Nature conservation implications of a Severn tidal barrage – A preliminary assessment of geomorphological change

ARTICLE IN PRESS Journal for Nature Conservation 17 (2009) 183—198

www.elsevier.de/jnc

Nature conservation implications of a Severn tidal barrage – A preliminary assessment of geomorphological change John S. Pethicka,, Roger K.A. Morrisb, David H. Evansb a

17 Highgate, Beverley, East Yorkshire HU17 0DN, UK Natural England, Northminster House, Peterborough PE1 1UA

b

KEYWORDS Morphological evolution; Eastern Schelde; Severn Estuary; Tidal barrage

Summary A tidal barrage across the Severn Estuary (UK) was first proposed in the 1970s and similar ideas have re-emerged in recent years as the pressure for sources of renewable energy increase. Claims that the barrage would deliver ecological and flood defence benefits based on the tidal power project at La Rance in France are examined. This analysis suggests that the range and scale of ecological and geomorphological impacts will be considerably more deleterious than has hitherto been documented. Monitoring of the Eastern Schelde tidal barrage, constructed to reduce flood events in Holland, provides important pointers about the possible effects of a barrage across the Severn Estuary. The published outcomes of detailed monitoring on the Eastern Schelde provide a robust analogue that suggests significant detrimental effects on nature conservation are likely and that some of the functional changes also have important implications for flood defence structures around the estuary and its tributaries. & 2009 Elsevier GmbH. All rights reserved.

Introduction Tidal power has the potential to generate substantial electrical output with greater consistency of supply than existing renewable energy sources such as wind energy; it is therefore very attractive as a new source of renewable energy. Corresponding author.

E-mail addresses: [email protected] (J.S. Pethick), [email protected] (R.K.A. Morris), [email protected] (D.H. Evans).

There are various options for harnessing tidal power but the concept of a tidal barrage across the Severn Estuary is the most prominent proposal in the UK. Options for, and the feasibility of, a Severn tidal barrage were first investigated by a committee chaired by Sir Herman Bondi which reported in 1981 (Bondi 1981). There followed a programme of studies which were, for the time, extremely comprehensive. By the early 1990s, however, the project was abandoned as it was considered to be prohibitively expensive. Concerns about climate change and the need to find renewable resources of

1617-1381/$ - see front matter & 2009 Elsevier GmbH. All rights reserved. doi:10.1016/j.jnc.2009.04.001

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Fig. 1. Approximate location of the Cardiff-Weston barrage option.

electricity have re-vitalised interest in the Severn tidal barrage and this has manifested itself in two separate groups promoting schemes along a line between Lavernock Point (in south Wales) and Brean Down (Somerset, south-west England) (Clark 2007; Kerr 2006; Severn Tidal Power Group 2006) (Fig. 1). This orientation is usually referred to as the Cardiff-Weston scheme. Proponents of a Severn tidal barrage present a very positive case that suggests environmental and flood defence benefits. These claims are contentious however, and there have been ongoing concerns since the early 1990s that they downplay the environmental impacts whilst purported environmental benefits are open to question (e.g. Nature Conservancy Council response in Severn Tidal Power Group report on public consultations 1991).

Application of expert geomorphological analysis This study has been conducted using geomorphological techniques developed and described by the EMPHASYS Consortium (2000) – a project funded by the UK Government’s Department for Environment, Food and Rural Affairs otherwise referred to as Defra. Two key approaches have been applied. Firstly the use of a regime model (Pethick & Lowe

2000), which is designed to predict the form an estuary seeks to attain (i.e. ‘‘most probable state’’). This is accompanied by the application of ‘‘expert geomorphological analysis’’ using relevant analogues usually referred to as ‘‘top down’’ modelling. This modelling takes account of the hydrodynamics and sedimentology that contribute to geomorphological evolution and consequently the overall package of assessment is referred to as geomorphology rather than hydrodynamics and sedimentology.

Choice of analogues and models The tidal power station at La Rance in France is frequently used to support positive environmental messages (Kirby & Retie `re 2007; Kirby & Shaw 2005; Severn Tidal Power Group 2006). This project was constructed long before it was common practice to establish a pre-construction baseline and as a consequence the nature of changes within the estuary cannot be correlated (Prater 2007). Empirical data do not exist for absolute changes as a result of construction at La Rance. Furthermore, La Rance is a ria (Desroy & Retie `re 2004) which has steep sides, was predominantly sandy and had low suspended sediment loads (Kirby & Retie `re 2007). As such, it makes a poor analogue for evaluating

ARTICLE IN PRESS Nature conservation implications of a Severn tidal barrage geomorphological responses in a sediment-laden, coastal plain estuary such as the Severn with wide mudflats and extensive saltmarshes. On the other hand, the storm surge barrage constructed on the Eastern Schelde in The Netherlands between 1983 and 1987 was properly studied and reported. Although the Eastern Schelde barrage was constructed to minimise the impacts of storm surges, its geomorphological implications are not dissimilar to those of a tidal power barrage because the overall design results in reduced tidal range and changes to sediment pathways. Construction was accompanied by detailed pre- and post- project monitoring that has been placed in the published literature. These studies provide an important baseline for predicting the possible impacts of the proposed Severn tidal barrage and need to be given far greater prominence in the debate about its possible implications. The best known alternative example, Annapolis Royal in the Bay of Fundy (Canada), is poorly represented in the published literature but similar responses to the Eastern Schelde do appear to have occurred (Tidmarsh 1984). This paper also takes account of the outputs of a regime model run to establish a first order estimation of the likely response of the Severn Estuary to a barrage orientated along a line between Lavernock Point and Breen Down. This was the same model used in the development of the Severn Proto-Champ report (Haskoning 2004) and subsequently reviewed by ABPmer (2006). The model is based on the relationship between tidal prism and channel cross section area, first proposed by O’Brien (1969) and subsequently developed by a number of authors (see review by HR Wallingford et al. 2006). It does not involve the same level of detail that would be used in traditional hydrodynamic modelling applied to determine changes in flow speeds and directions, but is better-suited to the analysis offered in this paper because it provides a robust first order indication of the way an estuary could be expected to evolve under ideal conditions. Where conditions deviate from ‘‘ideal’’ their implications can then be evaluated to explain the expected response pattern.

