AHTI LTD & MARINE SPACE LTD

RIVER HAMBLE SOFT SEDIMENT HABITAT RETENTION

Feasibility Study

DATE OF ISSUE 12 August 2016

RIVER HAMBLE SOFT SE DIMENT HABITAT RETEN TION FEASIBILITY STUDY

PREPARED FOR PREPARED BY

Hampshire County Council AHTI Ltd. River Hamble Harbour Authority Unit 16, Highcroft Industrial Estate Harbour Master’s Office Enterprise Road Shore Road Waterlooville Warsash Portsmouth Hants. Hants. SO31 9FR PO8 0BT http://www3.hants.gov.uk/hambleharbour.htm www.ahtigroup.co.uk

Phone: 01489 576387 Phone: 07765 384272

Email: [email protected] Email: [email protected]

DOCUMENT RELEASE AND AUTHORISATION RECORD Job Number AHTI_J2015_004 Client Name River Hamble Harbour Authority Client Contact Alison Fowler Date 12/08/2016 Status Date Issued Final 12/08/2016 This Version Authorised By Name Date Signature

Dr Simon Bray Authors Dr Ilse Steyl 12/08/2016 Dr Dafydd Lloyd Jones

Technical Reviewer Dr Ilse Steyl 12/08/2016

Quality Checker Mrs Joanna Gillman 12/08/2016

COPYRIGHT

The copyright and intellectual property (IP) rights in this document are the property of the client, River Hamble Harbour Authority (RHHA) and of the consultants AHTI Ltd & MarineSpace Ltd with express agreement from RHHA. This document shall not be copied, nor IP infringed, without the express consent of the client or its consultants.

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Contents

ABBREVIATIONS...... viii EXECUTIVE SUMMARY...... ix 1. INTRODUCTION AND BACKGROUND...... 1 1.1 Project Context ...... 1 1.2 Solent and River Hamble Overview ...... 2 1.2.1 Solent ...... 2 1.2.2 River Hamble Background ...... 6 1.3 Project Questions ...... 10 2. SALTMARSH EXTENT AND HABITAT QUALITY ...... 12 2.1 Data Sources ...... 12 2.2 Change Analysis ...... 12 2.2.1 Hacketts Marsh ...... 17 2.2.2 Lincegrove Marsh ...... 21 2.2.3 Mercury Marsh ...... 24 2.2.4 Satchell Marsh ...... 27 2.2.5 Little Marsh ...... 30 2.2.6 Hamble Common Marsh ...... 33 2.2.7 Hook Marsh ...... 36 2.2.8 Bunny Meadows South Marsh ...... 38 2.2.9 Bunny Meadows North Marsh ...... 41 2.2.10 Crableck Marsh ...... 44 2.2.11 Universal Marsh...... 47 2.2.12 Swanwick Marsh ...... 50 2.3 Habitat Quality ...... 53 2.4 Summary ...... 56 3. FACTORS CONTRIBUTING TO AREA AND QUALITY REDUCTION ...... 57 3.1. Introduction ...... 57 3.2 Reasons for saltmarsh decline ...... 57 3.3 Impact of dredging on sediment draw-down ...... 64 3.3.1 A conceptual model ...... 64 3.3.2 How do estuaries respond to dredging? ...... 65 3.4 Potential for reduced sediment supply ...... 67

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4. SEDIMENT DEPOSITION ...... 72 4.1 Introduction ...... 72 4.2 Sediment composition and quantities from dredging ...... 72 4.3 Suitability of River Hamble dredge material for reuse ...... 74 4.4 Summary ...... 78 5. SUITABILITY OF HAMBLE ESTUARY FOR HABITAT IMPROVEMENT ...... 79 5.1 Introduction ...... 79 5.2 Suitability of intertidal areas for reuse of dredge spoil ...... 79 5.2.2 Remarks on MCA ...... 122 5.3 Potential for other sediment management techniques ...... 122 5.3.1 Flow Modification ...... 122 5.3.2 Wave energy reduction ...... 124 5.3.3 Bunds to hold material on the foreshore...... 125 5.3.4 Vegetation Planting ...... 127 5.3.5 Scheme Risks ...... 128 5.4 Potential environmental benefits of sediment management schemes ...... 131 5.5 Potential adverse environmental impacts ...... 134 5.5.1 Environmental Effects Associated with Deposition of Dredged Material ...... 135 5.5.2 Direct Environmental Effects ...... 136 5.5.3 Indirect Environmental Effects ...... 137 5.5.4 Long-term Benefits ...... 138 5.6 Potential for increased accretion from disposal of dredge spoil ...... 139 5.6.1 Foreshore recharge ...... 139 5.6.2 Direct pumping or sediment placement ...... 140 6. POTENTIAL SOURCES OF FUNDING ...... 149 6.1 Introduction ...... 149 6.2 Esmée Fairbairn Foundation ...... 149 6.3 The Landfill Communities Fund ...... 150 6.3.1 The conservation of a natural habitat or of a species in its natural habitat ...... 150 6.3.2 Heritage Lottery Fund – Land and Natural Heritage – Landscape Partnerships ...... 150 6.4 UK Government - Knowledge Transfer Partnerships with local Universities ...... 151 6.5 INTERREG V ...... 152 6.6 Operator groups ...... 152 6.7 Other Sources ...... 153

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6.8 Indicative Costs ...... 154 6.9 Understanding and Value ...... 154 6.10 Costs ...... 155 7. DISCUSSION ...... 157 8. CONCLUSIONS ...... 161 REFERENCES ...... 163

Figures

Figure 1.1: Solent and Hamble for context ...... 3 Figure 1.2: Designations along the lower River Hamble Estuary ...... 4 Figure 2.1: Saltmarshes in Hamble Estuary ...... 14 Figure 2.2: Decline in total saltmarsh area along Hamble Estuary between 1870 and 2014 ...... 17 Figure 2.3: Change in area of Hacketts marsh between 1870 and 2014 ...... 19 Figure 2.4: Volumetric change at Hacketts marsh between 2007 and 2014 ...... 20 Figure 2.5: Change in area of Lincegrove Marsh between 1870 and 2014...... 22 Figure 2.6: Volumetric change at Lincegrove Marsh between 2007 and 2014 ...... 23 Figure 2.7: Change in area of Mercury Marsh between 1870 and 2014 ...... 25 Figure 2.8: Volumetric change at Mercury Marsh between 2007 and 2014 ...... 26 Figure 2.9: Change in area of Satchell Marsh between 1870 and 2014 ...... 28 Figure 2.10: Volumetric change at Satchell Marsh between 2007 and 2014 ...... 29 Figure 2.11: Change in area of Little Marsh between 1870 and 2014 ...... 31 Figure 2.12: Volumetric change at Little Marsh between 2007 and 2014 ...... 32 Figure 2.13: Change in area of Hamble Common Marsh between 1870 and 2014 ...... 34 Figure 2.14: Volumetric change at Hamble Common Marsh between 2007 and 2014...... 35 Figure 2.15: Saltmarsh extent in 1946 at Hook ...... 36 Figure 2.16: Volumetric change at Hook between 2007 and 2014 ...... 37 Figure 2.17: Change in area of Bunny Meadows (South) between 1870 and 2014 ...... 39 Figure 2.18: Volumetric change at Bunny Meadows (South) between 2007 and 2014 ...... 40 Figure 2.19: Change in area of Bunny Meadows (North) between 1870 and 2014 ...... 42 Figure 2.20: Volumetric change at Bunny Meadows (North) between 2007 and 2014 ...... 43 Figure 2.21: Change in area of Crableck Marsh between 1870 and 2014 ...... 45 Figure 2.22: Volumetric change at Crableck Marsh between 2007 and 2014 ...... 46 Figure 2.23: Change in area of Universal Marsh between 1870 and 2014 ...... 48 Figure 2.24: Volumetric change at Universal Marsh between 2007 and 2014 ...... 49 Figure 2.25: Change in area of Swanwick Marsh between 1870 and 2014 ...... 51 Figure 2.26: Volumetric change at Swanwick Marsh between 2007 and 2014 ...... 52 Figure 3.1: Marina developments on lower River Hamble ...... 59 Figure 3.2: Factors affecting estuary margin habitats ...... 62 Figure 3.3: Average annual volume of sediment required from marine exchange to balance the Hamble Estuary sediment budget 1894 to 2008 ...... 68

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Figure 3.4: The deviation in the components of the Hamble Estuary sediment budget between 1783 and 2008 ...... 69 Figure 4.1: Sediment samples from River Hamble marinas, analysed for Williams et al. (2010) and plotted on a Mud-Sand-Gravel tri-plot ...... 73 Figure 5.1: Historic change in saltmarsh area at Hacketts Marsh ...... 90 Figure 5.2: Buffer zones for potential saltmarsh ...... 91 Figure 5.3: Historic change in saltmarsh area at Lincegrove Marsh ...... 92 Figure 5.4: Existing Lincegrove saltmarsh and potential saltmarsh buffers ...... 94 Figure 5.5: Historic change in saltmarsh area at Mercury Marsh ...... 95 Figure 5.6: Existing Mercury Marsh saltmarsh and potential saltmarsh buffers ...... 96 Figure 5.7: Historic change in saltmarsh area at Satchell Marsh ...... 98 Figure 5.8: Existing Satchell Marsh saltmarsh and potential saltmarsh buffers ...... 99 Figure 5.9: Historic change in saltmarsh area at Little Marsh ...... 101 Figure 5.10: Existing Little Marsh saltmarsh and potential saltmarsh buffers ...... 102 Figure 5.11: Historic change in saltmarsh area at Hamble Common Marsh ...... 103 Figure 5.12: Existing Hamble Common saltmarsh and potential saltmarsh buffers ...... 105 Figure 5.13: Existing Hook feature and potential saltmarsh buffers ...... 107 Figure 5.14: Historic change in saltmarsh area at Bunny Meadows Marsh (South) ...... 108 Figure 5.15: Existing Bunny Meadows (South) saltmarsh and potential saltmarsh buffers ...... 109 Figure 5.16: Historic change in saltmarsh area at Bunny Meadows Marsh (North) ...... 111 Figure 5.17: Existing Bunny Meadows (North) saltmarsh and potential saltmarsh buffers ...... 112 Figure 5.18: Historic change in saltmarsh area at Crableck Marsh ...... 114 Figure 5.19: Existing Crableck Marsh saltmarsh and potential saltmarsh buffers ...... 115 Figure 5.20: Historic change in saltmarsh area at Universal Marsh ...... 117 Figure 5.21: Existing Universal Marsh saltmarsh and potential saltmarsh buffers...... 118 Figure 5.22: Historic change in saltmarsh area at Swanwick Marsh ...... 119 Figure 5.23: Existing Swanwick Marsh saltmarsh and potential saltmarsh buffers ...... 121 Figure 5.24: Marine ecosystem services as defined by Potts et al. (2014) ...... 132 Figure 5.25: Typical spring and neap plume dispersal envelope for the lower Hamble ...... 147

Tables

Table 1.1: Tasks as amalgamated from above list and allocation ...... 10 Table 2.1: Data sources for calculating saltmarsh change ...... 12 Table 2.2: Tidal data for secondary ports in Hamble Estuary relative to Ordnance Datum ...... 15 Table 2.3: Lower Hamble Estuary Saltmarsh change over time ...... 16 Table 2.4: Error bands for different data sources ...... 16 Table 3.1: Major factors associated with saltmarsh decline ...... 60 Table 3.2: Hamble Estuary Sediment Budget 1783-2008 - volumes in x103m3 (Blue is an input to the sediment budget and pink is an output) ...... 70 Table 4.1: Average siltation rates in River Hamble marinas 1986-2010 ...... 74 Table 4.2: Metal, TBT and DBT analyses for sediments dredged from River Hamble marinas ...... 77 Table 5.1: River Hamble marshes and potentially relevant affecting factors ...... 84

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Table 5.2: GIS based Multi Criteria Analysis for River Hamble marshes ...... 89 Table 5.3: Sediment volumes required to extend current Hacketts Marsh saltmarsh extent by 5 m, 20 m and 50 m buffers ...... 91 Table 5.4: Sediment volumes required to extend current Lincegrove Marsh saltmarsh extent by 5 m, 20 m and 50 m buffers ...... 93 Table 5.5: Sediment volumes required to extend current Mercury Marsh saltmarsh extent by 5 m, 20 m and 50 m buffers ...... 96 Table 5.6: Sediment volumes required to extend current Satchell Marsh saltmarsh extent by 5 m, 20 m and 50 m buffers ...... 99 Table 5.7: Sediment volumes required to extend current Little Marsh saltmarsh extent by 5 m, 20 m and 50 m buffers ...... 102 Table 5.8: Sediment volumes required to extend current Hamble Common Marsh saltmarsh extent by 5 m, 20 m and 50 m buffers...... 104 Table 5.9: Sediment volumes required to extend current Hook saltmarsh extent by 5 m, 20 m and 50 m buffers ...... 106 Table 5.10: Sediment volumes required to extend current Bunny Meadows Marsh (South) saltmarsh extent by 5 m, 20 m and 50 m buffers ...... 109 Table 5.11: Sediment volumes required to extend current Bunny Meadows Marsh (North) saltmarsh extent by 5 m, 20 m and 50 m buffers ...... 112 Table 5.12: Sediment volumes required to extend current Crableck Marsh saltmarsh extent by 5 m, 20 m and 50 m buffers ...... 115 Table 5.13: Sediment volumes required to extend current Universal Marsh saltmarsh extent by 5 m, 20 m and 50 m buffers ...... 118 Table 5.14: Sediment volumes required to extend current Swanwick Marsh saltmarsh extent by 5 m, 20 m and 50 m buffers ...... 120 Table 5.15: Site assessment in terms of sediment retention aspirations ...... 129 Table 5.16: Time–space matrix of potential environmental effects associated with dredged material placement ...... 135 Table 5.17: Mean annual siltation rates in River Hamble marinas 1986-2010 ...... 148 Table 6.1: Suggested costs for services/items for supporting studies and material/equipment ..... 156

Plates

Plate 1.1: Bursledon Pool, “Moodys” Marsh, and Lands End, circa 1959 ...... 8 Plate 1.2: “Moodys”/Premier Marina 2013 with 1870 saltmarsh extents as baseline reference ...... 9 Plate 2.1: Spartina spp. colonising top edge of remnant jetty between 2007 and 2014 ...... 50 Plate 5.1: Brushwood and fence structure to enhance sediment retention, Lymington Marshes .. 123 Plate 5.2: Brushwood and fence structure showing enhanced sediment retention, Lymington Marshes ...... 124 Plate 5.3: Clay bund at Wallasea Island ...... 125 Plate 5.4: Development of salt marsh at Wallasea Island ...... 126 Plate 5.5: Gravel bund retaining finer grained sediment on the foreshore at Shotley ...... 126 Plate 5.6: Little Marsh with semi permeable bund to left (south) of marsh – 2006 ...... 127

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Plate 5.7: Floating pipeline carrying fine grained dredge material to recharge site ...... 141 Plate 5.8: Fine grained material being pumped directly onto intertidal area ...... 142 Plate 5.9: Construction of sediment retaining polder ...... 142 Plate 5.10: Sediment retaining polder following sediment placement ...... 143 Plate 5.11: Sedimentation fence following sediment placement ...... 143

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ABBREVIATIONS

AA - Appropriate Assessment CCO - Channel Coast Observatory CD - Chart Datum Cefas - Centre for Environment, Fisheries and Aquaculture Science CHaMP - Solent Coastal Habitats Management Plan EA - Environment Agency EIA - Environmental Impact Assessment EMS - European Marine Site EU - European Union GIS - Geographic Information System HAT - Highest Astronomical Tide HCC - Hampshire County Council JNCC - Joint Nature Conservation Committee LAT - Lowest Astronomical Tide LiDAR - Light Detection and Ranging MCA - Multi Criteria Analysis MCZ - Marine Conservation Zones MHW - Mean High Water MHWN - Mean High water Neap MHWS - Mean High Water Spring MLW - Mean Low Water MLWN - Mean Low Water Neap MLWS - Mean Low Water Spring MMO - Marine Management Organisation MPA - Marine Protected Area MSL - Mean Sea Level NMR - National Monuments Record Centre OD - Ordnance Datum PAH - Polycyclic Aromatic Hydrocarbon PCB - Polychlorinated Biphenyls PSD - Particle Size Distribution RHHA - River Hamble Harbour Authority SAC - Special Area of Conservation SCOPAC - Standing Conference on Problems Associated with the Coastline SINC - Site of Importance to National Conservation SMP - Shoreline Management Plan SPA - Special Protection Area SSSI - Sites of Special Scientific Interest TBT - Tributytltin WFD - Water Framework Directive

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EXECUTIVE SUMMARY

The River Hamble Harbour Authority (RHHA) commissioned research to investigate spatial and temporal change, possible causal factors and opportunities for management or restoration of saltmarsh in the lower River Hamble (below the A27 road bridge) on the UK south coast. Following a review of literature which places saltmarsh decline and related factors into global and regional context, the research was approached through a series of pre-determined questions designed to address concerns and opportunities related to the marshes.

The questions related to the extent of saltmarsh decline to date and in the future. Factors potentially relating to the decline or growth were considered with specific attention to any role river dredging may have. Dredged sediment types and potential suitability for beneficial re-use in saltmarsh restoration schemes were identified, as well as which of the river’s saltmarshes may be suitable. In addition, consideration of impacts and benefits, other sediment management approaches and the possible costs of such schemes and potential funding sources were considered.

Geographical Information System analysis of aerial imagery and aerial LiDAR data (obtained from the Channel Coastal Observatory and the Environment Agency) identified the significant decline in the lower River Hamble (below the A27 road bridge) saltmarsh features since the available 1870 baseline data to the last available (2014) data set. The majority of decline is strongly associated with the major marina building period of the 1960s and 1970s. Since that time, marsh areas have continued to decline, though less markedly, and to some extent fluctuated in area with some localised accretion in some of the locations.

On an individual basis, the major factor identified as potentially impacting the river’s marshes is sea level rise and coastal squeeze and related waterlogging due to human infrastructure at the river edge on either bank, preventing the natural migration of the marshes landwards. Other factors affecting the marshes include Spartina die-back and loss of sediment binding, localised dredging effects and potentially vessel (wave) impacts, and algal smothering; often related to excess nutrients associated with human habitation and river use. The algal factor, in conjunction with erosion, is cited in saltmarsh condition assessments undertaken by Natural England, in which marshes in the river are largely assessed as unfavourable recovering.

Globally researchers have shown that dredge drawdown and slumping of fine sediment through spoil removal does occur. However, specific consideration of dredging as a major factor in marsh decline in the River Hamble provides ambiguous conclusions, although dredging and sediment supply alteration during the marina building period will have exacerbated marsh loss. In addition to ambiguity, the inherent variance in the available data sets for spatial analysis also inhibits definitive conclusions. Localised small scale drawdown effects are apparent and in some locations offer a potential for management to highlight local beneficial effects. In the future, close scale terrestrial LiDAR with minimal data error is recommended to enable accurate monitoring of spatial and temporal marsh change.

Whilst data were not available for all locations, assessment of sediment suitability for use in beneficial placement schemes to promote marsh regrowth in the Hamble indicates that the dredge arisings will be suitable in physical quality. Sediments also need to be assessed for contaminant levels and whilst Hamble sediments have contamination, none of the available data showed levels above Action Level 2, which would require disposal in landfill. There may, however, be residual pockets of contamination thus

AHTI_J2015_004 ix RIVER HAMBLE SOFT SE DIMENT HABITAT RETEN TION FEASIBILITY STUDY replicated testing would be required. Organic material needs to be at sufficient levels in sediment to promote biological interaction by colonising species, thus this will also require assessment should sediment placement options be pursued.

Detailed consideration of the Hamble marshes through multi criteria analysis did not indicate one site in particular as a suitable location for a major beneficial use location, though a minor scheme is suggested. There are considerable constraints on such schemes in the Hamble associated with potential infrastructure impacts and the effects of such placement on highly protected natural habitats and species, potentially necessitating detailed assessment. In addition, some locations are not declining and deliberate placement of sediment may adversely affect marshes which are relatively healthy.

