Study on the Geo-Environmental Evolution of the Laolonggou Lagoon Under the Impacts of the Caofeidian Reclamation Project in Hebei Province

YIN Cong,YE Siyuan, FENG Xiuli, and YIN Yanhong

Study on the Geo-Environmental Evolution of the Laolonggou Lagoon Under the Impacts of the Caofeidian Reclamation Project in Hebei Province

YIN Cong1), 3),YE Siyuan2), 4), FENG Xiuli1), *, and YIN Yanhong2), 4)

1),,,266100,2),,266071,3),,266033,4),,266071,

The Caofeidian Reclamation Project has been the biggest reclamation project in China so far, in which 310km2sea areas were reclaimed alongthe coast of Hebei Province, and it also bring about many problems and defects for large reclaiming area. The study focuses on the influences of the engineering exerted on evolution of the Laolonggou Lagoon with the methods of topographical measurement and surface sediment analysis. The results demonstrate that the topographical changes in the Laolonggou Lagoon had been controlled obviously by the engineering at three stages. Besides, blocking and reopening of the shoal tidal channel also affected the geological environment of the lagoon area. In the aspect of topographical change, the Laolonggou Inlet first experienced deposition after the shoal tidal channel was blocked, followed by short-time eroding for quarrying and cofferdam construction in the east, then depositing slowly after the reclaiming activity ceased, and finally eroding after the shoal tidal channel was reopened. The project, particularly cofferdam construction led to the movement of the Laolonggou Inlet axis from west to east for 50–100m. In the aspect of sediment variation, the reclamation project and hydrodynamic change resulted in the variation in compositions and distribution pattern. The western lagoon area has become land mainly constituted by silt, while the sediments in the eastern area have turned finer in size. After the shoal tidal channel was reopened, the current velocity in the Laolonggou Inlet has been enhanced, making the sediments at the bottom become coarser. The sediments around the Caofeidian foreland went through a process of changing in grain size from fine to coarse and back to fine again, and the sediments are mainly constituted by silt at present.

Laolonggou Lagoon Inlet; topographical evolution; Caofeidian Reclamation Project; shoal tidal channel

1 Introduction

1.1 Caofeidian Reclamation Project

The Caofeidian Reclamation Project, whose working area is located on the Caofeidian shoal, the coast of Tangshan city in Hebei Province, has been the largest reclamation project in China so far. It was almost finished with 310km2sea area reclaimed, actually 230km2if the area of harbor basins was excluded, requiring 1.1 billion cubic meter of earthwork. At present, the sea area has been turned into the Caofeidian industrial zone featuring by large terminals, large iron and steel plants, big chemical factories and big power works. In 2004–2005, according to the initial recla- mation planning, a highway (18.4km long, called the lead- ing-to-island highway) linking the land and the Caofei- dian barrier islands was built, and at the same time the ma- ssive reclamation project was proceeding to the west of the highway (Fig.1). Yin (2007) pointed out the distinct defect of the planning that the reclamation area was too huge, and the highway had blocked an important channel, named the Shoal Tidal Channel (Nachao River) in this sea area, ruining the nearly east-west tidal system, which imposed an obvious damage on the Laolonggou potential harbor zone and marine environment. Suggestions given for this project were keeping the shoal tidal channel, decreasing the reclamation area, and protecting the potential harbor zones (Yin, 2007). The Hebei Government and the former State Oceanic Administration of China at that time adopted these suggestions, and made a new updated reclamation planning in the second half of 2007. The new revised planning stated the improved measures of opening the shoal tidal channel (Nachao River) to recover the nearly east-west tidal system and building a large harbor basin (No.3 Harbor Basin) on the north end of the Laolonggou Inlet. In this way, the damages caused by the project were mitigated (Yin, 2009), and the newly built large harbor basin played a part in decreasing the reclamation area effectively. The channel was not really opened until the bridge construction at the blocked part of the shoal tidal channel was completed in 2016, and afterward the tidal system in nearly east-west direction was recovered gradually (Yin., 2016), which will be conducive to keeping the silting and scoring balance and maintaining the stability of harbors. The reclamation project can be divided into three stages. They are the stage of reclaiming the area to the west of the leading-to-island highway (2004–2007), the stage of reclaiming the area to the east of the leading- to-island highway (2008–2011), and the stage of restoration and maintenance after the reclamation (2011 to now).

