1. * * * * * * * * * * * INTERMAR * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * INTERNATIONAL ACTIVITIES AND MARINE MINERALS DIVISION Physical Impacts of Marine Aggregate Mining David R. Hitchcock Ph.D., M.R.I.C.S. Integrated Study on the Impact of Marine Aggregates Mining on the Physical and Biological Resources on the Seabed
2. Overview This presentation reviews the work carried out for the Minerals Management Service over the past three years under contract 1435-01-99-CT 30980 Principal Contractor is Coastline Surveys Limited, a UK based survey contractor and dredging research Consultancy Biological and statistical analysis has been carried out by Marine Ecological Surveys Limited, also based in the UK, specialising in benthic identification, ecological assessments and impact analysis
10. What is aggregates dredging? Terrestrial sources of aggregate (sand and gravel) are increasingly limited in the UK as other pressures for land use increase. Hence marine dredged aggregates are an increasingly important asset to the economy, currently providing around 21 percent of UK national aggregate needs. Some 2500 mariners are employed on the UK vessels alone. A total of 23 Million tonnes was removed from the seabed within the UK zone (out of a licensed 38 million tonnes) 7.3 million tonnes was exported to the near continent and a further 4 million tonnes used for beach replenishment. There are currently 72 licences in the UK with a further 30 applications in the pipeline. Current licences cover only 0.12% of the UK jurisdiction seabed, with only 15% of that being actively dredged in any one year (BMAPA 2001).
11. UK Statistics Summary Information (Crown Estate 2002: A total of 23.05 million tonnes of sand and gravel were dredged from Crown Estate licences in England & Wales during 2000.(1999 – 23.67 million tonnes dredged) The total area of seabed licensed in 2000 increased by 10%, to 1506km2. (1999 – 1371km2) The area of seabed dredged during 2000 reduced by 25%, to 179km2. (1999 – 238km2) Over 90% of dredging from licences in England & Wales took place from an area of 11.89km2. (1999 – 21.5km2) The reduction in the area of seabed dredged is primarily a result of improved resource and operational management by the marine aggregates industry.
12. Project Components Keyword INTEGRATED Impact on Physical Resources Impact on Biological Resources Although types of impact are largely generic and similar the world over, scales of impact are generally site specific, certainly regionally specific In UK impacts can be broadly categorised into geographical regions of extraction e.g. North Sea, English Channel, Bristol Channel
13. But Why? What are the underlying forcing functions? Extraction methodology? Geological conditions? Oceanographic conditions? Natural Disturbance? TSHD Arco Severn
14. How? In its most simple terms, the potential impacts on sea bed resources depend on the type of deposit on the sea bed and the amount of sorting which is required to obtain a cargo suitable for the needs of particular end-users Where deposits are of a high quality, a cargo can be obtained with little or no requirement for rejection of unwanted material. However, where deposits are mixed sand and gravels (more normal), significant quantities - often amounting to up to 1.5 x the normal cargo load of 5,000 tonnes - may be returned to the sea bed through overboard screening chutes It is reasonable to consider that the severity or significance of the impact will be largely determined by the comparative relationship between disturbances by the dredging activity and natural disturbances by storms etc i.e. consider whether shallow deposits are better adapted to short term recovery than deeper, less disturbed deposits?
15. Methods of Mining The type of mining operation is dictated by a number of factors. Where the geological deposits are extremely restricted, mining is commonly from a dredger at anchor. This leads to relatively deep dredge pits on the sea bed, and is consequently restricted in many areas for environmental reasons. A more efficient and widespread operation is for a trailer dredger to remove sea bed deposits whilst underway. This leads to shallow dredge furrows of up to nominal 0.5 metre depth formed by each pass of the drag head, the worked area spread over a wider expanse of the seabed. Sometimes these furrows will impinge on each other forming ‘sweet spots’ or areas of increased deepening. This form of dredging is sometimes carried out by several vessels operating close to one another in intensively worked production licence sites. One of the objectives of our work has been to compare the impact of anchor dredging with that of trailer dredging on benthic biological resources.
