3.1.

Remote-Sensing Image Processing

Image registration is one of the basic image processing operations in remote sensing. The most common approach to image registration is to choose, in both input and reference images, some well-defined ground control points (GCPs), and then to compute the parameters of a deformation model.¹⁶ Generally, GCPs are acquired from in situ data, topographic maps, or higher precision remote-sensing images that have been registered and corrected. In this study, the six temporal remote-sensing images were geometric corrected and registered based on SPOT5 fusion images of snapshots of the PRE around the year 2005, which had been registered and had 2.5-m resolution (higher than the six images in Fig. 1).

After true color synthesized fusion, we extracted the coastlines from the six images, and then superimposed and contrasted them. After comprehensive interpretation, the coastal land change chart of the PRE in the period from 1986 to 2011 was obtained (Fig. 2).

Fig. 2

Coastal land change of the PRE from 1986 to 2011. O-1: Humen Outlet, O-2: Jiaomen Outlet, O-3: Hongqimen Outlet, and O-4: Hengmen Outlet.

3.2.

Nautical Chart Processing

By scanning the two nautical charts and geometrically correcting them, the data were vectorized. Then, they were registered to acquire bathymetric contour and sounding points. The scattered set of points was interpolated using Kriging interpolation method to reconstruct the digital elevation model (DEM) of our study area. The DEM maps of 1992 and 2008 were matched to the coastal land change data and overlaid by the coastal land of 1990 and 2011, respectively, to conveniently analyze depth change (Fig. 3).

Fig. 3

Digital elevation model (DEM) of the PRE: (a) 1992 and (b) 2008.

Subtracting the DEM depth of 1992 from that of 2008, a depth-change chart from 1992 to 2008 was obtained, which could visually reflect the erosion-accretion evolution of the PRE. The coastal land of 1990 was chosen as the overlaying map, and thus a larger water area and more depth information were illustrated (Fig. 4).

Fig. 4

Depth change of the PRE from 1992 to 2008.

4.1.

Coastline Change of the PRE

The coastline change can be analyzed from two aspects: the change of coastline length and geometry and the change of coastline types.¹⁷ In order to facilitate the analysis, the study area was divided into five parts (see Fig. 2) over five periods from 1986 to 2011, namely 1986 to 1990, 1990 to 1995, 1995 to 2000, 2000 to 2005, and 2005 to 2011. Our discussion stresses changes of coastline length, coastal land area, and the maximum coastline seaward extension. In this article, the maximum coastline seaward extension is defined as a maximum vertical distance between the coastline extended to the sea and the previous coastline over a period.

Figure 2 shows that the PRE coastline extended to the sea and rarely retreated back to the land because of erosion, while the coastline became longer, water area became smaller, and the funnel-shaped PRE narrowed. As we can see from Table 3, in the period from 1986 to 2011, the total length of the PRE coastline increased by 149.2 km, which is equivalent to a growth of 0.57% per year. The coastal land increased by $251.76{\mathrm{km}}^2$, which is equivalent to a growth of 0.23% per year, with the maximum distance of seaward extension being about 13 km. Between 1986 and 1995, the coastline and coastal land increased much more and faster than any other periods, and we suspect it was promoted by the rapid economical development of many Economic and Technological Development Zones (ETDZs) in Guangdong Province.¹⁸ After 1995, the changes slowed down. Since 2000, the winding coastline has turned into a straight one. We suspect it was caused by increasing anthropogenic activity. With new land extended to the sea, the coastline length changed slightly while the area expanded greatly.

Table 3

Changes of the whole study area during different periods.

Changes in different parts of the study area during different periods are illustrated in Fig. 5. Of the whole PRE region, the most significant change occurred along the Nansha Economic Development Zone to Hengmen Island in the northwest (Part 1) and from the Shekou Peninsula to Shenzhen Bay in the east (Part 4); and changes along Jiaoyi Bay to Shenzhen Airport in the northeast (Part 2) and Sidong Shoal to Qi’ao Island in the southwest (Part 3) took second place.

Fig. 5

Changes in different parts of the study area during different periods: (a) coastline length, (b) coastal land area, and (c) the maximum distance of seaward extension.

4.1.1.