In comparison with beneficial use schemes, sediment management through retention methods may offer localised scale opportunities to enhance some River Hamble saltmarshes. The likelihood of success cannot be guaranteed, not least as marshes are complex ecological systems which are not fully understood in terms of reason for decline, or change due to human impact. However, retention schemes have met with some success (as have large scale beneficial use schemes where appropriate) and offer an opportunity for several locations to be used as trial or small scale project sites. The ecosystem service benefits of marshes are now more understood, such as passive coastal protection and ecological enhancement, therefore efforts to improve marsh habitat, where feasible, should be considered. This should be undertaken with due caution to ensure no adverse effects on these features and nearby human infrastructure. Early stakeholder engagement is advised with suitable monitoring schemes developed.

The project also called for a brief assessment of possible funding sources with indicative costs. Whilst costs are problematic to predict, not least as commercial consultants will have different pricing regimes, an outline has been supplied where reasonable estimates are available. Possible funding sources have been indicated, but it should be noted that the level of funding required will depend on schemes considered with some of the smaller options offering potential for useful outcome with relatively limited input. In comparison, one scheme, (the Hamble Marsh option) will require greater funding but may offer significant benefit if successful.

Accordingly, several smaller scale retention schemes, plus the Hamble Common Marsh beneficial use option, have been suggested. Whilst small in scale, if taken up, cumulatively they may offset marsh losses and have a positive effect on saltmarsh area in the Hamble Estuary. These are:  Satchell Marsh – management options to promote marsh regrowth through artificial channel control and drainage development;  Little Marsh – drainage development and retention structures to promote lower marsh growth at “cliffing” edge;  Hamble Common Marsh – sediment retention structures plus localised beneficial use of dredge arisings and management of localised dredging effect.

In due course, dependent on finance and development of options, larger schemes may be feasible at:  Bunny Meadows – change in culvert system to slow erosive flows plus sediment retention and larger scale beneficial use in Bunny Meadows north or south;  Swanwick Marsh – possible larger scale sediment retention structures project with appropriate monitoring. Recent small scale Spartina growth suggests this site may be marginal towards promoting plant growth on retained sediment.

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1. INTRODUCTION AND BACKGROUND

1.1 Project Context Following a tender request from the River Hamble Harbour Authority (RHHA) in April 2015, AHTI Ltd and Marine Space Ltd successfully bid to take forward work on “Assessing the viability of sustaining, restoring, enhancing, or creating saltmarsh and mudflats through sediment management measures, including the reuse of maintenance dredge arisings, within the Hamble Estuary”.

This work considers the current state of the lower River Hamble’s saltmarsh features, including factors affecting their temporal change and the feasibility and possibility of the beneficial use of River Hamble (or locally external) dredged sediments for soft habitat “restoration” or promotion (see: Environment Agency, 2010) within the river. In addition it considers potential sediment management techniques such as retention through physical methods (see Williams et al., 2010). The study also aims to consider potential alternative use for dredge arisings (beneficial use) in the region, and the wider geomorphological system of the near-field marine environments.

This study is set against a long term background of saltmarsh decline in the UK, but these habitats now benefit from aspirations and drivers from central Government via European Union requirements, to firstly minimise and subsequently reverse loss of such soft sediment habitats in the UK (and Europe). However, with specific respect to the Solent, in which the River Hamble sits, Foster et al. (2014) noted that “the evidence demonstrates an abundance of research and consultation for legislation and policy development purposes, with a relative lack of practice to actively conserve and sustainably use intertidal mudflats and saltmarshes across the Solent”. This perhaps reflects the difficulty of such aspirations in the Solent (and River Hamble) region in that there may be spatially limited opportunities to undertake such “active conservation”.

In a legislative context, saltmarsh is protected by European Union directives including the Habitats Directive (European Commission, 1992) and the Water Framework Directive (WFD) which requires member states to “avoid the deterioration of natural habitats within Special Areas of Conservation” (SACs) (Hayes, 2016). The WFD identifies saltmarsh as an important component of the assessment of ecological status of associated water bodies (Hayes, 2016) and this relates to ecosystem services which saltmarsh is increasingly acknowledged as playing an important role in (Luisetti et al., 2014).

As part of the vision for England’s Biodiversity 2020 strategy, Defra (2011) states as an outcome that “By 2020, we will see an overall improvement in the status of our wildlife and will have prevented further human-induced extinctions of known threatened species”. The reasons for saltmarsh decline are multifaceted and need to be contrasted against valid attempts to restore these habitats in the UK and beyond. The value of these habitats to nature and humans has relatively recently been realised and restoration or retention is a valid and valuable aim, where feasible. This is particularly relevant in relation to the wider ecosystem service benefits that humans receive from such soft sediment habitat. The value of saltmarsh to humans and ecosystems is now more understood in terms of ecosystem services such as passive coastal protection, absorption of pollutants, provision of nutrients to marine systems and carbon sequestration (e.g. see: Temmerman et al., 2013; Brisson et al., 2014; Curado et al., 2014 and UK National Ecosystem Assessment.

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The factors leading to saltmarsh decline, and the methods by which restoration, retention or augmentation may be achieved, require careful consideration and, if possible, clarification. Accordingly, this document considers the background of decline, research gaps, and where more research may be required. This is with the aim to look into the potential and feasibility of methods to restore or retain and promote the longevity of saltmarsh in the River Hamble.

Numerous studies have considered saltmarsh decline and condition worldwide. For the UK, Williams et al. (2010) noted that “European interest in saltmarshes has risen dramatically since seawall loss resulted in marsh development”. More recently the Marine Management Organisation (MMO, 2014) considered the beneficial use of dredge spoil for soft sediment habitat restoration. Garbutt et al. (2015) considered the status of English saltmarsh and Spartina habitats (including some River Hamble sites) and Haynes, (2016) recently authored a report on the status of saltmarsh in Scotland.

Thus it is evident that saltmarsh and soft sediment intertidal habitat status, restoration or re-creation is being considered where feasible and expedient for both ecological and ecosystem service interests. However, fourteen years ago, it was stated that the need for saltmarsh recreation or passive regrowth was “clear” for England and Wales, but that “this was not going to be an easy or cheap process” (Boorman et al., 2002).

In relation to dredging, it is useful at this stage to clarify the difference between types of dredging so that the availability and source of dredged material in the Hamble can be understood. The Marine Management Organisation (MMO) state that “Capital dredging is defined by the MMO as material arising from the excavation of the seabed, generally for construction or navigational purposes, in an area or down to a level (relative to Ordnance Datum) not previously dredged during the preceding 10 years”. Further to this, and applicable to Hamble sediments “maintenance dredging is defined as material (generally of an unconsolidated nature) arising from an area where the level of the seabed to be achieved by the dredging proposed is not lower (relative to Ordnance Datum), than it has been at any time during the preceding 10 years; or from an area for which there is evidence that dredging has previously been undertaken to that level (or lower) during that period” (Defra, 2012). However, due to later interpretation, the MMO effectively deals with capital and maintenance dredges in a similar manner including for the River Hamble. This is because it is agreed that the capital dredge 10 year interval is not appropriate when a maintenance dredge interval could be slightly longer, thus this leads to a pragmatic interpretation. A view is taken on capital material as being removal of non-recent deposits, thus in this case gravel and chalk would usually be considered capital material.

1.2 Solent and River Hamble Overview

1.2.1 Solent The Solent is a widely recognised body of water broadly in the centre of the English south coast (Figure 1.1). Sometimes known colloquially as “the playground” by the sailing fraternity, the Solent is heavily influenced by one of the highest concentrations of recreational craft (and associated infrastructure) in the UK. Other anthropogenic influences include major oil terminals and ports with naval, passenger and freight vessel movements, general industry, coastal protection, housing and social/recreational infrastructure. As a contrast to this, the Solent region is subject to local, national and international conservation designations (Figure 1.2) making the balance between large residential and industrial areas against highly valuable and protected habitats and species a problematic goal to achieve and maintain.

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Figure 1.1: Solent and Hamble for context Numerous studies have considered the Solent and the complex interrelationships between its natural assets and character, and the relationship between these and anthropogenic demands. Amongst the competing factors potentially adversely affecting saltmarsh and mudflat, Cope et al. (2008) noted that some 80% of the north Solent coast has shoreline protection for longer term sustainability of human developments and to minimise general coastal asset loss in the face of changing weather patterns, isostatic recovery (see Williams et al., (2010) for overview) and sea level rise (Cope et al., 2008). Coastal protection has been shown to adversely affect saltmarsh and mudflat through coastal squeeze. This has been suggested as a prevailing reason for saltmarsh decline in the Solent region (Cope et al., 2008) and wider areas where saltmarsh occurs. In further relation to saltmarsh decline, several documents have considered potentially influencing natural and anthropogenic factors (e.g. see EA, (2007); for Solent perspective see Cope et al. (2008) and New Forest District Council web search facility). Interestingly, in the face of knowledge regarding saltmarsh decline, several trials and practical projects have been undertaken to restore saltmarsh to meet EU and UK Government goals. However, for the Solent aspirations have not yet led to major practical outcomes for significant beneficial use projects, although well intentioned research projects and smaller trials and practical placements, plus numerous discussions, have taken place (Foster et al.,2013; Foster et al., 2014).

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Figure 1.2: Designations along the lower River Hamble Estuary

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Considering research and related saltmarsh aspirations, whilst the Solent as a whole is not the primary focus of this study, it is pertinent to give a brief overview of the water body to point interested parties to appropriate literature and resources should further information be required. Detailed descriptions relevant to natural and anthropogenic factors affecting the Solent are available in a variety of documents and web resources such as:

 Cope et al. (2008) provides detail for the Solent Shoreline Management Plan on intertidal loss and options for recreation of habitat/environment. The wider document provides a useful overview of coastal squeeze, saltmarsh change, coastal protection, and conservation factors etc.;  North Solent Shoreline Management Plan (2010) is a non-statutory approach to coastline management and providing context to the decisions aiming to “balance the management of coastal flooding and erosion risks, with natural processes and the consequences of climate change”. The documents contain useful background environmental data and information on the Solent region;  Standing Conference on Problems Associated with the Coastline (SCOPAC) is a “network of local authorities and other key organisations that share an interest in the management of the shoreline of central southern England” and provides an excellent resource for much of the related research in the south coast region and the rationale behind the region’s Shoreline Management Plans and coastal management;  The Solent Forum was formed in 1992 and was “set up to consider and provide advice on strategic issues” affecting the Solent region. The Forum is a useful resource and promoter of science and research in the region which feeds into studies regarding management of the region and the interactions between human and conservation resources. The Solent Forum was a major driver of the Solent soft sediment habitat restoration study (Williams et al., 2010).

In relation to saltmarsh and soft sediment habitat management research in the region, the Solent has been the focus of several interested attempts to promote studies and potential trials. As more recent examples, there are studies from:

 Foster et al. (2014) gives a useful academic review and summary of aims and attempts to promote sustainable use and restoration of (overall) declining soft sediment habitat in the Solent region. This document provides context to the overall aims of soft sediment habitat management in the region, the conservation policy and drivers/potential barriers and potential practical implementation against a background of previous attempts;  Inder & Ansell (eds.) (2008); the proceedings of the Solent Protection Society one day conference on the future of Solent saltmarsh. The report provides local and regional perspective on the Solent in context with wider UK efforts to conserve saltmarsh, relative biodiversity of these habitats and investigates aspects affecting decline, loss and potential restoration. Further works promoted by the Solent Protection Society include a presentation (Mossman, 2014) on redox and marsh height and drainage affecting restoration and re- growth trials;  Williams et al., (2010) provides a brief overview of the Solent wave and sediment system (see section 1.5) and gives a feasibility study for beneficial use of dredge spoil and saltmarsh restoration methods for creation or restoration of Solent soft sediment habitats. The report

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gives an overview of physical factors affecting Solent coastlines and as with Cope et al. (2008), it considers historic saltmarsh change in the region. It focusses in detail on specific sites identified by the Environment Agency (EA) and Hampshire County Council (HCC) as potential areas for restoration and/or augmentation with dredge spoil.

The Solent is approximately 30km in length, ranges between 1-8km wide and within its complex coastal and tidal regime, contains four major estuaries. Sediment transport in the Solent is acknowledged as “complicated” and in relation to the decline in saltmarsh cover (due potentially to numerous co-related factors) it has been noted that cliff erosion and subsequent beach replenishment are important sediment sources/sinks (Bray et al., 2000). In relation to this report, Bray et al. (2000) suggested “human activities including inshore dredging and coastal stabilisation have significantly affected some [sediment] pathways and have contributed to problems of beach erosion”. In this work it was also commented that intertidal restoration through replenishment schemes had been achieved, but this was presumably for coarser sediment areas (e.g. Hurst Spit – see Brown & Riley, 1998) rather than through use of dredge spoil to recharge eroding marsh or for mudflat creation.

Some schemes have been trialled in the Solent, (e.g. Hythe Marshes; see Colenutt, 1999) and latterly some restoration and sediment retention works have been achieved at Lymington Marshes (Lowe, 2012, 2013), plus a major managed realignment project at Medmerry (Burgess et al., 2015); Medmerry accepted as part of wider system adjacent to, and often (in text) included with the Solent. However, as discussed by Foster et al. (2013, 2014) a consistent approach to beneficial use is lacking and perceived as problematic to achieve due to conservation restrictions and possible conflict with other waterway users in which saltmarsh are located. This not least applies to the Hamble, where trials have been discussed (Tosswell, pers com; pers obs), but not undertaken and where the conflict between human uses and natural habitat is evident (e.g. see McAuliffe et al., 2014).

1.2.2 River Hamble Background The focus of this study are the saltmarsh and mudflat habitats, and physical/biological influence and management and restoration potential in the River Hamble up to the A27 road bridge and including Hook Marsh just outside the River mouth, to the south east (see Section 2.2). The Hamble is a relatively small estuary which, for its circa 8.5 km navigable length (10 km tidal length (SCOPAC, 2006), follows a broadly N/NNE direction from Southampton Water (Figure 1.1).

As with many coastal areas where humans interact closely with the resource and where demands are placed on managers (e.g. see O’Mahoney et al., 2009), the physical and biological environment of the River Hamble has received attention through research papers/university projects and related management reports (e.g. see Critchley et al., 1983; Maskell & Raybould, 2001; Thomas et al., 2001; Grant, 2006; McAuliffe et al., 2014). The research papers/university projects are often related to the impacts and effects of human interactions in the river (e.g. coastal erosion, sediment and water quality and changes to river ecology), the management reports are generally concerned with sustainable options for the river environment in the face of competing anthropogenic demands, related potential conflicts (Rees et al., 2010) and, perhaps contentiously, “naturally” derived change such as sea level rise and isostatic recovery (Williams et al., 2010 for overview).

Similarly to many heavily utilised waterways, the River Hamble has numerous competing demands; not least as it is located in the central south coast which is one of the most populous areas of the UK. In

6 AHTI_J2015_004 RIVER HAMBLE SOFT SE DIMENT HABITAT RETEN TION FEASIBILITY STUDY relation to human use, the Hamble has one of the highest recreational vessel concentrations in the United Kingdom (Bray, 2005, Williams-Hopley, 2014). The competing needs to serve this valuable industry with, in particular, the need to manage highly conserved habitats (including saltmarsh and mudflat) demonstrates the “balancing act” that must be achieved by the regulatory authorities, statutory conservation bodies and the river users themselves (see McAuliffe et al., 2014).

As an overview of the many anthropogenic pressures that management of the Hamble has to consider, they broadly comprise: multiple adjacent residents (many of whom own saltmarsh river frontage); commercial use (marinas etc.); sea defence and flood management; navigation maintenance (marina dredging); in-channel mooring; tourism (including vessel use and bankside recreation), and public value of the natural environment from residential and visitor basis.

Considered against the above pressures, which may compete for both perceived and actual significance (McAuliffe et al., 2014), the Hamble has many habitats with international, national, and local conservation status which, until just above the M27 road bridge, interact closely with marinas, housing and public infrastructure along the river banks (Figure 1.2). Saltmarsh/soft sediment habitats are the primary focus of this study and within the Hamble are subject to Ramsar, Natura2000 (Special Protection Areas [SPA] and Special Areas of Conservation [SAC] – (the Solent Maritime SAC) and Sites of Special Scientific Interest [SSSI] designation. In addition the UK as a whole, and thus management of River Hamble marshes, is targeted with the UK BAP (post 2010, UK Biodiversity Framework) goal of no further net loss of saltmarsh or mudflat habitat.

Saltmarsh loss has been notable in the UK and not least the Solent (Foster et al., 2014), though in relation to this study some sites were shown as accreting in the River Hamble (Williams et al., 2010). Through research, saltmarsh decline has been related to several factors (e.g. see Garbutt et al., 2015 and Section 3.2 below for overview), but identifying clear relationships at each site where loss is prevalent can be problematic. As an example in the Hamble, however, some work relevant to this has been undertaken noting that landward “migration” space was limited in the upper Hamble and that “land use is a significant constraint” in the lower Hamble (Gardiner et al., 2007). This shows that coastal squeeze is an issue along the river (though see Hughes & Paramoor, (2004) and Wolters et al., (2005) for differing views on the significance of coastal squeeze).

Furthermore, with regard to saltmarsh decline and condition, recent work has identified that saltmarsh condition assessments have been set at a level higher than necessary to achieve Favourable Condition under the Habitats Directive (Haynes, 2016) thus decline in condition may be overestimated at sites assessed. However, overall loss is evident and historically in the Solent, River Hamble and elsewhere, this has not least been due to the historical lack of understanding and undervaluation of saltmarsh habitat importance in general marine ecology (Bulleri & Chapman, 2010) and latterly ecosystem services (Foster et al., 2013).

It is the economic growth and development of ports, marinas, industry and housing and related infrastructure coupled with a lack of understanding of the value of saltmarsh that led to significant removal of the habitat in the UK, and by association, the River Hamble. There is a long history of boatyards and housing along the river banks. However, since a considerable rise in marina development and growth in the 1960s and 1970s (see Williams-Hopley, 2014 for time line) the Hamble’s character and natural features have altered significantly (e.g. see Plate 1.1 and Plate 1.2). In servicing the needs of the boating industry and river users dredging is required within some of the marinas and berthing areas.

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By association this will have increased the level of sediment removed from the geomorphological system and disposed of offshore at local dredge spoil dumpsites, or if above Cefas (Centre for Environment, Fisheries and Aquaculture Science) Action Level 2 (see Section 4.3 below and Williams et al. 2010, pp. 114-115 for explanation), then disposed of in landfill. Dredging has been implicated in saltmarsh and mudflat loss (van der Wal & Pye, 2004; Morris, 2007), as have numerous other factors, and has been perceived as a factor in River Hamble saltmarsh decline by some local residents (pers comm this project).

Plate 1.1: Bursledon Pool, “Moodys” Marsh, and Lands End, circa 1959

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Plate 1.2: “Moodys”/Premier Marina 2013 with 1870 saltmarsh extents as baseline reference (Source: Environment Agency, 2013 Aerial Photography)

Overall, it is evident from GIS analysis, and local personal observations encompassing general opinion, that the spatial extent of total saltmarsh in the River Hamble has declined (e.g. Plate 1.1 and Plate 1.2). Work shows the significant likelihood for coastal squeeze/sea level rise effects (Gardiner et al., 2007) and there is indication that dredge drawdown (over-steepening of subitdal/intertidal soft sediment features leading to slumping and loss) and sediment reduction (through settlement in dredged areas rather than on saltmarsh/mudflat) may also be implicated (Williams-Hopley, 2014). This may be in conjunction with other factors recognised as potentially responsible for saltmarsh decline, but not necessarily all applicable in the Hamble Estuary.

The competing demands in the Solent and more particularly in the Hamble itself have been considered from a boater point of view (McAuliffe et al., 2014). This work highlights the juxtaposition in views of what is good for a major river user group and what is good for the environment, i.e. in this case,

AHTI_J2015_004 9 RIVER HAMBLE SOFT SE DIMENT HABITAT RETEN TION FEASIBILITY STUDY saltmarsh. In their work on the River Hamble considering the impacts of Marine Protected Areas (MPA) and Marine Conservation Zones (MCZ) through the eyes of the boating community McAuliffe et al. (2014) noted that, “It is important to remember the underlying obligation to protect the marine and coastal environment [and that] directly or indirectly, it is [in this context] for economic reasons that the degradation occurs in the first place”. Whilst the latter statement may not be entirely proven in relation to the Hamble, where natural factors can also apply, it is pertinent to consider anthropogenic effects on the River’s saltmarsh habitats and the potential for restoration.