Fig.1 The location of the Caofeidian Reclamation Project in Hebei Province of China and the project progress till 2007 (according to the satellite image in 2007).

1.2 Regional Setting

The reclamation area is the Caofeidian shoal including the large lagoon area. The Caofeidian shoal formed in the mid Holocene (8000–3000 years ago), was originally a fandelta of the ancient Luanhe River. Under the tidal and ero- sive effect for a very long time, the present barrier-lagoon system came into being with its distinct topographical characteristics and hydrodynamic conditions (Jia., 1999; Wang., 1999; Yang, 2005; Jin., 2006; Li and Sun, 2011). The Caofeidian shoal is at the transition stage of evolving from the semi-closed barrier-la- goon to the gulf-lagoon. The main features of this stage are that the barriers are gradually retreating from the shore, the islands have deep and firm bases, a vast lagoon is formed due to subsidence inside, and a large amount of tidal prism effectively keeps the shoal and inlet system stable (Zhang, 1995; Wang and Zou, 2006; Wang., 2007).The lagoon area named Laolonggou Lagoon is a semi-closed lagoon with developed barrier islands. Its original image before the reclamation is shown in the Fig.2. From the west to the east in the Laolonggou Lagoon, there are some barrier islands and sand bars dotted on the Caofeidian shoal, orderly named Caofeidian Island, Yaotuo Island, Caomutuo Island, Hatuo Island, Dongkeng- tuo Island and so on (Fig.1),distributing along a banded zone 10–20km away from the shore. During the spring tides, some barrier islands are submerged and only a small part of the Caofeidian Island, about 4km2, is above the surface, while the vast shoal emerges from the sea during the neap tides. The Laolonggou Lagoon is 450km2in area, and the water depth is 0–5m on average. It connects to the sea with two tidal inlets. One is named as Laolonggou Inlet which is 10–22m in depth, running nearly north-south and stretching south to the open seas; the other is named as shoal tidal channel (also called the Nachao River later), with the total length of 20km and 1.5km in width, running nearly east-west and gradually turning into SWW to the open seas. The east end of the shoal tidal channel running in NEE direction connects with the Laolonggou Inlet. Before the highway was built, even at low tide, the water depth of the shoal tidal channel was still 1.5m in depth, and over 2.5m at high tide (Yin, 2007). This channel has made a great contribution to the formation and maintenance of the Laolonggou Lagoon.

After tens of years reclamation, the reclamation area is up to 230km2, covering almost all the planned reclamation area of 310km2, excluding the three harbor basins. The backfill soil of 1.1×109m3was used for reclamation, and the reclamation region is about 2m higher than the sea level. The status quo of the project is shown in the Fig.3. After large-scale reclamation, a part of Laolonggou Lagoon (mainly for the western part of the lagoon) has become the land, cofferdam and harbor basins, totally accounting for 40% of the original lagoon water area, but what was worse, the shoal tidal channel had been blocked by the highway during the period from 2004 to 2016. All these factors have led to great geo-environmental changes of the Laolonggou Lagoon. As for such a large reclamation project, people always concern the issue about how the project influences the topographical evolution and the marine environment in the Laolonggou Lagoon, which is also the main subject of this study.

1.3 Previous Studies and Significance

From the current researches on lagoons, numerous studies have attempted to explain the issues such as evolution and development of lagoons (Duck and da Silva, 2012; Adlam, 2014; Benallacka., 2016), sedimentary characteristics and facies of lagoons (Isaack., 2016; Badesab., 2017; Donnici., 2017; Flemming, 2017), dynamic geomorphology of lagoons (Wells., 1990; Noormets., 2006; Gao, 2008; Mohanty., 2018), mutual im- pacts between lagoons and their tidal inlets (Hoque., 2010; Bruneau., 2011; Panda., 2013) and impacts of human activities on lagoons (Cañedo-Argüelles., 2012; Haghani, 2016). Moreover, some researches focus on the impacts of reclamation projects on the marine environment (Lee., 2006; Yu., 2016; Gao., 2017), and there are also some studies specialized on the Caofeidian shoal, the barrier island-lagoon system and its tidal system (Kuang., 2012; Zhang, 2012; Chu., 2016; Liu and Li, 2016; Liang., 2018; Zhu., 2020). However, the researches on how the large marine projects influenced the evolution of lagoons are very rare. Actually, it is quite difficult to investigate the influence exerted by the ocean engineering on the topographical changes of the lagoons and the shoal along the coastal regions, because the water is too shallow to conduct the field researches with large vessels and multi-beam echo sounder. Instead the research can only be conducted with small boats for water depth measuring. Even with these difficulties, this research still had been conducted by us for more than ten years with the aid of National Natural Science Foundation of China and China Geological Survey Program. Some research achievements with great in- terests and significance have been obtained in the process of the research. According to the research, the different phases of topographical evolution have very good responses to the different stages of engineering progress. This research will provide reference for the future study on evolutionary trend of lagoon topography under the influence of the large marine projects, as well as for the government to make reclamation planning and similar decisions.

