Sea Wave Attenuation by Salt Marsh Vegetation

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Nederlands Instituut voor Oecologisch Onderzoek

Sea Wave Attenuation by Salt

Marsh Vegetation

Photo: Spartina vegetation, Paulina Salt Marsh,Westerschelde, Zeeland,the Netherlands.

by

Elinor Low

A Dissertation

submitted in part fulfilment for the degree of Master of

Science in Environmental

Proteetion

& Management

University of Edinburgh

Scotland

2002

Project undertaken

at the Centre for Marine and Estuarine

Ecology

(Netherlands

Institute for Ecological Research)

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THE UNIVERSITV OF EDINBURGH

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ABSTRACT OF THESIS

(Regulation3.5.13)

Name of Candidate ELINOR LOW

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AddressDegree

5 WALLIS CLOSE CROWBOROUGH, EAST SUSSEX, TN6 2YA •••••••••••••••••••••••••••• 1••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••.•

MSC ENVIRONMENTAL Date 17 SEPT 2002

PROTECTION & MANAGEMENT

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Title of Thesis SEA WAVE ATTENUATION BY SALT MARSH VEGETATION

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No. of words in the main textof Thesis 9022

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Ab

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tract

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The main topic of this dissertation is concerned with potential sea wave attenuation by salt marsh vegetation. The overall structure of the dissertation is divided into a literature review and an experimental paper, which is further divided into an in-situ and ex-situ

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study.

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Salt marshes have some protected status within the EU and much study is being undertaken to determine the ecological functioning of this habitat within the estuarine ecosystem. The literature review reveals that ecological services of salt marshes may include storm buffering and coastline stabilisation and that this may have direct implications in Dutch coastal management. The experimental work quantifies the wave attenuation in salt marshes as maximal when there is high density vegetation and where water levels do no greatly exceed the vegetation canopy. Analysis of wave height data collected in the field (Paulina salt marsh, Westerschelde, the Netherlands) revealed a reduction in wave height within the pioneer salt marsh canopy. These findings suggest a significant role to be played by salt marsh vegetation in preventing minor damage to sea defence structures through attenuation of sea wave energy.

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PGS/ABST/94 Use this side only

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Contents

Sea Wave Attenuation

by

Salt Marsh Vegetation

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General Introduetion

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Review Paper Abstract Introduetion

Brief Overview of Salt Marsh Biology and Distribution Status of salt marshes in the Netherlands

Hydrodynamics of the Delta region

Attenuation of sea waves by salt marsh vegetation Importance of sea wave attenuation in coastal proteetion Conclusion

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References Experimental Paper Abstract

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Introduetion In-situ Study Study site Methods Ex-situ Study Methods Results In-situ Results Ex-situ General Discussion

Recommendations for Further Research

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References Appendices

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Acknowledgements

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1 3 3 4 4 6 8 9 10 11 13 15 15 16 16 16 21 23 23 30 33 43

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Sea Wave Attenuation

by Salt Marsh Vegetation

General Introduction

Often regarded as wastelands, the goods and services that wetlands provide, such as water storage and storm buffering (Crooks & Turner, 1999), have often been greatly undervalued leading to systematic drainage of wetlands for competing agricultural, residential and industrial land uses (after Barbier, Acreman & Knowler, 1997). Over the last century these competing land uses have led to major wetland losses, especially in industrialised regions where it is estimated that up to 60% of wetlands have been destroyed over the last 100 years (Bergkamp & Orlando, 1999).The Netherlands for example,between 1950 and 1985 alone, lost 55 % ofits total wetland area (after Barbier et al. 1997a).Inparticular estuarine wetland environments are under pressure from land reclamation especially in the Netherlands where salt marshes have been extensively diked and drained in order to create additional agricultural land (after Chapman 1974a as cited by Queen, 1977). Nevertheless, more recently there has been widespread recognition of the valuable services that wetlands provide (Woodward & Wui, 2001). Wetlands provide many goods of significant economie value, including clean water and fisheries (after Bergkamp & Orlando, 1999) besides providing habitat for many migratory and other bird species and possess diverse functions such as water purification, aquifer recharge and shoreline protection. Estuarine wetlands (salt marshes and mangroves) are particularly important in shoreline stabilisation and storm buffering. In the U.S.A., estimates indicate that the natural salt marsh defences of Boston Harbour save $17 million a year in flood proteetion works (Holdgate, 1997). Salt marshes may therefore present an important coastal proteetion tool and as such are the focus ofthis study.

This dissertation wi11look at the attenuation of sea wave energy by salt marsh vegetation and the implications this has for coastal defence. The review paper will give an overview of the importance of salt marshes in coastal proteetion with special reference to the Netherlands, in particular Paulina salt marsh in the Westersehelde estuary in the province of Zeeland. The experimental paper is divided into a field (in-situ) and a flume (ex-situ) based study focussing on the direct effects of salt marsh vegetation on sea wave attenuation. The field study carried

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out on Paulina salt marsh aims to demonstrate sea wave attenuation In the natural environment whereas the wave flume study seeks to quantify sea wave attenuation under different experimental conditions of varying plant density height and species.

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References

Barbier, E. B., Acreman, M. & D. Knowier (1997a). Chp 1 : Background to the global wetlands management problem. In Economie valuation of wetlands: a guide for policy makers and planners. Ramsar Convention Bureau, Gland, Switzerland. http://ramsar.org/lib_val_e_5.htm

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Bergkamp, G. & B. Orlando (1999). Exploring collaboration between the Convention on Wetlands (Ramsar, Iran, 1971) and the UN Framework Convention on Climate Change. Wetlands and Climate Change Background Paper from IUCN. www.ramsar.org/key_unfcc_bkgd.htm

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_{Crooks, S} & R. K. Turner (1999). Integrated Coastal Management: Sustaining Estuarine Natural Resources. Advances inEcological Research 29:241-281.

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_{Holdgate, M. (1997).From Care to Action. Earthscan Publications Ltd.,London,U.K.}

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Queen, W. H. (1977).Chp 17 Human Uses of Salt Marshes.In Ecosystems of the World 1: Wet Coastal Ecosystems. Ed. V.J. Chapman. Elsevier Scientific Publishing Company, Amsterdam, the Netherlands.

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Woodward R.T. & Y. S. Wui (2001). The economie value of wetland services; a meta analysis. Ecological Economics 37:257-270.

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Abstract Review Paper

Often undervalued, salt marshes are now gaining increased popularity as the various ecological services that they may provide, such as shoreline stabilisation and storm buffering, are becoming more widely accepted and appreciated. Besides being important habitat for bird and other wild life, salt marsh vegetation has currently been afforded some proteetion status for its shore stabilising vegetation under Annex 1 of the EU Habitat Directive. This vegetation type also serves as a dampener for sea waves that hit the coastline.