Predicted immediate physical changes arising from barrage construction Broad-scale predictions of changes to tidal propagation, water levels and stand times are available for the Severn Tidal Power Group (STPG) design (Kerr 2006; Severn Tidal Power Group 2006)

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but the ‘‘Severn Lake’’ proposals (Clark 2007) are not accompanied by any such predictions. For the purposes of this paper the analysis is confined to the STPG proposals for the Cardiff-Weston scheme. These are outlined in the recent review of options by Black and Veatch (2007):

1. A reduction in tidal heights of approximately 0.5 m immediately upstream of the barrage rising to high water level reductions of 1 m at Avonmouth and 1.5 m at Sharpness. 2. An approximate tidal range reduction from 12 to 5 m at Avonmouth and from 11 to 4.5 m for the area between the barrage and Cardiff. The reduction in range in the inner estuary is due mainly to a rise in the low water elevation which is predicted to rise by 6 m at Avonmouth. 3. A reduction in tidal range is also predicted for the area seaward of the barrage. This reduction is principally due to a fall in high water elevations of approximately 0.5 m for the area within 25 km of the barrage but with a reduction in sea levels extending to Ilfracombe on the Devon coast and Port Eynon on the Gower coast. 4. Reductions in the levels of suspended sediments within the estuary by as much as 85% (Kirby & Shaw 2005). Reduced turbidity is expected to increase opportunities for photosynthesis and biological productivity. 5. High water stand times extended across the tidal cycle to create a head of water that is sufficient to justify release for power generation, depending upon the nature of the tide (floods or neaps).

Whilst the Sustainable Development Commission’s study (Black & Veatch 2007) identifies a number of direct morphological consequences, the reduction in the extent of inter-tidal habitat exposed over the tidal cycle (Table 1) is the most obvious visible impact. This is the issue that has focussed most attention in the past because of its possible consequences for migratory waterfowl that use the Severn Estuary each winter (see Table 2). Table 1. Predicted changes in extent of inter-tidal habitats landward of the Cardiff-Weston Barrage. Tide

Predicted Change in % change in Current inter-tidal inter-tidal inter-tidal inter-tidal area (ha) area (ha) area (ha) area (ha)

Spring 18,898 Neap 9,881

4,469 4,039

14,428 5,842

76.34 59.12

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Interpreting the implications of barrage construction on geomorphological evolution 1. Applying a regime model In order to provide a first order estimate of the probable magnitude of this process of morphological adjustment to the new tidal range, a regime Table 2.

model was applied (Pethick 2007). The lack of any detailed bathymetric surveys of the inter-tidal zone of the Severn Estuary means that accurate measurement of tidal prism is not possible and estimates in the literature vary from 6.8  109 m3 (data underpinning information in JNCC’s ‘‘Estuaries Review’’ (Buck 1993)) to 15  109 m3 (ABPmer 2006), a range of estimates that is also due to

International wildlife interest of the Severn Estuary.

Special Area of Conservation (English Nature & Countryside Council for Wales 2002)

Special Protection Area (English Nature & Countryside Council for Wales 2003)

Ramsar Site (JNCC 1997)

Atlantic Salt Meadows

Annex 1 species: Bewick’s Swan Cygnus columbianus (4.1% of GB and 1.7% of NW European population)

Criterion 1 (contains a representative, rare, or unique example of a natural or near-natural wetland type) – the estuary is hypertidal with the second largest tidal range in the World.

Internationally important populations of regularly occurring migratory species Shelduck Tadorna tadorna 1.2% NW Europe Dunlin Calidris alpine alpine 2.9% East Atlantic Flyway Redshank Tringa tetanus 1.3% East Atlantic Flyway European White-Fronted Goose Anser albifrons albifrons 1% NW Europe

Criterion 3 (populations of plant and/or animal species important for maintaining the biological diversity of a particular biogeographic region) – the estuary supports unusual estuarine communities with reduced diversity and high productivity. Criterion 4 (supports plant and/or animal species at a critical stage in their life cycles, or provides refuge during adverse conditions) – important for migratory fish. Criterion 5 (regularly supports 20,000 or more waterbirds) – supports in excess of 68,000 waterbirds.

