Conditions indicating favorable accommodation space for dune development
Accommodation space is considered favorable for dune development when it is sheltered
from storm impacts and experiences a steady accumulation of wind-blown sand. The latter
condition is not disputed, as the reinforcing feedback between the growth response of
marram grass and burial by wind-blown sand is well documented
and recognized to be fundamental to coastal dune development
in temperate regions around the world e.g.,. The positive feedback mechanism
originates from a trait that all beach grasses of the genus Ammophila possess,
namely potentially unlimited horizontal and vertical growth through its rhizomes
. Whether marram grass grows horizontally or
vertically subsequently depends on the amount of wind-blown sand, which makes it so
particularly advantageous to dune building. After establishment, by seed or rhizome
dispersal, marram grass first produces leafy shoots along newly developing horizontal
rhizomes. When wind-blown sand is trapped by the leafy shoots, the immediate sand surface
is raised and a small embryo dune is formed . The leafy shoots are
capable of growing up through a moderate thickness of sand by elongation of individual
leaves. If, however, a leafy shoot is overwhelmed by sand deposition, one or more of its
axillary buds develop into a vertical rhizome that will continue to grow until the
surface is reached. Adventitious roots are produced from the nodes of the vertical
rhizome and the horizontal rhizomes gradually die, so that the vertical rhizomes become
independent of one another. This process may be repeated as long as aeolian supply is
abundant and marram grass continues to trap sand. The capacity to trap sand, as noted
before, is enhanced by the growth response of marram grass to sand trapping, which
introduces the positive feedback mechanism driving coastal dune development
. Using very high-resolution data,
showed that marram grass on foredunes along the Zandmotor
appears to thrive best under a sand trapping rate of approximately 0.3 m of sand per
growing season and that marram grass can withstand sand burial of up to 1 m of sand.
However, while this demonstrates how positive plant–sand feedback steers dune
development, it must be noted that the physical size of a developing dune and predominant
wind regime also controls its morphology . As dunes grow, for
example, a limit is imposed on its height because the wind force required to transport
sand upslope increases significantly
e.g.,. Coastal foredunes
therefore tend to expand in width rather than height, which emphasizes the importance of
the wide favorable accommodation space the Zandmotor provides for foredune
development.
Bimodal probability density curve of sea water levels (in m ± m.s.l.)
measured by a buoy near Scheveningen since the construction of the
Zandmotor in 2011 until 2017. Included are the instances when sea water level exceeded
the 1.6 m a.m.s.l. boundary height for dune development along the Delfland
coast.
The condition that accommodation space is considered favorable when it is sheltered from
storm impacts warrants closer inspection, because the impacts of a storm surge depend
both on the magnitude of the storm as well as the geometry of the beach
. Wind stress due to atmospheric pressure differences drive
storm surge levels and offshore wave conditions, but the vertical dimension of the beach
profile, in particular, exerts great control on shoreline parameters such as wave setup,
swash and run-up e.g.,. This is significant because the dissipation of
kinetic energy of breaking waves is responsible for the highest rates of coastal erosion
and dune decline e.g.,. However, while
empirical models can calculate wave run-up levels and wave breaking energy from
parameters such as offshore wave conditions and beach profile (see
, and , for details), those
relations only return approximations as often not all required model input is available
or because of inherent model uncertainties. Having said that, the results suggest that
dunes along the Delfland coast are sheltered from storm impacts above a beach height of
1.6 m a.m.s.l. This finding is examined in relation to offshore sea water levels
measured by a buoy in close proximity to the Zandmotor mega-scale beach
nourishment. Figure shows the probability density curve (which is bimodal
because of tidal dynamics) of those sea water levels (in m ± m.s.l.), measured
every 10 min from 2011 until 2017. Included are the instances when sea water levels
exceeded the apparent 1.6 m a.m.s.l. boundary height for dunes to be sheltered from
storm impacts. It is clear from Fig. that this did not occur frequently,
only during about 0.4 % of the measurements. Those measurements, however, were
relatively clustered together, meaning that the boundary height was exceeded over
(relatively) prolonged periods of time. Although, over the course of 6 years this
happened for no more than 10 full days. On average the exceedance was about 20 cm up to
a sea water level of 1.8 m a.m.s.l., but on a few occasions sea water levels almost
doubled compared to the boundary height to 3.10 m a.m.s.l. This is excluding the wave
run-up onto the beach, which can be significant for natural beaches in the Netherlands.
