Rock armour for birds and their prey

16 Pages·2017·1.82 MB·English

MaritimeEngineering ProceedingsoftheInstitutionofCivilEngineers
Volume170IssueMA2 MaritimeEngineering170June2017IssueMA2
Pages67–82 http://dx.doi.org/10.1680/jmaen.2016.28
Rockarmourforbirdsandtheirprey: Paper201628
ecologicalenhancementofcoastal Received15/11/2016 Accepted09/10/2017
Keywords:conservation/coastalengineering/environment
engineering
Naylor,MacArthur,Hampshireetal.
PublishedwithpermissionbytheICEundertheCC-BY4.0
license.(http://creativecommons.org/licenses/by/4.0/)
Rock armour for birds and their prey:
ecological enhancement of
coastal engineering
LarissaA.Naylor MartinA.Coombes
Lecturer,UniversityofGlasgow,Glasgow,UK(correspondingauthor: Lecturer,UniversityofOxford,Oxford,UK
[email protected]) JimD.Hansom
MairiMacArthur Reader,UniversityofGlasgow,Glasgow,UK
DoctoralResearchStudent,UniversityofGlasgow,Glasgow,UK RowanByrne
StephanieHampshire PrincipalMarineEnvironmentalScientist,MottMacDonald,Cambridge,UK
CoastalScientist,MottMacDonald,Croydon,UK;currentlyatC2MH, TristanFolland
Swindon,UK
SeniorEcologist,MottMacDonald,Cambridge,UK;currentlyatNIRAS
KieranBostock ConsultingLtd,Cambridge,UK
PrincipalEngineer,HartlepoolBoroughCouncil,Hartlepool,UK
The authors present key design, construction and ecological enhancement criteria for sustainable coastal defence
structures at Hartlepool, UK, a high-energy wave climate. Such ‘ecologically favourable’ coastal defences fulfil the
habitats directive and key engineering and cost criteria. Bird, rocky intertidal ecological and biogeomorphological
data underpin recommendations for ‘passive’ enhancement mitigation to maximise ecological potential involving
rock armour material choice (partially enhanced) and its smart positioning (enhanced). Within 12–18 months of
installation, key intertidal species (e.g. limpets, barnacles, fucoid seaweeds) had successfully colonised the rock
revetment, matching the initialbaseline biotope.However, speciesabundanceandoverallmobile andsessilespecies
were not significantly different between the two enhanced treatments after 12–18 months. Importantly, key prey
species (the limpet, Patella vulgata) on enhanced rock armour showed statistically significant abundances similar to
the baseline shore platform and significantly higher than partially enhanced rock armour. These preliminary data
showthatwell-chosenrockarmourmaterialandboulderenhancementusingpositioningcanmatchbaselinebiotope
conditions in 12–18 months and that for some key prey species, positioning-enhanced rock armour rapidly matches
baselineconditions.Thisfacilitatesrapidrockrevetmentcolonisation,enablinggoodrecruitmentoffoodspeciesand
favourableconditionsforinternationallydesignatedwaterbirdspecies.
1. Introduction niche habitat designs, yet also satisfy engineering performance
Increasing storminess, sea levels and coastal urbanisation is requirements. Research worldwide shows the operational appli-
fuelling demand for hard defence infrastructure such as sea- cationsofthesetechniquestobesuccessful–thatis,theecologi-
walls and revetments. Such structures must withstand harsh cal goals of ‘environmental friendly’structural engineering have
environmental conditions (e.g. storm waves and deteriorative been met without reduction in the protection capability of
salts) and typically require expensive, on-going maintenance. schemes.Yet,theadoptionoftheseapproachesintomainstream
In parallel, there is a growing requirement from the govern- engineering practice remains limited to a handful of examples
ment for grey infrastructure, including coastal defences, to be across Europe (Devon, Isle of Wight, the Mediterranean), in
multi-functional, sustainable, resilient and towork with nature North America (Seattle, New York, Vancouver) and Australia
to provide ecosystem services (EA, 2012). However, urbanised (Sydney)(forareview,seeNayloretal.(2011)).
coastlines often have lower biodiversity value than equivalent
natural habitats and remain some of the least-studied ecosys- There also remains a need for best practice case studies and
temsworldwide(BulleriandChapman,2010). guidance on how assets that need to remain ‘grey’ for their
primary function can be ‘greened’ (Naylor et al., unpublished
A growing body of ecological and geomorphological research report, 2016). Such greening of hard maritime infrastructure
demonstrates that hard coastal infrastructure can be inexpen- (e.g.outfallpipes,ports,harbours,bridgefootings)andestuar-
sively designed to sustain greater biodiversity (e.g. Coombes ineandcoastalprotectionstructuresaretypicallymissingfrom
et al., 2015; EA, 2008; Firth et al., 2014; Strain et al., 2017). greeninfrastructurepolicyandguidance.Similarly,theexisting
These ‘ecological enhancements’ improve structural engineering guidance for working with natural processes (also called
throughtheselectionofecologicallyfavourablematerialsand/or nature-basedsolutions)typicallyfocusesonsoftmaterialssuch
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MaritimeEngineering Rockarmourforbirdsandtheirprey:
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Naylor,MacArthur,Hampshireetal.
