Samenvatting

This study explores the effect of precipitation, evaporation, animal burrowing and the changing climate on the permeability of the clay cover layer on a dike. It also showcases the possibility of utilizing finite difference flow models, such as MODFLOW, for the simulation of various environmental impacts on the dike’s clay cover. The study is divided in two phases. The first phase focuses on the setup and calibration of the model that is based on a case study. The modelled dike section is dp753, located on trajectory N29-3. It is known that the dike consists of three soil layers, however, the data about their exact properties is unavailable. The calibration of the model consists of a total of 15
iterations with varying soils parameters, namely the horizontal hydraulic conductivity of each soil type. The iteration yielding the closest results to the actual groundwater levels at dp753 is chosen for the simulations in phase two.

The second phase focuses on answering the research questions of this study. The answers to those questions come in the form of model results combined with knowledge obtained fromother related studies. First, two historical precipitation events are simulated, one of which is defined as an extreme precipitation event where 119 mm of rain accumulated in just one week. Whereas the second precipitation event is considered to be more prolonged, due to the fact that the same amount of rain is spread over 41 days. For the second type of simulation, the effect of cracks is introduced to the model. Cracks with two different depths, 50 cm and 100 cm, are imported into the crest of the dikes and their impact on the groundwater level is observed. Moreover, the sensitivity of the areal coverage of the cracks is tested as well. The simulation that follows investigates the impact of animal burrows on the permeability of the clay cover. In this type of simulation animal burrows from five different animals are tested. Apart from the type of burrowing, the sensitivity of the location and the number of burrows is also examined.
Finally, the effect of climate change on the conditions influencing the permeability of the clay cover is investigated. Based on KNMI’s predictions for the future of the regional climate, namely the precipitation rate and the potential evapotranspiration, adjustments to the base model are made, thus representing those changes.
The results from the calibration of the model show that the last iteration represents the real situation the best, having a total deviation for the minimum, average and maximum water level inside the dike of 11.9%, 12.3% and 6.4%, respectively. Furthermore, it was found that the model is much more sensitive to precipitation events than what is observed from the monitoring sensors at dp753. The field observations show that the rise of the groundwater inside the dike experiences a certain delay after a period of precipitation events have taken place. The outcome of the first simulation indicates that the low permeability characteristic of the clay cover is much more crucial in case of extreme precipitation, as most of the introduced water leaves the model as runoff. On the hand, the results also indicate that in case of prolonged precipitation, the clay cover conducts more water in general, but the groundwater level remains
stable due to the drainage of the ditch behind the dike. From the second simulation, it was found that the permeability of the clay covering layer is not particularly affected by the arial coverage of the cracks, but rather by their depth and location along the dike. It was also observed that the impact of animal burrows, no matter the type and the number, is similar to those of 100-cm deep cracks. Nevertheless, with enough time and high temperatures the cracks can occupy a large area of the dike, unlike the animal burrows that are dependent on the behavior of the animal and its interaction with the surrounding environment. Lastly, it was concluded that the effect of climate change on the permeability of the clay cover is extremely limited, as increased precipitation and evapotranspiration does not alternate the geohydrological properties of clay.
Despite the promising results of this study, it is still recommended that a more refined model to be developed, with data obtained from a proper investigation of the soil’s properties at dp753.