802050
Description
Mind Map by amyycartwright, updated more than 1 year ago
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Created by amyycartwright
over 10 years ago
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IMPACTS OF AGGREGATE DREDGING
- 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 on seafloor
- Weathered dredge tracks - marks on 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.
3 ways;
- Plumes of suspended
material from action of
dredge head itself -
underwater
- Outwash of material
from spillways from
vessel generate much
more
- Rejection of unwanted sediment
- These 2 are termed "surface plumes"
- Extent depends
on particle size,
volume, speed of
discharge and
local
hydrodynamics
- Impact depends on habitat type
- 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
sedimenst
- Encrusting epifaunal taxa affected
by abrasive action of suspended
sedimenst
- Levels of natural disturbance high with
high levels of suspended sediment due
to waves and tidal action - less impact
- Sand particles usually within few
100m, finer particles transported much
greater distances from point of
discharge
- Impact depends on habitat type
- Extent depends
on particle size,
volume, speed of
discharge and
local
hydrodynamics
- These 2 are termed "surface plumes"
- Rejection of unwanted sediment
- Outwash of material
from spillways from
vessel generate much
more
- Indirect impacts
may be more
important than
direct impacts
- Results in reduced set
of species, low
densities, reduced
biomass
- Results in reduced set
of species, low
densities, reduced
biomass
- Plumes of suspended
material from action of
dredge head itself -
underwater
- Direct - removal of sediment
- 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
- Removal of habitat
- Immediate loss of
fauna in extraction
area
- Changes in sediment
composition, associated
communities change
- Organisms 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 associated with certain
sediment types/particle size. Warwick &
Uncles 1980 - Severn Estuary
- Full restoration of fauna and sediments can
take 2-3 years from cessation of dredging -
short term dredge areas
- But in UK most licences 15 years
- 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 years
(Newell et al. 2004)
- Study by 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 of species
humbers, population density,
biomass and average body size
- Decreased
"equilibrium"
species such as
bivalves and
echinoderms
- More "opportunistic" species well adapted to disturbed sediments
- An impoverished community dominated by polycheates
- An impoverished community dominated by polycheates
- More "opportunistic" species well adapted to disturbed sediments
- Decreased
"equilibrium"
species such as
bivalves and
echinoderms
- Major suppression of of species
humbers, 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 2k
- "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 2k
- Impacts on
bathymetry and
sediment
composition
- Studied impacts of
dredged sites,
surrounding areas
and axis of transport
of discharged
materials
- Study by Newell et al. 2004
- But restructuring of
biomass dominants and
age structures could take
longer than other
community attributes,
perhaps 15-20 years
(Newell et al. 2004)
- Recovery often only
assessed on parameters
of biomass, abundance
and diversity
- But in UK most licences 15 years
- Areas near dredging site may be of
conservation significance or important nursery
and feeding grounds for commerically valuable
species such as the Queen Scallop
(Aequipecten opercularis) and edible crab
(Cancer pagurus)
- Immediate loss of
fauna in extraction
area
- 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
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