HBM Force Sensors Measure the Impact of Waves on 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.