Overflow and wave overtopping induced failure processes on the land-side slope of a dike

Overflow and wave overtopping induced failure processes on the land-side slope of a dike

Ponsioen, L.A.

Van Rhee, C. (mentor)
Van Damme, M. (mentor)
Hofland, B. (mentor)
Peeters, P. (mentor)

Civil Engineering and Geosciences

Hydraulic Engineering

2016-05-11

This thesis discusses the breach initiation processes at the land-side slope of a dike, due to overflow and wave overtopping. To date still little is known about the actual failure process of land-side slope covers. Therefore, starting in 2007, numerous wave overtopping experiments have been executed in The Netherlands and Belgium using a wave overtopping simulator. This simulator simulates different kinds of storm events by releasing volumes of water over the land-side slope according to a predetermined schedule. Flanders Hydraulics Research and Delft University of Technology per- formed another wave overtopping experiment in November 2015. The wave overtopping simulator as well as a new designed overflow simulator were used to test the breach initiation processes of the land-side slope of a river-dike adjacent to the river Scheldt, near the Wijmeers-II polder in Belgium. The data obtained from this experiment is presented in this thesis, and was used to evaluate current damage initiation theories. Currently the most advanced approach for determining the amount of erosion, or ’damage’, to the land-side slope is the ’Excess volume approach’ by Hughes [17, 18] who based his model on earlier developed models by Van der Meer [36] and Dean et al. [10]. This approach determines damage on the land-side slope based on overflow theories and overflow experiments (CIRIA experiments [13]). An overtopping wave is thereby modelled as a very short overflow event in which damage is assumed to be bottom shear stress induced. The Excess volume approach has therefore been applied on the Wijmeers-II data in order to verify the predictive accuracy of this theory. Contrary to the experimental outcome, the Excess volume approach predicted a heavier damage on the overflow test sections than on the wave overtopping test section. Due to these test results the correct representation of reality of the Excess volume approach is questioned in this thesis. Extensive investigation of video material obtained during the experiments showed that overtopping waves separate from the slope before reattaching with the slope. This separation time leads to a jet flow impacting the slope under an angle. Due to a decrease in discharge during a wave overtopping, the impact location moves towards the crest within the overtopping time of a wave. This discovery led to the development of a new modelling approach in which this phenomenon is considered. In this new ’Wave impact approach’, damage on the grass cover is assumed to be caused by wave overtopping induced jet pressures in cracks in the grass cover. These cracks are present due to weather conditions and temperature influences in the supporting clay layer. Wave overtopping induced jet pressure in the crack pushes the walls further aside leading to growth of the crack. This crack growth continues until the pressure in the crack becomes too high and grass aggregates are lifted from the cover. This crack growth is assumed to only occur when the wave overtopping induced jet pressure in the crack exceeds a critical pressure threshold Pc. The location at which the highest cumulative excess impulse load occurs is the location at which the most damage is expected. Verification of the theory against other experiments confirms the theory.

wave overtopping
wave impact
dike
land-side slope failure
breach initiation
breaching process
overflow

http://resolver.tudelft.nl/uuid:a6ef81da-53a2-4e84-8829-b9a08afea2ef

Student theses

master thesis

(c) 2016 Ponsioen, L.A.