Fig.2 The Laolonggou Lagoon in 1993 before the reclamation project.

Fig.3 The reclamation area of Laolonggou Lagoon in 2019.

2 Materials and Methods

The methods such as the single beam sounding, remote sensing and surface sampling at the seafloor of the reclamation area were adopted to study the evolutionary trend of the Laolonggou Lagoon since the project started in 2004.

2.1 Topographical Investigation

In order to study the topographical changes of the Lao- longgou Lagoon, three east-west profiles transecting the Laolonggou Inlet were set (Fig.4). The three profiles boast representative characteristics of the Laolonggou Lagoon topography. The A-B profile is crossing the entrance of the Laolonggou Lagoon, where is the narrowest place along the inlet channel with a stronger constraining intensity. The A-B profile was taken as the basis, and on its north and south side respectively located the E-F profile and G- H profile transecting the lagoon, both with a same distance of 2.5km away from it.From their arrangement and extension, the three profiles almost covered the key points on the topography of the Laolonggou Lagoon seabed, and the measurement along the profiles can reflect the impacts of the engineering on the evolutionary trend of the lagoon at different stages. The depth sounding data were acquiredin October of 2006, October of 2008, October of 2011 and July of 2017. Through comparative analysis on these data, the topographical changes caused by the engineering at different phases became clearer. The first three times of measurement were conducted two, four and seven years after the shoal tidal channel had been blocked respectively, and the last time was conducted one year after the channel has been reopened. The parameters about the length, location and direction of the three profiles from north to south are given in Table 1.

Fig.4 The three profiles set in the Caofeidian survey region.

Table 1 The parameters of the three profiles

The 320M sounder (measuring error<0.10m) and the DGPS of DSM212 (locating error<1m) produced by Trim- ble Company of the USA were used when the sounding were taken in 2006, 2008 and 2011. The sounding taken in 2017 was conducted by the HY-1600 sounder (measuring error±0.01m+0.1%D, D represents the water depth measured at real time) and the DGPS of SPS351 (locating error<1m) produced by Trimble Company. Beijing 54 Coordinates and Gauss-Krueger (6 degree zones) projection were adopted in this work, and the local theoretical depth datum was taken as the water depth datum.

2.2 Surface Sediment Sampling and Analysis

In 2016 and 2017, 260 sediment samples were obtained in the Caofeidian reclamation area and its adjacent area (Fig.5) to analyze and compare the types and distribution of the sediments before and after the reclamation. In the process of sedimentary analysis, several steps were taken in order as follows: drying, salt-leaching, removing organic matter, eliminating calcium cement, neutralizing and cleaning calcium ions and chloride ions, dispersing samples, and analyzing by laser particle analyzer Mastersizer 2000 (measurement range of 0.02–2000μm, deviation<1, and reproducibility50<1%). Due to the Shepard classification of sediments was used in the earlier studies of this region, in order to make a better comparison on the composition and distribution of the surface sediments be- fore and after reclamation, this classification was still adop- ted in this study.

Fig.5 The reclamation region and the surface sampling stations.

3 Results

3.1 The Topographical Changes

Actually, as early as in 2005 when the project has been just proceeding for one year, the large lagoon area was affected by the reclamation project significantly. The sea area on both side of the leading-to-island highway become shallower and sediments accumulated more compared with the situations in 2003 before the project started (Chu., 2019). The Laolonggou Inlet is the main part of the Laolonggou Lagoon and also the place with the deepest water depth, so it is chosen in this paper to de- monstrate the topographical changes in the process of re- clamation. Through measuring the water depth along three profiles, it was found that the topographical changes of the Laolonggou Lagoon were significant and behaved dif- ferently at different reclamation stages.