Mudflats and sandflats not covered by seawater at low tide Estuaries Sandbanks which are slightly covered by seawater all the time

Reefs

Twaite Shad

An internationally important assemblage of waterfowl 68,026 birds comprising 17,502 wildfowl and 50,542 waders

Sea Lamprey

Nationally important bird populations within an internationally important waterfowl assemblage Wigeon Anser penelope 1.6% GB Teal Anas crecca 2% GB Pintail Anas acuta 2.1% GB Pochard Athya farina 3.8% GB Tufted Duck Athya fuligula 1.5% GB Ringed Plover Charadrius hiaticula 1% GB Grey Plover Pluvialis squatorola 3.7% GB Curlew Numenius arquata 3.4% GB Wimbrel Numenius phaeopus 4.9% GB Spotted Redshank Tringa erythropus 1.5% GB

River Lamprey

 5 year mean peak 1988/89 to 1992/93.

Criterion 6 (supports 1% of the individuals in a population of one species or subspecies of waterbird) – see SPA criteria Criterion 8 (source of food for fishes, spawning ground, nursery and/or migration path on which fish stocks depend) – over 110 species of fish recorded and migration path for at least seven species of migratory fish.

ARTICLE IN PRESS Nature conservation implications of a Severn tidal barrage

A – Early Holocene form B – Partial in-filling C – Equilibrium morphology

Fig. 2. Simplified depiction of estuarine evolution towards dynamic equilibrium: A immediate post-glacial form (early Holocene); B partial in-filling; C Dynamic equilibrium.

differences in definition of the seaward boundary of the estuary. An existing tidal prism value of 6.0  109 m3 was assumed in the model runs for this study and this was assumed to be halved following barrage construction. In view of these initial assumptions, it is emphasised that the model predictions are to be interpreted as indicative values only. The dimensions of an equilibrium or regime estuarine channel are determined by its tidal discharge and associated current velocities. A regime condition is met when, over a geomorphologically significant time period (years/decades), the estuary experiences no net change in sediment volumes. In order to achieve this, there must be a balance, over time, between erosion and accretion processes with current velocities just above the

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critical deposition stress and below the critical erosion stress. Channel cross sectional areas adjust until, again averaged over a significant period of time, this critical velocity and associated bed stress is attained. The proposed reduction in the tidal prism of the Severn would mean that existing channel cross section areas are oversized, reducing tidal current velocities to below the critical depositional stress. Assuming sediment is available, it will accrete, reducing channel dimensions until a new equilibrium is established (see Fig. 2). 2. Outputs from the regime model Model results for pre- and post- barrage conditions are shown in Figs. 3 and 4. Under pre-barrage, that is existing conditions, assuming a tidal prism of 6  109 m3, the regime model shows that the Severn is approximately in regime with a predicted mouth width of 15.7 km compared to the actual width of 15.6 km. There are two major discrepancies between observed and predicted values that coincide with the tributary estuaries of the Usk and Wye. Reduction of the tidal prism by 50% to 3  109 m3, however, results in a model prediction for the mouth width of 12 km, a reduction of 3.6 km. At Sharpness, 55 km upstream of the barrage, estuarine width is predicted to decrease from its existing 2.3 to 1.76 km a decrease of 0.54 km. Similarly, equilibrium channel depth at the mouth is predicted to decrease from 30 to 21 m and at Sharpness from 5.1 to 3.9 m. The model predictions show that adjustment of the estuary to the tidal conditions imposed by the barrage would necessitate a total volumetric decrease in the channel of 5.2  109 m3. Most of this adjustment would take place by sub-tidal deposition since low water is predicted to rise, for example, by 6 m at Avonmouth. 3. Analysis of regime model outputs The process of depositional adjustment of the estuary to the new tidal conditions depends, critically, upon sediment availability. There is considerable controversy in the literature as to whether the Severn Estuary is a closed or open sediment system but most research suggests that inputs are either extremely small or non-existent. Few quantitative estimates are available but Gao and Collins (1992) suggest that between 1.6  106 m3 and 3.1  106 m3 of sediment enters the estuary annually and Velagrakis et al. (2001) estimate that 1.0  106 m3 of this input is of fine grained sediment from fluvial sources, suggesting that between 0.6  106 m3 and 2.1  106 m3 per year is of non-cohesive sediment derived from sources in the Bristol Channel. Assuming that all of this sediment, both fine and coarse fractions, is available for sub-tidal deposition, the outputs of

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Fig. 3. Output of regime model showing the estimated regime width based on the existing tidal prism. This output suggests that the estuary is currently very close to regime. Note: The red line is the predicted High Water Mark (HWMNT) and the blue line is the observed High Water Mark (HMWST). The zone between the two is potential salt marsh. 210000

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Fig. 4. Output of regime model showing the estimated regime width assuming 50% reduction in tidal prism compared to HWMST. (Note that post-barrage HWMST is not shown). This shows the theoretical regime width (red line) that the estuary would seek to attain after barrage construction assuming there was a sufficient sediment supply.

