Remobilizing stabilized island dunes for keeping up with sea level rise?

Abstract

Dune spits of the island of Sylt (North Sea) persisted through millennia by mobile dunes transferring sand from exposed to sheltered shores. However, 150 years of systematic planting on washovers, blowouts and migrant dunes have almost completely stopped this sand conveyor belt. Spits lost sand until sand replenishments to the exposed side compensated for losses during the last three decades. Based on maps since 1878, aerial images from the 1930s onward as well as botanical ground surveys, we document a long-term shift from loose grass cover to dense heather dominated vegetation. Bare sand areas almost vanished and high shrubs proliferated. Only three dunes continue migrating with about 3 m a-1 since 1936. Mainly artificial dune stabilizations and introduced plants have facilitated succession to an almost complete plant cover. Thus, revitalization of aeolian dynamics would be desirable, and pilot projects may acquire the appropriate knowledge. For a 1.7 × 2.5-km corridor across the island spit, we therefor discuss two suggested experiments: (1) intensify sand nourishments to the eroding beach and stop stabilization measures to allow excess sand blown inland and washovers entering deflation plains, and (2) examine the potential of tunnels for roads to let mobile dunes passing over. Such experiments may not only contribute to dune biodiversity but also to long-ranging adaptations of crowded barrier islands to accelerating sea level rise.

Residual dune ridges: Sedimentary architecture, genesis, and implications for palaeo‐climate reconstructions

“…1720 to 1936 CE (corresponding to 80 and 300 a). This rate is low but still in the range of migration rates of 1.7 to 5.0 m yr À1 as reported for transgressive dunes on Sylt (Costas et al, 2012;Lindhorst & Betzler, 2016;Osswald et al, 2019). The uncertainty of the age model and the limited number of dated ridges, does not allow for differentiating in migration rate between the younger and the older part of the sequence.…”

“…Most of these dunes are stabilized by vegetation due to anthropogenic measures against sand drift. These measures were locally initiated in the late 19th and early 20th century, but reached their peak in the 1930s and 1940s (Osswald et al, 2019). Stabilization measures on the transgressive dune in downwind direction of the studied residual dune-ridge sequence started in 1930 CE (Müller & Fischer, 1938).…”

“…Historical migration rates for the largest active dune range between 1.7 to 5.0 m yr À1 (average 4.1 m yr À1 ) over the time period 1936 to 2009 CE (Costas et al, 2012;Lindhorst & Betzler, 2016). The overall migration rate of all three active dunes over the same time period is 2.9 AE 1.2 m yr À1 (Osswald et al, 2019). The general stratigraphy of the barrier system of Sylt comprises two main units.…”

Is ‘re-mobilisation’ nature restoration or nature destruction? A commentary

“…Over the past three decades there has been increasing recognition, particularly in Western Europe, of a relatively rapid increase in the vegetation cover of coastal dune systems, leading to a decline in the extent of mobile dunes and of bare sand cover (Provoost et al 2011). This process has been described from many countries (Rhind et al 2001;Jackson and Cooper 2011;Arens et al 2013;Darke et al 2013;Miot da Silva et al 2013;Pye et al 2014;Moulton et al 2019;Osswald et al 2019) and has in turn given rise to studies that attempt to determine the causes of this phenomenon and others which have focused on describing what effects the changes in vegetation and reduced dune mobility have had, or may have, on the dune ecology and geomorphology. In particular dune sealing (loosely defined here as the growth of vegetation across the front of, or within active dunes) has been identified as leading to a decrease in ecological diversity and species richness, as well as posing a threat to rare or endangered species (Smith 2009;Jones et al 2010;Howe et al 2010;Houston 2016).…”

Comment on ‘Is ‘re-mobilisation’ nature restoration or nature destruction? A commentary’ by I. Delgado-Fernandez, R.G.D. Davidson-Arnott & P.A. Hesp

“…The paradigm shift in coastal dune management has been discussed for some years now (Van Boxel et al 1997;Arens et al 2004, Arens and Geelen 2006, Arens et al 2013aGeelen et al 2015;De Groot et al 2017;Martínez et al 2013;Provoost et al 2011;Psuty and Silveira 2013;Osswald et al 2019). Most of the Dutch dunes are the result of coastal erosion of an extensive barrier coast, induced by sea-level rise, resulting in the development of smaller or wider dune belts with transgressive dunes (Klijn 1990).…”