3.1.1 E-F profile

The E-F profile is the longest one among the three. It is located 2.5km north of the A-B profile and the entrance of Laolonggou Lagoon, crossing the No.3 harbor basin from the west to the east, and reaching the east part of the lagoon. From different sounding surveys along the profile, it was found that the topography of the eastern Laolonggou Lagoon and the shoal changed little. While the topography of the shoal in the western Laolongguo Lagoon changed obviously. Moreover, the depth of the bottom and the slopes on either side of the inlet also had evident changes.

From Fig.6, the topography images measured along the profile in October 2006 and October 2008 are similar, except for a more erosive bottom in 2008 than in 2006, with the deepest erosive depth of 2.95m. The deepest part of the inlet in 2006 is at the station nearly 6500m away from the starting point, and moved eastward to 6650m away from the starting point in 2008. The inlet bottom of the profile became shallower in both 2011 and 2017 than in 2008, but the bottom was eroded more in 2017 compared with in 2011. The maximum thickness of the deposition increased 2.09m in 2011 compared to 2008, then deepening by 2.27m in 2017 compared to 2011. The last two measurements (in 2011 and 2017) recognized some large pits on the west part of the profile, one in 2011 and three in 2017, with the depth varying from 8.01 to 12.78m, mainly resulting from engineering activities. The deep pit on the profile in 2011 was dug by quarrying for reclamation. The reclamation activity finished in 2011, so the big- gest pit in the profile of 2017 was formed under the integrated effects of wind and waves imposed continuously on the pit in 2011, cutting off the protruding parts and filling the hollowing parts. The other two in the westernmost of the profile were related to the human activities of digging the No.3 harbor basin. The bottom of the profile in 2017 moved eastward obviously for about 250m.

Fig.6 Comparison among the water depth changes in October 2006, October 2008, October 2011 and July 2017 along the E-F profile.

3.1.2 A-B profile

The A-B profile is the most important one among the three. It is located at the entrance of the Laolonggou Inlet which is a tidal inlet, the most sensitive and representative place in the process of topographical evolution. As early as in 1992, a detailed survey on marine geology, hydrodyna- mic force and marine topography with the working scale of 1:50000 was done within the Caofeidian shoal and Lao- longgou Lagoon, and field sampling and sounding along the profiles were implemented, collecting completed data before reclamation in this region. Generally speaking, by comparing the topographical curves along the A-B profile before and after the reclamation project, it can be found that the Laolonggou Inlet and its two slops on the east and west sides have changed obviously; while the shoal area to the east of the inlet has changed little.

As the Fig.7 shows, the topography profile in October of 2006 is very similar to that in June of 1992, but the bottom of the inlet in 2006 is more silty than that in 1992, with the maximum depositional thickness of 1.49m. The deepest station of the inlet in 1992 is 1660m away from the starting point, but moved westward to the place about 1560m away from the starting point in 2006. According to the measuring results, the bottom of the inlet in 2008 and 2011 was deepened apparently compared with 2006, and the strongest erosive period appeared in 2008, with the deepening of 7.31m, at the position about 1460m away from the starting point. The data of 2011 indicate that the inlet had a depositional trend, thickening by 1.57 m compared to 2008. The bottom of the profile in 2017 indicated that the inlet was erosive obviously compared with that in 2011, with the maximum erosive depth of 3.27m at the place of 1560m from the starting point, and the bottom moved eastward for about 100m. The topographical profile curves in 2008, 2011 and 2017 are very similar, but they had some fluctuations on the west part relative to the curves of 1992 and 2006.

Fig.7 Comparison among the water depth changes in June 1992, October 2006, October 2008, October 2011 and July 2017 along the A-B profile.

3.1.3 G-H profile

The G-H profile is located about 2.5km south of the A-B profile, near the lagoon entrance, transecting the south end of the lagoon estuary. The west end of the profile is very close to the present cofferdam region, and the east end is stretching to the east part of the lagoon. The topography to the west of the lagoon inlet changed little. While the topography to the east of the inlet had an obvious change, mainly due to eastward moving of the east slope of the Laolonggou Inlet.

