Seasonal variation of suspended-sediment transport through the southern Bohai Strait, China

1,2 1,2 1,2 3 AGU Chapman Conference Naishuang BI ,, Zuosheng YANG , Houjie WANG , Yoshiki SAITO Supported by MOST, China Jan. 24-27, 2011 Oxnard, USA (2010CB951202) and NSFC (41006023) 1 College of Marine Geosciences, Ocean University of China, 238 Songling Rd., 266100, China ([email protected]) Poster Number: T3 2 Key Laboratory of Submarine Science and Prospecting Techniques, Ocean University of China, Qingdao 266100, China 3 AIST, Geological Survey of Japan, IGG, Central 7, Higashi 1-1-1, Tsukuba 305-8567, Japan 0 0 0 0 0 0 0 0 0 0 5 0 5 0 5 0 5 0 0

41° 500 5 4 4 3 3 2 2 1 1 5 0 Grid stations mg/L Time-series stations Liaodong Bay SSC = exp(2.15 *R645/R555) * 0.95 How much sediment is transported from the Liaohe -20 Isobath N = 23 400 RMS=4.10 38.5° to the annually ? 40° R 2 = 0.88 The Bohai Strait connects the Bohai Sea and the Yellow Sea (Fig. 1) and is the pathway for material exchange between 38° 300 ) the two seas. The strait is roughly divided into two parts by North Huangcheng Island. Yellow Sea water flows into the Bohai -1 37.5° Jan. Feb. Mar. 39° Luanhe Sea through the northern Bohai Strait, and Bohai Sea water flows out to the Yellow Sea through the southern part . Thus, the Haihe 37° Bohai Bay Bohai Sea southern Bohai Strait is the primary pathway for sediment transport from the Bohai Sea to the Yellow Sea. 200 SSC (mg L The Huanghe suspended sediment transported eastward to the Yellow Sea through the southern Bohai Strait is considered 38.5° 38° A’ Yellow Sea a major source of the muddy deposits (e.g., mud wedge or distal subaqueous delta off the eastern Peninsula) in the Huanghe delta Bohai Strait 100 38° Yellow Sea. However, the quantity of sediment that is transported from the Bohai Sea to the Yellow Sea through the southern A28 Bohai Strait is still under debate due to the absence of long-term and multistation observations, especially in winter. Huanghe A 37.5° Apr. May Jun. 37° 0 41° Shandong Peninsula 37° Liaohe (B): Winter 117° 118° 119° 120° 121° 122° 123° 1 1.5 2 2.5 3 (A) Fig. 2. Bathymetric map of the Bohai Sea and the Bohai Strait with the loca- tions of the survey stations in 2006 and 2007. The dashed box indicates the Fig. 3. The SSC as an exponential function of the ratio of reflectance values 38.5° R645/R555 Liaodong Bay Peninsula 40° area of interest for retrieval of SSC from MODIS imageries. Transect A-A’ indi- of band 4 to band 1. R645 and R555 indicate the reflectance values of band 1 cates the locations for WF and SSF calculations. and band 4, respectively. 38° ～ Bohai Sea9 1.0x10 t/yBohair Haihe Luanhe Bohai Sea Strait Liaodong Peninsula 37.5° Jul. 39° Significant seasonal variation of suspended sediment transport through Aug. Sep. Huanghe Bohai Bay 37° Huanghe NCH island Bohai the southern Bohai Strait was detected Strait 38° Yellow Sea 38.5° Huanghe delta