ARTICLE IN PRESS Nature conservation implications of a Severn tidal barrage the regime model suggest that geomorphological adjustment to the new tidal conditions would take a minimum of 1500 years. This time frame is not appreciably altered if it is further assumed that the lower current velocities following barrage construction result in the deposition of all the existing suspended sediment in the estuary as suggested above. Since the volume of this sediment is estimated at 30  106 m3, the reduction in time period for morphological adjustment would be in the order of 10 years. This time frame is similar in many respects to that envisaged for the Eastern Schelde. A similar situation obtains in the Eastern Schelde where Mulder and Louters (1994) suggest that an additional 400–600  106 m3 of sediment will be required to achieve regime; a process which they estimate will take several centuries. In comparative terms, the tidal range decreased by 27% in the Eastern Schelde (4.5 to 3.3 m) compared to the predicted (circa) 50% reduction for the postbarrage Severn. It is also important to note that the Eastern Schelde is two orders of magnitude smaller in surface area than the Severn (3.31  108 m2 compared to 2.43  1010 m2) and therefore the time required for the Severn to return to regime morphology can be expected to be considerably longer. 4. Application of expert geomorphological assessment to erosion and deposition patterns A process of sediment re-distribution can be anticipated as the Severn Estuary adjusts to new regime conditions. The basis for this judgement is provided by evidence on the Eastern Schelde where monitoring has shown that mudflat levels have already dropped by 30 cm (E. van Zanten pers. comm.). The topography of mudflats and sandbars has flattened where wind generated wave action is high, but in the most sheltered locations accretion is occurring. Accretion on the Eastern Schelde is vastly exceeded by erosion and the extent of intertidal is predicted to decline from 11,000 to 1500 ha by the 2070s (E. van Zanten pers. comm.). This pattern of erosion was reported shortly after completion of the Eastern Schelde barrage (Mulder & Louters 1994) but has now reached such proportions that it is a recognised matter of concern (Ministry of Transport, Public Works and Water Management 2007). Similar responses can be expected to happen in the Severn Estuary for the following reasons:

 

The majority of the estuary comprises soft sediments (sands, silts and muds) that are vulnerable to wave erosion. Wind driven wave erosion is a well established

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Photo 1. Wind-driven wave erosion on the inside face of a former sea wall surrounding the realignment site at Tollesbury in Essex. This illustrates the erosive effects of relatively small waves over long time-frames. Photograph Roger Morris/Natural England. Initial HWM

Ebb dominant Initial channel

New HWM

Erosion

Erosion Deposition

Flood dominant

Inter-tidal deposition

Fig. 5. Theoretical development of estuarine crosssection in response to reduction in tidal prism and high water elevation.



process that is countered by extensive armouring of sea walls and eroding shorelines. Evidence of wind-driven wave erosion as a critical factor is provided in Photograph 1 which takes the impact of wind-driven waves in a small tidal enclosure at Tollesbury (Essex, UK) as a relevant analogue.

A schematic representation of these changes is depicted in Fig. 5. The absolute rate of erosion may differ, but it is worth noting that the predicted area of erosion within the Eastern Schelde was between

ARTICLE IN PRESS 190 10–15% of the total estuary (1,000 to 1,500 ha) over a period of 30 years after the storm surge barrage construction (Brinke 1994; Brinke et al. 1994) and there were authors who predicted the complete loss of salt marsh habitat in the absence of remedial measures (Smaal & Nienhuis 1992). As in the Eastern Schelde, however, the volume and rate of sediment redistribution in the Severn would be too small to have any major impact on the time frame for a depositional adjustment to the new tidal conditions. The mechanism for enhanced erosion within the upper inter-tidal zone follows two pathways. Firstly, increased stand times at high water will

Photo 2. Typical block failure resulting from wave energy hitting consolidated clays forming the former sea wall at Tollesbury, Essex. Photograph Roger Morris/ Natural England.

J.S. Pethick et al. lead to increased wind-generated wave action upon the upper inter-tidal with sufficient time for mobilised sediment to be carried down-slope and deposited in deeper, sub-tidal water (Brinke et al. 1994). Since high water elevations are predicted to fall by approximately 0.5 m at the barrage and 1.5 m at Sharpness, much of this erosion will take place at the foot of existing salt marsh cliffs causing accelerated erosion. Erosion is likely to be in the form of block failures, forming a scree of fallen blocks on the upper mudflat and only slowly breaking down to form suspended sediment. This process is illustrated in Photograph 2, which shows block-failure on a former sea wall in Essex caused by wind-driven waves. Importantly, this fine sediment will be deposited in deeper water and will therefore be permanently removed from the system and later re-deposition will not occur. In addition, the claimed positive benefit of reduced turbidity within the impounded area will also involve a reduction in the levels of sediment imported and deposited upon upper inter-tidal habitats and an increase in sediment deposition in deeper waters. It is therefore difficult to reconcile assertions by Kirby and Shaw (2005) who argue that the estuary would assume a profile consistent with mudflat building. Photograph 3 which shows foreshore erosion within the Eastern Schelde clearly illustrates the absence of soft sediment and high saltmarsh cliffs during the erosional phase. The erosional phase identified in this analysis is a temporary one and, provided that sufficient sediment is available, it would be followed by a period of accretion of salt marsh at a lower level than

Photo 3. Foreshore erosion within the Eastern Schelde. This illustrates the loss of fine sediments and rotation of the foreshore to form a cliff in front of former saltmarsh that is no longer subjected to tidal inundation. Photograph Eric van Zanten.

