Monitoring

Llanelli Sand Dredging Ltd

TECHNICAL SUMMARY REPORT UPON DREDGING ON HELWICK BANK

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Helwick Bank sediment transport paths

Extensive research over the past 30 years, much of it published in scientific journals, has shown that the predominant sand transport pathway under the normal tidal regime south of the Gower coastline is towards the west. Helwick Bank is down-transport in the sand pathway and much of the material from the east is swept along the southern flank of the Bank and beyond it to the west.

Sediment transport studies have shown that there is a clockwise movement of materials around Helwick Bank. There is a gyre at the western end of Helwick Bank and some fine sediment may feed into Carmarthen Bay. At the eastern end there is a link between Helwick Bank and the Port-Eynon sub-tidal area. Under particular weather conditions, for example during south-westerly storms, sediment will travel into the embayment, and possibly onto Port-Eynon beach. However, under normal weather and the ebb-dominant tidal conditions, sediment will tend to move out from Port-Eynon Bay towards the south and west.

Helwick Bank Dredging Licence, Area 373

This Licence permits up to 150,000 tonnes (equivalent to 100,000 cubic metres) per annum to be dredged from the area outlined in red on the attached plan for 5 years from 23 June 1998. The attached plan also shows Helwick Bank contours and the locations of the beach profiles referred to in Monitoring below.

Dredging is carried out within part of the Licence area for only a very limited period each year (about 8 to 10 weeks including weather delays).

Monitoring

The following monitoring is carried out in accordance with the Licence conditions:

an annual bathymetric survey of part of Helwick Bank but extending into Port-Eynon Bay; usually carried out in August/September
biannual surveys of 10 No. beach profiles (3 No. at Rhossili, 1 No. at Mewslade, 3 No. at Port-Eynon and 3 No. at Oxwich); usually carried out each March and September
an annual airborne laserscanner (LIDAR) survey, which allows measurement of beach surfaces; usually carried out in August/September
2 No. sets (1 No. spring/summer and 1 No. autumn/winter) pre- and post-storm beach profile surveys (1 No. at Mewslade and 1 No. at Port-Eynon)

For each of the above surveys, data reports are produced and submitted to the National Assembly for Wales (NAW). These reports are available for inspection at the NAW.

Annual review of monitoring

An annual review of monitoring is undertaken by Llanelli Sand Dredging Ltd (LSD)’s consultant marine geologist, Dr C M Davies, and is submitted to the NAW in order to recommend whether dredging should continue beyond one further year. These reports are available for inspection at the NAW.

Review of annual monitoring surveys of Helwick Bank and adjacent Gower beaches, 1998 to 1999

The 1998 and 1999 bathymetric surveys of Helwick Bank and the sub-tidal area of Port-Eynon Bay indicated an overall sediment loss of just over 9.5 million m3 (cubic metres) within the limits of the survey area.

The 1998 and 1999 LIDAR data (obtained by the National Remote Sensing Centre) showed that the adjacent Gower beaches of Rhossili Bay, Mewslade Bay, Port-Eynon Bay and Oxwich Bay experienced an overall sediment gain of 1.1 million m3.

Oxwich Bay and Port-Eynon Bay probably gained sediment as a result of storm damage to Helwick Bank and wave-dominant transport to these embayments. Analysis of weather in the Bristol Channel region in 1998 and 1999 corroborated this conclusion, with particular focus on conditions during the last week of October 1998, when several local meteorological records were broken.

Rhossili beach is more likely to have gained sediment from Carmarthen Bay during storms, but it is down-transport of Helwick Bank in the normal tidal regime. Some sediment loss in Oxwich Bay was attributable to freshwater run-off across the foreshore, in addition to marine processes.

During the time period covered by the data, Helwick Bank (excluding the sub-tidal Port-Eynon Bay) would have lost over 8.5 million m3 of sediment in the absence of any dredging.