From the Fig.8, the topographical curves along the profile in October 2006, October 2008, October 2011 and July 2017 are very similar, expect for small different fluc- tuations on the east of the profile. The bottom of the inlet had been a little bit erosive or silty sometimes in the past ten years. The location of the bottom had changed faintly, just moved eastward slightly from the place at 1650m to the place at 1700m from the starting point. The bottom of the profile in 2008 was deepened by 0.26m compared to 2006; while it became depositional in 2011, with a rising trend of 0.18m at the bottom surface, and it became erosive in 2017 from the latest measurement, with a deepening of 0.58m. The east slope of the inlet had been eroded little by little, becoming more and more erosive in the successive measurements (Fig.8).

3.2 Geological and Sedimentary Changes

3.2.1 Geological and sedimentary conditions before reclamation

The coastal region to the south of barrier islands, once the ancient coastline of the ancient Luanhe River, contains some coarse and dense sediments such as shell medium sand, fine sand, silty fine sand and so on, with the grain diameters of 0.06–0.25mm, which could resist the erosion caused by tides and waves, so the barrier islands were formed. To the north of the barrier islands, apart from a small part of the sediments composed of fine sand, the sediments are mainly composed of silty clay, clay silt and silt, with the main grain diameters of 0.016–0.12mm. The sediments are thin and loose with a weaker capacity to resist the erosion by tides and waves, so the shoal and the lagoon were formed. The sediments in the Caofeidian Channel are mainly the medium-fine sand of 0.2–0.3mm in diameter with the power to resist tides and waves, so the Caofeidian foreland is formed. The sediment types in Caofeidian sea area before reclamation are shown in Fig.9. The types of the sediments surrounding the Laolonggou Inlet from the land to the sea were sandy silt, silty sand, medium sands and clay silts in turn. Generally, the sediments from the land to the sea demonstrated the trend of fine to coarse and to fine again before reclamation.

Fig.8 Comparison among the water depth changes in October 2006, October 2008, October 2011 and July 2017 along the G-H profile.

Fig.9 The sediment types in the Caofeidian sea area before reclamation (revised from Jia et al., 1999). The red dotted line enclosed the later reclamation area. MS, medium sand; FS, fine sand; TS, silty sand; ST, sandy silt; T, silt; YT, clay silts; TY, silty clay.

3.2.2 The geological and sedimentary changes after reclamation

Based on the analysis to the surface sediments sampled at the bottom of the sea area in 2016 and 2017, it was found that the sediment features had obvious changes after the reclamation (Fig.10). The western part of the Laolonggou Lagoon has been reclaimed to the land by the backfill of silt quarried from the surrounding sea region and the harbor basins. The eastern part of the lagoon is still the sea area. For most part in this area, the sediments were the fine sand before reclamation but turned into the silt sand at present. The sediments in the sea area were diversified before the present No.3 harbor basin has been built in the eastern Laolonggou Lagoon, composed of medium sand, silty sand, sandy silt and silt. Now, after the No.3 harbor basin has been constructed and the reclamation has been finished, the bottom sediments have mostly turned into sandy silts. The sediments in the western Caofeidian sea area, where the present No.1 and No.2 harbor basins are located, were constituted by fine sand and sandy silt in 1996 before reclamation, but now have changed into sandy silt and silty sand with a small percentage of clay. After the reclamation project, the sediments around Caofeidian foreland are mainly the silt. The area 10km to the south of the foreland occurred a large band of stones stretching nearly in the EW direction and the size of stones became finer in the SN direction gradually.

Fig.10 The sediment types in the Caofeidian sea area after reclamation. FS, fine sand; S, sand; TS, silty sand; GS, gravel sand; ST, sandy silt; T, silt; SM, sandy mud.

4 Discussion

The erosive and depositional changes and the geomorphology in the Caofeidian area have been controlled by comprehensive factors of tide, wind, wave, sand transport,. Since the large-scale reclamation work started, the human activities such as constructing the leading-to-is- land highway, blocking the shoal tidal channel (Nachao River), enlarging the reclamation area, and digging new harbor basins, have imposed some impacts on the tide, wind, wave and sand transport. So human factors and natural conditions are combined to bring changes for the marine geological environment of the Caofeidian sea area in the past ten years.

