Beach Morphology and Coastline Evolution in the Southern Bohai Strait

ZHANG Wei, WU Jianzheng, LI Weiran, ZHU Longhai, HU Rijun,, JIANG shenghui, SUN Yonggen, and WANG Huijuan

1) College of Marine Geoscience, Ocean University of China, Qingdao 266100, P. R. China

2) Qingdao Marine Geological Engineering Investigation Institute, Qingdao 266071, P. R. China

3) Key Laboratory of Submarine Geosciences and Prospecting Techniques, Ministry of Education, Ocean University of China, Qingdao 266100, P. R. China

4) First Institute of Oceanography, SOA, Qingdao 266061, P. R. China

5) Qingdao Boyan Marine Environment Science & Technology Co. Ltd., Qingdao 266101, P. R. China

Beach Morphology and Coastline Evolution in the Southern Bohai Strait

ZHANG Wei1),2), WU Jianzheng1),3), LI Weiran1),3), ZHU Longhai1),3), HU Rijun1),3),*, JIANG shenghui1),3), SUN Yonggen4), and WANG Huijuan5)

1) College of Marine Geoscience, Ocean University of China, Qingdao 266100, P. R. China

2) Qingdao Marine Geological Engineering Investigation Institute, Qingdao 266071, P. R. China

3) Key Laboratory of Submarine Geosciences and Prospecting Techniques, Ministry of Education, Ocean University of China, Qingdao 266100, P. R. China

4) First Institute of Oceanography, SOA, Qingdao 266061, P. R. China

5) Qingdao Boyan Marine Environment Science & Technology Co. Ltd., Qingdao 266101, P. R. China

? Ocean University of China, Science Press and Springer-Verlag Berlin Heidelberg 2015

The beach studied in this paper spans a length of 51 km and is one of several long sandy beaches in the southern Bohai Strait. Due to the obstruction of islands in the northeast and the influence of the underwater topography, the wave environment in the offshore area is complex; beach types and sediment transport characteristics vary along different coasts. The coastlines extracted from six aerial photographs in different years were compared to demonstrate the evolving features. Seven typical beach profiles were selected to study the lateral beach variation characteristics. Continuous wind and wave observation data from Beihuangcheng ocean station during 2009 were employed for the hindcast of the local wave environment using a regional spectral wave model. Then the results of the wave hindcast were incorporated into the LITDRIFT model to compute the sediment transport rates and directions along the coasts and analyze the longshore sand movement. The results show that the coastline evolution of sand beaches in the southern Bohai Strait has spatial and temporal variations and the coast can be divided into four typical regions. Region (I), the north coast of Qimudao, is a slightly eroded and dissipative beach with a large sediment transport rate; Region (II), the southwest coast of Gangluan Port, is a slightly deposited and dissipative beach with moderate sediment transport rate; Region (III), in the central area, is a beach that is gradually transformed from a slightly eroded dissipative beach to a moderately or slightly strong eroded bar-trough beach from west to east with a relatively moderate sediment transport rate. Region (IV), on the east coast, is a strongly eroded and reflective beach with a weak sediment transport rate. The wave conditions exhibit an increasing trend from west to east in the offshore area. The distribution of the wave-induced current inside the wave breaking region and the littoral sediment transport in the nearshore region exhibit a gradual weakening tendency from west to east, which is opposite to the trend of the wave conditions outside the breaking region. The presence of submerged shoal (Dengzhou Shoal), deep trough (Dengzhou Channel), islands and irregular topography influnces the wave climate, beach types, wave-induced current features, littoral sediment transport trends and coastline evolution patterns in the southern Bohai Strait. Human activities, such as the sand exploitation of Dengzhou Shoal and other coastal engineering projects, also influence the beach morphology and coastline evolution.

sand beach; coastline evolution; profile feature; wave hind-casting; wave-induced current; littoral sediment transport