Coastline change along Nansha Economic Development Zone to Hengmen Island

The Nansha Economic and Technological Development Zone (NETDZ) established in 1993 is in the geometric center of the Pearl River Delta economic zone, where four outlets are located and three rivers meet. The area was underdeveloped and remote before the early 1990s. However, there have been rapid changes since the start of the NETDZ, and from then on, several new polices were enacted to facilitate economic development and foreign investment, and land demand for construction was on the increase.¹⁹ As a result, the coastline change there is the most significant. Between 1986 and 2011, the coastline increased by 85.5 km, the coastal area increased by $103.96{\mathrm{km}}^2$, and the maximum distance of seaward extension was about 13 km. Combining remote-sensing images, we can see that there were great change of coastline morphology around Jibaosha, Wanqingsha, and Hengmen Island because of the development in the NETDZ [Fig. 6(a)]. Jibaosha was at the top of the funnel-shaped Lingding Sea, where the land area increased due to reclamation for the construction of large ports and shipbuilding base, and so all the scattered small islands around Jibaosha had been connected to form the new land.²⁰ Wanqingsha and Hengmen Island exhibited a constant southeastward growth trend. Oriented toward agriculture and ecological tourism, reclaimed land in Wanqingsha was mostly used for agriculture, mariculture, and ecology.²¹ The Zhongshan Linhai Industrial Park is located on Hengmen Island, so the reclaimed land there was mostly used for industry.²²

Fig. 6

Coastal land change: (a) along the Nansha Economic Development Zone to Hengmen Island and (b) from the Shekou Peninsula to Shenzhen Bay.

4.2.

Erosion-Accretion Evolution of the PRE

The funnel-shaped PRE has widened from the north to the south and deepened from the west to the east. Under the interaction of tidal currents and runoffs, a unique configuration was formed so that the three shoals (West Shoal, Mid Shoal, and East Shoal) were separated by two channels (Fanshi Channel and Lingding Channel).²⁶

The data in Fig. 4 represent the depth change from 1992 to 2008. The negative values indicate that water depth decreased from 1992 to 2008 (namely accretion), and the positive values indicate that it increased from 1992 to 2008 (namely erosion). In the period of 1986 to 2011, the erosion-accretion evolution of the PRE was mainly reflected in two aspects: the continued siltation in most water areas, such as tidal flat and shoal, and the continued deepening in channels due to flow washout action or dredging, namely the foreshores became shallower and deep channels became deeper.

The water depth decreased by 2 to 4 m in Jiaoyi Bay, Shenzhen Airport, Jibaosha, the southwest of Wanqingsha, Hengmen Island, Sidong Shoal, Qianhai Bay, and the northeast of Qi’ao Island. Around the northwest of Jibaosha, the south of Qi’ao Island, both sides of Humen Channel, and the southwest of Shekou Peninsula, the water was 4 to 6 m shallower. Parts of Jinkou Shoal and Sidong Shoal are about 1 m shallower. Fanshi Shoal is 1 to 3 m shallower. The cross profile of Fanshi Shoal (Line A in Fig. 4) shows that the west side in 2008 was deeper than that in 1992, which was probably caused by sand excavation [see Fig. 7(a)]. Furthermore, there were depositions of different degrees on both sides of most waterways and channels. In order to ensure the capacity of navigation channel, annual dredging made the channels deeper, while mud dumping along the channels made both sides shallower. The cross profile of Tonggu Channel (Line B in Fig. 4) shows the most significant example: because of mud dumping, the west is 2 m shallower, while the east is 4 m shallower [see Fig. 7(b)].

Fig. 7

Examples of water depth: cross profiles of (a) Fanshi Shoal, (b) Tonggu Channel, (c) Lingding Channel, and (d) Longxue Channel. The solid line for 1992 and the dotted line for 2008 represent the depth value of these years, while the dashed line for 1992 to 2008 means the depth change over the period of 1992 to 2008.

In contrast, the water depth of waterways, channels, and deep water ports increased by 2 m. One cross profile in the northern Lingding Channel (Line C in Fig. 4) is shown in Fig. 7(c). The channel was deepened by 6 m due to dredging, while its west side was 2 m shallower due to mud dumping, and part of its east part was deepened by 2 m. As a result of flow washout action or dredging, the depths of Chuanbi Channel, Longxue Channel, Hongqimen Channel, and Hengmen Channel were increased by 2 to 4 m (see Fig. 4). The cross profile of Longxue Channel (Line D in Fig. 4) shows that in the period of 1992 to 2008 its depth was increased by more than 1 m, but the north side was deepened by 7 m, where the flows from Humen Outlet and Jiaomen Outlet meet [see Fig. 7(d)]. The 2008 nautical chart demonstrates that there was a 14-m deep channel whose northeast part was adjacent to Chiwan Bay and the southwest part met Lingding Channel in the southeast of Neilingding Island. In the 1992 nautical chart, however, we could only see Fanshi Shoal, whose depth was 4 to 6 m. This manmade channel, known as the Tonggu Channel was a significant change to the PRE topography, and was dug between 2004 and 2007.²⁷ Along Qianhai Bay to Chiwan Bay, the water depth was deepened by 4 to 6 m during 1992 to 2008 owing to the construction of wharfs and ports.²³

In addition, we found that the water depth was deepened in patches with an average increase of 1 to 2.5 m, especially on the east side of the Lingding Channel. Changes at some spots are as much as 5.5 m. We suspect this was caused by frequent sand excavation activities.

