HBM Force Sensors Measure the Impact of Waves on Flood Defenses

Due to rising sea levels, the need for construction of flood defenses is becoming increasingly urgent. A major component in the development of new flood defenses is measuring the impact of waves on structures. Brazilian Ermano de Almeida carried out a project in the Fluid Mechanics Laboratory of the Faculty of Civil Engineering at TU Delft during the summer of 2018 to measure the impact of waves on flood defenses with overhangs. He used force sensors from HBM to do this. Additionally, the project also involved other partners such as the Netherlands Organisation for Scientific Research (NWO), Rijkswaterstaat, Witteveen+Bos, PT Structural, and Deltares, the Dutch knowledge and innovation institute for water and subsoil. The results of the research will be relevant in developing new and upgraded hydraulic structures, such as the water outlets on the Afsluitdijk.

Problem: Floods and dike breaches are causing considerable problems and damage across the world. 
Solution: By collecting more information about the power of the impact of waves, research should be able to make sturdy and safe structures without making them oversize (as could be the case today). 
Results: The raw data from the physical tests will be processed and analyzed. The result will be valuable information for the engineering and construction of new flood defenses.

Measure the strength of the impact that waves exert on vertical structures

Ermano de Almeida is currently working on the impact of waves on models of water defenses made of steel as part of the research for his PhD in the Hydraulic Engineering department at the Delft University of Technology. He studied Civil Engineering in Madrid and Aachen and obtained his master’s degree from TU Delft. When he got the chance to carry out the research project on flood defenses, he seized it with both hands. This was not only because he found the project interesting both theoretically and practically but also because managing and controlling water is one of the most important challenges in the coming decades.

De Almeida’s current research focuses on vertical steel and concrete hydraulic structures. Such structures can be found in ports and at locks and water outlets. They have also been used extensively in Dutch flood defenses along the coast of the North Sea. They are used there in combination with other hydraulic and mechanical installations to close the water outlets or waterways at high tide or during a storm surge. As a flood defense, this type of construction is particularly suitable for places where the opening of the gates for water flow and transportation are required, while also being able to remain closed and provide safety against flooding during storms.

The aim of the research project is to measure the strength of the impact that waves exert on vertical structures. These forces are particularly massive on vertical structures with an overhang because the strength of the wave cannot be diverted upwards. Furthermore, this also increases the pressure on the vertical portion of the structure. Additionally, the waves also exert an enormous force on the overhang. The overhang could be, for instance, a concrete protective edge above the flood defense or the ceiling of the opening from which partitions are suspended. According to De Almeida, investigating the effect of the resonance and vibration of the relatively thin metal plates under such load conditions on the service life of the structure and the material used is highly interesting.

“The number of situations in which such hydraulic structures has failed is not very high,” he explains, “but there are several cases known in which a lack of clear knowledge about the load characteristics and the structural response has led to unsafe conditions, and sometimes even to structural failure. By collecting more information about the power of the impact of waves, we are better able to make sturdy and safe structures without making them oversize as could be the case today. As a result, we save time, material and costs during construction. Perhaps we can even develop new ways of building this type of flood defense, based on the research.”

Water basin and force sensors

The tests were carried out in the Fluid Mechanics Laboratory at TU Delft. The laboratory has an area of approximately 5,000 m², 1,700 m² of which are used for carrying out experiments. There are eight water channels available for experiments which can provide a water flow of 2 cubic meters per second. De Almeida’s experiments were carried out in one of the largest water channels in the laboratory which is 42 m long, 80 cm wide and 1 m deep. At the end of the channel, there is a solid concrete block of 80 x 80 x 100 cm, on which an aluminum plate 1 cm thick is mounted using aluminum sections. Nine HBM U3 force sensors are installed in a row between the aluminum frame and the metal plate, with a range of 1.0 kN. HBM recommended that the researchers use these particular load cells because they are made from stainless steel, are not susceptible to temperature effects and are, therefore, ideally suited to the wet test setup. Moreover, they have high intrinsic stiffness, compensate for bending moments and are insensitive to lateral forces, making the measurement results highly reliable.

The sensors are linked to measuring amplifiers developed by TU Delft itself. They record the impact of waves at a sample rate of 5000/s. De Almeida can view the measurement data in real time on a monitor screen which shows the height of the waves and the impact on the structure. The setup is flexible, thereby allowing him to adjust the height of the metal plate, the dimensions of the overhang and the positions of the force sensors.

A series of regular and irregular waves are created in the water channel using a wave generator, which is equipped with active reflection compensation that neutralizes returning waves in order to not influence the measurement data. Eight sensors, which measure the characteristics of the waves, hang about forty centimeters apart in the water, a few meters in front of the metal plate. Three cameras record the images of the tests, thereby making it possible to study the associated images for special measurement results.

Afsluitdijk discharge sluices as a reference

During the design of the test setup, the discharge sluices of the IJsselmeer in the Afsluitdijk were used as the initial reference because the types of waves and their impact examined in the research strongly resemble those observed in these structures. There are water outlets underneath part of the dike which are closed with steel gates during high tides and storms. The waves that hit the gates in the outlets under the horizontal defense beam are suddenly blocked during storms and, thereby, exert enormous forces on the structure. This structure was designed 80 years ago and is being renovated, along with the entire Afsluitdijk. De Almeida also aims to apply the knowledge developed during his research to other locations worldwide, such as in the United States, where there is a great deal of interest in the results of the research due to the incidents of serious flooding in the country and the standing reputation of the TU Delft in the field of hydraulic engineering.

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