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).

To evaluate the effects of the projected measures and to check the hydraulic calculations and assumptions of the Rhesi project, VAW of ETH Zurich has been commissioned with hybrid model experiments on behalf of the International Rhine Regulation (IRR) body. These investigations consist of two main parts: 1) experiments in a physical hydraulic model and 2) accompanying numerical simulations.

“Two key project sections are replicated consecutively at a scale of 1:50 in extensive hydraulic models. For each section, a flow length of approximately 5 km is replicated (around 110 m in model scale) with watercourse widths ranging from 250 m to 350 m (around 8 m in model scale),”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).

3D terrain modelling for flood modelling

“在洪水事件中，由于水排放和流速较高，河床会发生重大变化。因此，沉积物可以沉积在多个位置，导致水位上升，也可以侵蚀，例如在桥墩或河岸周围。两种情况都可能是危险的，对防洪产生负面影响。为了复制这些形态学变化，液压模型配备了可移动的河床。”Hinkelammert-Zens说。

为了观察不同的沉积物负荷和各种情况的影响，进行了大量具有不同参数（例如水排出和沉积物负荷）的科学实验。通过激光扫描仪，在每个实验之前和之后都测量模型地形。然后，获得的数据用于创建地形模型，以确定河床中沉积和侵蚀的区域的基础。

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

To capture the topographical data before and after each experiment, the research team of ETH Zurich relies on aLeica ScanStation P20, Leica Geosystems targets and a Leica TS02total station为了地理参考，激光扫描15个参考点。扫描仪P20安装在移动三脚架上，并部署在四个扫描位置上以捕获整个型号。扫描高度约为2.7 m-如果观看角度太陡并避免死角，则最大程度地减少阴影效果 - 在与设备的径向距离为10 m的径向距离下，分辨率为3 x 3 mm，可以获得具有非常低噪声的高质量数据。

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 m²is represented with approximately 250 million points. The point cloud is then ‘trimmed’ using polygons to cut-off the data points outside of the model boundaries. The remaining data points are then transformed into grid cells with a cell size representing 50 cm x 50 cm in real life. Finally, the topographical data is converted into the Swiss National Coordinate System.

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,”explains Hinkelammert-Zens.

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

• Surface views: The grid values of the scan made at the beginning of the experiment are deducted from those made at the end of the experiment. In this way, theETH teamcreates a view where the relative differences in the height of the model riverbed are visible.

• 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.

• Longitudinal profiles: The extracted cross profiles are averaged for the longitudinal profile. By comparing the averaged riverbed elevations before and after the experiments and by observing the changes in nature, the team of experts can validate the hydraulic model.

Intermediate results and future steps

苏黎世Eth的VAW的调查已经为进一步发展的Rhesi项目提供了重要的投入。首先，通过复制过去的洪水事件对模型进行了校准。在此过程中，将液压模型中获得的水位和河床地形与全面的事件中捕获的数据进行了比较。成功完成此步骤后，液压模型适应了Rhesi预测的河流的未来形状。从那时起，已经模拟了各种长期情景和高洪水事件，以研究Rhesi项目对河流形态和水位的影响。

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?
• 河岸必须支持多深tected against erosion and scouring?
• How can bridge piers be secured against erosion and scouring?
• What is the amount of driftwood clocked at bridges during flood events? What will be the effect on the water levels?

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.