assandscaping,theuseofdredgedmaterial,saltmarshcreation along the Durham coastline show that the area impacted by
and managed realignment (e.g. EA, 2012). Apart from Naylor this scheme represents some of the most important feeding
et al. (2011), the government guidance on improving the eco- sites for designated species (Cadwallender and Cadwallender,
logicalvalueofhardcoastalinfrastructureisscarce. 2013), an environment also under threat from coastal squeeze
undera‘holdtheline’policy(NaturalEngland,2014).Ifinter-
2. Aims tidal invertebratespecies on which thebirdsfeed cannot adapt
This paper reportsthe first known ecological enhancements of and migrate inland with future sea-level rise, then the locally
hard coastal structures in the UK that provide mitigation availablehabitat theydepend on will be reduced and/or lostin
under the EU habitats directive (EC, 1992), to ensure that the future (Jackson and McIlvenny, 2011). This will impact
there are no adverse effects on the integrity of a Natura 2000 negatively on the condition status of the qualifying features
site designated for its internationally important waterbirds. and consequently adversely affect site integrity. In England
The UK government’s implementation of article 6(3) of the and Wales, the Conservation of Habitats and Species
habitats directive to use habitat creation as mitigation within Regulations 2010 (as amended) (the ‘Habitat Regulations’),
the Natura 2000 site, as in this project, may not be strictly article 6(3) of the EU habitats directive, requiresthat aproject
compatible with the directive (see case C521/12 Briels v. ‘design appropriate mitigation measures that will cancel or
Minster of Infrastructuur en Milieu). Nevertheless, this project minimise the adverse impacts’ (EC, 2002). The scheme, within
aimed to mitigate the expected habitat and natural substrate the Natura 2000 site, therefore had to consider the design
loss associated with improving the standard of both new and implications of direct and indirect habitat loss on the qualify-
pre-existing coastal defences within the Natura 2000 site. This ing features, alone and in combination with the climate
mitigationalso soughtto minimise futurehabitatlossesdueto change-relatedsea-levelrise.
thesea-levelriseandcoastalsqueeze.
The approval process from the nature conservation body
To date, it is also the largest known operational ecological NaturalEnglandrequiredanyproposedmitigationtobesigned
enhancement of hard coastal infrastructures in the UK (after off at the planning permission stage (the final design did not
Shaldon, Devon, Isle of Wight and Bournemouth) (Arc need the Natural England official sign off). The Natural
Consulting, 2016; Naylor et al., 2012) and as such it provides Englandapprovedmitigationformedpartoftheapprovedplan-
animportant‘proofofconcept’demonstrationofhowecologi- ning permission for the scheme and as such was a delivery
calenhancementresearchandinnovationhasbeenoperationa- requirement.Anychangestotheproposeddesignand/ormitiga-
lised in the UK (e.g. Coombes et al., 2015; Evans et al., 2016; tion planswould require avariation to the planningconditions,
Firthetal.,2015;Nayloretal.,2012). triggerare-consultationwithNaturalEnglandandobjectionsif
theschemewasnotabletodelivertheoriginalmitigation.
The authors provide an appraisal of the rationale, approval
process, design criteria and building phase considerations 4. Site description
related to meeting the habitat mitigation requirements of the The headland coastal defences at Hartlepool protect 562 resi-
Hartlepoolheadlandcoastalprotectionscheme(thescheme)in dential and commercial properties, and key heritage features,
Hartlepool, Teesside, UK. Currently under construction, the including the Heugh gun battery scheduled monument
long-term (>1·5 years) colonisation patterns are not yet avail- (Figure 1). The defences consist of north-east facing vertical
able but pre- and post-construction ecological data are avail- masonry and concrete walls, built over the last 150 years, and
able from the areas of rock revetment that have been installed now in poor condition, being frequently overtopped during
to date. The authors highlight the lessons learned and discuss storms (e.g. significant damage during the winter 2013/2014
thewiderapplicationofsuchintervention. storms (Thorne, 2014)). Funded by way of the Project for
Accelerated Growth Scheme, funding partners include the
3. Legislative imperatives forecological Environment Agency, Hartlepool Borough Council and PD
enhancement Ports, with support from Natural England for ecological
The headland foreshore coastal defence scheme at Hartlepool enhancement ‘proof of concept’. The current defences are
iswithintheTeesmouthandClevelandCoastNatura2000site, fronted predominately by a magnesian limestone intertidal
designated under the EU birds directive (Council of the shore platform, with limited areas overlain by perched beach
European Union, 1979) as a special protected area (SPA) for deposits.The uppershorezone(0–10mfromtheseawall)also
internationally important numbers of waterbirds (JNCC, displays considerable evidence of active abrasion with a
2016).AlsodesignatedforwaterbirdsundertheConventionon reducedecology(Nayloretal.,2014).
