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Satellite-based observations of unexpected coastal changes due to the Saemangeum Dyke construction, Korea
Marine Pollution Bulletin 97 (2015) 150–159
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Marine Pollution Bulletin journal homepage: www.elsevier.com/locate/marpolbul
Satellite-based observations of unexpected coastal changes due to the Saemangeum Dyke construction, Korea Yoon-Kyung Lee a, Joo-Hyung Ryu a,⇑, Jong-Kuk Choi a, Seok Lee b, Han-Jun Woo c a
Korea Ocean Satellite Center, Korea Institute of Ocean Science & Technology, 787 Haean-ro, Ansan 426-744, Republic of Korea Physical Oceanography Division, Korea Institute of Ocean Science & Technology, 787 Haean-ro, Ansan 426-744, Republic of Korea c Marine geology & Geophysics Division, Korea Institute of Ocean Science & Technology, 787 Haean-ro, Ansan 426-744, Republic of Korea b
a r t i c l e
i n f o
Article history: Received 30 September 2014 Revised 8 June 2015 Accepted 11 June 2015 Available online 20 June 2015 Keywords: Tidal flat Coastal water Morphological change Suspended sediment GOCI Landsat
a b s t r a c t Spatial and temporal changes around an area of conventional coastal engineering can be easily observed from field surveys because of the clear cause-and-effect observable in the before and after stages of the project. However, it is more difficult to determine environmental changes in the vicinity of tidal flats and coastal areas that are a considerable distance from the project. To identify any unexpected environmental impacts of the construction of Saemangeum Dyke in the area, we examined morphological changes identified by satellite-based observations through a field survey on Gomso Bay tidal flats (15 km from Saemangeum Dyke), and changes in the suspended sediment distribution identified by satellite-based observations through a hydrodynamic analysis in the Saemangeum and Gomso coastal area. We argue that hydrodynamic changes due to conventional coastal engineering can affect the sedimentation pattern in the vicinity of tidal flats. We suggest that the environmental impact assessment conducted before a conventional coastal engineering project should include a larger area than is currently considered. Ó 2015 Elsevier Ltd. All rights reserved.
1. Introduction Tidal flats are characterized by broad low-gradient muddy areas that are often exposed during low tide and include the unvegetated bottom, salt marshes, and the river mouth (Dyer et al., 2000; Fagherazzi and Mariotti, 2012). Tidal flats are valuable ecosystems with productive flora and fauna that support large populations of birds and serve as the nursery and feeding areas of coastal fisheries (Costanza et al., 1997). Intertidal ecosystems act as a repository of carbon, a buffer zone from storms, and a filter for pollutants (Chmura et al., 2003; Kirwan and Murray, 2007). However, tidal flats are threatened by sea-level rise and conventional coastal engineering that has been conducted on a large scale (Kirwan et al., 2010; Ryu et al., 2004). Most large port cities (e.g., New York, New Orleans, Shanghai, Inchoen, and Tokyo) have expanded through the large-scale reclamation of river deltas, tidal flats, and estuaries (Temmerman et al., 2013). Due to land reclamation and dredging, 57% of China’s coastal wetlands have disappeared since the 1950s (Qiu, 2011), and the total area of tidal flats in South Korea decreased from approximately 2800 km2 in 1990 to 2393 km2 in 2005 (MLTM, 2010). Additionally, half of the world’s coastal wetlands are projected to be submerged during this ⇑ Corresponding author. E-mail address: [email protected] (J.-H. Ryu). http://dx.doi.org/10.1016/j.marpolbul.2015.06.023 0025-326X/Ó 2015 Elsevier Ltd. All rights reserved.
century due to sea-level rise, although salt marsh has a capacity to adjust to rises in sea level (Kirwan et al., 2010). Conventional coastal engineering such as the construction of sea walls, dykes, and embankments exacerbates land subsidence by soil drainage (Syvitski et al., 2009) and hinders the natural accumulation of sediments by tides, waves, and wind (Temmerman et al., 2013). Environmental changes such as the degradation of natural habitats, morphological changes, and the decline of water quality on reclaimed tidal flats are often identified by environmental impact assessments (Temmerman et al., 2013); however, the target area is typically limited to the surrounding landscapes (Hong et al., 2010). Time lags between conventional coastal engineering and environmental changes in the vicinity of tidal flats and make it difficult to determine the impacts of conventional coastal engineering using field survey data. A comprehensive investigation including a field survey, remotely sensed data, and hydrodynamic modeling is needed to reveal the spatial, temporal, and quantitative changes in the vicinity of tidal flats. Using satellite-based analysis with a hydrodynamic approach, we resolve environmental changes over 20 years in the vicinity of a tidal flat that was not included in the original environmental impact assessment. In Korea, coastal reclamation was constructed as a project to create vast rice fields while simultaneously serving a symbolic role demonstrating the country’s capacity for reconstruction that began in 1953. Although demanding for agricultural land was in declined
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because of the increasing dependence on the cheap imported food in the late 1960s, coastal reclamation was repeated due to the politically appeasing rural populations marginalized from sharing in the nation’s growing wealth (Choi, 2014). The Saemangeum reclamation project involved the conversion of 28,300 ha of tidal flats and 11,800 of shallow estuaries, equivalent to 60% of the size of Singapore or two thirds the area of Seoul, into rice-fields and new land for industry and tourism (Cho et al., 2008; Hong et al., 2010). Construction of the first and third dykes began in 1991, temporarily stopped because of water quality issues in 1999, and was finally completed on April 21, 2006 (Lie et al., 2008). Two sluice gates, Sinsi and Garyeok, are located on the southern part of the dyke, allowing an exchange with the open sea (Min et al., 2012). A lawsuit relating to the reclamation project gained international attention in light of global efforts to preserve wetlands, although the Supreme Court rejected appeals filed by NGOs and local residents (Cho, 2007). Once constructed, the 33.9-km-long, 6-m high Saemangeum Dyke changed the morphology of the tidal flat located within the dyke and the adjacent current (Lee and Ryu, 2008; Lee et al., 2008). On Byeonsan beach, located below the first dyke, sandy areas have noticeably expanded during 2003–2006 (Lee and Ryu, 2008); however, sandy areas have since been greatly decreased owing to the significant reduction in tidal currents after the gaps were closed (Lee and Lee, 2012). We focused on the Gomso Bay tidal flat, which is located below the Byeonsan peninsula and about 15 km south from the southern end of the dyke, which was not included in the target area of the project’s environmental impact assessment. Since January 2010, the Gomso Bay tidal flat has been protected under the Ramsar Convention because of its biological diversity and the habitats it provides for endangered species (www.ramsar.org). In 2011, the annual catch of Ruditapes philippinarum, one of the most commercially exploited clams in the world, was 37,929 metric tons in Korea (www.kostat.go.kr), of which approximately 9,000 metric tons originated from the Gomso Bay tidal flats. Some studies have considered the possibility that changes to the sedimentary facies in Gomso Bay may have occurred following construction of the Saemangeum Dyke (Chang et al., 2007), and some local residents also claim that there is a correlation between environmental changes in Gomso Bay and construction of the dyke. To determine the relationship
151
between environmental changes on Gomso Bay tidal flats and construction of the Saemangeum Dyke, we conducted an analysis of changes in morphology, sedimentary facies, distribution of suspended sediment (SS), and tidal currents using a variety of remote sensing images by dividing the project into three periods: (1) before dyke construction (before November 1991); (2) during dyke construction; and (3) after dyke construction (after April 2006). A modified version of the Princeton Ocean Model (POM) was used to estimate changes in the tidal current system. To evaluate the impacts of the construction of Saemangeum Dyke, we considered three separate periods of the engineering project: before, during, and after construction. The direction of the tidal current and the sediment flux from the river were directly influenced once the dykes were completed and connected to one another, blocking the flow of seawater. Therefore, we used the day of the second dyke’s completion, when all four dykes were connected, as the time of dyke completion. The coastal area was divided into six regions based on the location of the Saemangeum Dyke, Gogunsan Islands, and Wi Island, as shown in Fig. 1(a). The tidal flats were divided into four regions based on the Jujin stream and the elevation of the mean low-water neaps in Gomso, as shown in Fig. 1(b).