Laizhou Bay 31 .4 31 .5 31.6 31.7 31 .4 31 .42 31.44 31 .46 31.48 31 .5 The observational data shows that the water Yellow Sea Salinity (PSU) 38° Yellow Sea Huanghe Shandong Peninsula column is well mixed vertically for the physico- 4 8 12 16 SSC (mg/l) 40 50 60 70 Abandoned 37° 0 0 chemical parameters (e.g., salinity, density, and 37.5° 41° Sigma-t Dec. O ld Liaohe Oct. Nov. China 9 (C): Summer SSC SSC) (Fig. 4). The depth-averaged SSC is 48 37° Huanghe ～ 0.1x10 t/yr 2 2 Temp. Salinity mg/L, more than 4 times that in the summer. The 118° 119° 120° 121° 122° 118° 119° 120° 121° 122° 118° 119° 120° 121° 122° 3 Fig. 5. Seasonal and spatial variations of monthly averaged SSC based on the model of SSC retrieval. The white areas indicate clouds. 40° Liaodong Bay 4 4 WF and SSF are estimated to be 2.37 m /s and 1.5 Yangtze kg/m/s, equal to 1.2 and 5.0 times those in summer Changjiang The annual sediment flux through the southern Bohai Strait was 6 6 East China Sea Haihe Luanhe (Table 1). East China Sea Bohai Sea Liaodong Peninsula 9 39° The intermonthly variation of the MODIS- reestimated to be approximately 40.0 Mt/yr ～ Bohai Bay 8 0.5x10 t/yr 8 Depth (m) Depth (m) NCH island derived surface SSC indicates that highly turbid Bohai According to the seasonal variation of the monsoon activity, the year can be simply divided into two seasons, namely summer Strait 10 10 waters are formed off the Huanghe delta starting in 38° Yellow Sea Huanghe delta January and disperse eastwards through the north- (April–September), when southerly and southeasterly winds prevail, and winter (October–March), when northerly and north- Laizhou Bay 12 12 westerly winds dominate (Liu, 2006). During the winter, the water column is well mixed and the SSC has little variability with SeaWiFs image(7 Oct.1998) Huanghe middle Laizhou Bay some distance off its south Shandong Peninsula depth (Fig. 4). Thus, the MODIS retrieved SSC is used as the depth-averaged SSC in the present study. The averaged SSC in the 37° coast. They are then driven along the coast of 117° 118° 119° 120° 121° 122° 123° 14 14 summer in the present study is taken to be 10.5 mg/L calculated by Martin et al. (1993) based on 6 cruises data collected A: Summer B: Winter Shandong Peninsula, with the SSC decreasing from 1985-1991. It indicates that winter is the major season for sediment transported from the Bohai Sea to the Yellow Sea Fig. 1. (A) Location of the Bohai Sea and Bohai Strait and the schematic map of sediment transport through the southern Bohai Strait (after Yang 16 Sigma-t (g/l) 16 gradually towards the Yellow Sea through the and Liu, 2007) and general pattern of the circulation in the Bohai Sea and the Bohai Strait in (B) winter (after Fang et al., 2000) and (C) summer through the southern Bohai Strait. 21 .7 21 .8 21 .9 22 22 .1 23 .4 23.42 23 .44 23.46 23 .48 southern Bohai Strait. After April, the extension of (after Guan, 1994) and . NCH indicates northern Chenghuang Island. ℃) 2.5x10 11 100 8.0 Temperature ( (A) (B) 20 20.4 20 .8 21.2 21 .6 22 13 .8 13 .84 13.88 13 .92 13.96 14 highly turbid waters is narrowed and limited to the ) nearshore area close to the coast of the Huanghe 3 Fig. 4. Vertical profiles of sigma-t, SSC, temperature and salinity in (A) summer and (B) winter 2.0x1011 80 Field observational data and monthly averaged MODIS data were used at station A28. delta. As a result, the highly turbid water belt in the 6.0 southern Bohai Strait disappears gradually. During Hydrographic surveys at a time-series station (A28) near the southern Bohai Strait were conducted on November 13, 2006, Table 1. Comparison of salinity, SSC, WF and SSF at station A28 during two surveys, one in the flood season of the Huanghe (July–Septem- 1.5x1011 60 summer 2007 and the other in winter 2006. and July 7, 2007 (Fig. 2). The observations in summer (July 2007) were conducted when wind speeds were relatively low (~4 ber), the turbid river plume extends seaward and 4.0 Summer Winter 1.0x1011 40 m /s); the winter (November 2006) observations were made 4-5 days after stormy weather with wind speeds of ~15 m/s. In ad- Item becomes more identifiable than it is for April- dition, SSC data were collected at eight time-series stations (sampled for 13- or 25-h periods) and on a twenty-four grid sta- Maximum Minimum Average Maximum Minimum Average June, but it has little impact on the SSC distribu-

Annual Sediment flux (Mt) 2.0 Salinity 10 tions off the Huanghe delta and in the adjacent Bohai Sea on cruises during June 29–July 7, 2007 and November 7–13, 2006 31.65 31.59 31.63 31.41 31.20 31.36 5.0x10 20

tions in the southern Bohai Strait. The same distri- flux (m Monthly averaged Water -1 Monthly averaged SSC (mg/L) to establish models of SSC retrieval from the MODIS data. SSC (mg l ) 40 4 11.55 20 100 48.1 bution pattern of highly turbid waters for Janu- Water flux a (m3 s-1) / / 1.91 / / 2.37 Cloud-free MODIS L1b data with spatial resolutions of 250 m and 500 m were obtained for days corresponding to the field ary–March is formed again in November. The 0.0x10 0 0 0.0 Sediment flux (kg m-1 s-1) / / 0.3 / / 1.5 surveys. Reflectance data were extracted from pixels centered on the in-situ measurements to establish the MODIS retieved waters are more turbid in December, with an en- D ec Jan Feb M ar Apr M ay Jun Jul Aug Sep O ct N ov D ec Jan Jan Feb M ar Apr M ay Jun Jul Aug Sep O ct N ov D ec Month Month SSC model (Fig. 3) if the time gaps between the acquisition of MODIS imagery and field sampling were less than 30 minutes a The directions of the water and sediment fluxes are flowing out of the Bohai Sea to the Yellow larged extension of highly turbid waters that in coastal areas within the 15-m isobath and 3 hours in offshore areas deeper than 15 m. Moreover, daily cloud free MODIS Sea. Fig. 6. Seasonal variations of (A) monthly averaged water discharges (Lin et al., 2004), SSC, and (B) SSF in the southern Bohai Strait. Note that the averaged almost covers the whole area of this study (Fig. 5). SSC from April to September is from Martin et al. (1993), while those from January to March and October to December were calculated from the MODIS-retrieved imageries in 2007 were collected to yield composite monthly averaged MODIS data. SSC along transect A-A’ (Fig. 2).