Against this background, the Harbour Authority aim to consider if saltmarsh management and restoration though sediment retention or addition is feasible. In addition to management, the RHHA wish to understand, as best as reasonably possible, the reasons for saltmarsh decline and how to address the concerns of residents, industry and river users and to explore possible mitigation options.

1.3 Project Questions Through this study the RHHA seeks to answer specific questions regarding the future of the River Hamble’s saltmarshes, management options, threats, risks and opportunities. The questions posed follow an iterative path to aid in clarifying possible factors affecting the marshes, where data are available. This will then consider restoration or augmentation possibilities and the possible risks (or otherwise) to natural features, conserved habitats and anthropogenic infrastructure should such ideas be taken forward. In addition, as the reasons for saltmarsh decline can be multifaceted (see EA, 2007), where data gaps are evident the RHHA seeks to achieve recommendations for further research, as and if warranted.

The questions as posed by RHHA are presented below (Table 1.1). These have been amalgamated to related queries to enable related and sequential research and to facilitate a useful flow of answers.

Table 1.1: Tasks as amalgamated from above list and allocation AMALGAMATED TASKS TASKS AS GIVEN BY RHHA a – Section 2 a: The extent to which the Hamble’s saltmarsh and mudflats are reducing (and may continue to reduce) in area and habitat quality, and the potential need for sustainment, restoration, enhancement or creation. b (d, e) – Section 3 b: Factors contributing to any reduction in area or quality of the Hamble’s saltmarsh and mudflat habitats, and the extent to which this may or may not be related to dredging activities within and outside the Hamble. d: Whether dredging may contribute to sediment draw-down from the mudflats and saltmarshes, and if so to what degree. e: Whether the maintained dredge basins reduce the sediment supply onto Hamble Estuary saltmarsh and mudflats, and if so to what degree. c (f) – Section 4 c: Type, sources, and relative quantities from each source of the sediment which deposits in the marinas. f: The suitability of Hamble Estuary’s maintenance dredge arisings for reuse, including in beneficial disposal and habitat management. g (h, I, j, k) – Section 5 g: The suitability of Hamble mudflats and saltmarshes for direct or indirect reception of maintenance dredge arisings for habitat sustainment, restoration, enhancement or creation. h: The suitability of Hamble mudflats and saltmarshes for sediment management techniques other than the reuse of dredged material. i: Potential environmental benefits of the disposal of maintenance dredge

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AMALGAMATED TASKS TASKS AS GIVEN BY RHHA arisings and other sediment management techniques on the Hamble Estuary system and its margins. j: Potential adverse environmental impacts of the disposal of maintenance dredge arisings and other sediment management techniques on the Hamble Estuary system and its margins. k: Potential for disposal at one site to cause increased accretion at other locations (e.g. marinas, navigable channels or neighbouring habitats). l – Section 6 Potential sources of funding for any subsequent programme of research and/or practical projects.

Sections 7 and 8 provides discussion and concluding remarks for the report.

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2. SALTMARSH EXTENT AND HABITAT QUALITY

Question a: The extent to which the Hamble’s saltmarsh and mudflats are reducing (and may continue to reduce) in area and habitat quality, and the potential need for sustainment, restoration, enhancement or creation

2.1 Data Sources Change rates of saltmarsh in the River Hamble are driven by a variety of factors (see Section 3.2), e.g. wave attack, sea level rise, dredging, reclamation, development and pollution (Williams et al., 2010). Geographic Information System (GIS) software was used to assess saltmarsh and mudflat erosion or accretion from a variety of sources (Table 2.1) between 1870 and 2014. All data were incorporated in an ArcGIS (v10.3.1) geodatabase.

Table 2.1: Data sources for calculating saltmarsh change

YEAR DATA SOURCE CAPTURE SCALE 1870 County Series 1st Edition 1:10,560 1946 Cope et al. 2007 1:9,800 – 1:10,000 1971 Cope et al. 2007 1:10,000 1984 Cope et al. 2007 1:7,500 2000 Cope et al. 2007 1:10,000 2007 Extracted from EA LiDAR 1 m resolution 2014 Extracted from EA LiDAR 1 m resolution

Data accuracy varies between the different data sources, and factors such as capture scale of the original data and digitising error should be borne in mind. See Table 2.4 for error bands and error description.

2.2 Change Analysis Change analysis or change detection in GIS has been used in this study to give a better understanding of how a given geographical area has changed over time. This involved examining the change in spatial distribution of selected intertidal areas during different time periods. This technique is referred to as the “overlay method”. Using this technique, the sum of the total area of saltmarsh was compared between the years for which data was available and percentage change s of areas were calculated and compared. Spatial change, i.e. location of change in saltmarsh was also compared. Change in total area between the years 1870 (baseline year) and 2014 will be presented, as well as volumetric change between 2007 and 2014 (using both horizontal and vertical change).

Thirteen marshes were identified within the lower Hamble Estuary. Bursledon, Hook and Bunny Meadows North are not present in all years, due to saltmarsh being lost completely (Bursledon and Hook) or being created (Bunny Meadows North). Figure 2.1 shows the location of the different marshes. Bursledon Marsh has not been included in the assessment, since the site has been developed (now occupied by Deacons marina).

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Sea level rise requires that marsh soils accrete vertically to maintain their position in the tidal frame. There is a threshold above which marshes are not able to continue growing, in which case the system must migrate to higher elevations found on adjacent upland areas (Chmura, 2011) or the edges of the marsh must be raised to higher elevations to encourage accretion. If this is not possible, the marsh will decline.

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Figure 2.1: Saltmarshes in Hamble Estuary

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Mudflat exists between lowest astronomical tide (LAT) and mean high water neap (MHWN), whilst saltmarsh colonises between MHWN and highest astronomical tide (HAT). The range of individual species found within the marsh is a combination of their relative ability to tolerate tidal submergence, or factors related to tidal submergence such as soil anaerobics, and competition with increasing elevation (Gray, 1992). Based on this, LiDAR data were “flooded” to corresponding tidal elevations to determine expected ranges of intertidal habitat coverage. Tidal levels for the estuary were interpolated for the different saltmarsh areas from 2007 and 2015 tidal data (Proudman Oceanographic Laboratory, 2007; CCO, 2016) for the secondary ports of Bursledon and Warsash (Table 2.2). Tidal levels on tide tables are quoted relative to Chart Datum (CD) [approximately the lowest level due to astronomical effects and excluding meteorological effects]. To use these data in conjunction with the LiDAR data, CD figures were converted to Ordnance Datum, Newlyn (ODN) [defined as the Mean Sea Level (MSL) at Newlyn in Cornwall]. To calculate the tidal levels for the secondary ports of Warsash and Bursledon, CD values were added to -2.74 m, which is the height of CD relative to ODN in the standard port of Southampton. E.g. the LAT value for Warsash in CD is 0.34 m, whilst in ODN it is -2.40 m, which is 0.34 m plus -2.74 m.

Table 2.2: Tidal data (in m) for secondary ports in Hamble Estuary relative to Ordnance Datum

PORT HAT MHWS “MHW” MHWN LAT EASTING NORTHING NAME 2007 2015 2007 2015 2007 2015 2007 2015 2007 2015 Warsash 449272 105905 2.13 2.26 1.86 1.86 1.46 1.46 1.06 1.06 -2.40 -2.40 Bursledon 449237 109611 2.29 2.36 1.76 1.76 1.41 1.41 1.06 1.06 -2.58 -2.58

The overall variability of saltmarsh area change between 1870 and 2014 is depicted in Table 2.3 and Figure 2.2 below, with the horizontal and vertical error (where applicable) shown in Table 2.4.

The saltmarsh extent for 1870 was digitised from the County Series 1st Edition Maps. The expected relative accuracy of these maps is ±3.5 m (68% confidence level) to ±8.8 m (99% confidence level) over a maximum measured distance of 500 m (Ordnance Survey, 1998). The 1946 to 2000 extents were sourced through the Channel Coast Observatory (CCO) from The Solent Dynamic Coast Project (Cope et al., 2007) who quantified saltmarsh loss through historic aerial photography interpretation. The aerial photography was obtained from a variety of sources, including the National Monuments Record Centre (NMR) and local authorities. In the study by Cope et al. (2007), aerial photographs were scanned, geo- rectified and mosaiced and saltmarsh areas digitised. The majority of aerial photographs were taken between April and September at low tide. The average [relative] error for the historic photography geo- rectification and digitising was approximately ±6 m to 12 m (1940’s – 1991) and ±2.2 m for photography taken after the year 2000 (Cope et al. (2007).

Data for the most recent years (2007 and 2014) were extracted from EA aerial Light Detection and Ranging (LiDAR) surveys with a resolution of 1 m. The relative horizontal accuracy (x,y), with an average survey height of 1,000 m above ground level is ±18.18 cm and ±5 cm in the vertical (z) (EA, 2016). Intertidal areas were extracted from the LiDAR surfaces through tidal level interpretation (Williams et al., 2010) and then compared to aerial photography (2007 and 2014) to verify the extents of saltmarsh and mudflat.

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These error bands should be borne in mind when assessing the area calculations, e.g. in the case of the 1870s maps where the relative accuracy is ±8.8 m with a 99% confidence level, the features on the map may deviate more or less than 8.8 m for one percent of the time.

Table 2.3: Lower Hamble Estuary Saltmarsh change over time

MARSH SALTMARSH AREA (m2) NAME 1870 1946 1971 1984 2000 2007 2014 Bursledon 31,461 2,610 0 0 0 0 0 Hacketts 101,113 79,880 69,044 65,213 59,170 57,077 55,218 Lincegrove 192,215 137,545 108,265 89,610 85,517 79,156 78,493 Mercury 60,928 45,114 27,328 16,702 13,005 14,096 13,325 Satchell 66,107 46,479 33,276 31,286 20,900 26,375 25,984 Little 7,937 6,078 6,472 6,621 6,170 6,614 6,685 Hamble 46,904 73,636 35,791 15,581 12,612 16,636 14,112 Common Hook 0 8,380 0 0 0 0 0 Bunny Meadows 9,248 35,446 24,780 25,436 25,311 26,760 22,422 North Bunny Meadows 71,830 62,848 127,270 106,254 109,111 113,030 102,787 South Crableck 43,624 25,800 10,478 0 550 6,033 5,670 Universal 23,086 14,928 10,784 7,700 5,206 6,965 6,931 Swanwick 81,707 25,157 9,568 1,404 0 2,402 1,931 TOTAL 736,160 563,901 463,056 365,807 337,552 355,144 333,558

Table 2.4: Error bands for different data sources YEAR HORIZONTAL ERROR VERTICAL ERROR ±3.5 m over 500 m (68% confidence level); and 1870 N/A ±8.8 m over 500 m (99% confidence level) 1946 ±6 m to ±12 m (unknown confidence levels) N/A 1971 ±6 m to ±12 m (unknown confidence levels) N/A 1984 ±6 m to ± 12 m (unknown confidence levels) N/A 2000 ±2.2 m (unknown confidence levels) N/A 2007 ±18.18 cm ±5 cm 2014 ±18.18 cm ±5 cm

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Figure 2.2: Decline in total saltmarsh area along Hamble Estuary between 1870 and 2014 (Error bands are shown in Table 2.4)

It can be seen that while the majority of marshes show a decline in area since 1870 (used as baseline for this study), there has been a period of relative stability in marsh areas from 1984 to the present day. In order to get a better understanding of how the different marshes have changed over time, they will be considered individually below.

2.2.1 Hacketts Marsh The marsh has a natural transition from saltmarsh to unimproved pasture without any artificial delineation or tidal control and as a result is very species rich (Hantsweb, 2015). The marsh is closed to the public and the western section is managed through Hampshire County Council (HCC), whilst the eastern section is privately owned.

It has a number of designations, including European SPA and international Ramsar status, whilst the mudflat area falls within the European Solent Maritime SAC. It is also a Local Nature Reserve (LNR), as well as a SSSI. The SSSI is divided into two units, both of which are classified as being in unfavourable recovering condition (the condition assessment was undertaken in 2014 and updated in 2015) (GeoStore, 2016). In the presentation “Natural England’s work on the Hamble”, Crane (2014), noted that threats from diffuse water pollution and smothering from algal mats were evident.

Figure 2.3 shows the change in area between 1870 and 2014. In the 100 years between 1870 and 1971, approximately 32% of the saltmarsh area was lost. Between 1971 and 2014, a further 20% of the saltmarsh was lost – of this 3% was lost between 2007 and 2014.

Volumetric change for the whole intertidal area (i.e. saltmarsh and mudflat) was calculated between 2007 and 2014 (Figure 2.4). The total deposition (blue shaded area) was approximately 9,769 m3 over an

AHTI_J2015_004 17 RIVER HAMBLE SOFT SE DIMENT HABITAT RETEN TION FEASIBILITY STUDY approximate area of 40,964 m2. However, the loss in elevation (red shaded area) was approximately 15,185 m3 mostly on mudflat along the riverine edges, and at the north eastern edge of the saltmarsh. The total area of the marsh eroded is approximately 10,540 m2. Areas of saltmarsh loss between 2007 and 2014 are on the edges of the saltmarsh coinciding with areas of edge erosion. The area unchanged was approximately 162 m2. Saltmarsh area loss is mainly along the edges, coinciding with a decrease in elevation, whilst small areas of saltmarsh gain are along areas of deposition. The area of loss exceeds the area of gain.

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Figure 2.3: Change in area of Hacketts marsh between 1870 and 2014

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Figure 2.4: Volumetric change at Hacketts marsh between 2007 and 2014

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2.2.2 Lincegrove Marsh Similar to Hacketts, this is a mature saltmarsh that is very species rich without any significant artificial delineation or tidal control. A footpath runs along the western edge of the marsh, but otherwise the marsh is closed to the public.

It also has a number of European designations, including SPA and SAC and has Ramsar status. The marsh is also a SSSI consisting of one unit, classified as being in unfavourable recovering condition (the condition assessment was undertaken in 2014 and updated in 2015) (GeoStore, 2016). In the presentation “Natural England’s work on the Hamble” (Crane, 2014), noted that threats from diffuse water pollution and smothering from algal mats were evident.

Figure 2.5 shows the change in area between 1870 and 2014. In the 100 years between 1870 and 1971, the saltmarsh loss was approximately 44%. Between 1971 and 2014, a further 27% of the saltmarsh was lost – of this 1% was lost from 2007 to 2014.

Volumetric change for the whole intertidal area (i.e. saltmarsh and mudflat) was calculated between 2007 and 2014 (Figure 2.6). The total deposition (blue shaded area) was approximately 14,992m3 over an approximate area of 52,840 m2. The loss in elevation (red shaded area) was approximately 15,014m3, particularly along the eastern sections of the marsh and the upper marsh. The total area of the marsh eroded is approximately 25,211 m2 and the area unchanged was 372 m2. Saltmarsh area loss is mainly along the edges, coinciding with a decrease in elevation, whilst small areas of saltmarsh gain are along areas of deposition. The area of loss exceeds the area of gain.

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Figure 2.5: Change in area of Lincegrove Marsh between 1870 and 2014

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Figure 2.6: Volumetric change at Lincegrove Marsh between 2007 and 2014

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2.2.3 Mercury Marsh The marsh has open access to the public with footpaths, and a slipway for local residents is present.

Similar to the other marshes, it also has a number of European designations (SPA and SAC) and has Ramsar status. It is also a Local Nature Reserve (LNR), as well as a SSSI which has been classified as being in unfavourable recovering condition (the condition assessment was undertaken in 2010 and updated in 2013) (GeoStore, 2016).

Figure 2.7 shows the change in area between 1870 and 2014. In the 100 years between 1870 and 1971, the saltmarsh loss was approximately 55%. Between 1971 and 2014, a further 51% of the saltmarsh was lost – of this 5% was lost from 2007 to 2014.

Volumetric change for the whole intertidal area (i.e. saltmarsh and mudflat) was calculated between 2007 and 2014 (Figure 2.8). The total deposition (blue shaded area) was approximately 1,804m3 over an approximate area of 4,903 m2. The loss in elevation (red shaded area) was approximately 9,413m3 covering extensive areas. Erosion totalled an area of 8,383 m2 and the area unchanged was 20 m2. Saltmarsh area loss is mainly along the edges, coinciding with a decrease in elevation, whilst small areas of saltmarsh gain are along areas of deposition. There are areas of saltmarsh gain where the elevation has decreased at the upper end of the marsh. The area of loss exceeds the area of gain.

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Figure 2.7: Change in area of Mercury Marsh between 1870 and 2014

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Figure 2.8: Volumetric change at Mercury Marsh between 2007 and 2014

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2.2.4 Satchell Marsh Housing is present along the western boundary of the marsh and a number of cuts have been made through the marsh during the years to allow access for small boat launching.

The northern sections of the marsh, bordering Mercury marsh, are designated. This includes European SPA and SAC designations, as well as Ramsar status. The northern section is part of Mercury Marsh LNR and holds SSSI status. The SSSI is classified as being in unfavourable recovering condition (the condition assessment was undertaken in 2010 and updated in 2013) (GeoStore, 2016).

Figure 2.9 shows the change in area between 1870 and 2014. In the 100 years between 1870 and 1971, the saltmarsh loss was approximately 50%. Between 1971 and 2014, a further 22% of the saltmarsh was lost – of this 1% was lost from 2007 to 2014. Note that Figure 2.9 shows an increase in marsh area between 2000 and 2007, however this cannot be further commented on as the data quality for the 2000 data set was questionable.

Volumetric change for the whole intertidal area (i.e. saltmarsh and mudflat) was calculated between 2007 and 2014 (Figure 2.10). The total deposition (blue shaded area) was approximately 4,237m3 over an approximate area of 8,403m2. The loss in elevation (red shaded area) was approximately 6,437m3 covering most of the site. The total area of the marsh eroded is approximately 17,498m2 and the area unchanged was 55m2. Saltmarsh area loss is mainly along the edges, coinciding with a decrease in elevation, whilst small areas of saltmarsh gain are along areas of deposition. The area of loss exceeds the area of gain.

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Figure 2.9: Change in area of Satchell Marsh between 1870 and 2014

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Figure 2.10: Volumetric change at Satchell Marsh between 2007 and 2014

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2.2.5 Little Marsh The marsh borders private property to the west, close to the mouth of the estuary.

It also has a number of European designations (SPA and SAC) and has Ramsar status. It is also a Local Nature Reserve (LNR), as well as a SSSI. The SSSI is divided into two units, both of which are classified as being in unfavourable recovering condition (the condition assessment was undertaken in 2014 and updated in 2015) (GeoStore, 2016).

Figure 2.11 shows the change in area between 1870 and 2014. In the 100 years between 1870 and 1971, the saltmarsh loss was approximately 18%. Between 1971 and 2014, there was a slight increase in area of 3% – of this 1% was gained from 2007 to 2014.

Volumetric change for the whole intertidal area (i.e. saltmarsh and mudflat) was calculated between 2007 and 2014 (Figure 2.12). The total deposition (blue shaded area) was approximately 593m3 over an approximate area of 5,285m2 towards the west. The loss in elevation (red shaded area) was approximately 89m3. The total area of the marsh eroded is approximately 1,728m2 and the area unchanged was 31m2. Saltmarsh area loss is mainly along the edges, including the upper sections of the marsh, generally coinciding with a decrease in elevation. Areas of saltmarsh gain are mainly along areas of deposition. The area of gain slightly exceeds the area of loss.

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Figure 2.11: Change in area of Little Marsh between 1870 and 2014

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Figure 2.12: Volumetric change at Little Marsh between 2007 and 2014

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2.2.6 Hamble Common Marsh The marsh is in between Little Marsh and Hamble Point Marina and very fragmented. Significant sections of it have been lost due to marina development over the years and only remnants remain. It is bordered by Hamble Common to the west and southwest, with a footpath skirting the edge.

The marsh has Ramsar status, as well as the European designations of SPA and SAC. It is a designated SSSI classified as being in unfavourable recovering condition (the condition assessment was undertaken in 2010 and updated in 2013) (GeoStore, 2016).

Figure 2.13 shows the change in area between 1870 and 2014. In the 100 years between 1870 and 1971, the saltmarsh loss was approximately 24%. Between 1971 and 2014, a further 61% of the saltmarsh was lost – of this 15% was lost from 2007 to 2014.