Although generally found in sheltered areas pioneer salt marsh vegetation has been proven to significantly attenuate wave energy. This may have implications for coastal management, particularly in the Netherlands, by reducing the wave energy reaching sea defences and decreasing the frequency of costly dike renewals. Dike renewal is currently taking place in the Netherlands to ensure continued proteetion of low-lying parts of the country from potential storm flooding conditions. Salt marsh sea wave attenuation cannot protect from extreme high water but may prolong the life of a dike structure by reducing minor damage.

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Review Paper

The Role of Salt Marshes in Coastal Proteetion with Special

Reference to the Netherlands

Introduetion

The Netherlands IS a densely populated and intensively farmed country. It is also a particularly wet country, with nearly half of its territory consisting of agricultural land lying below sea level (Wolff, 1993).Although much wetland habitat has been lost, approximately 16% of the national territory is considered as internationally important wetland and 7 % is designated as Wetlands of International Importance under the Ramsar Convention on Wetlands (afterBest, Verhoeven &Wolff, 1993). Dutch salt marshes alsomake up 7 %ofthe world' s total salt marsh cover. Although historically large tracts of these estuarine marshes were diked and drained to provide additional agricultural land (Wolff, 1993) more recently there has been increased recognition of the goods and services that these wetlands may provide (Woodward&Wui, 2001).In particular salt marshes have the potential toprotect and stabilise coastal areas (after Crooks & Turner, 1999). Much research has been undertaken on this theme and this paper will review the role salt marshes play in coastal protection. Special reference will be made to the Netherlands, in particular Paulina salt marsh in the Westersehelde estuary in the province of Zeeland. Before reviewing the sea defence value of salt marshes a brief overview will be given of salt marsh distribution and biology including wetland definitions.

Brief o

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erview of salt marsh distribution and biology

The Ramsar Convention on Wetlands of International Importance defines wetlands very broadly as: 'areas of marsh,fen, peatland or water, whether naturalor artificial,permanent or temporary, with water that is statie or flowing, fresh, braekish or salt, including areas of marine water, the depth of whieh at low tide does not exeeed six metres' (Barbier et al.,

1997a).

Wetlands are found at most latitudes and may be classified as marine, estuarine, riverine, lacustrine or palustrine (after Williams, 1990).Marine wetlands include coral reefs and kelp beds,estuarine wetlands comprise salt marshes and mangrove swamps, riverine and lacustrine

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are wetlands associated with riverbanks and lakeshores respectively and palustrine wetland

habitat may defined as any inland wetland not associated with a deeper water body such as a peat bog (after Williams, 1990).

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Salt marshes are part of the estuarine ecosystem developing intertidally in sheltered places where silt and mud may accumulate (Mann, 2000). They are characterised by a bare seaward mud or sand flat followed by pioneer vegetation (after Chapman, 1977), such as cord grasses

(Spartina spp. ) and glassworts (Salicornia spp.), which bind and stabilise the sediment. This

colonising vegetation progressively increases the ground level, which successively supports more plant species. Although primarily confined to temperate latitudes salt marshes mayalso be found in the tropics occurring landward of mangrove swamps mostly in arid or monsoonal regions (Chapman, 1977). There are also regions where both salt marsh and mangrove are found including the Gulf coast of the U.S.A, New Zealand and the southem coasts of Australia and Japan (Chapman, 1977).Figure 1 illustrates the general world distribution of salt marshes (and mangroves).

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16'CJanuaIy

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~ """ ..

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\": ,': .;.~'...'.:. ~

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Figure 1 Worldwide distribution of estuarine wetlands (areas shaded black=salt marsh occurrence).(Chapman, 1977).Salt marshes are found where the winter water temperature is below 10°C, mangroves occur where winter temperatures are above 16°C and both are found in water with a winter temperature between 10&16°C (after Mann,2000).

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Establishment and maintenance of salt marsh vegetation will occur only in areas where there is sufficient lack of wave action to enable seedlings or seeds to put out roots before being removed by the tide (after Chapman, 1977).Salt marsh plants must also obtain enough water through roots that are immersed in salt water with high osmotic pressure and must get oxygen to their roots, which are often submerged in waterlogged oxygen deficient soil (Mann, 2000). Spartina, a dominant genus on salt marshes worldwide (Chapman, 1977), has the C4

photosynthetic pathway which enables higher potential productivity, higher water use efficiency and more efficient use of nitrogen. Spartina spp. are thought to accurnulate proline, a free arnino acid, and the quatemary ammonium compound glycinebetaine in order to take in enough water from the soil to maintain physiological functions. However for efficient photosynthesis in the leaves salt content of the intemal fluids must be reduced, which is achieved by salt glands in the epidermis of the leaves, abundant on the side of the leaves facing the stern (after Mann, 2000). For oxygen to reach the roots of salt marsh plants different plants adopt different approaches. In soils inundated less frequently some plants may carry out limited periods of anaerobic metabolism in their roots, others transport oxygen through the stomata of the leaves down through the stern and rhizome to the roots through loosely packed cells termed aerenchyma tissue (after Mann, 2000). Nevertheless high salinity and waterlogged anaerobic soils are not stressful to salt marsh plants as these are environrnental conditions within the normal range encountered by such plants (after Otte, 2001).

Status of salt marshes in the Netherlands

Common salt marsh plant species in the Netherlands include Spartina anglica, Salicornia europaea and Puccinellia maritima.A more detailed list of Dutch salt marsh species and their common narnes may be found in table 1 (the list is not exhaustive).

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Latin name Commonname

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_{Spartina anglica}

Sa/icornia europaea Scirpus maritimus Puccinellia maritima Aster tripolium Spergularia marina Spergularia media Plantago maritima Limonium vulgare Suaede maritima Glaux maritima Halimione portulacoides A triplex prostrata Festuca rubra

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_{Juncus gerardii} Phragmites austra/is Atriplex littoralis Juncus bufonius Artemsia maritima

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Cornmon cord grass Glasswort

Sea club rush

Sea poa (marsh grass) Sea aster Marine spurrey Spurrey Sea plantain Sea lavender Aceous seablite Sea milkwort Sea purslane Orache Creeping fescue Mud rush Reed Shore orache

Toad rush Figure 3 Sali_{Paulina salt marsh}corniaeuropaea (foreground) on Sea wormwood

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Table 1 Salt marsh plant species found in the Netherlands,in particular the Westerschelde.Latin names (pres. Comm.., Koutstaal, B.,2002).Common names (KNAW, 1982;Clapham, Tutin&Warburg, 1958)

At present, total salt marsh area in the Netherlands represents 7 % of European salt marsh cover (Wolff, 1993). Historically salt marshes covered much greater areas than today. Since the advent of drainage and embankment in the Middle Ages much wetland coverage in the Netherlands has disappeared. Drainage by digging ditches was necessary to create sufficiently dry conditions for agriculture and the subsequent subsidence of the land surface, which rendered it more accessible to sea and river water, then necessitated the construction of dams and dikes (Wolff, 1993). Due to the introduetion of cord grass (Spartina anglica) to the Netherlands in 1925 salt marsh has increased by approximately 700 hectares (ha) in the Westeschelde however through diking and draining of salt marsh and receding of the outer salt marsh there has been an overallloss of salt marsh..

