长江口南港航道沙波群研究

A Study of Sandwaves in the South Channel of the Yangtze Estuary

本文利用Ｉｎｎｅｒｓｐａｃｅ４４９热敏式双频测深仪和ＧＰＳ（全球定位系统）在长江口南港和南北槽主航道走航观测结果，确认南港（吴淞口以下）上段航道在洪季正常风况下存在尺度较大的沙波群，并对沙波的几何特征、形成条件、迁移速率以及台风后的变化做了初步分析。结果表明：（１）在统计的６５２个沙波中，平均和最大波长分别为２１ｍ和１０５ｍ。平均和最大波高分别为０．８ｍ和２．７ｍ，波高和波长间具有明显的正相关关系；沙波中７２％为单一沙波，复合沙波的波长和沙波指数较大；单一沙波最大、最小和平均沙波指数分别为３６．５、１１．２和２１．８（复合沙波最大可达６９．３）；随着沙波度的增大，沙波指数呈增大趋势；绝大多数沙波（波高＜１ｍ）是对称或近于对称的，但波高＞１ｍ的沙波通常向海倾斜。（２）沙波沉积物为分选好的细沙，Φ５０为２．７０。（３）沙波现场水流费劳德数Ｆｒ小于０．２，与某些理论相符而与某些水槽实验结果相冲突。（４）大潮和寻常潮一个涨（落）潮过程沙波来回迁移的距离约为１～５ｍ；小潮期间沙波的迁移距离接近０；落潮迁移距离总是大于涨潮。（５）台风后沙波有被掩埋的迹象。结果包括：①沙质床底是形成沙波的重要基础。南北槽虽具有沙波形成的?

Field surveys of sandwaves were undertaken in March,July and August 1997, with Innerspace 449 Thermal Depth Sounder Recorder and Del 1008/586 Globe Positioning System(GPS).During the period, measurements of hydrodynamics and deposits were also carried out.Based on these data, sandwaves were studied with respect to their geometric feature,sedimentary composition, hydrodynamic environment and migratory velocity.According to 652 sandwaves,the mean and maximum length was 21m and 105m;the mean and maximum height was 0 8s and 2 7m,respectively.The height and the length of the single sandwave were positively correlated to each other with the equation being H =0 0763 L 0 8299 ( R =0 8116, N =469).72% of the sandwaves were singly situated.The minimum,mean and maximum ratio of L/H was 11 2,21 8 and 36 5,respectively.The ratio of L/H of complex sandwaves was normally larger than that of single sandwaves(the latter approaches to 69 3).Most of the sandwaves higher than 1 0m inclined to downstream while those lower than 1 0m were normally symmetric.The sandwaves were composed of fine sands,with d 50 being 0 154mm. F r=V/(gh ) 1/2 in the field where sandwaves were found was less than 0 2,confirming the theory of Karahan et al and disproving that of Simons et al.The migratory distance of sandwaves was calculated to be 1m to 5m in each flood or ebb tidal cycle in spring and normal tides and near 0m in neap tide.The migratory distance during ebb was always larger than that during flood.After a storm,the sandwaves were partly covered by muddy deposits.Sandy bed was proved to be essential for sandwave formation.Although F r in the North Passage and the South Passsage is larger than that in the South Channel,no sandwave was found there for the absence of sandy bed due to the development of maximum turbidity.Most of the study results from this bi directional tidal estuary were comparable with those from rivers except for the good symmetry and more complex processes in the evoluation and migration of the sandwaves.