Morphological Evolution of the Kwinte Bank Central Depression Before and After the Cessation of Aggregate Extraction

Abstract

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 ( I) 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 ON er 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 inure KBMA monitoring zone is observed until the cessation of dredging in February 2003 Subsequently the deepening slotted down and the variation in sediment volumes became similar to that of the adjacent non-exploited sandbank From this marine aggregate extraction appeals to hate only a local impact