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Figure 4 shows the recent reductions in salt marsh cover in the Westersehelde estuary, Zeeland. Under current EU wetland policy salt marshes and mudflats are protected areas, mud and

sand binding salt marsh species are listed in Annex 1 of the EU Habitat Directive (94/43/EEC) as a natural habitat type whose conservation requires the designation of special areas of conservation (http 1). The EU Wise Use of Wetlands Communication (COM(95)189),an important set of guidelines, has also set goals

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5000 2 .5 4000 !! ~ 3000 ~ 2000 o

shallow water mudflats sand banks salt marshes

Figure 4 Changein cover(ha)of shallowwater,mudtlats, sand banks andsalt marshesin the Westersehelde from 1960 to 1998.

(SIC. 1999)

such as 'no net loss' of wetlands (http2).The majority of Dutch salt marshes are found in the Wadden Sea area but the Delta region in the province of Zeeland still has significant tracts of salt marsh.

Hydrodynamics of the Delta region

The Delta area in the south-west of the Netherlands is divided into the Oostersehelde and the Westersehelde branches of the Scheldt Estuary, the drainage basin of which covers 21,850 km2 of north-west France, Belgium and the southem Netherlands (after Nihoul,

Ronday, Peters &Sterling, 1978).The flow of the Scheldt River is slow but tidal motions in the estuary are large (Nihoul et al., 1978). Waves that originate from the North Sea are attenuated by bottom frictional energy losses as they enter the shallow estuaries of the Ooster-and Westersehelde (http 3). The estuary is also a highly managed environment. In 1986 work was completed on the Delta Barrier, constructed across the mouth of the Oostersehelde as proteetion against storm surges ofseawater after fatal floods in 1953.Itis an open barrier with 62 moveable steel gates that that are lowered during times of high flood risk (http 4). However the Westersehelde estuary remains entirely open to the sea and is a heavily used shipping channel, which is subject to continual deepening by dredging to maintain channel depth. In1996 legislation was introduced for the deepening of the Westersehelde (http 5) and the current dredging operation in the estuary,which began in 1997,will cause an increase in the water level to a maximum of 10 cm (http 6). The coast is lined with sea dikes, since most

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land is situated below sea level and would be inundated without the management measures to keep it dry. In the Westersehelde many salt marshes are found on the seaward side of sea dikes but also on sand banks in the estuary itself that become exposed at low tide. Figure 3 shows the area topography with regard to the Dutch ordnance level (NAP - Normaal Amsterdams Peil, the water level used as the reference point for tidal regimes).

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ZUID·BEVELAND '"

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DIeptegegevens t.c.v.NAP(m)1992

>+ 2.00 +-2.00

•2.00

,. .2.00

•2m Irtn.hoger gelegen

gebieden vallen droog tijdenslaagwater

ZEEUWSCH·VLAANDEREN

Figure 5 Depth data ofthe Westerseheldewith regard to NAP (NormaalAmsterdams Peil=Dutch Ordnance level(Blue=>

-2rn; Yellow=-2mB+2m;Green=>+2m) (after SIC,1999). Red box=Paulinasalt marsh,

Attenuation of sea waves by salt marsh vegetation

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Waves are created by wind and are an important factor in vertical mixing within estuaries (Mann, 2000). In the Netherlands waves also cause significant damage to coastal dikes at great financial co st to coastal proteetion bodies. Over the next 15 years €700 million will be invested in dike renewal in the Netherlands (http 3). In the UK more cost-effective 'soft' engineering approaches are being considered as sustainable options for coastal management (Möller, Spencer, French, Leggett & Dixon, 2001). Möller et al.(2001) describe 'soft' coastal engineering as the realignment of current 'hard' engineering defence lines, such as sea walls, further landward reintroducing formerly reclaimed land back into the tidal zone. These coastal setback areas result in an increase in the area available for salt marsh formation, which is assumed to reduce sea wave energy, allowing the new defence line to be constructed to a lower standard (after Möller et al.,2001). In the Netherlands, where land area is an expensive commodity it is unlikely that agriculturalland will be sacrificed to such coastal set back areas. Nevertheless salt marsh vegetation bordering the 'hard' engineering sea defences, characteristic of the Dutch coastline, may offer some proteetion from sea wave energy. A

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study carried out by Yang (1998) on wave attenuation by Scirpus mariqueter vegetation revealed that wave energy could be entirely dissipated when a wave moved shoreward approximately 50 m through the marshvegetation. Wave heights found over the marsh were on average 43 % less than those on the adjacent tidal flat. This is in accordance with results obtained by Möller, Spencer,French,Leggett&Dixon (1999) who state that wave attenuation does not vary linearly with distance across the salt marsh and that most wave energy is dissipated or reflected over the first 10 to 50 metres of salt marsh surface. Salt marsh canopies also Iower the speed of water flow. Shi, Hamilton & Wolanski (2000) describe salt marsh plants as capable of extracting momentum from tidal water via hydrodynamic drag and turbulence generation. Flow speeds are most reduced where the flowencounters the largest amount of plant material (Leonard & Luther, 1995). If this is the case then it would be expected that once the tidal water level is well above the vegetation canopy wave attenuation would no longer occur,as there is no obstacle to the water movement. However, according to results obtained by Möller et al.(1999) wave attenuation over salt marsh still remains 50 %

higher than attenuation over an adjacent sand flat irrespective of observed water depths.

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Importance of sea wave attenuation in coastal protection

Salt marshes have much potential for coastal proteetion by absorbing wave energy that consistently contributes to the wear and tear of sea dikes. Currently 57 km of sea dike along the coast of the Westersehelde is in need of renewal (http 3) due to inadequate stone protection.Figure 6 shows the location of dike areasin need of strengthening.

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UltMertng werken profect Zeewerlngen - R<.dl.. ...,.rdl~·2OOl - ~2001 - u-..inr;;KI(lZ M,qdeltoJrp -..., ......

