Coastal protection and the Barchan dunes of Espinho, Portugal
A case study analysis on the coastal protection efficacy of Barchan dunes.
It was on a very windy day that my husband and I took off to the beach with a fierce undulation caused by strong gusts of wind to produce this report.
This narrow strip of land bridges land and sea and harbors a notable and peculiar flora that fixes itself to the sand, locking it in space, and thereby protecting it and the shoreline from marine transgression. A dune with no vegetation is no dune, and will soon fall prey to wind fueled erosion, as well as large waves that will raze it to the ground.
Regarding fauna, it hosts many rare specimens, some of which may even be endangered! Above you can see a host of them, from organisms that attach themselves to the rocks and consume the algae that feed off of their minerals, to corals, to shelled ones that will one day become part of limestone and potentially marble.
One can see succession of sandstone rocks scattered throughout the beach, some with large marble crystals embedded in them. As marble is “metamorphosed limestone,” it is composed of recrystallized carbonate minerals, and it makes sense that you would be able to find it in a beach covered with the calcium carbonate shells of once living beings. The origin of the pressure and heat conditions that make the formation of these metamorphic rocks possible lies in the existence of a convergent plate boundary located south-southeast of the Iberian peninsula — the Iberian plate, and the African plate. Calcium carbonate is also the cementing material in sandstone. The fact that this is a continental beach (as we will discuss below) implies that these are quartz sandstones, the hardest type of sandstone.
These rocks also serve to add a layer of protection to the shoreline, by serving as a natural breakwater of sorts, decreasing the momentum of the waves much like an artificial one would. As a consequence of this, sand near the water is particularly coarse.
The sand is mostly fine, but one can easily notice a granularity gradient as they make their way down to the ocean, ranging from fine to very coarse gravel, and even cobbles. The grains are light brown, round, and even, but this homogeneity decreases the closer one gets to the sea, as mentioned earlier. Composition wise, upon closer inspection one can distinguish grains of quartz, feldspar, and mica, typical of continental sand, which is congruent with the fact Portugal is located along a passive, non-volcanic continental margin.
Beautiful as they may be, these Hottentot-figs are an exotic invasive species that are native to South Africa. They are traditionally considered a type of succulent, and they actively compete with native coastal species for nutrients, water, and light, impeding their natural development. They are being actively removed by volunteers.
Below is a compilation of the gorgeous flora that is native to this area’s dunes.
Though some areas seem barren and vegetation is spare, the dunes have come a long way.
They are divided by a small stream of fresh river water. This water is a good additional source of nutrients for the dune’s flora, and even its fauna. The force of its water is very gentle and it is unlikely to damage the surrounding dunes.
Protection Measures
The Passadiço
The “Passadiço,” as it is known in Espinho, is a boardwalk built with wooden planks whose goal is to prevent pedestrians from stepping on and thereby eroding the dunes and damaging its vegetation. It connects multiple beaches for 15 miles and it is used by hundreds of people every day.
The boardwalk has been so successful at protecting the dunes that the town hall has had to elevate it multiple times over the past couple of years.
The palisades
These wooden structures are used as defensive fences to protect the dunes from eolic erosion, as well as to increase the size of the sand deposits by accumulation. By being placed perpendicularly to the dominant winds of the region, they slow down the velocity of sand particles being transported by the wind, contributing to the growth of the dunes. These sand particles are then fixed in place by the dune’s native flora.
Above is the result of two year’s worth of sand collection, while below you can see what they normally look like, having been recently rebuilt. Note the difference in height.
To conclude,
I feel fortunate to reside minutes away from the beautiful beach and dunes surrounding the Passadiço. While the Passadiço is a lovely place to take walks, this is hardly the full extent of its importance. Since being built, it has prevented dire erosion of the dunes, which are crucial in protecting the coast of Espinho. The construction of the Passadiço and the palisades is not only a cost-effective method, but it has proven to be an impressive way to protect the integrity of the dunes, thereby forming a natural line of defense from the often agitated ocean.
I visited the Barchan dunes of Espinho for the first time about three years ago. Since then, I have witnessed the success that these measures have had on protecting the dunes. Sand has piled up over the palisades and even over the Passadiço, visibly highlighting the effectiveness and forcing the town hall to frequently elevate these structures. As time goes on, it will be interesting to track the progress of these protective measures.