ARTICLE IN PRESS Nature conservation implications of a Severn tidal barrage formerly. This would result in a reduction in channel width in order to attain the new regime conditions, a reduction in width estimated to vary from 3.6 km at the mouth to 0.54 km at Sharpness. Given the lack of available sediment for upper inter-tidal accretion, however, it is unlikely that any such marsh development will occur although initially some vegetation colonisation of the upper mudflats may take place. Instead, as sea levels rise over the next century so these upper inter-tidal surfaces, without continued accretion, will become lower in the tidal frame so that attainment of a regime morphology will become increasingly difficult. Where mudflats and salt marshes are currently extensive the implications are for a gradual reduction in exposed inter-tidal and supra-tidal sediments. However, in more confined situations, especially within the tributaries such as the Usk and Wye, which have deep narrow inter-tidal profiles, the erosion process is likely to lead to significant collapses affecting flood banks and waterside structures. Increased exposure to wind-driven waves can also be expected to result in a process of ‘‘winnowing’’ existing sediments and reducing the fine fraction within mudflats, thus leading to the inter-tidal environment becoming sandier. Evidence from the Eastern Schelde (Brinke et al. 1994) supports this prediction. Thus, where inter-tidal areas are subjected to high levels of wind-driven wave activity, a change in sediment composition may be anticipated and in turn this will have implications for the benthic assemblages that they support. Fine sediments winnowed from the inter-tidal are likely to be the principal source of sediment deposited into the sub-tidal environment and, as discussed above, these are important in the evolution of the sub-tidal environment. Seaward of the barrage, sediment supply will have important bearings upon coastal evolution. Reduced sediment mobility and the creation of a sink for fine sediment upstream of the proposed barrage will lead to shortfalls in the fine sediment supply to Bridgwater Bay since existing sediment supply to this area is understood to be from the inner estuary (ABPmer 2006; Long et al. 2003). As in the Severn, sediment starvation can be expected to affect the ability of salt marshes to develop at a new position within the tidal frame and to keep pace with sea level rise. The reduction in sediment supply is therefore likely to result in a decrease in the extent of the Parrett tidal delta seaward of the proposed barrage and, as a result, will allow greater wave propagation to the upper shore thereby increasing erosion of mudflats and salt marshes.

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Changes in the tidal prism of the Severn Estuary may have significant impacts on the functioning of the Bristol Channel sediment circulation system, seaward of the barrage. This circulation system has been reported (Gao & Collins 1992) to consist of an easterly wave-driven movement of sediment within the nearshore of the south Wales coast to a so-called bed-load parting at Nash Point where sediment is returned westward within the central Bristol Channel by ebb-dominant tidal currents. The reduction in tidal volume in the estuary may affect tidal currents, particularly these dominant ebb currents and hence bedload sediment transport. The resultant reduction in the rate of sediment circulation may lead to a short fall in supply both to sand banks and possibly inter-tidal beaches within the Bristol Channel. These changes to the large scale circulatory system and its repercussions are difficult to predict without detailed modelling but do appear on a priori grounds to represent a major impact of the proposed barrage.

The wildlife asset Contrary to some published assertions about its biological interest, the Severn Estuary does support plant and animal assemblages that are of considerable interest and judged worthy of conservation (English Nature and Countryside Council for Wales 2002; Morris et al. 2005). It is not, as has been portrayed in some documents, ‘‘predominantly barren’’ or ‘‘evolving towards a barren system’’ (Kirby 2006; Kirby & Shaw 2005). If this was true, the rationale for its conservation would be strictly confined to its geomorphological characteristics. The estuary is designated as a Special Protection Area under the EC Birds Directive (EEC 1979) and has been proposed (by the UK Government) as a Special Area of Conservation under the EC Habitats Directive (EEC 1992). Furthermore, it is classified as a Ramsar site in accordance with the Convention on Wetlands of International Importance. The primary interest features are listed in Table 2. This interest is set out clearly in advice by English Nature (now Natural England) and the Countryside Council for Wales to the UK Government’s Department for Environment, Food and Rural Affairs (Defra) and the Welsh Assembly Government (English Nature and Countryside Council for Wales 2002). These international designations are underpinned by national designation as Sites of Special Scientific Interest (SSSI) which recognise additional interest features such as sea grass, Zostera beds and breeding bird assemblages.

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Ecological responses to geomorphological change Proponents of the Severn tidal barrage acknowledge that there will be considerable changes within the estuary (Kirby & Shaw 2005; Severn Tidal Power Group 2006) but focus particular attention upon what they believe would be the positive outcomes. Their assertions are that: 1. There will be increased productivity within the water column as a result of reduced turbidity. 2. There are no grounds for concern about oxygen debt because the system will become more widely influenced by freshwater input and will consequently have improved oxygen carrying capacity. 3. Increased primary productivity will be followed by increased biomass within bethic assemblages. 4. Benthic assemblages will become more species rich and will replace existing assemblages of restricted species diversity. 5. Increased bethic biomass will offset the loss of mudflat extent and as a consequence the carrying capacity for birds will not be as adversely affected as might otherwise be expected. Impacts upon saltmarshes: The major predicted and recognised impacts involve a considerable reduction in extent of mudflats and a significant reduction in the extent of existing upper saltmarsh through lack of inundation with saline waters. At the moment there are around 1520 ha of saltmarsh in the Severn Estuary and Bridgewater Bay with 1130 ha upstream of the Cardiff-Weston scheme (Dargie 1999). This extent of saltmarsh (3.5%) of the Great Britain resource (Mitchel et al. 1981) is important in a southern English context because it is comparatively stable and not undergoing the massive levels of erosion shown to be occurring in eastern England where saltmarsh is receding at a rate of 1% per annum (University of Newcastle 2000). Evidence from the Eastern Schelde (Jong de & Pluijm 1994) indicates that where saline inundation ceases, the upper saltmarsh sward will undergo quite rapid changes involving the loss of halophytes and invasion by ruderal species such as Atriplex prostate, Suaeda maritima and Aster tripolium. Upon completion of construction and restoration of the tidal range to its new position, evidence for migration of halophytes down the foreshore was apparent. Jong de and Pluijm’s work (1994) showed that species composition within existing cordgrass