1 Introduction

Sand beaches are charming and offer beautiful sceneries and hydrophilic entertainment; they are also an important coastal zone for human activities. Domestic and foreign scholars have conducted extensive studies on sand beaches around the world (Pilkey and Richter, 1964; Hallermeier, 1980–1981; Wright and Short, 1984; Dean, 1991; Benedet and List, 2008). At present, domestic research mainly focuses on beaches in the South China Sea(Dai et al., 2007; Cai et al., 2009; Qi et al., 2010) and East China Sea (Chen et al., 2011; Qu et al., 2013). In addition, numerous studies (Kuang et al., 2010; Zhang et al., 2012) were conducted on the beach of Beidaihe, which is located on the west side of the Bohai Sea, China. The coastline of Shandong Province is the second longest in China, but research on sand beaches is lacking. With the increase in coastal development activities, the research and protection of sand beaches are becoming more and more important. Previous studies of sand beach morphology focused on investigations of the profile changes of the cross shore (Masselink and Pattiaratchi, 2001; Jeanson et al., 2013) and the coastline evolution along the beach (Schwarzer et al., 2003; Robinson, 2004). Com-monly used research techniques include field surveys (Rao et al., 2009), remote sensing image interpretation (White and Asmar, 1999; Liu et al., 2013), numerical simulation (Zheng and Dean, 1997; Takaaki et al., 2010), and theoretical analysis (Chiara et al., 2011; van Rijn, 2011).

In the present work, the study area in the southern Bohai Strait is characterized by a complex morphology (submerged shoal, a deep trough, and islands), coastal structures and irregular depth contours, as well as varied wave conditions and sand beach evolution. The relevant previous studies mainly focused on the sediment characteristics (Dong et al., 2011), hydrodynamics (Jiang and Gao, 2002), geomorphological features (Wu, 1992), and wave conditions (Chen and Hu, 1992) around Bohai Strait and Dengzhou Shoal. However, scholars only sparsely focused on the sandy beach in the southern Bohai Strait. Based on a field survey, Luan (2011) described the coastal sediment and geomorphological characteristics of the beach east of Luanjiakou Port and indicated that the beach is eroded and straightened. The evolution of Dengzhou Shoal and its association with the erosion on the west coast of Penglai has also attracted considerable attention (Wang, 1994; Wu et al., 1997; Li et al., 2004; Wang et al., 2005). These studies suggested that the sand exploitation of Dengzhou Shoal was the main reason underlying the coastal erosion along Penglai. However, these studies were mostly conducted before 2005 without considering the beach between Luanjiakou Port and Qimudao. Due to the complex geomorphological characteristics in the study area, the sediment dynamics and beach evolution process, which are useful for coastal protection and development, remain to be understood.

In this study, we attempt to elucidate the evolution of a sand beach and the controlling factors. More specifically, the research aims to 1) use satellite images to analyze the coastline variation, 2) compare in-situ measurements with historical profiles to study the lateral variation of the beach, 3) make use of the SW wave model to study the wave conditions in offshore areas, 4) apply the coastal sediment transport model (the LITDRIFT model) to simulate the wave-induced current and littoral sediment movement in nearshore areas.

2 Study Area

The study area is located in the southern Bohai Strait from the west side of Qimudao to the east side of Penglai Port (Fig.1). The northern part of the study area includes the Miaodao Archipelago and Daheishan Island, and the central part covers Sangdao Island. Freshwater inflow from the Huangshui River the east of Gangluan Port has a small discharge. The coastline of the beach runs through the study area with an overall direction of ENE–WSW and extends approximately 51 km. Unlike a normal straight coast, the bathymetry in the area is highly complex due to the presence of submerged shoal (Dengzhou Shoal), a deep trough (Dengzhou Channel), islands and coastal structures (Fig.1). The minimum water depth over the shoal is less than 5 m but the depth increases eastwards to approximately 47 m in the trough. In the study area, Dengzhou Shoal extends approximately 13 km northwestward from the Penglai Port. Along the coast, there are many ports and scenic spots, constructed with groins, breakwaters, and revetments to prevent coastal erosion. The characteristic sand beach width is mostly less than 70 m, although it is 140 m at the groyne root. The tide is small in the nearshore area with an average rage of 1.06 m.

Fig.1 The study area with bathymetry contours and the four main regions with seven typical beach profiles.

