Sand-strip characteristics on a narrow beach by using video imaging.

Pam Hage, Gerben Ruessink, Donker Jasper
Utrecht University, Utrecht, The Netherlands

During a period of strong aeolian activity, the beach can become covered with patches of light-coloured dry sand that move over darker moist sand. These patches can grow to form zebra-like stripes and are known as sand-strips. Although they are common bedforms in wet aeolian systems, their characteristics and dynamics are not well understood. Researching them will give insight about which wind conditions and beach characteristics result in high aeolian transport rates. This is especially important for narrow beaches, where many potential transport events (based on wind velocity) do not always result in actual events.
The goal of this research is, firstly, to characterise these sand-strips from a multi-annual dataset of video imagery, focussing on their wavelength, migration speed and aerial extent and, secondly, to determine how these characteristics depend on wind velocity and direction, as well as beach width.

The study site, located south of Egmond aan Zee in The Netherlands, is a N-S oriented, relatively narrow (100 m maximum at spring low tide), mildly sloping (~1:30) beach and consists of quartz sand with a median diameter of about 250μm. The site is monitored by an ARGUS video system mounted on a 50m high tower. ARGUS is an optical remote sensing system, here consisting of five RGB-colour cameras that offer an 180O beach view. The system has produced snapshots every half hour during daylight since 1998. The snapshots from 2005 to 2013 were used. The corresponding weather data (hourly mean wind velocity and direction) were measured with an automatic weather station in De Kooy, 35 km north of the study site.

The Argus images were searched for aeolian transport events. Auto- and cross-correlation were used to determine the wavelength of sand-strips, their migration speed and width of the beach they cover. It was found that sand-strips can form at wind velocities of 8 m/s or higher if the beach is wide enough. Sand-strips therefore occur more often during low tide. Their wavelength varies between 5 and 30m. They usually start to form close to the foredune and spread seaward from there during falling tide as the beach gets wider. However, offshore winds can initiate sand-strips close to the water line, but those events are rare. Sand-strips are best developed under oblique southwest winds, which makes them move in the alongshore direction (few metres per hour).

Argus has turned out to be a useful tool to study sand-strips, as long as their numbers are high and they are clearly visible. It has provided an extensive sand-strip database that we will next use to examine which wind events determine the long-term (> annual) input of wind-blown beach sand into the dunes.