Foundations for Physically-based, Multi-Dimensional River Hydrodynamic Models at the Watershed Scale

流域尺度的、经过校准的一维洪水模型与河段(小尺度)水动力模型在二维和三维上存在很大差距。后者对于水质、水生生境、沉积物迁移和地貌演变的模拟很有价值，但实际上不能扩大到整个流域。放大多维模型的主要方法问题是模型网格必须粗化以适应可用的计算机能力。对于粗略的模型网格，单个网格单元可能包含非常不同的底部类型和障碍物，因此需要特定于场地的网格相关校准。因此，扩大现有模型的比例，阻碍了过去50年来在实验室和现场研究中发展起来的关于小尺度湍流行为的知识的直接应用。这种知识很容易在现有的小规模模型中使用，以消除或减少校准要求。该项目将通过开发和测试“粗网格模拟(CGS)”的新理论，直接解决我们建模方法上的差距。对于这种新的建模方法，将大涡模拟(LES)形式应用于雷诺平均的N-S方程，建立了一组CGS方程，该方程明确地将网格相关的平均项与基本的湍流特性分开，这些基本的湍流特性可以由经验表征。需要证明/证伪的研究假设是，细网格模式中的细尺度过程可以在CGS模式中的任意网格尺度上表示，而不需要特别校准。该方法的未来发展和大规模验证将需要现场研究收集详细的速度和湍流数据，因此该项目包括使用最近开发的脉冲相干声学多普勒流剖面仪进行初步现场调查，以更好地了解可以合理收集的湍流和速度细节。这项研究对科学界的更广泛影响是，CGS为流域尺度的水动力学模型提供了一种新的实用方法，可用于驱动水质、水生生境和地貌研究。该项目还通过支持一名女性博士生，并为德克萨斯大学本科生(通过德克萨斯大学西班牙裔专业工程师协会或德克萨斯大学全国黑人工程师协会招募)提供机会，与NSF赞助的其他大学的REU学生一起进行实地研究，从而将教育和外联结合起来。这个项目的智能优点是一种全新的方法来框架湍流闭合问题，在粗略的网格分辨率下将网格尺度效应与湍流分开。这种新方法明确地处理了粗网格尺度的影响，并允许将未分辨(但经验上已知的)亚网格尺度特征产生的湍流积分到网格单元上。

There is a significant gap between catchment-scale, calibrated, one-dimensional flood models and river-reach (small-scale) hydrodynamic models in two and three dimensions. The latter are valuable for modeling of water quality, aquatic habitat, sediment transport, and geomorphologic evolution, but cannot be practically up-scaled to an entire river basin. The principal methodology problem for up-scaling multi-dimensional models is that the model grid must be coarsened to fit available computer power. With a coarse model grid, a single grid cell may contain very different bottom types and obstacles, therefore requiring site-specific grid-dependent calibration. As a consequence, up-scaling existing models prevents direct application of the knowledge of small-scale turbulent behavior that has been developed in laboratory and field studies over the last 50 years. Such knowledge is readily used in existing small-scale models to eliminate or reduce calibration requirements. This project will directly address the gap in our modeling methods by developing and testing a new theory for "Coarse Grid Simulation (CGS)". For this new modeling approach, the Large-Eddy Simulation (LES) formalism is applied to the Reynolds-Averaged Navier-Stokes (RANS) equations to create a set of CGS equations that explicitly separate grid-dependent averaging terms from fundamental turbulence properties that can be empirically characterized. The research hypothesis to be proved/falsified is that fine-scale processes in a fine-grid model can be represented at arbitrary grid scales in a CGS model without ad hoc calibration. Future development and large-scale validation of the method will require field studies that collect detailed velocity and turbulence data, so this project includes a preliminary field investigation with the recently-developed Pulse-Coherent Acoustic Doppler Current Profiler to gain a better understanding of the turbulence and velocity details that can reasonably be collected.The broader impact of the research on the scientific community is that CGS provides a new practical methodology for catchment-scale hydrodynamic models that can be used to drive water quality, aquatic habitat and geomorphological studies. This project also combines education and outreach by supporting a female Ph.D. student and providing an opportunity for a UT undergraduate student (recruited through the UT Society of Hispanic Professional Engineers or the UT National Society of Black Engineers) to work on the field study in concert with NSF-sponsored REU students from other universities. The intellectual merit of this project is an entirely new way of framing the turbulence closure problem that separates grid-scale effects from turbulence at coarse grid resolutions. The new approach explicitly treats the effects of a coarse grid scale and allows turbulence generated by unresolved (but empirically known) subgrid-scale features to be integrated over a grid cell.