3 Data and Methods

3.1 Basic Data

According to the distribution of the beaches and the data from a field investigation, the study area was divided into four main regions with seven typical profiles, which were measured in the shore-normal direction (Fig.1). By contrasting the depth data in the same positions of the profiles obtained from the field investigation with a large-scale nautical chart, the evolution of the selected beach profiles were characterized (Table 1). The field investigation used a Leica SR 500 Real Time Kinematic Global Positioning System (RTKGPS) with a position accuracy of ±1 cm and an elevation accuracy of ±2 cm as well as an SDE-28S Single Frequency Depth Sounder with a depth accuracy of ±1 cm ±0.1%D. A base reference station was established at Luanjiakou Port. The vertical coordinate was referenced to the mean sea level (MSL) for accurate comparisons. The nautical chart data are processed by the natural neighbor interpolation method (Yvonnet et al., 2006; Dinis et al., 2010).

To obtain the grain size parameter for sediment transport modeling, the sediment samples collected from all of the beach profiles were subjected to sieve analysis (Fernlund, 1998). For wave hindcast, the required field data for the boundary conditions of the regional spectral wave model were obtained from in situ wave recorders. SBA-2 Optical Wave Recorders were deployed at the Beihuangcheng ocean station (Fig.2) and the wave parameters were recorded at three-hour intervals from 1 January to 31 December 2009 (Fig.3). The wave parameters during a high wave event from 9 to 11 November 2009 at the Longkou ocean station were used to calibrate the modeling result.

3.2 Coastline Evolution and Comparison

For the multi-temporal analysis, we used six aerial photographs (Table 2) from 1979 to 2013 which were downloaded for free from http://glovis.usgs.gov/. Geometric correction was used in these Landsat images with a WGS-84 coordinate system and a Gauss-Kruger projection (with a RMS less than 0.5 pixels). Path radiance correction was used in dark object subtraction (DOS) (Chavez Jr., 1988). The deviation of the tidal level was ignored due to the small tides and steep slopes of the beaches. The coastlines were obtained by the single band density slicing method based on the NIR band (Liu et al., 2013).

Table 1 Location and data date of the beach profiles in the study area

Fig.2 Nested type of flexible mesh used in the regional spectral wave model for the study area.

Fig.3 Time series of offshore significant wave height (Hs), peak period (Tp) and peak wave direction (θ) at a 10 m depth from January 2009 to December 2009.

Table 2 Acquisition time, resolutions and the local tidal levels of the aerial images

3.3 Wave Hindcast

Wave hindcast for the study area was conducted by the MIKE 21 Spectral Wave (MIKE 21 SW) model (Byrnes and Hammer, 2004; DHI, 2005). This model can simulate the growth, decay and transformation of wind-generated waves and swell in offshore and coastal areas by dissipation due to white-capping, bottom friction and depth-i n-duced wave breaking, and by refraction and shoaling due to depth variation, wave interaction and the effect of time-varying water depth (Sabique et al., 2011). All forcing sources except tidal variation are considered in the model. A fully spectral formulation and instationary time formulation are used (Komen et al., 1994). Diffraction is included using the phase-decoupled refraction-diffraction approximation proposed by Holthuijsen et al. (2003) with a smoothing factor of α=1. Wave breaking is based on the formulation of Battjes and Janssen (1978) where α=1 and γ=0.73 (SWAN team, 2012). The Nikuradse roughness coefficient is used for the dissipation due to bottom friction, where KN=0.04. The dissipation due to white capping based on the theory of Hasselmann (1974) is found to be 4.5.

The wave simulation area spanned the region from western Qimudao to eastern Zhifudao Island, and the northern side extended to northernmost Bohai Strait island, i.e., Beihuangcheng Island (37?32′ to 38?25′N and 120?13′ to 121?25′E). The grid distribution is shown in Fig.2. The total simulated area is 8640.64 km2, which totals 26093 grids; the largest grid size is 0.90 km2. The area with concentrated grids (minimum size of 59.91 m2) was the region of our special concernment; the coastal zone extends approximately 63 km from Qimudao to the east of Penglai. Water depth data were obtained from the latest available nautical chart, and nearshore high-r esolution bathymetric data obtained from a field investigation.Wave heights, wave periods and wave directions were obtained from the numerical simulation results. The calibration results at Longkou station exhibited a strong correlation between the simulation results and the measured parameters (Fig.4). The annual wave distribution patternand the corresponding rose plots at the 10 m water depth of the in-situ measurement profiles are shown in Fig.9. The hindcast data on wave heights, wave periods and wave directions were utilized as input for the LITDRIFT modeling of sediment transport (DHI, 2005).

