Wendy Bonne

Recommendations for the sustainable exploitation of tidal sandbanks

A basic requirement for allowing marine aggregate (sand) extraction on the Belgian Continental Shelf (which takes place on sandbanks) is that it should not result in major environmental changes. However, a tidal sandbank (Kwinte Bank, Flemish Banks), exploited intensively since the 1970’s, has shown evidence of significant morphological changes with the development of a 5 m deep depression in its middle section; thus, since February 2003, sand extraction has ceased in this area in order to study the environmental impacts and the regeneration potential of the seabed. The present contribution synthesises the results of the multidisciplinary research, which has taken place in the area and, on the basis of these findings, considers the need for an efficient management framework, in both the planning and monitoring stages of the extraction. The investigation has shown that extraction has had significant impacts on the seabed sedimentary character and ecology and the local hydro-and sediment dynamic regime. Under these conditions, regeneration of the seabed is not likely in the short-term and, although modelling exercises have indicated possible recovery in the medium- and long-term, this is likely to be inhibited by the lack of appropriate sediments in the area. The results have provided the basis of the identification of a ‘suite’ of criteria, which can assist in the strategic planning/design of marine aggregate concession zones, the efficient management of marine aggregate extraction and the planning of effective environmental monitoring; these criteria are related to considerations on resource location, the nature/thickness of the targeted deposits, morphodynamics and sediment dynamics, biology and ecology and extraction practices. The Kwinte Bank investigation has demonstrated also the need for intensive monitoring schemes in order to identify the morphological, sedimentary and ecological impacts, related to the dredging activities. A critical part of these schemes should be the evaluation of the dredging-related effects, against the background of the natural dynamics of the seabed; thus, baseline information is crucial, as, in its absence, impact assessments are likely to remain inconclusive.

Morphological evolution of the Kwinte Bank central depression before and after the cessation of aggregate extraction

Analyses of the records of ships registers and Electronic Monitoring Systems, of the trailer suction hopper dredgers, operating on the Belgian Continental Shelf, reveal that since the beginning of extraction in 1976, 75% of the total extracted volume originates from only one sandbank, the Kwinte Bank. At present, two morphologically-distinguished depressions are observed along the two most dredged areas of this sandbank: one in the central; and one in the northern part of the bank. In order to limit the impact of sand extraction on the bathymetry, the central depression of the Kwinte Bank was closed for exploitation, in February 2003. An understanding of the morphological evolution of this central depression is based upon data obtained: (a) from November 1999, until the closure for extraction in February 2003; and (b) on the subsequent post-dredging evolution, until June 2005. During this 5-year period, a total of 17 surveys were carried out with a multibeam echosounder over the area of the central depression (KBMA) and over a reference zone on an adjacent non-exploited sandbank. The resulting time-series of bathymetrical digital terrain models, together with backscatter strength maps, permit a detailed comparison of the bathy-morphological and sedimentary evolution of both of the monitored areas. Since the commencement of multibeam monitoring in 1999, an overall deepening (by 0.5m) of the entire KBMA monitoring zone is observed, until the cessation of dredging, in February 2003. Subsequently, the deepening slowed down and the variation in sediment volumes became similar to that of the adjacent non-exploited sandbank. From this, marine aggregate extraction appears to have only a local impact.

Aggregate extraction from tidal sandbanks: is dredging with nature an option?: introduction

Sandbanks are considered as primary targets for the marine aggregate industry, not only because of considerations related to resource quality and operational advantages, but also due to the notion that natural sediment transport processes that form and maintain sandbanks are able to counterbalance the loss of sediment due to extraction. This paper introduces: (a) the problems related to the assessment of the impacts of aggregate extraction from tidal sandbanks; and (b) the multidisciplinary and integrated research that was undertaken on the potential for regeneration of the most intensively exploited area of the Kwinte Bank (Flemish Banks, Belgian Continental Shelf), following the cessation of extraction on this part of the sandbank. We assert that the results of a 30-year monitoring of exploitation effects along the Kwinte Bank have put in doubt the universal notion of ‘dredging with nature’. The elongated depressions that have been observed in the most heavily exploited areas provide a clear signal that more detailed information and thorough assessment are required in order to understand and predict the most likely evolution of the bank’s hydro-sedimentary regime and its natural and anthropogenically-induced dynamics.