Ocean Sci. J. (2010) 45(3):187-195 DOI 10.1007/s12601-010-0017-0
Available online at www.springerlink.com
Article
Preliminary Results on Suspended Sediment Transport by Tidal Currents in Gomso Bay, Korea Hee Jun Lee* Marine Environment Research Department, KORDI, Ansan P.O. Box 29, Seoul 425-600, Korea Received 10 August 2010; Revised 7 September 2010; Accepted 16 September 2010 © KSO, KORDI and Springer 2010
Abstract − This study briefly investigated sediment transport by tidal currents in Gomso Bay, on the mid-west coast of Korea during the summer season. Hydrodynamic measurements with benthic tripods (TISDOSs) show that near-bed suspended sediments are transported toward the bay mouth along the lowwater line of tidal flats in the southern part of the bay, while they are directed offshore in front of the major tidal channel at the bay mouth according to tidal asymmetry. However, suspended sediments in the main body of sea water, observed from transect and anchor-site measurements, indicate a consistent incoming toward the uppermost tidal flats. A brief episode of relatively strong winds from the west and southeast displays that wind waves can yield the near-bed suspended sediment concentrations (SSC) overwhelming the SSC by tidal currents alone in the remaining duration. Key words − suspended sediment transport, macrotidal current, benthic tripod, Gomso Bay, Yellow Sea
1. Introduction Gomso Bay is located on the mid-west coast, about 15 km south of the Saemangeum Dyke (Fig. 1). It has a large funnel shape like a typical estuary, but receives fresh runoff only from a disproportionately small stream, the Jujin. In the early 1990s, Gomso Bay was a focus of intense research in terms of sedimentological characteristics such as Holocene stratigraphy, distribution of surface sediments, and sedimentary bedforms (Lee et al. 1994; Chough et al. 2000). As a result, the tidal flats of the bay was found to consist of coarseningupward, late-Holocene sequences with surface sands which have accumulated since about 1800 yr BP (Kim et al. 1999). In addition, a shelly-sand ridge-like deposit, chenier, was *Corresponding author. E-mail:
[email protected]
reported to move onshore across the tidal flat mostly by winter storms and a summer typhoon. Movements of large bedforms on tidal flats suggest that Gomso Bay is intrinsically a high-energy environment, particularly during the winter season with prevailing winds and waves combined with macrotidal currents. In contrast, the summer season is represented by macrotidal currents only. A large dyke, Saemangeum, encompasses vast estuarine areas of the Mangyung and Dongjin rivers. The construction of the dyke has resulted in profound influences on the morphology and sediment facies of the seabed around the dyke (Lee and Ryu 2008). Since the extent of virtual dyke effects is little known, environmental questions associated with the dyke might be raised whenever Gomso Bay undergoes unexpected changes in physical and biological conditions such as rapid erosion or deposition and abrupt mass extinction of benthic animals, particularly shellfish. Furthermore, local residents insist that an input of warm water from offshore and abundant shrimp-cultivating plots by reclamation along the shore of Gomso Bay have caused the overall tidal-flat fertility of Gomso Bay to prominently deteriorate. To solve these environmental issues and predict environmental changes by probable tidal flat use in the future, essential knowledge may well be required concerning sediment and water movements under a regional hydrodynamic framework centered at Gomso Bay. We performed hydrodynamic measurements from a number of local sites in Gomso Bay during the summer (August) seasons of 2006 and 2008. The objectives of this study are to reveal a general pattern of tidal currents and associated sediment transport during the fair-weather conditions and determine whether tidal currents alone can
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supply fine-grained sediments to the ever growing innermost tidal flats of the bay.