Wetlands of International Importance (Ramsarconvention), it
is a Ramsar site (JNCC, 2008) and a site of special scientific The scheme aimed to upgrade the defences and ‘hold the line’
interest (Natural England, 1997). Overwintering bird patterns in accordance with the local shoreline management plan
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Naylor,MacArthur,Hampshireetal.
(MM, 2012: p. 7). Phase 1 includes 800m of low-level granite
rock revetment to dissipate wave energy and protect the toe
of deteriorated sections of existing seawall (Figure 2) and
prevent damage to foundations. However, the scheme also
aimed to ‘provide the same ecological function foroverwinter-
ing birds and as such there will be no overall loss of habitat
function for Annex II bird species’ (MM, 2014: p. 36) and
other species within the waterbird assemblage. Phase 1 of the
revetment works has been completed and phase 2 is currently
underway(autumn2016).Overall,theschemewilltake5years
toinstall.
5. Planning and mitigation design approval
5.1 Planningphase
Priortotheschemeplanningapplication,thepreferredoptions
forthewidercoastaldefencestrategy(theplan)weresubjected
to a habitats regulations assessment (HRA) (MM, 2012). The
HRA concluded that the strategy, including this scheme,
wouldnotadverselyaffecttheintegrityoftheNatura2000site
Hartlepool headlands coastal
protection scheme if: (a) the shore platform height was enhanced to maintain its
Seaton Carew extent;(b)therockrevetmentwasplacedontheshoreplatform
toincreaseitselevationandallowpotentialhabitatforbirdsto
N beexposedduringthetide,accountingforaprojectedsea-level
rise; (c) disturbance of qualifying features during construction
is avoided; and (d) reflective wave energy dissipation is mini-
0 0·2750·55 1·1km mised by the placement of rock blocks. Building on the con-
clusions of the strategic HRA, an HRA specifically for the
scheme was undertaken to support the planning application
(MM, 2012). Hartlepool Borough Council and Mott
MacDonald sought expert advice on the ecological enhance-
Figure1. Studyarealocationmapincludingthecomparisonsite
SeatonCarew ment design (Naylor et al., 2014); this was instrumental in
agreeing ecologically favourable design options with Natural
Englandandthussecuringplanningapprovalforthescheme.
(SMP)(RoyalHaskoning,2007).TheSMPhighlightsthechal-
5.2 Ecologicalenhancementdesigncriteria
lenges of maintaining ecological conditions while adopting a
hold the line policy: ‘The SMP supports the natural develop- 5.2.1 Keydesignparameters
ment of this SPA and Ramsar designated coastal habitat. Previous research indicates that ecological enhancements can
However, holding the line at Hartlepool Headland may result bedesignedtosupporttheassemblagesofmarineinvertebrates
inthelossofhabitatduetotheprovisionofenhancedtoepro- on which waterbirds might feed (Coombes et al., 2015; Evans
tectionoverthelittoralrocksub-feature’andthereis‘currently et al., 2016; Firth et al., 2014, 2015). These can be quite
a danger of short-term coastal squeeze and subsequent net simple andinexpensive ‘passive’techniques (e.g. choosing con-
lossesofSPAandRamsardesignatedforeshorehabitat’(Royal struction materials based on lithology and surface roughness
Haskoning, 2007: pp. 168 and 167). Public consultation on (Coombes et al., 2010, 2015)), or more ‘active’ multi-scale
allowingpartof the proposedareatonaturallyerodemet with enhancementsthat seek to better mimic the geomorphological
opposition; therefore, the decision to ‘hold the line’ required heterogeneityof natural rocky shores (e.g. Evans et al., 2016).
more focus on habitat mitigation than would be requiredwith Thiscanincluderockandconcreteblockswithfine-scalemilli-
anadaptationalpolicydecision,suchasmanagedrealignment. metre to centimetre textures, incorporation of sheltered and
overhanging areas and in-built water-retaining features such as
4.1 Coastaldefencescheme pools. To achieve a habitat outcome most closely mimicking
The scheme aimed to provide: ‘acoastal protection Scheme to the existing rocky shores at Hartlepool, and which offers
reduce coastal erosion risk to the community and increase feeding opportunities as the qualifying waterbird features, a
amenity value of the frontage over the next 100 years’ combination of passive and active multi-scale enhancement
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Average crest level 7·4 mOD
Proposed concrete encasement
Dowel bars
assumed at
850 centres
400
mm
Existing
seawall
3·3 mOD 100 years mean high water springs
3·0 mOD
Average toe crest
2·6 mOD 2·7 mOD mean high water springs
1500 mm
2
1
Proposed rock armour
m
m
Otoled concrete Approx. 1500 Average platform level
–0·5 mOD
600mm
Limestone platform
Underlayer rock
approx 650 mm dia.