Volumetric change for the whole intertidal area (i.e. saltmarsh and mudflat) was calculated between 2007 and 2014 (Figure 2.14). The total deposition (blue shaded area) was approximately 1,248m3 over an approximate area of 10,409m2. The loss in elevation (red shaded area) was approximately 1,054m3, along the upper parts of the marsh, as well as on the remnants in the northern section of the site close to Little Marsh. The total area of the marsh eroded is approximately 6,389m2 and the area unchanged was 64m2. Saltmarsh area loss is mainly along the edges, coinciding with a decrease in elevation, whilst areas of saltmarsh gain are along areas of deposition. Along the western edge however, a drop in elevation has resulted in saltmarsh gain. The area of loss exceeds the area of gain.

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Figure 2.13: Change in area of Hamble Common Marsh between 1870 and 2014

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Figure 2.14: Volumetric change at Hamble Common Marsh between 2007 and 2014

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2.2.7 Hook Marsh The section of Hook that has been included in this assessment (seaward side of the embankment) has not had any saltmarsh recorded since 1946 (Figure 2.15), with none apparent on the 1870 maps either. Figure 2.16 shows the volumetric and area assessments for the area. Hook has been included in the overall study due to the EA’s Regional Habitat Creation Programme. This has Hook as a potential Solent site for delivering intertidal habitat to compensate for the losses elsewhere in defending the coastline site, possibly through realignment of the flood defences south of Warsash.

Figure 2.15: Saltmarsh extent in 1946 at Hook

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Figure 2.16: Volumetric change at Hook between 2007 and 2014

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2.2.8 Bunny Meadows South Marsh Bunny Meadows has been divided into two sections, north and south, to take account of the lack of connectivity between the two as a result of the footpath that divides the northern from the southern section. The marsh is also separated from the main channel by an embankment with a footpath. During the 1800s and early 1900s the area to the east of the footpath was used for grazing. In 1946 the embankments were lowered and breached in places to allow tidal flooding, which created the saltmarsh.

The marsh has Ramsar status, as well as the European designations of SPA and SAC. It is part of the Hook-with-Warsash LNR and is a designated SSSI classified as being in unfavourable recovering condition (the condition assessment was undertaken in 2010 and updated in 2013 (GeoStore, 2016).

Figure 2.17 shows the change in area between 1870 and 2014. In the 100 years between 1870 and 1971, the saltmarsh increased in area by 77%, although a decline in the marsh on the western section of the embankment was evident. Between 1971 and 2014, a loss of 19% was recorded – of this 9% was lost from 2007 to 2014.

Volumetric change for the whole intertidal area (i.e. saltmarsh and mudflat) was calculated between 2007 and 2014 (Figure 2.18). The total deposition (blue shaded area) was approximately 5,698m3 over an approximate area of 44,337m2. A significant loss in elevation (red shaded area) of approximately 18,931m3 has been observed, mostly in the upper marsh sections. The total area of the marsh eroded is approximately 65,726m2 and the area unchanged was 348m2. Saltmarsh area loss is mainly along the edges, coinciding with a decrease in elevation, whilst small areas of saltmarsh gain are along areas of deposition. The area of loss exceeds the area of gain.

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Figure 2.17: Change in area of Bunny Meadows (South) between 1870 and 2014

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Figure 2.18: Volumetric change at Bunny Meadows (South) between 2007 and 2014

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2.2.9 Bunny Meadows North Marsh This is the northern section of Bunny Meadows, just south of Universal marina.

The marsh has Ramsar status, as well as the European designations of SPA and SAC. It is a designated SSSI classified as being in unfavourable recovering condition (the condition assessment was undertaken in 2010 and updated in 2013).

Figure 2.19 shows the change in area between 1870 and 2014. Saltmarsh change in this section was highly variable. In the 100 years between 1870 and 1971, the saltmarsh increased in area by 168%. This was due to the breaching of the embankment in 1946, which turned the then grazing marsh in the northern section to saltmarsh. The inlet of water caused saltmarsh retreat in the southern section. The saltmarsh was relatively stable between 1971 and 2007, but shows a loss of 16% between 2007 and 2014.

Volumetric change for the whole intertidal area (i.e. saltmarsh and mudflat) was calculated between 2007 and 2014 (Figure 2.20). The total deposition (blue shaded area) was approximately 2,380 m3 over an approximate area of 13,205 m2. The loss in elevation (red shaded area) was approximately 2,877 m3, again mostly in the upper marsh sections. The total area of the marsh eroded is approximately 12,629 m2 and the area unchanged was 84 m2. Saltmarsh area loss is mainly along the southern section, coinciding with a decrease in elevation. Saltmarsh area gain is minimal and the area of loss exceeds the area of gain.

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Figure 2.19: Change in area of Bunny Meadows (North) between 1870 and 2014

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Figure 2.20: Volumetric change at Bunny Meadows (North) between 2007 and 2014

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2.2.10 Crableck Marsh This marsh borders Universal marina to the south and east. Only remnants of the original marsh, lying to the river side of the footpath remains, with new marsh created since the 1800s due to breaching of the embankment (see section 2.2.8).

Most of the remnant saltmarsh area to the north is not designated, but the mudflat area to the south has Ramsar status, as well as the European designations of SPA and SAC.

Figure 2.21 shows the change in area between 1870 and 2014. In the 100 years between 1870 and 1971, most of the saltmarsh was lost (76%). Between 1971 and 2014, a further 46% of what remained was lost – of this 6% was lost from 2007 to 2014.

Volumetric change for the whole intertidal area (i.e. saltmarsh and mudflat) was calculated between 2007 and 2014 (Figure 2.22). The total deposition (blue shaded area) was approximately 209m3 over an approximate area of 2,809m2. The loss in elevation (red shaded area) was approximately 571m3 mostly in the upper marsh areas. The total area of the marsh eroded is approximately 2,804m2and the area unchanged was 33m2. Saltmarsh area loss is mainly along the edges, coinciding with a decrease in elevation, whilst small areas of saltmarsh gain are along areas of deposition to the north of the site. The area of loss exceeds the area of gain.

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Figure 2.21: Change in area of Crableck Marsh between 1870 and 2014

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Figure 2.22: Volumetric change at Crableck Marsh between 2007 and 2014

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2.2.11 Universal Marsh This marsh skirts around Universal marina. A public footpath runs alongside the marsh.

The marsh has no designations.

Figure 2.23 shows the change in area between 1870 and 2014. In the 100 years between 1870 and 1971, just over half of the saltmarsh was lost (53%). Between 1971 and 2014, a further 36% of what remained was lost – of this 0.5% was lost from 2007 to 2014.

Volumetric change for the whole intertidal area (i.e. saltmarsh and mudflat) was calculated between 2007 and 2014 (Figure 2.24). The total deposition (blue shaded area) was approximately 316m3 over an approximate area of 3,848m2. The loss in elevation (red shaded area) was approximately 208m3, totalling an area of 3,146m2. Currently the marsh seems to be in equilibrium, the area unchanged was 36m2. Saltmarsh area loss is mainly along the edges, coinciding with a decrease in elevation, whilst small areas of saltmarsh gain are along areas of deposition to the landward side. The area of loss exceeds the area of gain, but only slightly.

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Figure 2.23: Change in area of Universal Marsh between 1870 and 2014

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Figure 2.24: Volumetric change at Universal Marsh between 2007 and 2014

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2.2.12 Swanwick Marsh Only sparse remnants of this marsh still exist, the majority having been developed (now the site of Swanwick marina). A public footpath runs alongside the marsh.

The marsh has no designations.

Figure 2.25 shows the change in area between 1870 and 2014. In the 100 years between 1870 and 1971, 88% of the marsh was lost. Between 1971 and 2014, a further 74% of what remained was lost. The remnants that remain to the southeast have shown an increase in recent years (2007 to 2014) of 15%. The data for 2000 is unreliable and therefore the zero area in saltmarsh depicted should be questioned.

Volumetric change for the whole intertidal area (i.e. saltmarsh and mudflat) was calculated between 2007 and 2014 (Figure 2.26). ). The total deposition (blue shaded area) was approximately 130 m3 over an approximate area of 1,275 m2. The loss in elevation (red shaded area) was approximately 272m3. Erosion totalled an approximate area of 1,596 m2 and the area unchanged was approximately 6 m2. Saltmarsh area loss is mainly along the edges, including in the upper marsh, mostly coinciding with a decrease in elevation. There are small areas of saltmarsh gain along areas of deposition, with Spartina spp. noticed at the upper end of a remnant jetty (Plate 2.1). The area of loss exceeds the area of gain.

Plate 2.1: Spartina spp. colonising top edge of remnant jetty between 2007 and 2014 (Photo © AHTI Ltd)

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Figure 2.25: Change in area of Swanwick Marsh between 1870 and 2014

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Figure 2.26: Volumetric change at Swanwick Marsh between 2007 and 2014

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2.3 Habitat Quality The benefits of a restoration, recharge or beneficial use scheme in the Hamble Estuary have to be considered in the context of the current quality/condition of the constituent habitats, and the structure and function of the system in its entirety. The location of a recharge scheme will, in no small part, be driven by physical factors, e.g. sediment budget, position of the site within the water body and tidal prism (Carpenter and Brampton, 1996; Defra and Environment Agency, 2007). Areas of saltmarsh and intertidal mudflats that are being exposed to adverse effects, or which are deteriorating or damaged, have traditionally been chosen as locations for restoration (assuming they are located in areas with favourable hydromorphology and geomorphology to allow retention of deposited material and/or accretion of sediments) (Dearnley et al., 2007; Foster et al., 2013; Williams-Hopley, 2014).

Any deposition of dredged materials will result in short-term impacts to the habitats and species at that location (see Section 5.5). If there are currently no locations of damaged or deteriorating habitat, these short-term impacts have to be considered against the potential medium to long-term benefits of beneficial use of dredged materials: both in the context of providing resilience against adverse effects on intertidal and coastal habitats from climate change-induced sea level rise; and also ensuring that the hydromorphological and geomorphological functioning of the system is maintained or enhanced.

Considering these factors it is important to understand the quality and condition of the estuary’s physical system, and also of the habitats, communities and species dependent upon that system. The majority of the intertidal and coastal habitats within the lower Hamble Estuary are notified as part of the Lee-on-the Solent to Itchen Estuary SSSI and Lincegrove and Hackett's Marshes SSSI, or as part of the Southampton Water and Solent SPA and Ramsar site, or as features of the Solent Maritime SAC (collectively called the Solent European Marine Site (EMS)).

Under Regulation 35 of the Conservation of Habitats and Species Regulations 2010, Natural England have a responsibility to advise on what activities may cause damage or disturbance to special features. Updated and revised packages for the Solent Maritime SAC will be consulted upon in May 2016, whilst consultation on the SPA package is due in March 2017. These will be summaries of the baseline environment, with new and updated conservation objectives, but is meant to also now include supplementary advice on conserving and restoring site features. A short summary is here.

In addition, as part of its statutory duties, Natural England is required to monitor the condition of the notified SSSI features used to underpin the designation of the EMS, and of non-SSSI features of the EMS, and report these in a six year cycle to the Joint Nature Conservation Committee (JNCC); to deliver Article 17 favourable conservation status (FCS) reporting requirements of the EC Birds and Habitats Directive. Therefore Natural England (and JNCC) will be in possession of the latest information and data regarding the condition (quality) and conservation status of the intertidal and coastal habitats with the Hamble Estuary, along with the structure and function of the estuary itself, and within the context of Southampton Water.

Where data are available, Condition Assessment information is given for Hamble River Saltmarsh sites. This is presented in conjunction with, where known or recognised, relevant factors affecting condition and spatial extent. This is taken from site information from Natural England, plus observation from site visits where undertaken. These comprise:

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 Hacketts and Lincegrove Marsh are adjacent to each other and can broadly be treated as a continuum. Natural England assessed their status in 2014 (see here). The marsh units were assessed as Unfavourable Recovering, with erosion and algal matt smothering cited as threats to the sites. The Natural England assessment noted that Unit 2 (Unit ID 1007766) is being managed through a Higher Level Stewardship scheme and was “progressing towards favourable condition”. In addition encroachment by reeds (Phragmites spp.) and sea couch was noted, in units 1 and 3, though this had not occurred significantly. This is a noteworthy aspect as similar has been recorded during this study at Little Marsh where a linear feature of Phragmites spp. is evident parallel to a pipe that runs through the marsh, presumably retaining freshwater from the landward side thus making the salinity preferable for reed. This is clear evidence of the fragility of saltmarsh to change in abiotic factors. Furthermore, in this work, GIS analysis indicates habitat fragmentation. During a site visit at the Lands End area, the effects of erosion were noted, as was a highly localised attempt to retain sediment which appeared somewhat successful. Erosion due to lack of available landward migration and increasing inundation appear the major factors influencing the site, and algal mats should reduce in effect if water quality improvement efforts are continued;  Lee-On-The Solent to Itchen Estuary SSSI (here) covers the remaining majority of saltmarsh units within the Hamble. Specific details on condition were not readily available bar overview comments to state that the marshes in general appeared to be suffering from erosion and were known to be detrimentally affected by algal mats (as noted with Lincegrove and Hacketts).It should be noted that during the drafting of this report data were available, and status has been commented on, for each marsh section (Sections 2.2.1 to 2.2.12 – conditions generally recorded as unfavourable recovering) However, these data have now been removed from the Government portal (Geostore, 2016). Where available, further comment on marshes is noted through work for this report below:

o Mercury Marsh – has become partially terrestrial and has Phragmites spp. at upper edge. Significant erosion due to lack of space for rearward migration has resulted in cliff feature and remnant offshore soft sediment lower marsh which appear to be eroding. Channels and man-made features have to some extent bifurcated the marsh; o Satchell Marsh – appears to have been drained to some extent possibly causing habitat change to partial terrestrial. Channels cut through the marsh for boat owner access will have enhanced drainage. Lack of space for landward migration, plus to the south effects of encroachment and creation of Port Hamble marina will have caused acute loss of habitat with subsequent chronic effects listed above; o Little Marsh is protected to the south by a man made wall/bund feature which has minimised erosion as seen in the adjacent Hamble Common marsh. Little Marsh cannot retreat landwards due to land features and housing, is suffering toe erosion forming a cliff feature and some algal smothering was evident though it is not known if this is a regular occurrence on this marsh. The mid marsh is relatively healthy, but the site lacks pioneer/seaward characterising species and associated accreted soft sediment habitat. The upper marsh has an unusual linear feature of Phragmites spp. which appears to have been formed through freshwater drainage impingement by a pipe installed some time ago;

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o Hamble Common Marsh is backed by mature woodland on Hamble Common, and the common itself with open boggy grassland/scrub. The features suggest there is little opportunity for rearward migration from the marsh. The site has suffered significant temporal and spatial erosion with remnant islands on the mudflat remaining. Aerial imagery appears to indicate localised drawdown effect at the south east corner of the mudflat/former marsh area associated with dredging for the furthest upstream pontoon for Hamble Point marina. Access to the marsh remnants is hazardous, thus general condition was not considered widely in this work; o Hook Marsh was included in this study, but a site visit revealed no currently present saltmarsh/mudflat features; o Bunny Meadows (South and North) are problematic to assess for general condition due to safety of access. However, they have been commented upon as suffering erosion and algal smothering. Having converted grazing marsh to saltmarsh through the breach of the defences, the rebuilding of the coastal path resulted in the design of inlet/outlet culverts which have significant flows during both tidal cycles. Evidence of erosion through high water flows is apparent. In addition, rearward migration of the marshes appears largely unfeasible due to residential gardens, properties and topographical change; o Crableck Marsh is a remnant of a wider feature that has declined significantly. Apart from the factor of algal smothering, the marsh has been significantly affected by the creation of Universal Marina. Structures cross the marsh remnants which have been somewhat removed to facilitate walkways etc. and management of berthing for the marina may have an impact on sediment supply, but the overriding aspect will be a lack of rearward migration space due to the coastal path, behind which Bunny Meadows North lies, which, without the path, would create a continuum of the marsh feature; o Universal Marsh has remained relatively intact in comparison to Crableck Marsh. Algal smothering is evident as is some erosion at the toe of the marsh which perhaps prevents development of low marsh/pioneer species, but the mid marsh appears relatively intact. Rearward migration is problematic due to the coastal path, however this area regularly floods due to its low lying nature thus change in the nature of the terrestrial communities may be an eventuality; o Swanwick Marsh suffered significant loss through the creation of walkways/jetties and subsequently Moody (now Premier) Marina in the 1960s. There is none of the original large marsh extent, though a remnant does exist at the south east end at the apex of “Brooklands”. There is no space for landward migration, and management for the existing marina will have altered sediment regimes and flows. Thus the opportunity for the marsh to re-establish appears unlikely, although recently a very small stand of Spartina growth was noted just to the east of the Swanwick Shore car park.

The above summaries are based on local knowledge plus data from Natural England resources where available. Should this work lead to consideration of options to manage marshes at the local or wider

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2.4 Summary

This section examined the change in saltmarsh area in the lower River Hamble between 1870 and 2014, using a variety of data sources, including aerial photography, historical maps and LiDAR data. The general trend in the estuary is that of saltmarsh decline. Detailed data on saltmarsh condition are relatively sparse bar Lincegrove and Hacketts. General factors affecting all sites are similar, mainly comprising erosion, lack of space for landward migration and algal smothering, though the latter may be improving. More complex reasons for saltmarsh decline are known, however the above are apparently continuing aspects in the Hamble.

The next section will examine the potential factors contributing to the decline.

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3. FACTORS CONTRIBUTING TO AREA AND QUALITY REDUCTION

Question b: Factors contributing to any reduction in area or quality of the Hamble’s saltmarsh and mudflat habitats, and the extent to which this may or may not be related to dredging activities within and outside the Hamble

Question d: Whether dredging may contribute to sediment draw-down from the mudflats and saltmarshes, and if so to what degree

Question e: Whether the maintained dredge basins reduce the sediment supply onto Hamble Estuary saltmarsh and mudflats, and if so to what degree

3.1. Introduction The previous section demonstrated the change in saltmarsh area between 1870 and 2014 in the lower River Hamble. This section will examine the potential reasons for the decline aiming to answer the questions posed above.

3.2 Reasons for saltmarsh decline Saltmarsh decline and restoration possibilities are the prevalent subjects when the habitat is considered by current researchers. A recent report for Scotland (Haynes, 2016) noted the decline recorded was “related to the presence of built structures (like embankments) and the lack of natural landward transition habitats, which are issues that cannot be readily addressed through site management”; Cope et al. (2008) note the same for Hamble sites (see below).

Garbutt et al. (2015) (also see Tsuzaki, (2010)) looked at factors affecting UK saltmarsh and Spartina spp. They commented that losses in Southampton Water were due to site developments and that “there are indirect impacts such as changes in wave action and tidal currents, changes in sediment deposition and pollution”. Whilst at least some marsh decline has been related to the decline of Spartina alterniflora (see below) they did “not recommend re-introduction of S. alterniflora into the saltmarshes of the Hamble. The chances of success are too uncertain to justify disruption of some of the few remaining species-rich saltmarshes in Southampton Water”. Noting this, any works on Hamble saltmarshes need to carefully consider the value and condition of marsh areas already well established.

Thus factors affecting UK saltmarsh longevity and change have been suggested to include, amongst others, changing weather patterns, vessel wash, pollutants, soil chemistry, changes in infaunal cycling of sediment and bioturbation (for review see Foster et al. (2013)). But, as shown above, perhaps the most significant factor (post land reclaim) in the Solent region is coastal squeeze in a region where around 80% of the coastline is protected from flooding or erosion (Cope et al. 2008).

Saltmarsh extent in the River Hamble, in common with the habitat in much of the rest of the UK, has historically declined (Cope et al., 2008), though very localised accretion has been noted more recently (e.g. Williams et al., 2010). Perhaps reflecting the historical lack of understanding of the habitats value, Williams-Hopley, (2014) reported that saltmarsh loss between 1783 and 2008 derived from agriculture, rail development and urbanisation up to and including development work for marina growth. Anecdotally, decline in saltmarsh area in the Hamble is also a concern for locally affected landowners who cite differing reasons relevant to their particular land-holding (pers comm, this project). From

AHTI_J2015_004 57 RIVER HAMBLE SOFT SE DIMENT HABITAT RETEN TION FEASIBILITY STUDY inspection of aerial imagery and through the recorded marina developments, the effects on the acute saltmarsh loss in the 1960s-1970s are very clear (see Section 2 figures for marshes); other factors influencing decline (or accretion) since this period are more subtle.

