Assessment of Risk to Marine Mammals from Proposed Dreding
and Dumping at Sea Activity
1. Introduction
The Drogheda Port Company (DPC) has applied to the Department of Environment (DoE) for a
Foreshore Licence to permit maintenance dredging in the Boyne Estuary. The DoE has received
correspondence from the National Parks and Wildlife Service (NPWS) requesting that eight
mitigation measures be applied as a condition of the licence to ensure the protection of marine
mammals.
In connection with the proposed works DPC has also applied to the Environmental Protection
Agency (EPA) for a Dumping at Sea permit for the disposal of dredged material at offshore sites.
Similar correspondence from the NPWS requesting that the same eight mitigation measures be
applied as a condition of the Dumping at Sea permit, has also been received by the EPA.
This risk assessment covers all elements of the proposed works covered both by the Foreshore
Licence and by the Dumping at Sea permit and therefore addresses NPWS comments made on both
applications.
The Department of Arts, Heritage and the Gaeltacht (DAHG) has published Guidance to Manage the
Risk to Marine Mammals from Man-made Sound Sources in Irish Waters (Draft, March 2012). This
draft guidance sets out potential risks to marine mammals from man-made sounds. It states that
“An assessment of risk forms an important part of the decision-making framework for mitigating the
effects of anthropogenic sound in the marine environment. It is recommended that … [certain]
coastal and marine activities … should undergo a risk assessment for anthropogenic sound-related
impacts on relevant protected marine mammal species, to inform the consenting process.” The
NPWS recommendation to condition certain mitigation for marine mammals appears to have been
made in the absence of any risk assessment being undertaken. DPC therefore has undertaken this
risk assessment in order to inform the decision-making process, and as is concluded by this
assessment, to demonstrate that the recommended mitigation measures are not required in this
instance.
The NPWS draft guidance states:
A risk assessment for each marine mammal species of relevance to the proposed
works area needs to consider the nature of the sound source, its likely and/or
potential effects on individuals and/or populations and on their likely habitat…
Where an assessment identifies the likelihood of a risk to protected marine mammal
species, either by virtue of (a) the proposed operation or activity and/or (b) the
sensitivity of a particular site in which the sound-producing operation or activity is
proposed, it is recommended that appropriate risk management measures are
pursued by the relevant Regulatory Authority.
The guidance goes on to state:
Following the initial identification and assessment of risk arising from an operation or
activity … a menu of management options is available to Regulatory Authorities in
their decision making process (Fig. 2) and it includes:
A1. Consent without mitigation
A2. No consent given for the activity
A3. Avoid critical habitats for marine mammals (e.g., designated sites or known
sensitive areas)
A4. Avoid operations during key periods of the species’ life cycle (e.g.,
breeding/resting, migration)
A5. Avoid time periods when effective impact mitigation is not possible, and/or
A6. Risk minimisation measures, namely
A6.1. Minimise the duration over which the sound-producing activity is intended to
take place;
A6.2. Minimise the individual and cumulative sound pressure and exposure levels
delivered into the environment by the activity. If necessary the use of alternative,
lower impact equipment and methods should be explored (e.g., vibratory hammer,
gravity base piles).
A6.3. Incorporate the use of clear “ramp-up” or “soft-start” procedures, whereby
sound energy input to the marine environment is gradually or incrementally
increased from levels unlikely to cause significant behavioural impact on marine
mammals to the full output necessary for completion of the activity.
A6.4. Incorporate the use of fully enclosing or confined bubble curtains, encircling
absorptive barriers (e.g., isolation casings, cofferdams) or other demonstrably
effective noise reduction methods at the immediate works site, in order to reduce
underwater sound propagation from on-site operations. Studies have shown that
such methods can provide a significant reduction in sound input to the wider
aquatic environment in the order of 10-30 dB.
A6.5. The use of trained marine mammal observers (MMO’s) provides effective
means of detecting marine mammals in the vicinity of coastal and marine
operations. Associated operational considerations should also be taken into
account.
The guidance states that the consideration and/or application of activity-specific risk minimisation
measures as outlined in A6 above should be informed by the risk assessment. DPC believes that, as
demonstrated by this risk assessment, the application of risk minimisation measures is not necessary
in the case of the proposed dredging and dumping at sea activities and that option A1 (i.e. consent
without mitigation) is the most appropriate option from the menu of management options set out in
the NPWS guidance.
2. Information on the Proposed Activity
Full details of the proposed activity are set out in the Foreshore Licence application and the Dumping
at Sea permit application. However a summary of the key elements is provided here.
Maintenance dredging is proposed within the waterway under the jurisdiction of the Drogheda Port
Company in the following areas (refer to Figure 2 Loading Site Extent and Boundary included with
the applications for precise locations):

