Nearshore bedform instability in the eastern entrance to the Qiongzhou Strait, South China Sea

Abstract

Six bathymetric transect profiles were drawn from the nautical charts of 1942, 1962 and 1992 to show that the nearshore seabed remained unstable during the recent 50 years in the middle channel of the eastern entrance to the Qiongzhou Strait, South China Sea. Our results demonstrate that the multi-year averaged seabed aggradational rate was 25 cm/a and erosion rate was 12.5 cm/a. Lateral migration rate of the sea bedform identified from the historical contours was about 100 m/a in the SE direction. Bedform measurements were made using GPY Shallow Seismic Profiler in 1994 in the study area. The records revealed four types of distinctive bedforms that were composed of fine and medium sands. The average spacing of large and small-scale sand dunes is 416 m and 144 m and the average height remains 8.8 m and 4.9 m. The spatial and temporal equilibrium-range spectra of numerical bedform records were applied to estimate short term celerity of bedform movement. Results indicate that large and small dunes migrated at an average celerity of 0.02 cm/hr eastward and 0.09 cm/hr westward in the calm sea weather, while their celerity can reach 53 cm/hr eastward during typhoon season and is only 0.008 cm/hr westward when NNE winds prevail. The results also show that the larger the temporal and spatial scale is, the smaller the bedform movement celerity appears. On the other hand, the smaller-scale bedform celerity of the present study is much greater than that of flume, empirical and theoretical data, but close to the wind tunnel and field-measured data of similar grain size.