The broad scale removal of saltmarsh in the Hamble is historically apparent, particularly to those who have been associated with the River for a number of years. However, this does not clarify whether marshes are clearly continuing to erode or accrete (as indicated for some sites in Williams et al., 2010) and what factors may continue to be affecting them (though see Gedan et al., 2009; Gardiner et al., 2007; Cope et al., 2008) or how these may be “unravelled” (Hudson et al., 2008). However, there have been clear indications as to what the major factor(s) is (are) that affect the Hamble Estuary’s saltmarsh. In the Solent Dynamic Coast Project which was “designed to inform the North Solent Shoreline Management Plan (SMP)” Cope et al. (2008) considered, amongst other aspects, the loss of saltmarsh in the Solent region. In the Hamble it was reported that overall saltmarsh in the river had declined though seemingly not at such a significant rate as other areas in the Solent region. The total area loss was stated as 18% between 1940 and 2002 (Cope et al., 2008). Amongst less immediately apparent factors, the direct loss is clearly demonstrated through the level of reclamation which occurred since the 1940s, much of which relates to the creation of marinas (Figure 3.1) which, in the main, occurred in the 1960s- 1970s (Williams-Hopley, 2014).

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Figure 3.1: Marina developments on lower River Hamble

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In further Solent-based work on specific saltmarsh sites (Williams et al., 2010), locations in the river were investigated for spatial and temporal change; these were: Hook, Bunny Meadows, Mercury and Hacketts Marshes (Figure 2.1), all of which are reconsidered in this study, but briefly in the Williams et al. (2010) study were indicated as:

 Hook Marsh – locally accreting as the adjacent spit had extended;  Bunny Meadows – locally accreting within the seawall/footpath (though recently erosion was discussed [Garbutt et al., 2015]), significant loss in marsh affected by main channel to the west of the footpath;  Mercury Marsh – declined significantly, though the rate had slowed throughout the period considered (1947-2007). The dominant process in decline was apparently toe erosion, though a small increase was noted in the 2000-2007 data sets, but it was notable that the raised height of the marsh had resulted in significant change from saltmarsh to reed (Phragmites australis) bed community. Such features were noted by Gardiner et al. (2008) who commented that “in some locations the reed beds appear to be colonising areas of the intertidal where saltmarsh would be expected”. This may have been exacerbated by over- heightening of the marsh through dredged sediment disposal during the creation of Mercury Marina (Tosswell, pers comm, 2015).  Hacketts Marsh - showed significant loss between 1946 and 2007, but in the 2010 study was found to be locally accreting.

Saltmarshes are fine sediment derived habitats that have accreted in low flow/energy areas, often in estuary systems; for the Hamble, being sheltered from prevailing south-westerlys, significant marsh has developed. Sediment accretion is assisted by the presence of pioneer colonising halophytic plant species which create standing water areas allowing fine sediment to settle and further facilitate marsh growth. This can lead to characteristic marsh zonation in the established plant communities (Silvestri et al., 2005).

The River Hamble has faced a high level of interaction with human pressures in what would naturally be a relatively benign low energy system as largely noted above the M27 road bridge. This, coupled with other potential factors (see Table 3.1 below), means that whilst literature can identify what may be adversely affecting a saltmarsh system, clear apportionment of one or other to a site impact/effect will be problematic (Hudson et al., 2008). The RHHA aims requested in points b, d, e above were, in the main, to consider the relationship of dredging to marsh loss. Table 3.1 outlines commonly accepted factors implicated in saltmarsh decline, and takes forward those currently suggested in the River Hamble for consideration, based on relevant and available literature; a conceptualised flow chart of major factors is given below (Figure 3.2).

Table 3.1: Major factors associated with saltmarsh decline FACTOR REFERENCE HAMBLE REFERENCE See pp 2, Williams et al. (2010) Gardiner et al. (2007) “For the lower Hamble land Sea level rise For recent see: Kirwan & Megonigal, use is a significant constraint”; / increased (2013) – “sea-level rise is expected to ABPmer, (2011a) “This does not however indicate storminess / accelerate, with regional assessments that sufficient sediment will be available to isostatic predicting a 20–45% loss of salt marsh maintain the saltmarsh/mudflats should rates of recovery during the current century”. sea level rise increase in the future”. See Cundy and Croudace, (1995, 1996).

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FACTOR REFERENCE HAMBLE REFERENCE See Pontee (2013) for definition. Gardiner et al. (2008) “In the Lower Hamble For recent see: Foster et al. (2014) Estuary, coastal defences restrict the potential for Coastal “…compensate for the potential damage landward migration of saltmarsh leading to coastal squeeze to designated intertidal mudflats and squeeze, with few opportunities for migration saltmarshes across the [Solent] region except into private gardens [which] may create from coastal squeeze…”. [future] land-use conflicts”. Waterlogging as a result of the possible Targeted research on Hamble Spartina and marsh height loss / sea level rise / waterlogging as direct factor not apparent. Maskell isostatic recovery feedback loop causes & and Raybould, (2001) postulated that Solent negative impact on and loss of Spartina sites, including Hacketts and Lincegrove Marshes spp. Gazala et al. (2013) “Under saline- may have suffered waterlogging. Amongst other Waterlogging flooded conditions in the field and in the factors. It was suggested that “regular dredging of /Redox glasshouse, plants produced a higher deep water channels may be important”. However, concentration of antioxidants than under several combinatory factors may have a role in drained conditions”. Loss of sediment water logging of which dredging and “lateral creep” causing further Spartina loss, may or may not be a factor. exacerbates saltmarsh loss. Spartina hybridisation early 1800s. S. As above Maskell & and Raybould, (2001) martimia + S. alterniflora = S. anglica suggested waterlogging through higher water after intermediate phase (S. townsedii). retention through S. anglica may be a factor. Re- Spartina die- S. anglica spread, to other species survey of Lincegrove Garbutt et al. (2015) found no back decline. Saltmarsh erosion die back S. alterniflora at Hamble marshes. Whilst Hackets exacerbated, possibly due to S. anglica and Lincegrove The marshes were suggested for S. altering marsh drainage (Goodmann, alterniflora reintroduction, but not recommended. 1960; Raybould et al., 2000). Johnson, (2000a) “erosion of Williams-Hopley (2014) suggested negative [Lymington] saltmarsh adjacent to dredging relationships - “The decline of mudflat and navigation channels in more sheltered salt marsh between 1947 and 1965 appeared areas suggests that ship wash and draw- related to slumping of the intertidal associated with down exert a negative effect”. dredging”, and that “A period of salt marsh and Dredging and Also: van der Wal & Pye, (2004) “A mudflat accretion also occurred between 1996 and drawdown second effect of dredging is to cause 2008, this coincided with years in which channel (sediment progressive net movement of sediment accretion occurred indicating that the estuary was supply) from the intertidal flat into the dredged acting as a sediment sink following large dredges in channel, for instance by slumping”. the 1960s associated with marina development”. Also Price (2006) for suggested slumping Accretion also noted by Williams et al. (2010), not / drawdown mechanism. And Garbutt et notably increasing saltmarsh area. al. (2015) for Marchwood area loss pp 71. Excess nutrient pollution (which Recognised as a threat to Hamble marshes and saltmarsh can absorb) “this noted in this work (site visit). See Crane (2014) Algal accumulation of macroalgal biomass here, who noted Lincegrove and Hacketts as smothering may hinder S. alterniflora growth Unfavourable – Recovering, with algal matts as a through smothering and breakage of threat culms” (Newton & Thornber, 2013). Davis et al. (2009) – “results identify No direct comment for Hamble, but Garbutt et al. [vessels waves as] a potentially (2015) suggest wave propagation may affect Vessel important factor that may affect the Marchwood marshes in the adjacent Southampton impacts long term sustainability of the marsh”. Water. Hamble waves likely more attenuated and Also noted as strongest correlation with from recreational vessels. lateral marsh movement by Price (2006). Bulleri & Chapman, (2010) “Coastal Further land reclamation unlikely in context of Land reclaim landscapes are being transformed as a conservation and geological features. See North

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FACTOR REFERENCE HAMBLE REFERENCE consequence of the increasing demand Solent SMP (Policy Units 5C01-5C05), (here). See for urban infrastructure to sustain Eastleigh (here) and Fareham (here) local plans commercial, residential and tourist (Planning Policies DSP15 and DSP19). Eastleigh local activities”. plan currently un-adopted (consultation ended 7th Feb. 2016). Bunny Meadows marina application turned down in 1970s.

Figure 3.2: Factors affecting estuary margin habitats Price (2006) There has been considerable research on the decline of saltmarsh both in the UK and in other continents. What is often notable within research papers and condition reports is a commonality that the reasons for decline are difficult to categorise and often not understood. In the Hamble, the problem is seemingly little different, though there is perhaps a cascading, or linked, causality which appears to relate well to statements in the EA saltmarsh management manual (2007) which comprise: “…the vast majority of saltmarshes south of a line from (approximately) Suffolk to South Wales, although often displaying successional changes in the upper marshes, appear to be in decline. Far from being replaced by low marsh species, the pioneer zones of these marshes are often either eroding or absent. Reasons for this include the extensive die-back of Spartina marsh and the combination of rising relative sea levels causing submergence and coastal squeeze”.

Cope et al. (2008) noted that saltmarsh loss in the Hamble between 1946 and 2000 was “41.5% including reclamation and 23% excluding reclamation”. This equated to 0.8% and 0.4% loss per year respectively, the latter of which was noted as “remarkably low” when compared to other North Solent sites (e.g. Langstone Harbour). Excluding construction/reclamation works, which were prevalent for marina creation in the 1960s-1970s (Williams-Hopley, 2014), the main factor in saltmarsh loss was given

62 AHTI_J2015_004 RIVER HAMBLE SOFT SE DIMENT HABITAT RETEN TION FEASIBILITY STUDY as edge erosion for the periods 1971-1984 (1% loss per annum, greatest loss in Hamble (1.7%) in this period) and 1984-2000 (0.5% loss per annum due to low reclamation) (Cope et al., 2008). From this, it is evident that the major factor has historically been construction/reclamation, perhaps more prevalent before full understanding of the habitat’s value in the region and elsewhere (see: Johnson, 2000a).

As discussed, Cope et al. (2008) noted that the dominant process found in the loss of River Hamble saltmarsh was edge erosion (also for the Hamble, see Cundy & Croudace, (1995); Cundy & Croudace, (1996)), and through ongoing GIS analysis of data, Williams et al. (2010) recorded the same for sites investigated in the River. This effect has been recorded elsewhere in the northern hemisphere. Edge erosion has been predicted to worsen and result in the “loss of intertidal wetlands and their component wildlife species. In particular, impacts to salt marshes and their wildlife will vary both temporally and spatially and may be irreversible and severe” (Thorne et al., 2012). Thorne et al. (2012) also noted that “many salt marsh habitats are already impaired and are located where upslope transgression is restricted, resulting in reduction and loss of these habitats in the future. In addition, we conclude that increased inundation frequency and water depth will have negative impacts”. Whilst this work was undertaken in the USA on a “highly urbanised” estuary, which parts of the Hamble are not, it does have resonance with the findings and comments of Gardiner et al. (2007).

Gardiner et al. (2007) considered coastal site sea level rise scenarios for sites on the UK south coast from Portland Bill to Langstone Harbour. Here it was reported that for the upper Hamble (above the M27) there is a migration space for intertidal habitat to move laterally with changing tide levels, but at the expense of woodland habitat in the area. In the lower Hamble, for this study taken as below the A27 road bridge, GIS analysis indicated that “land use is a significant constraint. Here, intense development has meant that accommodation space is limited and by the 2080s, under most of the sea-level values, the majority of the saltmarsh areas will have disappeared” (Gardiner et al., 2007).

Specific research on the Hamble cannot currently fully identify the primary reason(s) for decline in River Hamble saltmarsh over and above acute reclaim loss. However, as suggested by Thorne et al. (2012) in studies on North American saltmarsh, it may be that for River Hamble habitats “synergistic effects caused by combining stressors with anthropogenic land-use patterns could create areas of significant [saltmarsh] loss”. When considered in conjunction, certain factors identified in Table 3.1 above are potentially co-responsible for decline with relationships between coastal squeeze, sea level rise and potentially water logging and Spartina loss, though caution should be ascribed to conclusions regarding Spartina interactions which may not be clear.

The self-limiting factor of increased redox through water retention by S. anglica may be significant in relation to both sea level rise and coastal squeeze. In addition, research has suggested that Spartina, and related coastal vegetation root systems are not necessarily responsible for binding sediment in the saltmarsh matrix per se. Alternately (Feagin et al., 2010) suggest that saltmarsh plant derived organic detritus binds sediment to “… control sediment dynamics in response to gradual phenomena like sea level rise or tidal forces, but is less well-suited to resist punctuated disturbances at the seaward margin of salt marshes, specifically breaking waves”. Further to the above Feagin et al. (2010) quoted previous research that showed above ground portions of plant stand can result in surface scouring in adjacent sediment less dense with plants and that this may be related to “cliff” edges as a result of relative differences in landward versus seaward flow velocities and wave stresses on either side of the edge. They went on to say that waves can cause large block detachment such as seen in edge erosion noted in

AHTI_J2015_004 63 RIVER HAMBLE SOFT SE DIMENT HABITAT RETEN TION FEASIBILITY STUDY the Hamble. The Hamble is not significantly wave dominated (see ABPmer, 2011a), but further research may indicate if the edge erosion seen in River Hamble saltmarsh could be related to vegetation structure and associated hydrodynamics.

Overall, the information provided in Table 3.1 indicates related factors as noted by Thorne et al. (2012). Whilst more research at individual sites may be justified, the synergies identified here through work by previous researchers potentially indicate the main causal factors for marsh erosion in the lower Hamble. This is subject to the caveat that cause and effect in saltmarsh decline (or accretion) is entirely problematic to accurately define on a site by site basis.

3.3 Impact of dredging on sediment draw-down Over the past 25 years, concern has grown about the loss of foreshores to a variety of factors, many of which are encapsulated within the concept of 'coastal squeeze' (for the Hamble, see Gardiner et al., 2007). However, there is also a further factor that can have a potentially substantial influence on inter- tidal extent and elevation: namely modification of the form and function of estuaries. This can be achieved either by deepening the channels and creating new structures such as berthing pockets and basins through a process of dredging; or by narrowing the cross-section of an estuary through the building of hard features into the inter-tidal and sub-tidal environment. Understanding the processes involved depends upon a basic knowledge of the ways estuaries have evolved and what they do in the event of modification.

3.3.1 A conceptual model The critical starting point for understanding the ways in which estuaries evolve is to reflect back on the past 10,000 years of estuarine evolution. Over this time, there has been substantial sea-level rise and reworking of the coastline. Depending upon the robustness of the rocks that make up the coast, it has eroded at varying rates, with very rapid inland transgression where rocks are unconsolidated clays or sands. These influences are most pronounced on the east coast, but are also important for parts of the south coast.

Paradoxically, coastal erosion is an essential part of estuarine evolution and the development of intertidal features such as saltmarshes and mudflats. Eroded sediment from soft cliffs enters the water column and is carried into low-energy situations where it is deposited as a result of a variety of mechanisms. Simple settling within the water column and deposition as a result of gravity is part of this process; but flocculation as waters become brackish is also very important and is responsible for the creation of the cohesive sediment layers (clays and silts) that make up much of the inter-tidal environment of British estuaries. In the River Hamble, most intertidal sediments come from a combination of marine sources and the re-working of existing sediments. Comparatively little comes from fluvial sources (ABPmer, 2011a; Williams-Hopley, 2014).

The regime state of an estuary is a theoretical dynamic equilibrium between sediment inputs and outputs (including human influences such as dredging, reclamation, engineering works etc.). At its simplest, sedimentation in estuaries seeks to conform to a very simple rule (O’Brien, 1931, 1969), in which the tidal volume is directly related to the cross-sectional area of the mouth. Estuaries seek to reach a point where there is sufficient energy to maintain an open channel (governed by the cross- sectional area of the mouth) and the energetics are “at equilibrium” (entropy). Any additional space will fill with sediment because the energy environment is lower than the shear stress required to mobilise

64 AHTI_J2015_004 RIVER HAMBLE SOFT SE DIMENT HABITAT RETEN TION FEASIBILITY STUDY sediment deposited. This form is known achieving “Regime Morphology”. The additional space that is filled by fine sediment is referred to as “Accommodation Space”. Eventually, in theory, the tidal prism (the volume of water in an estuary or inlet between Mean High Water (MHW) and Mean Low Water (MLW)) shrinks so that the estuary becomes a complex of subtidal and intertidal channels together with infrequently vegetated higher ground (saltmarsh and dry grassland followed by wet woodland). The prism-area relationship is very consistent for barrier enclosed estuaries, but less consistent for river mouth estuaries such as the Hamble.

All estuaries seek to achieve Regime form, but can only do so if there are sufficient sediment supplies. The classic estuary that fails to fill up is the Ria (drowned river valley), but there are many stages in the process and these are represented across the British Isles. Southampton Water (and its tributaries) was probably close to Regime by the late 18th Century, but has since changed dramatically in the wake of urbanisation and industrialisation. This 18th Century landscape is now gone, but the response to those changes is ongoing.

3.3.2 How do estuaries respond to dredging? If the natural depth of an estuary is changed by dredging, the system moves away from the state of dynamic equilibrium that it has sought to attain. The impact of dredging depends upon the degree to which the system is modified and the size of the estuary. There are several possible responses:

 Greater sedimentation within the channel. Once the equilibrium form has been attained, the average depth of the thalweg will vary relatively little (in geography and fluvial geomorphology, the thalweg is the line of lowest elevation within a watercourse). The position of the thalweg may, however, vary and may include ancillary channels for flood and ebb tides. These features tend to be lost as estuaries are incrementally changed by dredging;  Linked to channel sedimentation, there will be proportionately less sediment for deposition on the foreshore. This may not matter in sediment-rich systems such as the Humber or the Severn, but can start to significantly impact deficient systems such as those in the Greater Solent;  Substantial channel deepening in longer estuaries can lead to a rise in the heights of the tide at the landward end of the estuary, together with reductions in the low tide. Extreme examples are generally associated with big tidal rivers with major inland ports, e.g. the Elbe (Germany), Ems (Germany), Western Schelde (Belgium) and Seine (France) (Morris & Mitchell, 2012). Similar changes have also been noted in the Ganges-Brahmaputra Delta (Pethick & Orford, 2013). In the UK, this situation is not apparent, but there is documentary evidence of such changes in the Thames Estuary (Bowen, 1972), and also a record of deliberate engineering of the Clyde (Russell, 1838) to speed up the flood tide (which will have raised tide heights);  In extreme cases of loss of accommodation space and channel deepening, elevated tides are also accompanied by elevated suspended sediment levels. The most extreme example is the Ems (Germany). The high level of sediment import is termed “sediment pumping”.

The response to dredging by estuaries with naturally low suspended sediment loads is often described as “sediment drawdown” (Barton, 1979). Drawdown occurs as a long-term and gradual process in which sediment mobilised by wave activity is re-distributed by the tide. Some of this sediment will be re- deposited on the foreshore, but some will end up in the navigation channel and in berthing basins and

AHTI_J2015_004 65 RIVER HAMBLE SOFT SE DIMENT HABITAT RETEN TION FEASIBILITY STUDY pockets where it settles in the slow-moving water. Over time this sediment accumulates and has to be removed (dredged) and placed in a situation where it will not impact on navigation. Use of offshore disposal sites means that the sediment eroded from foreshores, as with other sediment transported into the Hamble, is eventually lost to the system.

The potential modification of the slope angle of the estuary bed as a result of dredging can exacerbate drawdown, through slope failure and slumping of sediments. Intertidal slopes near dredged areas in Southampton Water were reported by Barton (1979) to be frequently much steeper (9-13 degrees) than those more remote from dredged channels contributing, at least in part, to the failure and slumping of over steepened dredged channel margins.

The effects of drawdown are most keenly felt in estuaries where the background levels of suspended marine sediment are low. In estuaries with high levels of suspended sediment the effects of drawdown may be imperceptible, because there is sufficient suspended sediment to replace lost material. Thus, it is important to evaluate the sediment budget of estuaries. The effects of drawdown are most apparent in estuaries where levels of foreshore sedimentation are lower than levels of erosion.

