Inter-annual variability off sand banks and waves: Observations from

Inter-annual variability off sand banks and waves: Observations from Nash and Helwick Bank
Lewis*, M; Neill, S.
School of Ocean Sciences, Bangor University, Menai Bridge, Anglesey, LL59 5AB, UK.
*[email protected], tel: 01248 388114.
Theme 2 - Dynamics of shelf seas with reference to residual circulation and sand bank
growth. I would prefer an oral presentation.
It is important to understand the natural variability of offshore sand banks due to their role in
coastal flood protection, fisheries and as a source of marine aggregates. Annual bathymetric
surveys of two contrasting sand banks (Nash and Helwick) in the Bristol Channel (UK) were
analysed between 1991 and 2002. Both sand banks have dimensions of the order 10 km by 1 km, with
a crest height rising to around 20-25 m above the surrounding sea bed. The crest at Nash Bank is
exposed on the lowest spring tides, while Helwick Bank is always covered to a depth of at least 3 m.
The annual bathymetric data were interpolated onto a regular grid, and significant inter-annual
variability of sand bank bathymetry was calculated at both sites. The local wave climate was
simulated using a SWAN model of the northwest European shelf, and, using linear wave theory, the
number of hours between each survey period when waves alone could induce sediment transport over
the sand banks was calculated. The variability of sand bank volume correlated well to the number of
hours during which waves could induce sediment transport (R2 of 77% and 69%, with Pearson
correlations of -0.88 and 0.83 at Helwick and Nash respectively). Therefore, an increase in the
number of storm wave events appears to result in an increase in sand bank volume at the tidally
dominant Nash Bank, but a decrease in volume at the wave exposed Helwick Bank. Further, a
morpho-dynamic model (UNIBEST-TC) simulated storm wave-induced changes to sand bank
morphology that was consistent with observations. Such analysis indicates that inter-annual variability
within storm wave events could be responsible for the observed natural variability of sand bank
volume and morphology observed at both sites. Such a conclusion has implications for the clustering
of extreme events and coastal flood risk, as well as understanding environmental impacts on sand
banks within the context of natural variability.