The volume of sand dredged between the 1998 and 1999 LIDAR surveys was 76,393m3, which was equivalent to 0.8% of the sediment lost from Helwick Bank during that time, and 0.036% of the computed volume of Helwick Bank (estimated to be 209.4 million m3 in 1998).

With 8.5 million m3 of sediment lost from Helwick Bank by storm damage, it was not possible to identify any specific feature of sediment erosion and accretion attributable to dredging by comparing the 1998 and 1999 data for Helwick Bank and the Gower beaches.

Review of annual monitoring surveys of Helwick Bank and adjacent Gower beaches, 1999 to 2000

Bathymetric surveys of Helwick Bank and Port-Eynon Bay showed a sediment loss of 5.1 million m3 between 1 September 1999 and 12 September 2000. Dredging took place in April, May, and September 2000. The total volume of sand dredged from Helwick Bank during that period was 42,125m3. Storm damage was the likely main cause of sediment loss, with four protracted storms between the surveys, compared to only one protracted storm between 1998 and 1999.

The dredged volume was 0.83% of the total sediment loss of 5.1 million m3. Therefore, 5.06 million m3 of sediment would have been lost from Helwick Bank and Port-Eynon Bay in the same period in the absence of any dredging. Considering the major difference between the amount dredged and total sediment loss, it was not possible to attribute any changes in inter-tidal and sub-tidal morphology as a specific consequence of dredging Helwick Bank. No dredging “footprint” was observed in the bathymetric data.

The estimated volume of Helwick Bank in 1999 was 200.9 million m3 and a 5.1 million m3 volume loss represented a 2.54% reduction from 1999 to 2000.

Despite the volume loss, Helwick Bank remains very stable. It is likely that the reductions in volumes within the surveyed area of Helwick Bank between 1998 and 2000 have resulted in the northern flanks of the Bank beyond the survey area increasing in volume. The bathymetric survey area is to be extended to the north of its present boundary in 2001 to enable subsequent changes to be monitored.

Quantitative changes in beach elevation between summer 1999 and summer 2000 were not made because of an NAW decision to defer the summer 2000 LIDAR survey to February 2001, with the subsequent survey in summer 2001, both of which are being undertaken by the Environment Agency under NAW’s direction.

Beach profiling data provide an insight into beach processes and historical beach levels. Figures 2 to 11 show the variations in elevations of the surveyed profiles since February 1993 (when profiles started) and Figures 12 to 15 show mean beach elevations.

In summary:

the mean levels of Rhossili beach in the past few years have been higher than February 1993
the sediment budget of Mewslade beach has a predominant pattern of winter loss, sometimes down to bedrock, and summer gain, with an overall increase in the sand budget
the mean profile elevations of Port-Eynon beach have remained closer to the February 1993 levels than the other beaches; profile 1 in September 2000 being 0.01m lower, with profiles 2 and 3 respectively 0.02m and 0.06m higher
the budget of Oxwich beach has had an irregular pattern of loss and gain of sand, with a residual gain since February 1993.

Summary conclusion

Changes in Helwick Bank and Gower beaches are associated with a continuing weather pattern. Apart from 1996, high winter indices of the “North Atlantic Oscillation” have prevailed since 1987 and, theoretically, “destructive” coastal processes are more common in the “warm, wet, windy” weather of high index winters.

Nobel Banks dredging licence application

For some time, LSD has been actively looking at other potential resources within the Bristol Channel and Severn Estuary.

Following initial prospecting within the Outer Bristol Channel sediment environment, an application was submitted to the NAW in March 2001 for a Government View upon a proposal for up to 300,000 tonnes of material to be dredged from an area to be known as Nobel Banks each 12 months period from the commencement of the licence for a period of 15 years.

It is understood that the Outer Bristol Channel sediment environment may be less environmentally sensitive than the Helwick sediment environment.

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TECHNICAL SUMMARY REPORT UPON DREDGING ON HELWICK BANK