Williams-Hopley (2014) describes drawdown as a result of the failure (slumping) of over steepened dredged channels (to include intertidal banks and subtidal features), as occurring in Southampton Water and the River Hamble; drawdown and slumping are intrinsically related, with the former resulting in the latter. This was identified by accretion of the subtidal, leading to a conclusion that a drawdown of sediment into the dredged area through slope failure had occurred. For example, it was shown that between 1976 and 1996, and 1996 to 2001, channel dredging in Southampton Water coincided with frontal erosion of the entire length of the salt marsh and mudflat lining the Main Channel, erosion of the subtidal, and accumulation of sediment to the west of the Main Channel.

Williams-Hopley (2014) also indicates that, for the River Hamble, some evidence of this process can be identified in the data analysed. In particular, saltmarsh experienced frontal erosion (with an associated increase in mudflat volume), possibly associated with drawdown of the intertidal sediment, and following capital dredging of the channel associated with initial marina development in the 1960s. Williams-Hopley (2014) also identifies a decrease in the average height of mudflat, as well as saltmarsh frontal erosion, possibly due to drawdown linked to deepening and widening of the estuary in association with development of the boating industry in the 1970s (Williams-Hopley, 2014), which, in a very localised sense, appears to be continuing at some sites. More recently, Williams-Hopley (2014) indicates some evidence of small scale slumping of intertidal sediment into Mercury Yacht Harbour. Anecdotal evidence also suggests very localised, small-scale, slumping of sediments occurs into the dredged areas of Hamble Point Marina, Mercury Yacht Harbour and Deacons Boatyard (Lymington Technical Services, pers. comm.).

The complication of “sediment drawdown” is that it leads to the gradual loss of intertidal features. Saltmarsh cliffing and lateral regression releases sediment that may feed adjacent mudflats; but some will end up as dredging spoil. Mudflats may gain sediment from eroded saltmarsh whilst this source exists, but a proportion of the mudflat sediment will also be lost. These changes are represented by an overall loss of extent of saltmarshes and the lowering of mudflats. Mudflat lowering will ultimately lead to exposure of more heavily consolidated sediment that is far less prone to erosion, but which is also of little value to benthic organisms (benthos) that is the food source for fish and for migratory waterfowl.

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Similarly loss of saltmarsh itself can have cascading ecological effects as well as loss of associated ecosystem services.

3.4 Potential for reduced sediment supply The annual maintenance dredging within the estuary is approximately 16,000 m3 and, currently, all of this material is removed from the system and deposited at the dumping ground at Nab Tower and Hurst Fort (ABPmer, 2011a). Thus an argument may be made that, in the absence of the maintained basins, an additional 16,000 m3 of sediment would be available for the estuary’s saltmarshes and mudflats. However, alternately the River Hamble Maintenance Dredge Plan (ABPmer, 2011a) suggests that this argument is incorrect, as it ignores the modification of the estuary which created the marina basins. The Maintenance Dredge Plan indicates that:

 Large areas of intertidal and saltmarsh were removed to create the marinas;  This increased the tidal prism of the estuary;  There is an assumption that propagation of water in the estuary has not changed significantly; and, therefore  The volume of water passing in and out of the estuary is significantly increased, relative to the pre-marina situation.

ABPmer (2011a) indicates that “the average increase in tidal prism caused by the marina dredging represents a 7.4% increase in the average estuary tidal prism” i.e. the volume of water passing into and out of the Hamble has increased by an average of 7.4% over the pre-marina situation. Transport of fine- grained sediment in the River Hamble is dominated by the flood tide, and hence transport of fine- grained sediment into the estuary is also increased by approximately 7.4% over the pre-marina baseline. ABPmer (2011a) therefore conclude that the maintenance dredging is only removing the increased sediments brought in as a result of marina construction and the maintained dredge basins do not reduce the sediment supply to the saltmarsh and mudflats.

This view is challenged by Williams-Hopley (2014) who indicates that the increase in volume and area of the channel resulted in a decrease in the sediment stored in the system by approximately 10.5 x 106m3. Figure 3.3 shows the change in the volume of sediment required to balance the sediment budget over time, while Figure 3.4 shows the deviation in the contribution of the various components, excluding land claim, of the sediment budget to the mass balance of the estuary between 1783 and 2008.

Figure 3.3 indicates that the Hamble Estuary’s demand for sediment to balance the sediment budget increased between 1783 and 2008 and Figure 3.4 shows that the change in demand for sediment from marine exchange to balance the sediment budget was related to the demand for sediment from intertidal stores. The periods in which the greatest sediment demand is placed on the system relate to periods of intertidal habitat accretion, with Figure 3.3 and Figure 3.4 showing that the managed realignment of Bunny Meadows (1932-1947) placed a significant sediment demand on the system.

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Figure 3.3: Average annual volume of sediment required from marine exchange to balance the Hamble Estuary sediment budget 1894 to 2008 Blue shows periods where there was an increase in the sediment demand and pink a decrease in the sediment demand to balance the sediment budget from marine sources (after Williams-Hopley, 2014)

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Figure 3.4: The deviation in the components of the Hamble Estuary sediment budget between 1783 and 2008 Blue shows periods where there was an increase and pink a decrease in the sediment demand to balance the sediment budget from marine sources. Error for these amounts is given in Table 3.2 (after Williams-Hopley, 2014)

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Table 3.2: Hamble Estuary Sediment Budget 1783-2008 - volumes in x103m3 (Blue is an input to the sediment budget and pink is an output) (after Williams-Hopley, 2014)

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Williams-Hopley (2014) indicates, therefore, that the demand for sediment from marine exchange to balance the sediment budget is directly related to the demand for sediment from intertidal stores; that managed realignment sites place a sediment demand on the rest of the system; and that following dredging activities the dredged areas become a sediment sink, i.e. the dredge basins are intercepting sediment that would otherwise stay within the water column; a proportion of which would potentially be available for deposition on mudflats and saltmarsh, while some would inevitably be deposited subtidally.

Williams-Hopley (2014) concludes therefore that complete removal of dredged sediment from the system reduces a potential sediment supply required for saltmarsh and mudflats to recover from short term erosion events, to keep pace with rising sea levels, and to keep pace with their position in the tidal frame in the long term (see also Morris, 2007). Since most of the accommodation space in the River Hamble has been lost as a result of coastal squeeze, the intertidal habitats are flooded more often, which may lead to some surface accretion, and conversely are more prone to edge and drainage erosion. This will lead to a loss of sediment from the foreshore, and some of this sediment will be deposited within the dredged basins. Analysis of the intertidal areas examined in the current study has shown that during the years between 2007 and 2014, the volume of deposited material was approximately 66,000 m3, whilst the volume of eroded material was approximately 158,000 m3 (the net loss was therefore 43%) – see Section 2 for individual marsh area analyses.

It should also be remembered that sediment supply within the River Hamble is not isolated from that within Southampton Water, and the effects on the Southampton Water sediment supply will have an impact within the Hamble. The main supply of fine sediment to estuaries in the western Solent, including Southampton Water, comes from the eroding cliffs and seabed in Christchurch Bay (Bray et al., 1995; New Forest District Council, 2010b). Lawn (2001) calculates that despite continued active erosion this supply has reduced by 41% from 136,000 m3/yr, prior to 1932 to 80,000 m3/yr after this time due to increased coastal protection in the region.

Williams-Hopley (2014) indicates that Southampton Water has altered from a system which net exports sediment to one where sediments need to be imported to balance the sediment budget, largely through the impacts of dredging associated with port development. Williams-Hopley (2014) also suggests, therefore, that in the future it could be that marine sediment supplies are not sufficient to satisfy the sediment demand of the estuary, meaning that the sediment budget will only balance through the erosion of intertidal habitats. As this applies to the whole Southampton Water system, by extension, this also applies to the River Hamble. However, as there are arguments on both sides of the subject matter, it may be beneficial to undertake long term sediment tracer studies which aim to clarify erosion and transport trends both during normal cycles within the estuary and episodes of dredging. This may, or may not clarify matters as there will be numerous confounding factors in such a complex and managed system.

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4. SEDIMENT DEPOSITION

Question c: Type, sources and relative quantities from each source of the sediment which deposits in the marinas

Question f: The suitability of the Hamble Estuary’s maintenance dredge for reuse, including in beneficial disposal and habitat management

4.1 Introduction The previous section assessed the factors that could potentially have an impact on the decline of the Hamble Estuary’s intertidal area, both in area and habitat quality. This section will examine the sediment deposition in the Estuary and its suitability as a source for beneficial reuse.

4.2 Sediment composition and quantities from dredging Observations by Tosswell (1984) indicated that the main source of suspended sediments in the River Hamble was from the seaward direction, with the dominant mass transported by the flood tide, rather than by the ebb. The tidal wave within the River Hamble, as with that of Southampton Water, shows an unusual pattern with a long duration flood, a double high water, and short duration ebb. ABPmer (2011a) reports that the form of the tidal wave (in terms of tidal height and peak speeds) is preserved as the wave propagates into the estuary as far as the Mercury Yacht Harbour, after which the characteristics begin to alter. The long duration of the flood tide, and a settling lag occurring in the long slack water period during the double high water, provides a mechanism for the movement of fine grained sediments into the Hamble. Settling lag is a delay between the point at which fluid velocity falls below the threshold for suspension and the particle reaching the sea bed i.e. a particle continues to be carried landwards even after fluid velocity has fallen. The continued distance of travel partly depends on the settling speed of the particle and the duration before the subsequent reversal of flow direction. The shorter duration ebb currents may drive bedload transport of coarser particles towards the mouth of the Hamble, or the current speeds and settling times may simply not be conducive to deposition of fine grained sediment.

While suspended sediment measurements are rare, a prolonged period of suspended sediment measurements between 1980 and 1982, and reported in Tosswell (1984), indicated background suspended sediment concentrations of between 15 – 30 mg/l. More recent suspended sediment measurements were made by O’Mahoney and Weeks (2000), who found that observed concentrations ranged from 11.7 mg/l in early October to a maximum of 38.9 mg/l in late May.

Importantly, the general transport of fine grained sediment into the River Hamble from Southampton Water, means that since dredging of fine grained sediment from the marinas occurs upstream of the estuary mouth, then this dredging is unlikely to reduce the supply of fine grained sediment into the estuary. ABPmer (2011a) notes, however, that ongoing maintenance dredging (where sediment is disposed of outside the estuary, and hence removed), may potentially reduce supply of sediment to areas upstream of the marinas.

Particle size distribution (PSD) data for the sediment deposited in the marinas of the River Hamble is rare. Extremely limited particle size data (four samples) was analysed for Williams et al. (2010) and is presented in Figure 4.1 below. In order to maintain confidentiality of sample locations (as data were not

72 AHTI_J2015_004 RIVER HAMBLE SOFT SE DIMENT HABITAT RETEN TION FEASIBILITY STUDY in the public domain) the sample locations have been anonymised, but it can be seen that the sediments are classified as Sandy Mud and Slightly Gravelly Sandy Mud, according to the Folk classification scheme.

Gravel content within the sediments is extremely low, between 0.0 – 1.1%, and this is unsurprising since gravel sized particles are unlikely to be transported in the typical flows found within the river. Sand particles make up between 18.1 – 32.5% of the measured particles, while mud (<63µm diameter) makes up between 67.51 – 80.1% of the samples. Suspended sediment measurements within the River Hamble are scarce, and it is difficult therefore to make inferences between the proportions of mud, sand and gravel found within the marinas and the proportions of each sediment class suspended within the natural river flow. However, the Deacons Boatyard Dredge: Hydrodynamic Assessment (ABPmer, 2011b) does indicate that the majority of the sediments found in the vicinity of the Deacons Boatyard are fine grained muds, and the sediment types indicated in Figure 4.1Error! Reference source not found. are onsistent with the published data.

Figure 4.1: Sediment samples from River Hamble marinas, analysed for Williams et al. (2010) and plotted on a Mud-Sand-Gravel tri-plot The River Hamble Maintenance Dredge Plan (ABPmer, 2011a) also presents average siltation rates for the marinas (Table 4.1) and indicates a declining siltation rate upstream of the Mercury Yacht Harbour, but a relatively uniform rate of 0.1 – 0.2 m/yr below Mercury.

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Table 4.1: Average siltation rates in River Hamble marinas 1986-2010 (ABPmer, 2011a) MEAN ANNUAL DEPOSITION MEAN ANNUAL SILTATION RATE MARINA (x103m3) (m/yr) Swanwick Marina 2.9 0.05 Universal Marina 1.5 0.08 Mercury Yacht Harbour 3.6 0.1 Hamble Yacht Services 0.6 0.08 Port Hamble Marina 3.4 0.1 River Hamble HM Pier 0.9 0.11 Stone Pier Yard 0.2 0.03 Hamble Point Marina 5.3 0.17

ABPmer (2011a) also collated dredge data from 1986 – 2010 and concludes that in the region of 480,000 m3 of sediment was dredged during those years. This includes some capital dredge volumes, so the Plan concludes that the average annual maintenance dredge volume is in the region of 16,000 m3. Most of the marinas within the estuary are dredged once every 2 – 9 years, with Hamble Point Marina being dredged most frequently. These sediments are currently removed from the system, with the majority being deposited at the Nab Tower licensed disposal site and a smaller amount deposited at the Hurst Fort disposal site.

Boorman, (2003) points out that the successful use of dredge sediment to create saltmarsh depends not only on the availability of sufficient quantities of the right material, and a comparable level of organic content (EA, 2007), but it also has to be a similar, suitable, particle size as well as being of an appropriate density. This is obviously a significant issue when importing sediment into a system, however material dredged from the River Hamble marinas comprises that sediment which would otherwise have naturally deposited on the intertidal areas and mitigates the risk of importing otherwise unsuitable sediment.

4.3 Suitability of River Hamble dredge material for reuse Sediments obtained from the ongoing maintenance dredging programme in the River Hamble marinas are potentially an obvious source for beneficial reuse and habitat restoration within the system. In addition to the potential for habitat restoration, this possibility of keeping sediment within the geomorphological system from which it derived (Haynes, 2016) could make the system more “sustainable” in terms of sediment supply for marshes subject to ongoing stressors.

When assessing the suitability of dredged material for recharge schemes, consideration must be made of whether the sediment is contaminated by e.g. heavy metals and hydrocarbons, which can bioaccumulate to higher organisms or cause stress in affected plant species (see Section 5.5 below). In addition, the particle size and organic content of sediment also require consideration (EA, 2007) to facilitate colonisation by flora and fauna that would be associated with a more natural marsh (Leggett et al., 2004; Laegdsgaard, 2006; Hudson et al., 2008). Sediments dredged from areas close to industrial activity may contain increased levels of pollutants. Those from popular recreational boating areas in particularly can contain increased levels of pollutants associated with antifouling of which the River

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Hamble has a research history for tributyltin (TBT) and biocides, e.g. Diuron and Irgarol 1051 (Thomas et al., 2001), as well as copper after the 1987 TBT ban on recreational craft (Grant, 2007).

One means of assessing the usability of dredged sediments is to compare their analysed contaminant levels with the “Action Levels” for contaminants as defined by Cefas. Action Levels are used as part of a “weight of evidence” approach to assessing dredged material and its suitability for disposal at sea and hence by extension within this project, its suitability for use in recharge projects. In the Cefas decision making process, these values are used in conjunction with a range of other assessment methods, including bioassays, the disposal site characteristics and other relevant data. These data are then employed to aid management decisions regarding the fate of dredged material. These further methods are beyond the scope of this review, however some of these may be appropriate if this study was to lead to a full-scale trial project.

Table 4.2 shows a range of contaminants from River Hamble sediments, linked to a series of Action Levels; to maintain confidentiality of sample locations (as data are not in the public domain, but are kindly provided for this research) sample locations have been anonymised. In general, contaminant levels in dredged material below Action Level 1 (in Green) are of no concern to Cefas and unlikely to influence the licensing decision, while dredged material with contaminant levels above Action Level 2 is generally considered unsuitable for sea disposal. Where contaminant levels lie between Action Levels 1 and 2, further consideration must be made before disposal, but there is no prohibition that sediment exceeding Action Level 1 is unsuitable for further use, though advice will be necessary before beneficial use may proceed.

Table 4.2 shows the analyses for metals, TBT, and dibutyltin (DBT) for 25 samples from Hamble marinas, collected in 2011 and 2012 and tested for contaminants prior to disposal of those sediments in licensed disposal areas. All green cells show contaminant levels below Action Level 1, while orange cells show contaminant levels in excess of Action Level 1 but below Action Level 2. It can be seen that none of the sediment analysed contains contaminants that exceed Action Level 2, however levels in excess of Action Level 1 are found in all the samples tested. Chromium, Copper and Nickel are the most common metals above Action Level 1. The majority of the sediments show levels of TBT and DBT below Action Level 1, though where TBT (and its derivative DBT) are recorded this reflects the compound’s ability for long term persistence in fine organic sediments (e.g. see Maguire, 2000).

Data available for River Hamble sediments does not contain particle size analysis or organic content information. It gives a broad description for particle size of fines (those particles likely to be carried in suspension so typically material <2mm in diameter, with emphasis on the <63 micron fraction) which would be in accordance with the predominant very fine muddy silt found in the river and in the adjacent saltmarsh. Thus, as a broad assumption, it would seem that sediments available from dredged sources may be physically within the size class necessary for beneficial use. However, the organic content is unknown, and opinion on sediment contaminants would be required from the MMO, as well as liaison with other stakeholders as necessary, as to its suitability for a beneficial use scheme.

In addition to the data in Table 4.2, ABPmer (2011a) presents sediment quality data collected within the River Hamble from 1999 – 2009. These data indicate that sediment-metal concentrations are generally low, although some samples show values exceeding Action Level 1 (typically for Copper, Mercury and Zinc). Interestingly, in comparison, in the recently collected samples collected from marinas (summarised in Table 4.2), only a single sample shows Mercury levels above Action Level 1. As a possible

AHTI_J2015_004 75 RIVER HAMBLE SOFT SE DIMENT HABITAT RETEN TION FEASIBILITY STUDY causality, research has shown that dredging disturbance and bacterial methylation processes can reduce mercury levels (Bloom et al., 2004); note, though a plausible mechanism, the actual reasons for decline would require research. ABPmer (2011a) also indicates that where Action Level 1 is exceeded, this is usually by a relatively small amount compared with the range between Action Levels 1 and 2. Only a single sample exceeded Action Level 2 (for Mercury, collected in 2004), however no depth data were collected with this sample and it is therefore uncertain whether this represented recent or historic contamination.

The data summarised by ABPmer also indicated that for sediments collected from 1999 – 2009, a number of samples exceeded Action Level 1 for TBT, while three samples exceeded Action Level 2. Again, since no depth data were recorded for each sample it was not possible to make any conclusions regarding whether this represented historic contamination. Comparison with Table 4.2 suggests however, that TBT contamination may be declining, with most samples below Action Level 1 for TBT and none exceeding Action Level 2. DBT levels in samples collected from 1999 – 2009 were below Action Level 1, with the exception of two samples; while one sample collected between 2011 – 2012 exceeded Action Level 1 for DBT.

Finally, ABPmer (2011a) indicates that for polychlorinated biphenyls (PCBs), all samples were below Action Level 1; while for polycyclic aromatic hydrocarbon (PAHs), all samples showed at least some PAH substances that exceeded Action Level 1.

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Table 4.2: Metal, TBT and DBT analyses for sediments dredged from River Hamble marinas SAMPLE ARSENIC CADMIUM CHROMIUM COPPER LEAD MERCURY NICKEL ZINC TBT DBT 1 2 3 4 5 6 7 8 9 10 11 NO DATA NO DATA 12 NO DATA NO DATA 13 NO DATA NO DATA 14 15 16 17 18 19 20 21 22 23 24 25

Samples below Action Level 1 Samples in between Action Levels 1 and 2

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As has been indicated previously, further consideration must be made by the Regulator and its Advisors before disposal of sediments containing contaminant levels lying between Action Levels 1 and 2. As mentioned previously, it is important to note that there is no prohibition that sediment exceeding Action Level 1 for contaminants is unsuitable for further use. Since Licences have been granted to dispose of dredge arisings from maintenance works of River Hamble marina sediments, it is perhaps reasonable to conclude that disposal of these sediments does not pose an unacceptable environmental risk in terms of further pollution. It may also be concluded that, subject to further consideration by the Regulator and its Advisors, that these sediments may be suitable for placement (disposal) with the River Hamble system rather than by removal from the system. Whether the organic content and physical factors are suitable for a beneficial use scheme, will require research.

