Fathoming ‘Sand Waves’

Peter Waller of Felixstowe has been studying papers on Sand Waves and their mobility, and following much reading has come up with an interesting analysis.

His studies of numerous papers have shown that Sand Waves can move at various velocities depending on their location, the sea depth, tides, currents, wave climate and the surface wind speed and direction. The velocity of their movement has been noted as 5-10m per year and even as much as up to 600m per year. They can become more mobile if disturbed by the the sea bed being ‘reactivated’ in their vicinity by disturbance such as cable or pipe line laying, and of course dredging. Furthermore they can move in the direction of the tide or current or against it! Sand wave height is proportionate to sea depth; they can be up to and even over 10m in height in deep water.

Sand banks may have sand waves on their sides, and, if they are below water at high tide sand they can have further waves on top, the size dependent on depth and other variables noted above. Relict sand and gravel deposits such as that primarily targeted by the aggregate dredgers may also be covered with sand waves, so it seems reasonable to deduce that dredging relict deposits covered by sand waves will reactivate the sand wave field. These sand waves cover a large proportion of the North Sea bed, so it is reasonable to assume that due to the large areas dredged, sand waves have been compromised. (see page 28 of ‘Modelling offshore sand waves’ (see below) and in many papers that appear by searching Google for modelling+offshore+sand+waves.

The large “relict deposits” dredged to the east of the Scroby sandbank off Great Yarmouth are likely to have been covered by sand waves, whilst Scroby itself may also be covered with sand waves. Thus, by reactivating the sand wave field to the east of Scroby, Scroby itself would logically be compromised. Sand waves would move/fall down the sand bank slope with gravity as an additional catalyst) so with the impact of greater wave energy could lead to considerable sand bank loss. Under these conditions it is unclear whether Scroby will ever recover! If Peter’s theory is correct there are serious implications for other dredge areas and sand banks which absorb wave energy so protecting the coast!

Sadly, Peter has been unable to find any empirical studies of sand waves to date, those noted below are papers using mathematical/computer modelling formulae. But he will continue to seek and find hopefully with more information to come later.

For detailed information on Migrating Sand Waves see:

Migrating sand waves
Ocean Dynamics (2003) 53: 232-238
DOI 10.1007/s10236-003-0043-x
Giovanni Besio: Paolo Blondeaux: Maurifio Brocchini: Giovanna Vittori
http://www.usna.edu/Users/math/rmm/Papers/sand_waves.pdf

Modelling offshore sand waves
Samenstelling promotiecommissie:
prof. dr. ir. H.J. Grootenboer Universiteit Twente, voorzitter/secretaris
prof. dr. S.J.M.H. Hulscher Universiteit Twente, promotor
prof. dr. ir. H.J. de Vriend Universiteit Twente, promotor
dr. R.M.J van Damme Universiteit Twente
dr. A. Falqués Universitat Politècnica de Catalunya
prof. dr. A. van Harten Universiteit Twente
dr. A.J.F. van der Spek Nederlands Instituut voor Toegepaste
Geowetenschappen TNO
prof. dr. ir. G.S. Stelling Technische Universiteit Delft
dr. A. Trentesaux Université des Sciences et Technologies de Lille
http://doc.utwente.nl/41459/1/thesis_Nemeth.pdf

So what we have so far appears to be based upon computerised theory and as far as we can ascertain has not been tested by field studies nor backed up by empirical evidence, as expounded and backed up in by the article by Peter Lanyon ‘WORK IN PROGRESS AT SIZEWELL’— by the sea, by the dredging companies and by Marinet which appeared on our website in August 2007, (see page 3).

Amongst others this article quotes the following references:

  1. Roos CR et alia, “Linear Evolution of Sand Wave Packets and Relevance to Offshore Sand Extraction”, Proceedings of the 29th. International Conference, Coastal Engineering, 2004.
    “Abstract: Sand waves form a highly dynamic rhythmic bed feature in coastal seas like the North Sea. Various types of human intervention at the seabed, like large-scale offshore sand extraction, may interfere with sand waves. We investigate how a local topographic disturbance of a flat seabed may become morphodynamically active, according to the linear instability mechanism associated with sand wave formation. To that end, we present an analytical approach, using the essentials from existing models of sand wave formation. The results show how a linear sand wave packet evolves, gradually expanding and — in the case of tidal asymmetry or residual currents — migrating in time. Finally, we discuss the implications of our results for the long-term morphodynamic effects of sand extraction.”
    Abstract to be found at http://eproceedings.worldscinet.com/9789812701916/9789812701916_0200.html
    N.B. There is a fee of $19.90 for access to the full-text article.
  2. Magoon OT et alia, “Economic Impacts of Anthropogenic Activities on Coastlines of the United States”, Coastal Engineering, 2004

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