18. Common features All dredging operations result in a "plume" of dispersing material which may extend for up to 3 km downstream from the dredger on normal tidal streams (up to 2 knots) in UK waters. The plume will comprise entrained air bubbles, sediments and importantly organic material The impact of the dispersing plume depends to a large extent on whether screening or ‘all-in’ loading is taking place, and for how long the dredge site has been exploited The geographical impact on the sea bed depends on whether the vessel is at anchor or trailing over a site of 1-3km long and whether screened material is rejected overboard in the vicinity of the dredge site Anchor dredging results in pits on the sea bed which may be as much as 5-10 metres depth whilst only a few hundred metres in diameter, and which may take a significant time to infill Trailer dredging results in shallow furrows in the sea bed which may be rapidly infilled especially if large quantities of screened material are discharged from the dredger during the mining process
19. Impact on Biological Resources Community Structure Diversity Abundance Determination of biological footprint Dispersion of Organic Load Zones of Enrichment FOR MORE INFO... Marine Ecological Surveys ‘Biological Impacts of Marine Aggregate Dredging’ to follow…...
33. Limitations Because cargoes are non-screened (all-in), our study shows the impact of the dredging process itself but does not address the potential impacts of discharge of relatively large quantities of screened material which occurs in many production licence areas, especially in the North Sea
34. Limitations In common with many other production licence sites, the impact of dredging at North Nab may interact with other activities that affect sea bed resources. Other licences and a spoil dumping site are nearby, although not strictly downstream. Far-field impacts are therefore increasingly likely to reflect other uses of the marine environment including spoils disposal as one moves away from the immediate site of aggregate dredging Despite these limitations, the results of our survey for near-site impacts are unlikely to be significantly affected by activities outside the dredge site, and are probably applicable to other production licence areas where discharge of material by overboard screening is minimal
35. Physical Impact Fieldwork Real-time Sidescan Sonar mosaic (3) over 350 line km collected Seabed sampling (5 phases) over 180 samples collected full PSA analysis
36. Hamon Grab The Coastline Hamon Grab is type approved by CEFAS for offshore benthic investigations. With a dataset collected with constantly changing equipment it is difficult for comparisons to be made. Use of the standard Hamon Grab, as recommended in 1992 by CEFAS will improve nationwide consistency
41. ADCP on the NAB (June 2001) Key: Distances in metres downstream between dredger and survey vessel 45m 45m 95m 100m 105m 105m 105m 130m 155m 200m
42. ADCP on the NAB (June 2001) Key: Distances in metres downstream between dredger and survey vessel 45m 200m 210m
43. ADCP on the NAB (June 2001) Key: Longitudinal distances in metres downstream between dredger and survey vessel 45m 45m 820m 820m 35m 25m 820m
44. Water sampling (June 2001) Pump water samples obtained from frame which incorporated an UMI data logger, optical backscatter sensor and pressure transducer
46. City of Rochester Plume () Backscatter image of density current present within the plume and possible near bed benthic boundary layer
47. City of Rochester Plume () Backscatter image of density current present within the plume and possible near bed benthic boundary layer Image shows just the high levels of backscatter indicating the dynamic phase of the density current
48. City of Rochester Plume () Backscatter image of density current present within the plume and possible near bed benthic boundary layer
49. Geopotes Plume () Backscatter image of density current present within the plume and possible near bed benthic boundary layer
50. Geopotes Plume () Backscatter image of density current present within the plume and possible near bed benthic boundary layer
51. Geopotes Plume () Backscatter image of divisions within the plume with possible near bed benthic boundary layer and Surface expression of organics or light fraction
52. Geopotes Plume () Backscatter image of divisions within the plume with possible near bed benthic boundary layer and Surface expression of organics or light fraction
54. City of Chichester Plume () Backscatter image of density current present within the plume Cut away of the core of the plume
55. City of Chichester Plume () Backscatter image of density current present within the plume Section across the higher concentrations of the plume , tidal excursion is to the NE (top right) and SW (bottom left)
56. City of Chichester Plume () Backscatter image of density current present within the plume This image strips away the different regions of backscatter intensity, low levels to higher, allowing us to ‘see’ into the regions of high concentration near the centre of the plume.