Rock toe dug into
Approx. 9 m limestone platform
Section 8
Concrete wall encasement and low-
004
level granite rock revetment
(315 m)
Figure2. SchematicdiagramdepictingtheencasedseawallandrockrevetmentdesignusedfortheHartlepoolHeadlandsareaof
thescheme
wasconsidered.Thedesignneededto becost-effectiveanduse 5.2.2 Baselineecologicalsurveys
structurallyacceptableengineering materials; theseengineering Prior to any enhancement recommendations, a series of
and cost constraints favoured passive enhancement over active baseline ecological assessments included: 14 repeated bird
enhancement for the rock revetment. However, it resulted in surveys by Hartlepool Borough Council; a JNCC phase 1
granite being used instead of the more ecologically preferable habitat survey by Mott MacDonald of the existing defences
(but expensive) local limestone. Recommendations also influ- and foreshore; and a phase 1 habitat survey by Mott
enced the design of a proposed concrete step revetment and MacDonald of a comparable, recent (2002) rock revetment
concretewallcasingtooptimise post-construction colonisation scheme (cid:1)2km from the current scheme. These surveys helped
(Naylor et al., 2014; Perkol-Finkel and Sella, 2015). The con- to develop an understanding of how bird species used the
tractorsusedReckliformliners(Yukondesign)fortheconcrete intertidal habitat likely to be affected by the scheme (Table 1)
wall casing to mimic natural rock and provide enhanced and informed both the engineering design recommendations
texture (up to 27 mm deep) and improve the structural com- (Nayloret al., 2014) and the ecological mitigation required by
plexity of the wall, compared with plain cast concrete. The theHRA(MM,2012).
encased wall and rock revetment is currently under construc-
tion, while the stepped revetment is part of a later phase of 5.2.2.1 BASELINE BIRD SURVEYS
construction. This paper reports solely on the rock revetment The scheme area has international designations for inter-
element of the scheme currently being deployed (during con- nationally important bird species and the habitat (including
struction years 1–3) (Figures 1 and 4). Details of the rec- food sources) that supports them (Table 2). Rocky shore
ommended rock revetment mitigation are provided in Sections habitats typically provide refuge and overwintering sites for
5.2.4and6. these bird species (Cadwallender and Cadwallender, 2013;
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Naylor,MacArthur,Hampshireetal.
Table1. Summaryofbaselinehabitatmitigationdataneeds,datacollectionmethodsandkeyfindings
Question Datacollected Keyfindings
Whichpartsoftheforeshoredobirdsuse & Fourteenrepeatwinterbird & Birdspreferentiallyusedthelowerintertidalzone
mostandforwhatpurpose(e.g.foraging, surveysbetweenNovember & Oystercatchersweremoreprevalenthigherupthe
roosting)? 2010andJanuary2014 shorethanotherspecies
& Birdspeciescountsanduse
(e.g.foraging,roosting)maps
Whichofthekeyfoodspeciesforwaterbirds & Phase1habitatmapping & Morespeciesofkeyfoodforbirds(e.g.mussel
arepresentandwhereontheshoreare (February2014) spat)arefoundlowerontheshore
theylocated? & Higherontheshore,thenumberofspecies
providingfoodforwaterbirdsdecreases,although
somemolluscs,isopodsandgastropodsarefound
ontheshoreplatform
Istheecologyofrockarmouracomparableto & Phase1habitatmapping & Similarbiotopeswerefoundontherockarmourat
thecurrentshoreplatformbiotopesfoundin (February2014) SeatonCarewandthenaturalrockyshoreinthe
thewiderschemearea? upperintertidalzoneoftheschemearea
aPlacednearbyintheupperintertidalzone
Table2. Summaryofinternationallyimportantbirdspeciesandtheirkeyrockyintertidalpreyspecies(afterCrampetal.,2004;Rehfisch
etal.,1993)
Keyrockyintertidalpreyspecies
Birdspecies Molluscs Crustaceans Other
Oystercatcher(Haematopusostralegus) Bivalves(especiallymusselsMytilusedulis),limpets
(Patellavulgata)
Redshank(Tringatotanus) Periwinkles(Littorinaspp.),
Turnstone(Arenariainterpres) Periwinkles(Littorinaspp.),mussels(Mytilusedulis) Crabs(Carcinusmaenas),
barnacles
Redknot(Calidriscanutus) Periwinkles(Littorinaspp.),mussels(Mytilusedulis) Crabs(Carcinusmaenas), Greenseaweed
barnacles (Ulvaspp.)
