长江三角洲对流域输沙变化的响应:进展与问题

Changjiang Delta Sedimentation in Response to Catchment Discharge Changes:Progress and Problems

流域变化对河流三角洲的影响已成为全球性的问题。由于流域水库建设的影响,长江输沙率自1980年以来呈现明显下降趋势,引发了研究人员对长江三角洲地貌和沉积响应问题的关注。在地貌上,海图对比和海岸冲淤观测的证据显示,长江输沙率目前已经低于冲淤平衡的临界值,使三角洲进入了蚀退阶段。分析表明,根据水下三角洲部分范围的沉积速率与河流输沙率的关系进行计算,会过低估计临界输沙率。对于水下三角洲整体而言,应根据沉积物滞留指数来更准确地估算临界输沙率,而且需要区分2个"临界输沙率",一个是保持三角洲体积的临界值,另一个是保持三角洲陆上部分面积的临界值。在沉积响应上,来自长江水下三角洲沉积中心的柱状样显示出沉积速率自下而上减小的趋势,反映了流域输沙率变化的影响。今后,如将柱状样采集区域扩大到沉积中心之外的整个水下三角洲范围,则所获得的数据不仅有助于沉积速率响应特征的全面了解,而且还可应用于沉积物滞留指数特征和控制因素的分析以及河口区碳循环过程变化的研究。此外,在长江三角洲沉积记录的分析方面,为了更好地提取气候、环境演化信息,应充分考虑沉积记录形成后所经历的自身变化(如成岩作用变化、分解和衰变等)和相对于岸线迁移的空间位置变化。

The influence of catchment changes on river deltas is an issue of global importance.As far as the Changjiang River is concerned,the fluvial sediment discharge has been decreasing since the early 1980s in response to catchment human activities,which results in significant sedimentological and geomorphological changes over the deltaic areas.In the present contribution,some research topics are proposed on the basis of a review of the progress made in recent years.In terms of geomorphology,bathymetric changes and coastal accretion/erosion patterns indicate that the sediment discharge is now below the critical quantity for the maintenance of the delta,i.e.,the delta has reached an eoroding stage.Analysis carried out in the present study demonstrates that the critical value on the basis of chart comparison and localized deposition rate measurements is actually underestimated.For the entire subaqueous delta,accurate calculations for the critical value will depend upon a sound understanding of the sediment retention of the deltaic system.Moreover,it would be appropriate to consider two critical discharges,one for the maintenance of the delta volume,and the other for the maintenance of the surface area of the delta on land.In sedimentology,sediment cores collected from the depocentre of the subaqueous delta show a trend of decrease in accretion rate over the last 30 years,representing the effect of catchment sediment discharge changes.In the future,analysis of the sediment samples from the entire sub-aqueous delta will be beneficial to an improved understanding of the deltaic response to the cathcment changes.At the same time,such a data set will be useful for the study of the sediment retention index and the material cycling patterns（e.g.carbon cycling） over the estuarine waters.Furthermore,in order to improve the interpretation of the deltaic sedimentary record,e.g.,to identify climate and environmental changes using the record,the various parameters derived from analyses of the cores should be standardized by evaluating the temporal changes caused by early diagenesis and decay,and by determining the rate of deltaic shoreline changes which control the relative position of the record within the deltaic sedimentary system.