Morphological changes due to marine aggregate extraction for beach nourishment in the German Bight (SE North Sea)

Abstract

Facing the predicted rise in global sea level, sandy shorelines are under increasing pressure. In order to counteract the loss of material at eroding coastlines, beach nourishment is considered to be an environmentally friendly approach worldwide. This has resulted in a rising demand for aggregates, which are frequently extracted from the seafloor near the coast. In order to explore the long-and short-term morphological changes of such mining on the seabed, the largest extraction area in the German Bight (Westerland Dredging Area, established in 1984) was investigated in this study. Several measurement campaigns were conducted between the years 1994 and 2017 using a set of hydroacoustic techniques. The measurements revealed that up to 20-m-deep pits with diameters of more than 1 km were dredged into the seafloor. The depressions caused by this sand mining are still detectable more than 30 years later. Because of slope failures that mainly consist of fine sand, the formerly steep rims at fresh dredging pits smoothed within a few months. However, after approximately 1 year, muddy sediments dominated the deposition. Since the sedimentation rates are slow, a complete backfill of the post-dredging pits is likely to take many decades. A natural regeneration towards the former seafloor conditions is only visible at the shallow rims of the oldest dredging pits.

Persistent effects of sand extraction on habitats and associated benthic communities in the German Bight

“…The comparison of the 2019's bathymetry of the oldest pits with earlier measurements in 2016 and 2017 (Mielck et al, 2018) revealed no significant change indicating that the annual sedimentation rate was below the resolution of our multibeam device (~10 cm). This is in accordance with the very low sedimentation rate (2-18 mm per year) recorded from a muddy depression near the Island of Helgoland, ~80 km south of the study area (Dominik et al, 1978;von Haugwitz et al, 1988).…”

“…The dredging pits themselves reveal a muddy surface, even 35 years after sediment excavation. This is due to low sedimentation rates in the southern North Sea and the study area (Dominik et al, 1978;von Haugwitz et al, 1988;Mielck et al, 2018) brought about by the combination of a lack of mobile sediments and weak transport rates (Valerius et al, 2015).…”

“…Freshly dredged pits show a layer of fine sand which derived from the formerly steep rims of the pits (Zeiler et al, 2004;Mielck et al, 2018). After this initial phase, accumulation of muddy sediments prevails at very low sedimentation rates.…”

“…This is due to the combination of a lack of mobile sediments and low transport rates (Valerius et al, 2015). Accordingly, a complete backfill of the deep dredging pits was estimated to take many decades (Mielck et al, 2018).…”

“…In order to classify the seafloor and to map the prevailing benthic habitats at dredging sites, hydroacoustic devices are considered as an useful remote sensing tool. While multibeam echosounders give information about water depth and morphology, and can thus be used to calculate backfill rates at the extraction pits (Harris and Baker, 2012;Jones et al 2016;Mielck et al, 2018), sonar systems such as sidescan sonars allow to investigate the backscatter intensity as an additional parameter useful for seafloor classification (Blondel and Murton, 1997;Blondel, 2003;Mielck et al, 2014, Mielck et al, 2015. Extensive ground truthing (e.g.…”