2. Material and methods 2.1. In-situ measurement Field surveys were conducted to obtain in situ measurements of suspended sediment (SS) and the optical properties of the water surface. Since 2005, a total of 167 samples were collected to establish accurate measurements of SS not only in Gomso Bay but also around the western coast of the Korean Peninsula, as indicated in Fig. 2(a). A 25-mm-class fiber filter (GF/F) was used to collect SS. The weight of the filter paper was measured in the laboratory after 4 h of drying at 60 °C, and the water sample was filtered until the paper turned yellow or ochre. The filtered pad was flushed with 10 ml distilled water to remove salt (Min et al., 2012). The optical characteristics of the surface water were measured using a FieldSpec Dual VNIR spectroradiometer with a spectral range of
Fig. 1. (a) Landsat8 Operational Land Imager (OLI) image of the study area acquired on 16 September, 2013. Box A shows the changes in morphology and sedimentary facies in Gomso Bay tidal flats. Box B is an enlargement of the study area showing the tidal current around Saemangeum and Gomso Bay. (b) The blue line represents an echo sounding transect. Levelling was conducted along lines K–H and K–M. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
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Fig. 2. (a) A mosaic Landsat TM image of the western Korean Peninsula. The colored dots indicate sampling stations in each cruise since 2005. (b) The SS algorithm used in this study, derived from 167 observations of optical properties and the concentration of SS acquired around the western coast of the Korean Peninsula. (c) A comparison of Rrs (560) between Landsat and in situ measurements using the 11 samples acquired on May 29, 2005. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
350–1050 nm. All optical properties were measured three times, and the results were averaged. Optical properties were measured at 135° in a counterclockwise direction from the sun to minimize the glinting effect. The total water-leaving radiance (LwT, W/m2/nm/sr) was measured on the surface at a 30° incidence angle from the nadir. The sky radiance (Lsky, W/m2/nm/sr) was obtained in the same direction as the LwT. The downwelling irradiance (Ed, W/m2/nm) was observed using a remote cosine receptor (RCR) to gather the full-sky radiance in a hemispheric field of view. For measurements acquired since 2010, the sky glint radiance was computed as Lsky multiplied by a constant factor (0.025) and then subtracted from Lsky. A white offset was subtracted to make the average of Rrs over 850–900 nm null, as recommended by Moon et al. (2012) for the quality control of Rrs measurement. An empirical SS algorithm was generated based on the relationship between in situ measurements of Rrs and in situ measurements of SS concentrations obtained from the western coast of the Korean Peninsula. Min et al. (2012) generated an empirical SS algorithm to observe changes in levels of SS in the vicinity of Saemangeum Dyke. This empirical algorithm was generated based
on the remote sensing reflectance at 560 nm and was derived from 40 observations made in the Saemangeum coastal area. In this study, an empirical algorithm was derived from the 167 observations around the west coast of the Korean Peninsula, including the highly turbid area, based on the relationship between the Rrs value at 560 nm collected from FieldSpec and SS concentrations from the in situ measurements made at the locations shown in Fig. 2(b).
g ss ¼ 0:7199 e149:43Rrs ð560Þ ; m3
R2 ¼ 0:864:
ð1Þ
Although numerous field surveys have been conducted on the west coast of the Korean Peninsula, it is difficult to match in situ measurements with the Landsat TM/ETM+ overpass (Doxaran et al., 2006). May 29, 2005 was the only date when in situ measurements were made concurrently with the Landsat ETM+ overpass. We obtained a correlation coefficient of 0.79 between in situ SS and Landsat-derived SS for the 11 in situ measurements at the locations shown in Fig. 2(c).
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Table 1 Summary of satellite remotely sensed data, field survey data and the history of Saemangeum dyke construction.
Ground levelling along the lines indicated by K–H and K–M in Fig. 1(b) has been conducted since 1991 and 2000, respectively. Levelling was conducted between February and March to avoid seasonal variation in the sedimentation due to the monsoon. The K–H line measured in 1991 was slightly rotated (by about 3° anticlockwise) from that measured in 2000. To avoid the Gaetboel (tidal flat) site, levelling on the K–H line conducted since 2011 was slightly different from that on the K–H line conducted in 2000. The elevations of the K–H and K–M lines were measured using a Pentax Pal 2S Level by Chang and Choi (1998) in 1990 and Lee and Chu (2001) in 2000. These elevations were measured using a GNSS viva GS10/GPS (Leica Geosystems, Switzerland), with 10-mm and 20-mm horizontal and vertical accuracy with real time kinematic (RTK) since 2011. The depth of the water was also measured in November 2011 using an echo-sounder (Raytheon, Waltham, MA, USA). The water depth data were calibrated using tide gauge data surveyed by the Korea National Oceanographic Research Institute (NORI) at the nearest station to the study area. Calibrated water depth data were converted to the BM (bench mark) standards to match the elevation provided by the GNSS/GPS. A shipboard Van Veen grab sampler was used at flood tide to acquire 80 samples for grain-size analysis in August 2012. A GPS (eTrex Vista HCx, GARMIN) with a horizontal accuracy of 1 m was used for accurate positioning of the sampling sites. The sand and mud fractions were separated by a 4u stainless-steel sieve after removing the organic material and carbonate by immersion in a solution of 10% H2O2 and 0.1 N HCl. The grain-size distribution was determined by standard sieving and a Sedigraph-5100 (Micrometrics, Norcross, GA, USA) for the sand and mud fraction. According to Folk’s classification (Folks, 1968), grain-size data were classified into sand, silty sand and sandy silt. 2.2. Satellite images All of the Landsat TM/ETM+ images from 1988 to 2012 in Table 1 were geometrically rectified by image-to-image
co-registration using a reference Landsat TM image that was rectified by a topographic map at a scale of 1:5000. For the generation of intertidal DEMs, the waterline was extracted from bands 4 or 5 according to the tidal conditions, based on the density slicing method suggested by Fraizer and Page (2000). Absolute elevation was then assigned to each waterline using levelling data at the K–H line. The levelling data acquired at the K–M line were used to validate the accuracy of the intertidal DEM that was generated. The waterline acquired in 1993 was required to be included in DEM1990, although it was acquired after construction began on the Saemangeum Dyke due to the lack of a suitable image of the exposed tidal flats. Converted echo sounding data were assigned to the waterline located at the outer tidal flats because they were not included in the K–H line. Atmospheric correction was applied to the Landsat TM/ETM+ for the analysis of SS using a cosine approximation (COST) model suggested by Chavez (1996). The digital number (DN) was converted to a radiance value. A dark pixel corresponding to the Lsky was selected from the top 1% of DNs in the histogram because DNs within the bottom 1% were considered to be errors (Chavez, 1996). Lsky was subtracted from the radiance values and then converted to the Rrs by dividing the reflectance value into p. 2.3. Hydrodynamic model The numerical model was a modified version of the Princeton Ocean Model (POM), which is widely used in studies of coastal water circulation (Blumberg, 1987). This model is similar with the original version of POM as using primitive equations and mode split method but mainly different as using wetting–drying scheme and semi-implicit external mode calculation (Lee et al., 2008). For application in the Saemangeum area, where a huge tidal flat existed prior to construction of the dyke, the wetting–drying scheme by Flather and Heaps (1975) was adapted to represent the tidal flat, and a semi-implicit scheme for the calculation of the external mode was applied to reduce numerical instability.
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The numerical experiment was conducted for several cases during construction of the dyke to determine the change in tidal current. The model is calibrated at the open boundary using sea-level observations in the whole Yellow Sea and verified with tidal variation analyzed from sea-level and tidal current observations near the Saemangeum area. Unfortunately, sea-level data around the Gomso Bay had not observed before the Saemangeum dyke construction because scientists had not recognized the possibility of the sea-level change due to the dyke construction. Tidal modification near the dyke as calculated by the model was well agreed with the results of field observation (Lee et al., 2008). Based on this coincidence, the model results around Gomso Bay were applied in this study because that there is no available observed data in the Gomso Bay. Numerical model results reported by Lee et al. (2008) were reanalysed to assess any changes in the tidal current system near Gomso Bay. The horizontal range of the model domain was 77 km from south to north and 72 km from east to west, which included the whole of Saemangeum Dyke and Gomso Bay. The model grid system consisted of a regular 300-m horizontal grid and a 10-layer vertical sigma grid. The numerical model was conducted using only tidal forcing, as described by sea-level variation in an open boundary. The sea-level variation consisted of mean sea level and the harmonic constants of eight major constituents in the study area (M2, S2, N2, K1, O1, M4, MS4, and MSf), which were calculated by a numerical model on a larger model domain including all of the Yellow Sea. The simulated time of the experiment was 30 days, with data for the last 28 days used in harmonic analysis to calculate the mean tidal residual current and tidal ellipse parameters of the eight constituents. The mean tidal residual current was generated by non-linear effects such as bottom friction and complex
geometry. Although the unidirectional residual current was much weaker than the oscillating tidal current, it played an important role in the water and material flux in the coastal area. Therefore, the direction of the residual current can be considered to be the direction in which suspended sediment was transported. The maximum tidal current, which has an effect on the reworking of bottom sediments in shallow coastal water, was indexed by the major axis of the M2 tidal ellipse.
