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Application of Numerical Modeling to Study River Dynamics: Hydro-Geomorphological Evolution Due to Extreme Events in the Sandy River, Oregon

机译：数值模拟在河流动力学研究中的应用：俄勒冈州桑迪河极端事件引起的水文地貌演化

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摘要

The Sandy River (OR) is a costal tributary of the Columbia River and has a steep hydroshed 1316 square kilometers which is located on the western side of Mount Hood (elevation range 3 m to 1800 m). The system exhibits highly variable flow: Its average discharge is ~40 m3/s, and the highest recorded discharge was 1739 m3/s in 1964. In this study I model the geomorphic sensitivity of an 1800m reach located the downstream of the former Marmot Dam, which was removed in 2007. The hydro-geomorphic response to major flood has implications for system management and aquatic life.Studying hydro-geomorphic change requires a systematic approach. Here, I define flows and flood hydrographs for specified return interval based on the observed hydrologic record, and then examine potential hydro-geomorphic changes using a numerical model. A Pearson Type III distribution is used to calculate 100, 75, 50, 25, 10, and 2 year return periods. Extreme event hydrographs are derived by fitting derived and observed flood hydrographs to the gamma distribution curve. Sediment transport and geomorphology are then modeled numerically with Nays2DH, a solver that is part of iRIC software. Because the model is computationally intensive, I model the domain with five different spatial grid resolutions, to find proper grid resolution. The grid resolutions used are 1.5 m, 2 m, 3 m, 4 m, and 5 m. We choose 4 m as optimum grid resolution, based on the convergence of model results. The model is run for extreme event hydrographs with six above return periods. For result visualization and analysis, we focus on flow properties and bed elevation at peak flow and at the end of each event. For both times for each event, important flow and sediment transport parameters are visualized for the entire domain in plane form and eight cross-sections at 200 m intervals. Finally, we divide the geomorphic response into areas of erosion and deposition. Linear regression analyses of mean values of erosion and deposition at peak flow for all extreme events yield R2 of 0.981 for erosion and 0.986 for deposition. The mean erosion and deposition depth at the end of the events is modeled by nonlinear regression with correlation coefficient of 0.965 for erosion and 0.998 for deposition. The regression models provide direct understanding of impacts of different floods on the geomorphic response of the river domain. examination of the model as a whole suggest that the amount of erosion and deposition in the bed and banks is a function of channel geometry, bank and bed geology, riparian area condition and strongly depend on the amount of flow through the channel.
机译：桑迪河（OR）是哥伦比亚河的沿海支流，位于胡德山（Mount Hood）西侧（海拔3 m至1800 m），有陡峭的分水岭1316平方公里。该系统显示出高度可变的流量：其平均流量约为40 m3 / s，1964年的最高记录流量为1739 m3 / s。在本研究中，我对位于前土拨坝下游1800m的地貌敏感性进行了建模。 ，该水文地貌对大洪水的响应对系统管理和水生生物都有影响。研究水文地貌变化需要系统的方法。在这里，我根据观察到的水文记录定义了指定返回间隔的流量和洪水水位图，然后使用数值模型检查了潜在的水文地貌变化。使用Pearson III型分布来计算100、75、50、25、10和2年的回报期。极端事件水位图是通过将派生的和观测到的洪水水位图拟合到伽马分布曲线而得出的。然后使用iRIC软件的求解器Nays2DH对沉积物的传输和地貌进行数值建模。由于该模型需要大量计算，因此我使用五种不同的空间网格分辨率对域进行建模，以找到合适的网格分辨率。使用的网格分辨率为1.5 m，2 m，3 m，4 m和5 m。基于模型结果的收敛性，我们选择4 m作为最佳网格分辨率。该模型适用于具有六个以上返回期的极端事件水位图。为了进行结果可视化和分析，我们重点研究峰值流量和每个事件结束时的流量属性和床高。对于每个事件，两次都以平面形式和200 m间隔的八个横截面显示整个区域的重要流量和泥沙输送参数。最后，我们将地貌响应分为侵蚀和沉积区域。对所有极端事件在峰值流量下的侵蚀和沉积平均值进行线性回归分析，得出R2为0.981，侵蚀为0.986，沉积为R。事件的平均侵蚀和沉积深度通过非线性回归建模，侵蚀的相关系数为0.965，沉积的相关系数为0.998。回归模型可以直接了解不同洪水对河域地貌响应的影响。对模型的整体检查表明，河床和河床的侵蚀和沉积量是河道几何形状，河床和河床地质，河岸区域条件的函数，并且很大程度上取决于通过河道的流量。

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