Monitoring 2003

Llanelli Sand Dredging Ltd

Review of annual monitoring surveys of Helwick Bank and adjacent Gower beaches, 2002 to 2003

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Data quality of the bathymetric surveys and the airborne laserscanner surveys in 2002 and 2003 are within the specifications outlined in the dredging licence for Area 373.

A comparison of the 2002 and 2003 bathymetric surveys of Helwick Bank and Port-Eynon Bay indicates an overall sediment gain of 16,300 m³ within the new boundary limits and a 2,300 m³ loss within the extended new boundary. The low residual values signify that the volume of Helwick Bank remained in a relatively steady-state. Principal features of morphological changes were (1) extraordinary linear erosion along the SSE part of the crestal platform of West Helwick, (2) prominent sand wave migration along the southern flank of the whole sandbank, (3) prominent sand migration along the north flank and crestal platform of East Helwick and Helwick Swatch (in part), and (4) pathways of sand wave trains into Helwick Swatch and East Helwick from a Helwick Channel sand wave field.

Between 01 September 2002 and 31 August 2003, the volume dredged from Helwick Bank by Llanelli Sand Dredging Limited was 79,056m³ (118,584 tonnes) of building sand.

LIDAR data indicate that some 157,728m³ of sand was gained on the four monitored Gower beaches in 2002-03, with the ballpark estimate from beach profiling data suggesting a 105,889m³ gain. Some 171,000m³ was gained on Rhossili beach, principally in compensation for 157,000m³of sand that was lost in 2001-02 as a result of the development of a rip channel in the foreshore. The two beaches in Mewslade Bay benefited also from the benign weather in the five quarters from summer 2002 to summer 2003 inclusive, continuing the underlying trend of overall growth in sand volume (irrespective of the fact that in winter months of recent years, all of the sand is capable of being eroded from the foreshore, to return in the summer months). In January 2004, the major sand incursion on Port-Eynon foreshore that started early in 2001 still continues to contribute to the decreasing exposure of older Holocene sediments, albeit with evidence of seasonal fluctuations. Evidence continues to corroborate a hypothesis for weak, bi-directional littoral drift on Port-Eynon foreshore, involving the main sand area of the beach and the region where older sediments are exposed. In contrast, Oxwich beach including ThreeCliff Bay, continued to lose sand from the foreshore. The Oxwich beaches experience fluvial processes in addition to marine processes, and from meteorological data, autumn 2002 experienced very high rainfall, and sand loss probably relates to freshwater flushing from Parkmill Stream and Oxwich Marsh running across the foreshore.

In contrast to autumn/winters 2000-01 and 2001-02, autumn/winter 2002-03 was extremely tranquil. The year 2002-03 (i.e. between surveys) was the second most benign period after 1995-96 in the past ten years, and a marked contrast to the two previous years; gale and storm force conditions in 2000-01 stand out because of their severity and conditions in 2001-02 stand out because of their longevity. Whereas autumn/winter 2002-03 was relatively calm, autumn 2002 was, however, extremely wet, being the second wettest quarter in the past ten years, with autumn 2000 being the wettest quarter year. According to the UK Meteorological website, the year from 01 April 2000 to 31 March 2001 was the wettest year in England and Wales since records began in 1766.

Morphological changes in 2002-03 reflect the tranquil conditions (as evident by tidal-current dominant processes on Helwick Bank, and sand gains on Rhossili beach and Mewslade beaches) and very wet autumn period (contributing to the sand loss at Oxwich and ThreeCliff beaches). Despite the extraction of 79,056m³ of building sand (118,584 tonnes) in Area 373, an overall positive sediment budget existed for 2002-03, manifest as a relative steady-state in Helwick Bank’s volume and the four monitored Gower beaches in total experiencing overall accretion.

Several provisos have been added to the 2004 monitoring specifications, including a "first stage" volume threshold for Helwick Bank (prior to additional studies) of 14.5M m³ above the 10m contour and below the 5m contour; in August 2003, the equivalent volume was 14.653M m³.

The review of the 2002-03 bathymetric and beach data shows that there is no reason why dredging of Helwick Bank should not continue. Specific reasons for this recommendation are:

Comparing the 2002 and 2003 surveys, volume calculations indicate that Helwick Bank experienced an overall sediment gain of 16,300 m³ within the limits of the new boundary survey area and a 2,300 m³ loss within the limits of the extended new boundary; such numerical values lie within the margins of survey error, but nevertheless indicate that over the year, the sandbank remained in a steady-state condition
During 2002–03, Rhossili, Mewslade, and Port-Eynon gained sand, Rhossili completely replacing, with significant interest, the sand loss of 2001-02 that arose from the development of a major rip channel on Rhossili North; Oxwich and ThreeCliff beaches lost sand, but the principal cause of the loss was the extremely wet autumn 2002 and fully-charged, freshwater runoff over the beach surface flushing sand off the foreshore
No dredging footprint is evident in the 2003 bathymetric data; previously, the absence of a dredging footprint has been attributed solely to dredging operations being conducted in campaigns, allowing the seabed area time to redevelop the sand wave morphology; whilst this explanation remains perfectly valid, the prominent development of the tidal regime imprint on Helwick Bank in 2002-03 illustrates that the main dredging area also contains two pathways of sand wave trains from Helwick Channel sand wave field onto Helwick Bank
Since early 2001, Port-Eynon beach (the foreshore that has been the major focus of criticism of dredging) has experienced extensive sand incursion, and this continues to be the situation in early 2004; evidence of weak, bi-directional littoral drift has existed for a number of years; for example, the main sand area of Port-Eynon beach lost 9,555m³ in 2002-03 whereas the sand gain in the area of the exposed, older sediments was 9,821m³ in the same period.

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Llanelli Sand Dredging Ltd

Review of annual monitoring surveys of Helwick Bank and adjacent Gower beaches, 2002 to 2003