Analyzing long-term sand wave patterns in the Marsdiep Inlet

Abstract

The Marsdiep inlet is a part of the Wadden Sea, located between Den Helder and Texel in the Netherlands. It is four kilometers wide, has a depth of maximally 27 meter and is dominated by semi-diurnal tidal currents. Sand waves are present on the sea bed of the Marsdiep, and they migrate under the influence of tidal currents. It is important to investigate these bed form features, to gain better understanding in their characteristics and migration. Two Acoustic Current Doppler Profilers (ADCPs) are mounted under a ferry, which crosses the inlet up to 32 times a day. The ADCPs measure the depth of the sea at 8 different locations per second when the ferry is crossing the inlet, this data was collected from 2010 to 2021. The data was corrected for tides and geometry, placed in grid cells and averaged over one month. The study area was divided into 3 parts, where subsequent sand waves were clearly visible. The sand wave characteristics were analyzed and tracked over time. The most northern area has symmetrical sand waves with an average height and wavelength of 2.16 m and 144 m, respectively. The sand waves did not migrate in the direction of the currents, probably because the area is deep, and currents are weaker at deeper levels. This was not observed by previous research. A seasonal cycle was found in the first four years of the data, with higher sand waves during summer with respect to winter months, which was also observed by previous research. The second area was located in the south of the inlet, but was hard to analyze because one large anomaly was present during the first 6 years of the analysis. The sand waves had a height and wavelength of 3.64 m and 187 m, respectively, and they migrated about 70 m per year, on average. The most southern part analysed in this research was the most stable during the 11 years. It had a sand wave height and length of 2.9 m and 114 m, respectively, and the sand waves migrated on average 80 m per year. The methods for determining the sand wave heights worked quite good, but the methods to find the other characteristics can be improved, for example by cross-correlation techniques.