Numerical modelling of the growth of offshore sand waves: A Delft3D study

Numerical modelling of the growth of offshore sand waves: A Delft3D study

Choy, D.Y.

Stive, M.J.F. (mentor)
Hasselaar, R.W. (mentor)
Borsje, B.W. (mentor)
Hoekstra, R. (mentor)
Blom, A. (mentor)
Ooi, S.K. (mentor)
Cheong, H.F. (mentor)
Raaijmakers, T.C. (mentor)

Civil Engineering and Geosciences

Hydraulic Engineering

2015-02-17

Offshore sand waves of several meters high and hundreds of meters long are formed in shallow seas due to the tidal currents. These sand waves can, amongst others, influence the burial depth of cables of offshore wind farms and obstruct navigation channels. For the Far Large Offshore Wind programme (FLOW), one of the goals of Deltares is to increase the knowledge on sand wave growth and behaviour. Since only a limited amount of bathymetrical data is available in these offshore areas, numerical models can give some insights to predict sand wave development. So far, all numerical models used to estimate the growth of offshore sand waves predict unrealistic values for either the sand wave height or the sand wave length. These values are highly overestimated. Most models initially predict a sand wave length that is realistic under the modelled conditions. This is the length of the fastest growing mode (LFGM). However, when extending the model simulation time, the sand waves grow up to the length of the entire computational domain. When limiting the length of the domain to one sand wave length, the sand waves become extremely high. No model has yet been able to correctly predict both the wave length and the wave height. The main objective of this study was to understand the processes behind sand wave growth and modelling the sand waves up to an equilibrium state. The effects of two different aspects on the growth of offshore sand waves in a sand wave field were studied. First, the impact of the bed slope effect was assessed, since in previous studies the influences of this parameter were not studied in much detail. The bed slope effect is the (numerical) phenomenon where sediment grains are transported downhill more easily than uphill. This parameter is very important under the applied conditions and has a strong influence on the model predictions. Secondly, the influence of different initial bathymetries on the growth of the sand waves was determined. Both initially random and large sinusoidal beds were used. Long-term model simulations (morphological time > 200 years) were run to assess the impacts of the studied aspects on a potential equilibrium. Equilibrium is found for growing sand waves, when the bed with sinusoidal perturbations equal in length to the LFGM is applied. With these sinusoidal perturbations, the bed slope effect only influences the growth rate and the shape. The equilibrium height found is similar to that predicted with other numerical models where the domain has the length of one sand wave and is, thus, overestimated. In both cases, the equilibriums are forced. The sand waves cannot grow further in length and have reached a maximum height for the corresponding wave length. Model simulations with other initial beds led to even larger and longer sand waves. Though, at some point during the simulation, a mean wave length of approximately the length of the fastest growing mode is dominant in the sand wave field. Eventually, after the modelled 750 years, no equilibrium is found with these initial beds as the increase in wave length or wave height is on-going. This suggests that essential processes are not taken into account or are modelled incorrectly.

sand waves
numerical modelling
modelling
North Sea

http://resolver.tudelft.nl/uuid:35e168b6-4d39-4e12-becd-4bbd701372d0

Student theses

master thesis

(c) 2015 Choy, D.Y.