Dependent on whether the beach profile is dissipative or reflective, both
and show that wave run-up may
reach to heights from 0.85 to 1.45 m above still water level, which is the level that
would occur in the absence of waves. This implies that, since the construction of the
Zandmotor in 2011, the Delfland coast may have experienced coastal erosion by
storm surge levels reaching heights up to at least 4 m a.m.s.l.
The distribution of (embryo) dunes on the Zandmotor, suggesting a
correlation to marine dispersal of rhizome fragments as a large number of
embryo dunes are present between the identified boundary height of 1.6 m a.m.s.l.
and the (current) maximum expected storm surge height of 4 m a.m.s.l.
The observation that in 2017 quite a large number of embryo dunes were present on the
beach at heights well below the maximum experienced storm surge levels points to the
capacity of established dunes to withstand and recover from hydrodynamic storm impacts as
well as to the pivotal role marine dispersal of rhizome fragments likely plays to dune
establishment processes. As remarked by various researchers
e.g.,, the ability of embryo dunes to recover from storm impacts largely
depends on the extent to which the above- and belowground structural integrity of marram
grass remains intact after a storm event. This depends, in turn, on the severity of the
storm impacts on the dune, which can be caused by wave erosion (scarping and overwash)
and swash inundation . Wave erosion may
completely remove all sand from an embryo dune (so it is no longer raised from the beach
surface) and have an abrasive effect on the leaves of marram grass, causing either minor
damage or complete removal of all aboveground biomass. Most of the belowground root
system of marram grass, however, has been observed to largely remain intact after wave
scarping or overwash . Potential damage of swash inundation to
marram grass depends on the duration of the inundation period, but as
demonstrate, marram grass displays no visible decomposition of
stems, roots or rhizomes after being immersed with sea water for 20 days. This is well
beyond the period a beach will be inundated after a storm event, which implies that
inundation has a limited, if any, negative effect on the structural integrity of marram
grass. Given that storm events occur more frequently in winter, it has been observed that
embryo dunes on dissipative beaches undergo a classic seasonal cycle of erosion during
the winter and accretion during the summer .
Their presence on the beach, however, would remain persistent throughout the year and
often show a yearly net growth when aeolian supply was sufficient
. This not only indicates that embryo dunes have
the capacity to withstand storm impacts and quickly recover to prestorm conditions, but
also that the above- and belowground structure of marram grass often remains largely
intact after a storm event. Marine forcing, at the same time, has been shown to be an
important agent in the dispersal of marram grass rhizome fragments and subsequent dune
establishment via clonal growth . The
distribution of (embryo) dunes on the southern part of the Zandmotor, as shown
in close-up in Fig. , suggests a correlation to marine dispersal of rhizome
fragments as a large number of embryo dunes are present between the identified boundary
height of 1.6 m a.m.s.l. and the (current) maximum expected storm surge height of
4 m a.m.s.l. The embryo dunes around the small dune lake, while technically located in
the same elevation zone, have likely mostly established by seed germination as the high
constructed base of the Zandmotor has completely blocked all storm surge impacts
until now. Over the years there has been a steady build-up of a freshwater lens under the
Zandmotor and the salinity of the dune lake has significantly decreased as a
result . This fresh water availability, in combination with
moderate burial dynamics, have been shown by to be
beneficial to marram grass seed germination and subsequent dune establishment. The
specific distribution of embryo dunes around the dune lake may therefore correlate best
to seed dispersal by wind coming from the dominant south-western wind direction, either
pushing the seeds over the lake towards the north-east corner of the lake or depositing
it on the south-west lee side where the beach slopes downwards towards the lake. In
effect, Fig. illustrates that, even though the Zandmotor may
provide wide favorable accommodation space and thus a high potential for dune
development, the conditions required for successful (natural) dune establishment must
also be considered. Having said that, showed in a field
transplant experiment that planted marram grass (consisting of a rhizome fragment with
one shoot) thrived on most parts of the Zandmotor except when exposed to direct
wave action. This suggests that conditions that limit marram grass growth and subsequent
dune development (e.g., high salinity, drought, low nutrient status) are mostly absent on
the Zandmotor and likely along the entire Delfland coast.
Overview of total alongshore changes in dune cover by marram grass between 2016
and 2017 (in m2 m-1 yr-1) in relation to anthropogenic activities that
impact (positively and negatively) dune development along the Delfland coast. Aerial
photographs courtesy of René Oudshoorn (a) and Google
Maps (b).
