Morphodynamic evolution of a sand extraction excavation offshore Vale do Lobo, Algarve, Portugal

Gonçalves, D.S.; Pinheiro, Luís Menezes; Silva, Paulo A.; Rosa, João; Rebelo, Luís; Bertin, Xavier; Teixeira, Sebastião Braz; Esteves, Rita

doi:10.1016/j.coastaleng.2014.02.001

Cited by 7 publications

(6 citation statements)

References 16 publications

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“…Moreover, joint research projects (e.g., SANDPIT (Sand Transport and Morphology of Offshore Sand Mining Pits/Areas; Van Rijn et al 2004) and EUMARSAND (European marine sand and gravel resources; Bonne 2010)) dealt with this topic. However, information on backfill processes regarding the extraction pits after a few months (Cooper et al 2007;Gonçalves et al 2014) is still sparse, especially in areas with low sedimentation and weak transport rates as given in the area west of Sylt (SE North Sea).…”

Section: Introductionmentioning

confidence: 99%

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

et al. 2018

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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.

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Section: Introductionmentioning

confidence: 99%

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

et al. 2018

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“…If that stress depends on roughness due to bedforms and biogenic features, this adds significant prediction uncertainty. Effects of bed roughness will need to be considered during the modelling of seabed sediment recovery after sand dredging (Gonçalves et al ., 2014) and assessing risks of subsea pipelines or cables from scouring (Mohr et al ., 2016; Tom et al ., 2018).…”

Section: Discussionmentioning

confidence: 99%

Wave‐influenced deposition of carbonate‐rich sediment on the insular shelf of Santa Maria Island, Azores

et al. 2022

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Formulae for sediment thresholds of motion are commonly based on flume experiments on rounded quartz particles and it is unclear how well they predict thresholds in natural settings. Here, sediment threshold shear stresses were calculated from one such formula using surface grain‐size data from 112 sites around Santa Maria Island, Azores. To compare with those stresses, a Simulating Waves Nearshore model was run for three typical winter months to predict shelf stress maxima due to waves. As wind‐driven and other circulations also increase stresses, the model predictions are under‐estimates. Comparison of the two stress estimates suggests that the whole shelf of the island was mobile during extreme conditions. However, three forms of evidence contradict this. First, 129 rollovers of sandy clinoforms lying in 30 to 200 m water depths around the island were identified from boomer seismic data. It has been suggested that such rollovers mark depths at which hydrodynamic stresses fall beneath the sediment threshold of motion. Second, differences in grain‐size diversity between carbonate‐free and whole sediment indicate where carbonate particle fragmentation occurs. Third, seabed images reveal variations in ripple character and presence. The combined data suggest that deposition has occurred in the middle and outer shelf, overlapping where the model predicts sediment mobilization. However, by decreasing the model bottom shear stress or increasing the shear stress at threshold of motion by a factor of two to three, deposition is predicted to have occurred immediately deeper than the shallow active rollovers. Therefore, in practice, the ratio of wave‐imposed shear stress to stress at threshold of motion is two to three times smaller than predicted. This is speculated to be due to the presence of widespread hard substrates and other features shielding particles between them from wave stresses. Alternatively, the threshold of motion is higher than expected from the formulae for these sediments dominated by bioclastic particles.

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“…Based on the realized distribution across sediment types in the eastern North Sea, most benthic species seem to be restricted to a species-specific spectrum of sediment composition (Armonies, 2021). However, since sediment composition correlates with many other factors such as hydrodynamic stress and sediment stability (Snelgrove and Butman, 1994), or oxygen supply and biogeochemistry (Giere, 2008;Giere et al, 1988), the causes for these restrictions are not clear. Therefore, we can only state that the sediment types occupied in the dredging pits coincide with the sediment types occupied in the surroundings (see Table S2 in the Supplement).…”

Section: Discussionmentioning

confidence: 99%

“…As a result, there is a worldwide high demand for marine aggregates needed especially for coastal protection. It has reached a high level on a worldwide scale and further increase is expected (Hamm et al, 2002;Kubicki, 2007;Danovaro et al, 2018;Schoonees et al, 2019). For the northern European continental shelf, the extracted volume rose from altogether 53×10 6 m 3 between 1998 and 2002 to a total of 73.2 × 10 6 m 3 in 2018 (ICES, 2016(ICES, , 2019.…”

Section: Introductionmentioning

confidence: 99%

“…Sea-level rise, with ever-increasing rates in the near future, demands protection measures of endangered coastlines crucial for the local population (Nicholls and Tol, 2006;Hinkel et al, 2014). In many cases, ecological awareness and sustainability considerations have led to the preference for "soft" coastal protection measures like beach nourishment over "hard" protection approaches such as dikes or revetments (Hamm et al, 2002;Pranzini et al, 2015;Staudt et al, 2020). As a result, there is a worldwide high demand for marine aggregates needed especially for coastal protection.…”

Section: Introductionmentioning

confidence: 99%

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

et al. 2021

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Abstract. Sea-level rise demands for protection measures of endangered coastlines crucial for the local population. At the island of Sylt in the SE North Sea, shoreline erosion is compensated by replenishment with sand dredged from an offshore extraction site. We studied the long-term effects of sand extraction on bathymetry, geomorphology, habitats and benthic fauna. Sand extraction created dredging holes about 1 km in diameter and up to 20 m below the ambient seafloor level. Directly after dredging the superficial sediment layer, inside the pits was dominated by coarse sand and stones. Hydroacoustic surveys revealed only minor changes of bathymetry > 35 years after sand extraction. Obviously, backfill of the dredging pits was very slow, at a rate of a few millimeters per year, presumably resulting from low ambient sediment availability and relatively calm hydrodynamic conditions despite high wave energy during storms. Thus, a complete backfill of the deep extraction sites is likely to take centuries in this area. Hydroacoustic surveys and ground truthing showed that the backfilled material is mainly very fine sand and mud, turning the previously coarse sand surface into a muddy habitat. Accordingly, grab samples revealed significant differences in macrozoobenthos community composition, abundance and species density between recently dredged areas (< 10 years ago), recovery sites (dredging activity > 10 years ago) and undisturbed sites (control sites). Overall, dredging turned the original association of sand-dwelling species into a muddy sediment association. Since re-establishment of disturbed benthic communities depends on previous re-establishment of habitat characteristics, the low sedimentation rates indicate that a return to a pre-dredging habitat type with its former benthic community and habitat characteristics is unlikely. Since coarse sand is virtually immobile in this area, a regeneration towards pre-dredging conditions is also unlikely without human interference (e.g., mitigation measures like depositing coarse material on the seafloor to restore the sessile epifauna).

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Morphodynamic evolution of a sand extraction excavation offshore Vale do Lobo, Algarve, Portugal

Cited by 7 publications

References 16 publications

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

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

Wave‐influenced deposition of carbonate‐rich sediment on the insular shelf of Santa Maria Island, Azores

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

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