691276
Beschreibung
Mindmap von Madison Burt, aktualisiert more than 1 year ago
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Erstellt von Madison Burt
vor mehr als 10 Jahre
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Impacts of Aggregate
Dredging
- Removal of habitat
- Areas near dredging site may be of
conservation significance or important nursery
and feeding grounds for commercially
valuable species e.g. Queen Scallop
(Aequipecten opercularis) and edible crab
(Cancer pagurus).
- Full restoration of fauna and sediments can take
2-3 yrs from cessation of dredging - short term
dredge areas. But in the UK most licensesfor 15
years
- Changes in sediment composition,
associated communities change.
- Organisms can be associated with certain
sediment types/particle size (Warwick &
Uncles 1980, Severn Estuary)
- Different faunal composition
of silty sand to coarser sand
- Different faunal composition
of silty sand to coarser sand
- Increase in the proportion of fine
sands on increase in gravel
- Organisms can be associated with certain
sediment types/particle size (Warwick &
Uncles 1980, Severn Estuary)
- Long term effects not
known but areas around
UK still not recovered after
7 years
- Recovery often only
assessed on parameters
of biomass, abundance
and diversity
- But restructuring of biomass
dominants and age
structures could take longer
than other community
attributes, perhaps 15-20 yrs
(Newell et al, 2004)
- Newell et al,
2004.Studied impacts of
dredged sites,
surrounding areas and
axis of transport of
discharged materials
- Impacts on
bathymetry
and sediment
composition
- Significant differences
in sediments in heavily
and lightly dredged sites
- Benthic communities
significantly different
between dredged and
undredged sites
- Major suppression of species
richness, population density,
biomass and average body
size
- Decreased
'equilibrium' species
e.g. bivalves &
echinoderms
- More 'opportunistic
species well adapted to
disturbed sediments
- An impoverished
community dominated
by polychaetes
- An impoverished
community dominated
by polychaetes
- More 'opportunistic
species well adapted to
disturbed sediments
- Decreased
'equilibrium' species
e.g. bivalves &
echinoderms
- Major suppression of species
richness, population density,
biomass and average body
size
- Benthic communities
significantly different
between dredged and
undredged sites
- Significant differences
in sediments in heavily
and lightly dredged sites
- A 'footprint' of the
impacts of dredging and
screening in surrounding
areas
- Majority of sediments settle
within 500m but evidence of
2km
- 'Footprint' can extend 2-3km along
axis of net sediment transport
- Distance depends on net
sediment transport at the
seabed
- Distance depends on net
sediment transport at the
seabed
- Majority of sediments settle
within 500m but evidence of
2km
- Impacts on
bathymetry
and sediment
composition
- Newell et al,
2004.Studied impacts of
dredged sites,
surrounding areas and
axis of transport of
discharged materials
- But restructuring of biomass
dominants and age
structures could take longer
than other community
attributes, perhaps 15-20 yrs
(Newell et al, 2004)
- Recovery often only
assessed on parameters
of biomass, abundance
and diversity
- Changes in sediment composition,
associated communities change.
- Immediate loss of
fauna in extraction
area
- Areas near dredging site may be of
conservation significance or important nursery
and feeding grounds for commercially
valuable species e.g. Queen Scallop
(Aequipecten opercularis) and edible crab
(Cancer pagurus).
- Impacts on Marine Life
- Direct -
removal of
sediment
- Severe physical disturbance of
seabed. Physical effects seen for
up to 7 years after dredging ceased
(at a moderate energy site)
- Weathered dredge tracks -
marks the seafloor
- Weathered dredge tracks -
marks the seafloor
- Severe physical disturbance of
seabed. Physical effects seen for
up to 7 years after dredging ceased
(at a moderate energy site)
- Indirect -
Sediment
resuspension
- Indirect impacts may
be more important than
direct impacts. Can
result in reduced
species richness, low
densities & reduced
biomass
- Plumes of suspended
material from action of
dredge head itself -
underwater
- 1) Outwash of material from spillways
from vessel generate much more. 2)
Rejection of unwanted sediment
- These two termed "surface plumes"
- Extent depends on particle size,
volume, speed of discharge and local
hydrodynamics
- Sand particles usually within few 100m,
finer particles transported much greater
distances from point of discharge
- Sand particles usually within few 100m,
finer particles transported much greater
distances from point of discharge
- Impact depends on
habitat type
- Low levels of natural
disturbance - more significant.
Encrusting epifaunal taxa
affected by abrasive action of
suspended sediment
- Levels of natural
disturbance high with high
levels of suspended
sediment due to waves
and tidal action - less
impact
- Low levels of natural
disturbance - more significant.
Encrusting epifaunal taxa
affected by abrasive action of
suspended sediment
- Extent depends on particle size,
volume, speed of discharge and local
hydrodynamics
- These two termed "surface plumes"
- Indirect impacts may
be more important than
direct impacts. Can
result in reduced
species richness, low
densities & reduced
biomass
- Direct -
removal of
sediment
- Suggested Mitigation Measures
- Reduction of overboard screening; material accumulates
at dredge site, means more dredging and more discharge
required in the exploited area over time
- Adjustment of time for
exploitation; dredging only
when tidal stream will carry
sediments away
- Establishment of refuge
areas to enhance
recolonisation;
non-dredged areas left
between areas of
dredged seabed
- Measures need to be
cost effective to really be
implemented
- Reduction of overboard screening; material accumulates
at dredge site, means more dredging and more discharge
required in the exploited area over time
- 21% of the sand and
gravel used in England
and Wales is extracted
from the marine
environment
- Extracted using huge ships - in UK usually trailer
dredging, sometimes static suction hopper dredging
- Useful materials are washed and screened, rest thrown
back overboard
- Useful materials are washed and screened, rest thrown
back overboard
- Extracted using huge ships - in UK usually trailer
dredging, sometimes static suction hopper dredging
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