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Figure 6 Location of dikerenewal works.Blue=workcarried out in 1999-2000,Red=work for 200 I, Green=work for

2002 (http3).Red box denotesPaulina salt marsh,OrangeboxdenotesSaeftingesalt marsh.

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Approximately 10.5km of these stretches win be replaced in 2002. After the severe flooding in the region in 1953 all sea dikes were brought up to the height of the Delta Barrier in the Oosterschelde. However under severe weather conditions the concrete blocks are becoming dislodged too easily (http 5).

Figure 7 Loosened stone boeks on a sea dike A 2,900 m region of sea dike is currently being

worked on due east of Paulina salt marsh but work on the sea dike behind Paulina polder will not take place unti12003. Work is prioritized so that dikes in most urgent need ofrenewal are strengthened first (pers. comm. - Jakobsen, J., 2003). Since salt marsh vegetation has been described in many studies as a significant shock absorber for wave energy it might seem surprising that the presence of Paulina salt marsh has not prevented the renewal of Paulina Polder sea dike. However, in the Netherlands, tests carried out to determine whether sea dikes are in need of renewal are scaled to whether the dike is sufficiently strong to prevent flooding even under the most severe storm conditions and extreme high water levels (http 7). Salt marsh vegetation may help to prolong the life of the dike but cannot proteet the land from exceptionally high water levels. Most violent storms are also likely to occur during the winter when salt marsh vegetation has died back and there is minimal obstruction to water motion. Nevertheless, behind the Saeftinge salt marsh, a far larger vegetated area than Paulina salt marsh (see figure 6), only parts of the sea dike are in need of complete renewal and Paulina Polder dike renewal will consist only of the replacement of the top layer of stone cover (pers. comm.- van Elzinga,2002).

Conclusion

Since recognition of the ecological services provided by salt marshes much research has been focussed on the functioning of this habitat. Although the salt marsh environment has some proteetion status there is still a growing need to quantify the value of wetland services

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(Woodward & Wui, 2001) so that the benefits provided by salt marsh habitats may compete with removal for other financial gains. This can be an important tool for coastal habitat proteetion where an area of land kept as wetland may be shown to be the best economie use of the land area rather than, for example, drained for agricultural use. Although afforded some proteetion status from development salt marshes are recognised to be under threat from potential sea-level rise. Dredging in the Westersehelde is set to artificially increase local water levels and the effects of this are to be closely monitored (http 6) so that mathematical models may be constructed for future management of salt marshes should sea levels increase. These dredging activities move more than 10 million m3 sediment each year (http 6).

Uncontaminated dredged materials are potentially valuable resources for wetland habitat creation or restoration (Costa-Pierce & Weinstein, 2001). Adding uncontaminated dredged material on receding wetlands may increase their elevation so that marsh vegetation may be re-established (Costa-Pierce&Weinstein,2001). Whether this is feasible or not any increase in the 'soft' coastal defence lines of salt marsh cover in the Westersehelde will mean reduced wave energies reaching dike structures, whilst at the same time restoring previously lost habitat.

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References

Review Paper

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Barbier, E. B., Acreman, M. & D. Knowier (1997). Chp 1 : Background to the global

wetlands management problem. In Economie valuation of wetlands: a guide for policy

makers and planners. Ramsar Convention Bureau, Gland, Switzerland. http://ramsar.org/lib_val_e_5.htm

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Bergkamp, G. & B. Orlando (1999). Exploring collaboration between the Convention on Wetlands (Ramsar, Iran, 1971) and the UN Framework Convention on Climate Change. Wetlands and Climate Change Background Paper from IUCN. www.ramsar.org/key_unfcc_bkgd.htm

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Chapman, V. J. (1977). Chp 1 Introduction. In Ecosystems of the World 1: Wet Coastal

Ecosystems. Ed. V. J.Chapman. Elsevier Scientific Publishing Company, Amsterdam, the Netherlands.

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Costa-Pierce, B. A. & M. P. Weinstein (2001). Use of dredged materials for coastal

restoration _ Editorial. Ecological Engineering 19:181-186.

Crooks, S & R. K. Turner (1999). Integrated Coastal Management: Sustaining Estuarine

Natural Resources. Advances in Ecological Research 29:241-281.

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Holdgate, M. (1997). From Care to Action. Earthscan Publications Ltd., London, U.K.

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KNAW - Royal Netherlands Academy of Arts and Science (1982). The Dutch Delta: A

compromise between environment and technology in the struggle against the sea.

Eds. Duursma, E. K., Engel, H. & J. M. Martens. Koninklijke van Poll, Roosendaal,

the Netherlands.

Leonard, L.A. & M. E. Luther (1995). Flow hydrodynamics in tidal marsh canopies.

Limnol. Oceanogr. 40(8):1474-1484.

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Mann, K. H. (2000). Chp 3 Salt Marshes. In Ecology of Coastal Waters _ With Implications

for Management. Blackwell Science Inc., Massachusetts, U.S.A.

Möller, I, Spencer, T., French, J. R., Leggett, D. J. & M. Dixon (2001). The Sea-Defence Value of Salt Marshes: Field Evidence from North Norfolk. J.CIWEM 15: 117.

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Nihoul, J. C. J., Ronday, F. C., Peters, J. J. & A. Sterling (1978). Hydrodynamics of the Scheldt Estuary. In Hydrodynamics of Estuaries and Fjords. Ed. Nihoul, J. C. J.

Elsevier Scientific Publishing Company, Amsterdam, the Netherlands.

Otte, M. L. (2001). What is stress to awetland plant? Botany 46(3):195-202 Abstract only.

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Queen, W. H. (1977). Chp 17 Human Uses of Salt Marshes. In Ecosystems ofthe World 1:

Wet Coastal Ecosystems. Ed. V. J.Chapman. Elsevier Scientific Publishing Company, Amsterdam, the Netherlands.

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_{Shi, Z., Hamilton, L.J. & E. Wolanski (2000). Near-Bed Currents and Suspended Sediment} Transport in Salt marsh Canopies. Joumal ofCoastal Research 16(3):900-914

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SIC (Schelde Informatie Centrumeen beeld van een estuarium. (The Schelde Atlas, a tableau of an estuary). LnO- Schelde Information System) (1999). De Schelde Atlas, drukkerij/uitgeverij, Zierikzee, the Netherlands.

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Williams, M. (1990). Understanding Wetlands. In Wetlands: A Threatened Landscape. Ed Williams, M. Alden Press Ltd, Oxford, U.K.

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Wolff, W. J. (1993). Netherlands-Wetlands. Hydrobiologia 265:1-14. In Netherlands-Wetlands. Eds. Best, E. P. H. & J. P. Bakker. Kluwer Academie Publishers, Dordrecht, the Netherlands.