4.4 Summary The suitability of River Hamble dredge material for beneficial reuse schemes in the intertidal areas of the Estuary has been assessed in this section. Limited data on PSD and contaminant levels were available for this study, and no data on organic content were available. However, from the data obtained, the assessment is that there is the potential for beneficial reuse of dredge material, but further assessment and consultation, as discussed above, will be necessary.

The next section will assess the potential for beneficial reuse of dredge material within the intertidal areas of the lower River Hamble through the means multi-criteria analysis (MCA), as well as what other methods could be used to slow the decline of saltmarsh areas.

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5. SUITABILITY OF HAMBLE ESTUARY FOR HABITAT IMPROVEMENT

Question g: The suitability of Hamble mudflats and saltmarshes for direct or indirect reception of maintenance dredge arisings for habitat sustainment, restoration, enhancement or creation.

Question h: The suitability of Hamble mudflats and saltmarshes for sediment management techniques other than the reuse of dredged material.

Question i: Potential environmental benefits of the disposal of maintenance dredge arisings and other sediment management techniques on the Hamble Estuary system and its margins.

Question j: Potential adverse environmental impacts of the disposal of maintenance dredge arisings and other sediment management techniques on the Hamble Estuary system and its margins.

Question k: Potential for disposal at one site to cause increased accretion at other locations (e.g. marinas, navigable channels or neighbouring habitats).

5.1 Introduction This section considers the feasibility of intertidal soft sediment features in the River Hamble for direct placement of dredge arisings, the possibility of more passive sediment management techniques and the benefits and possible adverse implications of these approaches.

5.2 Suitability of intertidal areas for reuse of dredge spoil Efforts to restore marsh should be undertaken with due caution in relation to potential damage to the existing and already beleaguered habitat (see Section 5.5) or possible infringement of conservation regulations, water quality aspects and the potential for impact on nearby infrastructure or other habitat. Williams et al. (2010) in their work on potential Solent saltmarsh restoration found a precautionary approach recommended by Natural England (re: conservation aspects), the Environment Agency (re: water quality, trace elements and possible impacts on flora and fauna, including possible bioaccumulation through saltmarsh plants – e.g. see Weis & Weis, (2004); Curado et al. (2014)).

Globally, soft sediment habitat restoration using passive or semi-passive approaches through failure or removal of sea defences; and active through beneficial use placement of dredged material or sediment retention approaches, has been undertaken with varying success. The active placement of dredge spoil is perhaps the more problematic approach, as it seeks to simulate the habitat needed for the development of pioneer and upper marsh plant species, and to provide suitable conditions for colonising biota e.g. aerating infauna (Boyer & Fong, 2005).

Factors that need consideration when examining the viability for beneficial use placement include: physical and ecological condition; socio-economic impacts and, significantly for the highly protected Hamble marshes, conservation status (also see section 5.4). Whilst in theory success in soft sediment habitat restoration should be apparent, the reasons for failure may be less so, and can interplay between biotic and abiotic factors as with the initial marsh decline. The information below, (modified)

AHTI_J2015_004 79 RIVER HAMBLE SOFT SE DIMENT HABITAT RETEN TION FEASIBILITY STUDY from Williams et al. (2010), summarises some of the difficulties that can lead to marsh restoration failure as an overview of the variability of success:  Significant difference between dredged material organic content and pre-existing saltmarsh sediment. Oxygenating invertebrate (infauna) recolonisation is sensitive to organic content differences. UK guidance (EA, 2007; also see Laegdsgaard, 2006) suggested this be assessed during initial decisions. Note that these data were not made available for assessment for this study;  Change in marsh structure where recolonising plant communities become different from surrounding natural areas. For example, where marshes are over-heightened by beneficial use sediment deposition and the natural plant communities change to Phragmites spp domination (Warren et al., 2002). With reference to the River Hamble, see Gardiner et al. (2008) and Section 3.2 above, also see EA (2007;  Difference in nutrient content from dredged sediment compared with the natural system may affect development of a new marsh community (Langis et al., 1991). In particular nitrogen is a primary limiting nutrient in many saltmarshes (Oczkowski et al., 2016), a lack of which may affect plant species/communities, also see EA (2007);  Insufficient dewatering (drainage and gradient) of placed sediments leading to waterlogging and conditions detrimental to recolonisation or re-establishment of marsh systems (see Table 3.1) and previous consideration of “some” Hamble marshes in Williams et al. (2010), also see EA (2007);  Saltmarsh flora and fauna recolonisation may be adversely affected by sediment pollutants (e.g. heavy metals/hydrocarbons) the introduction of which can impact saltmarsh communities (Windham et al., 2003). However, some studies indicate that saltmarsh halophytes are robust enough for bioremediation (Carvalho et al., 2010). Available Hamble river sediment analysis indicates levels below limits required for offshore or inland disposal, thus in principle suitable for restoration work/trials but will require site to site consideration in relation to Action Levels (see Williams et al. (2010) for description) and required resampling prior to any dredge/beneficial use works as contaminants can exist in “pockets” at high levels.

In the UK, projects have been carried out on passive and active restoration of saltmarsh. The majority have been undertaken through removal or passive failure of seawalls, i.e. managed realignment, of which circa 60 have been recorded (see http://www.omreg.net/query-database/#tabs-2). Around 17 beneficial use of dredge spoil schemes (from the most readily available list (MMO, 2014)) have been completed. These were mainly in Essex and Suffolk (MMO, 2014), with two schemes on the south coast at Lymington River (Lowe, 2012, 2013a, 2013b) and Poole Harbour (MMO, 2014).

As this section considers beneficial use of spoil, the main difficulties related to spoil placement, apart from site access and legislation and social aspects, are its physical and chemical consistency, organic content and the geomorphological complexity (drainage/gradient) that is achieved upon placement. In a critical review, albeit undertaken in 2000, Streever (2000) noted that “Although it is clear that dredged material marshes provide habitat for birds, limited data suggest that dredged material marshes may provide habitat for a different community of birds than natural marshes. Similarly, limited data suggest that geomorphological features found in natural marshes are not duplicated in dredged material marshes. In short, data summarised from the literature suggest that dredged material marshes provide

80 AHTI_J2015_004 RIVER HAMBLE SOFT SE DIMENT HABITAT RETEN TION FEASIBILITY STUDY some of the functions of natural marshes, but probably do not replace all of the functions of lost natural marshes”. A search for opinion on more recent success did not reveal such an encompassing statement, but did suggest some successes in specific factors.

Foster et al. (2013) also indicate that options for the reuse of dredged materials for the recharge of existing, impoverished mudflats and saltmarshes, and for the creation of new mudflats and saltmarshes (usually where they have previously existed or nearby) include:

 Direct placement onto intertidal areas (constrained or unconstrained) to raise the elevation relative to the tidal frame and/or to increase the lateral extent;  Sub-tidal placement at a single point or at a series of points along the shore (called ‘trickle charge’), or dispersion into the water (called ‘water column recharge’) to recycle the sediment onto mudflats and saltmarshes by natural hydraulic processes; and  Direct placement into managed realignment sites to build up the bed level prior to breaching of coastal defences.

As also noted by Williams et al. (2010), Foster et al. (2013) indicate that three techniques are commonly used to disperse dredged sediment in beneficial use projects, being:

 Pumping via ‘rainbowing’, which describes the process of sediment placement whereby a special bow jet sprays the sediment onto the shore or into the water with a lateral movement resulting in a rainbow effect;  Pumping via pipeline, in which sediment may be either pumped from the dredger to the shore via rigid hydraulic pipes or directly into the water from the dredger; and  ‘Grab and place’, where placement is accomplished by unloading the sediment mechanically, usually with the same apparatus that was used to dredge the material.

The technique employed is often dependent upon the type of dredging plant used, i.e. whether suction or mechanical, the applicability of the receiving environment, and confirms that most UK schemes use fine-grained materials from maintenance dredging of existing ports and navigation channels. Foster et al. (2013) also indicate that while uncertainties in their current status and trends make it difficult to assess the overall net change, it is apparent that losses due to erosion continue to exceed gains from Intertidal Mudflat and Saltmarsh Reparation (IMSR) schemes in south-east and southern England. They also confirm that IMSR schemes in the UK have been generally limited to relatively small-scale trials in comparison to elsewhere in Europe and in the USA, with particular note that the Solent region has been the focus of much discussion, but limited action.

Williams et al. (2010) undertook a short review to demonstrate success and failure in beneficial use schemes, for example, a dredge spoil rainbow (spraying) scheme in Louisiana yielded poor results. This was ascribed to over-heightening of the marsh and an increase in sediment sulphides causing redox (Schrift, 2006). Mossman (2014) comments on this for the Solent and for Mercury Marsh (see Section 1.2 above). Other problems associated with use of spoil included inadequate drainage (see above). Also highlighted was the height of marshes which, if too low, did not demonstrate full recolonisation, contrasting findings by Schrift (2006) and perhaps reflecting over inundation. This adequately highlights the factors that can lead to failure, and conversely success, and that there is a need for case by case assessment as stated by Natural England in Williams et al. (2010).

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As highlighted in Table 3.1, saltmarshes in the Hamble appear likely to be affected by cumulative factors comprising natural aspects interacting with human mediated stressors. Reclamation, sea level rise and coastal squeeze from human infrastructure leaving no room for shoreward migration, plus possible localised dredging, Spartina die back and edge erosion all combine to lead to overall historical decline. This is seemingly ongoing in places, particularly in relation to sea level rise and related effects, equally some of the remaining River marshes appear to temporally fluctuate in area e.g. Hacketts.

In the study by Williams et al. (2010) Natural England commented that they “would prefer a trial study to look at sites where there is minimum risk and maximum benefit”. This could be contrasted with the view by Foster et al. (2014) that considerable high level research and discussion have taken place in the Solent region (e.g. see Colenutt, 2001) on the possibility of saltmarsh restoration and management (also see Inder & Ansell, 2008), but that this has led to limited progress (though see Lowe, 2012, 2013a,b).

Cope et al. (2008) used a questionnaire approach (put to local (Solent) coastal managers) for possible managed realignment site suitability assessment. This was combined with topography and tidal elevation data for eleven North Solent locations assessed to possibly offset intertidal soft sediment habitat losses from coastal squeeze. The Cope et al. (2008) study found that “even with managed realignment, there would only be a near balance between gains and losses of intertidal habitat from coastal squeeze alone across the north Solent” (Williams et al., 2010). Of interest to the question put forward by RHHA regarding suitability of Hamble sites for possible restoration is that it was noted by Cope et al. (2008) that “of the 2025ha of potential habitat creation sites, only 552ha were considered suitable” and none of the sites considered suitable were in the Hamble.

Williams et al. (2010) investigated sites in the River Hamble amongst general potential Solent locations and found that, of the eleven marshes they were tasked to investigate in the Solent, the Hamble marshes were not amongst those recommended for restoration using dredge spoil. A multi-criteria analysis (MCA) was undertaken (see below for MCA in this study) which indicated other sites considered in the region (e.g. Keyhaven) were more suitable for beneficial use of sediments/passive intervention than the River Hamble locations. This is not to suggest that Hamble marshes are not suitable for such options, but rather that in a wider regional context, they were less favourable from the analysis undertaken.

As demonstrated in this document, the lower Hamble River has significant constraints upon its geomorphological status in the form of abundant bordering residences and commercial infrastructure resulting in coastal squeeze (Cope et al., 2008). In addition, the numerous marinas are a significant feature in the river and the effective management of sediment through dredging is an important commercial aspect of their operation. The ongoing program of maintenance dredging undertaken by the marina operators indicates that the marina basins are sinks for sediment, with sediment in the River Hamble mostly coming from a combination of marine sources and the re-working of existing material; comparatively little comes from fluvial sources (Hopley, 2014; ABPmer, 2011b).

Previous studies of Southampton Water and the River Hamble, summarised in this report, have shown a potential link between erosion of the local intertidal saltmarsh and mudflats, and deposition in the sub- tidal parts of the estuary. Therefore retention of maintenance dredged sediment, by placement on local saltmarshes, may potentially be seen as contributing to a continued requirement to dredge the marina basins.

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The Hamble marshes are generally opposite, or adjacent to, marinas: e.g. Hackets and Lincegrove opposite Universal; Mercury Marsh adjacent to Mercury Marina; Satchell Marsh adjacent to Port Hamble; Little Marsh and Hamble Common Marsh adjacent to Hamble Point Marina; Crableck and Universal Marshes by Universal Marina and the remnants of Swanwick Marsh downstream from Swanwick Marina. Thus, with the possible sensitivity surrounding re-deposition of sediments as part of a beneficial use project, any placement plans must consider these aspects carefully.

There are several “perceived barriers” to the beneficial use of sediment (Foster et al., 2013) which may persist in terms of a cautionary or reluctant approach to this method of marsh and mudflat regeneration. In addition there can apparently be an oversupply of capital and maintenance dredged material, including in the Solent region. This has been noted in work undertaken by ABPmer where in a meeting at the Solent Forum a comment was recorded that “a great deal is being done to try to use spoil, however it is not always possible to find a suitable destination for dredgings” (Solent Forum, 2015). This may be for various reasons (conservation legislation or the unsuitability of the dredged material which may be too coarse), thus a well-intentioned beneficial use aspiration may not find a “home” for dredge arisings. Should such restrictions or sensitivities be viewed as barriers in the Hamble, the alternative approach of sediment management (retention) can be investigated. However, also at the above Solent Forum (2015) meeting a question was asked as to “how mudflat [and by extension saltmarsh] can be retained”. In response it was stated that “the three banks of fences used at Lymington had been very helpful to overall stability”. This highlights the possibility that other methods exist which promotes, retain, or slow the erosion of saltmarsh and mudflat features, and these techniques may be suitable for use within the Hamble system.

The constraints placed on potential River Hamble marsh restoration using recharge suggest the need for a cautionary and well thought out approach as per regulator guidance. If beneficial recharge use or sediment retention is finally considered feasible approaches for any location within the River, it may still prove impractical due to nearby infrastructure, conservation value and homes. Restrictions may comprise the availability of suitable sediment material, or the apparent likelihood (risk) of success against factors which have led to marsh decline, or that the marshes being considered should not be interrupted in their natural/semi-natural progression. For example, whilst not in relation to dredge spoil recharge, Garbutt et al. (2015) noted that “the chances of success [of planting S. alterniflora at Hacketts and Lincegrove Marshes] are too uncertain to justify disruption of some of the few remaining species- rich saltmarshes in Southampton Water”; a local HCC warden has also previously expressed caution with regard to potential beneficial use at these marshes (see Sections 2.2.1 and 2.2.2 for assessment of saltmarsh change).

Against this background of useful aspirations, but little progress in the region, assessment of whether saltmarsh restoration, augmentation or enhancement in the River Hamble is a viable or realistic proposition is necessary. Table 5.1 (see also section 2.3) considers the marshes in relation to factors potentially affecting them and relevant comments and personal communications, where available. The information given in Table 5.1 is partially used to inform the MCA below.

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Table 5.1: River Hamble marshes and potentially relevant affecting factors (NOTE: marsh names with * are created for this study due to lack of information on formal names) MARSH FACTORS POTENTIALLY AFFECTING RELEVANT RESEARCH OR COMMENT Hacketts Marsh Sea level rise/coastal squeeze Local residents interested in marsh restoration. Edge erosion Garbutt (2015) felt it inappropriate to “disrupt” Algal matts (NE Condition Assessment) a species rich marsh with attempts at Spartina restoration. But, local area at Lands End offers sediment retention possibility. Lincegrove Sea level rise/coastal squeeze Garbutt (2015) felt it inappropriate to “disrupt” Marsh Edge erosion a species rich marsh with attempts at Spartina Algal matts (NE Condition Assessment) restoration. Localised drawdown/slumping Mercury Marsh Over heightened in places, drainage altered Anecdotal information that dredge spoil from Sea level rise/coastal squeeze Mercury Marina was placed on site during Edge erosion construction. May have led to upper band of Loss of drainage Phragmites and “fen vegetation” (Williams et al., 2010; ABPmer, 2011a). Remnant marsh grades to terrestrial. Satchell Marsh Sea level rise/coastal squeeze Limited information available. Appears affected Loss of natural drainage features by local residents having created linear Loss during marina construction channels, presumably for boat launch. Marsh community would require detailed survey to assess viability and drainage engineering needed. Little Marsh Sea level rise/coastal squeeze Limited information available. Local resident Edge erosion interested in marsh restoration, though marsh Loss of drainage relatively intact, potentially due to down shore Protected to south by bund/wall “wall” structure which may have impeded lateral erosion, but pioneer species largely absent (though on adjacent mud area at Hamble Common Marsh), drainage requires optimisation. Linear change in upper marsh community to fen/Phragmites affected by pipeline works suggests marsh on “edge” of community change in this area. Hamble Sea level rise Limited information on marsh itself. Adjacent Common Marsh* Edge erosion Little Marsh is relatively intact potentially due Localised drawdown/slumping to “wall” structure thus indicates what may be achieved at the site. Water outflow and sediment impedance may have led to decline. Local comment suggests change and loss, though evidence of localised Spartina colonisation Hook Marsh Is being created through policy change inland of Inland of sea wall, marked for slow change to footpath. Outer area currently intertidal intertidal habitat as compensatory habitat. channel Seaward side is currently not viewed as feasible for marsh creation Bunny Meadows Created through sea wall breach in 1940s. Numerous high value houses back on to eastern Marsh (South)* Marsh locally eroding due to tidal edge of marsh, evidence of some gardens being influx/outflow. protected by brushwood etc. Garbutt (2015) Sea level rise recognised erosion problems at Bunny Coastal squeeze Meadows as did Dawson & Henville (1992) (scouring of bed and banks) who also noted

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MARSH FACTORS POTENTIALLY AFFECTING RELEVANT RESEARCH OR COMMENT change Phragmites in south east corner, and lack of protection for marsh/mud features from erosive forces of inlet/outlet channels. Bunny Meadows Created through sea wall breach in 1940s. Topographical change has high value houses Marsh (North)* Marsh locally eroding due to tidal protected by height above river. Remainder to influx/outflow. north is smallholding land and mature Sea level rise woodland. Garbutt (2015) recognised erosion Coastal squeeze problems at Bunny Meadows, erosive forces of inlet/outlet channels evident. Crableck Marsh* Formerly linked to Universal Marsh – only Linear band of lower and pioneer marsh remnants left adjacent to foot path, bisected by marina Sea level rise access. Also remnants of pioneer marsh with Coastal squeeze Salicornia sp. Intermixed with Atriplex Sediment supply (Halimione) portulacoides and isolated stands Edge erosion of Spartina. Interruption of sediment supply through marina structure evident and squeeze against footpath. Universal Sea level rise Isolated by Universal Marina structures to Marsh* Coastal squeeze southwest and residential “hard” to north east. Loss of drainage Interrupted sediment supply. Edge erosion and Edge erosion squeeze evident, upper marsh to lower marsh, Sediment supply pioneer species largely absent. Swanwick Sea level rise Formerly part of the much larger marsh now Marsh* Coastal squeeze the site of Swanwick Marina. Sediment supply Sediment supply now significantly interrupted, seemingly by Swanwick Marina, though sediment pathway may require research. Remnant of the marsh remains.

5.2.1 Multi-Criteria Analysis A description of each marsh (i.e. location and change) is provided in Section 2, whilst factors affecting marsh decline are discussed in Section 3.2 This information has been used, in conjunction with knowledge of sites, and feeds into a Multi-Criteria Analysis (MCA). MCA (undertaken here using a “traffic light” approach) is a comparative decision making technique allowing assessment or evaluation to be made between multiple, often differing, criteria; and where multiple stakeholders and interested parties are involved. It is particularly useful where both quantitative and qualitative data are compared (for examples see Williams et al. (2010); Kvile et al., (2014); Zanuttigh et al. (2015)), and allows different types of data to be combined and compared. It provides a framework for decision makers to identify potentially suitable sites.