3. Results 3.1. Morphological change For consistency in a study of morphological change by Ryu et al. (2008), DEM2000 was generated using the same waterlines, and the other DEMs were also generated using the same minimum-curvature interpolation technique as that used for DEM2000. DEM1990 and DEM2010 were also generated using the waterlines listed in Table 1. The absolute elevation was assigned using the crossing points of the extracted waterline and the K–H line, and the accuracy of the intertidal DEM was assured using levelling data at the same K–M line as that used in DEM2000. Calculation of the overall root-mean-square error (RMSE) of the DEM1990 was impossible because of the lack of levelling data at the K–M line. The overall r.m.s error of the DEM2010 along the K–M line was 11.9 cm, with a standard deviation of 4.1 cm. By comparing two intertidal digital elevation models (DEMs), an estimation of the quantitative morphological changes over the three time periods was made. Fig. 3(a) shows the morphological
Fig. 3. A map showing the morphological change between (a) DEM1990 and DEM2000 and (b) DEM2000 and DEM2010. (c) Waterline comparison at similar tidal height in different years. (d) Elevation along the line K–H in October 1991, March 2000, and February 2012.
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Y.-K. Lee et al. / Marine Pollution Bulletin 97 (2015) 150–159 Table 2 Estimation of volumetric changes in the intertidal DEMs. Regions
Area (m2)
Morphologic change
Sedimentation/erosion rate (cm/10yr)
Annual mean budget of morphologic change (m3/yr)
Outer bay Lower flat
11,063,164
DEM2000–DEM1990 DEM2010–DEM2000
22.9 51.5
253,346 569,752
Upper flat
10,550,700
DEM2000–DEM1990 DEM2010–DEM2000
8.5 10.8
89,680 113,947
Inner bay Lower flat
14,463,900
DEM2000–DEM1990 DEM2010–DEM2000
30.5 29.7
16,534 16,100
Upper flat
542,110
DEM2000–DEM1990 DEM2010–DEM2000
10.0 7.8
144,639 112,818
Fig. 4. Surface sediment distribution in (a) a 2006 map (after Chang et al. (2007)) and (b) August 2012.
change in Gomso Bay tidal flats during construction of the Saemangeum Dyke, which was calculated by subtracting DEM1990 (intertidal topography before construction of the dyke) from DEM2000 (intertidal topography during construction of the dyke). During construction of the dyke, the whole tidal flat area displayed a tendency toward accretion. Sedimentation was especially dominant on the lower flat in the outer and upper bays. The volumetric changes on the lower flat over a 10-year period were 22.9 cm in the outer bay and 30.5 cm in the inner bay (Table 2). Fig. 3(b) shows the morphological changes that have occurred since completion of the Saemangeum Dyke, determined by subtracting DEM2000 from DEM2010 (intertidal topography after completion of the dyke). After construction of the dyke, the tendency for accretion was only maintained on the upper flat in the inner bay. The tendency for sedimentation changed to erosion in the other regions, with a maximum erosion rate of about 51.5 cm over a 10-year period on the lower flat in the outer bay (Table 2). To examine the waterline alteration due to the changes in sedimentation patterns, three waterlines acquired at similar tidal height but in different years (1991, 2000, 2011) were compared. Waterline extracted from the image acquired on October 6, 1991 at a tidal height of 250 cm was located in higher tidal flat than waterlines extracted from the images acquired on September 4, 2000 at a tidal height of 237 cm and on March 11, 2011 at the tidal height of 202 cm, as shown in Fig. 3(c). Tidal height difference between 1991 and 2000 was only 13 cm. The difference of the exposed surface areas indicates the changes in topography of the tidal flats. This difference is agreed with the sedimentation tendency in the outer flat analyzed from the levelling data and DEM comparison during construction of the dyke. Although tidal height
in 2011 was 35 cm lower than 2000, waterlines have little difference because of the accretion/erosion tendency change since completion of the dyke. A sand shoal, easily identified as a bright feature in the satellite images until 2000, had been located at the end of the K–H line on the lower flat. However, this sand shoal was not detected in satellite images and field surveys. Levelling data collected during the field survey revealed this change in the sedimentation/erosion pattern in the outer flat, as shown in Fig. 3(d). This implies that the change in tidal energy in the outer bay contributed to a change in the sedimentation/erosion pattern where no river input into Gomso Bay exists. The morphological changes in the tidal flats indicate that construction of the Saemangeum Dyke contributed to the change in tidal energy in Gomso Bay. 3.2. Sedimentary facies change A sedimentary facies map was produced based on the field survey data collected in August 2012, which was then compared with a map generated based on field survey data collected in July 2006 by Chang et al. (2007). According to Folk’s classification, grain-size data were classified into three facies types: sand, silty sand, and sandy silt (Folks, 1968). Although the field survey was conducted in July 2006, just after the completion of the dyke, the sedimentary facies map represented the status during construction of the dyke because sedimentary facies are not substantially changed within a short time, with the exception of the border, which can be altered due to seasonal effects. Sandy silt facies were dominant in the inner bay, and silty sand facies were dominant in the outer bay during construction of the dyke, as shown in Fig. 4(a). After the
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Fig. 5. The reddish area of Box B in Fig. 1(a) indicates ratios greater than 100%, where the tidal current after construction of the dyke was weaker than the tidal current before and during construction. (a) The ratios of maximum tidal current speeds before and after construction of the dyke. (b) The ratios of maximum tidal current speeds during and after construction of the dyke. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Fig. 6. Maps of the SS concentration derived from Landsat TM/ETM+ band 2 based on the empirical SS algorithm and the distribution of the M2 tidal energy flux from numerical modeling at the time of image acquisition. (a) SS distribution before construction of the Saemangeum Dyke, and (b) distribution of the M2 tidal energy flux. (c and e) SS distribution at different stages in the dyke construction process; (d and f) distribution of the M2 tidal energy flux at the time of the image acquisition for (c and e), respectively. (g) SS distribution after the dyke was completed; (h) distribution of the M2 tidal flux.
dyke was completed, the area of silty sand facies expanded substantially, whereas the area of sandy silty facies noticeably declined in the study area (Fig. 4(b)). A relatively large area of silty sand facies in the mouth of Gomso Bay changed to sand facies after construction of the dyke. Except for the upper flat in the inner bay, where sandy silt was dominant and an accretion tendency was maintained since construction of the dyke began, the sedimentary facies of the flats in the inner and outer bays were coarse, and the pattern of sedimentation changed from accretion to erosion after the dyke was completed. Sand flats are more vulnerable to morphological change than are mud flats due to the small amount of clays that act as a lubricating agent (Lambrechts et al., 2010; Ryu et al., 2008). Overall, the tidal flats with a tendency for erosion to occur had coarser sedimentary facies. This implies that the changes in sedimentary facies have increased the erosion of the lower flat by reducing the critical bed shear stress.
