Harbour porpoise...Principles of Underwater Sound, 3rd Edition. New York. McGraw-Hill. 2. Arons A B, Yennie D R and Cotter T P (1949). Long range Shock Propagation in Underwater Explosion

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Assessing the Impact of Explosive Blast on Marine Life

by Peter D Ward Kongsberg Maritime Ltd

Abstract

XO (Unexploded Ordnance) poses an obvious risk to the marine environment. The options are to leave it in place, detonate it in-situ, or remove it to

a safe place prior to detonation. If detonation is required then this must be carried out in a place where any potential environmental impacts are minimised as far as possible. For instance, the detonation site should be sufficiently far from, for instance, seal haul-out sites, fish spawning grounds, and whale migration paths. The critical distance for survivability and the likely outcome on fish and marine mammals may be estimated by modelling the effect of explosive blast on marine life.

This involves a two-stage process. The first part is concerned with the propagation of explosive blast which is represented using a set of semi-empirical equations, based on the explosive charge weight. This is followed by modelling the impact of the impulsive blast wave on fish and marine mammals where the key parameter is their body weight.

In order to illustrate the potential impact of blast on marine life, a number of case studies are used. These are the SS Richard Montgomery which is currently resting on the seabed off Sheerness with around 1500 tonnes of explosive on board; a WWII sea mine dredged up off Sheerness and containing 680kg of high explosive which was later detonated in the English Channel in November 2012; and the Kyle of Durness incident where a number of pilot whales were beached following the controlled detonation of a series of 1000lb bombs during an underwater munitions disposal exercise in July 2011.

Introduction

While ordnance remains unexploded, it poses a risk to the environment. At the moment of detonation the risk becomes real and this can result in an immediate threat to life and property. It is widely acknowledged that explosive blast energy can have a deleterious effect on marine life. If circumstances permit, the UXO must be removed to a site sufficiently far away from any environmental sensitivities prior to a controlled explosion. The question of "How far is far enough?" has been the subject of some investigation and this is reviewed below.

Theory

The signature of an underwater explosion consists of an initial outgoing shock wave which eventually collapses in on itself before oscillating a number of times [1]. The peak value, which occurs around a microsecond after detonation, is given by the empirical expression:

PaμR

WxPpeak

13.13/1131024.5

where W is the charge weight in kilogrammes and R is the distance in metres from the detonation site. Subsequently, the propagation of sound generated by an underwater explosion may be modelled using the

Figure 1: Peak pressure as a function of charge weight (refer to the Case Studies below)

semi-empirical approach developed by Arons et al [2], Rogers [3] and Gaspin[4]. From this work, it is possible to generate a very simple model of the environment in which the blast wavedecays in range and time. Figure 1 demonstrates this relationship for a number of charge weights.

The effect of explosive blast on animals has been studied [5] [6]. From such work, models are available for estimating distances at which lethality or significant organ damage occurs when animals are subjected to blast from charges of given size. The models themselves are somewhat limited in scope being based on small terrestrial animals submerged in shallow water and where only gross injuries were considered post-mortem. Given current ethical constraints, it is unlikely that such tests would ever be repeated or extended.

The models are used to demonstrate the potential environmental impacts related to three case studies involving exploding ordnance.

Case Study 1: SS Richard Montgomery

The SS Richard Montgomery was a WWII US Liberty ship that broke its back on a sandbank before sinking in 15 m of water around 2.5km off Sheerness. The cargo consisted of a large quantity of ordnance. A salvage operation commenced immediately and carried on until the ship broke in two and the salvage had to be abandoned. Some 1400 tonnes of high explosive remain on board.

In the event that the remaining cargo detonated simultaneously, Figure 1 indicates that the peak pressure at a distance of 1m from the wreck could be as high as 320dB re 1μPa.

Figure 2 shows the potential impact on three species of marine life that are found in and around the Thames estuary. In order to survive the blast, the peak pressure impinging on a harbour porpoise (Phocoena phocoena) must not exceed 244dB re 1μPa. For this level, Figure 1 shows that the animal must lie beyond 2.5km from the detonation site. Similarly, there is a 1% probability of mortality occurring at 1.7km from the blast and a 50% probability of mortality at 1.4km. Being larger, the seals may get closer to the detonation site yet still avoid injury. The no-injury distances are 2.1km and 1.8km for harbour seals (Phoca vitulina) and grey seals (Halichoerus grypus) respectively.

continued over page

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28 soundings Issue 64 Summer 2015

Figure 2: Impact criteria for Case Study 1

Figure 3: Impact criteria for Case Study 2

Figure 4: Impact criteria for Case Study 3

Case Study 2: Sheerness sea mine

In November 2012 an air-dropped 'GC' mine containing 1500lbs (680kg) of high explosive was dredged off the seabed at a site some 10km north of Sheerness. A Royal Navy Mine Disposal team despatched from Portsmouth worked through the night to free the sea mine which had got caught up in the dredge head. Subsequently, the mine was towed out to sea and detonated at a depth of 10m. The resulting explosion threw up a plume of water and mud 76m high.

There was no evidence of marine casualties following the explosion. It is possible therefore to use the impact models to estimate how far an animal would have to be in order to survive the blast. Impact thresholds for a charge weight of 680kg are shown in Figure 3. Using these data together with those given in Figure 1, it may be determined that for a harbour porpoise to survive the blast, it would need to be beyond a distance of 633m from the detonation site while the largest harbour seals and grey seals would survive at distances of 505m and 447m respectively.

