation is the sum of sea enclosure and sea reclamation. Sea enclosure refers to the formation of a sea area which is fully or partially closed by embankment or other means resulting in partially change the natural attributes of the sea. Sea reclamation (or filling sea) refers to the construction of embankments to enclose sea areas and fill them into land areas, whose behavior is to completely change the natural attributes of the sea (National Administration of Quality Supervision, Inspection and Quarantine, 2013; Li, 2020a).

The remote sensing interpretation of LZB reclamation was first developed built on the spectral and spatial information discrepancies between different types of reclamation purposes. Then the visual interpretation was applied to extract vector maps of the reclamation changes during each period of 1968–1984, 1984–2002, 2002– 2009 and 2009–2015 respectively. The mask file of each period was developed based on the corresponding extracted vector map, and then was used to compare with the remote sensing image prepared in Section 2.2.1 to get the remote sensing patch images for each reclamation period. These remote sensing patch images were then imported into the ENVI5.2 software for further analysis to finally get the spatial distributions and area change information of the reclamations during each study period.

3 Results

3.1 Changes of the Areas and Water Types of the Bay

3.1.1 Area changes in different years

The area of the bay decreased continuously from 1968 to 2015, as shown in Fig.2 and Table 3. The total decrease area was 1253.2km2in 2015 compared with that in 1968, which was 17.4% of the area in 1968, and the mean annual decrease rate was 26.6km2yr−1. The largest annual decrease rate, 37.3km2yr−1, was found during 2009–2015, and the smallest annual decrease rate was 18.5km2yr−1during 1968–1984, half of that during 2009–2015.

Fig.2 Area changes of five different types of waters in Laizhou Bay from 1968 to 2015.

Table 3 Reduction rate of surface area and different water types in Laizhou Bay from 1968 to 2015 (km2yr−1)

3.1.2 Area changes of different types of water

The area changes of different types of water in the selected five discrete years were shown in Fig.2 and Table 3. The natural wetland area showed a monotonic decrease trend from 1968 to 2015. Compared with the year 1968, the natural wetland area in 2015 was decreased by 17.2%, with an average annual decrease as 11.1km2. There also had been a progressive decrease of the intertidal zone area during the study period with 56.1% of the intertidal zone lost from 1968 to 2015, showing an average annual decrease as 21.5km2. The area of the water between 0 and 6m contour lines increased from 1968 to 2009, and decreased from 2009 to 2015 with the area increased by 38.7% during the study period, indicating an average annual increase as 10.4km2. The increase trend during 1968–2009 was mainly due to the fact that the sediment load delivered from the Yellow River increased the area of this type of water in the southern part of the Yellow River Mouth, which was larger than the area loss as a result of coastal reclamation. The area of the water >6m decreased monotonically by 17.7% from 1968 to 2015, and the average decrease was 15.5 km2. The area of water >0m decreased in time except for 1968–1984, when the area increased by 2.8km2yr−1, and the overall decrease was 4.5% and the average decrease was 5.1km2. During 1968–2015, the average water depth in LZB generally decreased from 9.05 m (in 1968) to 8.16 m (in 2015) at low tide level.

3.2 Changes of the Bay Shape

3.2.1

Changes of the bay shape bywere shown in Fig.3. In general,had few variations during 1968–1984 but showed a sharp increase afterwards. It was mainly because the aquaculture and salt pan occupied the natural coastline and the intertidal zone, which caused the slight decrease ofduring 1968–1984.increased during 1984–2009 because the coastal reclamation projects such as ports and dams had increased the complexity of the coastline, meanwhile the sea reclamation such as aquaculture and salt pan projects had simplified the bay shape. During 2009–2015, new coastal reclamation further intensified the complexity of the bay, and the sea reclamation activities became less simultaneously, soincreased the most significantly during this period.

Sea reclamation and salt pans occupy the natural coastline and intertidal zone, leading to a dramatic loss of the natural wetlands, a biodiversity decrease and a degeneration of the marine ecosystem services. Although the coastal engineering projects such as jetties, breakwaters and artificial islands may increase the complexity of the coastline and consequently increase the biodiversity, they cannot compensate the biodiversity loss caused by the reclamation.

Fig.3 The SIB change of Laizhou Bay from 1968 to 2015.