3.3. Current change with SSC An artificial change in tidal direction following construction of the dyke resulted in a change to the sediment transport conditions (Lee and Ryu, 2008). A hydrological model was used to identify changes in the tidal current in the mouth of Gomso Bay following construction of the Saemangeum Dyke, as shown in Fig 5(a and b). The construction of the Saemangeum Dyke influenced the tides and tidal currents not only in the vicinity of the dyke but also throughout the Yellow Sea (Lee et al., 2008). After construction was completed, clear changes in tidal currents were apparent in the vicinity of the dyke. The tidal current speed increased near Garyeok gate; however, speeds decreased elsewhere along the dyke. Increased tidal current speeds were recorded in the mouth of Gomso Bay, which had not been expected by the project engineers. Before 1980, the average maximum speeds in the Gomso
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Y.-K. Lee et al. / Marine Pollution Bulletin 97 (2015) 150–159 Table 3 Summary of environmental change in Gomso Bay. Tidal flat* Duration
Outer bay Lower flat
Upper flat
Coastal water** Inner bay
Lower flat
Upper flat
Before the dyke construction
Under the dyke constr uction
3rd and 4th disco nnect ed 3rd and 4th conn ected
After the dyke construction
Accretion (silty Sand)
Erosion (Sand/silt y Sand)
Accretion (Sand/san dy Silt)
Erosion (silty Sand)
Accretion (silty Sand /sandy Silt)
Erosion (silty Sand)
Inner coast
Outer coast Northern Middle area area 1.40/ 2.11/ 150150180
210
Southern Northern Middle area area area 2.65/ 3.84/ 3.30/ 12018090-150 180 210
0.80/ 150-
0.91/ 150-
1.51/ 120-
180
180
180
1.13/ 180210
2.28/ 90-120
Southern area 5.64/
3.62/ 120-
Jul. 97
180
Dec. 98
90-180 Nov. 91
rd
3 dyke completion st
Accretion (sandy Silt)
Accretion (silty Sand/ sandy Silt)
The SMG dyke construction began
1 dyke completion
May 99 Environmental lawsuit
2.04/ 180-
1.64/ 210-
2.82/ 180-
1.70/ 210-
2.45/ 210-
4.26/ 180-
210
240
240
240
240
240
1.33/ 180-
1.40/ 180-
3.32/ 180-
2.26/ 150-
1.90/ 180-
8.89/ 180-
210
210
210
210
240
210
May 01 Jun. 03
th
4 dyke completion
(Garyeok gate completion) nd
Apr. 06
2 dyke completion
Apr. 10
The SMG dyke construction completion
(Sinsi gate completion)
* sedimentation tendency (dominant sedimentary facies) ** mean of SS concentration / dominant direction of tidal currents
tidal channel were 1.50 m/s and 1.15 m/s at ebb and flood tide, respectively, according to a National Geospatial Information (NGI) basic survey (NGI, 1981); however, it is difficult to determine the exact location and date of this survey. The maximum speeds of 1.25 m/s and 0.7 m/s in the mouth of Gomso Bay were observed during ebb and flood tide on 25 August 2006 over a 12-h measurement period (Lee, 2010). Tidal asymmetry, with the ebb tide dominant and increased tidal current speed, is a likely cause of the change in sedimentary facies and erosion around the lower flat in outer bay. This indicates that the blockage of tidal current transportation resulting from to the Saemangeum Dyke altered the tidal current speed in the mouth of Gomso Bay, and the sedimentary facies therefore became coarser, with an increase in erosion due to the reduction in the critical bed shear stress. The movement of suspended sediments across the coast explains its topography (Allen, 2000). To understand the changes in the concentration of suspended sediments (SS) and the movement of the current according to the dyke construction process, the spatial pattern of SS from the Saemangeum Dyke to Gomso Bay was estimated using Landsat TM/ETM+ band 2, and the distribution of the mean tidal residual current at the time of image acquisition was generated from numerical modeling, as shown in Fig. 6. The seasonal erosion of tidal flats due to the monsoon was prevented by SS deposited during the spring/summer season when the tendency for sedimentation is seasonally superior. Tidal conditions during image acquisition were the same as at the end of ebb tide, which reduced the variation in the levels of SS due to resuspension of the bottom sediment (Choi et al., 2014). Before construction of the dyke, highly concentrated SS (mean range: 3.30– 3.84 g/m3) flowed from the Geum, Mankyung, and Dongjin rivers. The highly concentrated SS also flowed toward the mouth of Gomso Bay with the dominant direction of currents being 90– 180°, as shown in Fig. 6(a and b). During construction of the dyke, the highly concentrated SS from the Mankyung and Dongjin rivers was blocked. Amounts of SS accumulated around the Gogunsan islands because construction began there. The dominant direction currents were almost similar with before construction of the dyke, as shown in Fig. 6(c and d). The SS plume between the third and fourth dyke disappeared after the completion of the northern dyke; however, highly concentrated SS moved south-westward and was observed at the gate of Sinsi and Garyeok, as shown in Fig. 6(e and
f). The flow of SS was totally blocked following completion of the southern dyke. The current direction became shifted to 180– 210°, as shown in Fig. 6(g and j). This means that the dyke construction changed the tidal current characteristics in Gomso Bay. The high turbidity area in front of the mouth of Gomso Bay could be partially affected by the Heuksan Mud Belt located at the southern part of the mouth of Gomso Bay (Lee and Chu, 2001) because the spatial distribution of this turbid area displayed seasonal variation with its maximum extent during winter and minimum extent during summer (Min et al., 2014). However, the increased levels of SS around the mouth of Gomso Bay indicate that resuspension of bottom sediment increased due to the enhanced tidal current speed following construction of the dyke. 4. Conclusion and discussion To investigate the cause-and-effect of environmental change on the vicinity tidal flat by the reclamation project, morphological change with hydrodynamic approach is needed. In this study, we focused on the Gomso Bay tidal flat which was not included in the target area of the project’s environmental impact assessment before the Saemangeum reclamation project. The morphological change and sedimentary facies change in Gomso Bay tidal flat were estimated. Moreover, changes in SS patterns and currents in coastal from the Saemangeum to Gomso Bay was evaluated (Table 3). The conclusions drawn from this study are as follows: (1) Morphological changes in Gomso Bay tidal flats according to the Saemangeum dyke construction were quantitatively estimated based on the DEMs generated by waterline method. The tendency for accretion was only maintained on the upper flat in the inner bay with a sedimentation rate about 10.0 cm and 7.8 cm, respectively before and after construction of the dyke. The tendency for sedimentation changed to erosion in the other regions. Especially, significant erosion was observed on the lower flat in the outer bay after construction of the dyke. We found that the sedimentary facies of Gomso Bay were coarser and covered a larger area of the lower tidal flats in the outer bay, where sediments deposited during dyke construction were eroded after its completion.
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(2) After the completion of the third and fourth dykes, the SS concentration was increased in the southern area of the inner coast and the dominant direction of the tidal current in the mouth of Gomso Bay changed from 90–180° to 180– 240°. The speed of the tidal current also increased in the mouth of Gomso Bay after the completion of the dyke. The Saemangeum Dyke has remarkably played an artificial role in sediment transport. Our analysis revealed that construction of the Saemangeum Dyke altered not only the levels of SS and the speed and direction of the tidal current in Gomso Bay coastal water but also the sedimentary facies and accretion/erosion tendencies the previously characterized Gomso Bay tidal flats. For example, the direction of the tidal current changed, and the levels of SS increased in the southern area of the inner coast. The area dominated by sand increased, and the sedimentation trend changed from accretion to erosion on the lower flat in the outer bay. The morphological and sedimentological changes in Gomso Bay and the alterations to SS and the current in the Gomso Bay coastal area indicate that environmental impacts such as the reduced habitat for fish and shellfish, decline in water quality, and degradation of natural habitat (Temmerman et al., 2013) due to conventional coastal engineering have occurred on a large sale in the area surrounding the engineering work. The Saemangeum Dyke Changes induced by natural factors such as the increased sediment flux from upriver locations and the seasonal variation of monsoons were difficult to distinguish from anthropogenic activities. In Gomso Bay, the amount of terrestrial sediment supplied directly by the Jujin stream to the bay is negligible due to the presence of upstream weirs. The freshwater discharge of Galgok stream is also negligible because of the upstream reservoir. Therefore, we concluded that most of the changes to the morphology and sedimentary facies in the Gomso Bay tidal flats were caused by changes in energy flux from the low tidal area. We used a modified version of the POM for the numerical modeling. Although the sea-level variation were calculated by a numerical model on a larger model domain including all of the Yellow Sea, tidal currents were analyzed on a smaller model domain included the whole of Saemanangeum Dyke and Gomso Bay. The tidal current change cause by the Yeonggwang nuclear power plant located about 14 km south from the southern part of study area was excluded. Thus, more detailed analyses of sediment dynamics with tidal current change including southern part of study area are needed to reveal the nature of seasonal or artificial environmental process in the Gomso Bay coastal area. Conventional engineering projects in coastal areas have become a worldwide issue due to the demand for urban, industrial, and agricultural expansion (Wang et al., 2010). Satellite-based observation strategies can be used to overcome the limitations of field surveys when determining the environmental status following conventional engineering projects. Unexpected morphological and sedimentological changes in the vicinity of tidal flats affected by conventional engineering require interventions to protect living organisms and support the local economy. The results of this study should further encourage the use of satellite-based studies for the big data, and should motivate governments and NGOs to extend the target area used in environmental impact assessments.
Acknowledgements This research was supported by a project titled ‘‘Research for Application of Geostationay Ocean Color Imager’’ funded by the Ministry of Ocean and Fisheries (MOF), Korea and ‘‘Development of satellite based ocean carbon flux model for seas around Korea’’
funded by the Korea Institute of Ocean Science and Technology (KIOST).
