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

了解水和沉积物如何通过三角洲系统的不同部分移动是至关重要的资源管理者负责关键沼泽和湿地栖息地的可持续未来。 河道分叉（在主河道中水流分为两个独立的下游河道的位置）是河流三角洲的常见自然特征。 尽管它们丰富，河流分叉尚未得到广泛的研究，因为它是传统的假设，悬浮泥沙“随流”-悬浮泥沙之间的下游分支分配成比例的水流量到每个下游分支。这一假设与众所周知的事实相反，即由于二次流和河道内源汇面积的变化，悬浮泥沙在河道上的分布通常是不均匀的。 这个博士论文研究改进项目的目标是通过识别水文和形态参数，通过分叉影响悬浮沉积物的划分，以解决这些相反的概念。 将在位于加州中部的萨克拉门托-圣华金三角洲受潮汐影响的河流分叉处收集高分辨率的现场数据。 仪器将包括一个船载三维声学多普勒剖面仪、若干光学后向散射传感器、一个全球定位系统以及一系列其他电子传感器和手动装置，以测量各种水文条件下整个分叉处的水流和悬浮泥沙分布。 将计算分叉的每个分支的剪切应力、湍流强度、动量通量和悬浮泥沙输运能力，并将其与观测到的泥沙浓度模式相关联。 现场结果将通过使用二维流体动力学模型进行补充，这允许外推到更长的时间框架（月到年）和替代地点，以产生关于分叉形态的周期性和长期演变的概括。河道中的分叉代表了河道网络中的“决策点”，其中悬浮泥沙优先流向下游分支而不是另一分支。 科学家们还没有破译这个复杂的“决定”是如何做出的，尽管它在河流三角洲和河口的发展中起着关键作用。 悬浮沉积物提供了潮汐沼泽栖息地生长的基础，知道为什么一个下游分支接收更多的悬浮沉积物比另一个将指导管理人员选择成功的沼泽恢复策略。 当防洪闸、屏障和水坝等流量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正处于巨大的环境压力下，这项研究有助于更好地了解塑造这一重要生态系统的过程。 虽然通道分叉处的过程很复杂，但现场技术和计算机建模的最新发展使研究人员能够首次解决基本科学理解中的这一失误。 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的研究生涯。

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.