57. City of Chichester Plume () Backscatter image of density current present within the plume This series of vertical cross sections from NW to SE clearly shows the central core of high backscatter levels (red) which we believe closely depicts the behaviour of the dynamic phase of the density current
58. City of Chichester Plume () Backscatter image of density current present within the plume 10m deep pit is clearly visible possibly with overspilled sediments flowing back out of hole near seabed The NAB plume data represents a composite body of water up to a few hours old. Tidal currents reversed during the monitoring causing a plume in both directions
59. Sidescan Sonar Mosaic Sidescan sonar mosaic of the eastern approaches to the Solent. This encompasses the NAB Area 122/3 shown in RED and the active dredging zones in BLUE and GREEN. Also NAB Tower.
60. June 1999 sidescan sonar mosaicing Enlarged portion of the previous mosaic. Clearly shows the NAB Tower, different sediment types in the shipping channel, and the dredge holes formed by anchor dredging. Some of the sample locations are also shown.
61. June 2001 sidescan sonar mosaicing The June 2001 sidescan sonar mosaicing clearly shows the increased area of active dredging and the localised test loads and trail dredging operations. There is a small development of ripples to the North West of the site but this is across the tide and probably was present during the baseline study in 1999, surveyed at a coarser resolution.
62. June 2001 sidescan sonar mosaicing Despite a finer resolution than the baseline survey in 1999, there is no evidence of sand ripple development upstream or downstream from the dredge site. The topographic lows formed by the dredging are clearly seen on the sidescan, and it has been possible to carefully check the seabed around the dredge hole for signs of any overspilled material.
63. Sediment analysis Powerful statistical tools are now available to determine whether marine aggregate mining has an impact on sediment composition within dredged areas These methods include Group Average Sorting techniques and Multi-dimensional Scaling (MDS) methods. These non-parametric multivariate analytical methods are similar to those which are now widely used to distinguish biological communities on the sea bed.
64. Sediment analysis The sediments all have a high degree of similarity. Station 134 - a mined site - is quite distinct in its sediment characteristics. The sandy sediments fall into a distinct group 2 - coded blue here. Other sediment groups include a group 3 -coded green which represents the gravels of the licence area. Finally there is a large group 4 - coded red representing the sediments in the surrounding deposits 1 134 64 28 46 44 80 45 63 81 2 82 99 27 47 49 86 84 85 142 117 95 96 94 115 126 97 136 57 78 137 58 119 123 70 3 127 77 90 135 140 118 41 75 122 139 17 18 54 56 72 113 125 147 8 148 25 43 67 98 Stress = 0.04 131 68 88 9 10 89 93 24 1 116 138 141 4 59 108 4 134 69 65 2 42 87 144 11 35 62 49 132 108 132 86 87 1 85 59 143 84 11 35 62 146 34 65 32 69 71 20 144 36 38 142 42 150 27 59 24 151 1 6 71 129 9 36 18 116 131 98 68 118 47 138 7 93 109 133 89 10 80 39 44 73 17 139 149 6 23 28 75 106 46 53 41 99 120 45 92 104 63 88 91 79 40 119 76 121 122 54 124 58 145 129 81 95 82 125 72 113 148 56 135 8 123 141 124 127 90 147 39 94 70 96 57 25 115 76 140 64 77 73 78 92 136 106 137 67 97 43 126 23 120 117 121 145 34 40 3 104 7 109 91 53 149 38 146 20 32 143 37 150 79 133 151 100. 90. 80. 70. BRAY-CURTIS SIMILARITY
67. Impact on sediments 24 23 trailing dredge patch since 1998 anchor dredging patch since 1994 anchor dredging patch since 1991 number Hamon grab station 134 centre of dredge pit May 1999 115 42 117 41 116 50 ° 40' N 40 39 118 119 59 58 121 120 57 56 122 123 77 76 125 138 75 124 136 135 134 139 127 137 141 126 94 140 93 92 91 90 0 ° 58' W Neither anchor dredging nor trailer dredging areas are characterised by deposits which differ from those outside the boundaries of the dredge areas. Station 134 is the only known dredge pit area where deposits are significantly different from those elsewhere. This is due to a high proportion of coarse material. Dredging of all-in cargoes without overboard screening does not therefore appear to be associated with major changes in sediment composition in this particular survey site
