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

流域尺度的一维洪水模型与河段（小尺度）的二维和三维水动力模型之间存在着很大的差距。 后者是有价值的水质，水生生物栖息地，泥沙输运和地貌演变的建模，但实际上不能放大到整个流域。 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，因此需要特定于站点的网格相关校准。 因此，现有模型的升级阻止了在过去50年中在实验室和现场研究中开发的小尺度湍流行为知识的直接应用。 这种知识很容易用于现有的小规模模型，以消除或减少校准要求。 这个项目将通过开发和测试一种新的“粗网格模拟（CGS）"理论来直接解决我们建模方法中的差距。 对于这种新的建模方法，大涡模拟（LES）的形式主义应用于雷诺平均的Navier-Stokes（RANS）方程，创建一组CGS方程，明确分离的网格相关的平均项的基本湍流特性，可以凭经验表征。 要证明/证伪的研究假设是，细网格模型中的细尺度过程可以在CGS模型中以任意网格尺度表示，而无需特别校准。 该方法的未来发展和大规模验证将需要收集详细的速度和湍流数据的实地研究，因此，该项目包括一个初步的实地调查与最近开发的脉冲-相干声学多普勒海流剖面仪，以更好地了解湍流和速度的细节，可以合理地收集。更广泛的影响，科学界的研究是，CGS提供了一个新的流域尺度水动力学模型的实用方法，可用于推动水质，水生生境和地貌研究。 该项目还通过支持一名女博士，将教育和外联结合起来。学生，并提供了一个机会，为UT本科生（通过西班牙裔专业工程师UT社会或黑人工程师UT国家社会招募）在现场研究工作与NSF赞助的REU学生从其他大学。该项目的智力价值是一种全新的方式来构建湍流闭合问题，该问题将粗网格分辨率下的网格尺度效应与湍流分离。 新的方法明确对待粗网格尺度的影响，并允许未解决的（但经验已知的）亚网格尺度的功能所产生的湍流被集成在一个网格单元。

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.