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Woodward R.T. & Y. S. Wui (2001). The economie value of wetland services: a meta analysis. Ecological Economics 37:257-270.

Yang, S.L. (1998). The Role of Scirpus Marsh in Attenuation Hydrodynamics and Retention of Fine Sediment in the Yangtze Estuary. Estuarine, Coastal and Shelf Science 47:227-223.

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Internet sites

httpl :lleuropa.eu.inticomm./environmentinaturel (European Commission - Environment) http2:llesl.jrc.it/envind!meth_shtlMs_we043.htm (Ecomission Joint Research Centre

-Wetland Loss)

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http3:llwwwNewsletter).waterland.net/dzl/zeeweringenlinfospec2002.html (Sea dike Project Bureau

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http4:1Iwww.knag.nlSociety) /pagesuk/ geography/naturalsettings.html (Royal Dutch Geo graphical

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http5:1Iwww.minvenw.n1/cend! dvo/intemational/English/ summariesl eng08 96.html\Shi pping (Dutch Ministry of Agriculture, Nature Management and Fisheries Press Releases) http6:1Iwww.nioo.knaw.nl/cemolisled!english.htm (ISLED homepage )

http7:llwww.waterland.netldzl/zeeweringenidijkversterk.html (Sea dike Project Bureau)

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Abstract

Experimental Paper

In the Westerschelde, Zeeland, the Netherlands, salt marsh vegetation is a potential natural sea defence structure in a highly unnatural coastal environment where sea dikes border most of the coastline. Experimental investigations in the field(in-situ) and in a controlled flume environment (ex-situ) were carried out to establish wave height attenuation by salt marsh vegetation. Field results of wave height measurements in pioneer Spartina vegetation (Paulina salt marsh, Westerschelde, Zeeland) revealed that wave amplitude was most reduced when the water level was inferior or equal to the vegetation canopy height. Results from the wave flume study (WL Delft Hydraulics) corroborated this finding. Cultivated vegetation treatments of Spartina and Salicornia, field Spartina vegetation and artificial substrates of different densities were tested under an identical regular wave regime at low and high water. Results revealed that greatest wave attenuation occurred at the low water level with high density vegetation.

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Experimental Paper

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A Small-scale Field and Flume based Study on Sea Wave Height

Attenuation

b

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Salt Marsh Vegetation.

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Introduetion

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Salt marsh habitats are currently the focus of much research tobetter understand the various ecological functions that this habitat provides. Previous studies have demonstrated that salt marsh vegetation may significantly dampen sea waves and water motion (Möller, Spencer, French, Leggett & Dixon, 1999;Shi, Hamilton & Wolanski, 2000; Yang, 1998). Salt marsh vegetation therefore is a potential natural sea defence structure in the highly unnatural Dutch coastal environment where sea dikes border most of the coastline. The experiments carried out for the experimental part of this dissertation aim to demonstrate wave attenuation in the field and quantify the field results with controlled flume measurements of predetermined

vegetation treatments.

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Data used in this small scale study on sea wave attenuation by salt marsh vegetation is only a subset of a larger dataset collected for the DELFT CLUSTER project whose main aim is to include biology in the existing physical and hydrodynamic models ofthe Schelde estuary. The study is being carried out in partnership between the Centre for Estuarine and Marine Ecology (CEMO-NIOO),Yerseke, Zeeland, WL Delft Hydraulics, the Technical University ofDelft, and the Royal Institute for Coast and Sea (RIKZ), Middelburg.

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Stud

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site

Paulina salt marsh (51°25'N,

3°40'E) is situated on the south coast of the Westersehelde estuary, approximately 6 km

northeast of Terneuzen, in the

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{}

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..'

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' ... " I--~---'--.!.I.i.,. ..···· :•••••J ..•••••• :_.bOldera·HL ._.

..

, : BELGIUM Rup.1 10km

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Figure 8 Map ofthe westerschelde,inset map ofthe Netherlands (http 1). Red boxdenoteslocation of Paulinasalt marsh

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province of Zeeland, the Netherlands (see figure 8).Itis a small marsh in comparison to other salt marshes of the Westersehelde and the Netherlands in general but is a suitable site for study as it has easy access form the Paulina polder sea dike. The Westersehelde estuary experienees approximate lunar semi-diurnal tides (Nihol, Ronday, Peters & Sterling, 1978), (essentially a 50 minute progression in the time of each succeeding high water). Figure 9 shows Paulina salt marsh location in more detail.

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___ Paulinapolder

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Figure 9 Hydrographic map showing location of Paulina Polder and surroundings, Westersehelde (Koninklijke MarinelDienst der Hydrografie,2000). Red box denotes location of Paulina salt marsh study site.

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Delft Cluster project

The primary aim of the DELFT CLUSTER project is to fit biological processes into existing hydrodynamie modeis. Involvement in this project at Paulina salt marsh consisted of aiding in the set up of four measuring frames and visiting the study site every two weeks (from 14 June 2002- 26 August 2002) to offload data from and reprogram the start up time for these frames. The four automated measuring frames were located across a sea to land transeet as shown in figure 10 and collected data on saltmarshlmudflat hydrodynamie interactions.

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Paulina Polder

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Figure 10Satellite image of Paulina salt marsh. Measuring frame location indicated by yellow diamonds, M 1-4 denote DELFT CLUSTER location of measuring frames.(Colour:Orange=salt marsh vegetation, Green=Schelde water and water in creek networks).

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Measuring Frame Origin Location

AMF

DON RIZA2 RIZAI RIZAI

(Automated Measuring Frame,WL Delft) (Directie Oost Nederland)

(RIZA, Dordrecht) (RIZA,Dordrecht)

Interface of schelde water/mudflat On mudflat near mudflat/veg interface In high order (large) creek

In low order (smalI) creek

Moved to pioneer vegetation 2/7/02

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MI M2 M3 M4

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Table 2 Measuring frame name, origin and location. RIZA = Rijksinstituut voor Integraal Zoetwaterbeheer en Afvalwaterbehandeling (Royal Institute of Integral Freshwater Management and Wastewater Treatment).

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Figure 11 a) AMF frame,b) DON frame,c) RIZA2 frame

Each measuring frame consisted of a datalogger, memory module and computer hard drive encased in a cylindrical waterproof container. Batteries were either enclosed in the same container as the datalogger (RIZA frames) or in adjacent waterproof containers (DON &

AMF). The data logger measured water pressure, water turbidity, water flow via a pressure sensor, an OBS (optical back scatter) sensor and an EMF (electro-magnetic flowmeter), respectively (see figure 12).