3.2.2 Migration of the bay centroid

The migration of the bay centroid and its moving speed can reflect the general characteristics of the shape change of a bay. The locations of LZB centroid and its moving speed were shown in Figs.4 and 5. The bay centroid migrated 5835 m in total during 1968–2015, with an average annual move distance as 124m. The moving trajectory in general directed northeast and showed a shifting away from land to ocean trend, which was in consistent with the spatial coastline movement due to the intense reclamation activities in the western and southern bay (Fig.4). The moving speed of the bay centroid (Fig.5) increased sharply before 2002 and decreased afterwards, and the largest average annual speed was found during 1984–2002, which was 342myr−1. In contrast, the smallest speed was found during 1968–1984 with an annual average of 78myr−1.

Fig.4 The locations of Laizhou Bay centroid from 1968 to 2015.

Fig.5 The locations of Laizhou Bay centroid moving speed from 1968 to 2015.

4 Discussion

4.1 Influence of Coastal Reclamation

The detailed analysis of the sea enclosure, sea reclamation, coastal reclamation areas and the proportion of different types of waters occupied can be found in Table 4 and Fig.6. Remote sensing interpretation results demonstrated that the total reclamation area during 1968–2015 was 1201.7km2, and 94.1% of it,.., 1130.8km2was deployed in the intertidal zone. The occupied areas of the intertidal zone in four different periods were 248.5, 476.2, 232.8, and 173.3km2, respectively. From 1984 to 2002, reclamation activities occupied the largest area in the intertidal zone (Fig.6b), accounting for 42.1% of the total occupied area of the intertidal zone in 47 years. While from 2009 to 2015, the occupied areas of waters between 0–6m and above 6m were both the largest among the four different periods (Fig.6d). The former account for 65.5% of the total occupied area of waters between 0–6m and the latter account for 98.4% of the total occupied area of waters above 6m. From the perspective of reclamation purpose, the coastal reclamation activities in LZB had gradually shifted from the original purpose of salt fields and aquaculture to the mixed purpose including salt pan, aquaculture, port engineering and city development. The coastal reclamation projects were mainly in the southern and western LZB as well as the Longkou Bay (a part of LZB and located in its east), in which sea enclosure were mainly for salt pan and aquaculture on the southern and western bay, whereas the filling sea projects were mainly for port terminal, artificial island and other city development (Xu, 2019). Such intense reclamation activities may lead to a series of coastal environmental problems including coastline change, decrease of the coastal natural wetlands, degradation of seawater self-purification capability, alteration of coastal hydrodynamics and sedimentation, as well as intensive eutrophication and organic pollution (Ma, 2014b; Strokal, 2014; Gao, 2018). Coastal natural wetlands play a unique role in climate change mitigation, coastline protection, pollution degradation and climate regularization (Melville, 2016; Yang, 2019), and the replacement of wetlands by reclamation will unavoidably cause the degradation of coastal ecology and ecosystem services (Shen, 2015; Shen, 2016). Li(2016) demonstrated that the replacement of the natural wetlands by reclamation had resulted in a substantial loss of the ecosystem regularization service, the support service as well as the cultural service, which will cause the irreversible loss of human welfare at the coastal region and gradually affect the coastal economic development in the future. Furthermore, the reclamation also influences the population and behavior of the aquatic birds in the wetlands (Melville, 2016). Therefore, the protection of coastal natural wetlands, particularly the intertidal zones, should be streng- thened to reduce the occupation of reclamation projects.

Table 4 The area of different waters occupied by coastal reclamation in different periods

Fig.6 Distribution of various types of waters and occupancy status of coastal reclamation in different periods in Laizhou Bay.