Fig.4 Calibration of spectral wave model results for a) significant wave heights b) wave directions and c) wave periods at Longkou station (located on the 16.8 m depth contour).

3.4 LITDRIFT Model

The Annual Sediment Drift and Longshore Current model of LITDRIFT has been applied to calculate the longshore transport of non-cohesive sediment and waveinduced currents on a long uniform beach (DHI, 2009). LITDRIFT provides a detailed deterministic description of longshore sediment transport (LST) for an arbitrary bathymetry and calculates the net and gross littoral transport amounts for a section of coastline over a specific design period considering wave breaking. The input data used for the LITDRIFT simulation are the cross-shore profiles, the coast-normal angle, wave heights, wave directions and wave periods at a reference water depth of 10 m. For all profiles, the profiles’ length used to calculate the LST amount was 650 m with maximum boundary depths from 8.3 m to 10.0 m. The simulation results of a wave-induced current inside a breaking wave area of a 2.0 m depth are described in Section 4.4. The boundary wave parameters of the profiles are derived from wave hindcast in Section 3.3. The model considers Battjes and Janssen’s approach of wave propagation from deep water (Battjes and Janssen, 1978).

4 Results and Discussion

4.1 Coastline Evolution

The changes of the coastlines since 1979 as extracted from the satellite images are shown in Fig.5 and Fig.6. The coastline of Region (I) is flat and extends from east to west. The coast was continuously eroded; the maximum erosion distance was 60.6 m (2.75 m yr-1) from 1979 to 2000. Then, the coast was in a dynamic equilibrium state with negligible variation between 2000 and 2009. With the construction of breakwaters and other influencing factors, the coast exhibited an erosional trend again in which the maximum erosion distance was 52.8 m (5.28 m yr-1). An accretion trend with a nearly equivalent distance can be observed on both sides of the eastern breakwater root.

Fig.5 Each coastline was extracted from the aerial photo only by visual analysis. These are examples of extracted coastlines in 1979 (a), 1984 (b), 1989 (c), 2000 (d), 2009 (e), and 2013 (h).

Fig.6 Coastline changes at various coasts since 1979.

Overall, Region (II) presents a coastline extending from northeast to southwest. A continuous natural sand beach existed before 2000 that showed a gentle erosion trend at an average speed of approximately 0.45 m yr-1. Currently, many artificial buildings such as groynes, breakwaters, scenic spots, reclamation engineering and revetment have been constructed along the coast. The coastline distribution pattern seems to be significantly influenced by these buildings. For example, the presence of the east port led to the disappearance of the original beach coastline after 2009 (Fig.6). However, the establishment of a tourism landscape, built on the east side of the region, formed a typical artificial static equilibrium headland bay (Moon Bay) (Hsu et al., 1989; González and Medina, 2001) and sand beach. The beach at the root of the northeast groyne shows significant accretion, but erosion occurred along the coastline southwest of Gangluan Port, which implies a longshore sediment transport from northeast to southwest. The natural sand beach on the west estuary of the Yongwen River disappeared due to the construction of the revetment after 2009.

The coastlines of Region (I) and Region (III) have similar distributions. Three elliptic reclamation engineering projects and Gangluan Port are located on the west coast. The sand beach that extends along the coast to the west of Luanjiakou Port showed continuous erosion, with a maximum erosion distance of 77 m (3.5 m yr-1) from 1979 to 2000. Then, the erosion speed slowed with a maximum erosion distance of 36 m (2.57 m yr-1) between 2000 and 2013. The reclamation engineering in the west remarkably modified the coastline, which caused serious erosion between the engineering projects and deposition in the shadow zones.

