Abstract
The Sand Motor is a very large (20 million m3) nourishment constructed along the coast in The Netherlands. The huge volume of sand is redistributed along the coast by natural forces stemming from tidal currents and waves. For environmental evaluation of this large construction, the benthic subtidal fauna has been sampled prior to the construction of the Sand Motor, and at 1, 2, 4, and 6 years after construction. Although some significant differences between years were detected, overall the total density, total biomass and average number of species per sample were surprisingly constant over this time period. However, large differences were found in the species accumulation curves over samples, and in the rank-biomass and rank-abundance plots. These were related to two important trends in the communities. First, the invasive mollusk Ensis leei, the biomass dominant in the years before construction of the Sand Motor, dwindled in importance in later years. Recruitment of the species failed, but it is unclear whether, and how, this is related to the construction of the Sand Motor. Second, the correlation structure between depth, grain size, bottom shear stress due to waves and currents, which is very tight along a linear coast, was disrupted by the Sand Motor. The community composition was shown to depend strongly on these physical factors. The nature of the dependencies did not change, but the range of different combinations of factors after construction of the Sand Motor was widely larger than before. Although samples had similar number of species per sample before and after construction, the average difference between samples after construction was much larger than before. The Sand Motor is a very large construction, leading to loss of a substantial area (order 100 ha) of submarine area, which recovers at a long time scale. Total disturbance of benthos by burial, expressed as area∗(time before full recovery) was shown to be similar for the Sand Motor and for other coastal nourishment schemes when expressed per unit volume of sediment applied. However, in contrast to beach and shoreface nourishments, the Sand Motor led to a habitat diversification in the coastal zone.
Highlights
One quarter of the world’s beaches is experiencing significant erosion (Luijendijk et al, 2018)
Summarizing, the increase of biodiversity after the Sand Motor construction, as shown by the species accumulation curves, was not caused by a much higher number of taxa per sample, nor by a qualitative jump in the regional species pool or a fundamental change in the relation between species and their environment. In contrast to these invariant factors, we observe a larger range in scores of the first axis of the Principal Component Analysis (PCA) after construction of the Sand Motor, and we find combinations of the important environmental variables depth, grain size and bottom shear stress that did not occur before (Figure 9)
In terms of direct ecological losses by burial of habitats and organisms, different nourishment strategies have comparable effects that scale linearly with the volume of sediment applied in the nourishment
Summary
One quarter of the world’s beaches is experiencing significant erosion (Luijendijk et al, 2018). As the coastal zone is squeezed between the rising sea level and urban development (Schlacher et al, 2007), future climate conditions and coastal urban growth will exacerbate the coastal erosion problem and increase the need for coastal defense (Temmerman et al, 2013; Tessler et al, 2015). Hard coastal defenses, such as groins and breakwaters, require high capital investment and have several problems, such as inflexibility and scouring (van Rijn, 2011) that can lead to failures or require very high maintenance costs. Return times are between 2 and 10 years, depending on local conditions
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