Graduation of Kay Boomaars

Sea level rise causes more difficulties in maintaining the coastline. In the past, hard structures were built
in order to prevent for flooding. Due to the rising sea level, these hard structures should be maintained
more often. More recently nature based solutions got therefore the preference over these hard structures.
However, for the application of these nature based solutions a better understanding in the coastal
processes and variability is desired. Especially the restoration of the coast after a storm is not well
understood, which is mainly driven by the wind.

Sand is deposited on the beach with hydrodynamic forces. On the beach this sand dries, and subsequently
it is transported towards the dunes as a form of aeolian transport. One of the most visually clear signs of
aeolian sand transport are organised, low, slipfaceless bedforms of dry sand over a moist beach, referred
to as sand strips. In order to improve the knowledge of aeolian transport in the form of sand strips, this
study focussed on the properties of these sand strips and the environmental conditions that occurred
while sand strips were present at the beach, using data of a terrestrial laser scanning (TLS) device located
in Noordwijk.

For the time period considered, sand strips are detected with a Fourier transform based on the reflectance
values of the TLS-data. With these reflectance values, the moisture content can be derived, causing a
moisture-content based detection. Sand strips are detected dependent on the amount of energy in the
variance density spectrum and the corresponding wavenumber. Subsequently, the wavelength,
orientation and height of the sand strips are determined according to the Fourier transform. These shape
properties are analysed based on the average shape properties occurring in each scan. The wavelength
of the detected sand strips is around 13 m, the height ranges between 2 cm and 12 cm, and the orientation
of the sand strips is alongshore to oblique-alongshore. This is in correspondence with other studies
conducted on sand strips. According to sand samples acquired on the beach, the grain size of the sand
strips varied in transverse direction, comparable to the grain size variation of aeolian sand ripples.
Furthermore, the moisture content of the sand strips differed significant compared to the moisture
content of the surrounding beach, in correspondence with the reflectance-based detection.

In addition, environmental conditions are linked to sand strip occurrence. This analysis showed that sand
strips mainly occurred during alongshore winds with a wind velocity of at least 8 m/s, while they form
with a minimum wind velocity equal to 10 m/s. Other studies also concluded that sand strips mostly
occur with alongshore winds of 8 m/s. Based on the detected height of the sand strips, they do not
necessarily disappear during precipitation events, while the moisture content of the sand strips increases
and detection based on the reflectance values is no longer possible. Furthermore, the sand strips mostly
formed during falling tide and disappeared during rising tide, where they possibly are destroyed caused
by the increasing water level.

Dynamic properties of the sand strips are analysed based on one life cycle of the sand strips lasting in
hours. These results suggest that sand strips show dynamic behaviour, which could be caused by
topographic steering since the weather conditions remained mostly constant. The results obtained for
the dynamic properties are however only indicative and encourage further study of dynamic behaviour
of sand strips since the results obtained in this study are only based on one life cycle of the sand strips,
from formation until disappearance. Additionally, the reliability of the outcome will increase with more
events considered.