Hydrodynamic stability of a 3D printed concrete artificial reef design
D. CHRONOPOULOU (TU Delft - Civil Engineering & Geosciences)
M.F.S. Tissier – Mentor (TU Delft - Civil Engineering & Geosciences)
Marcel van Gent – Graduation committee member (TU Delft - Civil Engineering & Geosciences)
S. Haage – Graduation committee member (Royal Boskalis)
N. Fani – Graduation committee member (Coastruction)
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Abstract
This thesis explores the hydrodynamic stability of an artificial reef unit, designed by Coastruction. The design was previously used in CREST, a project led by TU Delft in collaboration with Plymouth University, Deltares, Boskalis, which aimed to study the hydrodynamic response of a fringing reef environment with artificial reef elements installed on the reef flat.
Small scale experiments were conducted in the Western Scheldt flume in Deltares , The bathymetry used in the experiments was based on the CREST configuration, scaled down by a factor of 1:6.25, and it consisted of a foreshore with a slope of 1.4:5, to recreate fringing reef conditions. Two artificial reef units with the same shape but a different scale were tested, with nominal printed heights of 4 and 5 cm. The analysis focused mainly on the stability of isolated elements, although full reef configurations were also explored experimentally.
To assess the stability of Coastruction’s design, two sub questions are formulated. The first examines how the offshore wave climate affects the hydrodynamic stability of individual reef units, with particular attention to the transformation of offshore waves over the reef and the role of infragravity waves on the reef flat. The second evaluates the consistency of stability predictions obtained using the Shields and Morison methods, based on laboratory time series data and comparison with observed stability outcomes.
The analysis of the hydrodynamic forcing showed that intense wave breaking over the reef slope, combined with non linear interactions contribute to the enhancement of the infragravity waves (IG), which become the dominant wave motion in the low water level conditions WL05. For WL11 the presence of IG is magnified however to a lesser extent, resulted in a bimodal spectrum. As velocity was the main forcing parameter in the stability formulations, the higher values observed for WL05 were consistent with the corresponding unstable conditions.
Concerning the stability methods, the Shields' method fails to predict the stability patterns observed, likely due to violations of several assumptions underlying its derivation. The effectiveness of the Morison method depends on the definition of the input parameters, where representing the bimodal forcing through an equivalent monochromatic wave appears to reduce predictive ability. Instead, it is more effective if the contribution of each frequency band in shaping the force components is explored separately. Force time series utilizing velocity measurements showed that infragravity waves impose a low frequency oscillation, which combined with the persistent short scale oscillation of the sea swell waves contributes in initiating motion. Therefore, the traditional Morison formulation showed limited predictability, while a modified approach using the infragravity wave velocity component and the sea swell acceleration component appeared more suitable.
Further research is required to evaluate the proposed approach. A first recommendation would be to use direct force measurements to calibrate the force coefficients and expand the experimental dataset, focusing in creating varying infragravity wave contributions. If this is not feasible, applying a safety factor to the estimated required weight is recommended.