Monitoring 2001

Llanelli Sand Dredging Ltd

Review of annual monitoring surveys of Helwick Bank and adjacent Gower beaches, 2001 to 2002

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The data quality of the bathymetric surveys and the airborne laserscanner surveys are within the specifications outlined in the extraction licence.

A comparison of the 2001 and 2002 bathymetric surveys of Helwick Bank and the sub-tidal area of Port-Eynon Bay indicates an overall sediment gain of ~4.5M m3 within the new boundary survey area and ~6.1M m3 within the limits of the extended new boundary. Principal features of this sediment gain were (1) a progressive increase in sandbank volume from East Helwick to West Helwick, and (2) more accretion on the flanks of the sandbank than in the central, crestal platform areas.

Between 01 September 2001 and 30 August 2002, the volume dredged from Helwick Bank by Llanelli Sand Dredging Ltd was 55,402m3 of sand (83,103 tonnes). Comparing the volume dredged to the volume of sediment gained on the sandbank and adjacent sub-tidal Port-Eynon Bay in 2001-02, absence of dredging would have added only 1.22% more sediment onto the sandbank, in theory. The dredged quantity represents an extraction of 0.027% of the 2002 estimated volume of Helwick Bank, and the 4,522,991m3 sediment gain (new boundary) represents a 2.27% increase in computed sandbank volume from 2001 to 2002.

As beach profiling and LIDAR surveys compute elevations by different geodetic methods, the 2002 LIDAR data was snapped to the profiling data, via adjustment of elevations to provide a best fit. In 2002, the LIDAR and profiling surveys were undertaken on the same day for Port-Eynon beach and one or two days difference for the other beaches.

LIDAR data indicate ~188,000 m3 of sand was lost from Gower beaches in 2001-02, with the beach profiling estimate being a 179,000 m3 loss. Of this total, 157,000 m3 was lost from Rhossili beach, relating mainly to the development of a large rip channel. The southern half of the beach lost only 28,000 m3 of sand. Throughout 2002, Rhossili beach developed a series of shoreline-parallel sandbars along the north and central areas of the beach, a development conducive to the formation of rip currents and rip channels. In contrast, Port-Eynon beach continued its trend of gaining sand, ~6,000 m3. By early 2003, the exposure of older sediments had reduced dramatically to only 14% of the surface area exposed some 26 months earlier.

The stormy autumn 2000 was followed by another stormy autumn/winter 2001-02, which included 16 days of maximum sustained wind speed in excess of Gale Force 8, including 4 days exceeding Strong Gale 9. The main difference was that the storms did not attain the intensity in 2001-02 compared to 2000-01, but the occurrence of stormy weather was greater, with 40 days when the maximum sustained wind speed was Gale Force 8 or stronger in 2001-02 compared to only 26 days equivalent in 2000-01. In context, 2000-01 and 2001-02 experienced storm force conditions far in excess of other years in the recent past at Pembrey Sands.

Given the excessive stormy nature of 2000-01 and 2001-02, it is not surprising that the west-facing Gower beaches experienced sediment losses in these years, which would have happened irrespective of dredging activity on Helwick Bank. In contrast, the major sand incursion on Port-Eynon foreshore continues, albeit with evidence of seasonal fluctuations. Such an incursion demonstrates that the beaches are capable of recovery during sand dredging activities on Helwick Bank.

Considering the substantial difference in scale between dredging on the one hand and sediment gains and losses on the sandbank and adjacent beaches on the other, it is impossible to attribute directly any changes in inter-tidal and sub-tidal morphology as a specific consequence of dredging on Helwick Bank.

The review of the 2001 and 2002 bathymetric data and beach measurements shows that there is no reason why dredging of Helwick Bank should not continue. The principal reasons for this recommendation are

between the 2001 and 2002 surveys, Helwick Bank increased in volume by over 4.5M m3 (new boundary), irrespective of the small volume of sand extracted by dredging, 55,402 m3
during 2001-02, Rhossili, Mewslade, and Oxwich foreshores lost sand and Port-Eynon and Three Cliff Bay gained sand, much of the loss being attributable to the development of a major rip channel on Rhossili; the autumn/winter of 2001-02 followed on from the several severe storms in autumn 2000, and these events have a far bigger impact on sediment re-distribution than dredging, controlling the sediment budget of Helwick Bank and adjacent Gower beaches
there is no evidence that dredging significantly exacerbates sediment gains and losses, and no dredging footprint is evident in the 2002 bathymetric data
in 2001 and 2002, Port-Eynon beach (the foreshore that has been the major focus of criticism of dredging) has experienced extensive sand incursion, even during sand dredging activities on Helwick Bank; there appears to be a pattern that whenever Helwick Bank experiences major sediment gains, Port-Eynon Bay experiences sediment gain as well.

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Llanelli Sand Dredging Ltd

Review of annual monitoring surveys of Helwick Bank and adjacent Gower beaches, 2001 to 2002