The regional coastline distribution pattern of Region (IV) is similar to that of Region (II): it extends from northeast to southwest. However, the beach of Region (IV) is short and narrow, and its distribution is not continuous. Moreover, there are also many eroded cliffs and houses and roads on the verge are swallowed (Fig.7). Additionally, this coastal segment undergoes changes because of a large amount of coastal engineering, such as breakwaters, groynes, ponds and revetments (Fig.6). Previous studies have been conducted on Dengzhou Shoal in the offshore area and suggest that the sand exploitation at the end of the last century severely damaged the shoal, which led to serious coastline erosion (Li et al., 2004; Wang et al., 2005). Most of these coastal constructions were established during the last decade to prevent serious coastal erosion. The beaches scattered around the breakwaters and groynes are mainly present at a small scale with primary sand types of coarse sand to gravel. The comparison in Fig.6 shows a dynamic variation between erosion and accretion from 1979 to 1984. This trend has been confirmed to relate to the existence of Dengzhou Shoal, which can weaken the wave influence significantly. However, the shoal was generally eroded, with an average erosion distance of 30 m (5.0 m yr-1) from 1984 to 1989. This result correlates well with that of Wu et al. (1992, 1997). Since 2009, the coastline has changed insignificantly, indicating that these coastal engineering constructions have played a certain role in preventing coastal erosion.

Overall, the coastline evolution of sand beaches in the southern Bohai Strait exhibit spatial and temporal variations. Prior to the year 2000, natural sand beaches experienced serious coastline erosion in Regions (I) and (III) at a maximum speed of 3.5 m yr-1. The coastline erosion speed in Region (III), which is located in the middle of the region, was greater than that in Region (I) located in the west. After 2000, the speeds decreased in the two re-gions. The coastal evolution in Region (II), which showed gentle erosion previously, changed due to the coastal engineering after 2000. The coast exhibited a dynamic variation with the existence of the Dengzhou Shoal which weakened the wave influence significantly before 1984. Then, the coast was seriously eroded, with a maximum speed of 5.0 m yr-1between 1984 and 1989, due to the crude sand exploitation of Dengzhou Shoal. Since 2009, the coastline has changed insignificantly due to the coastal protection projects.

Fig.7 Field photographs showing severe erosion of coastline in Region (IV).

4.2 Profile Features

Based on the hydrodynamic processes and the relative contributions of different mechanisms to sediment transport and morphologic change, Wright and Short (1984) classified sand beach types, such as dissipative, reflective and several intermediate states (e.g., bar-trough, rhythmic bar-trough, transverse bar-rip and ridge and runnel). The temporal morphological variability of the specific crossshore transects are shown in Fig.8, where the upper figures indicate the profile obtained in different years, and the lower figures show the difference between temporally varying profiles.

Comparisons revealed that profile 1 of Region (I) (Fig.8) was in a slightly eroded state and exhibited very low mobility between 2007 and 2012. Two or three gently sloping sandbars formed by the storm surge were located below the water in Profile 1 and Profile 2. The beach of Region (I) can be classified as a slightly eroded, dissipative beach (Ojeda et al., 2011; Thiebot et al., 2012).

Profile 3 in Region (II) is characterized by a deposition trend (Fig.8) from 1975 to 2012, with a maximum deposition thickness of approximately 2.8 m (0.56 m yr-1). The occurrence of different trends in Region (I) and Region (II) may be due to the constructed groynes along the coast. These groynes may cause heterogeneous wave circumstances and thus block the sediment transport and enhance the shoreward sediment accumulation process.

As observed in Region (III), the profile changes were similar for Profile 4 and Profile 5 from 1975 to 2004; both of the profiles were characterized by erosion with a maximum value of approximately 1 m (0.03 m yr-1) (Fig.8). However, the profiles have showed different changes after 2004. The beach having Profile 4 therein with a gentle slope and a sand bar formed below 3 m in a storm surge, is a micro-erosion dissipative type. For Profile 5, a sand bar also existed below 5 m, and a 150 m wide trough with a relatively steeper slope is present. Many smallsized beach cusps exist along the coastline. All of these features suggest that the beach under Profile 5 is a bar- trough beach, which is the intermediate state between a dissipative and reflective beach. The maximum erosion value above a 4.3 m water depth in Region (III) is approximately 2.1 m (0.3 m yr-1) after 2004 but less than 1 m (0.14 m yr-1) under the same water depth before 2004. In general, the sand beach has been transformed from a slightly eroded dissipative beach to a moderately or somewhat strong eroded bar-trough beach from west to east in Region (III), both of which are intermediate states between dissipative and reflective ones.