4.2 Influence of the Yellow River Runoff

The influence of the Yellow River runoff to the bay was analyzed from the Yellow River Mouth migration and river sediment load. Since the 1960s, the Yellow River course to the ocean has experienced several changes. In 1964, the Yellow River course was changed southwards from Lao Shen Xian Canal to Diao Kou He Canal (Fig. 7a). It was changed to Qing Shui Gou Canal in 1976 (Fig. 7b), then to Qing Ba Cha (Fig.7c and Fig.7d) in 1996, and was shifted 2.28 km northwards (Fig.7e and Fig.7f) in 2007 (Kuenzer, 2019). Simultaneously, the Yellow River sediment load to the ocean decreased gradually with the annual sediment load as 10.98, 6.21, 1.62 and 0.83×108ton during the periods of 1964–1976, 1976– 1996, 1996–2007 and 2007–2016, respectively (Peng, 2013; Ministry of Water Resources of People’s Republic of China, 2017). The sediment load from the Yellow River is mainly deposited at the river mouth and in the coastal area (Peng, 2013). The change of the Yellow River course and the sediment load decrease influences the Yellow River Delta area, which further affect the morphology of LZB, especially the coastline and the bathymetry on the northwest. Sun(2017) studied the evolution of the intertidal zone in the Yellow River Delta, and the results showed a positive correlation between the wetland area and the annual sedimentation from the river. The remote sensing interpretation results demonstrated that the changes of the Yellow River course and its sediment load during 1968–2015 had caused a decrease of 57 km2in LZB, accounting for 4.6% of the total area loss during this period. More specifically, the decrease of LZB area induced by the Yellow River changes were respectively 34.0km2during 1968–1984, 22.7km2during 1984– 2002 and 0.6km2during 2002–2009, accounting for 11.5%, 4.6% and 0.2% of the total decrease during each period. These results indicated that LZB was mainly influenced by the Yellow River in the study periods of 1968–1984 and 1984–2002. In recent years, the implementation of the Yellow River Water-Sediment Regulation and the improvement of the Yellow River watershed ecosystem protection have made the Yellow River course and sediment flux more stable from 2002 to 2015, so since 2002 the influence of the Yellow River changes to the bay has become significantly smaller and limited in the northwest of the bay.

Fig.7 The evolution images of the Yellow River course from 1968 to 2015.

4.3 Cross-Comparisons Against Other Bays

To better understand the relative morphological chang- es of LZB, we further analyzed the variation of some other bays during the study period for cross-comparison. For example, the water area of the Bohai Sea reduced from 78998.8km2in the 1970s to 75569.0km2in 2014 (Hou, 2018), meanwhile the area percentage of LZB in the Bohai Sea dropped from 9.1% to 7.9%, indicating that the decrease rate of LZB area was greater than that of the Bohai Sea. Hou(2016a) analyzed the area variations of 85 coastal bays in China during the past 70 years, and concluded that the surface areas of most bays have decreased by 10%–60%, mainly caused by the anthropogenic activities especially by the coastal reclamation. The surface area of the Jiaozhou Bay was 470.3km2in 1966, yet turned to 343.1km2in 2012, which was decreased by 27.0% during the past half century (Ma, 2014a). The cross-comparison between LZB and the Jiaozhou Bay implied that the intensity of the surface area variation is influenced by the original surface area of the bay as well as by the intensity of other anthropogenic activities such as urbanization.

Song(2018) pointed out that theof the Bohai Sea increased from 2.98 in the 1970s to 4.11 in 2014, showing a trend of slow and mild increase at the beginning and a rapid increase afterwards. At the same time, theof LZB gradually increased from 1.67 to 2.37.computation is significantly influenced by the scale of the marine charts and the spatial resolution of the remote sensing images (Hou, 2016a). Compared with Song(2018), this study utilized the remote sensing images with a higher spatial resolution. Additionally, the profiles of the dams and artificial island in Longkou (a county in the city of Yantai, Shandong Province, China) were also considered as the coastline. All these result in a larger coastline length of LZB in this study than that of Song(2018), and it is the reason why thevalues in this study are higher than those in Song(2018) given a similar surface area of the bay. Despite the discrepancies in numbers, both studies showed the same increasing trend of coastline by SIB in LZB.

The variations of coastline length, type and location as well as the surface area determine the morphological change of a bay, which will alter the hydrodynamic condition and the water exchange ability, and further affect ecosystem of the bay. So, the morphological change of a bay, which quantifies the influence intensity of climate change and the anthropogenic activities, could be used as an important indicator to show the overall changes of the bay.

4.4 Uncertainty Analysis

1) In the process of reclamation area extraction from the remote sensing images in this study, we did not take the changes of the reclamation types into consideration during the selected different periods. For example, a specific water region in LZB may be used for sea enclosure during 1968–1984, but it might have been shifted to filling sea during some other period after 1984. This led to a smaller change in sea enclosure area than the reality in the 47 years. Considering that the total coastal reclamation area is the sum of sea enclosure and sea reclamation, the total coastal reclamation area shown in this study can reflect the overall reclamation situation in LZB.