Case Study 3: Kyle of Durness incident

In late July 2011, an underwater munitions disposal exercise took place at a site close to the entrance of the Kyle of Durness, Scotland, where a number of 1000lb (453kg) bombs underwent a controlled explosion. The following day, around 70 long-finned pilot whales (Globicephala melas) entered the Kyle. When the tide turned, around 35 whales were beached. Some of these were returned successfully to the sea but 20 whales eventually died.

On post-mortem, no damage to the ears or other soft body organs of the whales, that would have been consistent with exposure to the blast, was seen. However, circumstantial evidence pointed to acoustic disturbance and navigational error arising as a result of the underwater explosions.

On the basis that the whales did not succumb directly from the explosion, it is possible to estimate how far they would have to be from the detonation site in order to survive. Figures 4 and 1 indicate that the smallest full-grown whales would have to be at least 300m from the point of detonation. From the published data on behavioural reactions in cetaceans exposed to sudden loud noises, startle reactions may be observed at noise levels in the range 160-180dB re 1μPa. For the simple model used to illustrate sound propagation at Kyle of Durness, sound pressure levels could remain above 180dB re 1μPa out to a range of 180km.

However, it is noted that insufficient data exists in the published literature from which to draw meaningful conclusions regarding behavioural reactions. The reactions themselves are often context-driven and vary species to species, day to day, and noise type to noise type.

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soundings Issue 64 Summer 2015 29

news●bulletin

'Valeport in Action' competition winners announced

A stunning image of a Weddell Seal in the Antarctica carrying a Valeport CTD sensor and a hands-on video showing the pilot deployment of a Valeport Midas directional wave recorder at the mouth of the UK's Humber Estuary, have been selected as the winning entries in Valeport's first-ever product in action competition.

Earlier this year the Devon-based designer and manufacturer of instrumentation for the oceanographic, hydrographic and hydrometric communities invited its globally distributed customers to submit videos and photographs of its products in action for a chance to win an iPad Air 2 (video category) and iPad Mini 3 (picture category).

In late June the company announced that Daniel Costa from the University of California had won the prize for best picture category with his image of a Weddell Seal, carrying a Valeport CTD sensor integrated into a tag built by the Sea Mammal Research Unit, St Andrews. The winning image (shown above) was captured in the Ross Sea, Antarctica.

The prize for the video category was awarded to Ross Jennings of the University of Hull. His footage depicted a Valeport Midas DWR being deployed at Spurn Point, at the mouth of the Humber Estuary. The video was taken as part of a pilot project to test the device and deployment methodology for later fieldwork in more energetic sites upstream.

Valeport was particularly surprised and impressed with the quality, quantity and variety of the entries submitted for the inaugural competition. They are planning on showcasing a selection of the entries on their website and social media networks over the coming months.

Visit www.valeport.co.uk for more information.

Conclusions

In the context of acoustic impacts, it is necessary to determine distances from a detonation site over which a subsequent controlled explosion gives rise to minimal (or at least manageable) damage to the environment.

From the simple modelling presented herein, it is possible to estimate distances from the detonation site at which marine life may or may not survive when exposed to explosive blast. Typical sea mines containing 450-680kg of high explosive generally cause mortality over distances of 300m to 630m. Much larger explosions lead to mortality over a greater range. The critical distances are a function of the body weight of the animal exposed to the blast; the larger the animal the greater the chance that it will survive the blast.

The author gave a presentation based on this feature at the Society's recent UXO Surveys Seminar. A review of the event appears on page 32. The presentation will be also available on the Downloads page at www.ths.org.uk later in the year.

References

1. Urick, Robert J (1983). Principles of Underwater Sound, 3rd Edition. New York. McGraw-Hill.

2. Arons A B, Yennie D R and Cotter T P (1949). Long range Shock Propagation in Underwater Explosion Phenomena II. US Navy Dept. Bur. Ord. NAVORD Rep. 478.

3. Rogers P H (1977). Weak Shock Solution for Underwater Explosive Shock Waves, Journal of the Acoustical Society of America, 62(6):1412-1419.

4. Gaspin J B (1983). Safe Swimmer Ranges from Bottom Explosions, NSWC/WOL TR-83-84, Naval Surf. Weap. Cent. DTIC AD-B086375.

5. Yelverton, J T, Richmond, D R, Hicks, W, Saunders, K and Fletcher, E R (1975). The Relationship Between Fish Size and Their Response to Underwater Blast. Report DNA 3677T, Director, Defense Nuclear Agency, Washington, DC.

6. Yelverton, J T (1981). Underwater explosion damage risk criteria for fish, birds and mammals. Manuscript presented at 102nd Meeting of the Acoustical Society of America, Florida, USA.

The Author

Peter Ward has over 27 years' experience in underwater acoustic propagation studies including 17 years' experience in environmental impact assessment with reference to the impact of man-made noise on the marine environment.

He has provided advice on acoustic impacts to the UK MoD for their sonar procurement programmes as well as contributing to studies on oil rig decommissioning, marine renewables construction and operation, offshore aggregate dredging and port and harbour construction.