Collaborative Research: A mechanistic understanding of hydrograph shape influence on temporal variations in bedload transport, grain size distributions, and armor persistence

世界各地的自然和人工管理的河流显示了各种各样的水文曲线，从浮华和突然到更缓慢的流量变化。水文曲线形状（通过峰值大小和排放变化率量化）显著影响可供人类利用和水生生态系统的水量、时间和水质。不确定性仍然存在，不稳定流如何影响河床通量和滞后，粒度分布，以及装甲层的持久性。缺乏对水文影响的机制和预测性理解也可能部分解释了为什么砾石河床中的河床输运方程往往是不准确的。研究小组提出了互补的水槽、野外和数值模拟实验，以量化水文对沉积物运动的影响。具体来说，他们假设水线形状会影响以下因素：(1)给定剪切应力下的床质输运率、滞后和可移动粒度，以及(2)床质粒度分布和床层装甲的持久性/程度。在水槽实验中，河床通量、粒度分布和护甲持久性的时间变异性将在一系列水流曲线形状下进行测量。野外试验将进一步研究水流线形状对河床输运速率、河床粒度变化、装甲持久性、流动性和移除的影响。水槽和现场的联合测量将用于测试和验证河床输运的数值和分析模型。水文曲线表示河流中的水量如何随时间变化，可以用流量变化的时间、幅度和速率来表征。人类对这些水文特征的改变对经济发展有积极影响，但对水生物种和河道形态有消极影响。这一点很重要，因为北半球大型河流总水量的77%受到水坝和改道的中度至严重影响。水资源管理者通常寻求通过河流恢复、水电再调节以及在某些情况下拆除大坝来评估和减轻这些影响。例如，砾石增加通常用于减少沉积物供应和改善水坝下方的鱼类栖息地，但由于缺乏对可变流量引起的沉积物运输的预测性理解，这种方法的结果往往好坏参半。管理人员可以利用这项研究的结果来确定水流曲线的形状，以潜在地减轻水流调节对受威胁或濒危水生生物的影响。此外，这项研究将导致对水流、沉积物运输和河床沉积物大小的时间变化之间相互作用的预测和机制理解。这些知识对于正确设计河流基础设施和修复项目，以及预测长期河道切割率和景观变化是必要的。此外，该项目将为研究生和本科生提供独特的教育机会，并支持“女性户外科学营”，鼓励年轻女性进入STEM领域。

Natural and managed rivers throughout the world display a wide range of hydrographs, from flashy and abrupt to more gradual changes in flow. Hydrograph shape (quantified by peak magnitude and rate of discharge change) significantly influences the volume, timing, and quality of water available for anthropogenic use and aquatic ecosystems. Uncertainties remain as to how unsteady flows that characterize hydrographs impact bedload fluxes and hysteresis, grain size distributions, and the persistence of armor layers. The lack of a mechanistic and predictive understanding of hydrograph influences may also partially explain why bedload transport equations in gravel-bed rivers are often inaccurate. The research team proposes complimentary flume, field, and numerical modeling experiments to quantify the impact of hydrographs on sediment movement. Specifically, they hypothesize that hydrograph shape influences the following: (1) bed load transport rates, hysteresis, and mobile grain sizes for a given shear stress, and (2) bed grain size distributions and the persistence/degree of bed armoring. In flume experiments, temporal variability in bedload fluxes, grain size distributions, and armor persistence will be measured for a range of hydrograph shapes. Field experiments will further investigate the influence of hydrograph shape on bedload transport rates, bed grain size changes, and armor persistence, mobility and removal. The combined flume and field measurements will be used to test and validate numerical and analytical models for bedload transport. Hydrographs represent how water quantities in rivers vary over time and can be characterized by the timing, magnitude and rates of change of flow. Human modifications of these hydrograph characteristics have positively influenced economic development but negatively impacted aquatic species and channel morphology. This is important because 77% of total water discharge from large northern hemisphere rivers is moderately to severely impacted by dams and diversions. Water resource managers commonly seek to assess and mitigate these impacts through river restoration efforts, hydropower re-regulation, and in some cases dam removal. For example, gravel augmentation is commonly used to mitigate for low sediment supplies and improve fish habitat below dams but this often has mixed results because of a lack of a predictive understanding of sediment transport caused by variable flows. Managers could use the results of this research to determine flow hydrograph shapes that potentially mitigate for flow regulation influence on threatened or endangered aquatic organisms. Further, this research will result in a predictive and mechanistic understanding of the interactions between flow hydrographs, sediment transport, and temporal variations in the size of sediment on the channel bed. Such knowledge is necessary to properly design river infrastructure and restoration projects, and predict long-term channel incision rates and landscape changes. Additionally, the project will contribute towards unique educational opportunities for graduate and undergraduate students and support for a Women Outdoors with Science Camp will encourage young females to enter STEM fields.