The coastline of Demak, Indonesia, has been eroding during the
last 15 years. Coastal retreat in Demak is caused by a combination
of mangrove deforestation and local subsidence due to groundwater
extraction in the nearby city of Semarang. To restore the lost mangrove
forest, permeable dams, consisting of bamboo poles with a
brushwood filling, have been built to attenuate the waves, facilitate
sedimentation at their land side, and thus create a suitable habitat
for mangroves. However, existing designs required frequent brushwood
maintenance. Therefore, a new type of design is proposed,
consisting of only vertical bamboo poles without a filling of brushwood.
Nevertheless, the hydrodynamic performance of this type of
structure is not known. This study assesses the wave transformation
through structures formed by bamboo poles for the physical
conditions of Demak, Indonesia, with the numerical wave model
SWASH. Field measurements and WaveWatch III data are analyzed
to obtain the design conditions for the structures in Demak.
SWASH is validated against laboratory experiments, and applied
to investigate different structure designs. The model shows that for
a structure consisting of two rows of bamboo poles, the transmission
rate Et/Ei decreases from 75% to 55% when the row spacing
in the wave direction is increased from sx = 0.42 m to sx =5.8
m. Even larger spacings do not result in less transmission, and
at least three rows are needed to have a transmission rate lower
than 50 % - a common wave reduction target used in restoration
efforts with structures. This study thus identifies potential strategies
to maximize wave attenuation by bamboo structures, which
can be used to reduce wave attack along muddy coasts without
the need of a brushwood filling. Hereby it provides an economically
and user friendly alternative with respect to the previous
brushwood structure designs, as it requires less material costs and
maintenance. ... Read more

Mangrove forests, that often act as natural coastal defences, enormously suffered due to ongoing climate change and human disturbances. Thus, it is necessary to have a countermeasure to mitigate the loss of mangroves. Wooden fences are becoming a viable nature-based solution to protect vulnerable replanted mangrove forests. However, the wooden fence's hydraulic characteristics are not yet fully understood due to the complication of branches arrangement. In the present study, a small-scale wave flume modelling of wave damping by a wooden fence was constructed using the inner branches as an inhomogeneous arrangement tested in earlier flow-resistance experiments. The physical model results indicate that the wooden fence is highly effective on wave transmission and that the effectiveness in wave reduction depends on the relative fence thickness, B/Hi. To understand the scale effect on wave transmission further, the numerical model SWASH was used with the laboratory wave data. By applying the prior experiments' drag coefficient on steady flow, the uncalibrated numerical model gave a good agreement with the wave model results, with a root-mean-square error for the total transmitted wave heights of 4.7%. After validation, potential scale effects for small scale tests were determined from scaling simulations at both full scales and the applied 1:5 model scale. These simulations were performed for a fence porosity of 0.81, and different fence thicknesses to understand scale effects between model- and full-scale. Both wave reflection and transmission at model-scale are about 5% higher than full-scale results due to the increased drag coefficient and viscous effects. The effects of fence thickness and porosity were the same in large and small scale, and much larger than the error due to scale effects. Hence testing fence efficiency at physical small scale is regarded as a useful tool, together with numerical modelling. ... Read more

While the significance of infragravity waves (IG) in many—often-hazardous—nearshore processes is widely-recognized, many of the empirical and numerical models used in dike safety assessments do not (directly) consider their contribution. Here, we combine physical and numerical modelling to better understand the factors that contribute to the dominance of IG waves over higher-frequency waves at the dike toe. Findings show that IG-wave dominance increases as the ratio of local water depth to offshore significant wave height decreases. Therefore, it is critical that any tool used to assess the safety of dikes fronted by very and extremely shallow foreshores accurately describe IG-wave dynamics. ... Read more