Dune development potential in relation to anthropogenic impacts
The results highlight the overall importance of the Zandmotor for dune
development along the Delfland coast. First, this is because its beach provides very wide
favorable accommodation space that therefore supports a high potential for new embryo
dune development. And second, because of its sand-feeding effects, the Zandmotor
has likely contributed to creating more favorable accommodation space for dune
development along the entire Delfland coast. The coastline directly north of the
Zandmotor, for example, experienced a significant accumulation of sand between
2013 and 2017 even though it has not been nourished with sand in the years before.
Although the amount of sand accumulation was less compared to the coastline that has been
nourished between 2009 and 2011, the overall positive sand budget illustrates the
intended dynamical nature of the Zandmotor, where its sand is redistributed
along the coastline causing a seaward broadening of the beach and dunes. In fact,
Fig. c suggests that the unnourished northern part of the Delfland coastline
supports a higher potential for dune development compared to the nourished southern
coastline. In part this may be due to the fact that the Delfland coast is characterized
by a net northward sediment transport regime , which is reflected
in the sand-feeding budget of the Zandmotor. In the first 18 months after its
completion, for example show that up to 40 % more sand of
Zandmotor was transported in a northward direction rather than southward towards
Rotterdam harbor. At the same time, because the 2009–2011 nourishment strategy consisted
(for a large part) of foredune reconstruction that included plantings of marram grass,
the created favorable accommodation space along the nourished coastline may not
accommodate much new dune development. As such, even though the coastline south of the
Zandmotor has been reinforced with sand nourishment, it is quite possible for
the unnourished northern Delfland coastline to experience more pronounced dune
development in the years to come.
Interestingly, however, the positive effect of the Zandmotor on the northern
Delfland coastline, in terms of sand accretion, is not reflected in the changes in cover
by marram grass between 2016 and 2017. Even though it is shown that the coastline north
of the Zandmotor provides ample favorable accommodation space, it appears that
the potential for dune development is currently not being realized. There are two main
anthropogenic impacts that may hamper dune development along this urbanized coastline,
namely persistent disturbances arising from recreation and leisure as well as a
(increasingly prevalent) nature management practice that is aimed at remobilizing the
dune landscape. Figure gives an overview of total alongshore changes in dune
cover by marram grass between 2016 and 2017 (in m2 m-1 yr-1) and aims to
relate it to anthropogenic activities that may impact (both positively and negatively)
dune development along the Delfland coast. In the broadest sense, the coastline can be
divided first according to whether or not it has recently been nourished with sand and
what type of sand nourishment has been implemented. A distinction can be made between the
sand nourishment that was carried out between 2009 and 2011 to reinforce most of the
Delfland coastline, the nature compensation project Spanjaardsduin implemented
at the same time and, finally, the Zandmotor mega-scale beach nourishment that
was completed at the end of 2011. Further, within the nourished coastline there is a zone
where the dune development appears to lag behind compared to the rest of the nourished
coastline. This zone, as Fig. b illustrates, can be characterized by a higher
concentration of disturbances arising from recreation and leisure. Then, finally, there
is the northern part of the Delfland coastline that has not been nourished with sand in
recent years. Within this zone, as Fig. c shows, relatively large dune areas
have been excavated between 2011 and 2013 aimed at rejuvenating the dune landscape by
reinitiating aeolian dynamics. In the following paragraphs, each identified zone of the
Delfland coast and how dune development potential may be impacted by the various
anthropogenic activities are discussed.
As Fig. c shows, the overall positive effect of the three sand nourishment
schemes on dune development is clearly reflected in the changes in marram grass cover
between 2016 and 2017. Within the nourished zone, however, there are three clear dips
where the dune cover appears to have decreased over the course of a year. Upon closer
inspection it seems that each dip coincides with a beach entrance where the public can
enter the beach. A number of natural processes and human activities may be involved here
in the observed decline in marram grass cover. First of all, as Fig. b shows,
the seaward side of a beach entrance is commonly paved with concrete slabs and cuts
relatively deep into the stoss slope of the foredune. This, effectively, mimics a through
foredune blowout e.g.,, in which wind erosion is enhanced
because of local wind speed acceleration and pronounced turbulent flow structures such as
corkscrew vortices . Because the floor is paved, these
wind-driven forces will in particular erode (i.e., widen) the slopes of the beach
entrance and this susceptibility to lateral erosion may have lead to the observed decline
in marram grass cover. Second, as can also be seen in Fig. b, there is often
a hospitality establishment (e.g., a beach bar or restaurant) directly beside a beach
entrance. And although their placement on the beach is often seasonal, their presence is
numerous. In the summer of 2017, for example, only 3 of the 23 beach entrances along the
Delfland coast did not have one or more hospitality establishments directly placed
besides it. Perhaps not coincidentally, two of those three entrances gave access to the
more isolated parts of the Zandmotor. The presence of hospitality establishments
puts additional pressure on the dunes as people may flock around the beach entrances and
motorized vehicles are more common, e.g., to resupply the establishment. Even though
walking or driving in the foredunes is prohibited along the Delfland coast, several
studies e.g., show that vehicles and
people on the beach may have a significant negative effect on dune development.
