Doctoral Dissertation Research: Spatial and Temporal Patterns of Suspended Sediment Transport at a Tidal River Bifurcation

博士论文研究：潮汐河分叉处悬浮泥沙输送的时空模式

• 批准号: 0402104
• 负责人: John Wilson
• 金额: $ 1.2万
• 依托单位: University of Southern California
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 2004
• 资助国家: 美国
• 起止时间: 2004-03-15 至 2006-02-28
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=0402104&HistoricalAwards=false
• 关键词: Doctoral; Dissertation; Research; Spatial; Temporal

Understanding how water and sediment move through different parts of a deltaic system is of fundamental importance to resource managers who are charged with sustainable futures of critical marsh and wetland habitat. Channel bifurcations (locations in a primary channel where the flow divides into two separate downstream channels) are common natural features of fluvial deltas. Despite their abundance, river bifurcations have not been studied extensively because it is conventionally assumed that suspended sediment "goes with the flow" -- that suspended sediment is partitioned between the downstream branches in proportion to water discharge into each downstream branch. This assumption is contrary to the well-known fact that suspended sediment is typically distributed unevenly across channels because of secondary flows and because of varying source-sink areas within a channel. The goal of this Doctoral Dissertation Research Improvement project is to address these contrary notions by identifying the hydrologic and morphologic parameters that influence the division of suspended sediments through a bifurcation. High-resolution field data will be collected in a tidally influenced river bifurcation on the Sacramento-San Joaquin Delta located in central California. Instrumentation will include a boat-mounted, three-dimensional Acoustic Doppler Profiler, several Optical Backscatterance Sensors, a Global Positioning System, and a range of other electronic sensors and manual devices to measure water flow and the distribution of suspended sediment throughout the bifurcation over a variety of hydrologic conditions. Shear stress, turbulence intensity, momentum flux, and the suspended sediment transport capacity of each branch of the bifurcation will be calculated and related to the observed pattern of sediment concentrations. The field results will be complemented by the use of a two-dimensional hydrodynamic model, which allows extrapolation to longer time frames (months to years) and to alternative sites in order to produce generalizations about cyclic and long-term evolution of bifurcation morphology.A bifurcation in a river channel represents a "decision point" in a channel network where suspended sediment is preferentially directed towards one downstream branch over another. Scientists have yet to decipher how this complex "decision" is made despite its critical role in the development of river deltas and estuaries. Suspended sediment provides the base on which tidal marsh habitat grows, and knowing why one downstream branch receives more suspended sediment over another will guide managers in selecting successful marsh restoration strategies. This is particularly important when flow control structures such as floodgates, barriers, and dams modify the natural dynamics of the system. The Sacramento-San Joaquin Delta-Estuary system is under immense environmental stress and this study contributes to a larger understanding of the processes that shape this vital ecosystem. While the processes at a channel bifurcation are complex, recent developments in field techniques and computer modeling allow researchers to address this lapse in basic scientific understanding for the first time. As a Doctoral Dissertation Research Improvement award, this award also will provide support to enable a promising student to establish a strong independent research career.

了解水和沉积物如何在三角洲系统的不同部分移动，对于负责关键沼泽和湿地栖息地可持续未来的资源管理者来说至关重要。河道分岔（河流在主河道中分成两个独立的下游河道的位置）是河流三角洲常见的自然特征。尽管河流分岔丰富，但由于传统上认为悬浮泥沙“随流而动”——即悬浮泥沙在下游支流之间按向每个下游支流的水量成比例划分，因此尚未对其进行广泛研究。这一假设与众所周知的事实相反，即由于二次流动和河道内不同的源-汇面积，悬浮泥沙通常在河道间分布不均匀。本博士论文研究改进项目的目标是通过确定通过分岔影响悬浮沉积物划分的水文和形态参数来解决这些相反的概念。高分辨率的野外数据将在加州中部萨克拉门托-圣华金三角洲潮汐影响的河流分叉处收集。仪器将包括一个安装在船上的三维声学多普勒分析器，几个光学背散射传感器，一个全球定位系统，以及一系列其他电子传感器和手动设备，用于测量水流和各种水文条件下整个分岔处悬浮沉积物的分布。将计算分岔各分支的剪应力、湍流强度、动量通量和悬沙输运能力，并将其与观测到的泥沙浓度模式联系起来。实地结果将由使用二维水动力模型加以补充，该模型允许外推到更长的时间框架（数月至数年）和其他地点，以便对分岔形态的循环和长期演变作出概括。河道的分叉代表了河道网络中的一个“决定点”，在这里，悬浮沉积物优先流向下游的一个分支，而不是另一个分支。尽管这个复杂的“决定”在河流三角洲和河口的发展中起着至关重要的作用，但科学家们尚未破译它是如何做出的。悬浮沉积物为潮汐沼泽栖息地的生长提供了基础，了解为什么一个下游分支比另一个分支接收更多的悬浮沉积物将指导管理者选择成功的沼泽恢复策略。当闸门、屏障和水坝等水流控制结构改变系统的自然动力学时，这一点尤为重要。萨克拉门托-圣华金三角洲-河口系统承受着巨大的环境压力，这项研究有助于更深入地了解塑造这一重要生态系统的过程。虽然通道分叉的过程很复杂，但现场技术和计算机建模的最新发展使研究人员第一次能够解决基础科学理解中的这一缺陷。作为博士论文研究改进奖，该奖项还将提供支持，使有前途的学生建立一个强大的独立研究生涯。