The changes of Profiles 6 and 7 in Region (IV) indicated strongly erosion landward and accretion seaward at 6 m to 7 m depths, the maximum erosion value being 3.5 m (0.11 m yr-1). There is a strong wave energy environment in Region (IV) where the waves clearly influence the depth more than in other regions. During 2004 to2012, these profiles were eroded landward and accreted seaward at 8 m to 8.5 m depths with a maximum erosion value of 1.6 m (0.18 m yr-1) and an increasing wave influence depth. All of these features indicate that the beachin Region (IV) is a strongly eroded reflective beach.

Fig.8 Variations of the selected profiles along coast of the study area.

4.3 Wave Climate

The wave simulation results indicate that there are distinct regional characteristics over different profiles in our study area (Fig.9 and Table 3). Most observed waves in Region (I) are in the directions of N to NNE, then in WNW and NNW, scarcely with the directions of W and E. Region (II) is affected by the northeastern Sangdao Island, and the frequencies of NW to W waves are significantly higher than those of the NNW to ENE waves; the highest frequency waves are in the W direction. In Region (III), the most frequent wave directions are NNW and N. The wave frequency in the W direction is relatively high as well, it being impacted by the southern waterway on the west side of Sangdao Island. In contrast, due to the blocking of the Miaodao Archipelago, the NE and ENE waves rarely occur. Also, due to the Miaodao Archipelago, N and NNW waves occur less often, and W waves are frequently observed in Region (IV).

Wave height statistics suggests that the frequency of wave heights greater than 1.8 m is higher in Regions (III) and (IV) on the eastern side of the study area, than those in Regions (I) and (II). An eastward trend increasingly occurs, corresponding to a wave climate of a dissipative or reflective beach from west to east (Table 4).

Fig.9 Rose plots for wave heights and directions for offshore locations at a 10 m depth off the coast.

Table 3 Frequency statistics of waves for different stations and directions

Table 4 Frequency statistics of waves of various heights and stations

4.4 Wave-Induced Current

The wave-induced currents generated in coastal regions are directly responsible for sediment transport and morphology evolutions (Xie, 2012). Based on the wave simulation results, the wave-induced current inside the breaking region of a 2.0 meter depth in the selected seven profiles can be calculated by the LITDRIFT model (Fig.10). Similar variation tendencies and strong currents occur in C1 and C2 of Region (I). The max-speed was higher than 4.0 m s-1. The current speed in C3 of Region (II) is negligible due to blocking by the breakwaters around it. The current curves of C4 and C5 in Region (III) are similar in that the maximum values are greater than 3.0 m s-1(with opposite coastal sloping directions). The current at the east station (C5) is stronger than that in the west. The similar variation tendencies in C6 and C7 of Region (IV) can also be seen in Fig.10. However, the max current speeds of 2.3 m s-1in C6 and 1.4 m s-1in C7 are obviously smaller than those in other regions. Meanwhile, the occurrence frequency of wave-induced current in C7 is higher than that at other points. In all, the distribution of the wave-induced currents inside the wave breaking region exhibits a gradual weakening tendency from west to east, which is opposite to the trend of wave conditions outside the breaking region. The non-correspondence is mainly caused by the complex morphology. In the west region, the morphology is flat, such that waves break closeto the coast with strong energy to generate a strong longshore current. In the east region, the morphology is complex and includes shoals, which can wake the waves break in the offshore area, weaken the energy and generate a weak longshore current.

Fig.10 Time series plots of mode-simulated wave-induced current for nearshore locations (C1 to C7).

4.5 Littoral Sediment Transport

The littoral profile sediment transport amount is shown in Table 5. Strong transports take place at Profiles 2 and 5 with large net and gross LST amounts. Moderate transports can be observed at Profiles 1, 3 and 7; meanwhile, weaker transports occur at Profiles 2 and 6. Comparing with the wave-induced current conditions in Fig.10 shows an incomplete coincidence trend. The strong longshore current at C1 and moderate LST amount at Profile 1 can be ascribed to the deep water depth, which causes weak turbulence and transportation of sediment. Otherwise, the weak longshore current at C3 and moderate LST amount at Profile 3 are due to the low water depth, which enhances the turbulence and the transportation of the sediment. Thus, topography is also an important factor for the longshore sediment transport.