References

1. Ashley G M (1990). Classification of large-scale subaqueous bedforms: anew look at an old problem. Journal ofSedimentary Petrology, 60(1): 160–172
2. Berne S, Castaing P, Drezen E L, et al. (1993). Morphology, internal structures,and reversal of asymmetry of large subtidal dunes in the entranceto Gironde Estuary (France). Journal ofSedimentary Petrology, 63(5): 780–793
3. Cai S Q, Long X M, Wu R H, et al. (2006). A three-dimensional sanddiffusion model in Qiongzhou Strait. Journalof Tropical Oceanography, 25(2): 1–5 (in Chinese)
4. Chen D S, Chen B, Yan J H, et al. (2006). The seasonal variation characteristicsof residual currents in the Qiongzhou Strait. Transactions of Oceanology and Limnology, (2): 12–17 (in Chinese)
5. Cheng H Q, Kostaschuk R, Shi Z (2004). Tidal currents, bed sediments, andbedforms at the South Branch and the South Channel of the Changjiang(Yangtze) Estuary, China: implications for the ripple-dune transition. Estuaries, 27(5): 861–866
6. Cheng H Q, Li M T, Xue Y Z, et al. (2001). High-resolution detectionand study of sub-aqueous micro-geomorphology movement in the ChangjiangEstuary. Advances in Natural Science, 11(10): 1–7 (in Chinese)
7. Cheng H Q, Song B, Xue Y Z, et al. (2000). Mechanics on transport ofcoarser silt and very fine sand in the Changjiang Estuary_episodicre-suspension and large-scale bedform movement. Journal of Sediment Research, (1): 20–27 (in Chinesewith an English abstract)
8. Dalrymple R W, Knight R J, Lambiae J J (1978). Bedforms and their hydraulic stabilityrelationships in a tidal environment, Bay of Fundy, Canada. Nature, 275: 100–104. doi:10.1038/275100a0
9. Dong Z B, Chen G T, Yan C Z, et al. (1998). Discipline of sand dunemotion along the Oil Highway in the Talimu desert. China Desert, 18(4): 328–333 (in Chinese)
10. Dou G R (1999). Discussion on motivated flow velocity of sediments. Journal of Sediment Research, (6): 1–59 (in Chinese)
11. Dyer K R (1986). Coastal and Estuarine Sediment Dynamics. New York: John Wiley & Sons, 1–342
12. Feng W K, Li W F (1994). Sandwave morphology on the seabed in the northern part of South ChinaSea. Tropical Sea, 11(4): 655–662 (in Chinese)
13. Fenster M S, Fitzgerald D M, Bohlen W F, et al. (1990). Stability of giant sandwaves in Eastern Long Island Sound, USA. Marine Geology, 91: 207–225. doi:10.1016/0025‐3227(90)90037‐K
14. Harris P T (1989). Sandwave movement under tidal and wind-driven currentsin a shallow marine environment: Adolphus Channel, northeastern Australia. Continental Shelf Research, 9(11): 981–1002. doi:10.1016/0278‐4343(89)90003‐4
15. Hino M (1968). Equilibrium-range spectra of sand waves formed by flowingwater. Journal of Fluid Mechanics, 34: 565–573. doi:10.1017/S0022112068002089
16. Huang Q Z, Huang Y H (1999). Characteristicof Suspended Load Concentration Transport in the Qiongzhou Strait. In: Liang S, Ed. Selections of the Studies of the Resources and Environment in SouthChina Sea . Guangzhou: Zhongshan University Press, 157–164 (in Chinese)
17. Ke P H (1986). Preliminary study on exchange of currents and freshwaterin the Qiongzhou Strait. Tropical Geography, 6(4): 253–346 (in Chinese)
18. Levey R A (1980). Comparison of bed form variance spectra within a meanderbend during flood and average discharge. Sedimentary Petrology, 50(1): 149–155
19. Li Z H, Ke X K (2000). Preliminarystudy on tidally-induced sediment fluxes of the Qiongzhou Strait. Marine Science Bulletin, 19(6): 42–49 (in Chinese)
20. Liu Z X, Berne S, Wang K Y, et al. (1998b). Tidal bedforms in easternpart of the Bohai Sea. Acta OceanologicaSinica, 17(2): 209–231
21. Liu Z X, Xia D X, Wang K Y (1998a). Tidal depositional systems and patternsof China's continental shelf. Oceanologiaet Limnologia Sinica, 29(2): 141–147
22. Nordin C F (1971). Statistical properties of dune properties of dune profiles. United States Geological Survey Professional Paper[C], 41
23. Parsons D R, Best J L, Orfeo O, et al. (2005). Morphology and flow fieldsof three-dimensional dunes, Rio Parana, Argentina: Results from simultaneousmultibeam echo sounding and acoustic Doppler current profiling. Journal of Geophysical Research, 110: 1–9
24. Sleath J F A (1984). Sea Bed Mechanics. NewYork: John Wiley & Sons, 1–335
25. Wang S Y, Li D M (1994). Dynamicsand wave on shelf and slope of Zhujiang basin in the South ChinaSea. Acta Oceanologica Sinica, 16(6): 122–132(in Chinese)
26. Wang W J (2000). Propagation of tidal waves and development of sea-bottomsand ridges and sand ripples in northern south China Sea. Donghai Marine Science, 19(1): 1–7 (in Chinese). doi: 10.1287/mksc.19.1.1.15181
27. Wang Y H, Dai C F, Richard A C (1993). Observations of bed forms off SouthernTaiwan using a ROV. Acta OceanographicaTaiwanica, 30: 1–9
28. Wu N Y (2000). Successful application of GIS to the detection of submarinetopography. Development of Marine Geology, 7: 8 (inChinese)
29. Yang S L, Zhang Z T, Xie W H (1999). Research of sandwave group in theSouth channel of the Changjiang Estuary. Ocean Engineering, 17(2): 79–88 (in Chinese)
30. Zhu Z D (1962). Characteristics of wind sand geomorphology experiment. Journal of Wind Sand management, 4: 89–93 (in Chinese)
31. Zhu Z D, Chen Z P, Wu Z (1981). Wind sand morphology in Takelamagan. Beijing: Science Press: 1–62 (in Chinese)
32. Zhu Z D, Lu J H, Jiang W Z (1988). Preliminary study on formation andenvironmental evolution in the downstream of the Yahe River in Takelamagandesert. Chinese Desert, 8(2): 1–10 (in Chinese)