Purplesandpiper(Calidrismaritima) Molluscs(especiallyLittorinaspp.) Somecrustaceans
Curlew(Numeniusarquata) Mussels(Mytilusedulis) Crabs(Carcinusmaenas)
RingedPlover(Charadriushiaticula) Molluscs(Littorinaspp.)
Sanderling(Calidrisalba) Deadstorm-damagedmolluscs(Mytilusedulis) Crabs(Carcinusmaenas)
Dunlin(Calidrisalpina)
Rehfisch et al., 1993). Fourteen repeat winter intertidal bird summary map (Figure 4) showing feeding and roosting activi-
surveys were carried out between 2010 and 2014 to determine ties taking place seaward of the zone directly impacted by the
the number of species, number of individuals and bird usage scheme(0–10 m)andthe10–20mbufferzone.
(e.g. foraging or roosting) across all areas of the proposed
scheme. 5.2.2.2 PHASEI HABITATSURVEYS: WHICHOF THEKEY FOOD
SPECIESFOR WATERBIRDS AREPRESENTAND WHERE ON
The results show the highest density of individuals across the THESHOREARE THEYLOCATED?
entireintertidalzonesurveyed(Figure3)areforoystercatchers, The rocky intertidal prey species listed in Table 2 respond to
redshanks, turnstones and knots, and these also have a much passive and active ecological enhancements elsewhere in the
higher abundance in the upper intertidal zones most affected UK (including Elmer, West Sussex and Lyme Regis, Dorset
by the scheme. Purple sandpipers were found in low numbers (Moschella et al., 2005); Colwyn Bay and Penthyn Bay (Firth
acrossalltheshorezonessampled,andbirdspeciesabundance et al., 2014); Porthleven and Zennor (Coombes et al., 2015)).
increased with distance from the seawall (nine taxa occurred Baseline ecological surveys were undertaken to identify key
beyond 20m from the seawall, compared with four taxa at species and their location relative to the proposed footprint of
0–10m). These results are supported by the bird usage data the scheme. Surveys involved a combination of desk-based
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Naylor,MacArthur,Hampshireetal.
45 qualifying waterbirds including red knot, oystercatchers and
redshanks(Figure4).
40
35 Eleven biotopes were recorded across all transects. However,
ed the upper intertidal (0–20m from the seawall) zone had only
v
ser 30 0–10 m one biotope across all 14 transects (ephemeral green seaweeds,
b
mber o 25 1A0ve–r2a0g em per 20 m LinRte.rFtiLdRal.Ezpohn.Eent(,0–C2o0nmno)r edtataal.,w2e0r0e4;cFonigsuisrteen5t).aTchroesuspptheer
u 20 from 20 to 100 m entire length of the proposed scheme, providing only limited
n
ge food species for birds (e.g. barnacles, limpets and periwinkles)
a 15
er and containing fewer prey species of interest (e.g. marine
v
A 10 isopods (Jaera albifrons), limpets (Patella vulgata) and
periwinkles(Littorinaspp.).Thetotalnumberofbiotopessup-
5
porting prey species of interest was greater lower down the
shore (Figure 5) where the number of biotopes increased to
0
Oystercatcher KnotRedshankTurnstoneSanderliRinngged pPluorvpelre sandpiper CurlewCormorant Dunlin hb3–air5ddsaincocrorfesasisnetthdeerensmutmid(b-ee.gtros. alLonwRde.MrdeiLnnRstiet.ryMtidouafslFpzroebynieos.tpoTepcheiiessczofoonnrtaesihnaoilnsroge
substantive colonies of mussels (Mytilus edulis) and mussel
Species
spat).
Figure3. Averagebirdspeciescountacrosstheproposedscheme
footprint(0–10m),theadjacent10–20mlikelytobeimpacted Food availability was more prevalent in the lower intertidal
duringconstructionandanaverage20mwidthvalueforthe zonewherebirdshavebeen documented moreoften (Figure 4)
remainingmid-to-lowerintertidalareassurveyed suggesting that birds favour the lower intertidal zone,
probably due to food availability, open sightlines and less
disturbance than near the seawall (Cadwallender and
Cadwallender,2013).
