沉积学报2025，Vol.43Issue(3)：894-911,18.DOI:10.14027/j.issn.1000-0550.2024.087
平衡指数解释深水、浅水三角洲地貌的差异
Principal Differences Between Deep-water and Shallow-water Delta Landforms:An interpretation by the Grade Index Model
摘要
Abstract
[Objective]Recent research suggests that the basin water depth governs the morphodynamics of deltas that formed at the basin margin,because water depth affects the amount of subaerial and subaqueous sediment that are deposited.Shallow-water deltas contain more sediment than deep-water deltas in the subaerial region.As a result,aggradation of distributary channels takes place more rapidly in shallow-water deltas,making the channel more active in terms of migration and avulsion.The grade index(Gindex)model is proposed to quantitively illustrate this process.[Methods]This study elaborates the origin,theoretical modeling,experimental validation and application of the grade index model,and discusses its limitations.In this context,grade refers to the state of a river stream in which sedimentary material is all transported by the river flow without net deposition or erosion taking place.The grade in-dex is defined as the ratio of the volume of sediment allocated subaerially to the total volume of sediment input in per unit time.By this definition,Gindex is a dimensionless number between 0(no deposition subaerially)and 1(complete deposition subaerially).[Results]Theoretical analyses confirm a negative relationship between Gindex and water depth.It is also related to the geometry of the delta(e.g.,delta plain radius,topset slope and foreset slope).In basins with deeper water,Gindex → 0,which means that decreasing volumes of sediment are deposited subaerially per unit time,forming a more stable channel that approaches the equilibrium condition or'state of grade'of the alluvial river,when neither erosion nor deposition takes place.Conversely,Gindex → 1 for deltas developed in shallower water ba-sins,and the delta plain becomes increasingly unstable.The value of Gindex reflects basic morphodynamic parameters of the delta(e.g.,rates of progradation,aggradation and channel migration,and the timescale of channel avulsion).Each of these parameters can be calculated as the product or quotient between Gindex and their counterparts obtained with negligibly small basin water depth,while the former is determined by the delta's geometrical parameters and ba-sin water depth and the latter is determined by the delta's geometrical parameters and total sediment supply rate.This means that for a particular deltaic system with specific geometrical parameters,sediment supply rate and basin water depth,it has theoretical values for the grade index and geomorphodynamic parameters,both of which can be calculat-ed.This speculation was verified by tank experiment.The Gindex model is derived from the global mass balance of the deltaic system.Local and/or tentative depositional,erosional and dispersal processes(e.g.,backwater effect and coastal processes including waves,tides and longshore currents,as well as effects unrelated to the depositional system(vegetation and/or anthropogenic processes))were not considered.[Conclusions]The grade index model iso-lates the effect of basin water depth from other variables in describing delta morphodynamics,and reveals the princi-pal differences between the formation of deep-water and shallow-water delta landforms.It also goes some way toward explaining the influence of factors other than water depth.The model has the potential for general application to modern alluvial-deltaic systems.Its application to ancient systems has yet to be explored.关键词
三角洲/水深/地貌动力学/平衡指数/水槽实验Key words
Delta/water depth/morphodynamics/grade index/tank experiment分类
天文与地球科学引用本文复制引用
王俊辉,张伟,李莉,鲜本忠,周源..平衡指数解释深水、浅水三角洲地貌的差异[J].沉积学报,2025,43(3):894-911,18.基金项目
国家自然科学基金项目(42172108) （42172108）
中国石油大学(北京)科研基金项目(2462020BJRC002,2462022YXZZ010)[National Natural Science Foundation of China,No.42172108 （北京）
Science Foundation of China University of Petroleum,Beijing,No.2462020BJRC002,2462022YXZZ010] （）