4.3.

Relationship Between Coastline Change and Erosion-Accretion Evolution of the PRE

Overlapped images of the coastline-change chart and erosion-accretion evolution chart are used to show the relationship between coastline change and erosion-accretion evolution of the PRE (Fig. 8).

Fig. 8

Relationship between coastline change and erosion-accretion evolution of the PRE from 1986 to 2011.

With increasing deposition and reclamation in the PRE, the coastline continued to extend seaward. Comparing coastline change and erosion-accretion evolution, we found that the areas where coastline extended to the sea coincided with areas where water depth decreased by 1 to 6 m. These water areas became land under reclamation activities, such as in the southwest of Jibaosha, the south of Hengmen Island, Jiaoyi Bay, Shenzhen Airport, Qianhai Bay, and Sidong Shoal.

As shown in Fig. 8, the biggest depth change took place in the west Chiwan Bay (Line A in Fig. 8) with a reduction of more than 10 m, which was a water body thicker than 5 m in 1992 but has become land now, and is used as Chiwan container terminal. In addition, there appears to be a new deposition around the reclamation land to different degrees; channels continued to be extended in length, narrowed down in width, and deepened especially near the outlets. Many scattered small islands between larger islands had been connected to form new land due to reclamation of tidal flat. So, the outlets are narrower. Take Hengmen Channel (Line B in Fig. 8), for instance, in 1992, it was 75-m wide and 6-m deep, and it became 60-m wide and 9-m deep in 2008. This was because Wanqingsha and Hengmen Island had been extended to the sea, and there was also a 1-m deposition along both sides of the channel.

Anthropogenic digging deepened some parts of the water greatly. This was most significant in southeast Jibaosha and around the Shekou Peninsula. During 1999 to 2008, the first and second stages of Nansha Harbor District Port Project were in the east and south Jibaosha, where Guangzhou Shipbuilding Base was under construction.²⁰ Many excellent harbors in the Shekou Peninsula were built, where inland ships and giant oceangoing ships were allowed to berth, and therefore, this area water depth was deepened by 3 to 10 m. Jinxing Harbor, south of Qi’ao Island, was 2 to 4 m shallower, thanks to the construction of Qi’ao Bridge that connects Qi’ao Island and Zhuhai City.²⁸

4.4.

Causes and Impacts of Coastline Change and Erosion-Accretion Evolution

The Pearl River Delta is an important drive of China’s economic growth. It is also known as the wealthiest and the fastest growing region in the mainland. Human activities play an important role in changing the landform of the PRE and extending the seaward coastline, and increasing erosion-accretion had tremendous impacts on the PRE environment.

Small area reclamation will contribute to scouring action, for it only partially transforms the morphological structure of the coast, making the rough and winding coastline smoother and straighter. Many islands have been connected together, outlets have become narrower, and deposition has increased. The navigation capacity of silted outlets and channels has reduced, and then dredging is needed to deepen them; but the mud from dredging is piled up nearby, which causes more scouring.

With the increasing urbanization, reclamation from the sea can be an effective means to satisfy different land demands such as urban construction, industry, and agriculture. New reclamation areas appear continuously, and coastlines are extended to the sea constantly. More and more inning land has been developed to build factories or used for infrastructure construction. Industrial wastewater and aquaculture wastewater from the reclamation land become the major factors of coastal water pollution,²⁹ which may also influence the ecosystem of mangrove wetlands. Thus, both the life of coastal residents and the ecological environment are damaged. During a field survey of the coastal area of Shenzhen, we saw many cement plants that were constructed near mangrove wetlands. Wastewater from the plants poured into the sea may impact the environment of mangrove forest and the biological diversity. Furthermore, the construction of sea-crossing bridges, port projects, and illegal sand excavation break the erosion-accretion balance of channels and greatly intervene with the natural variation of coastline and erosion-accretion.