Sediment transport on various depth contours of the 'Holland Coast' shoreface

P.P. Knook
Ministerie van Infrastructuur en Milieu (IenM)

The scientific foundation to maintain the Holland Coast shoreward of the -20m depth contour is limited. It is assumed that profile perturbations shoreward of the -20m depth contour influence the coast within a time scale of 50 to 200 years. Hence, seaward of -20m NAP dredging companies are allowed to dredge sand. The dredged material is amongst others applied in nourishments near the beach. In order to naturally preserve the shoreface of the Dutch Coast, coastal policy in The Netherlands prescribes that the sediment volume of Holland Coast should be preserved shoreward of the -20m depth contour. The volume required to accomplish this can be significantly reduced in case a shallower depth contour can be assumed. In order to investigate the influence of perturbations shoreward of -20m NAP and to validate the scientific foundation of the -20m depth contour, cross-shore sediment transport on various depth contours will be analysed. The emphasis of this research will lie on sediment transport on the lower shoreface (deeper than -10m NAP).

Sediment transport will be evaluated with the model Unibest-TC. First, sediment transport sensitivity on a straight slope due to varying parameters (wave height, wave period, grain size, slope steepness, water depth and magnitude of Longuet-Higgins streaming) is analysed. Besides, non-dimensional numbers and transport equations were considered to extend the analysis. In the analysis the dominance, direction and magnitude of the wave related and current related transport of both the bed load and suspended load is investigated. Subsequently, sediment transport due to variable wave conditions on various depth contours is examined from the shoreface profile of Noordwijk. Also the situations with a variable wave angle and a tidal current was considered. Finally, morphological simulations, including a 100-year profile evolution and profile perturbations (e.g. sand pits) located on the lower shoreface, were performed.

Shoreface sediment transport depends on the wave steepness in combination with the slope steepness for every depth contour. This was concluded by analysing sediment transport sensitivity for a range of parameter settings. Sediment transport due to wave action is particular present on the upper shoreface. Although, the onshore directed bed load transport is dominant on the lower shoreface (provided that the orbital velocity induced shear stress exceeds the critical shear stress), its relative contributions is negligible in case a tidal current is included. It was found that low amplitude waves are responsible for the largest profile changes. On the lower shoreface, the tidal current induced offshore sediment transport is dominant. Onshore transport on the upper shoreface and offshore transport on the lower shoreface induce a lower shoreface flattening and an upper shoreface steepening. Perturbations located at the -15m, -20m and -25m NAP depth contours propagate shoreward caused by tidal current induced concentration gradients. Sediment transport induced by waves result in diffusion of profile perturbations and a limited shoreward shifting. Only at -15m NAP a clearly visible propagation is visible. A situation including a tidal current, results in a larger shoreward propagation of the sand pits/artificial ridges at -25m NAP than at -15m NAP. A larger depth dependent tidal velocity induced sediment transport gradient at -25m NAP is responsible for this phenomenon. So, the interaction of waves and tidal currents is of great importance on the entire shoreface profile and may have a large impact on cross-shore sediment transport.