A dam-break wave can result in considerable damages and casualties. Events that can lead to such waves include dike and dam breaches, storm surges and impulse waves. Dutch history is familiar with a number of coastal dike breaches but also river dike breaches have occurred, resulting in dam-break waves reaching far inland. Beyond the Netherlands, propagating walls of water could occur when dams of water reservoirs collapse. More specific knowledge of the hydrodynamic behaviour of dam-break waves is necessary in the battle against water.
The impact of a dam-break wave can be reduced through a better understanding of the currently poorly understood link between bed roughness and wave hydrodynamic properties. The objective of the present study is therefore to study experimentally the effect of bed roughness on the hydrodynamic properties of dam-break waves, on both dry and wet beds. The approach was to generate dam-break waves through a lift-gate and a reservoir with a depth d0 = 0.4 m and to measure the water levels over time in the downstream channel. 7 different bed roughness configurations were made (range of ks = 0.6 to 48 mm) in addition to smooth bed reference test. Dry bed tests were performed, 5 wet bed tests with an initially still water level (h0) (range of h0/d0 = 0 to 0.125) were performed. The analysis focused on wavefront celerity, inundation depths and roller length which are used to characterise the hydrodynamic properties.
Visual observations showed substantial differences in wavefront behaviour between surges and bores as well as between smooth and rough beds. Validation of the generated waves was done by successfully comparing the reconstructed water levels over the flume length with a new approach.
Results showed that an increase in bed roughness resulted in a decrease in wavefront celerity for surges and that the initially still water level can work as a lubricant for bores. The initial still water level is dominant over the bed roughness when h0 >0.5ks.
The maximum water level increased with increasing bed roughness for surges, while bores showed an opposite behaviour. Increasing the initially still water level reduced the relative maximum water level for rough bed configurations.
In the present study, only some test configurations revealed the presence of a plateau height. If there was one, an increase in bed roughness showed a decrease in plateau height for bores with h0/d0 = 0.125. Three sections in the h0/d0 range with different plateau height behaviours were noticed.
The roller lengths were determined with the wavefront celerity and the plateau height. No trivial relations were found for the influence of bed roughness or initial still water level.
Overall, this project revealed some interesting behaviours that need to be further investigated in the future. It is recommended to conduct a full quantitative analysis once precise values of the bed roughness configurations are determined. Nevertheless, these results present some preliminary, yet interesting, findings that will contribute to a better understanding of the behaviour of dam-break waves over rough bed.
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In this assignment we simulate a two meters wide dike stretch which is subjected to overflow. Three failure mechanisms could occur due to overflow: full erosion of the soil of the dike, macro instability of the levee if too much soil is eroded away, leading to insufficient counterweight on the inside of the levee, and surface slip failure due to reduction of strength of the dike by flow of water in the dike core. Prevention of failure can be achieved by either keeping the water away from the berm surface or holding the soil particles in place while the water flows alongside it. The desired repair is an emergency repair that can placed quickly during a storm, or between two storms to prevent a damaged levee from failing. This is a temporary repair that will later be removed for full reconstruction of the levee. This provides constraints to the installation and repair procedure.
To decide which solution is best, 10 concept solutions are compared based on their score in the Multi Criteria Analysis with 8 weighted criteria. The solution with the highest score is the best solution according to the Multi Criteria Analysis. The selected repair option is to locally cover the damaged area with flexible overlapping sheets of Tyvek®. This is a light and thin material that can easily be transported and cut to size at the location of the repair. These sheets can be easily secured against flow loads using Gripple anchors, while pins are used to keep the Tyvek® in place during windy conditions. An installation plan and cost breakdown was made. Using 3 m wide Tyvek® rolls, the costs would be around 19.19 euros per meter height.
The damaged areas of the levee are covered with Tyvek® which is a waterproof, damp open material. This prevents the penetration of water into the core through the damaged areas. The Tyvek® also prevents the exposed soil from being eroded. Moreover, Tyvek® is a very strong material that won’t be torn off. As there is no strength reduction due to water penetration or further loss of surface material the chance for macro instability is minimized.
First, a design was created, with components dimensioned for a dike stretch in the Hedwigepolder. Subsequently, the design was validated with tests. Including two types of damage. The first was removal of a 2 by 2 m grass layer (0.1 m thick) at the top of the dike and the second was a dug-out step-section across the entire width of the dike with a height difference of 0.5 meters at the toe of the dike. Only the second damage went through the clay layer and into the sand core of dike.
During and after the tests some important observations were made. During all test volumes the plastic sheets were kept in place due to the anchors and pins. Some pins however, came loose due to vibration of the sheets (we presumed the cause of vibration is flow of water between two sheets or turbulence of water). The trench of the lower damage was partly washed away by the water. The erosion of soil in the trench did not result in changing the stability of the top sheet. At the lower damage the plastic sheet was torn off after a tear of 1 m was made with a knife.
Our solution is designed to protect any dike from macro-instability by keeping the water flow away from the berm. This is done by layering multiple sheets of plastic foil over the inner berm, fastened with ground anchors and pins. The solution is a durable and reliable design due to proven waterproof, damp open and UV resistant properties, together with Gripple's demonstrated anchoring capabilities. Another major advantage of our solution is that the repair of the damage is selective, i.e. the solution is applied only to the parts of the levee where damage has occurred. In case of only localized and small damage to the levee, the costs and workload are very minimal compared to other solutions that are applied to the entire surface area of the levee. ... Read more