J.S. Pethick et al. Spartina anglica swards also changed and that species displaced from upper saltmarshes partially or wholly displaced the Spartina spp. Thus, whilst it may be expected that there will be a short-term decline in the extent of saltmarsh habitats, colonisation further down the shore will occur and this may partially offset losses. There are two complications to this hypothesis, however. Firstly, the extent of freshwater influence is expected to move downstream by between 5 and 30 km (Black & Veatch 2007). This can be expected to displace existing saltmarsh vegetation and replace it with common reed Phragmites australis, thus permanently reducing the potential for saltmarsh development and a reduction in the likely post-construction extent. The second factor is that greater exposure of the foreshore to wind-driven waves is likely to accelerate saltmarsh erosion and without the same levels of suspended sediment the extent of inter-tidal habitat can be expected to narrow and drop lower in the tidal frame. This will further limit the possibilities for saltmarsh recolonisation which relies upon sediment deposition for establishment of pioneer species and in order to keep pace with sea level rise. The extreme tidal range and the sediment regime of the Severn Estuary also mean that it exhibits unusual characteristics in saltmarsh evolution. Thus, there are places where saltmarshes undergo episodes of erosion followed by deposition at a lower point in the tidal frame to create terraces (Allen 1992) such as at Stert Point, Aust and Oldbury. Elsewhere, localised failures and slippages occur, as illustrated by Jacobs (2006). The two processes are very different and there are differences in biological responses to changes in the tidal range. Where Spartina spp. occurs, it frequently exists as a monoculture without significant areas of herb-rich swards close by. Colonisation of this sward by upper saltmarsh species will therefore be dictated by the availability of propagules, which will have already been depleted and as a consequence the possible re-colonisation of Spartina spp. swards, is less certain than the Dutch data suggests. Where saltmarsh terracing occurs, enhanced wave erosion is likely to be particularly profound as these are areas where erosion has been an ongoing feature of morphological evolution. This will ultimately lead to a new environment in which a shallower, flatter foreshore is achieved. The prospects for the less common or unusual upper saltmarsh plants such as slender hair’s ear Bupleurum tenuissimum, bulbous foxtail Alopecurus bulbosus, sea clover Trifolium squamosum, and corn parsley Petroselinum segetum are therefore more

ARTICLE IN PRESS Nature conservation implications of a Severn tidal barrage questionable as the conditions they favour are likely to be severely restricted. Short-term reductions in the extent of saltmarsh and long-term re-establishment of saltmarsh lower down the existing inter-tidal profile will combine with predicted changes in tidal range to substantially limit the extent of inter-tidal mudflats. The data presented in Table 1, which is derived from Table 6.7(2) in Black and Veatch’s report to the Sustainable Development Commission (Black & Veatch 2007), show that the extent of inter-tidal habitat is likely to be reduced by around 76%. This is a very significant change in relation to the likely carrying capacity for waterbirds and has rightly been the focus of attention in many presentations. Optimistic forecasts presented (Kirby & Shaw 2005) in defence of the barrage are highly unlikely to be realised because the duration of foreshore exposure is also expected to be reduced as it will be necessary to hold water within the upstream ‘‘lake’’ for several hours until a sufficient head of water has been created. Impacts upon migratory waterfowl: Responses of waterbird populations to habitat loss have been given considerable attention in recent years and much emphasis has been placed on the creation of compensatory habitat (Morris & Gibson 2007) where it is lost to port development. This precautionary approach has been adopted because of evidence that displaced waterfowl have been shown to lose condition and suffer increased mortality (Burton et al. 2006) and that bird numbers in adjacent areas do not automatically rise and maintain any gains in numbers. Thus, displacement is likely to lead, over time, to an overall reduction in water bird populations (Clark 2006). Barrage proponents have placed great emphasis upon an argument constructed around the possibility that reduced tidal energy within the Severn Estuary will have a number of biological outcomes that can be regarded as benefits. In particular, it has been argued that greater productivity within the water column will lead to higher biomass within remaining mudflats and that this increased biomass will be sufficient to support displaced birds (Kirby & Shaw 2005) or indeed to actually increase the numbers of birds (both in terms of species and actual numbers) (Severn Tidal Power Group 2006). This argument is reinforced by the contention that the Severn Estuary does not compare well against other UK estuaries for overall bird densities (Kirby et al. 2004) and that the Severn Estuary is ‘‘evolving towards a barren estuary’’ (Kirby & Shaw 2005). Too much faith is being placed on increased biomass within mudflats as a result of improved