At the same time, as laid out in more detail by , the structure
of the hospitality establishment itself may alter (i.e., block) aeolian transport from
the beach and retard foredune development. This plays an even larger role in the
nourished zone north of the Zandmotor where, besides a large number of
hospitality establishments, a high amount of seasonal beach cabins are also placed along
the toe of the foredunes from March until October (from 2016 onward). Their placement
close together, as Fig. a shows, has the additional effect that the airflow
can constrict and accelerate between the beach cabins, which increases the likelihood of
local scour resulting in aeolian deposition farther landward
. Although in some instances this may actually be beneficial
to the foredunes, the clear decline in cover by marram grass (see Fig. c)
indicates that the placement of beach cabins had an overall negative impact on dune
development between 2016 and 2017. And while the beach cabins were in fact raised
slightly from the surface (∼50 cm), show that
such a modest height may not have been sufficient to minimize interference with the wind
flow and resultant aeolian dynamics. Further, another important anthropogenic disturbance
with a highly negative impact to dune development is that the beach directly north of the
Zandmotor is mechanically raked during the summer to remove wrack line material
and human litter. Even though it is a common practice to accommodate beach recreation
, this severely hampers embryo dunes from establishing
themselves on the beach. Not only can the machinery destroy any sprouting seedlings or
rhizomes of marram grass, the removal of wrack deposits also deprives marram grass from
potential hospitable locations to establish itself on the beach .
As a result, these anthropogenic disturbances combined have likely contributed to reduced
dune development compared to the rest of the nourished Delfland coastline.
Then, as Figs. c and e show, the decrease in marram grass cover
along the unnourished northern part of the Delfland coastline suggests that the foredunes
have been in decline between 2016 and 2017. This is unexpected considering the positive
sand-feeding effect of the Zandmotor on this stretch of coastline. Upon closer
inspection, the main candidates for the observed foredune decline are a number of dune
excavations aimed at rejuvenating the dune landscape. When the focus of Dutch coastal
policy widened, to also include preserving the spatial quality and natural values of the
coastal zone, it was recognized that traditional flood safety measures had led to
over-stabilized dune systems that were characterized by a markedly reduced biodiversity
compared to younger and more dynamic dune systems
e.g.,. For that reason, in places where
coastal safety could be guaranteed, remobilizing dune systems by removal of dune
vegetation and topsoil has become a key management practice for maintaining a high
biodiversity in the dune landscape. Reinitiating aeolian dynamics is hereto essential, as
deflation and deposition zones creates habitat diversity and renewed opportunities for
specialized pioneer vegetation species e.g.,. Nowadays, in
order to maintain or even increase dune mobility, the rejuvenated dune systems are often
connected to the beach and foredunes through the excavation of foredune notches. This has
been shown to result in a sustained input of wind-blown calcareous beach sand and more
diverse living conditions for pioneer vegetation, e.g., due to higher levels of sand
burial, wind speeds or salt spray .
However, as Fig. c shows, dune excavation practices appear to adversely
affect foredune development. While there were no foredune notches explicitly excavated
along the Delfland coast, Fig. b shows that the paved beach entrances may
similarly act as conduits for aeolian transport into the dune excavations. As a result it
is quite possible that the narrow foredunes in front of the excavated dune are
experiencing a net deflation of sand that negatively affects the growth of marram grass
(e.g., by root exposure). Sand deposition then likely occurs deeper landward where no
marram grass is presently growing to benefit from an increase in sand burial. Marram
grass cover, as a result, has been in decline between 2016 and 2017, indicating a decline
in the foredunes as well. However, with the Zandmotor feeding the coastline and
providing an effective flood defense, this localized foredune decline should not pose an
imminent threat to coastal safety.