analysisofprevioussurveys(2003,2010byBMTCordah)and
new baseline phase I habitat mapping surveys conducted by 5.2.3 SeatonCarewdefencescomparison
MottMacDonaldinJanuary–February2014. To evaluate the local feasibility of colonisation of rocks at the
seawall–land boundary in the upper intertidal zone, a nearby
Established phase I mapping protocols were followed (Wyn, comparison with similar wave exposure and aspect was
2000) and rocky shore ecology communitieswere mapped and required. Seaton Carew, (cid:1)2km from the scheme, provided the
their biotopes assessed as feeding resources for waterbirds. comparison of a rock revetment installed in 2002 on the upper
Fourteensystematic-randomselectedtransectsweresurveyedat intertidal zone at the seawall toe (Figure 1). Visited on
(cid:1)500mspacingalongtheshoreplatform,fourinBlockSands 1 February 2014, the rock revetment rests on a red Triassic
and ten in North Headland areas, and the biotopes present sandstone rocky shore platform and sandy beach, where fucoid
weremappedover20mintervalsupto100macrosstheinter- seaweeds and mussels (M. edulis) were found. Importantly, the
tidal zone. The presence of key rocky intertidal species upperintertidalbiotope(ephemeralgreenseaweedcommunities,
(Table2)aspossiblepreysourceswasalsorecordedalongeach onunstableuppereulittoralrock(FLR.Eph.Ent),Connoretal.,
transect. Transects were geo-referenced to facilitate future re- 2004) observed on the rock revetment at Seaton Carew wasthe
survey work and, where possible, transects were positioned to same as that found on the upper intertidal shore platforms at
overlap with previous surveys by BMT Cordah in 2003 and Hartlepool (Figure 5). Foraging species were present and
2010.The2014dataarepresentedhere. coastal birds (oystercatchers, Haematopus ostralegus) were
observedinteractingwiththecoastalprotectionstructureduring
Across the intertidal zone, species assemblages known to surveys conducted by the authors, suggesting that rock armour
providefoodforoverwinteringbirdspecieswerefound,includ- can be colonised in a similar manner to the upper intertidal
ing mussel spat, gastropods and molluscs, marine isopods and zone of natural rocky shores in this area (see (MM, 2012) for
crustaceans (Cramp et al., 2004; Rehfisch et al., 1993). These moredetails).
findings are consistent with that expected for a relatively
exposed, high-energy North Sea coastline and the substrate 5.2.4 Designgoalsandparameters
and morphological features of each transect (BMT Cordah, The mitigation sought to maintain the extent of key habitat
2004). The whole intertidal zone supports feeding habitats for sub-features through ecological enhancement of the proposed
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N
Main feeding site:
regularly 100–300 oystercatcher
Main feeding site: feeding at low tide
main feeding site for knot,
several species use at low tide
Main feeding site:
redshank feeding area around
seaweed covered pools
Some use by birds:
typically some oystercatcher,
redshank and turnstone
Some use by birds:
occasionally flocks roost here
temporarily at high tide
Occasional use by birds:
occasionally feed at mid-to-low tide
Some use by birds:
highest point used by several
species such as: shag, cormorant,
oystercatcher, redshank, turnstone
0 0·125 0·25 0·5 km
Figure4. Birdusagemapderivedfrom14individualbirdusagesurveysbetween2010and2014,wheretype(feedingorroosting),
locationandindicativeintensityofuseisclassifiedas:main=darkgreen(darkgrey),moderate=mid-green(mediumgrey)andlow
use=lightgreen(lightgrey).ContainsOSdata©CrowncopyrightEdinaDigimapsubscription.Afull-colourversionofthisfigurecanbe
foundontheICEVirtualLibrary(www.icevirtuallibrary.com)
structures. The aim of these enhanced habitat surfaces and To satisfy the mitigation requirements for Natural England,
characteristics was to facilitate colonisation and establishment the scheme was required to provide the same ecological func-
of breeding populationsof preyspecies favouredbySPAbirds. tion for over-wintering birds and ensure no overall loss of
Although ‘quantitative predictions of the effects [of hard habitat function for Annex II bird species. Pre-construction
defences]onindividualspeciesandassemblagesatanyparticu- surveys (Section 5.2.2) found little evidence of roosting by
lar location are more difficult…’ (Airoldi et al., 2005: p. 1075) overwinteringbirdsinthisareaoftheSPA,allowingmitigation
and the ecological outcomes of any design are uncertain, the measures to focus on ensuring that opportunities for feeding
enhancement measures recommended for this scheme were areimprovedwherepossible.
informedbythebestavailablescientificevidence.