2. Geologic Setting Gomso Bay consists of a major tidal channel (hereafter, the Gomso tidal channel) to the north and extensive tidal flats to the south (Fig. 1). The latter are largely backed by a number of segments of dykes which demarcate reclaimed tidal flats into paddies, salt ponds, or shrimp farms. Sand bars are frequently found on the lower intertidal flats, whereas tidal creeks are abundant on the middle and upper flats (KORDI 2007). Although occasional cheniers rest on the uppermost flats, mud tends to increase landward (Lee et al. 1994; Chough et al. 2000). The Gomso tidal channel reaches about 15 m in depth below lowest low water level and is up to 900 m wide. It is morphologically asymmetric in cross section with the northern flank steeper than the
Fig. 1. Map showing study area in Gomso Bay with sites and a transect for various hydrodynamic measurements. The measurements include benthic tripod (TISDOS) deployments (open square) and 12-hour onboard observations of currents and suspended sediment concentrations (SSC) along a transect (line and heavy square) and anchor sites (heavy triangle). Asterisk indicates a weather station in Wolsan from which wind data in Fig. 5 are derived
southern flank. Tide is semi-diurnal and macrotidal with a mean range of 4.33 m, with ebb-dominant tidal currents on the Gomso tidal channel with the average maximum speeds of 1.15 and 1.50 m/s at flood and ebb, respectively (NGI 1981). Concentrated in the summer seasons (June-August), annual average precipitation reaches 1100-1300 mm (KMA 1998). Winds are generally from the south in summer and from the north in winter with storms (wind speeds >13.9 m/s) occurring for 32.6 days a year, which are concentrated in winter (KMA 1998). In contrast, typhoons take place during summer at a recurrence rate of 1.2/year (Yang et al., 2005). Some of the typhoons generated large waves (>4 m high) close to Gomso Bay (NGI 1981).
3. Materials and Methods A set of benthic tripods, called TISDOS (TIdal Sediment Dynamics Observational System), were deployed at three stations (GS1-3) on 24-28 August 2006 and at two stations (GS4-5) on 19-25 August 2008 (Fig. 1). Both measurement periods were around the peak spring tide. In addition, on 25 August 2006, a transect was occupied across the mouth of Gomso Bay to collect a time-series of currents and suspended sediment concentrations (SSC) in the water column for 12 hours. In the middle and upper reach of the Gomso tidal channel, two stations (GSA and GSB) were simultaneously anchored to obtain 12-hour current and SSC measurements in the water column on 19-20 August 2008. In the TISDOS measurements, currents were recorded at 0.7-1.0 m above the bed (mab) with a 5-MHz acoustic Doppler velocimeter (ADV, SonTek) as 30-min averages or with an acoustic Doppler current sensor (DCS3620) as 1min averages. To measure SSC, two synchronized optical backscattering sensors (OBS, Seapoint) were installed at 0.1-0.4 and 0.7-1.0 mab. Differential subsidence of the TISDOS on the seabed occurring at the deployment sites resulted in variable measuring levels for currents and SSC. The changes in water surface elevation were measured with a pressure sensor (Digiquartz) at 4 Hz, and were used to evaluate water depth with corrections for depth attenuation effects (Wang et al. 1986) and wave statistics such as the significant wave height (Hs) and peak wave period (Tp). Wave statistics were calculated by applying the zero-down crossing method to the oscillatory part of the demeaned water-surface elevation records. Temperature and salinity
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were measured with a conductivity-temperature-depth (CTD) sensor (SBE37, Sea-Bird) at 2 Hz. As the ship moved along the transect for 12 hours, currents were measured with a Sontek Aquatic Doppler Profiler (ADP, 1 MHz), mounted aboard ship for 1-m-depth bins at 10-second intervals. Water samples were taken with a vacuum pump from 3 levels (surface, middle, and about 1 mab) at three stops (A, B, and C) along the transect (Fig. 1). In the laboratory, water samples (about 1 L) were filtered through 0.45-µm Millipore membranes and dried at 60 °C. The SSC were calculated with the dry weight of the filtered residues.