at the river mouth and seaward approaches

at commercial berths and ship swing basins (i.e. Drogheda Port town berths No. 1, 2, 3 & 4,
Knaggs Head and swing basin, Maxol/Flogas hydrocarbon berth, Premier Periclase berths,
Drogheda Port Company Tom Roes Point berths and ship swing basin)
On average it can be assumed that there will probably be two maintenance dredging campaigns per
year at the river entrance and seaward approaches, generally in Q2 and Q4 of each year. However,
given the weather sensitive nature and effects of storm events, unplanned maintenance dredging
also takes place to maintain safe navigation. In some years over the previous decade there have
been three annual campaigns. A typical campaign takes about three weeks, working each tide, twice
daily, generally from three hours before the high water to about one hour after the high water.
Maintenance dredging within the estuary (i.e. river channel, river bends, berths and swing basins) is
pre-planned with timing determined by plant availability, opportunity dredging from passing plant,
operational requirements, market conditions etc. It is commercially more favourable to tie the
berths, swing basins and channel maintenance dredging (if and when required) into a river entrance
and seaward approaches campaign but this will very often depend on plant suitability as the
dredging of sand and silts present different operational difficulties.
It is proposed that a portion of the dredged material (up to a maximum of 60,000m3 which modelling
has determined is a sustainable quantity from the point of view of coastal cell sediment dynamics)
from the river mouth and seaward approaches will be beneficially re-used by the construction
industry. The balance of dredged material (quantities exceeding 60,000m3 from the river mouth and
seaward approaches and all materials dredged from the river channel, river bends, berths and swing
basins) will be disposed of at one or more of a combination of three seaward dump sites (refer to
Figure 1 Maintenance Dredging Dump Sites included with the applications for precise locations).
The proposed dredging may be carried out by a range of dredging plant equipment due to the layout
of the port, material types (sand/silts), plant constraints and accessibility issues. However the
primary item of plant is the trailer suction hopper dredger. Of the 25 maintenance campaigns
undertaken by DPC over the last 11 years, 24 have been carried out using a trailer suction hopper
dredger. As is noted in correspondence from the Irish Ports Association (copy included in Appendix
A), trailer suction hopper dredgers are commonly recognised as being the type of dredgers with the
highest sound pressure level and many other dredging equipment (e.g. cutter suction dredging,
mechanical dredging using a grab or a backhoe, water injection dredging and plough dredging) may
be substantially lower. Therefore it can be taken that the most likely equipment, a trailer suction
hopper dredger, represents the worst case scenario in terms of sound pressure levels.
The proposed dredging will be timed with tides, working twice daily, generally from three hours
before the high water to about one hour after the high water. In between dredging the dredger will
be transporting material either to a town berth to go for re-use in the construction industry or to the
seaward dump sites, or will be idle for periods where the dredger is unloading at the town berth.
Therefore in any 24 hour cycle the active dredging will only be taking place for approximately 8
hours (4 hours around each high tide). For up to an additional 10 – 16 hours, the dredging vessel will
be moving between the dredge site and town berth/seaward dump site (but will not be actually
dredging). In the case of where materials are to be re-used in the construction industry there will be
up to 6 hours when the dredger will be moored at the town berths an idle (i.e. engine switched off)
during the unloading operations.
As dredging is timed to coincide with high tides it should be noted that works may take place during
daylight or darkness dependent on when high tide occurs.
3. Information Marine Mammals within the Area of the Proposed Activity
The NPWS draft guidance states:
There are at least 26 species of marine mammal known to occur in Irish waters. Two
seal species, the Grey Seal (Halichoerus grypus) and Harbour Seal (Phoca vitulina)
breed around all shorelines of Ireland and use the coastal and offshore waters in their
daily lives for foraging, transit between terrestrial resting places (known as haul-out
sites), and other behaviours linked to their annual life cycles (e.g., social behaviour,
territoriality).
Twenty-four species of cetacean (i.e., whales, dolphins and porpoises) have been
recorded from Ireland, 18 of which are more commonly observed, while the remaining
six species have rarely been recorded and are currently classed as vagrant (i.e.,
species well outside their normal natural range).
Some species can occur close to shore, and may be found within enclosed bays,
harbours and estuaries, such as Dingle Harbour or the Shannon Estuary. Others (e.g.,
Blue Whale, Sperm Whale, Humpback Whale) may be highly migratory and show a
preference for deeper water offshore habitats, or travel hundreds or thousands of
kilometres between winter breeding and summer foraging locations, occupying Irish
waters during part of their annual cycle.
The following sources of records of marine mammals have been checked for occurrence of marine
mammals within and around the area of the proposed dredging and disposal activities:




The National Parks and Wildlife Service online database of species (accessed at www.npws.ie
on 9th October 2012)
The Irish Whale and Dolphin Group online database of sightings of marine mammals
(accessed at www.iwdg.ie on 9th October 2012)
Appendix 4 of the NPWS draft guidance (NPWS, draft March 2012) which provides
generalised maps of marine mammal distribution and habitat in Irish waters
Consultation with DPC staff of sightings in the Boyne Estuary
Results of these searches are provided below. It is acknowledged that absence of records does not
necessarily provide conclusive results that marine mammals are absent.
The National Parks and Wildlife Service Online Database of Species
The proposed dredging and disposal activities take place mainly within National Grid Square (NGS)
O17 with a small element of the dredging also falling within NGS O07. These two NGSs as well as the
five surrounding marine NGSs were checked for records of marine mammals with results shown
below:


NGS O17: one record of Harbour Seal at Callaighstown & Shallon, Drogheda (no grid
reference provided) on 04/06/1991
NGS O07, O16, O18, O26, O27 and O28: no records of marine mammals
The Irish Whale and Dolphin Group Online Database of Sightings of Marine Mammals
Results from within a 10km distance of the proposed activities (dredging and/or disposal) in the
marine and estuarine environment are provided below. All records are validated and available on
www.iwdg.ie
Sighting ID
19480
19355
Species
Bottlenose
Dolphin
"dolphin" species,
possibly harbour
porpoise
Location/area
Clogher Head,
Louth
Platform type
Other
vessel/unspecified
vessel
Date
19690
19671
19376
Harbour porpoise
Harbour porpoise
"dolphin" species,
possibly harbour
porpoise
Port Oriel, Louth
Clogher Head,
Louth
Clogher Head,
Louth
Mornington
Beach, Meath
Land
Headland/spit
Unknown
Land
18 Aug 2012 15:00 15 Jul 2012 15:00
6 Sep 2012 11:35
7 Sep 2012 12:00
18 Jul 2012 21:30
Number
Best est: 3 (min 3,
max 3)
Best est: 1 (min 1,
max 2)
Best est: 1 (min 1,
max 1)
Best est: 3 (min 2,
max 4)
Best est: 2 (min 1,
max 3)
Adults
2
Unknown
1
Unknown
Unknown
Juveniles
1
Unknown
Unknown
Unknown
Unknown
Appendix 4 of NPWS Draft Guidance (NPWS, draft March 2012)
This publication provides generalised maps of marine mammal distribution and habitat in Irish
waters. These maps indicate that there is habitat suitable for the following species off the
Meath/Louth coastline:
Baleen Whales



Fin whale
Minke whale
Humpback whale
Toothed Whales and Dolphins





Northern bottlenose whale
Long-finned pilot whale
Killer whale
Risso’s dolphin
Common Bottlenose dolphin





White-beaked dolphin
Striped dolphin
Striped dolphin
Short-beaked common dolphin
Harbour porpoise
Pinnipeds


Harbour seal
Grey seal
Some of the above species are known only in deeper waters and therefore are extremely unlikely to
occur within the shallow coastal waters where the proposed dredging and disposal activities will be
taking place (e.g. Fin, Minke, Humpback and various other whale and dolphin species).
Consultation with DPC staff of sightings in the Boyne Estuary
DPC staff are present on the estuary on a daily basis and have been consulted for any sightings of
marine mammals they have observed over the years. The only marine mammals that DPC staff have
observed within the estuary have been of seals (species unknown). These have not been observed
within the area directly to be affected by the dredging proposals (i.e. within the river walls) but have
been observed in other areas within the estuary. Sightings are infrequent (i.e. less than 1 per year)
and sightings have only ever been of single individuals.
Summary of Desktop Data Records of Marine Mammals
In summary, the only confirmed records of marine mammals within or in close proximity of the
proposed activities are