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Marine Pollution Bulletin journal homepage: www.elsevier.com/locate/marpolbul
Satellite-based observations of unexpected coastal changes due to the Saemangeum Dyke construction, Korea Yoon-Kyung Lee a, Joo-Hyung Ryu a,⇑, Jong-Kuk Choi a, Seok Lee b, Han-Jun Woo c a
Korea Ocean Satellite Center, Korea Institute of Ocean Science & Technology, 787 Haean-ro, Ansan 426-744, Republic of Korea Physical Oceanography Division, Korea Institute of Ocean Science & Technology, 787 Haean-ro, Ansan 426-744, Republic of Korea c Marine geology & Geophysics Division, Korea Institute of Ocean Science & Technology, 787 Haean-ro, Ansan 426-744, Republic of Korea b
a r t i c l e
i n f o
Article history: Received 30 September 2014 Revised 8 June 2015 Accepted 11 June 2015 Available online 20 June 2015 Keywords: Tidal flat Coastal water Morphological change Suspended sediment GOCI Landsat
a b s t r a c t Spatial and temporal changes around an area of conventional coastal engineering can be easily observed from field surveys because of the clear cause-and-effect observable in the before and after stages of the project. However, it is more difficult to determine environmental changes in the vicinity of tidal flats and coastal areas that are a considerable distance from the project. To identify any unexpected environmental impacts of the construction of Saemangeum Dyke in the area, we examined morphological changes identified by satellite-based observations through a field survey on Gomso Bay tidal flats (15 km from Saemangeum Dyke), and changes in the suspended sediment distribution identified by satellite-based observations through a hydrodynamic analysis in the Saemangeum and Gomso coastal area. We argue that hydrodynamic changes due to conventional coastal engineering can affect the sedimentation pattern in the vicinity of tidal flats. We suggest that the environmental impact assessment conducted before a conventional coastal engineering project should include a larger area than is currently considered. Ó 2015 Elsevier Ltd. All rights reserved.
1. Introduction Tidal flats are characterized by broad low-gradient muddy areas that are often exposed during low tide and include the unvegetated bottom, salt marshes, and the river mouth (Dyer et al., 2000; Fagherazzi and Mariotti, 2012). Tidal flats are valuable ecosystems with productive flora and fauna that support large populations of birds and serve as the nursery and feeding areas of coastal fisheries (Costanza et al., 1997). Intertidal ecosystems act as a repository of carbon, a buffer zone from storms, and a filter for pollutants (Chmura et al., 2003; Kirwan and Murray, 2007). However, tidal flats are threatened by sea-level rise and conventional coastal engineering that has been conducted on a large scale (Kirwan et al., 2010; Ryu et al., 2004). Most large port cities (e.g., New York, New Orleans, Shanghai, Inchoen, and Tokyo) have expanded through the large-scale reclamation of river deltas, tidal flats, and estuaries (Temmerman et al., 2013). Due to land reclamation and dredging, 57% of China’s coastal wetlands have disappeared since the 1950s (Qiu, 2011), and the total area of tidal flats in South Korea decreased from approximately 2800 km2 in 1990 to 2393 km2 in 2005 (MLTM, 2010). Additionally, half of the world’s coastal wetlands are projected to be submerged during this ⇑ Corresponding author. E-mail address: [email protected] (J.-H. Ryu). http://dx.doi.org/10.1016/j.marpolbul.2015.06.023 0025-326X/Ó 2015 Elsevier Ltd. All rights reserved.
century due to sea-level rise, although salt marsh has a capacity to adjust to rises in sea level (Kirwan et al., 2010). Conventional coastal engineering such as the construction of sea walls, dykes, and embankments exacerbates land subsidence by soil drainage (Syvitski et al., 2009) and hinders the natural accumulation of sediments by tides, waves, and wind (Temmerman et al., 2013). Environmental changes such as the degradation of natural habitats, morphological changes, and the decline of water quality on reclaimed tidal flats are often identified by environmental impact assessments (Temmerman et al., 2013); however, the target area is typically limited to the surrounding landscapes (Hong et al., 2010). Time lags between conventional coastal engineering and environmental changes in the vicinity of tidal flats and make it difficult to determine the impacts of conventional coastal engineering using field survey data. A comprehensive investigation including a field survey, remotely sensed data, and hydrodynamic modeling is needed to reveal the spatial, temporal, and quantitative changes in the vicinity of tidal flats. Using satellite-based analysis with a hydrodynamic approach, we resolve environmental changes over 20 years in the vicinity of a tidal flat that was not included in the original environmental impact assessment. In Korea, coastal reclamation was constructed as a project to create vast rice fields while simultaneously serving a symbolic role demonstrating the country’s capacity for reconstruction that began in 1953. Although demanding for agricultural land was in declined
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because of the increasing dependence on the cheap imported food in the late 1960s, coastal reclamation was repeated due to the politically appeasing rural populations marginalized from sharing in the nation’s growing wealth (Choi, 2014). The Saemangeum reclamation project involved the conversion of 28,300 ha of tidal flats and 11,800 of shallow estuaries, equivalent to 60% of the size of Singapore or two thirds the area of Seoul, into rice-fields and new land for industry and tourism (Cho et al., 2008; Hong et al., 2010). Construction of the first and third dykes began in 1991, temporarily stopped because of water quality issues in 1999, and was finally completed on April 21, 2006 (Lie et al., 2008). Two sluice gates, Sinsi and Garyeok, are located on the southern part of the dyke, allowing an exchange with the open sea (Min et al., 2012). A lawsuit relating to the reclamation project gained international attention in light of global efforts to preserve wetlands, although the Supreme Court rejected appeals filed by NGOs and local residents (Cho, 2007). Once constructed, the 33.9-km-long, 6-m high Saemangeum Dyke changed the morphology of the tidal flat located within the dyke and the adjacent current (Lee and Ryu, 2008; Lee et al., 2008). On Byeonsan beach, located below the first dyke, sandy areas have noticeably expanded during 2003–2006 (Lee and Ryu, 2008); however, sandy areas have since been greatly decreased owing to the significant reduction in tidal currents after the gaps were closed (Lee and Lee, 2012). We focused on the Gomso Bay tidal flat, which is located below the Byeonsan peninsula and about 15 km south from the southern end of the dyke, which was not included in the target area of the project’s environmental impact assessment. Since January 2010, the Gomso Bay tidal flat has been protected under the Ramsar Convention because of its biological diversity and the habitats it provides for endangered species (www.ramsar.org). In 2011, the annual catch of Ruditapes philippinarum, one of the most commercially exploited clams in the world, was 37,929 metric tons in Korea (www.kostat.go.kr), of which approximately 9,000 metric tons originated from the Gomso Bay tidal flats. Some studies have considered the possibility that changes to the sedimentary facies in Gomso Bay may have occurred following construction of the Saemangeum Dyke (Chang et al., 2007), and some local residents also claim that there is a correlation between environmental changes in Gomso Bay and construction of the dyke. To determine the relationship
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between environmental changes on Gomso Bay tidal flats and construction of the Saemangeum Dyke, we conducted an analysis of changes in morphology, sedimentary facies, distribution of suspended sediment (SS), and tidal currents using a variety of remote sensing images by dividing the project into three periods: (1) before dyke construction (before November 1991); (2) during dyke construction; and (3) after dyke construction (after April 2006). A modified version of the Princeton Ocean Model (POM) was used to estimate changes in the tidal current system. To evaluate the impacts of the construction of Saemangeum Dyke, we considered three separate periods of the engineering project: before, during, and after construction. The direction of the tidal current and the sediment flux from the river were directly influenced once the dykes were completed and connected to one another, blocking the flow of seawater. Therefore, we used the day of the second dyke’s completion, when all four dykes were connected, as the time of dyke completion. The coastal area was divided into six regions based on the location of the Saemangeum Dyke, Gogunsan Islands, and Wi Island, as shown in Fig. 1(a). The tidal flats were divided into four regions based on the Jujin stream and the elevation of the mean low-water neaps in Gomso, as shown in Fig. 1(b).
