Protecting the Alpine Rhine Valley from flooding

Author: Renata Barradas Gutiérrez

The Rhine, one of the main rivers in Europe, sources from the Swiss Alps in the canton of Grisons. The Alpine Rhine Valley extends over 90 km along the Rhine from its source in Switzerland via Liechtenstein to Austria. The Valley has a history of devastating flood events that date back to the 11th century. Today, around 300,000 people live in the lower Rhine Valley and numerous companies, including Leica Geosystems, flourish in this area. Due to the intense population and major economic activities in the Rhine Valley, damage potential from major flood events is estimated at EUR 10 billion.

To protect people, settlements, and as economic activities in the Valley, more room for flood runoff and water retention needs to be given to the Alpine Rhine. Therefore, the flood protection project “Rhein – Erholung und Sicherheit” (“Rhine - Recreation and Safety”) – or shortRhesi- seeks to increase the flow capacity of the Alpine Rhine from 3,100 m³/s to at least 4,300 m³/s on the international stretch between kilometre 65 at the junction of the tributary river Ill and km 91, where the Alpine Rhine discharges into Lake Constance. The project costs, funded equally by Austria and Switzerland, are currently estimated at EUR 1 billion.

“To achieve the requested level of flood protection, the channel geometry of the Alpine Rhine needs to be altered to enhance flood protection along the project perimeter. In the Rhesi project, a very modern approach has been chosen: instead of raising the river’s levees to take account of the elevated discharge of 4,300 m³/s，所需的流程部分将通过将河流宽度从目前的60至70 m增加到将来增加数百米来创建。由于过去150年中各种河流恢复措施，目前的河道河道目前具有非常技术性的形状，它将通过这种情况，以模仿人类干预前模仿河流状态的近天然状态，”explains Florian Hinkelammert-Zens, environmental engineer at the Laboratory of Hydraulics, Hydrology and Glaciology (VAW) at the Swiss Federal Institute of Technology Zurich (ETH).

为了评估预计措施的效果，并检查了恒河部项目的液压计算和假设，苏黎世Eth的VAW已代表国际莱茵河调节（IRR）机构进行了混合模型实验。这些研究由两个主要部分组成：1）在物理液压模型中进行实验和2）伴随的数值模拟。

“在广泛的液压模型中，两个关键项目部分以1:50的比例连续复制。对于每个部分，复制大约5 km的流长（模型尺度约为110 m），水道宽度范围为250 m至350 m（型号尺度约为8 m），”says Hinkelammert-Zens. “At the same time, numerical computer models of the project were created to provide and evaluate the boundary conditions of the hydraulic models, to validate the results and to carry out sensitivity analyses.”

As a result, these two hydraulics models are among the biggest models of alpine rivers ever built, with average dimensions of 110 x 9 m. Both are located in an old factory building in Dornbirn, Austria, where ETH Zurich designed a water circuit with a discharge of 400 l/s. The system consists of a high-level tank, inlet and outlet basins, a water return line in the basement and a deep tank, from which the water is pumped back into the high-level tank (max. 400 l/s).

洪水建模的3D地形建模

“在洪水事件、河床significant changes due to high water discharges and flow velocities. Hence, sediment can be deposited at several locations, leading to rising water levels, or can be eroded, e.g. around bridge piers or along the river banks. Both scenarios can be dangerous and have a negative effect on flood protection. To replicate these morphological changes, the hydraulic models are equipped with movable riverbeds.” says Hinkelammert-Zens.

To observe the impact of different sediment loads and various scenarios, a large number of scientific experiments with varying parameters (e.g. water discharge and sediment load) are conducted. By means of a laser scanner, the model topography is measured before and after every single experiment. The acquired data is then used to create terrain models which serve as a basis for the determination of areas where sedimentation and erosion occur in the riverbed.

从数据捕获到可操作的数据

右：Alpine Rhine的一部分的3D地形模型（在流动方向上查看） /左：实验结束后液压模型中的可移动河床

为了捕获每个实验之前和之后的地形数据，ETH Zurich的研究团队依赖于Leica ScanStation P20, Leica Geosystems targets and a Leica TS02total stationto geo-reference the laser scans with 15 reference points. The ScanStation P20 is mounted on a mobile tripod and deployed on four scanning positions to capture the whole model. With a scanning height of approx. 2.7 m - to minimise shadowing effects if viewing angles are too steep and to avoid dead angles – and a resolution of 3 x 3 mm at a radial distance of 10 m to the device, high-quality data with very low noise can be obtained.

After each experiment, the data is imported intoLeica Cyclone3D point cloud processing software to register the data and merge the point clouds. At this point, an area of 4000 m²约有约2.5亿点代表。然后使用多边形“修剪”点云，以切断模型边界之外的数据点。然后将其余的数据点转换为网格单元，其细胞大小代表现实生活中的50 cm x 50 cm。最后，将地形数据转换为瑞士国家坐标系。

Right: visualisation of the observed changes in the riverbed in the hydraulic model after evaluating the laser scan (red: erosion on the outside of the curve, blue: sedimentation on the inside of the curve, viewed in flow direction)/ Left: laser scan in the experiment hall (viewed in flow direction)

“The 3D point clouds are used to create grid datasets with approximately 15 million grid cells with a resolution of 0.5 x 0.5 m, each of them representing one distinct point of time during the experiments. This data is then further processed in geo-information systems in order to create surface views as well as longitudinal and lateral profiles of the mobile riverbed. This enables us to compare different points in time of the experiment with each other,”Hinkelammert-Zens解释.

The referenced grid dataset can be used in GIS applications for various evaluations, including:

• 表面视图：在实验开始时进行的扫描的网格值是从实验结束时扣除的。这样，ETH团队创建一种观点，使模型河床高度的相对差异可见。

• Transverse profiles: The team creates cross profiles at certain positions, extracting grid values to create lateral profiles. Using the scans before and after the tests, the experts can visualise the observed changes and compare them to the project goals.

• 纵向曲线：提取的跨轮廓平均用于纵向轮廓。通过比较实验前后的平均河床高程以及观察自然界的变化，专家团队可以验证液压模型。

Intermediate results and future steps

The investigations by VAW of ETH Zurich already led to significant inputs for the further development of the Rhesi project. At first, the model was calibrated via the replication of past flood events. During this process, the water levels and riverbed topography obtained in the hydraulic model were compared to data captured during those events in full-scale. After successful completion of this step, the hydraulic model was adapted to the future shape of the river, as projected in Rhesi. Since then, various long-term scenarios and high flood events have been simulated to investigate the effects of the Rhesi project on the river morphology and water levels.

随着调查仍在进行中，只能引用中间结果。到今天为止，结果表明，瑞典项目的假设和预测是正确的，并且是详细详细阐述未来项目阶段的扎实基础。混合模型实验将持续到2022年夏季，探讨以下技术问题的答案：

• 砾石银行将在哪里定位？
• Where will depressions resp. scours occur and what will be their maximum depth?
• 必须多深river banks be protected against erosion and scouring?
• How can bridge piers be secured against erosion and scouring?
• 在洪水事件中，桥梁在桥梁上的浮木数量是多少？对水位有什么影响？

The findings of these scientific experiments, supported by reality capture technology from Leica Geosystems, are the basis to ensure sustainable river planning and assure that the Rhesi flood protection project is technically and economically viable. This integrated flood risk management approach will significantly reduce flood risks and improve the ecological and recreational value of the Alpine Rhine in the international stretch.