Article: Skagen – at the top of Denmark with the port of the future

Weitten by: Willy B. Hansen, Executive Director, the Port of Skagen. Karsten Vognsen, Business Manager, Geopartner Inspections and Niels Broge, Project Manager, Geopartner Inspections

The article was published in Geoforum, September 2022 – Theme section PORTS

The Port of Skagen with its unique location at the northernmost point of Denmark is an important hub for the international marine traffic and the Danish fishing fleet. The easy access to the vast fishing areas of the North Sea, Kattegat and Skagerrak has made Skagen to the largest fishing port in Denmark.

In the last 10 years the Port of Skagen has experienced a significant surge in demand for land available for businesses in the port area. The port has invested heavily in its expansion, where it has undergone two expansion rounds – first to the South, and then to the East adding a new land area of 190.000 m2. The expansion will create settings for the five core business areas – fishing and processing, maritime services, goods and bunkering, cruise and leisure economy as well as commercial areas. In close collaboration, these business areas constitute a complete maritime business structure with strong value chains comprising all relevant players.

Skagen’s unique location is also relevant in terms of climate adaptation and sea level rise. The northernmost position means that the area experiences an uplift of 3mm/year as the result of the adjustment from the last ice-age glaciers (reference: Per Knudsen, DTU-Space), while the average rate of the sea-level rise around Skagen is estimated to be 3-4mm/year for the reference period 1990-2021. A preliminary conclusion is that the land and the water generally rise with the same velocity, and the relative sea level has thus remained unchanged over the last 30 years. This trend, however, is not expected to last, as the sea level will continue to rise.

Just like the most of the Danish port areas, a large part of the Port of Skagen is located on the fill site.  A heavy load from a large amount of the fill deposit causes consolidation, compression of the sea bottom and thus different rates of short-term or long-term settlement and vertical ground deformation, depending on the geological settings of a given location and fill. The regular pattern of settlement is characterized by higher deformation rates in the beginning with the exponential deceleration observed over a longer time until the settlement reaches a constant velocity.

Repeated elevation measurements of the old part of the Port of Skagen indicate persisting settlement rates between 1 and 5 mm/year, predominantly in the Northern part of the port. The measurements have been conducted once every three years since 1990’ies using high precision levelling equipment over a number of benchmarks.  The results show a general linear trend, in other words the vertical ground deformation rates remain constant over the past 10 years.  The deformations in the Northern part of the port are still at the level of the sea-level rise, making it crucial to determine their velocity and incorporate into a future climate-proofing of the port.

Geopartner Inspections in collaboration with the Port of Skagen has developed a novel innovative concept to monitor and document these vertical ground deformations in the port area. The concept is based on the use of satellite data provided by the EU free of charge until 2030, with expectations of a further prolongation of that period.  In connection to the recent port extensions, 4 radar corner reflectors have been installed in the area.  

A corner reflector is a passive device without fragile electronics used to reflect the radar signal from a satellite, and that can function as a benchmark with precise coordinates. Every 6 days a satellite passes over The Port of Skagen and the rest of Denmark acquiring radar images.  After several months, vertical ground motion can be computed with millimeter level accuracy based on the time series of these images. Along with the signal from the radar reflector, the radar imagery provides a large number of ground motion measuring points, which are typically highly reflective and stable objects such as buildings, constructions, stone moles etc. reflecting radar signal. The radar imagery resolution of 5 x 20 m makes it impossible to pinpoint the exact reflection source on a single building or a mole, but one can be sure, that the reflection and the time series of the ground motion always come from the same point. 

Geopartner Inspections has named its radar reflectors “The dynamic benchmark” since the elevation of the benchmark can be maintained and updated alone with the help of satellites. Moreover, a GPS base station can be mounted on the radar reflector, which makes it possible to measure elevations using GPS in relation to the Danish vertical datum, DVR90, with the accuracy better than 1cm. In addition, the base station can be used under airborne mapping, for instance as a reference station for UAV photogrammetry or LIDAR campaigns.

The reflectors were set up at the end of March 2021 in the new part of the port, and by June 2022 four ground motion computations have been performed for the whole port area.

The calculations, seen in Figure 4 below, show the ground motion of the new port area in the period March 2021 – June 2022 between 1,6cm at the northernmost radar reflector (RRF01-00027) and 17cm at the southernmost (RRF01-00030). As expected, the calculations show a decelerating deformation velocity. The ground motion is computed in respect to the satellite geometry. Based on that, the vertical component of the ground deformation is estimated to be between 1.9 and 20 cm in the above-mentioned period. The further development of the deformation rate is going to be closely monitored to ensure that the quay does not sink below the projected minimum elevation.

The computations have shown, that the newly built stone moles settle as expected, as well as the old part of the Port of Skagen still experiences some level of subsidence. It should be pointed out that the ground deformation of the old harbour and port areas is not specific to Skagen, but a phenomenon that is observed in nearly every Danish port.

The installation of the radar reflectors and the application of the satellite data have given the Port of Skagen an innovative and efficient tool to perform near real-time monitoring of settlement/subsidence at the port areas. The rising sea-level together with the local land settlement/subsidence will require demand-driven investments into climate-proofing in order to protect the port’s businesses and the infrastructure.

The Port of Skagen and Geopartner Incpections therefore assess that the use of satellite data in combination with the radar reflectors will equip the port with the best possible decision support in the face of the future climate challenges.