2. Material and methods 2.1. In-situ measurement Field surveys were conducted to obtain in situ measurements of suspended sediment (SS) and the optical properties of the water surface. Since 2005, a total of 167 samples were collected to establish accurate measurements of SS not only in Gomso Bay but also around the western coast of the Korean Peninsula, as indicated in Fig. 2(a). A 25-mm-class fiber filter (GF/F) was used to collect SS. The weight of the filter paper was measured in the laboratory after 4 h of drying at 60 °C, and the water sample was filtered until the paper turned yellow or ochre. The filtered pad was flushed with 10 ml distilled water to remove salt (Min et al., 2012). The optical characteristics of the surface water were measured using a FieldSpec Dual VNIR spectroradiometer with a spectral range of
Fig. 1. (a) Landsat8 Operational Land Imager (OLI) image of the study area acquired on 16 September, 2013. Box A shows the changes in morphology and sedimentary facies in Gomso Bay tidal flats. Box B is an enlargement of the study area showing the tidal current around Saemangeum and Gomso Bay. (b) The blue line represents an echo sounding transect. Levelling was conducted along lines K–H and K–M. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
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Fig. 2. (a) A mosaic Landsat TM image of the western Korean Peninsula. The colored dots indicate sampling stations in each cruise since 2005. (b) The SS algorithm used in this study, derived from 167 observations of optical properties and the concentration of SS acquired around the western coast of the Korean Peninsula. (c) A comparison of Rrs (560) between Landsat and in situ measurements using the 11 samples acquired on May 29, 2005. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
350–1050 nm. All optical properties were measured three times, and the results were averaged. Optical properties were measured at 135° in a counterclockwise direction from the sun to minimize the glinting effect. The total water-leaving radiance (LwT, W/m2/nm/sr) was measured on the surface at a 30° incidence angle from the nadir. The sky radiance (Lsky, W/m2/nm/sr) was obtained in the same direction as the LwT. The downwelling irradiance (Ed, W/m2/nm) was observed using a remote cosine receptor (RCR) to gather the full-sky radiance in a hemispheric field of view. For measurements acquired since 2010, the sky glint radiance was computed as Lsky multiplied by a constant factor (0.025) and then subtracted from Lsky. A white offset was subtracted to make the average of Rrs over 850–900 nm null, as recommended by Moon et al. (2012) for the quality control of Rrs measurement. An empirical SS algorithm was generated based on the relationship between in situ measurements of Rrs and in situ measurements of SS concentrations obtained from the western coast of the Korean Peninsula. Min et al. (2012) generated an empirical SS algorithm to observe changes in levels of SS in the vicinity of Saemangeum Dyke. This empirical algorithm was generated based
on the remote sensing reflectance at 560 nm and was derived from 40 observations made in the Saemangeum coastal area. In this study, an empirical algorithm was derived from the 167 observations around the west coast of the Korean Peninsula, including the highly turbid area, based on the relationship between the Rrs value at 560 nm collected from FieldSpec and SS concentrations from the in situ measurements made at the locations shown in Fig. 2(b).
g ss ¼ 0:7199 e149:43Rrs ð560Þ ; m3
R2 ¼ 0:864:
ð1Þ
Although numerous field surveys have been conducted on the west coast of the Korean Peninsula, it is difficult to match in situ measurements with the Landsat TM/ETM+ overpass (Doxaran et al., 2006). May 29, 2005 was the only date when in situ measurements were made concurrently with the Landsat ETM+ overpass. We obtained a correlation coefficient of 0.79 between in situ SS and Landsat-derived SS for the 11 in situ measurements at the locations shown in Fig. 2(c).
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Table 1 Summary of satellite remotely sensed data, field survey data and the history of Saemangeum dyke construction.
Ground levelling along the lines indicated by K–H and K–M in Fig. 1(b) has been conducted since 1991 and 2000, respectively. Levelling was conducted between February and March to avoid seasonal variation in the sedimentation due to the monsoon. The K–H line measured in 1991 was slightly rotated (by about 3° anticlockwise) from that measured in 2000. To avoid the Gaetboel (tidal flat) site, levelling on the K–H line conducted since 2011 was slightly different from that on the K–H line conducted in 2000. The elevations of the K–H and K–M lines were measured using a Pentax Pal 2S Level by Chang and Choi (1998) in 1990 and Lee and Chu (2001) in 2000. These elevations were measured using a GNSS viva GS10/GPS (Leica Geosystems, Switzerland), with 10-mm and 20-mm horizontal and vertical accuracy with real time kinematic (RTK) since 2011. The depth of the water was also measured in November 2011 using an echo-sounder (Raytheon, Waltham, MA, USA). The water depth data were calibrated using tide gauge data surveyed by the Korea National Oceanographic Research Institute (NORI) at the nearest station to the study area. Calibrated water depth data were converted to the BM (bench mark) standards to match the elevation provided by the GNSS/GPS. A shipboard Van Veen grab sampler was used at flood tide to acquire 80 samples for grain-size analysis in August 2012. A GPS (eTrex Vista HCx, GARMIN) with a horizontal accuracy of 1 m was used for accurate positioning of the sampling sites. The sand and mud fractions were separated by a 4u stainless-steel sieve after removing the organic material and carbonate by immersion in a solution of 10% H2O2 and 0.1 N HCl. The grain-size distribution was determined by standard sieving and a Sedigraph-5100 (Micrometrics, Norcross, GA, USA) for the sand and mud fraction. According to Folk’s classification (Folks, 1968), grain-size data were classified into sand, silty sand and sandy silt. 2.2. Satellite images All of the Landsat TM/ETM+ images from 1988 to 2012 in Table 1 were geometrically rectified by image-to-image
co-registration using a reference Landsat TM image that was rectified by a topographic map at a scale of 1:5000. For the generation of intertidal DEMs, the waterline was extracted from bands 4 or 5 according to the tidal conditions, based on the density slicing method suggested by Fraizer and Page (2000). Absolute elevation was then assigned to each waterline using levelling data at the K–H line. The levelling data acquired at the K–M line were used to validate the accuracy of the intertidal DEM that was generated. The waterline acquired in 1993 was required to be included in DEM1990, although it was acquired after construction began on the Saemangeum Dyke due to the lack of a suitable image of the exposed tidal flats. Converted echo sounding data were assigned to the waterline located at the outer tidal flats because they were not included in the K–H line. Atmospheric correction was applied to the Landsat TM/ETM+ for the analysis of SS using a cosine approximation (COST) model suggested by Chavez (1996). The digital number (DN) was converted to a radiance value. A dark pixel corresponding to the Lsky was selected from the top 1% of DNs in the histogram because DNs within the bottom 1% were considered to be errors (Chavez, 1996). Lsky was subtracted from the radiance values and then converted to the Rrs by dividing the reflectance value into p. 2.3. Hydrodynamic model The numerical model was a modified version of the Princeton Ocean Model (POM), which is widely used in studies of coastal water circulation (Blumberg, 1987). This model is similar with the original version of POM as using primitive equations and mode split method but mainly different as using wetting–drying scheme and semi-implicit external mode calculation (Lee et al., 2008). For application in the Saemangeum area, where a huge tidal flat existed prior to construction of the dyke, the wetting–drying scheme by Flather and Heaps (1975) was adapted to represent the tidal flat, and a semi-implicit scheme for the calculation of the external mode was applied to reduce numerical instability.
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The numerical experiment was conducted for several cases during construction of the dyke to determine the change in tidal current. The model is calibrated at the open boundary using sea-level observations in the whole Yellow Sea and verified with tidal variation analyzed from sea-level and tidal current observations near the Saemangeum area. Unfortunately, sea-level data around the Gomso Bay had not observed before the Saemangeum dyke construction because scientists had not recognized the possibility of the sea-level change due to the dyke construction. Tidal modification near the dyke as calculated by the model was well agreed with the results of field observation (Lee et al., 2008). Based on this coincidence, the model results around Gomso Bay were applied in this study because that there is no available observed data in the Gomso Bay. Numerical model results reported by Lee et al. (2008) were reanalysed to assess any changes in the tidal current system near Gomso Bay. The horizontal range of the model domain was 77 km from south to north and 72 km from east to west, which included the whole of Saemangeum Dyke and Gomso Bay. The model grid system consisted of a regular 300-m horizontal grid and a 10-layer vertical sigma grid. The numerical model was conducted using only tidal forcing, as described by sea-level variation in an open boundary. The sea-level variation consisted of mean sea level and the harmonic constants of eight major constituents in the study area (M2, S2, N2, K1, O1, M4, MS4, and MSf), which were calculated by a numerical model on a larger model domain including all of the Yellow Sea. The simulated time of the experiment was 30 days, with data for the last 28 days used in harmonic analysis to calculate the mean tidal residual current and tidal ellipse parameters of the eight constituents. The mean tidal residual current was generated by non-linear effects such as bottom friction and complex
geometry. Although the unidirectional residual current was much weaker than the oscillating tidal current, it played an important role in the water and material flux in the coastal area. Therefore, the direction of the residual current can be considered to be the direction in which suspended sediment was transported. The maximum tidal current, which has an effect on the reworking of bottom sediments in shallow coastal water, was indexed by the major axis of the M2 tidal ellipse.
