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Relations for clay erosion by wave overtopping based on the fire hose method

Rozendaal, I.M.J. van (2022) Relations for clay erosion by wave overtopping based on the fire hose method.

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Abstract:One of the failure mechanisms for grass-covered dikes is wave overtopping, which occurs during extreme events when waves overtop the crest of the dike and flow over the landward slope. These waves cause high hydraulic loads on the cover of the dike and when these loads exceed the strength of the cover material, the material erodes. Dikes are commonly constructed with a sand or clay core and an erosion resistant clay and grass layer to protect the dike against waves. By performing experiments, the erosive behaviour of the cover materials, such as clay, can be investigated. Due to a lack of knowledge, the erosion resistance of clay is currently not taken into account in the assessment of dikes. Clay used in dikes in the Netherlands is classified based on its capacity to resist erosion into three categories: Erosion resistant, Moderately erosion resistant, and Little erosion resistant. This categorisation is based on Rotating Cylinder Tests (RCT) performed by Grondmechanica Delft between 1985 and 1987. In 2017, this categorisation was updated based on new insights to optimise the use of clay in dikes. Next to RCT, other small-scale experiments are used to inspect the erosive behaviour of materials. These tests can be performed in laboratories or in situ and are relatively cheap and logistically easy to perform. The disadvantages of using small-scale experiments is that the hydraulic loads exerted on the soil could differ from the loads occurring during wave overtopping and the heterogeneous character of the material is not taken into account during a single test. Consequently, multiple experiments must be performed to gain enough insight in the erosive behaviour of the clay. Large-scale experiments overcome these problems. However, these experiments are logistically challenging and expensive to perform. Deltares has designed a new test method commissioned by Rijkswaterstaat, which overcomes the aforementioned problems: the fire hose method. Experiments with the fire hose method have previously been performed, however the test on bare clay had to be aborted after only three minutes due to the high erosion rate. The experimental setup has been adjusted in this study, such that it can be used to determine the erosion resistance of clay. The aim of this study is to indicate to what extent the fire hose method can be used to accurately determine the erosion resistance of clay in dikes against wave overtopping. The adjusted setup is used to perform experiments on clay in the Hedwige-Prosperpolder. During these experiments, the grass cover is removed and the nozzle of the fire hose method setup is placed above the clay. Flow velocities between 1.25 m/s and 5.25 m/s are exerted on the clay. After each interval, the depth of the hole is measured and used to calculate the erosion rate. Using the flow velocity and the erosion profile of the hole, the stresses on the clay are calculated. This information is needed to determine the parameters ‘erodibility coefficient’ (k) and ‘critical stress’ (τc), which are commonly used to define the erosion resistance of clay. The results of the fire hose method experiments are analysed to find a relation between the applied normal and shear stresses and the observed erosion rate. This has been done for the ‘erosion rate of the depth’ (Ed), ‘average erosion rate’ (Eav), ‘maximum erosion rate’ (Emax), and ‘erosion rate of the volume’ (EV ). The analysis indicated that when using the maximum erosion rate and erosion rate of the volume a negative trend is observed between the applied stress and observed erosion rate. This contradicts what is observed during large-scale field experiments and to theory. Using the erosion rate of the depth and average erosion rate, a positive trend is observed. However, the erosion rate of the depth results in a large spread in the outcome. This spread is lowered when using the average erosion rate. However, this significantly reduces the amount of data, which increases the uncertainty in the trend. Wave overtopping experiments are performed on clay as well in the Hedwige-Prosperpolder. The erosion rate is analysed for 20 waves of 500 L/m, 10 waves of 1500 L/m, and 10 waves of 2000 L/m and used to calculate the normal and shear stresses. These results have been used to indicate to what extend the results of the fire hose method can be correlated to the results of wave overtopping experiments. Both experiments showed a large spread in the observed erosion rate, which can be caused by the heterogeneous character of the clay. Over the depth of the clay layer and length of the dike, the structure of the clay highly varies, which can result in large variations in the observed erosion rate under similar hydraulic loads. The magnitude of the calculated normal stresses and observed erosion rate are of the same order for both experiments. Since the shear stress cannot be calculated for the fire hose method experiments, this is expressed as the flow velocity of the jet and overtopping wave. The flow velocity of the waves of 1500 L/m and 2000 L/m is larger than the maximum flow velocity used during the fire hose method experiments. However, the flow velocity of the waves of 1500 L/m is close to 5.25 m/s, which was the maximum flow velocity used during the fire hose method. The data show approximately the same range for the erosion rate for both situations. The flow velocity for the waves of 500 L/m lays between 3.25 m/s and 4.42 m/s, which were used during the fire hose method experiments. However, hereby the magnitude of the erosion rate observed during the wave overtopping experiments is larger than those observed during the fire hose method experiments. Uncertainties arise for both test methods. The biggest uncertainties for the fire hose method experiments arise during the analysis of the data. Multiple assumptions are made to calculate the normal stress, which are shown to be incorrect. Furthermore, the effect of the remaining water in the erosion hole is not taken into account. The main uncertainty during the analysis of the wave overtopping experiments is that using photogrammetry, clay and water cannot be distinguished from each other, which results in an underestimation of the erosion speed. Consequently, it is expected that the actual erosion speed of the wave overtopping experiments is larger after the results are analysed further. The hydraulic loads exerted on the clay during the fire hose method experiments are representative for the loads occurring during wave overtopping. The current results showed a large spread, which make it not possible to define a value for the parameters erodibility coefficient and critical stress. This makes the current setup still unsuitable to determine the erosive behaviour of clay. It is recommended to recalculate the normal stresses and erosion rates after the complete analysis of the results of the wave overtopping experiments are completed. Secondly, the spread in the outcome of the fire hose method should be minimised, such that it can be used to define the erosion resistance of clay. If this is both possible, it is recommended to perform more fire hose method experiments on clay to verify whether the method can be used to define the erosion resistance of clay.
Item Type:Essay (Master)
Clients:
Ministerie van Infrastructuur en Rijkswaterstaat
Faculty:ET: Engineering Technology
Subject:56 civil engineering
Programme:Civil Engineering and Management MSc (60026)
Link to this item:https://purl.utwente.nl/essays/90651
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