Figure 12 a) OBS and EMF b) pressure sensor ofRIZA 1when in the tidal creek

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conneetion box

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pressuresensor laptop PC

The datalogger was programmed to record three hours before and three hours after high tide. Data was recorded in 9 minute bursts at 4 Hz (see Table 3) and offloaded bi-monthly from the computer hard drive (where datalogger stored all data recorded from the sensors) to a laptop hard drive in the field via the conneetion box (see figure 13 for layout; for protocol see Appendix 1). The datalogger was then given a new start up time so that measurements could be kept as close to the three hours before and after real high tide as possible, this was not always possible however as the tidal cycle fluctuated away from the lunar semi-diurnal pattem.

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EMF

Direction of flowing tide

Figure 13 Schematic layout of the RlZA I frame for offloading and startup

one measuring cycle period is 12 h 25 min => 6 h 00 on+6 h 25 min off 6 h on => 24 cycles of 15 min

15 min cycle=> 9 min. on (recorded at 4 Hz)+6 min. off

1 data file=consists of a 15 min. measuring cycle=>downloaded on the PC • thus: per 24 h+50 min => 48 data files

(before each 15 min. cycle => 30 sec tilt +compass +30 sec startup sensors)

Table 3 Modified excerpt from protocol in Appendix I describing frame data measurements

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Methods - Wave height attenuation

by

salt marsh vegetation

The in-situstudy upon which this paper is based is concemed only with pressure data from the DON and RIZAI frame over a six week period. From 2 July 2002 to 15 August 2002 RIZAI was relocated from the small creek to an area of pioneer salt marsh vegetation (Spartina anglica) at the seaward edge of Paulina salt marsh to permit this comparative study of wave heights over the mudflat (DON) and in the vegetation (RIZAl). Pressure data was collected in 9 minute bursts (see table 3) at 4 Hz.

Pion eer vegetation Mud flat

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m

Rizal

DON

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Figure 14 Distancesbetween theRIZA 1and DONmeasuringfra~es duringthewaveheightstudy

Wave height was calculated from pressure readings taken at the two measuring stations. The two most windy days (when waves are most likely to be present) near a spring tide and a neap tide were selected for comparison but due to time constraints only the spring tide was analysed. According to meteorological data supplied by Hydro Meteo Centrum Zeeland for July and August 2002 (see Appendix 2 for detailed wind data), the following day was selected:

11 August 2002

High Water at 17h12, +268 cm NAP Mean Wind Speed

=

6.8 m sec-I Mode Wind Speed

=

7.2 m sec

From the 24 data (DAT) files (of 15 minute measuring cycles) corresponding to the PM tide occurring at each station on 11 August 2002, the tidal curve and wave heights at different water heights were calculated as follows:

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!DATfile converted to ASe file using RUNSPLIT (WL Delft, 2002) programm~ .J,

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!Ase file opened in Exce~

.J,

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kaw data for pressure sensor in mVJ

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@ataconverted to total pressure in mBar using formula~ DON 1000+mV * 0.24203=mBar

RIZA1 976+mV*0.1210=mBar .J,

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~otalpressure=water + air pressure :. water pressure =total pressure- air pressur~ (Air pressure value derived from measurements made by the pressure sensor when out of water)

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!Waterpressure in mBar=WATER HEIGHT in c~

1 atm -1 bar =10 m :. 1 mBar =1 cm

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(for every lOm decrease in sea level, pressure increases by 1 atm)

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ATER HEIGHT =increase and decrease in total water level due to flow and ebb of tide!Watersurfac~ +wave heights o~ .J,

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WOR WAVE HEIGHT ONL Y=water pressure values - moving average of water leveij

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Data are displayed as a tidal curve over the chosen tide period and for water heights 10cm, 20 cm, 40 cm and 80 cm the corresponding wave heights at the RIZAI station are plotted over a 120 speriod.

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Ex

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study

Both natural salt marsh vegetation and artificial substrates were exposed to wave energy in a wave flume at WL Delft. The main aim of the study was to look at the effects of plant height, density and flexibility on wave energy absorption. A study was also carried out on laminar flow in salt marsh vegetation in the NIOO (Netherlands Institute for Ecological Research) flume.

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egetation preparation

Natural vegetation: Salicornia europaea and Spartina anglica

Thirty-six stainless steel boxes were made to specific dimensions (100 cm x 23.5 cm x 14.5 cm) according to the flume dimensions. Four drainage holes (0 5 mm) were drilled at equal distances along the bottom rim of the two longest box sides. Each box was filled with a mixture of clay and sand (mixing ratio 2: 1 respectively) and per box 60 g of slow release fertiliser was added. Soil was well compacted to avoid subsidence when in flume water.

Figure 15Stainless steel boxes in which salt marsh plants cultivated

Seed germination

On 22 May 2002, Salicornia europaea and Spartina anglica seeds, coUected previously, were placed on moistened filter paper in glass trays covered with a glass plate and left to germinate for a week. Care was taken to keep the filter paper moist at all times. Once germinated (after approximately one week), the seedlings were carefully planted at 5 cm equi-distance in the thirty-six boxes: eighteen boxes of S.anglica and eighteen boxes ofS. europaea each with 100 plants per box, a density of 417 plants m-2• Treatments (each consisting of six boxes)

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of mature vegetation at high and low density were tested against a 4cm amplitude wave regime at 12 cm and 25 cm flume water height. Spartina vegetation was also taken from the field so that the effects of natural vegetation density on wave attenuation could be investigated. The field plants were trimmed to 23 cm and 10 cm.

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Artificial vegetation:Six perforated stainless steel metal boards were cut to size (100 cm x 48 cm) and each boardSoft and hard plastic strips had 1,000 holes (0 5 mm) at 3 mm intervals. Two different types of artificial substrate were chosen, one of a flexible 'soft' nature (plastic folder cut to 4.7 mm width and 100 mm length) and the other non-flexible or 'hard' (plastic tie wraps 4.7 x 195 mm cut to 100 mm length). Each 'leaf was 4.7 mm in diameter and 10 cm substrate protruded from the metal board. High density denoted one 'plant' fixed every third hole and low density one plant every sixth hole.

Table4 describes the different treatments.

