June 2010

A couple of weeks ago, in the midst of the series of posts on Gulf of Mexico
berms, I encountered one of those odd coincidences that are odd only because we
forget the statistics behind coincidences.

The travel section of the newspaper I was reading had a piece extolling the
delights of the endless expanses of dunes and beaches on the German island of
Sylt - the tile of this post immediately jumped into my deranged mind. And then,
browsing through the geoblogosphere, I came upon an enjoyable post by Ole
Nielsen on hooked
spits - which highlighted the dramatic sedimentary architecture of Sylt. Ole
referred to a recent paper in Sedimentology, and so, intrigued, I took
myself off to the Geological Society Library and looked it up. The story is,
indeed, intriguing.

Sylt is a form of barrier island and, given the role that these
shape-shifting piles of sand play in the ongoing saga in the Gulf of Mexico,
they link in to the inexhaustible theme of coastal dynamics. As Ole commented, "
Barrier islands are sedimentary islands separated from the mainland by bays or
lagoons, and some originate as sand spits. A hooked spit is a landform created
by the water currents and may be described as a curved sand bar. In fact barrier
island systems often terminate with hooked spit morphologies." Barrier islands
come in many shapes and sizes, and the conditions for their formation and their
evolution, regardless of whether they terminate in a hooked spit or not,
continue to retain many mysteries.

Sylt faces the North Sea and is the northernmost German island - indeed, the
mainland across from its northern half is Denmark and the place was Danish until
1864 ; it’s 38 kilometres from one end to the other (when last measured) and is
connected, artificially, to the mainland. At its northern tip is a beautiful
hooked spit. Sylt is geologically young, its core built from debris left over
from the melting of ice sheets at the end of the last ice age: these materials
were reworked by the onslaught of the rising seas and the main island began to
develop around 6000 years ago. Today, the strength and complexity of tides,
waves, and currents produces a dynamic playground for sand, dramatically
illustrated in low and high tide images, the Wadden Sea to the east festooned
with ever-shifting sand bars:

This playground has, as usual, been a difficult place to live. Historically,
villages on Sylt have been swept away in storms and inundated by migrating and
growing sand dunes, and lighthouses have found themselves in locations no longer
effective for their original purpose. The west coast, facing the energy of the
North Sea, is subject to constant erosion, the sand being flushed northwards or
swept out into offshore bars. The western shoreline of Sylt is now perhaps 10
kilometres east of where it used to be, and today erosion rates reach more than
a meter a year; the southern tip of the island has on occasion been severed
after a violent storm. But it’s an important tourist destination, and so
something must be done, and that something, as usual, is beach
nourishment. According to Wikipedia, which cites the reference
(in German), " Since 1972 an estimated 35.5 million cubic metres of sand have
been flushed ashore and dumped on Sylt. The measures have so far cost more than
€134 million in total, but according to scientific calculations they are
sufficient to prevent greater loss of land for at least three decades, so the
benefits for the island’s economic power and for the economically underdeveloped
region in general would outweigh the costs." Aha - those scientific calculations
again… But these measures require annual replenishment, and so understanding
the sand dynamics and sediment budgets of Sylt is important, and this where the
four authors, from institutes in Hamburg and on Sylt, of the paper in
Sedimentologycome in. While the paper itself is not directly
available, the PhD thesis of the lead author, Sebastian Lindhorst, that formed
the basis for this and other publications, is available to read
online.

This work has demonstrated that the sedimentary systems of the main part of
Sylt and the hooked spit that forms its northern tip are quite distinct - the
spit is growing at the cost of erosion further south; but the way in
which the spit grows is far from simple. The work was conducted using
ground-penetrating radar, a relatively simple and inexpensive technique that
nevertheless has powerful applications in engineering and archaeology (there’s
quite a good
summary on Wikipedia and various commercial companies such as GeoModel). Essentially, the reflections of
the radar energy from subsurface layers and objects are recorded when they
bounce back to the surface; the reflections are caused by changes in the
velocity of the radar waves as the nature of the material they are travelling
through changes. Use some clever computer processing and an image of what these
subsurface reflections look like can be generated as a profile, a vertical
“slice”; the vertical scale on such an image is actually *time,*in that
what you are measuring is the time taken for the radar pulse to travel
downwards, be reflected, and return to your recording instrument on the surface
(“two-way time”, TWT). But, if you know fairly well the nature of the materials
underground, then this can all be converted to a depth scale. Importantly,
Lindhorst and his colleagues calibrated the radar data with cores and so the
character and velocity of different sands, wet, dry, fine-grained,
coarse-grained, could be integrated with the radar data.

