Observations of a narrow zone of high suspended particulate matter (SPM) concentrations along the Dutch coast Carola M. van der Hout n , Theo Gerkema, Janine J. Nauw, Herman Ridderinkhof Royal Netherlands Institute for Sea Research (NIOZ), Postbus 59, 1790 AB Den Burg, The Netherlands article info Article history: Received 7 January 2014 Received in revised form 12 December 2014 Accepted 6 January 2015 Available online 14 January 2015 Keywords: Turbidity maximum zone In-situ measurements Periodic stratification ROFI Cross-shore SPM transport Resuspension abstract The objective of the study described in this paper is to localize the transport path of suspended parti- culate matter (SPM) in the Dutch coastal zone in the southern North Sea. It is known that a large mass of SPM is transported northward from the Strait of Dover, which is however mostly hidden from satellite and other surface measurements. The study area is located at 80 km north of the Rhine-Meuse estuary mouth in the far-field plume of the region of freshwater influence (ROFI). We investigate the occurrence and persistence of a turbidity maximum zone (TMZ) in an area closer to the coast than studied in pre- vious observational programs. Shipboard measurements of vertical profiles of SPM concentrations, density and current velocities with a high cross-shore spatial resolution are presented. A turbidity maximum zone is found at a distance between 0.5 and 3 km from the coast along 30 km of the coastline. Observed concentrations are shown to vary strongly within a tidal cycle, and also between contrasting meteorological conditions in terms of the spring-neap tidal cycle, the significant wave height and the wind force. Temporary stratification is observed during spring tides, and occurs on the ebb phase of the tidal cycle. Cross-shore transports at a transect perpendicular to the coast show an accumulation of SPM in the TMZ within one tidal cycle. Possible mechanisms for this accumulation close to the coast are discussed. & 2015 Elsevier Ltd. All rights reserved. 1. Introduction Along the Dutch coast a large mass of suspended particulate matter (SPM) is transported northward, but it is often hidden from satellite and other surface measurements (Visser et al., 1991; Joordens et al., 2001; Geyer et al., 2004; Fettweis et al., 2007; Pietrzak et al., 2011). SPM in the Dutch coastal zone originates predominantly from the Strait of Dover. A well-accepted estimate of the yearly average SPM transport though the Strait of Dover is 44 Mton/yr, with seasonal variability between 22 and 58 Mton/yr (McManus and Prandle, 1997; Fettweis and Van den Eynde, 2003). Other studies show a wide range of the yearly averaged SPM transport between 13 and 44 Mton/yr (Fettweis and Van den Eynde, 2003; De Kok, 2004). The variability in these estimates reflects both the limited nature of the available field data and the high yearly variability of the transport itself (Van Alphen, 1990; McManus and Prandle, 1997; Fettweis and Van den Eynde, 2003). The SPM transport though the Strait of Dover splits about 50–50 over the coasts of the UK and Belgium (McManus and Prandle, 1997; Fettweis et al., 2007). Following the SPM northward, it re- mains close to the coast and only temporary settling occurs in the Belgian turbidity maximum zone at the Flemish banks and in the Rhine-Meuse estuary (Fettweis and Van den Eynde, 2003, De Nijs, 2012). Possibly erosion of the Flemish banks contributes up to 3 Mton/yr (Van Alphen, 1990; Fettweis et al.,2007). The resulting estimate for the transported mass of SPM along the Dutch coast is 22 710 Mton/yr. The study of SPM has become urgent in recent decades as the human pressure on coastal systems has increased, and negative effects of SPM – decreasing optical depth of the water column, carrying pollutants, harbor siltation – need to be mitigated. One of the gaps in the present day knowledge is the transport pattern of SPM near the shore. Large engineering works in coastal areas around the world might intervene with the SPM transport, but the effects are not always well understood. The use of bottom landers (Souza and Howarth, 2005; Cacchione et al., 2006; Manning and Bass, 2006) and satellite technology (Fettweis et al., 2007; Eleveld et al., 2008; Moreira et al., 2013) for monitoring in-situ con- centrations of SPM has enormously increased our understanding of the transport of SPM. With bottom landers, details of the pro- cesses determining the vertical flux of SPM have been uncovered, by observing floc formation, settling velocities and turbulent fluxes (Friedrichs et al., 2000; Howarth et al., 2002; Manning and Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/csr Continental Shelf Research http://dx.doi.org/10.1016/j.csr.2015.01.002 0278-4343/& 2015 Elsevier Ltd. All rights reserved. n Corresponding author. E-mail addresses: carola.van.der.hout@nioz.nl (C.M. van der Hout), theo.gerkema@nioz.nl (T. Gerkema), janine.nauw@nioz.nl (J.J. Nauw), herman.ridderinkhof@nioz.nl (H. Ridderinkhof). Continental Shelf Research 95 (2015) 27–38