基于径-潮流能耗的长江口南支河势稳定性研究

A stable river regime is one of the main targets in channel regulation. Hydrology and energy are important physical parameters in estuary evolving process. The seasonal variations of riverine discharges cause a series of complex interactions between river and tide, thus forming complicated feedback mechanism among river, tide and geo-morphology. The Yangtze estuary is a widely recognized “golden channel” for navigation in the world, intensively influenced by forces from both river and tide. Under great impact of climate change and human activities, the morph-dynamics conditions in Yangtze estuary become even more unpredictable. Therefore, river-tide interaction and human activities are the key factors to understand regime development and changes in Yangtze estuary. Based on the above background, this proposal is aimed to assess flow regime equilibrium as well as evolution in the south branch of Yangtze estuary by the approach of achieving optimized morphology self-organizing simulation and developing a series of indexes to quantify morphology, hydrology and energy (flux). This process applies field measurements and historical records, in addition with numerical modeling and analytical modeling, which is conducted under consistent framework of both river and tide, on the principle of most efficient energy dissipation. The study is intended to produce two outcomes, firstly to evaluate regime stability under river discharges of averaged wet season and dry season; secondly to analyze inter-annual regime equilibrium variation with the influence of human activities (estuary engineering). This study will contribute to reaching further understanding of regime equilibrium, bring values on managing channels as well as enhance the engineering safety.

河势稳定是河道整治的重要目标之一。水文和能量是河势演变过程中重要的物理量，径流的季节性变化引起潮流和径流之间一系列复杂相互作用，形成河流、潮流和地貌之间的复杂反馈机制。长江口是世界闻名的黄金水道，受径流和潮流作用强烈，加之人类工程扰动，导致动力条件变得十分复杂。因此，径-潮流交互作用及人类工程扰动对长江口河势的变化与发展至为重要。本项目以此为背景，以长江口南支为研究对象，采用实测及历史时期的数据和模型，在径-潮流统一的计算框架下，运用数值模拟和解析模拟两种水动力模式，根据最优化能耗率原则，实现最优化地形自组织模拟，同时发展定量化计算地形、水文和能量（通量）指标，评价长江口南支河势平衡与演变趋势，实现对①年内平均洪季和平均枯季径流作用下河势稳定性评估；②强烈人类活动（河口工程建设）影响下年际间河势稳定性变化研究。本研究将对理解长江口河势稳定性及河道整治和工程安全具有重要学术意义和应用价值。

河势稳定是河口安全与健康的重要指标。受季节性径流、周期性风浪和强烈人类活动影响，许多大河河口三角洲的水、沙动力条件发生了巨大变化，形成河流、潮流和地貌之间的复杂反馈机制。长江口是世界闻名的黄金水道，受径流、潮流和风浪作用强烈，加之人类工程扰动，导致动力条件变得十分复杂。本项目以此为背景，以长江口600km河段（包括长江航道、南支和水下三角洲）为研究对象，采用实测、卫星遥感技术和历史数据分析，在径-潮流统一计算框架下，运用数值模拟法和解析模拟法，根据最优化能耗原则，实现地貌自组织模拟，研究内容含盖了①径流、潮流和风浪等自然驱动力对长江口河势稳定性影响；②强烈人类活动（三峡建坝、河口围垦、水库建设和入海泥沙锐减）对河势稳定性变化的影响；③实现了数值模拟法和解析模拟法的对比研究，分区域探讨了河流段、南支和水下三角洲河势稳定性，总结不同区域演变规律；④从影响河势演变基本动力因素着手，发展了沉积动力理论、系统熵增理论和自组织理论，实现河势局部演变与全局变化相联系的研究。结果表明（1）近半个世纪以来（1958~2010）影响长江口河势稳定性的人类工程措施，导致系统远离平衡态达2.5~3%，然而1987年至1997年间河势自然演化具有一定的恢复（平衡）作用；（2）长江口大规模围垦前，1958年长江特大洪水导致系统脱离平衡态达3~6%，但经过关键结点（徐六径）围垦治理、束水攻沙，加大了潮下带体积，使系统略微趋向于平衡态1~2%；（3）在季节性风浪、径流作用下，长江口水下三角洲波-流耦合模拟结果分析表明河口拦门沙具有“冬输夏储”的重要特征，这一规律对长江口“亚三角洲”开发及“深水航道”建设具有重要意义；（4）长江三峡工程洪、枯季调蓄作用和长江口下游“回水效应”引起的潮波不对称性对近期长江口河势重新调整及演变至关重要。总体而言，长江口作为一个大河口系统具有一定的自动调节能力（自组织能力），能够抵抗一定强度的外部扰动，实现受约束条件下河势的动态平衡和调整。