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Llanelli Sand Dredging Ltd
Review of annual monitoring surveys of Helwick Bank and adjacent Gower beaches, 2003 to 2004
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AUTOCAD DWF drawings:
Location of Beach Profiles
Variation in beach elevations: Rhossili 1
Variation in beach elevations: Rhossili 2
Variation in beach elevations: Rhossili 3
Variation in beach elevations: Mewslade Bay
Variation in beach elevations: Port-Eynon 1
Variation in beach elevations: Port-Eynon 2
Variation in beach elevations: Port-Eynon 3
Variation in beach elevations: Oxwich 1
Variation in beach elevations: Oxwich 2
Variation in beach elevations: Oxwich 3
Mean beach elevations: Rhossili Bay
Mean beach elevations: Mewslade Bay
Mean beach elevations: Port-Eynon Bay
Mean beach elevations: Oxwich Bay
Data quality of the bathymetric surveys and the airborne laserscanner surveys in 2003 and 2004 are within the specifications outlined in the dredging licence for Area 373.
A comparison of the 2003 and 2004 bathymetric surveys of Helwick Bank and the sub-tidal area of Port-Eynon Bay indicates an overall sediment loss of 1.47M m3 within the sand limits (34.8km2) of the Extended New Boundary (42km2). Principal features of morphological changes on Helwick Bank for 2003-04 were (1) continued prominent sand wave migration along the southern flank of the whole sandbank, (2) continued prominent sand migration along the north flank and crest platform of East Helwick, extending slightly into the north flank of Helwick Swatch, (3) continued migration of sand wave trains from Helwick Channel into Helwick Swatch and East Helwick, (4) widespread slight erosion, and (5) accretion in the SW apex, north flank of West Helwick, and Port-Eynon Bay.
Between 01 September 2003 and 31 August 2004, the volume of sediment dredged from Helwick Bank by Llanelli Sand Dredging Limited was 76,298m3 of building sand (114,447 tonnes).
From the LIDAR data, some 21,118m3 of sand was lost from Gower beaches (excluding Three Cliff Bay) in 2003-04, with a ballpark estimate from beach profiling data suggesting a 37,943m3 loss. However, if Three Cliff Bay LIDAR data is included (normally, Three Cliff Bay is not included because there is no beach profiling there), the sand loss for all beaches is only 3,472m3. Of this 21,118m3 figure, 19,372m3 (91.7%) was lost from Oxwich beach, and the morphological imprint on the Oxwich beach and Three Cliff foreshore suggests that continued freshwater run-off remains a major contributor to sand loss. The other Gower beaches surveyed (Rhossili Bay, Mewslade Bay, and Port-Eynon Bay) exhibited little morphological change from the 2002-03 comparison.
Apart from the severe weather on 21 March 2004, meteorological conditions experienced in the Gower region in 2003-04 were relatively similar to 2002-03 but with slightly more gales and more days of tranquil weather: [2002-03] 199 days @ <Force 5, 10days @ Force 8, 1day @ Force 9, and 1day @ Force 10, compared to [2003-04] 209 days @ <Force 5, 18days @ Force 8, 3days @ Force 9, and 1day @ Force 11. Therefore, although the effects of the tidal current regime upon bedload transport processes were relatively dominant, there would have been a partial wave-dominant counteraction from the severe weather on 21 March 2004, when the joint highest maximum sustained wind speed (60.2kts; also on 06 February 2001) was recorded in the past 10 years.
The Welsh Assembly Government included additional requirements to the seabed and coastal monitoring specifications and the data review procedures for a two-year extension of dredging operations. One requirement has been activated, viz. “Should the volume of Helwick Bank above 10m depth and below 5m depth fall below 14.5M m3 in the survey of (August) 2004, additional analysis of the causes supported by modelling of the Bank will be required. The dredging programme will be considered by AG subject to the results of this survey.”
The volume above –10m CD and below –05m CD fell from 14,653,261m3 in 2003 to 14,297,584m3 in 2004. Examination of the survey data indicates that there is no primary conspicuous reason for the shortfall of 220,416m3 in meeting the sediment volume threshold; therefore, it is likely that the contributing factors are a combination of the following: (1) Predominance of the tidal current regime, (2) sediment transport from the Violent Storm Force 11 of 21 March 2004 contributing to an increase of 101,452m3 above the 2002-03 accretion in Port-Eynon Bay, (3) Survey error, with the margins of error equating to about 0.5M m3 in volume error for the –10m CD isobath within the “Old Boundary”, and (4) Shoal accretion above –05m CD on West Helwick shoal.
Dredging in Area 373 occurs mainly in water depths from just above –10m CD to –20m CD, with the more intense activity focused along the –11m CD to –12m CD isobaths. Dredging extracts little sediment from above –10m CD directly.
As the two-year extension of the 1998-2003 dredging licence for Area 373 expires in October 2005, the “continuity” recommendations of previous reports are not applicable.
However, Llanelli Sand Dredging has applied for a 15-year licence for a continuation of dredging in Area 373.
Therefore, should a positive Government View be given for Helwick Bank, in light of the previous experience for the monitoring surveys, the following recommendations are suggested in relation to proposed monitoring requirements:
Beach monitoring. The quality of the 2004 LIDAR survey data was no better than the 2001, 2002, and 2003 data for the foreshore areas; the decimetre level of accuracy produces unacceptable high margins of error, the data being “too coarse” to be used on a repetitive basis. The basic cause of the problem is that the extent of flight coverage is beyond the range of procedures whereby dual-frequency dGPS receivers can be used. As advocated for a number of years, annual beach topographies should be measured by use of a mobile RTK GPS system, which provide the highest available levels of accuracy for horizontal and elevation measurements derived from all variants of GPS. The U.S. Department of Defense is modernising the GPS system, with the introduction of the L2C code and L5 carrier wave starting in 2005. Therefore, triple-frequency dGPS systems will be available within 1-3 years, thereby enhancing RTK capability.
Beach monitoring. If the biannual beach monitoring were to continue, as iterated in previous reports, there is no specific scientific benefit derived from the existing requirement for pre- and post-storm surveys. The viewpoint presented in the previous annual monitoring report remains valid, viz.
Therefore, as no systematic morphological change has ever been identified over the years of pre- and post-storm surveys, the continuation of the present arrangement for pre- and post-storm surveys is not warranted for any scientific purpose.
Bathymetric surveys. It is recommended that the present format of bathymetric surveys is continued. Whilst the use of multibeam echo sounding is increasing, it is not a panacea methodology. As advised by Titan Environmental Surveys for the Helwick Bank Links project, although there are obvious benefits, there are also specific critical aspects in measuring sandbank morphology by this method:
- highly variable swathe widths due to steep topography
- magnification of slope errors due to steep flanks
- probable requirement for concurrent single-beam sounding for calibration cross-checks
- greatly increased cost
- increased post-processing time.
The experience since 2001 indicates that the present procedures are adequate for monitoring purposes.