Monitoring 2001

Llanelli Sand Dredging Ltd

Review of annual monitoring surveys of Helwick Bank and adjacent Gower beaches, 2000 to 2001

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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 2000 and 2001 bathymetric surveys of Helwick Bank and the sub-tidal area of Port-Eynon Bay indicates an overall sediment gain of just over 3.1 million m3 within the limits of the 1998-2003 survey area. The sediment gain is equivalent to an overall increase of 0.12m in seabed elevation over Helwick Bank (excluding the sub-tidal area of Port-Eynon Bay).

The volume of sand dredged from Helwick Bank by Llanelli Sand Dredging Limited between the bathymetric surveys was 62,146 m3. Comparing the volume dredged to the volume of sediment gained on the sandbank and adjacent sub-tidal Port-Eynon Bay in 2000-01, absence of dredging would have added only 2% more sediment onto the sandbank, in theory. In the past, the substantial sediment losses experienced on Helwick Bank (~9.5 million m3 in 1998-1999; ~5.1 million m3 in 1999-2000) have been attributed to storm damage; however, considering that the worst weather experienced at Helwick Bank since dredging operations began occurred in autumn 2000, the gain of 3.1 million m3 of sediment suggests that storms can contribute also as a major mechanism in sediment accretion.

As there was no LIDAR survey in the summer of 2000, the review of beach changes between summer 2000 and summer 2001 is based upon beach profiling estimates. It is estimated that ~ 54,685 m3 of sand was lost from the four beaches surveyed between summer 2000 and summer 2001.

The autumn of 2000 was noted for its bad weather, with the storms of 29 and 30 October, 25 and 26 November, and 12 and 13 December 2000 being the most severe, extended storm events (i.e. lasting more than one day) in the Pembrey Sands meteorological data archived at NOAA since 01 January 1994. To place the monitoring period 2000 - 2001 into perspective, the autumn storms of 2000 had 5 days when maximum sustained wind speeds were Storm Force 10 and 2 days of Violent Storm Force 11, compared to the six previous years in total, when there were only 4 days of maximum sustained wind speeds reaching Storm Force 10 and 1 day of Violent Storm Force 11. It is likely that the severe weather had a major impact upon sediment losses and gains on Gower beaches.

The beach profiling data indicate that Gower beaches did show a good recovery between the March 2001 and early October 2001 surveys, which was a period of relatively tranquil weather conditions. These data indicate that the beaches are capable of recovery during sand dredging activities on Helwick Bank.

Having examined all four LIDAR datasets, a re-iteration of a statement in the last annual monitoring report remains valid, viz. the margin of error of LIDAR data, with "decimetres" level of accuracy, is too coarse for the purpose of assessing topographic changes in beach elevations on a repeated annual basis.

It is recommended that use of airborne laserscanner (LIDAR) surveys should be discontinued in 2002 and, as an alternative technique, beach topographies should be measured in 2002 by use of a mobile RTK GPS system.

The review of the 2000 and 2001 bathymetric data and beach measurements shows that there is no reason why dredging of Helwick Bank should not continue during 2002.

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Llanelli Sand Dredging Ltd

Review of annual monitoring surveys of Helwick Bank and adjacent Gower beaches, 2000 to 2001