Mine burial in the seabed of high-turbidity area—Findings of a first experiment

• Published in: Continental shelf research 2012-07, Vol.43, p.107-119
• Main Authors: Baeye, Matthias, Fettweis, Michael, Legrand, Sebastien, Dupont, Yves, Van Lancker, Vera
• Format: Article
• Language: English
• Subjects: Fluid dynamics; Fluid flow; High-concentrated mud suspension; Hydrodynamics; Marine; Mine burial experiment; Mines; Scouring; Sea beds; Sediments; Slack tides; Suspended particulate matter; Tides
• ISSN: 0278-4343; 1873-6955
• DOI: 10.1016/j.csr.2012.05.009

The seabed of the North Sea is covered with ammunition dating back from World Wars I and II. With increasing human interference (e.g. fisheries, aggregate extraction, harbor related activities), it forms a threat to the safety at sea. In this study, test mines were deployed on a sandy seabed for 3 months to investigate mine burial processes as a function of hydrodynamic and meteorological conditions. The mine experiment was conducted in a shallow (9m), macrotidal environment characterized by highly turbid waters (yearly and depth-averaged suspended particulate matter concentration of 100mgl−1). Results showed some variability of the overall mine burial, which corresponded with scouring processes induced by a (sub-) tidal forcing mechanism. The main burial events however were linked to storm-related scouring processes, and subsequent mine roll into the resulting pit. Two storms affecting the mines during the 3-month experiment resulted in enduring increases in burial volume to 60% and 80%, respectively. More cyclic and ephemeral burial and exposure events appear to be linked to the local hydrodynamic regime. During slack tides, suspended sediment settles on the seabed, increasing the burial volume. In between slack tides, sediment is resuspended, decreasing the burial volume. The temporal pattern of this never reported burial mechanism, as measured optically, mimics the cyclicity of the suspended sediment concentration as recorded by ultrasonic signals at a nearby benthic observatory. Given the similarity in response signals at the two sites, we hypothesize that the formation of high-concentrated mud suspensions (HCMS) is a mechanism causing short-term burial and exposure of mines. This short-term burial and exposure increase the chance that mines are ‘missed’ during tracking surveys. Test mines contribute to our understanding of the settling and erosion of HCMS, and thus shed a light on generic sedimentary processes. ► Sandy seabed is responsible for the well-known mine burial process related to scour by storm waves. ► Superimposed on the mine burial by scour signal, a more cyclic (quarter–diurnal) signal occurred. ► Suspended particulate matter dynamics are responsible for this cyclicity. ► During slack tides, the mine is covered with high concentrated mud suspension (HCMS) layer. ► Mine burial by transient HCMS is introduced and discussed for the Belgian coastal zone.