3. Results 3.1. Morphological change For consistency in a study of morphological change by Ryu et al. (2008), DEM2000 was generated using the same waterlines, and the other DEMs were also generated using the same minimum-curvature interpolation technique as that used for DEM2000. DEM1990 and DEM2010 were also generated using the waterlines listed in Table 1. The absolute elevation was assigned using the crossing points of the extracted waterline and the K–H line, and the accuracy of the intertidal DEM was assured using levelling data at the same K–M line as that used in DEM2000. Calculation of the overall root-mean-square error (RMSE) of the DEM1990 was impossible because of the lack of levelling data at the K–M line. The overall r.m.s error of the DEM2010 along the K–M line was 11.9 cm, with a standard deviation of 4.1 cm. By comparing two intertidal digital elevation models (DEMs), an estimation of the quantitative morphological changes over the three time periods was made. Fig. 3(a) shows the morphological
Fig. 3. A map showing the morphological change between (a) DEM1990 and DEM2000 and (b) DEM2000 and DEM2010. (c) Waterline comparison at similar tidal height in different years. (d) Elevation along the line K–H in October 1991, March 2000, and February 2012.
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Y.-K. Lee et al. / Marine Pollution Bulletin 97 (2015) 150–159 Table 2 Estimation of volumetric changes in the intertidal DEMs. Regions
Area (m2)
Morphologic change
Sedimentation/erosion rate (cm/10yr)
Annual mean budget of morphologic change (m3/yr)
Outer bay Lower flat
11,063,164
DEM2000–DEM1990 DEM2010–DEM2000
22.9 51.5
253,346 569,752
Upper flat
10,550,700
DEM2000–DEM1990 DEM2010–DEM2000
8.5 10.8
89,680 113,947
Inner bay Lower flat
14,463,900
DEM2000–DEM1990 DEM2010–DEM2000
30.5 29.7
16,534 16,100
Upper flat
542,110
DEM2000–DEM1990 DEM2010–DEM2000
10.0 7.8
144,639 112,818
Fig. 4. Surface sediment distribution in (a) a 2006 map (after Chang et al. (2007)) and (b) August 2012.
change in Gomso Bay tidal flats during construction of the Saemangeum Dyke, which was calculated by subtracting DEM1990 (intertidal topography before construction of the dyke) from DEM2000 (intertidal topography during construction of the dyke). During construction of the dyke, the whole tidal flat area displayed a tendency toward accretion. Sedimentation was especially dominant on the lower flat in the outer and upper bays. The volumetric changes on the lower flat over a 10-year period were 22.9 cm in the outer bay and 30.5 cm in the inner bay (Table 2). Fig. 3(b) shows the morphological changes that have occurred since completion of the Saemangeum Dyke, determined by subtracting DEM2000 from DEM2010 (intertidal topography after completion of the dyke). After construction of the dyke, the tendency for accretion was only maintained on the upper flat in the inner bay. The tendency for sedimentation changed to erosion in the other regions, with a maximum erosion rate of about 51.5 cm over a 10-year period on the lower flat in the outer bay (Table 2). To examine the waterline alteration due to the changes in sedimentation patterns, three waterlines acquired at similar tidal height but in different years (1991, 2000, 2011) were compared. Waterline extracted from the image acquired on October 6, 1991 at a tidal height of 250 cm was located in higher tidal flat than waterlines extracted from the images acquired on September 4, 2000 at a tidal height of 237 cm and on March 11, 2011 at the tidal height of 202 cm, as shown in Fig. 3(c). Tidal height difference between 1991 and 2000 was only 13 cm. The difference of the exposed surface areas indicates the changes in topography of the tidal flats. This difference is agreed with the sedimentation tendency in the outer flat analyzed from the levelling data and DEM comparison during construction of the dyke. Although tidal height
in 2011 was 35 cm lower than 2000, waterlines have little difference because of the accretion/erosion tendency change since completion of the dyke. A sand shoal, easily identified as a bright feature in the satellite images until 2000, had been located at the end of the K–H line on the lower flat. However, this sand shoal was not detected in satellite images and field surveys. Levelling data collected during the field survey revealed this change in the sedimentation/erosion pattern in the outer flat, as shown in Fig. 3(d). This implies that the change in tidal energy in the outer bay contributed to a change in the sedimentation/erosion pattern where no river input into Gomso Bay exists. The morphological changes in the tidal flats indicate that construction of the Saemangeum Dyke contributed to the change in tidal energy in Gomso Bay. 3.2. Sedimentary facies change A sedimentary facies map was produced based on the field survey data collected in August 2012, which was then compared with a map generated based on field survey data collected in July 2006 by Chang et al. (2007). According to Folk’s classification, grain-size data were classified into three facies types: sand, silty sand, and sandy silt (Folks, 1968). Although the field survey was conducted in July 2006, just after the completion of the dyke, the sedimentary facies map represented the status during construction of the dyke because sedimentary facies are not substantially changed within a short time, with the exception of the border, which can be altered due to seasonal effects. Sandy silt facies were dominant in the inner bay, and silty sand facies were dominant in the outer bay during construction of the dyke, as shown in Fig. 4(a). After the
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Fig. 5. The reddish area of Box B in Fig. 1(a) indicates ratios greater than 100%, where the tidal current after construction of the dyke was weaker than the tidal current before and during construction. (a) The ratios of maximum tidal current speeds before and after construction of the dyke. (b) The ratios of maximum tidal current speeds during and after construction of the dyke. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Fig. 6. Maps of the SS concentration derived from Landsat TM/ETM+ band 2 based on the empirical SS algorithm and the distribution of the M2 tidal energy flux from numerical modeling at the time of image acquisition. (a) SS distribution before construction of the Saemangeum Dyke, and (b) distribution of the M2 tidal energy flux. (c and e) SS distribution at different stages in the dyke construction process; (d and f) distribution of the M2 tidal energy flux at the time of the image acquisition for (c and e), respectively. (g) SS distribution after the dyke was completed; (h) distribution of the M2 tidal flux.
dyke was completed, the area of silty sand facies expanded substantially, whereas the area of sandy silty facies noticeably declined in the study area (Fig. 4(b)). A relatively large area of silty sand facies in the mouth of Gomso Bay changed to sand facies after construction of the dyke. Except for the upper flat in the inner bay, where sandy silt was dominant and an accretion tendency was maintained since construction of the dyke began, the sedimentary facies of the flats in the inner and outer bays were coarse, and the pattern of sedimentation changed from accretion to erosion after the dyke was completed. Sand flats are more vulnerable to morphological change than are mud flats due to the small amount of clays that act as a lubricating agent (Lambrechts et al., 2010; Ryu et al., 2008). Overall, the tidal flats with a tendency for erosion to occur had coarser sedimentary facies. This implies that the changes in sedimentary facies have increased the erosion of the lower flat by reducing the critical bed shear stress.