4.1 Topographical Changes

The reclamation project experienced three stages as mentioned above. The topographical changes of the Lao- longgou Lagoon are mainly reflected by the topographical changes of Laolonggou Inlet, and these changes also went through three stages which are corresponding to the three stages of reclamation project. Generally speaking, in these years, the Laolonggou Inlet topography has changed orderly from deposition to erosion, then deposition and erosion again. These changes have close relation with the different reclamation events at different stages, for example, that the shoal tidal channel was blocked, the harbor basins were constructed and the shoal tidal channel was reopened.

1) The stage of reclaiming the area to the west of the leading-to-island highway (2004–2007)

From the A-B profile, it can be found that the inlet deposited obviously in 2006, and the sediments were thickened by 1.49m relative to that in 1992. It is very likely that the deposition was formed during the period of 2004–2006 as the result that the shoal tidal channel was blocked at the beginning of the reclamation. The channel was blocked by the highway, which gave rise to the changes of hydrodynamic conditions. According to the data mea- sured by Hebei Bureau of Geology and Mineral Resources Exploration (2015), the average current velocity of the tidal current in Laolonggou Lagoon was decreasing as a whole after the highway was constructed, which led to the deposition in the Laolonggou Inlet. At the beginning of the blocking for the shoal tidal channel, sediments deposited rapidly, and then became slower gradually. Moreover, the bottom of the inlet on the profile in 2006 had an obvious trend of moving westward for about 100m compared with that of 1992 (Fig.7). The reason for this westward movement is that the current flowing through the bottom became slow with the blockage of the channel, which would make the convex bank (the west bank) erosive and the concave bank (the east bank) depositional gra- dually. This phenomenon is consistent with the rule to the rivers on land that the erosion and the deposition along the concave and convex river banks changed with the flow velocity.

2) Stage of reclaiming to the area the east of the leading-to-island highway (2008–2011)

From the curves of the three profiles, the Laolonggou Inlet was deepened in 2008 and 2011 compared with that in 2006. Among all the profiles, the bottom on the A-B profile was deepened most prominently with the maximum increased depth of 7.31m in 2008. There are two reasons for the bottom changing from deposition to obvious erosion after the shoal tidal channel was blocked. One reason is the construction of No.3 harbor basin as well as the reclamation work in the east part in the period of 2008–2011, and particularly, a large amount of soil was quarried for reclamation in this sea area. The other important reason is the cofferdam construction in 2009. The cofferdam was constructed in the east of the reclamation area (Fig.11), which narrowed the current flowing through the inlet, leading to high velocity and incision effect, so the inlet was deepened greatly. The two satellite images demonstrated in Fig.11 show that the cofferdam construction started in 2008 and proceeded mainly in 2009. The area highlighted by the red frame in the image is the nearly completed cofferdam at the end of 2009. Therefore, the inlet bottom was eroded obviously and temporarily in 2008 for the factors of human activities. When the reclamation project and quarrying activity were drawing to a close in 2011, the inlet was deposited slightly in 2011 compared with that in 2008 due to the blocked shoal tidal channel and the damaged tidal system.

3) The stage of restoration and maintenance after reclamation (2011 to now)

The reclamation activity was almost finished in 2011, except for some restoration work on the harbors and harbor basins, as well as the preparation for reopening the shoal tidal channel, such as bank revetment on either side of the shoal tidal channel, bridge construction,. After all the work was finished, the shoal tidal channel was reopened finally in August 2016 by removing the blocking part of the leading-to-island highway from the channel (Yin., 2016).

Fig.11 The project progress in the east reclamation area in 2008 and 2009, respectively. In the red frame was the newly-built cofferdam.

Since the shoal tidal channel was reopened in August 2016, the tidal system had recovered to some extent. From the field survey data along three profiles in 2017 and the physical modelling results, the hydrodynamic force of the Laolonggou Inlet was strengthened after the shoal tidal channel was reopened, with the average flood current ve- locity increased by 20% or so and the average ebb tide velocity increased by 15% or so. The topography of Lao- longgou Lagoon showed the feature of deepening, and it is estimated to reach fluvial equilibrium several years later.Generally, reopening the shoal tidal channel is very meaningful for tidal system recovery and inlet stability maintenance.

