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    INTERMAR
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    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
  • 3. Coastline Surveys Ltd
    Coastline operates the MV FlatHolm, a 22m research vessel equipped for full hydrographic, oceanographic, geophysical and sampling investigations and diving operations
  • Marine aggregate
    dredging for
    construction and
    coastal defences
    TSHD Geopotes
  • 9. TSHD ARCO Humber
  • 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.
  • 16. Operations
    Trailer suction dredger
    Anchor suction dredger
  • 17. Intensive activity, Southern North Sea
  • 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…...
  • 20. Impact on Physical Resources
    • Distribution of suspended sediments
    • 21. (underwater video, ADCP, water sampling)
    • 22. Distribution of deposited sediments
    • 23. (sidescan sonar, traps, sampling)
    • 24. Changes in seabed composition
    • 25. (seabed sampling)
    • 26. ? Compaction profiles of sediment (CPT)
    • 27. (effects on benthos)
    • 28. Longer term bed-load processes
    • 29. net sediment build-up
    • 30. Implications for coastal erosion
    • 31. modelling of wave and current dynamics
  • North NAB Project Site – why?
    Area 122/3 is located to the east of the Isle of Wight, on the south coast of the UK in the English Channel. Although the amount of aggregates removed from this area is quite low (150,000 tonnes per year), its is probably one of the most intensively dredged sites per unit area.
    The area had been exploited for almost 10 years prior to our survey, so any impact of intensive dredging at this site should be apparent in our surveys
    The material is not significantly screened, but of the 11 licence areas on the south coast of UK, 9 involve non-screened cargoes.
  • 32. North NAB
    The NAB area is therefore representative of the majority of the licence areas on the south coast of UK in contrast with those of the southern North Sea which are generally heavily screened
    Importantly this was also more representative of the US situation (although maybe not now?)
    • The North Nab site has a further advantage in that part of the area is mined by anchor dredging whilst another part is mined by trailer dredging. This allows direct comparisons of the impact of the 2 types of dredging in adjacent areas.
  • 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
  • 37. Distribution of suspended sediments
  • 38. ADCP Backscatter Surveys
  • 39. Multiple plumes
  • 40. ADCP Profiling June 2001
  • 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
  • 45. Water sampling (June 2001)
    Cast 3
    Cast 1
    Cast 4
    Cast 2
    Cast 5
  • 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
  • 53. City of Chichester
  • 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
  • 65. Sediment province by multivariate analysis
    50° 42' N
    Isle of Wight
    129
    6
    1
    28
    11
    47
    10
    27
    9
    46
    8
    25
    65
    7
    24
    45
    23
    64
    44
    43
    20
    63
    115
    42
    82
    18
    62
    117
    41
    116
    81
    40
    17
    39
    118
    119
    38
    59
    80
    50° 40' N
    Nab Tower
    37
    36
    121
    58
    120
    79
    57
    56
    78
    99
    35
    123
    122
    77
    76
    54
    98
    34
    125
    138
    124
    75
    97
    53
    135
    96
    136
    127
    139
    73
    137
    134
    95
    141
    94
    72
    126
    140
    71
    93
    32
    92
    91
    70
    90
    89
    108
    69
    106
    49
    land
    88
    68
    104
    151
    87
    sands at low tide
    67
    113
    148
    149
    86
    146
    depth < 10 metres
    85
    133
    109
    144
    132
    84
    143
    150
    147
    depth > 10 metres
    145
    131
    142
    number
    Hamon grab station
    1 nautical mile
    50° 38' N
    0° 55' W
    1° 05' W
    1° 00' W
  • 66. June 1999 sonar correlation with sediments
  • 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
  • 68. June 1999 sonar correlation with fauna
  • 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. Physical Impact - conclusions
    • There is no evidence from ADCP profiling of the development of a measurable draghead plume from the anchored dredge vessel.
    • 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 2500-3000m
    We 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….
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    INTERMAR
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    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