Hydrodynamic and ecological performance of a new modular unit for living breakwaters

Abstract

Living breakwaters are designed to protect the coast against flooding and erosion, whilst at the
same time they enhance the local ecological system by incorporating natural reef components.
This study investigates the design of a modular artificial reef developed by the company
called Reefy. Reefy breakwaters will consist of interlocking blocks with holes inside and
rounded corners. For the configuration of the breakwater no proper design guidelines exist
yet which incorporate both the hydrodynamic and ecological functionalities, as both this field
of engineering and this reef system are relatively new. Therefore, this study aims to provide
insight and develop preliminary design guidance on how to design a hybrid living breakwater
from Reefy blocks under wave loading. 
To this end, an experimental study was performed in the Eastern Scheldt wave flume of
Deltares investigating the impact of different design variables on both the 2D hydrodynamicand ecological performance under wave loading, in shallow water conditions. Both irregularand regular wave conditions are tested. This thesis focuses on submerged structures and
therefore the freeboard is defined to be positive for submerged structures. In total, 15 different
designs are tested amongst which 7 are 2DV configurations and the other 8 are complex 3D
configurations. Single as well as double 3D structures are tested and the space between a
double structure is referred to as ”channel”.
For the hydrodynamic performance, the impact of several design variable on the transmission
coefficient (Kt) and reflection (Kr) is quantified. In this study, Kt
is defined as the transmitted
waveheight behind the structure divided by the incoming waveheight at the same location
without a structure. These coefficients are based on the incident wave signals. As most of these
tests had shallow water conditions, with large Ursell numbers, the usual methods to determine
the incoming wave did not work. Therefore, to obtain the incoming wave signal, a new method
was used, based on a combination of the existing techniques. Lastly, from the irregular waves
the transmitted- and reflected energy density spectra are investigated and compared to the
results from the regular waves. The results of this study reveal that for the same number of blocks, a more complex structure
can be built without making a compromise in the hydrodynamic performance parameters.  Furthermore, the existing formulae for the Kt and Kr are compared to results of the hydrodynamic performance as measured. The ones with the best correlation are optimized using a non linear
regression analysis. 
For the ecological performance, the stream-wise peak velocities are investigated in the
wake behind the structure and in the channel. The performance is investigated based on a
tranquility index Tr. Tr increases if the flow is more tranquil. The outcomes of Tr showed that in general, Tr increases for an increase in the design variables
that were inversely proportional related to Kt
.
In conclusion, the performed tests and analysis provide insight into relevant
physical processes and design parameters for artificial reefs, and therefore assists the designer
of artificial reefs.