Modelling of the cross-shore grain size distribution in the intertidal zone

Abstract

The Building with Nature approach has been gaining ground in hydraulic engineering, increasing the importance of understanding the cross-shore morphodynamic processes. The intertidal zone, where marine and aeolian processes come together, is an important link in the transport of sediment from the sea towards the dunes. The grain size distribution affects the sediment supply in the intertidal zone. This research investigates the effect of marine processes on the cross-shore variations of the grain size distribution in the intertidal zone by using a one-dimensional non linear shallow water XBeach model.
The intertidal zone is subject to shoaling, surf and swash zone processes. The grain size influences the beach slope, the initiation of motion and settling to the bed. The cross-shore sediment transport is the combination of sediment that is stirred up from the bed and subsequently transported. Breaking induced turbulence enhances stirring of sediment from the bed and keeps sediment in suspension. The amount of stirring and the transport direction depends on the wave conditions.
Input and control data for the model study was provided by the Scanex 2020 fieldwork campaign at Noordwijk, the Netherlands. The ADV velocity data combined with a pressure signal has been used for the tidal and incoming wave signal. Cross-shore profiles have been determined in Matlab based on terrestrial laser scans. Soil samples of the intertidal zone were taken with a sand scraper and analyzed with a sieve tower. For the initial grain size distribution is the average distribution of 14 samples on a transect was used. Based on wave, wind and soil sampling data a model period from 29-2-2020 02:00 to 10-3-2020 13:00 was selected.
The XBeach model used is as described by Reniers et al. (2013), but with a time-averaged turbulent kinetic energy and a different implementation of the Riemann boundary. The model consisted of a 176 x 3 grid with a grid size of dx=1 m and dy=5 m. For the initial bathymetry the laser scan of 29-2-2020 02:00 was used. The initial grain size was imposed on all the model grid cells. Additional to the standard run, runs have been performed to research the effect of a storm, the model sensitivity and the effect of aeolian transport.
The model shows a pattern of cross-shore grain size variations with coarser sediment from x=20 to x=56 m, finer sediment from x=57 to x=105 m and fluctuating grain size from x=106 to x=136 m compared to the initial grainsize. After 24 h a grain size pattern establishes with a clear deposition of fine sediment on the upper beach. The pattern remained stable for nearly the full model period. After 200 hours the fines become less prominent and move onshore. On the intratidal scale sediment becomes coarser when submerged and finer when emerged, except near the high water line where fine sediment is deposited.
The model reproduced the same pattern of grain size variations over the cross-shore as was found in the soil samples of 10-3-2020. As the cross-shore grain size pattern remained stable during the model period, processes on the spring-neap time scale or storm time scale seem to govern the cross-shore variations of the grain size. For the aeolian transport this would imply that for this model period the fine sediment supply is controlled on the same time scales. Nevertheless, considering that aeolian transport could have resulted in coarsening of the fines in the upper intertidal zone, processes over a single tide, could be more important than was visible in the model result.