Here’s an example of one of the profiles from their work:

The profile at the top is the processed radar data; TWT in nanoseconds (1 ns
is a billionth of a second) is the vertical scale on the left, depth in meters,
with mean sea level as zero, is on the right. In order to display an image where
the components are more clearly shown and interpretable, the whole thing is
vertically exaggerated by a factor of five. If it weren’t displayed this way,
the true-scale version would look roughly like this - accurate, but difficult to
interpret:

So, back to the main figure. The profile runs at right angles to the coast of
the spit, with the surface topography at the top: the higher ground on the right
shows the dunes at the back of the low beach ridges that run parallel to the
shore. Now, with experience, local understanding, some cores (and a little
imagination), the darker layers that are the reflectors of the radar pulses can
be interpreted as successive sedimentary features, and the relationships between
them examined. The one significant “layer” that is nota sedimentary
feature is the strong, essentially horizontal, reflector that runs across the
profile just above sea level: this is the top of the groundwater table, the
boundary between dry sands above and wet sands below, a velocity contrast so
dramatic that it bounces back a lot of the radar energy. The lower part of the
figure is the geological interpretation of the radar data, with the main
components of the architecture of the spit identified (my apologies for this
only being in black and white, but it’s still quite clear). The main elements
are all inclined (dipping) towards the right, the shoreline, and represent
episodes of growth of the spit, building through time, younger swathes of sand
on top of older ones, from right to left (elements G to D).

But this growth is not continuous, it takes place in episodes, so that
surfaces such as F represent the sea floor following a phase of sand build-up
where the actions of waves and currents gently scour away at the sand surface.
The result is a discontinuity in the sedimentary pile, the next flush of sand
being deposited onto that somewhat modified surface. The boundary between the
two building events shows up as a radar reflection, with the geometries of the
sand layers above and below being slightly different - these surfaces are what
are marked on the profile as unconformities. These are vital as they divide the
foundation of the spit into successive episodes of deposition and quiescence.
But even more important are the features labelled “C.” These mark something far
more dramatic than the normal ebb and flow of sand deposition: the base of each of these
packages of sand is marked by a pronounced discontinuity - the layers beneath
are chopped off, scoured, removed, with new piles of sand then dumped on top of
them. These “cut and fill structures” are, as the notes on the figure record,
telling us of periodic major events, storm surges scouring into the edge of the
spit. They are essentially scoop-shaped excavations of the pre-existing sands, filled in, and, like the succession of layers beneath them, recording sequentially younger events from right to left (shorewards).

From numerous radar profiles and the core data, Lindhorst and his colleagues
construct a model of how the spit evolves, each major episode of growth,
ironically, spurred by an episode of erosion. Each storm surge event leaves a
scarp, a small cliff, at the back of the eroded section, and this then acts as a
sand trap, leading to the development of “embryonic dunes” along its length.
Sand transported along the coast builds up the beach and is periodically
recycled out to the edge of the spit. Fast-growing vegetation (largely marram
grass, Ammophila arenarea) stabilises the dunes which form “foredune
ridges,” common features of hooked spits, and a new element of the spit is now
firmly established. This sequence of events repeats itself every year or so, and
the spit grows.

This work also established that the hooked spit of Sylt is considerably younger
than the main island: it began forming perhaps as recently as 1300 years ago.
Its growth is supplied by sand eroding from the coast to the south, and that
supply has increased by a factor of two or three over the last few decades -
most probably as a result of the increased efforts at beach nourishment. So
tourists cause landscape changes…

I’m still not quite clear why this spit is hooked, but presumably
it’s a reflection of the currents swirling around the entrance to the Wadden
Sea, and the dramatically complex playground of sand.

[Header image: creative commons license, fromTemporalata;
satellite images from Eovision; reference for
the Sedimentologypaper: **Anatomy and sedimentary model of a
hooked spit (Sylt, southern North Sea),**SEBASTIAN LINDHORST, JÖRN
FÜRSTENAU, H. CHRISTIAN HASS, CHRISTIAN BETZLER, Sedimentology
Volume   57 , 
Issue   4 , Pages935  -  955;] SIGNATURE

Originally published at: https://throughthesandglass.typepad.com/through_the_sandglass/2010/06/index.html