Chapter 25 SPM variations in a harbour basin

Abstract

Abstract This paper deals with suspended particulate matter (SPM) variations in the Botlek Harbour basin located along the meso-tidal Rotterdam Waterway at the limit of saline water intrusion. Observations of the vertical structure of velocity, salinity and turbidity from a 7 hour boat survey in Rotterdam Waterway just down-estuary of Botlek Harbour and long-term time series of salinity and turbidity near the bed acquired with a measuring rig in the Botlek Harbour basin are presented. These data have been analyzed to determine the possible causes of the pattern of SPM variations. The existence of an estuarine turbidity maximum (ETM) in this part of Rotterdam Waterway is deduced from both data sets. On a tidal timescale, the ETM in Rotterdam Waterway is associated with a continuous cycle of processes that include settling, deposition, re-entrainment and advection of SPM near the head of saline water intrusion. High near-bed SPM concentration layers are observed due to tidal advection of the ETM towards the Botlek Harbour region during flood and low near-bed SPM concentration layers are observed at slack tide as a result of settling of background SPM. Recurring salinity and SPM increases around local HW in the long-term data records are attributed to advection of a part of the ETM into Botlek Harbour by salinity-induced density currents. The persistence of this pattern indicates that the supply of SPM to the ETM is sufficient to keep it maintained over time. Modifications of the “regular or periodic” siltation pattern correlate with changes in the hydrodynamic forcing at the sea and river boundaries, respectively, due to mean water level changes during and after rough weather episodes and fresh water discharges below the threshold value of the Haringvliet sluicing program. The aforementioned cycle of processes, the occurrence of non-capacity transport conditions and the fact that dredged material in Rotterdam Waterway hardly contains any mud indicate that the amount of SPM in the ETM is not stored in the sandy bed. This implies that significant change of the amount of SPM in the ETM is determined by advection of SPM from the sea and river boundaries, which has a larger timescale than the tidal period. Modifications of the “regular or periodic” siltation pattern are therefore attributed to displacements of the salt intrusion limit and ETM relative to Botlek Harbour and/or to a different functioning of the dominant (density driven) exchange mechanism of SPM due to temporary storage of saline water in the basin, but at timescales exceeding the tidal period.