Hydraulic stability of a living breakwater

Abstract

Coastal structures such as breakwaters can
mitigate the erosive effects of sea level rise by protecting shorelines from
wave impact. The Reef Enhancing Breakwater (REB) is a modular, permeable
structure designed to dissipate wave energy and boost marine biodiversity by
providing suitable habitats. This research investigates the hydraulic stability
of the REB under wave loading using a physical model.

 

Experiments were conducted in the Scheldt
flume of Deltares with a 1:20 scale model. Both 2 and 3 level structures with
simple and complex forms were tested on gravel underlayers of varying
roughness. Irregular waves with significant heights up to 12.5 cm and wave
steepness ranging from 2% to 4% were used, with varying water depths. As
failure was not reached on the horizontal foreshore (main location), the
structure was moved closer to the transition slope (alternative location) where
plunging breakers can emerge. Smart ReefBlocks with integrated mobility sensors
measured accelerations and angular velocities induced by wave motion.

 

Five main types of movements were observed:
sliding, rocking, shaking, tilting, and lifting. Sliding was the most common,
occurring mainly in the top layer, with a maximum of 4 sliding events per block
per 1000 waves. Three limit states were defined: start of motion, start of
damage, and failure. Start of motion is when blocks move for the first time,
damage is when a block loses interlocking, and failure is defined by a damage
level Nod=0.4Nod=0.4.

 

Expected and characteristic values of the
stability number (Hm0/ΔDnHm0/ΔDn) were determined for each limit state. Only
one displacement occurred at the main location, while failures were reached
near the transition slope due to plunging breakers. The 2 and 3 level
structures were stable on bed slopes without plunging waves. For non-plunging
waves, the start of movement limit state (Ns,mNs,m) ranged from 0.39 to 0.51.
When plunging waves occurred, the start of damage limit state (Ns,dNs,d) was
0.94, and the failure limit state (Ns,fNs,f) ranged from 0.94 to 1.11. The
start of motion was not determined for setups at the alternative location.

 

For setups at the main location, sliding
was analyzed in relation to wave height, wave steepness, water level, and
structure height. Higher water levels allowed for higher waves, leading to more
instabilities, but excessive submersion reduced movements. Waves with 2%
steepness caused more movements than 4% steepness due to higher energy in
longer waves. Other factors like underlayer irregularity and increased drag
from epifauna were also considered. Most sliding movements occurred for
Hm0/ΔDn>0.8Hm0/ΔDn>0.8. The model blocks, made of PLA with lower friction
than concrete, started sliding under smaller forces, making the model
conservative for sliding movements.

 

The impact of sliding movements on the
REB's structural performance was investigated by estimating stresses in the
protrusions from data collected by smart ReefBlocks. A conservative model was
used to calculate impact forces and check for protrusion rupture. The resulting
tensile stresses exceeded the concrete strength, but the model's
representativeness is questionable as it differs from the actual protrusion.
Further research is needed for a definitive answer.