2) The water depth data extracted from the marine charts were originally for navigation and did not have a consistent update frequency at different regions of each chart, which timely restricts the data. For example, the northwest region of LZB is greatly influenced by the Yellow River, but the water depth data of this region is far behind an on-time update, thus the surface areas of different types of waters may not well represent the real situation. However, considering that the current remote sensing images still do not contain the bathymetry information, the water depth from the historical marine charts will make up and provide an alternative data source in underwater terrain research for this study.

4.5 Relevant Effects of Reclamations During Different Periods

There have been several large-scale reclamation stages took place in China after 1949, which include 1) 1960s- 1970s (mainly for farming), 2) 1980s–1990s (mainly for aquaculture) and 3) after 2000 (mainly for industry, including transportation, port and industrial estate construction,.). The scale, location, method and (ecological, economic, and environmental) effects of the reclamation in different stages are significantly different. Similarly, LZB experienced mainly two different reclamation stages during 1968–2015, and the main reclamation activities and the corresponding effects also varied in different stages. Specifically, the main reclamation activities during 1968–2002 were carried out for aquaculture and salt pan; consequently, the yearly-averaged occupied area for the sea enclosure was 21.2km2; during 2002–2015, the main reclamation activities turned to sea reclamations, such as the developments of ports and industrial estate constructions, with a yearly-averaged occupied area as 8.8km2. The port development mainly happened during 2002–2015 and the developments of the industrial estate constructions mainly happened during 2009–2015. All these reclamation activities, although had different effects on the LZB in different time periods, together changed the coastline structure (length, types, locations and tortuosity) and the general area of LZB. For example, the port development and the constructions of industrial estate decreased the complexity of the coastline, but the development of the artificial islands actually showed an opposite effect. A comprehensive discussion about these different reclamation stages with more detailed data can be found in the author’s PhD. dissertation (Xu, 2019). This study case in LZB can be used as an example in reflecting the large-scale reclamation campaigns across the entire country.

5 Conclusions

Based on the remote sensing images and historical marine charts in LZB, this study conducted a systematic analysis on the coastline variations and the surface areas of different types of waters in LZB during 1968–2015. The results of the study reflected the general reclamation process and the morphological changes of the bay and demonstrated the influence of different reclamation activities as well as the Yellow River runoff to the morphology of the bay in different time periods. The main conclusions are summarized as follows.

1) The surface area of the bay had decreased by 1253.2 km2(17.4% of the total area) during the study period from 1968 to 2015 with the largest annual decrease as 37.3km2yr−1during 2009–2015. Natural wetlands and intertidal areas were decreased by 17.2% and 56.1% respectively with the largest annual decrease appearing during 2002–2015. 94.1% of the total reclamation area was employed in the intertidal zone. The average water depth varied from 9.05m (in 1968) to 8.16m (in 2015) at low tide level. For about half a century, the intensity of decrease in the surface area of LZB was greater than that of the entire Bohai Sea.

2) The shape variations of the bay had turned to be complex since the 1980s, and the SIB values showed an increasing trend (firstly gradual change and then rapid rise) in more recent years. The centroid of the bay generally migrated northeastwards, showing a moving trend of being close to the center of the Bohai Sea. Coastal reclamation played a major role in this process, and its high-intensity activities on the western and southern parts of LZB caused the coastline to move towards the sea. The shrinking direction of the bay was consistent with the migration direction of the coastline.

3) The variations of LZB in morphology and surface areas of different water types were mainly determined by the coastal reclamation projects, the Yellow River Mouth migration and the variation of the sediment load from the river, among which coastal reclamation was the main driving factor.

Acknowledgements

This study was financially supported by the National Science Foundation of China (NSFC)-Shandong Joint Funds (Nos. U1606404, U1906215), and the Ocean Special Funds for Scientific Research on Public Causes (No. 2012 05001). We thank the three anonymous reviewers for their reading of our manuscript and their insightful comments and suggestions that really helped improve and clarify this manuscript.

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(Edited by Ji Dechun)