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productivity in the water column and weaker tidal influences. Evidence from the Eastern Schelde indicates that sediment winnowing can be expected. A consequence of winnowing is the loss of fine sediments and increasing dominance of coarser grain sizes. This is important because the biomass is reduced in sandier substrates and the nature of the food resource for particular birds may also be depleted. For the Severn Estuary this is especially important because both shelduck Tadorna tadorna and dunlin Calidris alpina depend upon sediment regimes dominated by silts and clays rather than sand. Furthermore, it should also be noted that the operation of the barrage depends upon storage of water behind the barrage until there is a sufficient head of water to gain maximum output from the turbines. This must inevitably reduce the time when inter-tidal habitats are exposed and available for feeding birds. The impact of this additional complication has not been quantified but is an important cumulative effect that would reduce foraging times and which would also have to be offset by mudflat productivity. The importance of particle size to bird usage is clearly demonstrated by McCulloch and Clark (1992) who show that dunlin density rises in line with the silt and clay content on foreshores. Such differences are to be expected amongst other birds such as shelduck and there is strong evidence from the Humber Estuary to show that waterbird distribution as a whole is a function of the geomorphology of an estuary (Allen et al. 2000). This appears to be the case in the Severn Estuary (Clark & Prys-Jones 1994) where waterfowl distribution is concentrated in areas of finer sediments and where the elevation of the mudflats gives longer feeding times. In the case of the Humber, waders tend to concentrate in the outer estuary where the mudflats and saltmarshes are most extensive. Wildfowl occur upstream at the fresher end where mudflats are narrower and grazed saltmarshes are more extensive. This suggests that regardless of any gains in productivity, changes in particle size will significantly affect the ability of remaining mudflats to support waterfowl, especially dunlin, shelduck and redshank Tringa totanus, all of which currently occur in internationally important numbers. The link to geomorphological change, and the dangers of such changes, is reinforced by a series of studies (Austin et al. 1996; Holloway et al. 1996; Rehfish et al. 2000; Yates et al. 1996) that have been used to predict changes in waterbird numbers in relation to barrages. These geomorphological links have been used to question whether a postbarrage morphology might be engineered to deliver

ARTICLE IN PRESS 194 suitable sediment conditions (Prater 2007 quoting Clark & Prys-Jones 1994). This concept is highly questionable and would demand long-term engineering to remove wave-wash and winnowing; it is therefore an unrealistic solution to bird displacement. Furthermore, sediment stripping during storm events is likely to lead to permanent exposure of harder consolidated materials that are less suitable for colonisation by benthic organisms, rather than temporary changes as currently occurs (Ferns 1983). Sediment mobilised by wave action will settle in deeper water contributing to the change in bedform from deep channels to a shallower, flatter form with a layer of fine sediment smothering many existing surfaces. These changes are behind predicted improvements in the richness of benthic organisms (Kirby & Shaw 2005). This change in subtidal habitats will lead to smothering of sandbanks and also of reef communities associated with exposed rock in the existing lower inter-tidal and in the sub-tidal channels. These reef communities are by no means as species rich as some clear water estuaries such as those of south-west England, but they are of particular interest because they include structures created by the reef-building worm Sabellaria alveolata. These reefs are unusual because Sabellaria alveolata is normally found in inter-tidal environments and its occurrence in sub-tidal conditions is extremely unusual (Morris et al. 2005). In the absence of sufficient suspended sediment and a shift towards sediment deposition in former higher energy environments, this species and its associated assemblage is likely to be lost. Communities of benthic organisms associated with sub-tidal and inter-tidal sandbanks are also expected to be lost because the change in tidal range is likely to lead to less or little exposure of sandbanks, and over time the sandbanks will be smothered with finer sediments. This may lead to changes in the suitability of the estuary for commercially valuable fish species such as plaice Pleuronectes platessa, but possible changes in water quality and salinity may also be significant and potentially detrimental. Increased productivity in the water column is expected because conditions behind the barrage are expected to change significantly. The reduction in tidal range means that scope for sediment mobilisation is reduced, and this in turn means that when sediment settles in deeper water it cannot be re-mobilised. Therefore for much of the year turbidity will decrease and the suppressant effect of sediment in the water column will be reduced, thus allowing phytoplankton populations to increase significantly. This has been promoted as

J.S. Pethick et al. an improvement in biological terms (Kirby & Shaw 2005; Severn Tidal Power Group 2006). This assertion depends upon the biological objectives set for the estuary and upon the degree to which confidence can be placed on predictions of water quality outcomes; it requires detailed consideration by relevant specialists. Water clarity has also been raised as a possible benefit for diving ducks (Kirby & Shaw 2005), with the assertion that their feeding behaviour is sightdependent. On the Eastern Schelde the numbers of cormorant Phalacrocorax carbo, great crested grebe Podiceps cristatus and goldeneye Bucephala clangula have increased in the parts of the estuary that have been turned from a saline tidal habitat to fresh water impoundments (Smaal & Nienhuis 1992). These changes may, however, relate as much to changing salinity and fish ecology as to water clarity and therefore cannot be used to infer that the same results will arrive in the more saline environment predicted for the Severn Estuary. The feeding strategies of many diving ducks such as common scoter Melanitta nigra and eider Somateria mollissima are unlikely to be sight-dependent, however. Over-wintering populations on the east coast of England and Liverpool Bay where turbidity is also extremely high are evidence that turbidity is not an issue but water depth and prey densities are important (Kaiser et al. 2006). These birds have particularly tactile bills which would allow them to detect submerged food such as bivalve molluscs by touch; increased sub-tidal productivity may improve the attractiveness of the Severn Estuary if prey availability increases, regardless of water clarity. The extent to which any water clarity improvements might also benefit pisciverous species such as red-throated divers Gavia stellata is also debatable, as over-wintering red-throated divers are known to concentrate in internationally important numbers in the outer Thames Estuary which is relatively turbid.

