Modelling of spatial and temporal variations in offshore sandwaves

Background

On the offshore seabed, of e.g. the North Sea, extensive sand wave fields can be found. Sand waves are wave-like seabed features, whose elongated crests can be more or less parallel. The wavelength is about 500 meters and their heights can lead up to 10 meters, which takes a significant part of the local water depth. Sand waves are not static, there are strong indications that they migrate at a speed of tens of meters per year (Németh et al., 2002). Hulscher (1996) showed that observed sand waves are generated by tide-topography interactions. In order to use the insights gained by the idealized models, and extend it to model more realistic cases we started modelling sand waves using numerical and data-assimilation methods. Currently, we can simulate sand wave evolution due to periodic water motion using a idealized deterministic model in which the sand waves are all equal: differences in shape or variations in time are not accounted for.

The amplitudes, wavelengths and positions of sand waves show temporal and spatial variations. The reasons underlying these variations are not understood, neither from theoretical nor from observational point of view. Only recently, appropriate data-sets, i.e. accurate enough in space and overall positioning and covering a necessary time-span, are becoming available. The physical parameters, which are crucial for sand waves, as e.g. turbulent viscosity formulation, depth, strength of tidal motion, sediment characteristics and transport mechanisms, influences of storms and residual currents vary slowly in space and time. These variations are partly deterministic (having a known structure in space/time, e.g. the yearly storm/calm weather period) and partly stochastic (e.g. sediment pick-up or turbulent viscosity, and also weather variations), both leading to slow spatial/temporal variations in the sand wave-dynamics.

Objectives

In this project, the impact of both deterministic and stochastic variations will be investigated, starting with mathematical techniques such as stability analysis and averaging, subsequently extended with numerical approaches to assess a wider range of processes. At the end of this project both approaches will be applied to the same case, to investigate to what extent they can describe reality.

Publications

Berg, J. Van den, and Damme, R.M.J. Van, A simplified sand wave model, Marine Sandwave and River Dune Dynamics II 1-2 April 2004, The Netherlands

Berg, J. Van den, and Damme, R.M.J. Van, Sand wave simulation on large domains,

Bijsterbosch, L.W.W., Németh, A.A. and Hulscher, S.J.M.H.. Modelling offshore sand waves: Effect of suspended sediment transport, Marine Sandwave and River Dune Dynamics II 1-2 April 2004, The Netherlands

Knaapen, M.A.F., Measuring sand wave migration in the field. Comparison of different data sources and an error analysis, Marine Sandwave and River Dune Dynamics II 1-2 April 2004, The Netherlands

Németh, A.; Hulscher, S.; Van Damme, R., (2003) A sand wave simulation model, EAE03-A-08886. EGS-AGU-conference, Nice.

Németh, A.A. and Hulscher, S.J.M.H., 2003, Finite amplitude sand waves in shallow seas, IAHR-symposium on River, Coastal and Estuarine Morphodynamics, Barcelona, Spanje, editors Sánchez-Arcilla, S. en Bateman, A., 435-444

Németh, A.A., Hulscher, S.J.M.H. and Damme, R.M.J. Van, Sand wave migration and height, comparing model results with data, Marine Sandwave and River Dune Dynamics II 1-2 April 2004, The Netherlands

Németh, A.A., Hulscher, S.J.M.H. and Damme, R.M.J. Van, Modelling sand wave evolution: influence of suspended sediment transport and non-erodible layers. In PECS 2004, Hydrodynamics and morphodynamics in estuaries and open seas, October 18-24 2004, Merida Mexico

Németh, A.A., Blondeaux, P., Hulscher, S.J.M.H., Besio, G. et.al. (2004). Models of offshore sand waves and sand banks and their applications. In M.A. Losada & A. Baquirizo (Eds.), HUman interaction with large scale MORphodynamical features. Jornadas sobre Avances eb ingeriería de Costas y Oceanografía operacional. 2-3 December 2003, Grananda Spain. 10mo Aniversario Real Academia de ingeriería (pp. 53-73) Barcelona: Real Academia de ingeriería (ISBN 84-95662-29-9)

Meer, F.M. van der, Németh A.A., Hulscher, S.J.M.H. (2005) Modelling sand wave evolution using various sediment transport mechanisms. In: Book of abstracts, 5th International Conference on Coastal dynamics, 4-8 april 2005, Barcelona, Spain. Laboratori d'Enginyeria Maritima (LIM/UPC) Universitat Politechnica de Catalunya Barcelona (Spain)