The location of the bottom along the three profiles in 2017 all moved eastward compared with the previous measurements. This movement of the bottom at the A-B profile is more obvious than the bottoms at the E-F profile and the G-H profile. It indicated that the west slopes in the three profiles were depositional and the east slopes were erosive to different extents. The most important reason for their movement is that the tidal current was impeded then deviated by the cofferdam which was constructed in the eastern sea area during 2008–2010. The cofferdam is situated closely to the west end of the three profiles. The spring and the neap tidal currents were obstructed here and deviated a little with increasing flow velocities. Due to the increasing flow velocity, the flow would hit the concave bank (the east bank) in a high speed, which resulted in the erosion on the eastern slope of the inlet and the eastward movement of the inlet bottom.

4.2 Variation of Sediment Types

After the reclamation was almost finished, the west part of the Laolonggou Lagoon has been turned into land which was mainly constituted by the backfill of silt. The sediments in the east part of the lagoon were mainly the fine sand before the reclamation, but changed into silty sand, with smaller grain size, after the reclamation. The sediments in the harbor basins and the lagoon inlet are now finer than the sediments before the reclamation. Due to the weakened flow velocity in the reclamation area, along with the damaged tidal system caused by the blocked shoal tidal channel, the transporting capacity was decreasing, and the sediments with small grain size such as silt and clay were transported and deposited here. After the shoal tidal channel was reopened, the tidal system was somewhat recovered. In a short period the sediments will not change greatly, but gradually become coarsening as time goes by. The flow velocity around Caofeidian foreland is higher because of the cape effects. After the shoal tidal channel was blocked in the first stage, the flow velocity became higher, taking finer sediments away and leaving bigger particles deposited here, so the surface sediments became coarser. The channel now has been sharing some current after it was reopened, making the flow velocity decrease around the foreland, so some smal- ler particles deposited here and the surface sediment became finer. Generally, the sediment size here was bigger when the flow velocity was higher, and smaller when the velocity decreased. So the sediments around the foreland turned coarsening firstly, then refining again as the flow velocity was decreasing around the Caofeidian foreland after the reclamation.

5 Conclusions

The Caofeidian Reclamation Project, as the biggest re- clamation project in China, exerted significant and changing impacts on the topographical features and sedimentary environment in the Laolonggou Lagoon.

1) In the process of reclamation, the topography of the Laolonggou Lagoon, especially in the Laolonggou Inlet, demonstrated two deposition-erosion cycles, mainly controlled by the blocking and reopening of the shoal tidal channel, besides local manual quarrying as well as the harbors and cofferdam construction at special positions.

2) The axis of the Laolonggou Inlet was moving 50–100m from the west to the east with the proceeding of the project and its causing changes of the hydrodynamic conditions. Particularly, the newly-built cofferdam in the eastern lagoon constrained and narrowed the current channel, which enhanced the flow hitting on the concave bank (the east bank) and made the east bank erosive.

3) The west part of the Laolonggou Lagoon was filled into the land with silt. The sediments in the eastern lagoon generally decreased in their grain size from fine sands to silty sand due to the decrease of spring and neap tide current velocity caused by the blocking of the shoal tidal channel. The sediments around the foreland of Cao- feidian Island changed from fine to coarse for increased current velocity at the cape when the shoal tidal channel was blocked. After the tidal channel was reopened, opposite changes have occurred gradually. All the marine geo- environmental changes and influences mentioned above will last for a long time in this area, and researches should be conducted continuously in future.

From the Caofeidian Reclamation Project, some experience could be acquired. The reclamation projects should be discussed and surveyed carefully before they break the ground, and the reclamation area should be limited in a reasonable range. During the project proceeding, the ori- ginal marine dynamics and the key topography and geomorphology should be protected from being damaged, or the damage should be minimized as much as possible.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (No. 40876033), and the China Geological Survey Project (No. DD20160144). We thank Professor Tianfeng Wan for his instructive advice. We also give our honest appreciation to the anonymous reviewers for their constructive comments and suggestions that made improvements for the manuscript.

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. E-mail: fengxiuli@ouc.edu.cn

June 27, 2019;

October 14, 2019;

December 25, 2019

(Edited by Chen Wenwen)