Assessment of Risk to Marine Mammals from Proposed Dreding and Dumping at Sea Activity

Assessment of Risk to Marine Mammals from Proposed Dreding and Dumping at Sea Activity

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 This draft guidance sets out potential risks to marine mammals from man-made sounds.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The NPWS draft guidance states: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.

  1. 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 60,000m3which 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 60,000m3from the river mouth and seaward approaches and all materials dredged from theriver 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).

Theproposed dredging may be carried out by a range of dredging plant equipmentdue 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 highwater. 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.

  1. 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 andestuaries, 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 on 9th October 2012)
  • The Irish Whale and Dolphin Group online database of sightings of marine mammals (accessed at 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

Sighting ID / 19480 / 19355 / 19690 / 19671 / 19376
Species / Bottlenose Dolphin / "dolphin" species, possibly harbour porpoise / Harbour porpoise / Harbour porpoise / "dolphin" species, possibly harbour porpoise
Location/area / Clogher Head, Louth / Port Oriel, Louth / Clogher Head, Louth / Clogher Head, Louth / Mornington Beach, Meath
Platform type / Other vessel/unspecified vessel / Land / Headland/spit / Unknown / Land
Date / 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.

  1. 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 ofpulse 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 belowthat expected to cause injury to a marine mammal, they have the potential to cause lower leveldisturbance, masking or behavioural impacts.