Coastal erosion and accretion
In Denmark, you are never more than 52 km from the ocean (according to Wikipedia). The country consists of one peninsula and a whole lot of islands. This results in a rather large coastline, which, as coastlines do, shift around due to coastal erosion and accretion. It even states on the Wikipedia page that “the size of the land area cannot be stated exactly since the ocean constantly erodes and adds material to the coastline”.
This constant change is not necessarily something you walk around and ponder on every day. But for some people, the dynamic nature of the coasts can be cause for quite a headache. What do you do if your house is on its way into the ocean?
Technology as a tool for understanding
One approach to understand the changes is to track them with high-resolution aerial imagery. Let us take a look at one part of Lønstrup, a city on the west coast of Denmark.
The change is quite clear, and three houses didn’t make it. It should probably be mentioned here that they are removed before they actually fall into the ocean. Nevertheless, they are gone in 2022. While looking at images provides a good overview, one might also be interested in actual measurements at specific places. To do this, we can use Geographic Information System (GIS) software, such as QGIS. The images are georeferenced, so you can find the GPS coordinate of every part of the image, and thereby directly calculate real-world distances in the image.
It can be a bit difficult to initially see what’s going on, as we are looking at a mix of two images. To help a bit, the left red dot is at the edge as it was in 1999, and the right red dot is at the edge as it was in 2022. The distance between the two dots gives a measurement of the coastal erosion in the time span for that specific place. In this case, we measured it to 65.2 m, or roughly 2.8 m/year.
The human impact
As we are often reminded these days, humanity has a big impact on nature. This is also the case with coastal erosion, a subject often covered in news articles, especially regarding whether or not coastal protection should be implemented. Recently, there was case in Hesseløje on Funen (see this dr.dk article). Let us take a look at what it looks like from above.
The coastal protection was installed without permission from 2001 to 2018, was then approved in 2022, only to have the approval revoked again in 2023. The debate now centers on the best course of action for the next steps, and arriving at a consensus has proven challenging for both locals and politicians.
Should we just let nature be nature?
It’s difficult to determine how much we should try to control nature, and it’s most certainly difficult for groups of people to reach agreement on the subject. The goal of this blog post is not to give an answer to whether we should do more or less coastal protection. Instead, the aim is to show how technology can be used to monitor our coasts by taking a look at them from above. Hopefully, this can be a valuable input in the discussion on the degree of human intervention we think is needed. And if nothing else, at least it can be a fascinating view into how a country can “move around” so much that we cannot even have a precise estimate of its land area.
You are very welcome to try downloading your own images and investigate an area of your interest. Head on over to https://download.atla.ai/ and give it a go, and feel free to reach out to us at contact@atla.ai if you have any questions or would like to hear more.
The geographer Sylvain Genevois who runs the excellent website Cartonumérique points out that, since 1 August 2023, many French towns have chosen to join the list of municipalities subject to coastal erosion (see decree n°2023-698 establishing the list of municipalities whose town planning and development policy must be adapted to the hydrosedimentary phenomena leading to coastal erosion).
Map of French municipalities affected by erosion On the France Inter website.