3.3. Current change with SSC An artificial change in tidal direction following construction of the dyke resulted in a change to the sediment transport conditions (Lee and Ryu, 2008). A hydrological model was used to identify changes in the tidal current in the mouth of Gomso Bay following construction of the Saemangeum Dyke, as shown in Fig 5(a and b). The construction of the Saemangeum Dyke influenced the tides and tidal currents not only in the vicinity of the dyke but also throughout the Yellow Sea (Lee et al., 2008). After construction was completed, clear changes in tidal currents were apparent in the vicinity of the dyke. The tidal current speed increased near Garyeok gate; however, speeds decreased elsewhere along the dyke. Increased tidal current speeds were recorded in the mouth of Gomso Bay, which had not been expected by the project engineers. Before 1980, the average maximum speeds in the Gomso
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Y.-K. Lee et al. / Marine Pollution Bulletin 97 (2015) 150–159 Table 3 Summary of environmental change in Gomso Bay. Tidal flat* Duration
Outer bay Lower flat
Upper flat
Coastal water** Inner bay
Lower flat
Upper flat
Before the dyke construction
Under the dyke constr uction
3rd and 4th disco nnect ed 3rd and 4th conn ected
After the dyke construction
Accretion (silty Sand)
Erosion (Sand/silt y Sand)
Accretion (Sand/san dy Silt)
Erosion (silty Sand)
Accretion (silty Sand /sandy Silt)
Erosion (silty Sand)
Inner coast
Outer coast Northern Middle area area 1.40/ 2.11/ 150150180
210
Southern Northern Middle area area area 2.65/ 3.84/ 3.30/ 12018090-150 180 210
0.80/ 150-
0.91/ 150-
1.51/ 120-
180
180
180
1.13/ 180210
2.28/ 90-120
Southern area 5.64/
3.62/ 120-
Jul. 97
180
Dec. 98
90-180 Nov. 91
rd
3 dyke completion st
Accretion (sandy Silt)
Accretion (silty Sand/ sandy Silt)
The SMG dyke construction began
1 dyke completion
May 99 Environmental lawsuit
2.04/ 180-
1.64/ 210-
2.82/ 180-
1.70/ 210-
2.45/ 210-
4.26/ 180-
210
240
240
240
240
240
1.33/ 180-
1.40/ 180-
3.32/ 180-
2.26/ 150-
1.90/ 180-
8.89/ 180-
210
210
210
210
240
210
May 01 Jun. 03
th
4 dyke completion
(Garyeok gate completion) nd
Apr. 06
2 dyke completion
Apr. 10
The SMG dyke construction completion
(Sinsi gate completion)
* sedimentation tendency (dominant sedimentary facies) ** mean of SS concentration / dominant direction of tidal currents
tidal channel were 1.50 m/s and 1.15 m/s at ebb and flood tide, respectively, according to a National Geospatial Information (NGI) basic survey (NGI, 1981); however, it is difficult to determine the exact location and date of this survey. The maximum speeds of 1.25 m/s and 0.7 m/s in the mouth of Gomso Bay were observed during ebb and flood tide on 25 August 2006 over a 12-h measurement period (Lee, 2010). Tidal asymmetry, with the ebb tide dominant and increased tidal current speed, is a likely cause of the change in sedimentary facies and erosion around the lower flat in outer bay. This indicates that the blockage of tidal current transportation resulting from to the Saemangeum Dyke altered the tidal current speed in the mouth of Gomso Bay, and the sedimentary facies therefore became coarser, with an increase in erosion due to the reduction in the critical bed shear stress. The movement of suspended sediments across the coast explains its topography (Allen, 2000). To understand the changes in the concentration of suspended sediments (SS) and the movement of the current according to the dyke construction process, the spatial pattern of SS from the Saemangeum Dyke to Gomso Bay was estimated using Landsat TM/ETM+ band 2, and the distribution of the mean tidal residual current at the time of image acquisition was generated from numerical modeling, as shown in Fig. 6. The seasonal erosion of tidal flats due to the monsoon was prevented by SS deposited during the spring/summer season when the tendency for sedimentation is seasonally superior. Tidal conditions during image acquisition were the same as at the end of ebb tide, which reduced the variation in the levels of SS due to resuspension of the bottom sediment (Choi et al., 2014). Before construction of the dyke, highly concentrated SS (mean range: 3.30– 3.84 g/m3) flowed from the Geum, Mankyung, and Dongjin rivers. The highly concentrated SS also flowed toward the mouth of Gomso Bay with the dominant direction of currents being 90– 180°, as shown in Fig. 6(a and b). During construction of the dyke, the highly concentrated SS from the Mankyung and Dongjin rivers was blocked. Amounts of SS accumulated around the Gogunsan islands because construction began there. The dominant direction currents were almost similar with before construction of the dyke, as shown in Fig. 6(c and d). The SS plume between the third and fourth dyke disappeared after the completion of the northern dyke; however, highly concentrated SS moved south-westward and was observed at the gate of Sinsi and Garyeok, as shown in Fig. 6(e and
f). The flow of SS was totally blocked following completion of the southern dyke. The current direction became shifted to 180– 210°, as shown in Fig. 6(g and j). This means that the dyke construction changed the tidal current characteristics in Gomso Bay. The high turbidity area in front of the mouth of Gomso Bay could be partially affected by the Heuksan Mud Belt located at the southern part of the mouth of Gomso Bay (Lee and Chu, 2001) because the spatial distribution of this turbid area displayed seasonal variation with its maximum extent during winter and minimum extent during summer (Min et al., 2014). However, the increased levels of SS around the mouth of Gomso Bay indicate that resuspension of bottom sediment increased due to the enhanced tidal current speed following construction of the dyke. 4. Conclusion and discussion To investigate the cause-and-effect of environmental change on the vicinity tidal flat by the reclamation project, morphological change with hydrodynamic approach is needed. In this study, we focused on the Gomso Bay tidal flat which was not included in the target area of the project’s environmental impact assessment before the Saemangeum reclamation project. The morphological change and sedimentary facies change in Gomso Bay tidal flat were estimated. Moreover, changes in SS patterns and currents in coastal from the Saemangeum to Gomso Bay was evaluated (Table 3). The conclusions drawn from this study are as follows: (1) Morphological changes in Gomso Bay tidal flats according to the Saemangeum dyke construction were quantitatively estimated based on the DEMs generated by waterline method. The tendency for accretion was only maintained on the upper flat in the inner bay with a sedimentation rate about 10.0 cm and 7.8 cm, respectively before and after construction of the dyke. The tendency for sedimentation changed to erosion in the other regions. Especially, significant erosion was observed on the lower flat in the outer bay after construction of the dyke. We found that the sedimentary facies of Gomso Bay were coarser and covered a larger area of the lower tidal flats in the outer bay, where sediments deposited during dyke construction were eroded after its completion.
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(2) After the completion of the third and fourth dykes, the SS concentration was increased in the southern area of the inner coast and the dominant direction of the tidal current in the mouth of Gomso Bay changed from 90–180° to 180– 240°. The speed of the tidal current also increased in the mouth of Gomso Bay after the completion of the dyke. The Saemangeum Dyke has remarkably played an artificial role in sediment transport. Our analysis revealed that construction of the Saemangeum Dyke altered not only the levels of SS and the speed and direction of the tidal current in Gomso Bay coastal water but also the sedimentary facies and accretion/erosion tendencies the previously characterized Gomso Bay tidal flats. For example, the direction of the tidal current changed, and the levels of SS increased in the southern area of the inner coast. The area dominated by sand increased, and the sedimentation trend changed from accretion to erosion on the lower flat in the outer bay. The morphological and sedimentological changes in Gomso Bay and the alterations to SS and the current in the Gomso Bay coastal area indicate that environmental impacts such as the reduced habitat for fish and shellfish, decline in water quality, and degradation of natural habitat (Temmerman et al., 2013) due to conventional coastal engineering have occurred on a large sale in the area surrounding the engineering work. The Saemangeum Dyke Changes induced by natural factors such as the increased sediment flux from upriver locations and the seasonal variation of monsoons were difficult to distinguish from anthropogenic activities. In Gomso Bay, the amount of terrestrial sediment supplied directly by the Jujin stream to the bay is negligible due to the presence of upstream weirs. The freshwater discharge of Galgok stream is also negligible because of the upstream reservoir. Therefore, we concluded that most of the changes to the morphology and sedimentary facies in the Gomso Bay tidal flats were caused by changes in energy flux from the low tidal area. We used a modified version of the POM for the numerical modeling. Although the sea-level variation were calculated by a numerical model on a larger model domain including all of the Yellow Sea, tidal currents were analyzed on a smaller model domain included the whole of Saemanangeum Dyke and Gomso Bay. The tidal current change cause by the Yeonggwang nuclear power plant located about 14 km south from the southern part of study area was excluded. Thus, more detailed analyses of sediment dynamics with tidal current change including southern part of study area are needed to reveal the nature of seasonal or artificial environmental process in the Gomso Bay coastal area. Conventional engineering projects in coastal areas have become a worldwide issue due to the demand for urban, industrial, and agricultural expansion (Wang et al., 2010). Satellite-based observation strategies can be used to overcome the limitations of field surveys when determining the environmental status following conventional engineering projects. Unexpected morphological and sedimentological changes in the vicinity of tidal flats affected by conventional engineering require interventions to protect living organisms and support the local economy. The results of this study should further encourage the use of satellite-based studies for the big data, and should motivate governments and NGOs to extend the target area used in environmental impact assessments.
Acknowledgements This research was supported by a project titled ‘‘Research for Application of Geostationay Ocean Color Imager’’ funded by the Ministry of Ocean and Fisheries (MOF), Korea and ‘‘Development of satellite based ocean carbon flux model for seas around Korea’’
funded by the Korea Institute of Ocean Science and Technology (KIOST).
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