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Monitoring the impacts of marine aggregate extraction

 | 
Robert Lafite
, 
Michel Desprez

GIS SIEGMA: objectives and study sites

Texte intégral

  • 14 www.siegma.fr

1GIS “SIEGMA”14 — Scientific Interest Group “Monitoring the impacts of marine aggregate extraction”— was established in June 2003 in Normandy (NW of France) with the 2002-2006 State-Region Plan contract with the support of European ERDF funds. The objective was twofold:

  • to promote consultation with fishermen and foster coordination between scientists to provide transparent and deeper research assessing the overall impact of extraction on the Eastern English Channel’s marine environment;

  • to provide mining practices (fallow test, levelling) able to minimize environmental impacts and promote restoration.

2Previous technical information on the operational mode of extraction is necessary in order to finely characterize how the activity impacts the environment. In the Eastern English Channel, this operation is carried out by a trailing suction hopper dredger (TSHD) moving at reduced speed on the extraction site. A hydraulic pump is fixed to the vessel and connected to a seafloorlying draghead equiped with a suction pipe. The pump sucks a mixture of water and sediment into a storage hopper. Excess water — overflow — is returned to the sea through a well or through side doors. Screening of the mixture is forbidden. The deposit is exploited through successive passages of the draghead at low speed — approximately 2 knots — allowing for furrows to be dug.

3The analysis of literature shows a strong control of the local hydro-sedimentary conditions and the extraction methods on the impact magnitude and the restoration process of the environment. The studies conducted by GIS SIEGMA have focused on two sites featuring a different hydrosedimentary environment to obtain a better representation of the effects in the Eastern English Channel. Dieppe has been a commercial extraction site of gravels and pebbles over 6 km² since 1980 while the Baie de Seine location has been an experimental extraction site of gravelly sands over two areas of 0.6 km² since 2007. The site complies with the extraction procedures established by GIS.

Trailer suction hopper dredger in activity: infilling of the storage hopper in front, oversanding plume behind (source: GIE Graves de Mer).

Synthesis of the main impacts of aggregate extraction on the marine ecosystem (adapted from IFREMER).

Location of the two study areas of GIS SIEGMA in the Eastern English Channel.

4Although Dieppe undergoes stronger tidal currents than the Baie de Seine location, the data acquired on these two sites—including those for swell—are representative of a hydrodynamical context of moderate energy and of a medium intensity of extraction (according to the classification of Boyd et al., 2004). The research topics selected by the SIEGMA programme aim to characterise the environmental effects and their duration (restoration rate); they concern the following compartments:

  1. Turbidity in the water column,

  2. Morphology and sedimentary nature of the seabed,

  3. Benthic communities,

  4. Fish communities,

  5. Trophic relationships between benthos and fish,

  6. Habitats and biodiversity (structure and function).

5The objectives of these studies were:

  • to complete international knowledge on the impact of extraction and on seabed restoration namely through experimenting fallow temporal zoning;

  • to obtain new information on fish following several ICES recommendations (1992, 2001, 2009).

6These studies will enable to present good practices for environnemental monitoring and exploitation methods.

Site-specific experimentation in the Baie de Seine

Site-specific experimentation in the Baie de Seine

Experimental site locations (A et B) in the Baie de Seine, with position of the sampling stations for benthos*.
Colour code: control stations in orange and red, reference stations in green.

The originality of the experiment in the Baie de Seine lies in the fact of having two sites of equal surface (2x0.6 km²) on which the strategy of exploitation is different (site A = fallow mode for 1 year; site B = continuous mode for 3 years). Monitoring the two sites allows to:
compare the impact after only one year of extraction (11 control stations of site A in orange) with the impact of extraction after 3 years (14 control stations of site B in red);
study the restoration rate of the seabottom during the 3 years following the end of extraction on site A (fallow test);
compare the environmental impacts observed between this experiment and the long-term regular extraction practice in Dieppe—for more than 20 years—in a different hydrosedimentologic context.
The results that have been obtained from the impacted locations are compared to those obtained from 5 reference stations that are way apart from the extraction sites (in green). They have been monitored since 1989 and have thus allowed to report on the natural evolution of the environment (sediments and related benthic macrofauna).

Notes

14 www.siegma.fr

Table des illustrations

Légende Trailer suction hopper dredger in activity: infilling of the storage hopper in front, oversanding plume behind (source: GIE Graves de Mer).
URL http://books.openedition.org/purh/docannexe/image/114/img-1.jpg
Fichier image/jpeg, 112k
Légende Synthesis of the main impacts of aggregate extraction on the marine ecosystem (adapted from IFREMER).
URL http://books.openedition.org/purh/docannexe/image/114/img-2.jpg
Fichier image/jpeg, 124k
Légende Location of the two study areas of GIS SIEGMA in the Eastern English Channel.
URL http://books.openedition.org/purh/docannexe/image/114/img-3.jpg
Fichier image/jpeg, 172k
Titre Site-specific experimentation in the Baie de Seine
Légende Experimental site locations (A et B) in the Baie de Seine, with position of the sampling stations for benthos*.Colour code: control stations in orange and red, reference stations in green.
URL http://books.openedition.org/purh/docannexe/image/114/img-4.jpg
Fichier image/jpeg, 93k

© Presses universitaires de Rouen et du Havre, 2014

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