Modeling of spatial and temporal variations in offshore sand waves: process-oriented vs. stochastic approach


Type

PhD-research


Persons involved:

Ir. Fenneke Sterlini (PhD student)

Ir Joris van den Berg (PhD student)

Dr ir Attila A. Németh (Post Doc)

Dr ir Michiel A.F. Knaapen (Post Doc)

Dr Ruud M.J. van Damme (Daily supervisor)

prof. dr S.J.M.H. Hulscher (promoter)


Funding

STW (project TWO.5805)

Duration

2003 – 2009


Summary

The sea floor of shallow seas is rarely flat and often dynamic. A widely occurring bedform type is the sand wave. Sand waves form more or less regular wavelike patterns on the seabed with crests up to one third of the water depth, wave lengths of hundreds of metres and a migration rate of metres up to tens of metres per year.

Because of their migration speed and spatial dimensions, sand waves can interfere with anthropogenic activities. In relation to offshore activities especially the variation and extremes in sand wave characteristics are important. These variations can be caused by variation in environmental factors.

In this project, we validated a non-linear idealized process-based model (Sand Wave Code, SWC) and included relevant physical processes, to relate them to variations and extremes in sand waves characteristics. Our specific interest was firstly to understand which environmental factors cause the sand wave shape and variation and, secondly, to predict this variation.

First, a detailed comparison between the Golden Gate sand wave field and the SWC was carried out. The results of the SWC compare reasonably well with the observed sand waves when both an oscillating and a residual current are taken into account. Current velocity together with water depth, seem to be the most important factors influencing sand wave characteristics.

Including physical processes showed that suspended sediment in general (1) shortens and lowers the sand waves, (2) increases the growth and migration rate, and (3) decreases the crest/trough ratio for the sand wave length and height. Smaller grain sizes, stronger currents, or more asymmetric currents, increase the quantitative suspended sediment effects. Surface waves, in general, lower the sand wave height and cause migration in the direction of the surface wave propagation. The shape of the sand wave changes to a broader crest, milder slopes and a smaller trough.

Simulations with heterogeneous sediment indicate a coarsening trend towards the crest as observed in the North Sea. The heterogeneous sediment has no significant effect on the sand wave height and length.


Scientific output

Dorst, L.L, Roos, P.C., Van der Meer, F. M. and Hulscher, S. J. H. M. (2008). The analysis of migrating tidal sand waves in an anchorage area on the Netherlands Continental Shelf. submitted.

Roos, P.C., Hulscher, S.J.M.H., Meer, F.M. v. d., Dijk, T.A.G.P. v., Wientjes, I.G.M., and Berg, J. v. d. (2007). Grain size sorting over offshore sand waves: observations and modelling. In: River, Coastal and Estuarine Morphodynamics: RCEM 2007 , Dohmen-Janssen, C.M. and Hulscher, S.J.M.H. (eds.). Taylor & Francis Group, London.

Van den Berg, J. (2007). Non-linear sand wave evolution. PhD thesis, University of Twente.

Van den Berg, J. and Van Damme, R. (2006). Sand wave simulations on large domains. In: River, Coastal and Estuarine Morphodynamics:RCEM2005, Parker and Garcia(eds.).

Van den Berg, J. and Van Damme, R. (2008), Simulation of variations in a sand wave field, submitted.

Van den Berg, J., Van Damme, R. and Hulscher, S (2009), Full 3D simulations of sand wave evolution, in preparation.

Van der Meer, F., Németh, A., and Hulscher, S. (2005). Modelling sand wave evolution using various grain size dependent sediment transport equations. pp. 404-405.

Van der Meer, F., Hulscher, S.J.M.H., and van den Berg, J. (2007a). On the influence of suspended sediment transport on the generation of offshore sand waves. In: Particle Laden Flow:from geophysical to Kolmogorov scales, B.J.Geurts et al.(ed.), pp. 29-41. Springer.

Van der Meer, F., Hulscher, S.J.M.H., and Dodd, N. (2008a). On the effect of wind waves on offshore sand wave characteristics. In: Marine Sandwave and River Dune Dynamics III: MARID 2008 , Parsons, D. R., Garlan, T., and Best, J. L.(eds.), pp. 227-234.

Van der Meer, F., Hulscher, S. J. M. H., and Dodd, N. (2008b). Modelling surface wave effects on evolved offshore sand waves. In: PECS 2008: Physics of Estuaries and Coastal Seas, Liverpool, UK.

Van der Meer, F., Hulscher, S.J.M.H., and Hanes, D.M. (2008c). Non-linear modeling of offshore sand waves near San Francisco. In: 31st International Conference on Coastal Engineering, Hamburg, Germany.

Sterlini, F.M. and Hulscher, S.J.H.M. (2008). Exploring suspended sediment transport effects on sand waves: a model study. submitted.

Van der Meer, F.M., Hulscher, S.J.M.H., Hanes, D. M., and Elias, E. (2007b). San Francisco Bay sand waves: modelling and observation. In: River, Coastal and Estuarine Morphodynamics: RCEM 2007 , Dohmen-Janssen, C.M. and Hulscher, H.S.J. M.(eds.). Taylor & Francis Group, London.

Sterlini, F.M., Hulscher, S.J.H.M., and Dodd, N. (2008d). Modelling the effect of surface waves on offshore sand waves. submitted.

Sterlini, F., S. J. M. H. Hulscher, and D. M. Hanes (2009), Simulating and understanding sand wave variation: A case study of the Golden Gate sand waves, J. Geophys. Res., 114, F02007, doi:10.1029/2008JF000999.

Wientjes, I.G.M. (2006). Grain size sorting over sand waves. CE&M research report 2006R-004/WEM-005.