As stated in Sections 3 and 4, although the scheme fulfils the
The key engineering and ecological enhancement design rec- current ‘hold the line’ SMP policy, it does limit the capacity
ommendations for the rock revetment are summarised in for intertidal species to respond to coastal squeeze by moving
Table 3 (after MM, 2012; Nayloret al., 2014). Ensuring miti- inland. Although (cid:1)10 m of intertidal foreshore habitat was to
gation did not adversely affect the primary coastal protection be lost to the rock revetment, the revetment was designed to
performance of the scheme required close communication mitigate some of this loss (Figure 2). The passively enhanced
between the Mott MacDonald design team and the lead rock revetment provides a much larger habitat surface area
HartlepoolBoroughCouncilengineer. than the existing shore platform, including large numbers of
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pe 10
y
es (t 8
p
o
ot 6
bi
of
nt 4
u
o
C 2
0
20 40 60 80 100
FLR.Eph.Ent (2 var.) 14 2
FLR.Rkp.Cor 6 4 3 8
LLR.F.Fves. (2 var.) 7 9 9 2
LLR.Fves/HLR.MusB 1
MLR.MusF.MytFR 3
FLR.Rkp.FK/MLR.BF.Fser 1 3
LLR.MusF.MytFR 1
Biotope zone in 20 m intervals ( = m from current seawall)
Figure5. GraphandtablesummarisingthephaseIintertidalbiotopemappingdatacollectedin2014
crypticspacesthatprovideimportantintertidalhabitatforprey access (Basford et al., 2016). The key considerations
species(Sherrardetal.,2016).Therevetmentalsoincreasesthe included the engineering and biogeomorphology design
range of tidal elevations for species colonisation, providing criteria outlined in Table 3 as well as sensitive construction
physical space to allow species to adapt to sea-level rise. techniques on site, including timing, toe design and contractor
Without the revetment, there would be limited vertical space instructions. Work on the foreshore was allowed only
on the seawall and unsuitable habitat (lack of shade, water- between April and September to minimise adverse impacts on
retaining features, void spaces) for species to colonise on sea- the qualifying waterbird species, although work was permitted
level rise. Carboniferous limestone rock armour was added on the seawall from overhead during this period to help
into the upper areas of the revetment to create the necessary improve efficiency and reduce construction costs. To minimise
engineering void ratio and a more ecologically favourable sub- the footprint of the works and ensure its durability, the
strate than granite over the short term (e.g. Coombes et al., rock revetment was toed into the limestone platform to stabil-
2011) and long term (e.g. Plymouth breakwater). The revet- ise the structure and ensure it met the design criteria.
ment design allows additional layers of rock armour to be Intertidal limestone removed during this process was used as
added on top as sea levels rise, allowing the scheme (and the the core material (i.e. laid on top of the platform to aid
species it supports) to adapt to climate change. Within a ‘hold machinery) or, where sufficiently large, added to the rock
the line’ policy, these design features help mitigate the risks of revetment.
coastalsqueeze.
6.2 Contractorinstructions
This is the first known project worldwide aiming to employ
6. Building phase
‘passive’ ecological enhancement techniques on a rock revet-
6.1 Ecologicallysensitiveconstruction ment and deliver a simple, cost-effective deployment method-
considerations ology. However, the original detailed design criteria (Naylor
Foreshore operations at Hartlepool required careful manage- et al., 2014) were made without consultation with contractors
ment given the sensitive nature of the environment (i.e. soft (who had not yet been appointed). This original design
limestone platform supporting internationally important suggested interspersing areas of passively enhanced rock
habitat) and working in a dynamic tidal zone with limited armour (i.e. placed and oriented to optimise the ecological
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MaritimeEngineering Rockarmourforbirdsandtheirprey:
Volume170IssueMA2 ecologicalenhancementofcoastal
engineering
Naylor,MacArthur,Hampshireetal.
Table3. Outlineoftherecommendedengineeringandbiogeomorphologydesigncriteriaforrockrevetmentelementofthescheme
Designtype Detailedtype Rationale Detailedcriteria/recommendations
Engineering Crestlevelofthe Positioncrestoftherevetmentinthe Tomaintaintheextentofintertidalhabitat
revetment tidalzone(meanhighwater
springs–meanhighwaterneaps)
Engineering Topofrevetment Allowanceforsea-levelrise Designedtoallowmorerocktobeaddedduringthe
designlife,ifrequired
Engineering Slopeofrevetment Designedwitha1in2slopeatthe Toprovidetransitionalenvironmentalconditionsand
seawardedge(Figure2) thereforeacttoatleastmaintainbaselinespecies
richness
Biogeomorphology Passive: Toimproveecologicalsuitability Light-coloured,coarse-grainedgranitewithnaturally
specificationof (withinavailablegranite) topographicallycomplexfeatures(mm–cmscale)was
granitea preferredtooptimisethepotentialforgranitetoserve
asanecologicallyfavourablesubstrate
Biogeomorphology Passive:coreand Carboniferouslimestoneand Limestonerockshavebeenshowntoprovideimproved
voidspaceinfill intertidalmagnesianlimestone ecologicalsuitability(e.g.Coombesetal.,2011)and
material fromconstructionwasused arewhatthenaturalshoreiscomposedof.Using
limestoneforthecoreandvoidspacefillwillaid
ecologicalcolonisationofthescheme
Biogeomorphology Passive:rock Wherepractical,selectrockswith Toprovidegreaterrangeoftopographiccomplexity(i.e.
selection complexfeatures,toolingand cmscale)toimproveestablishmentofrockyintertidal
quarrymarks communities
Biogeomorphology Passive:rock Toimprovewater-holdinghabitat Position/orientationoftopographicallysuitablefeatures(e.
