R&D highlights edition 2019

PROJECTS Delta infrastructure WATER OVERFLOWING OUR STORM SURGE BARRIERS: WHAT CAN THEY HANDLE? W ith sea levels rising and safety requirements becomingmore demanding, our storm surge barriers and other structures have to be checked for situations in which water flows over the crest of the structure. The structures were often not designed for this amount of overflow, if at all. It generates hydrodynamic loads (or larger loads) on the bed protection on the inside of the structure, and the prevention of severe damage or even the complete failure of the structure depends on this bed protection. In order to identify the structures that may require attention, a method is needed for an initial assessment of future situations of this kind. Existing formulae used to assess the strength of bed protection often take the depth-averaged flow as a starting point. But the overflowing water creates a very different hydraulic situation: often a submerged (or: drowned) hydraulic jump (Figure 1). However, depending on the geometry and the water levels on either side, a normal hydraulic jump could also develop (Figure 2), potentially accompanied by a supercritical flow (Figure 3). Furthermore, pressure fluctuations also play an important role in the loads on the bed protection. The study consisted of exploratory tests on a simple geometry to measure the flow velocities and pressure fluctuations in the relevant area. Visual observations and PIV measurements (Particle Image Velocimetry, see a result in Figure 4 for a situation equivalent to Figure 1) show that, even with a small flow of water over the structure, the plunging jet reaches the bottom, leading to near-bed velocities that are far higher that the depth-averaged flow velocity. Drawing on theory, it was possible to draw up the outline of an assessment method to provide an estimate of the near-bed velocities based on known parameters only: the height of the crest, the bed level behind the structure and the water levels on either side. Using this near-bed velocity, it should then be possible to assess the suitability of an existing bed structure. The exploratory tests also provided information about the applicability of PIV in a situation with air entrainment. In the follow-up to this study, further tests will be conducted with other measuring techniques in order to collect more data on the near-bed velocities in various conditions. Furthermore, work will be done to establish a proper link with existing formulae on the stability of bed protection structures. Contact: Otto Weiler, Otto.Weiler@deltares.nl, t +31 (0)6 4691 1175 Femke Verhaart, Femke.Verhaart@deltares.nl, t +31 (0)6 4691 1186 Sea level rise could lead to water levels exceeding the height of stormsurge barriers. The integrity of the structure requires the bed protection on the inside to handle the loads associated with the overflowing water. Work is taking place on the development of a method for an initial assessment of situations of this kind. Results of the PIVmeasurement of the flow velocities in figure 1 (Figure 4) Photograph of the experiment with little flow over the structure: there is a heavily submerged hydraulic jump but the jet still reaches the bed (Figure 1) Photograph of the experiment with a higher flow over the structure: a ‘normal’ hydraulic jump develops, requiring all the available water depth (Figure 2) Photograph of the experiment with an even higher flow over the structure: the available water depth is no longer sufficient for a hydraulic jump to develop directly behind the structure and a supercritical flow is seen over a large distance (Figure 3) 50