Loss of geological conservation assets Whilst most of the emphasis of debate about the Severn tidal barrage has focussed upon the likely ecological and biodiversity implications, little effort has been made to quantify the impact upon geological interest of the Severn Estuary. Previous studies addressing the impacts of a barrage reviewed several scheme options (McKirdy 1982) and included sites in the hinterland of the schemes. Within the Brean Down to Lavernock orientation, there are nine or ten sites (depending of the exact

ARTICLE IN PRESS Nature conservation implications of a Severn tidal barrage Table 3.

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Geological assets that are vulnerable to the construction of a Severn tidal barrage.

SSSI

Geological interest

Anticipated impact

Flat Holm

Dinantian stratigraphy Carboniferous Hetangian – Sinemurian stratigraphy Jurassic

Partial submergence if the barrage is to the west of Flat Holm. Partial submergence of the extreme eastern end of the site. Potentially threatened by engineering at the landfall of the barrage. Partial submergence-reduced period of access during tidal cycle. Potentially threatened by engineering at the landfall of the barrage. Partial or complete submergence-reduced period of access during tidal cycle.

Lavernock to St Mary’s Well Bay Penarth Coast

Rhaetian stratigraphy Triassic

Lydney Cliff

Non-marine Devonian stratigraphy Silurian – Devonian fish Devonian Silurian Rhaetian stratigraphy Triassic Hettangian – Sinemurian stratigraphy (this is a potential Geological Conservation Review site). Jurassic Ludlow stratigraphy Silurian – Devonian fish Devonian Silurian Rhaetian stratigraphy Mesozoic fish Mesozoic reptiles Fossil insects Triassic Westphalian Variscan structures Non-marine Devonian Silurian Devonian chordata Carboniferous Devonian Quaternary/Pleistocene of SW England Pleistocene Carboniferous/Permian Igneous Carboniferous Carboniferous/Permian Igneous Carboniferous Quaternary/Pleistocene of SW England Pleistocene

Garden Cliff Hock Cliff

Purton Passage

Aust Cliff

Portishead-Black Nore

Middle Hope Middle Hope Spring Cove Brean Down

Access impeded Access impeded?

Partial or complete submergence-reduced period of access during tidal cycle.

Probably little change to the interest features.

Partial submergence – reduced period of access during tidal cycle.

Well above high-water – no impact Partial submergence – reduced period of access during tidal cycle. Partial submergence – reduced period of access during tidal cycle. Above MHW – no impact in relation to water-level changes, but potentially threatened by engineering as at the landfall of the barrage.

 Requires survey to fully assess impact.

position of the barrage) that are designated as geological SSSI that lie to the east of the Severn tidal barrage. Nearly all will be affected by changes to the tidal range and sedimentation patterns. Two of the sites lie at the land-fall of the barrage and may therefore also be impacted by engineering works. An attempt to provide a detailed assessment of these impacts would be premature as a more detailed survey of each site will be required. Table 3 provides an indication of the anticipated impacts. The nature of the interest features

on many of these sites is such that significant losses cannot be readily mitigated or compensated through the creation of new exposures elsewhere.

Conclusions In this analysis it has been argued that the use of La Rance as the model for predicting effects on the Severn Estuary is unhelpful. Better correlation

ARTICLE IN PRESS 196 is likely to occur in relation to the Eastern Schelde, the morphology of which is closer to that of the Severn. Until now the arguments have largely centred upon biological responses to immediate physical changes, but our analysis suggests that much more emphasis needs to be placed upon the likely geomorphological evolution if a barrage was to be constructed. ‘‘Expert geomorphological analysis’’ has raised a number of important issues that dispel many of the positive arguments that proponents portray. Increased erosion of foreshores has not received sufficient attention and it has potentially serious implications for coastal structures including flood defences. The implications for the large-scale circulation of sediment in the Bristol Channel and its associated features, such as sand banks and coastal beaches, is another major issue demanding careful attention. A barrage may only prove to be a short-term flood management benefit and may ultimately demand increased investment to avoid significant loss of coastal infrastructure as a result of erosion of foreshore and collapse of flood defence infrastructure. Changes will occur, and the magnitude of those changes will be considerable. Loss of existing biological interest can be expected to be widespread, and there are no grounds for placing faith in the argument that reduced turbidity will benefit wildlife in the long-term. This change will affect all of the existing wildlife interest, and involves the loss of a highly dynamic and unusual system. Whilst proponents of the barrage may argue that this is a wildlife benefit they fail to recognise the importance of range and variation, effectively arguing that conservation objectives should favour a shift towards homogeneity at the expense of existing heterogeneity. The impacts will undoubtedly lead to greater homogeneity but this does not constitute a sound argument in favour of a barrage.

Acknowledgements This paper has drawn upon the work of a variety of people and has benefited from helpful assistance from several Natural England colleagues, especially Chris Green who prepared the illustration of the location of the Cardiff-Weston Scheme, and Mark Wills who supplied important details concerning local features that may be affected by the proposed barrage. The development of a refined version of the text has been greatly assisted by constructive comments and useful suggestions

J.S. Pethick et al. from Rob Cooke (Natural England Policy Team), Tony Prater (RSPB) and Dr Maggie Hill (Countryside Council for Wales). Thanks to Eric Van Zanten, Leo Adriaanse and Jon Coosen (Rijkswaterstaat), for providing an up-to-date insight into the ongoing evolution of the Eastern Schelde, and the unnamed reviewers who provided helpful critique of an earlier version of this text. We also thank Eric Van Zanten for use of his photograph of foreshore erosion on the Eastern Schelde.

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