Mapping the recent bio-geomorphological evolution of the man-made Terschelling blowout complex using PlanetScope imagery and airborne LiDAR data

Abstract

For centuries, The Netherlands delivered a constant battle with the sea to protect the country from flooding and to secure its welfare. Its main defence system is the dunes, protecting the vulnerable Dutch lowlands. Throughout the twentieth century, the foredunes were maintained with the philosophy “The higher, the better”, resulting in dune stabilisation that, in turn, contributed to a decline in biodiversity in the landward dunes. To counter the loss of biodiversity a new management strategy aiming to remobilize the foredunes was brought to existence. This strategy included the initiation of blowouts. In 1995 eight foredune notches were excavated, in a pioneering project along the Terschelling coast (km 15-20) in order to create a blowout complex. Now 25 years later, the remobilisation could be marked as a successful project beneficial for both human and nature. In the early years, the area has expanded tremendously. During the last years, indications arose that the blowout complex slowly shifted towards a more ecological development phase. To quantify the bio-geomorphological changes a combination of high resolution 3x3 m PlanetScope multispectral imagery and annual airborne LiDAR data were used. Throughout the six-year study period, the dune area experienced a positive but declining volume growth trend, except for 2013/2014. The volume gain was mainly concentrated around the fore- and back dunes as erosion was found within the dune valley. Furthermore, notches showed signs of closure by filling-in. The landcover change maps showed that regrowth of vegetated area exceeded the area that gets buried by the sand deposition. In autumn most vegetation started to return, net 116.000 m2 landcover turned from sand in to vegetation within two years (2016-2018). By combining the two resulting products, the changing relationship between the abiotic and biotic mechanisms come to light. Abiotic effects decreased at areas where vegetation was returning, decreasing the overall dune dynamics. With the absence of increasing frequency of storm events, it is expected that the blowout complex continues to shift into the ecological development phase and slowly stabilises.