The LST directions off different coasts also exhibit a regional character. The LST direction is from east to west at Profile 1 and from northeast to southwest at Profile 3; it from west to east in Profile 2 at which the wave condition is influenced by the Sangdao Island located in the northeast. Affected by the Miaodao Archipelago and Dengzhou Shoal locating in the northwest, the LST directions at Profiles 4 and 5 are from west to east. Meanwhile, the LST directions at Profiles 6 and 7 are from northeast to southwest due to the coast span direction and the presence of Dengzhou Channal.

Comprehensively considering the wave conditions, wave-induced current and littoral sediment transport features, we can identify a very interesting pattern, in which the wave height is enhanced in the offshore area with a 10 m depth along the coast from west to east; however, opposite trends in the wave-induced current and littoral sediment transport are observed in the nearshore area. The enhancement of wave height in the east is mainly due to the deep water in the Dengzhou Channel. The weakening trends of the wave-induced current and littoral sediment transport in the nearshore area are mainly attributed to Dengzhou Shoal, which can weaken the wave energy in the off-shore area.

Table 5 Grain size (D50), shore slope and simulated amount of annual sediment transport at the beach profiles

Fig.11 Longshore and cross-shore transport directions in the study area obtained from the LITDRIFT model.

5 Conclusions

The coastline in the study area, which is affected by sand exploitation and complicated geomorphology (i.e., channels, shoals, islands, and ports), exhibits wide variations in its evolution, wave conditions, wave-induced current and longshore sediment transport patterns.

The coastline evolution of sand beaches in the southern Bohai Strait shows spatial and temporal variations. In the natural state, the west coast shows a serious erosion trend, while the coast in the eastern area that is shielded by Dengzhou Shoal exhibits a dynamic variation. Due to the sand exploitation of Dengzhou Shoal after 1984 and the coastal engineering after 2000, the coastline variations exhibits an obvious change: the erosion speed slows down in the west but becomes severe in the east. Currently, the coastline in the east is changing insignificantly due to the existence of the coastal protection projects.

The beaches eroded along the coastline, except in Region (II) where the sediment transport and wave field were affected by the groynes. The beaches can be divided into several types based on their profile morphologies and comparisons. The beach in Region (I), which is located in the west of the study area, is a slightly eroded, dissipative beach. A slightly deposited, dissipative beach exists in Region (II) southwest of Gangluan Port. Region (III) in the central area is a transition region in which the beach is transformed from a slightly eroded dissipative one into a moderately or somewhat strongly eroded bar-trough beach which is an intermediate state between dissipative and reflective beaches from west to east. The beach in Region (IV) located in the eastern area is a strongly eroded, reflective beach.

The various evolving coastline features and types reflect different dynamic environments. The wave hindcast results suggest that the frequency of strong waves has been obviously higher in Regions (III) and (IV) in the east than that in Regions (I) and (II). This trend corresponds to the process for dissipative beaches with low wave energy to be transformed into reflective beaches with high wave energy from west to east in the study area.

The distribution of wave-induced currents inside the wave breaking region and the littoral sediment transport in the nearshore region displayed a gradual weakening tendency from west to east, which is opposite to the trend of the wave condition outside the breaking region.

With the aid of aerial photography comparisons, field investigation, wave handcast, wave-induced current and littoral sediment transport simulation, this study determines the evolving features of sand beaches in the southern Bohai Strait and therefore provides useful guidance for coastal exploitation, beach regulation and beach protection.

Acknowledgements

The authors wish to express sincere thanks to Drs. Yongchen Xu and Peng Wang, and Mrs Chao Dong, Dongxiao Yin, and Xianfeng Zhang at the Ocean University of China for their hard work in the field investigation. This work is supported by the National Natural Science Foundation for the Youth (No. 41106039).

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(Edited by Xie Jun)

(Received December 25, 2013; revised March 10, 2014; accepted December 18, 2014)

J. Ocean Univ. China (Oceanic and Coastal Sea Research)

DOI 10.1007/s11802-015-2569-1

ISSN 1672-5182, 2015 14 (5): 803-815

http://www.ouc.edu.cn/xbywb/

E-mail:xbywb@ouc.edu.cn

* Corresponding author. Tel: 0086-532-66781882

E-mail: hrj@ouc.edu.cn