4. TISDOS Measurements The seabed at the TISDOS stations consists of a mixture of sand and mud with a mean grain size of 4.5-6.5φ. The coarsest and finest sediments occur at stations GS2 and GS3, respectively (KORDI 2007). The TISDOS data from the measuring stations off the tidal flats (stations GS3-5) all show the flood-dominant tidal current regime with a major tidal axis parallel to low-water line of the tidal flats in the NE-SW direction (Figs. 2 and 3). As seen in Figure 2, the difference between the peak speeds of flood and ebb flows (0.5-0.7 and 0.3-0.5 m/s, respectively) reaches about 0.2 m/s. However, current records from station GS2, off the bay mouth, display an ebb-dominant tidal current asymmetry (Fig. 2). The peak ebb flows attain speeds about 0.1-0.2 m/s stronger than flood counterparts. Here, tidal currents reversely flow in approximately the east-west direction (Fig. 3). Tidal currents at station GS1, about 4 km northwest of the bay mouth, characteristically rotate anti-clockwise, and thus show virtually no major tidal axis. A progressive vector plot of water particles indicates that they advance toward the northwest over repeated tidal cycles (Fig. 3). Most of the stations, particularly station GS2, show a good SSC dependency on tidal current speeds (Fig. 4). This implies that tidal currents alone can re-suspend and move the seabed sediments. However, waves are proved to be the most dominant forcing agent controlling SSC (Fig. 5). When relatively strong winds blew from the west and southeast at 4-10 m/s, wave heights exceeded 1.0 m. Even though the waves lagged about one day behind the winds, the resulting increase in SSC was remarkable compared to SSC of the remaining milder-weather period (Fig. 5). Water temperature varies well with tide at station GS3 such that it
Fig. 2. Tidal current time-series at the TISDOS stations. Note that stations GS3-5 are characterized by flood dominance, whereas station GS2 is ebb-dominant. At station GS1, tidal currents appear to vary intricately (for more details of current direction and movements, see Fig. 3). For station locations, see Fig. 1
increases during ebb and decreases during flood in the range of 23-25°C. However, water temperature dependency on tide diminishes off the bay mouth at station GS1 in which it gradually increases over time from 24.0 to 25.5 °C. At station GS1, salinity gradually decreases from 31 to 25 psu through the measurements. Near the bay mouth (station GS5), the 2008 measurements show no clear tidal dependency of either water temperature or salinity, which range between 25 and 27 °C and between 29 and 31 psu, respectively.
5. ADP Measurements and Water Sampling Transect A total of nine tidal current cross sections were obtained on 25 August 2006 from a 12-hour transect across the mouth of Gomso tidal channel (Fig. 1). Among them, two sections for peak currents each at flood and ebb are described below in detail (Fig. 6). Flood currents are directed to the east in most of the cross-sectional area except for the southwestern tip where currents return to the west or north in the middle of the flood phase (Fig. 6A). Maximum flood current
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Fig. 3. Progressive plot of water particle movement and suspended-sediment flux from the TISDOS stations. Note that the tidal major axis at each station can be seen in the plot of water particles. The direction N is upright. For station locations, see Fig. 1
Fig. 4. Example of relationships between tidal currents and SSC from station GS2. Note that the SSC are well dependent on tidal current intensities. For station location, see Fig. 1
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Fig. 5. Measurement time-series from station GS4 including tidal and residual currents, wave height, wind speed, and turbidity (not calibrated to real values). Residual currents were calculated with a 25-hour moving average method. Wind data was obtained from a weather station in Wolsan (Fig. 1). Note that the direction of residual currents changed concordantly with the direction of relatively strong winds during 22-24 August
speeds are recorded at 0.7-0.8 m/s at both the tidal channel and a trough close to Jukdo. During ebb, the maximum speeds of 1.25 m/s are observed at the surface of the tidal channel, whereas tidal currents diminish (0.25 m/s) on tidal flats and then slightly increase to 0.35 m/s at the trough (Fig. 6B). Ebb currents persistently flow toward the west across the entire cross section. For all three stops on the transect, the SSC range mostly between 10-30 mg/L with a slight tendency of either decreasing from ebb to flood or
remaining constant. Water temperature and salinity show no stratification through the measurements in the narrow range of 24.5-26.5 °C and 30-31 psu, respectively. Anchor site Surface sediments at the anchor sites, GSA and GSB, tend to be finer toward the bay head with a mean grain size of 4.5-5.0φ (KORDI, 2007). Figure 7 shows 12-hour measurements at station GSA on 19-20 August 2008.