positioning g.concavities):thesewereplacedfacingupwardson
thetopofthestructuretoenablewaterholdingto
mimicrockpoolswhilemaintainingthevoidratio
required
Biogeomorphology Active:modifyrocks Toimprovewater-holdinghabitat Retrofitadditionalholestoimprovehabitatprovisionb
aNootherrocktypeswerecosteffectivenorcouldtheyyieldthelargerockblocksneeded
bTobecarriedoutifpreliminaryecologysurveyssuggestfurtherenhancementsareneeded
potential of topographical complexity) with those that were enhancement by including more ecologically favourable rock
deployedusingconventionaltechniques.Thisrequiredcontrac- types(afterCoombesetal.,2011).
tors to separate the rock armour based on its suitability for
passive enhancement (i.e. physically complex surfaces) from 7. Ecological colonisation of phase I
unsuitable, smooth, featureless surfaces, and then alternating of thescheme
each load deployed. This proposal was focused on developing
an optimal ‘proof of concept’ design to measure colonisation 7.1 Methods
success. However, the contractors devised a more efficient Three different natural and artificial habitats (hereafter, treat-
andcost-effectivemethodofdeliveringtheenhancementwithin ments) were sampled along the length of the new scheme in
the timescale and working space constraints. The partially Hartlepool: natural shore platforms (baseline); optimally
enhanced rock armour was deployed on the lower (i.e. buried) selected rock revetment (based on material choice only; here-
layers of the rock revetment, and the passively enhanced rock aftertermedpartiallyenhanced);andenhancedrockrevetment
armour was placed on the top surface of the rock revetment (based on material choice plus further enhancement using
and positioned as per Table 3. The rock revetment was thus a smart positioning). Access and funding constraints at the
mixture of treatment types where the core structure comprised design and pre-build phase, and the rapid construction of a
normal (partially enhanced) rock armour and the top surface timetable, precluded ecological monitoring of the shore plat-
was a combination of passively enhanced or normal (partially forms and walls prior to construction year 1 and most of
enhanced) rock armour (Figure 6). The top surface of a rock year 2. All sampling thus occurred at the upper tidal levels
revetmenthas more challengingecological conditions for inter- (0–10m from the seawall) in August and September 2016.
tidal species and is thus where the enhancement need is the Exposed shore platform surfaces, not yet covered by the new
greatest (Sherrard et al., 2016). This was further supplemented schemeordamagedbymachinery,wereusedtocollectquanti-
by adding limestone rock armour to the top surface, to create tative baseline ecological data as control data for comparison
the required void space ratio and further optimise passive withthenewlyinstalledrockrevetment.
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MaritimeEngineering Rockarmourforbirdsandtheirprey:
Volume170IssueMA2 ecologicalenhancementofcoastal
engineering
Naylor,MacArthur,Hampshireetal.
(a) (b)
(c) (d)
Figure6. Passiveenhancementofrockarmourcomparedwithpartiallyenhancedrocks.(a)Positioningofapassivelyenhancedrockso
thatdepressionsareface-up.(b)Water-retainingcapacityofpassivelyenhancedrock.(c)Partiallyenhancedrock,withsmooth,
featurelesssurfaceand(d)rockrevetmentwithtwopassivelyenhancedrocksintheforeground/centre(blackarrows)separatedby
partiallyenhancedrocks.Therockshowninpart(a)wasinstalledduring2015(colonisationdataincludedinthispaper)andtherocks
showninparts(b),(c)and(d)wereinstalledduring2016,showingdifferencesinecologicalcolonisation
At each baseline plot (n=5), five quadrats (25(cid:3)25 cm) were platform 3 sub-divided into wet and dry areas (n=5 quadrats
randomly placed, leaving at least 50cm between adjacent per type). The results from five sampling areas (n=25 quad-
quadrats (Chapman and Bulleri, 2003; Firth et al., 2013; rats)on thehorizontalshoreplatformwereusedasthecontrol
Moreira et al., 2006). Four quadrats (25(cid:3)25cm) were baseline against which the new rock revetment was compared.
randomly sampled on enhanced and partially enhanced areas A preliminary survey of the newly colonised (12–18 months
ofthenewrockrevetment (n=1plotpertreatment).Datacol- post-deployment) rock revetment was made, where one
lected consisted of visual estimates of percentage cover for sampling area (n=4 quadrats) was surveyed for passively
sessile invertebrates andalgae,andcounts andlife stage (adult enhanced and partially enhanced (traditionally deployed) sur-
or juvenile) of the mobile organisms within each quadrat faces. More intensive monitoring surveys will take place in
(after Chapman and Bulleri, 2003). Quadrats were searched autumn2017and2018.
with plants and animals identified to species level where poss-
ible. Some stretches of shore platform habitat were relatively Post-construction bird surveyswill assess birduse of the struc-
short (e.g. <20m long); as such, each sampling area was ture, although anecdotal evidence from engineers and contrac-
between 5 and 8m long and spaced 10m apart (after tors shows that birds are using the new revetment during the
Chapman and Bulleri, 2003, Moreira et al., 2006) with shore constructionphase.
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