69. Underwater video imagery Two campaigns to obtain underwater imagery of the dredge area have been undertaken. The first and most successful used a sled designed by Coastline and towed whilst drifting – the following short avi file is from that session in 2000. A second attempt was made using a small inspection class ROV – however tides were too strong and even mounting the ROV in the grab frame didn’t improve the images
70. Towed video Frame grabbing of the video is inefficient – these 20 seconds are 32Mb – but the image shows a rough ground with gravelly sediments in between – the drift is in the middle of the heavily worked area, within the pits themselves. There is no evidence of benthic organism (as would be expected immediately following dredging) although some small fish and a crab are encountered The camera was originally pointing forward when deployed – but a minor snag with a discarded fishing net or lobster potline tilted the camera backwards – hence the image is upside down
71. Physical Impact Results Sidescan sonar mosaicing and swath bathymetry shows clear evidence of dredge trails and anchor dredging. Measurements show these pits to be 125m in diameter, with two pits superimposed adjacent to each other, and both up to eleven metres deep. Some trails are only 300m or so long but are poorly defined. Dredge imprint is not deep, considerably less than half a metre, suggesting trailing by smaller vessels with shallower penetrating dragheads. But there is anecdotal evidence (from the vessel themselves) that the in situ geology is such that the draghead cannot penetrate easily the surface crusting, probably due to cementaceous biological activity There is no evidence of the development of microtopographical features such as sand ripples etc. This suggests that the quantities of overspilling sand are sufficiently low not to allow development of such pathways. Underwater video and high resolution sidescan sonar supports this. Recovered grab samples do not show a thin surface layer of fine sediment, although it must be acknowledged that such fine sediments maybe displaced by the sampling action itself.
72.
73. There are no other distinctive topographical features which suggest an impact due to dredging. Dredge trails do not appear infilled. This is probably to be expected due to the nature of the dredge site and underlines the importance of repeating the exercise in a heavily screened area.
74. However, there is possible evidence from the statistical analysis of the sediments that samples along the dominant flow and ebb currents do display a tendency to be sandier than those off the plume excursion, extending some 2500-3000mWe may conclude therefore that, any significant PHYSICAL SEDIMENTARY IMPACT from non-screening operations ie that impact produced by the benthic draghead plume and overspill alone is probably minimal and most likely confined within a few hundred metres of the dredge area Gravels are non-mobile (HRW), with sands only mobile during peak tides and storm conditions. Return to baseline would therefore be slow.
75. Current Status With respect to the initial programme proposed to the US MMS and discussed with the industry the current status of this project is as follows The Biological Assessment of the non-screened site was accelerated and completed in one year rather than the two year period, reflecting the strategic importance attached to this aspect The Physical Assessment of the non-screened site has been completed (sidescan sonar and grab sampling). Underwater physical inspection by divers was abandoned due to re-allocation of funds to the biological component of the project. ADCP data of the dredge plume from the non-screening scenario has been completed, although not ideal due to the logistics of the short dredging operation. The original goals of the project have been exceeded in that a complete assessment of the non-screening case have been determined with great confidence in the first year. Final Draft reporting will be submitted to MMS by March 2002. It now remains to work with industry to expand on these successes and apply the proven techniques to heavily screened areas. This will be especially important for defining predictive models.
76. and finally…. * * * * * * * * * * * INTERMAR * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * INTERNATIONAL ACTIVITIES AND MARINE MINERALS DIVISION I formally would like to thank MMS and in particular Barry Drucker, COTR, for the continuing support and encouragement with this and previous projects Our hosts at ITM 2002 And the support of the UK Marine Aggregates Industry, in particular Dr A Bellamy and Mr S Luckett of United Marine Dredging, the crew of the FlatHolm and Mr S Bell for the development of the 3D capability
