Fine particles make up attractive – and easily eroded – beaches
The soft cliffs at the west coast were formed during the last ice age and consist of very fine particles such as clays and fine sands, made up of 100 meters of marine sediments deposited over 100.000 years (known as ‘Skærumhede series’).
Part of these layers are visible on the cliffs when walking on the beaches. The sand of the west coast consists mainly of these soft sediments and this is the main reason the beaches are so attractive.
Wind and wave forces ‘act’ on the beach and transport the sediments offshore – this is what we call coastal erosion. Of course, if the sediment is small and light like at the west coast, it will be easier to remove it.
Severe erosion happens mainly during storms, because the high pressure and winds typical of stormy weathers push the water on the shore and temporarily increase water level.
It is more or less the same thing that happens when we blow on our cup of tea: If we are not careful, the tea level on the opposite side of the cup will spill over the edge.
As a result, a beach where you walked or drove in summer may be completely flooded with waves during a storm in winter, and you may find that part of the beach is missing the following summer at your next visit.
Coastal erosion is a complex issue
The costal dynamics are complex and the reference timescale (seasons, years, decades, centuries or millennials) is very important to understanding what happens when the shoreline erodes.
In fact, the Danish west coastline has been moving both back (erosion) and forth (deposition) since the last ice age and even before.
Some claim that erosion in some places on the west coast has been accelerating during the past 20 years in unforeseen ways. This is the case, for example, in Nørlev Strand, which I have used as a research case.
We can count the missing meters of coastline
Humans can accelerate natural erosion. Because the sediment is not only removed but also added to the beach by the waves, erosion always occur when there is an imbalance in this removing-adding equilibrium (for example in storms where waves are stronger and water levels higher).
Most coastlines have a longshore current; this can be weak or strong and vary in time, but currents are also playing a role in moving sediments.
Along the Danish west coast there is a constant current from South to North that transport the sand and clays along the coast, just like a river.
In normal conditions, we can say that this is a source of sediments for the beaches, therefore counteracting erosion. But in Nørlev strand a coastal protection was built in 1981 and this stopped and deviated the flow of sediments.
In the few hundreds meters north after the protection, the coast line retreated 150 meters in 43 years vs 27 meters a few kilometers further north. This is considered to be a direct consequence of the manmade structures altering the sediment flow along the coast.
Steady situation over the last 40 years
The west coastline suffers from chronic erosion, meaning that the combination of waves, winds and currents, generate an erosion that is constant in time (so the sediments coming from the longshore transport are less than the ones taken away). Nevertheless we have been building on this coastline and house owners claim that the erosion is accelerating.
Is this true? What could be the causes?
In order to understand if the erosion is accelerating, my colleagues and I looked at the records, particularly to parameters that may relate to climate change, such as increased wind speeds and storminess.
We compared different aerial photos dating all the way back to before the time of the coastal protection construction, we studied water level and wave climate records (how often the beach was flooded and how big the waves have been during the past 43 years) and finally we used numerical models to predict different erosion scenarios.
The simulations can tell us that by a small increase in wave height and forces, there would be a dramatic increase in erosion. But our data did not reveal an increase in wave heights nor in water levels.
In other words, the situation has on average been quite steady over the past 40 years. Additionally, we found that waves hit the coast with a 90° angle more often than before, and this, with everything else being constant, should slightly slow down erosion! But we do not see signs or erosion slowing downand locals still believe it is increasing.
So why is the shore eroding?
It seemed we had to conclude that the known effects of climate change (increase in storm events and wind forces) did not affect our case study, and that the erosion in Nørlev strand today is natural, constant and steady, with an aggravated effect caused by manmade structures circa 10 years after their construction.
But if there were no significant changes in waves, winds and wave heights for the past years and the effect of manmade structures on erosion decreased to close to zero during the last 8 years why is the local community sure that an acceleration of the erosion is still occurring and their houses are at higher risk than 20 years ago?
This general opinion is shared not only in Nørlev Strand, but in different locations along the West Coast suffering from erosion.
Our knowledge in the field has progressed a lot in the last 20 years and we now know that the aggravating effects of manmade structures are only temporary. But there may still be other factors to consider.
This is when we found out that rainfall had increased significantly during the past 40 years. Higher amount of rain on sand can provoke more landslides and therefore expose more sand to waves.
Climate change can contribute to erosion
If this is true, it would mean that climate change is contributing to coastal erosion in an unexpected way – by causing more rainfall that generates more landslides on the beach – and it would finally give reason to the local community’s observations and fears.
We know that rainfall has increased, but we can only hypothesize that this can cause landslides from studies on different kinds of soils and sands.
But whether this in fact is the cause of the perceived increased rate of erosion, our data cannot say. We would need to look into detail into each heavy rain event of the past 40 years and hope to have some kind of assessment on landslides on the beach.
We need to research more before concluding anything
Our data tells us that the coastal erosion has, on average, been constant in the past 40 years and that manmade structures have accelerated that erosion temporarily, with obvious effects years after they have been constructed.
But if the increased rainfall was accelerating the erosion a few centimeters per year, we would not be able to tell, as we do not measure costal erosion with such a precision. The coastline position is not defined by the centimeter.
The dramatic Danish west coast is a landscape devoted to change, and it is very hard to argue with nature.
The shoreline is eroding and it is a natural event that we cannot stop without major interventions such as the ones in the Netherlands, where dikes offshore were built. However, erosion might still be accelerating due to circumstances that we are not sure of yet.
If we try to prevent the erosion, we might make it worse upstream, like it happened in Nørlev where erosion was accelerated by manmade structures built downstream.
Therefore, it is important that we keep an eye on this and study it further before jumping to conclusions.