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Vegetation Type Veg height Density In plants m-z Flume water height

1 Salicornia (cultivated) 18 cm HIGH (HD) 417 25 cm

2 Salicornia (cultivated) 18 cm HIGH (HD) 417 12 cm

3 Salicornia (cultivated) 18 cm LOW (LD) 104 12 cm

4 Spartina (cultivated) 17 cm HIGH (HD) 417 25 cm

5 Spartina (cultivated) 17 cm HIGH (HD) 417 12 cm

6 Spartina (cultivated) 17 cm LOW (LD) 104 12 cm

7 Spartina (from field) 10 cm FIELD (FD) -1250 25 cm

8 Spartina (from field) 10 cm FIELD (FD) -1250 12 cm

9 Spartina (from field) 23 cm FIELD (FD) -1250 25 cm

10 Spartina (from field) 23 cm FIELD (FD) -1250 12 cm

11 Soft plastic strips 10 cm HIGH (HD) 2000 25 cm

12 Soft plastic strips 10 cm HIGH (HD) 2000 12 cm 13 Soft plastic strips 10 cm LOW(LD) 500 12 cm

14 Hard plastic strips 10 cm HIGH (HD) 2000 25 cm

15 Hard plastic strips 10 cm HIGH (HD) 2000 12 cm

16 Hard plastic strips 10 cm LOW (LD) 500 12 cm

17 Empty Flume / / / 25 cm 18 EmptyFlume / / / 12 cm

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Table 4 Natural vegetanen and artificial substrate treatments for tlume study

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Flume water heights were chosen so that the effects of water level approximately at canopy height and water well above canopy height could be investigated. Wave height measurements were also made at both water heights with no plants present to establish any wave damping due to energy losses in the flume alone.

Methods - Flume experimentation

WLDELFT

Over a three week period (5 - 23 August 2002) each treatment was tested at the selected

densities and water heights in a wave flume tank at WL Delft Hydraulics. The flume tank

channel had a totallength of l3.5 m, the working section ofwhich began at 8.08 m. The length

of the working section in this experiment was 3 m.

Figure X Above Oliegoot wave flume, WL Delft, left Salicorniain working section

Waves were generated by a piston and at the opposite end of the flume dampened by a wooden rack. Two electromagnetic flowmeters (EMFs) and a conductivity meter, measuring water flow speeds and wave height respectively, were attached to a sliding carriage that could be manually

moved along the length of the flume.Two further conductivity meters were placed just after

the wave generator and after the working section. Measurements were made at 78 cm before

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the vegetation and at 22, 122 and 222 cm into the vegetation (1 m spacing between each measurement) and the flume was filled with either 12 cm or 25 cm freshwater. After daily experiments the flume was emptied and plants illuminated if present in the flume ovemight.

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The DELFT CLUSTER measurements consisted of 720 s readings of each sensor at different

height intervals in the flume (see Appendix 3 for protocol). The wave attenuation study upon which this ex-situ experiment is based is concemed only with data collected by the conductivity meter on the sliding carriage. Figure 17 shows a schematic diagram of the experimental set up in the flume.

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Side View -78 cm 22cm 122 cm 222 cm

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Top View lm

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0.48 m , , , , , ,_{, ,}

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13.5m Figure 17 Flume layout (not to scale)

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The conductivity meter was calibrated so that a 1 cm displacement of water would be recorded as 1 volt (positive values for an increase and negative values for a decrease in water height). In

a regular wave, peaks and troughs will occur more frequently over time than the deseending

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and ascending parts ofthe waves (see figure 18). From the most frequent maximum peak and trough values at each measuring point the most frequent wave heights may be calculated.

cm

2

1.5

1

0.5

o

-0.5

-1

-1.5

-2

...

+

vèvälüës

::::::::::

~

\

::::::

II

:::::::::::::::::::::::::

uu.

...

. uu. u.

u

:::Ve

u

valües

.

u.u

u.uu

.__ Conductivity meter

Figure 18 Calculation of most frequent wave heights.Wave curve a (wave peak) has longer time period than wave curve b, consequently more readings will be taken during the peak and trough periods.Green arrow represents relative time period.

Readings taken before, in the beginning, middle and end of the vegetation were analysed for most frequent wave heights by constructing frequency tables for the water heights recorded at each measuring point (see Appendix 4.1- 4.18). The maximum negative (trough) and positive (peak) values were selected. Only a single maximum value was selected for both trough and peak if the values either side of the maximum were+10 % less frequent.

900 1200 750 650 10 % of 1200 = 120 .--- Maximum value 1200- 120 =1080

.. both 900 and 750 values are>10% less frequent

1200may be selected as greatest frequency value

For most peaks and troughs the maximum values were clear and the standard error smalI, however in some cases an extra data set of 720 s was added to the water height values to make the peak and trough values more clear.Graphs containing extra datasets are marked with

*.

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there was uncertainty the bin values could be plotted as a frequency distribution and examined more closely.

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-78cm

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4000 >.CP IJ IJ C C CP I!:! :::I ... cr::::l CP IJ ot: IJ o

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-6 -4 -2 2 4 -1000 water height (cm)

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6 22 cm >.cp IJ IJ C C CP I!:! ::::I ... cr:::l Cl) IJ ... IJ ...0 4 -4 water height (cm)

Figure 19 Frequency distributions of water heights measured at78 cm before the vegetation and 22 cm into the vegetation for the 23 cm field Spartina treatment with 12 cm water. Red and green circles denote maximum trough and peak values respectively.

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For the -78 cm measurement the maximum frequencies are quite clear but for the 22 cm measurement there is much more variation especially in peak values.

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Bin values for the frequency tables were in intervals of O.lmm, which corresponded to all values from 0.09 before and up to the bin point (see Appendix 5). Therefore, for the most frequent trough and peak values the corresponding bin value had to be corrected for uncertainty by subtracting 0.05 mm (0.1 mm/2), the mean of the bin value. If more than one frequency value had to be taken for the trough and peak values the mean of the bin values was also taken and the uncertainty value increased by 0.05 per value selected. Standard error was calculated using the uncertainty values in the following formula:

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S.E. =

j

(X)2

*

(y)2 where x=trough uncertainty value, y=peak uncertainty value The amplitudes of the most frequent wave heights were then determined by calculating the di stance between the maximum trough and peak values for each measuring point in the flume. For each treatment the most frequent wave height before, in the beginning, middle and end of the vegetation was plotted with its standard error value and percentage attenuation (referring back to the measuring point before the vegetation) attached. These data were then transformed

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to percentage values only and a linear regression applied so that percentage wave attenuation per metre vegetation could be calculated.

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NIOO

Over a two week period (15 - 26 July 2002) a set oflaminar flowexperiments were carried out at the NIOO flume, a 17.55 m race track shaped channel, using two box es of both Salicornia

and Spartina. Each vegetation species was subject to high and low velocity freshwater flow, 30

cm sec·! and 5 cm sec" respectively. Measurements were prograrnmed (Flume Software version 1.7, NIOO) and recorded using an Acoustic Doppier Velocimeter (ADV; Nortek field version) mounted on a sliding carriage. At night the flume was drained and the plants illuminated.