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Fig. 6. Cross-sections for maximum tidal currents at (A) flood and (B) ebb from the 12-h onboard observations along a transect at the mouth of Gomso Bay. For current direction, north is upright. For transect location, see Fig. 1
Fig. 7. Time-series of tidal currents and SSC at the anchor site GSA for 12 hours on 19-20 August 2008. For site location, see Fig. 1
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Fig. 8. Time-series of tidal currents and SSC at the anchor site GSB for 12 hours on 19-20 August 2008. For site location, see Fig. 1
Located in the middle of the Gomso tidal channel, the station shows a tidal range of 6 m with water depths of 4-9 m. Tidal currents, which are more prolonged during flood than ebb, flow back and forth along the channel with peak speeds of 1.0 m/s. Accordingly, maximum SSC measure over 300 mg/l near the bed during flood compared with the ebb maximum of 100 mg/l. Both temperature (26.8-27.0 °C) and salinity (29.0-31.0 psu) exhibit weak stratification, indicating a well mixed sea-water state. Both tend to slightly increase toward high tide. Further landward in the Gomso tidal channel, simultaneous measurements at station GSB show similar patterns in all the parameters to those of station GSA (Fig. 8). Characteristically, the recorded values from station GSB of water depth, maximum current speeds, temperature, and salinity are all lower than those at station GSA; the former are in the range of 1-6 m, 0.8 m/s, 26.5-26.8 °C, and 27.5-31.0 psu, respectively. However, maximum near-bed SSC increase to over 500 mg/l at station GSB (Fig. 8).
6. Discussion and Conclusions Sediment dynamics investigations in the western coastal region of Korea have shown that winds and incident waves play a predominant role in sediment transport (Lee et al. 2004; KORDI 2005). When waves are superimposed on tidal currents, sediment flux is frequently and markedly enhanced by more than one order of magnitude than from tidal currents alone (Lee and Ryu 2007). This study also exhibits a measurement of windy-day suspended-sediment flux far greater than that of fair-weather from station GS4 (Fig. 5). During winter, when strong monsoonal winds consistently blow from the north or northwest, such a magnifying effect of wind waves on suspended sediment transport becomes highlighted. In this rough-sea season, even surface sea water maintains relatively high concentrations of suspended sediments that are consistently supplied from the seabed through resuspension processes by combined wave and tidal currents (Lee et al. 1999). Another important
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Fig. 9. Schematic diagrams of suspended sediment transport in Gomso Bay (A) within 1 m of the seabed and (B) in the main body of water above 1 m of the seabed. The diagrams are based on (A) TISDOS deployments and (B) 12-h onboard observations. Note that the two diagrams differ from each other in transport direction near low-water line of tidal flats. Diagram A is not scaled in contrast to the scaled diagram B.
role of winds is to generate mean currents. It is well known that, during winter, residual currents by persistent winds exist along the west coast of Korea by which coastal suspended plumes move southward on the whole (Lee and Chough 1989). In Gomso Bay, although measured during summer, the current data from station GS4 also display the existence of slight but distinctive wind-generated currents during high waves associated with winds from the west and southeast (Fig. 5). In this case, current direction is shown to be in good accordance with wind direction. The direction of the suspended-sediment flux, evaluated at each of all the stations and stops, is schematically diagrammed in Fig. 9. The resulting near-bed flux vectors from the TISDOS measurements show that, owing mostly to flood-dominant currents, nearshore suspended sediments are transported along the low-water line of tidal flats toward the mouth of Gomso Bay (Fig. 9A). However, tidal asymmetry is reversed to ebb dominance off the bay mouth causing suspended sediments to be exported offshore (Fig. 9A). Suspended sediment efflux through the bay mouth can
also be seen in the cross-sectional measurements over the Gomso tidal channel (Fig. 9B). By contrast, suspended sediments laden by the main body of sea water are transported landward over the tidal flats and in the middleto-upper reaches of the Gomso tidal channel (Fig. 9B). This supply to the bay of fine-grained sediments by tidal currents explains why the tidal flats in the innermost part of the bay have kept accumulating toward supra-tidal environments and why the upper tidal flats along the southern shore of the bay become muddier during summer. Nonetheless, further study is needed, particularly on the southern shore, to establish the long-term behavior of these muds associated with winter waves that most likely either winnow them offshore or displace them to the innermost mud flats. An issue to be addressed in the future is the location where tidal asymmetry turns from flood- to ebb-dominant in the Gomso tidal channel and the reasons of this tidal-asymmetry change. One of the reasons may be related to the geometry of tidal flats which are influenced significantly by the Jujin Stream. Therefore, further studies need systematic field observations
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supplemented by interactive modeling for bay-wide sediment transport.
Acknowledgements This research was part of a project titled “Saemangeum coastal research for marine environmental conservation” funded by the Ministry of Land, Transport and Marine Affairs, Korea (grant no. PM55630). It was additionally supported by the Korea Ocean Research & Development Institute (KORDI; grant no. PE98462). We thank S.W. Lee, J.S. Kee, and J.M. Lee for their technical assistance.
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