Bottlenose Dolphin
Harbour porpoise
Harbour Seal
Possibly Common Seal (species unconfirmed)
These species are also those most likely to be relevant as they regularly occur in shallow (i.e. <20m
water depths) coastal waters. Although confirmed records are not available it is possible that a
number of other whales and dolphins could occur within or in proximity to the proposed activities.
4. Assessment of Risk
The draft NPWS guidance states that the evaluation of risk to protected marine mammal species
arising from anthropogenic sound depends on three basic elements, namely the (1) Source, (2)
Species and (3) Environment. These three elements are discussed below.
The Source
The sources of sound include the operation of the dredging vessels (i.e. non-dredging activity) and
the dredging activity itself. Both of these sound sources are non-pulse types of sound that involve
intermittent and/or continuous sound events without rapid rise time of pulse type (i.e. unlike
explosions). Non-pulse producing activities such as dredging are generally of less concern for
impacts on marine mammals than single or multiple pulse sources of sounds.
Appendix 1 of the NPWS guidance provides information on the nature of sound from various sources
and states that the sound pressure level for trailer suction hopper dredgers is 177 dB with a
frequency range of 80-200Hz.
As is noted in correspondence from the Irish Ports Association (copy included in Appendix A),
shipping noise is characterised under the Marine Strategy Framework Directive as being between
57Hz and 141Hz. This correspondence also notes that operating trailer suction hopper dredgers are
marginally louder while dredging than shipping traffic. Therefore it can be assumed that the sound
pressure level for shipping noise is less than 177 dB.
While sound exposure levels from dredging operations are thought to be below that expected to
cause injury to a marine mammal, they have the potential to cause lower level disturbance, masking
or behavioural impacts.
The Species
The key species of relevance (i.e. which are either known or likely to occur within or in proximity to
the proposed activities) include: Bottlenose Dolphin; Harbour Porpoise; Harbour Seal and Common
Seal. Information on the likely sensitivity of these species to sounds associated with the proposed
activities has been put together from information provided in the NPWS draft guidance as follows:



Bottlenose Dolphin (150hz – 160Hz) fall within the frequency for trailer suction hopper
dredgers (80Hz - 200Hz) but outside of the frequency range for shipping noise (57hz – 141
Hz). This species is not believed to experience permanent injury at the sound pressure level
associated with dredging and/or shipping noise (≤ 177dB); permanent injury is predicted for
this species at 230dB. Disturbance/behavioural responses have however been recorded for
this species between 90-200dB; dredging and shipping noises fall within this range.
Harbour Porpoise (200Hz – 180Hz) fall within the frequency for trailer suction hopper
dredgers (80Hz - 200Hz) but outside of the frequency range for shipping noise (57hz – 141
Hz). This species is not believed to experience permanent injury at the sound pressure level
associated with dredging and/or shipping noise (≤ 177dB); permanent injury is predicted for
this species at 230dB. Disturbance/behavioural responses have been recorded for this
species between 90-170dB; the dredging and shipping noises are higher than this level.
Harbour and Common Seal (75Hz – 75Hz) fall just outside of the frequency for trailer suction
hopper dredgers (80Hz - 200Hz) but within of the frequency range for shipping noise (57hz –
141 Hz). These species are not believed to experience permanent injury at the sound
pressure level associated with dredging and/or shipping noise (≤ 177dB); permanent injury is
predicted for these species at 218dB. Disturbance/behavioural responses have however
been recorded for this species at ≥ 100dB; the dredging and shipping noises are higher than
this level.
In summary of the four species considered to be of most relevance, Bottlenose Dolphin and Harbour
Porpoise fall within the frequency range of trailer suction hopper dredgers but Harbour and
Common Seal fall just outside of this range and therefore may not be able to actually detect sounds
arising from trailer suction hopper dredgers.
On the other hand, Harbour and Common Seal fall within the frequency range of shipping noise but
Bottlenose Dolphin and Harbour Porpoise fall outside of this range and therefore may not be able to
actually detect sounds arising from shipping noise.
Permanent injury is not believed likely to occur to any of the four species considered to be of most
relevance, at the sound pressure level associated with dredging and/or shipping noise.
Disturbance/behavioural responses have been recorded for all four species at or below the sound
pressure level associated with dredging and/or shipping noise. However given that Harbour and
Common Seal may not be able to detect sounds arising from trailer suction hopper dredgers and
that Bottlenose Dolphin and Harbour Porpoise may not be able to detect sounds arising from
shipping noise not all species are likely to have responses to all elements of the proposed activities.
It is anticipated that Harbour and Common Seal may experience disturbance/behavioural responses
to shipping noise only and Bottlenose Dolphin and Harbour Porpoise may experience
disturbance/behavioural responses to dredging only.
It is possible that other species of mid-frequency cetaceans, in addition to the species considered to
be of most relevance listed above, may occur in the area which do fall within the frequency range of
trailer suction hopper dredgers only (i.e. not within shipping noise frequency range) and which are
known to display disturbance/behavioural responses at or below the sound pressure level associated
with dredging. However no species of any marine mammal is expected to experience permanent
injury at the level of either dredging or shipping noise.
It should also be noted that the frequencies of the proposed activities are outside of the frequency
range for all low frequency whale species (e.g. Fin, Minke and Humpback whales) and therefore
regardless of whether or not they may occur within the general area, they would not be able to hear
the proposed activities.
The Environment
The environment within which the proposed dredging will take place includes the estuarine portion
of the Boyne River as well as its mouth at the coast (refer to Figure 2 Loading Site Extent and
Boundary included with the applications for precise location). The coastal waters at the mouth of
the Boyne River form part of extensive shallow coastal waters. All areas in which the proposed
activities will take place are less than 10m in depth at all stages of the tide. The occurrence of any
marine mammals other than Harbour and Common Seal within the areas in which the proposed
activities will take place is extremely unlikely due to shallow depths.
While some cetacean species are likely to occur within proximity (i.e. within 10km) of the proposed
activities these are still within shallow coastal waters. As is noted in correspondence from the Irish
Ports Association (copy included in Appendix A), acoustic propagation in shallow waters is not highly
efficient and for some frequencies in depths of less than 20m does not occur at all or is attenuated
rapidly with distance. This is likely to be the case for lower frequencies such as shipping noise.
It should be noted that the proposed activity will take place within an existing busy shipping
environment. DPC receives approximately 700 vessels per annum and operates a pilot boat on the
estuary twice per day every day. In this context the addition of a single dredger for approx. 3 week
periods 2-3 times per year, represents a minor increase in terms of shipping noise and activity in
addition to the existing background shipping levels.
Conclusions of Risk Assessment
In summary DPC considers, as is demonstrated by this risk assessment, that the application of risk
minimisation measures is not necessary in the case of the proposed dredging and dumping at sea
activities and that option A1 (i.e. consent without mitigation) from the menu of management
options set out in the NPWS draft guidance is the most appropriate option.
Consent without mitigation may be given as the risk to marine mammals is considered extremely
low based on the following:



The sound sources associated with the proposed activities are amongst those of least
concern for impacts on marine mammals (i.e. non-pulse producing activities)
The duration of the sound sources will be temporary, limited to 2-3 times per year at approx.
three weeks per dredging campaign, and limited to 8 hours per day in the case of dredging
and up to 10 – 16 hours per day in the case of shipping noise. It should be noted that the
introduction of 10 -16 hours shipping noise per day, over an approx. 3 week period, 2-3
times per year, represents a minor increase in terms of shipping noise and activity in
addition to the existing background shipping levels.
There are extremely few existing records for marine mammals within or in proximity (i.e.
within 10km) to the proposed activities; the area within or in proximity to the proposed
activities does not appear to have high quantities or diversity of marine mammal species by
comparison to other parts of the country.





The area within and in proximity to the proposed activities is extremely shallow and
therefore few marine mammals are likely to occur.
Much of the activities will take place in the upper reaches of the estuary and will therefore
be at considerable distances from the open coastal waters where most marine mammals
would be expected to be more likely to occur (unless in exceptional cases where they may
accidentally enter the river).
No species of any marine mammal is expected to experience permanent injury at the level of
either dredging or shipping noise. Any impacts which might occur would be related to
disturbance/behaviour responses only.
Only four species are expected to be likely to occur within or in proximity to the proposed
activities; Bottlenose Dolphin, Harbour Porpoise, Common Seal and Harbour Seal. The
former two are expected to be able to hear dredging activities only (i.e. not shipping
activities) and the latter two are expected to be able to hear shipping noise only (i.e. not
dredging).
Acoustic propagation in the shallow waters within which the proposed activities will take
place is expected to be quickly attenuated and would not travel far; this is likely to be
particularly the case for lower frequencies such as shipping noise.
Appendix A
Correspondence from the Irish Ports Association – consultation response to draft NPWS
guidance