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

SeabedINTERMAR

** ** **** **

*

*

**

*

*

***

***

*

*

*

*

* *** ***

*

*

*

*

*

*

*

*

*

*

*

**

*

* *

*

*

INTERNATIONAL ACTIVITIES AND MARINE MINERALS DIVISION

Overview

This presentation reviews the work carried out forthe Minerals Management Service over the past threeyears 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

Coastline Surveys Ltd

•Coastline operates the MV FlatHolm, a 22m research vessel equipped for full hydrographic, oceanographic, geophysical and sampling investigations and diving operations

Coastline undertake Prospecting Bathymetry Sampling Geophysics Sonar

Marine aggregate

dredging forconstruction

andcoastal

defences

TSHD Geopotes

TSHD ARCO Humber

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).

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.

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

But Why?

•What are the underlying forcing functions?

– Extraction methodology?

– Geological conditions?

– Oceanographic conditions?

– Natural Disturbance?

TSHD Arco SevernTSHD Arco Severn

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?

Methods of MiningThe 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.

Operations

•Trailer suction dredger•Anchor suction dredger

Intensive activity, Southern North Sea

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

Impact on Biological Resources

•Community Structure– Diversity– Abundance

•Determination of biological footprint•Dispersion of Organic Load•Zones of Enrichment

Marine Ecological Surveys ‘Biological Impacts of Marine Aggregate Dredging’ to follow…...

FOR MORE INFO...

Impact on Physical Resources

Distribution of suspended sediments (underwater video, ADCP, water sampling)

Distribution of deposited sediments (sidescan sonar, traps, sampling)

Changes in seabed composition (seabed sampling)

? Compaction profiles of sediment (CPT) (effects on benthos)

Longer term bed-load processesnet sediment build-up

Implications for coastal erosionmodelling 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.

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.

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

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

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

Hamon GrabThe 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

Distribution of suspended sediments

ADCP Backscatter Surveys

Multiple plumes

ADCP Profiling June 2001

ADCP on the NAB (June 2001)

45m45m 95m95m 100100mm

105105mm

105105mm

105105mm

130130mm

155155mm

200200mm

Key:

Distances in metres downstream between dredger and survey vessel

45m

ADCP on the NAB (June 2001)

200m200m

Key:

Distances in metres downstream between dredger and survey vessel

45m

210210mm

ADCP on the NAB (June 2001)

25m25m

Key:

Longitudinal distances in metres downstream between dredger and survey vessel

45m

45m45m 820820mm

820820mm

35m35m

820820mm

Water sampling (June 2001)

Pump water samples obtained from frame which incorporated an UMI data logger, optical backscatter sensor and pressure transducer

0

1000

2000

3000

4000

5000

6000

0 100 200 300 400 500 600 700 800 900

Susp

ende

d so

lids (m

g/l)

distance downplume (m)

City of Chichester June 9th 2001 NAB Licence (anchor dredge, no screening)

1 2 3

4 5

Water sampling (June 2001)

Cast 3Cast 3

Cast 4Cast 4

Cast 5Cast 5

Cast 1Cast 1

Cast 2Cast 2

City of Rochester Plume ()Backscatter image of density current present within the

plumeand possible

near bedbenthic

boundarylayer

City of Rochester Plume ()Backscatter image of density current present within the plume

and possiblenear bed

benthicboundary

layerImage

shows just thehigh levels of

backscatterindicating the

dynamic phaseof the density

current

City of Rochester Plume ()Backscatter image of density current present within the

plumeand possible

near bedbenthic

boundarylayer

Geopotes Plume ()Backscatter image of density current present within the

plumeand possible

near bedbenthic

boundarylayer

Geopotes Plume ()Backscatter image of density current present within the

plumeand possible

near bedbenthic

boundarylayer

Geopotes Plume ()Backscatter image of divisions within the plume

with possiblenear bed

benthicboundarylayer and

Surfaceexpression of

organics orlight fraction

Geopotes Plume ()Backscatter image of divisions within the plume

with possiblenear bed

benthicboundarylayer and

Surfaceexpression of

organics orlight fraction

City of Chichester

City of Chichester Plume ()

Backscatter image of

density current present within

the plume

Cut away of the core of the

plume

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)

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.

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

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

Sidescan Sonar MosaicSidescan

sonar mosaic of

the eastern approaches

to the Solent. This encompasse

s the NAB Area 122/3

shown in RED and the

active dredging zones in

BLUE and GREEN. Also NAB Tower.

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.

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.

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.

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.

Sediment analysisThe 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

BRAY-CURTIS SIMILARITY100.

90.

80.

70.

98 67 43 25 148

151 133 79 150 37 143 32 20 146 38 149 53 91 109 7 104 40 34 145 121 120 23 106 92 73 76 39 124 129 6 71 36 1 132 62 35 11 144 87 42 65 69 108 59 141 138 116 24 93 89 10 9 88 68 131

8 147 125 113 72 56 54 18 17 139 122 75 41 118 140 135 90 77 127 70 123 119 58 137 78 57 136 97 126 115 94 96 95 117 142 85

64 134

84 86 49 47 27 99 82 81 63 45 80 44 46 28 1

3

4

2

134

1 67

8

910

11

1718

20

2324

25

27

28

323435

3659 38

39 4041

42

43

44454647

49

535456

57

58

5962

63

64

65

67

68

69

70

71

72

7375 76

7778

79 808182

84858687

8889

90

9192 93

94

95

96

97

9899104106

108

109

113

115

116

117

118119 120121122

123124

125

126

127

129 131

132

133

135

136137

138139

140

141

142143

144

145

146

147148

149

150151

Stress = 0.04

12

3

4

Sediment province by multivariate analysis

Isle of Wight

50° 40' N Nab Tower

7 8

9 10

11

17

18

20

23 24 25

27

28

34

35

36 37

38 39

40 41

42 43

44

45

46

47

53

54

56 57

58

59

62

63

64

65

68

69

70

71 72

73

75

76 77

78 79

80

81

82

88

89 90

91 92

93 94

95 96

97 98

99

104

106

108

109

113

115

116 117

118 119

120 121

122 123

124 125

126 127

129

6

1

32

49

67

84

85

86

87

131

132

133

140 137 139 134 136

138 135

141

50° 42' N

50° 38' N

land

sands at low tide

depth < 10 metres

depth > 10 metres

number Hamon grab station

1 nautical mile

142143

144 146

145 150

151

148

147

149

June 1999 sonar correlation with sediments

Impact on sedimentsNeither 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

50 ° 40' N

0 ° 58' W90

91

92

93

94

75

2324

39

40

41

42

56

5758

59

76

77

115

116 117

118119

120 121

122 123

124

126

125

127141

138

135

140

136

137139134

134

number

trailing dredge patch since 1998anchor dredging patch since 1994anchor dredging patch since 1991Hamon grab stationcentre of dredge pit May 1999

June 1999 sonar correlation with fauna

Underwater video imageryTwo 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

Towed video

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

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

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.

Physical Impact - conclusions•There is no evidence from ADCP profiling of the development of a

measurable draghead plume from the anchored dredge vessel.

•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.

•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 tidesand storm conditions. Return to baseline would therefore be slow.

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.

and finally….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

INTERMAR

** ** **** **

*

*

**

*

*

***

***

*

*

*

*

* *** ***

*

*

*

*

*

*

*

*

*

*

*

**

*

* *

*

*

INTERNATIONAL ACTIVITIES AND MARINE MINERALS DIVISION