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EX2erimentai set u2

Experiment Vegetation type Water velo city

Leading Edge Spartina High

Leading Edge Spartina Low

Leading Edge Salicornia High

Leading Edge Salicornia Low

3D Grid Spartina High

3D Grid Spartina Low

3D Grid Salicornia High

3D Grid Salicornia Low

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The Leading Edge experiment examined the first layer of vegetation in the working section of the flume and the 3D grid analysed a three dimensional region of vegetation further into the working section. Data acquired used in this paper originated from the 3D grid and included values for Reynolds stress and turbulence.

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Results

In-situ

study

The tidal curves constructed for the DON and RIZA1 locations (Figure 20.1 & 20.2) on the spring tide (11 August 2002) show that the inflowing tide moves at a faster rate than the receding tide. High tide remained higher for longer at the DON location than the RIZA1 location. The six hour measuring period was not in synchronisation with the six hour tidal period, measurements began 1 h before tidal water reached the DON frames and 1 h30 min before water arrived at the RIZA1 frame.

There is negligible wave motion on the inflowing tide at both locations. However at high tide and on the receding tide waves may clearly be seen as fluctuations of the graph line. Comparisons between waves found at both locations at similar water levels however is not possible as data is missing from the tail end of the DON tidal curve due to the premature onset of the measuring period. Figures 21.1-21.5 thus show only the wave heights for different water heights at the RIZA1 location. Each wave spectrum consists of 120 s of data (480 data points). Due to time constraints no statistical analyses could be applied to look at the significanee of the differences in wave height at the different water levels. Nevertheless there is a clear trend showing that as the water level decreases there is also a decrease in wave motion. The pioneer Spartina vegetation canopy height at the RIZA 1 location was approximately 40 cm. At water levels below canopy height there is a clear reduction in wave height.

·- -

--Ê 200 ~ 1: 150 Cl .ii -: 100 ~ 3: 50 o 0.00 Ê 200 ~ 1: 150 Cl ëij -: 100 Q) iá 3: 50

---Figure 20.1

TIDAL CURVE SPRING TIDE - DON 11 AUG 2002

250

.>

~

/

---.

/

/

60.00 120.00 180.00 Time (min) 240.00 300.00 360.00 Figure 20.2

TIDAL CURVESPRINGTIDE - RlZA111 AUG 2002

250 ~

-...

/

~

/

~ o 0.00 60.00 120.00 180.00 240.00 300.00 360.00 420.00 Time (min) 31

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Figure 21. 1 RlZA1 wave height at 80 cm water height

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3 "[2 i' ~ 0 ~ ·1 .. ·2 ;r:.3 ft A I" ft ft A

.

, • •

" '"

1111'" Allft 111111 "" " nN\1\1' "J .IVI 11 11111111 ,IV 1/\ 1111 .\ 1111.IJ J\ ' \1\1\ 11 I

_'-'" UU'lIVV VUL fnUIIV\J\..JV II:'nlI \/V \t'\1 IUlI I' lAl U\ 'V \j V \ltllO' 1 lAl U.U"\L

, " v V

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.

11 • •

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·5 Tlme(s)

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Figure 21.2

RIZA1 Wave height at 60 cm water height

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3 K 2 i' .~ 0 1!., ~ ·2 ;r: .3 -4 ·5 I I' IIA I\Ar /\ftI1 1\111\ J' .• 1 1\ J\ 1\

"

'1"

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,

_"\1 Vu," IIV ,11 11 \ lol11 IHj V \ ,>..} _{\II. ,,\IV \/'0}

j I_ V \j

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Time (5)

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Figure 21. 3 RlZA1 wave height at 40 cm water height

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_ 3 !2 :E 1 ~ 0 s:.1 !·2 ;r:.3

..

·5 A A I • I

"

ft

.

I A ft n IAAII 1\IU\ 1\ Jh.J\A AlL. AI' JI 11 f\m n AA. I' 111\n nnr f\nl! VI 11

V 11 V VV,,'V' 'V 11\1: 11 r V.II' V v \, lI,l _{,I V V} 1(",

,,\I

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V ' V

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Tlm.(s)

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Figure 21. 4 RlZA1Wave heightat20cm water height

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.A11 11 111\ /\ 1\.1\ nn1\ 11AI\ 1\1\ J\ AA 1\1\1\" l\ 1\f\ I1A 11AIIII 1\ t 11./\ 11 1\ .1\ 11ti J\ I1 VVV\lVVV\l;'''''Y\lWV''V~V\I V\/V ffi"VV''''V\JlI' VVYVVl.<m" \,JV "V \I.

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Tlrra(s)

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Figure 21. 5 RIZA1 Wave height at 10 cm water height

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/\ 1\1\1\11./\1\ I' 1\ 1\ 1\ _" AI\ ... /\/\ "'11./>#\11./1./01\",11." r-:AI\ AA A 1\ 1\ ~ v ~v ... v v",v \I '-'v v "'.n' V vV ~ ";r ... v V V v 00.-v v ... _{" VN} V ...- \I '" 'I,

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·2 -3

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·5 T'm.(a)

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Results Ex-situ study

Wave flume (WL Delft Hydraulies)

Figures 22, 23 & 24 give an overview of all flume data collected and reveal that most wave attenuation was occurring at the 12 cm flume water level with dense vegetation of a similar or superior height. With each set of vegetation treatment (cultivated, field and artificial) results the empty flume data have been added for reference. All percentage values marked on the graph refer to the change in wave height with reference to the 100 % wave value (described as the value obtained at the measuring point before the vegetation). When no vegetation was present in the flume at high water level th ere was no clearly defined damping but at low water there was a more defined downward trend with a total of 30 % wave energy loss over the working section. (The working section, although measuring 300 cm in total, when referred to in the results section defines the distance up to which measurements were made, here 222 cm.). In both cultivated vegetation treatments at high water there was an increase in wave height in the beginning of the vegetation after which in Salicornia only there was some wave attenuation. Wave attenuation for both high and low density at low water appeared to exceed general flume damping. For the tall field Spartina there was c1ear wave attenuation at both high and low water. For the shorter field Spartinathere was no well defined wave attenuation at high water but at low water there was c1ear wave energy loss. The only artificial vegetation (10 cm) to show any clear wave attenuation consisted ofhigh density hard strips at low water. Worth noting is the increase in most frequent wave height in the beginning of the vegetation for high density hard strips at high water as noted in the high water treatments for cultivated vegetation and the 10 cm field Spartina.

In general, standard error for each most frequent wave height was minimal. However the treatments did reveal different initial wave heights at the measuring point before the vegetation, varying from 3.9 to 5.8 cm for high water and 2.3 to 3.7 cm for low water. To investigate wave attenuation effects between vegetation treatments all data were normalised into percentage attenuation graphs with the initial wave height described as 100 %.

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-Figure 23 Empty flume and field vegetation wave attenuation