Graduation of Lennard Spaans

05 juni 2019 13:30 - Locatie: P&E lecture room 0.290, Faculty of Mechanical, Maritime and Materials Engineering (building 34-K) - Door: Webredactie

"Detecting and reproducing cross-shore sediment transport in theintertidal zone" | Professor of graduation: Prof.dr.ir. A.J.H.M. Reniers, supervisor: Dr. S.E. Vos.

The beach recovery process resembles the resilience of a sandy coast and is an important aspect of the coastal safety. Sediment stored underwater by storms is transported onshore by the migration of subtidal and intertidal bars under mild wave conditions. The intertidal zone is an important interface connecting the marine and aeolian zone, facilitating the transition of hydrodynamic to aeolian sediment transport. A recent terrestrial laser scanning (TLS) measuring campaign conducted at Kijkduin, the Netherlands, provides new insight in the intertidal bar behavior and is presented in this thesis. Subsequently, the findings are compared with two XBeach models which are used to reproduce the intertidal bar behavior.

An analysis on the dominant cross-shore sediment transport processes has shown that they can largely be divided into surf zone and swash zone processes. The surf zone processes are primarily determined by the balance of wave nonlinearities, undertow and infra-gravity waves, provided that wave-breaking induced turbulence mobilize the sediment into the water column. In the swash zone the cross-shore sediment transport is determined by the balance between the wave-breaking induced turbulent uprush and the gravity induced backwash. The swash zone processes are forced by the oscillatory wave motions of short waves and long (infra-gravity) waves depending on the offshore wave conditions and the nearshore bathymetry.

A conceptual model is constructed, in which four intertidal bar regimes are distinguished, to describe the dominant cross-shore processes for the migration and growth of intertidal bars. In the overwash regime, where the runup level overtops the bar crest but the rundown level remains below it, swash zone processes are dominant for the onshore migration and growth of intertidal bars. Surf zone processes are dominant in the submersion regime, where also the rundown level overtops the bar crest.

For a period of 6 weeks hourly data at Kijkduin is processed after which the results of one cross section are analyzed. Two distinctive intertidal bars develop, grow and migrate onshore during mild wave conditions. Within 5 days the upper intertidal bar migrates onshore over a distance of 25 m and grows with a height of 0.3 m. Onshore sediment transport fluxes reach values of nearly 2 m3 per meter width in one tidal cycle. As the overwash regime prevails for all nine tidal cycles the sediment transport can be attributed to swash zone processes.

Two hydrodynamic model options in XBeach, the surf beat model and the hydrostatic swash model, are applied to reproduce the observed morphological behavior of the upper intertidal bar. They both partly reproduce the onshore migration but do not show the final growth of the intertidal bar. The onshore migration distances in the models are less than two times the observed distance travelled according to the TLS measurements. Moreover, the onshore sediment transport fluxes are about four times smaller in the models. In the surf beat model, the onshore migration is mainly induced by surf zone processes prevailing in the submersion regime, not agreeing to the measurement results. The hydrostatic swash model shows a better resemblance in terms of the dominant cross-shore sediment transport processes as here the swash zone processes are mainly prevailing.

The findings presented in this study provide a better understanding of the intertidal bar behavior. Although the XBeach models did not reproduce the observed behavior completely, there are some pronounced similarities. Further research is required to improve its performance.