Wind-driven variability in longshore transport of SPM in the turbidity maximum zone along the Dutch coast

Abstract

Long-term measurements from a mooring off the Dutch coast, placed in the turbidity maximum zone (TMZ), are analyzed to study the temporal variability of the alongshore transport of SPM (Suspended Particulate Matter), covering a two-year period. In particular, the effect of wind speed and wind direction is investigated, as it is dominant over the tidal effect. The data reveal that the general net transport at the mooring location is northward; annually, the mean northward transport is 1 kton per meter in the cross-shore direction. For calm conditions, when wind speeds are less than 5 m/s, the overall effect of winds on the net transport diminishes to zero. Energetic conditions (wind speed higher than 5 m/s) are studied for a sequence of storms from opposing wind directions. Here, the gross tidal transports (i.e., transport during ebb or flood separately) show a delay with respect to the onset of the wind event, ranging from half a day to a day. The correlation between the net transport per tidal cycle and the wind characteristics during that cycle, is used to extrapolate the results to a 10-year period, using wind data from a nearby weather station. It is shown that annual transports of SPM can vary by a factor of two between years, due to different wind and wave conditions. Within a year the seasonal variability is larger, a factor of five exists between summer and winter months.