Background

Bars exist on the intertidal beaches along the Dutch sandy coast. These intertidal bars are highly dynamic and may vary in shape and size over a period of a tidal cycle. The bars are interrupted by so-called rip channels. The dynamics of the bars and the rip channels are coupled to the offshore hydrodynamics (waves and tides). The present study focuses on the seasonal variations in intertidal morphologies and volumes.

Objectives

To understand the short- to long-term behaviour of bar and rip patterns on the intertidal beach of Noordwijk aan Zee and Egmond aan Zee (Fig. 1). Short-term is defined here as daily and long-term as yearly. Length, position of the bar line and the bar shape can be indicators for bar patterns. Rip patterns can be characterized by the orientation and the width of the rip and by the spacing between the rips.

Methodology

Monthly field surveys using a DGPS to measure the morphology of the intertidal beach. Morphometric analysis of the intertidal beach morphology using the ARGUS video images.

Photo 1: Base receiver

Photo 2: Roving receiver

Photo 3: ARGUS video at Noordwijk

Results

Moderate storm conditions (H_m0 ~ 2 m) alternate with calm conditions (H_m0 ~ 1 m) in period I. Period II is divided in half; first half has moderate storm conditions, second half is calm. The conditions in period III are mainly moderate storm conditions. The moderate storm conditions involve waves mainly from south to southwest (Fig. 2).

Fig. 2: Bathymetric maps from the intertidal beach of Noordwijk aan Zee of four monthly DGPS surveys, with additional timeseries of the offshore hydrodynamic conditions

The first bathymetric map shows a small bar with one rip. The rip has migrated northward and the bar is better developed after period I (Fig. 2). The beach also has flattened; sand volume of the upper beach decreases, while it increases for the lower beach (Fig. 3). Period II leads to a beach with a faded rip and a bar placed further on the beach (Fig. 2). The lower part of the beach increases again a little bit in its sand volume (Fig. 3). The last map shows a beach with one bar interrupted by several rips after a period of uniform conditions, period III (Fig. 2). The sand volume response to these conditions is a Fig. 3 Computed sand volume above a certain decrease over the whole intertidal beach (Fig. 3).

Fig. 3: Computed sand volume above a certain height contour for the four DGPS surveys

Please contact Susanne Quartel for more info on this project.