PhD defence: Drude Fritzbøger Christensen
Drude Fritzbøger Christensen defends her PhD thesis
SURF ZONE TURBULENCE AND SEDIMENT DYNAMICS
Associate Professor Troels Aagaard, IGN
Professor Tom O'Donoghue, University of Aberdeen
Professor Nadia Senechal, University of Bordeaux
Associate Professor Aart Kroon (chair), IGN
After the PhD defence there will be a reception in Rød Stue, Øster Voldgade 10, Area 6, First floor – everybody is welcome.
The world's sandy beaches are constantly changing due to transport of sand which results in erosion or accretion of the beach. The frequent changes in beach width are of importance to local residents and coastal managers to whom computer models are an important tool for prediction of beach evolution. These models are, however, largely inaccurate as they cannot model onshore-directed sand transport with confidence. In this thesis, we studied whether turbulence (generated at the sea surface by wave breaking or at the bed due to friction between the wave motion and the seabed) affected the lifting of sand from the seabed into the water and thereby the sand transport by the waves. An effect which is not incorporated into the models.
Waves can both transport sand onshore and offshore depending on whether the largest amount of sand exists beneath the wave crest (onshore-directed motion) or trough (offshore-directed motion). In the thesis we show that turbulence correlates positively with the amount of sand in the water, which indicates that turbulence is an important mechanism for sediment suspension. Whether turbulence and thus the amount of sand in the water were larger beneath the wave crest or trough was shown to depend on the wave type. Plunging breakers generated the strongest turbulence and lifted the largest amounts of sand into the water. Turbulence was generated at the wave front of plunging breakers and therefore sand was lifted into the water beneath the wave crest and consequently transported towards the beach. Turbulence generated by wave breaking is thus of significance for beach accretion and should be incorporated into the models in order to make them more accurate. The wave period and grain size were also shown to be of significance for the sand transport. When the period was long and sand grains coarse, sand suspended beneath the wave crest settled before the wave trough arrived and the net transport was onshore-directed. For short wave periods and fine sand, the sand grains did not have sufficient time to settle and were transported by both the wave crest and trough, i.e. back and forth. Currents and infragravity waves (i.e. waves with a period above 20 seconds) were moreover shown to negatively affect onshore transport of sand by the waves.
The thesis is available from the PhD administration.