The process of saltmarsh creation should ensure that adverse impacts on the environment or activities in the surrounding area are minimised. The criteria used for the MCA (adapted from Williams et al., (2010) and partially based on EA, (2007), with the addition of factors considering risk, sediment volume and a modification of the socioeconomic aspect) are:

1) Presence of existing natural saltmarshes – this indicates the existence of favourable conditions for saltmarsh creation. This may also supply new sites with saltmarsh propagules and where colonisation is slow, assisted seeding can be considered. If areas of

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mid to low level saltmarsh are present, the criteria were coded GREEN, if upper saltmarsh is present, the criteria were coded AMBER; and if no or only remnant saltmarsh is present, the criteria were coded RED. 2) Elevation – as a general rule, saltmarsh forms at site elevations of 2-3m OD and mudflats elevations of <1m OD. The minimum elevation at the proposed sites should be around MHWN, or at a level that would experience 450-500 tidal inundations a year. In the past the most successful marsh creations have been at approximately 2.1m OD GREEN, when breached, with the height of established marsh being 2.34m OD (<300 tidal inundations). Marshes either side of this optimum height are coded are coded AMBER or RED based on assessment of over or under height in conjunction with other factors listed here. 3) Natural Drainage – an extensive creek system is required. Drainage increases sediment stability by supplying the marsh surface with sediment and nutrients. This reduces waterlogging, which is detrimental to plant colonisation and survival. Experience from the US concluded that sites which are too high do not develop adequate drainage systems and lack habitat diversity. Where possible the relic creek network should be enhanced. If natural creek development is slow, excavation of a drainage system should be considered. The creation of marsh at the Tollesbury managed realignment site found that creeks did not begin to develop until about 20-30cm of sediment had accreted on top of an agricultural site surface. This suggests the importance of excavating drainage channels in areas suitable for saltmarsh creation. Therefore, when choosing a site, access for earthmoving vehicles needs to be considered. Without such intervention it is recommended that sites slope gently to a level slightly lower than needed for saltmarsh development as natural saltmarsh drainage will form in the accreting sediment, producing marshes of better quality and diversity. Presence of natural creeks results in an existing natural drainage system GREEN, limited or non-natural drainage system AMBER, extremely limited or no drainage system, RED. 4) Surface gradient – site gradient determines species biodiversity, with more natural slope giving greater habitat diversity. Flat sites may result in poor diversity dominated by pioneer or low marsh species; optimum is 1-2% (<1:50) GREEN, marginally over or under this value AMBER, significantly over or under (e.g. negative slope inland) RED. 5) Sediment particle size – sediment grain size, composition and porosity affect drainage characteristics and organic content, and can influence the elevation of species colonisation and the outcome of plant competition. Finer sediments would be best to use in saltmarsh restoration. PSA grain size for future schemes unknown, but assumed to be relatively constant with current knowledge, AMBER. 6) Sediment volume – the quantity of sediment needed to increase the existing mudflat areas to a height that would encourage saltmarsh creation. To determine the volume of sediment required to convert the mudflat to saltmarsh, the existing mudflat area was converted to a 3D profile of specified height, and volumes were calculated for levels up to MLWN, MHW and MHWS. This is because pioneer species establish between MLWN and MHW and mid/upper species between MHW and MHWS. Sediment volumes required to extend marsh edge by 5 m, 20 m, 50 m over the mudflat were calculated to simulate accretion and the required sediment volumes calculated and presented in the MCA sections below. Where volumes calculated per site for all tidal levels are within the mean

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annual dredge volume GREEN, where 1 or 2 tidal levels are within the mean annual dredge volume, AMBER, where no tidal levels are above mean annual dredge volume RED. 7) Natural Sediment supply – a natural sediment supply must be available to supplement recharge sediment and help maintain an accretion rate sufficient to offset predicted sea level rise. The presence of healthy marshes close to a proposed site would indicate a suitable location in terms of sediment supply. Sediment supply in Hamble system judged to be sufficient for all sites (Williams et al., 2010), GREEN, though local studies may be required (e.g. Swanwick Marsh). 8) Accessibility – sediment retention at disposal sites may be problematic for access from river or shore. The behaviour of fine material is difficult to predict unless protected in some way or placed in quiescent locations. When selecting intertidal areas for recharge, consideration of the accessibility and costs for appropriate vessels/plant/machinery to handle the material and the possible indirect ecological impacts of sediment re-suspension must be considered; for example re-suspended material may smother adjacent saltmarsh or shellfish areas. Good access from shore and land GREEN, only good access from seaward or landward side AMBER, no access RED. 9) Contamination – areas away from major pollutant sources are preferable. All considered as AMBER as contamination data are available, but not replicated and location/depth not known. All sediment samples from Hamble Marinas show some contaminants above Action Level 1, but none show contaminants above Action Level 2, and all sediments have been licensed for disposal at sea. However, it should be noted that data for Premier (Moodys) marina were not supplied despite 3 requests, and it is known that a relatively recent capital dredge project did produce sediments above action level 2 which were required to be disposed of other than an offshore dumpsite; 10) Local Economic/Recreational Activities – all current activities checked to consider potential for adverse effects by the creation of the recharge site. Sites may have important recreational/amenity value that will be affected by a recharge scheme. Mitigation measures to offset associated impacts may be expensive. For example, at Freiston, the implications of managed realignment on oyster farms were not considered. Following the breach of the site, large volumes of sediment drained off and caused rapid channel deepening and erosion. Suspended sediment was washed through an oyster farm on to mudflats south of the site causing siltation and burial of oyster racks, which had to be moved at considerable cost (Williams et al., 2010). If there is no potential for local economic or recreational activity to be adversely affected GREEN, moderate potential to adversely affect economic or recreational activity AMBER, high potential to adversely affect economic or recreational activity RED. 11) Socio-economic – the value of intertidal mudflat and saltmarsh features in terms of their primary perceived “service” is considered. The most recognised service these features supply is of coastal protection and general wave reduction. Where significant value is being attained from saltmarsh features in terms of property and infrastructure protection GREEN, where protection is serving moderate value and infrastructure is important (e.g. footpaths), but not crucial AMBER, where the saltmarsh/mudflat feature is apparently serving no immediate infrastructure protection service RED. 12) Land Conservation Value – sites selected for saltmarsh creation should not have a high conservation value (such as SAC, SPA, Ramsar, SSSI). No designation GREEN, partial

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designation AMBER, full designation necessitating HRA/appropriate conservation consideration RED. 13) Risks - the risk potential for sediment augmentation or retention works may materially impact infrastructure and where immediate and/or future management is needed, this could incur additional costs. Where no immediate risk to infrastructure exist GREEN, where risk evident but mitigation feasible (e.g. retention) AMBER, where risk evident and localised with limited mitigation options RED.

When considering the results of the MCA below it should be noted that green does not mean “definitely go”, nor does red mean “definitely stop”. The colours simply allow the different criteria to be compared against one another. The qualitative assessments are highly simplified, and the judgements of the research team should be tested by a stakeholder group before decisions are made. It should also be noted that at this stage the criteria are unweighted, i.e. they are all assessed to be of equal value. When moving forward with site-selection a stakeholder group may decide that certain criteria are more important than other and should be weighted accordingly. This may alter the outcomes of the MCA.

Please note: in the assessments below buffers are calculated for all sites, though are clarified here for interpretation of the method and other options available. Buffers are suggested using work undertaken by Williams et al. (2010) where the amount of sediment required to “restore” target saltmarsh to former seaward extents was calculated.

There are other methods available such as charging the middle of the marsh with sediment (with retention structures), or placing sediment on the “cliffing edge” of the marsh to allow redistribution throughout the feature. However, for ease of interpretation in this report, the buffers model has been applied.

Should a site be selected for restoration, the appropriate methodology should be agreed on a site by site basis in conjunction with regulatory authorities and those with appropriate geomorphological and ecological knowledge.

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Table 5.2: GIS based Multi Criteria Analysis for River Hamble marshes (Modified from Williams et al. (2010) based on EA (2007) factors) EXISTING GRADIENT SEDIMENT SEDIMENT SEDIMENT ECONOMIC / SOCIO- MARSH ELEVATION DRAINAGE ACCESSIBILITY CONTAMINANTS CONSERVATION RISKS MARSH (modal value) (particle size) VOLUME SUPPLY RECREATIONAL ECONOMIC Hacketts Marsh

Lincegrove Marsh

Mercury Marsh

Satchell Marsh

Little Marsh

Hamble Common Marsh

Hook Marsh

Bunny Meadows Marsh (South) Bunny Meadows Marsh (North) Crableck Marsh

Universal Marsh

Swanwick Marsh

Not favourable Unknown / ambiguous Favourable

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Hacketts Marsh Hacketts Marsh has lost 45% of its area since 1870 (Table 2.3, Figure 2.3 and Figure 5.1). Edge erosion is a dominant process at Hacketts (Williams et al., 2010).

Figure 5.1: Historic change in saltmarsh area at Hacketts Marsh

Hacketts Marsh - MCA Presence of existing natural saltmarshes – Indicates the existence of favourable conditions for saltmarsh creation - There are existing saltmarshes at the site: GREEN

Elevation – Most successful marshes have been approximately 2.1 m OD – Majority of site between -2.2 m and 2.1 m relative to OD: GREEN

Drainage – An extensive creek system is required – Extensive existing creek system: GREEN

Surface gradient – Optimum is approximately 1-2% (<1:50). The calculated modal slope for Hacketts saltmarsh is 2%: GREEN.

Sediment particle size – Suitable for restorative works - Exact composition of dredged sediment to be used for recharge is unknown, but current data in this report (Section 4.2) indicate that fine grained sediments may be available as a result of marina dredging: AMBER.

Sediment volume - Sediment volumes required for recharge should be similar to the volumes available from maintenance dredging - Volumes of sediment required to extend the existing Hacketts Marsh by 5 m, 20 m, and 50 m buffers are shown in Table 5.3 and Figure 5.2.

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Table 5.3: Sediment volumes required to extend current Hacketts Marsh saltmarsh extent by 5 m, 20 m and 50 m buffers

TOTAL MUDFLAT VOLUME OF SEDIMENT REQUIRED BUFFER SURFACE 3 MUDFLAT SURFACE (m ) DISTANCE (m) AREA (m2) AREA (m2) CONVERTED MHWS MHW MHWN 5 22,337 89,347 25% 17,531 11,485 5,398 20 44,620 89,347 50% 55,437 41,035 26,408 50 79,284 89,347 89% 133,212 106,504 79,632

Figure 5.2: Buffer zones for potential saltmarsh Volumes required to achieve 20 m and 50 m extension are greater than the typical yearly sediment volumes available from maintenance dredging in the Hamble, however volumes required for a 5 m extension are within the typical yearly volumes for two of the tidal levels calculated: AMBER

Natural Sediment Supply – Levels sufficient to support aspirations - Natural sediment supply in Hamble system judged to be sufficient for all sites (Williams et al., 2010) and local saltmarshes are present: GREEN

Accessibility - When selecting areas for recharge, consideration of the accessibility and costs for appropriate vessels/plant/machinery to handle the material must be considered – Accessibility by vessel placement equipment and plant/machinery from land and river relatively good: GREEN

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Contamination – Levels below Action Level 2 and licensed for disposal - All sediment samples from Hamble Marinas show some contaminants above Action Level 1 but none show contaminants above Action Level 2, and all sediments have been licensed for disposal at sea: AMBER

Local Economic/Recreational Activities – All current activities should be checked that they are not likely to be adversely affected by the creation of the recharge site – Site has restricted access with permissions from Hampshire County Council or local landowners giving access only. The site has very shallow water fronting it thus vessel proximity unlikely to be a significant aspect: GREEN

Socio-Economics (public/private) – Value of protecting human infrastructure - Not immediately protecting high density housing, though gardens and private land fronted, no immediate access recreational value: AMBER

Land Conservation Value - Sites selected for saltmarsh creation should not have a high conservation value (such as SSSIs, Ramsar sites etc.) – Hacketts Marsh site designated SSSI, Ramsar, SPA and SAC ((Figure 1.2 and Figure 2.4): RED

Risks – Risks to infrastructure or areas with natural value should be considered - Recognised as a high value mid-level diverse marsh community thus risk of community change on main marsh, though Lands End may benefit. Commercial marina nearby although not immediately adjacent, liaison with marina operator may be appropriate based on possible project size: RED

Lincegrove Marsh Lincegrove Marsh has lost 59% of its area since 1870 (Table 2.3, Figure 2.5 and Figure 5.3).

Figure 5.3: Historic change in saltmarsh area at Lincegrove Marsh

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Lincegrove Marsh - MCA Presence of existing natural saltmarshes – Indicates the existence of favourable conditions for saltmarsh creation - There are existing saltmarshes at the site: GREEN

Elevation – Most successful marshes have been approximately 2.1m OD – Majority of site between -2.4 m and 2.1 m relative to OD: GREEN

Drainage – An extensive creek system is required – Extensive existing creek system: GREEN

Surface gradient – Optimum is approximately 1-2% (<1:50). The calculated modal slope for Lincegrove saltmarsh is 3%, making it sub-optimal: AMBER

Sediment particle size – Suitable for restorative works – Exact composition of dredged sediment to be used for recharge is unknown, but current data in this report (Section 4.2) indicate that fine grained sediments may be available as a result of marina dredging: AMBER

Sediment volume - Sediment volumes required for recharge should be similar to the volumes available from maintenance dredging - Volumes of sediment required to extend the existing Lincegrove Marsh by 5 m, 20 m, and 50 m buffers are shown in Table 5.4 and Figure 5.4.

Table 5.4: Sediment volumes required to extend current Lincegrove Marsh saltmarsh extent by 5 m, 20 m and 50 m buffers

BUFFER TOTAL MUDFLAT VOLUME OF SEDIMENT REQUIRED (m3) SURFACE DISTANCE 2 MUDFLAT SURFACE AREA (m ) 2 MHWS MHW MHWN (m) AREA (m ) CONVERTED 5 39,703 135,567 29% 33,812 22,932 11,812 20 90,758 135,567 67% 130,550 99,904 68,493 50 124,621 135,567 92% 216,504 173,201 128,835

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Figure 5.4: Existing Lincegrove saltmarsh and potential saltmarsh buffers Volumes required to achieve 20 m and 50 m extension are greater than the typical yearly sediment volumes available from maintenance dredging in the Hamble, however volumes required for a 5m extension are within the typical yearly volumes for one of the tidal levels calculated: AMBER

Natural Sediment Supply – Levels sufficient to support aspirations – Natural sediment supply in Hamble system judged to be sufficient for all sites (Williams et al., 2010) and local saltmarshes are present: GREEN

Accessibility - When selecting areas for recharge, consideration of the accessibility and costs for appropriate vessels/plant/machinery to handle the material must be considered – No easy land access, relatively easy river access: AMBER

Contamination – Levels below Action Level 2 and licensed for disposal - All sediment samples from Hamble Marinas show some contaminants above Action Level 1 but none show contaminants above Action Level 2, and all sediments have been licensed for disposal at sea: AMBER

Local Economic/Recreational Activities – All current activities should be checked that they are not likely to be adversely affected by the creation of the recharge site – Recreational value through walkers, bird watchers etc., regularly used by locals for recreational walking on footpath behind marsh: RED

Socio-Economics – Value of protecting human infrastructure – Marsh is protecting significant infrastructure (railway): GREEN

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Land Conservation Value - Sites selected for saltmarsh creation should not have a high conservation value (such as SSSIs, Ramsar sites etc.) – Lincegrove Marsh site designated SSSI, Ramsar, SPA and SAC (Figure 1.2 and Figure 2.6): RED

Risks – Risks to infrastructure or areas with natural value should be considered - Recognised as a high value mid-level diverse marsh community thus risk of community change on main marsh: RED

Mercury Marsh Mercury Marsh has lost 78% of its area since 1870 (Table 2.3 and Figure 2.7 and Figure 5.5), however it has remained relatively constant since 2000.

Figure 5.5: Historic change in saltmarsh area at Mercury Marsh

Mercury Marsh – MCA Presence of existing natural saltmarshes – Indicates the existence of favourable conditions for saltmarsh creation – Existing marsh but significant edge erosion and over-heightened with apparent community change to rear: AMBER

Elevation – Most successful marshes have been approximately 2.1 m OD – Majority of site between -2.3 m and 2.1 m relative to OD: GREEN

Drainage – An extensive creek system is required – Creek system on main marsh absent: RED

Surface gradient – Optimum is approximately 1-2% (<1:50). The calculated modal slope for Mercury saltmarsh is 2%: GREEN

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Sediment particle size – Suitable for restorative works - Exact composition of dredged sediment to be used for recharge is unknown, but current data in this report (Section 4.2) indicate that fine grained sediments may be available as a result of marina dredging: AMBER

Sediment volume - Sediment volumes required for recharge should be similar to the volumes available from maintenance dredging - Volumes of sediment required to extend the existing Mercury Marsh by 5 m, 20 m, and 50 m buffers are shown in Table 5.5 and Figure 5.6.

Table 5.5: Sediment volumes required to extend current Mercury Marsh saltmarsh extent by 5 m, 20 m and 50 m buffers

BUFFER TOTAL MUDFLAT VOLUME OF SEDIMENT REQUIRED (m3) SURFACE DISTANCE 2 MUDFLAT SURFACE AREA (m ) 2 MHWS MHW MHWN (m) AREA (m ) CONVERTED 5 4,214 29,365 14% 3,250 2,076 873 20 12,046 29,365 41% 16,162 12,077 7,890 50 20,927 29,365 71% 35,714 28,353 20,804

Figure 5.6: Existing Mercury Marsh saltmarsh and potential saltmarsh buffers Volumes required to achieve 50 m extension are greater than the typical yearly sediment volumes available from maintenance dredging in the Hamble, however volumes required for a 5 m extension are within the typical yearly volumes for all tidal levels calculated, and volumes required for a 20 m extension are within the typical yearly volumes for two of the tidal level calculated: GREEN

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Natural Sediment Supply – Levels sufficient to support aspirations – Natural sediment supply in Hamble system judged to be sufficient for all sites (Williams et al., 2010) and local saltmarshes are present: GREEN

Accessibility - When selecting areas for recharge, consideration of the accessibility and costs for appropriate vessels/plant/machinery to handle the material must be considered – Accessibility by vessel and plant/machinery from land and river relatively good: GREEN

Contamination – Levels below Action Level 2 and licensed for disposal - All sediment samples from Hamble Marinas show some contaminants above Action Level 1 but none show contaminants above Action Level 2, and all sediments have been licensed for disposal at sea: AMBER

Local Economic/Recreational Activities – All current activities should be checked that they are not likely to be adversely affected by the creation of the recharge site – Good recreational value, site regularly used by public and nearby to local slipway: RED

Socio-Economics – Value of protecting human infrastructure – Marsh protects relatively dense local housing: GREEN

Land Conservation Value - Sites selected for saltmarsh creation should not have a high conservation value (such as SSSIs, Ramsar sites etc.) – Mercury Marsh site designated SSSI, Ramsar, SPA and SAC (Figure 1.2 and Figure 2.8): RED

Risks – Risks to infrastructure or areas with natural value should be considered – Sediment placement risk for impact on adjacent marina, but may be suitable retention techniques: AMBER

Satchell Marsh Satchell Marsh has lost 61% of its area since 1870 (Table 2.3, Figure 2.9 and Figure 5.7), but shows accretion over the period 2000 – 2014 (although it seems to have declined slightly in area from 2007 – 2014).

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Figure 5.7: Historic change in saltmarsh area at Satchell Marsh

Satchell Marsh - MCA Presence of existing natural saltmarshes – Indicates the existence of favourable conditions for saltmarsh creation – There are existing saltmarshes immediately adjacent to the site: GREEN

Elevation – Most successful marshes have been approximately 2.1 m OD – Majority of site between -2.8 m and 2.1 m relative to OD (Figure 2.10): GREEN.

Drainage – An extensive creek system is required – Marsh has been affected by artificial channels for boat launch, possibly altering drainage regime, thus slower draining creek system would need creating: AMBER

Surface gradient – Optimum is approximately 1-2% (<1:50). The calculated modal slope for Satchell saltmarsh is 2%: GREEN

Sediment particle size – Suitable for restorative works - Exact composition of dredged sediment to be used for recharge is unknown, but current data in this report (Section 4.2) indicate that fine grained sediments may be available as a result of marina dredging: AMBER

Sediment volume - Sediment volumes required for recharge should be similar to the volumes available from maintenance dredging - Volumes of sediment required to extend the existing Satchell Marsh by 5 m, 20 m, and 50 m buffers are shown in Table 5.6 and Figure 5.8.

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Table 5.6: Sediment volumes required to extend current Satchell Marsh saltmarsh extent by 5 m, 20 m and 50 m buffers

BUFFER TOTAL MUDFLAT VOLUME OF SEDIMENT REQUIRED (m3) SURFACE DISTANCE 2 MUDFLAT SURFACE AREA (m ) 2 MHWS MHW MHWN (m) AREA (m ) CONVERTED 5 9,041 51,514 18% 6,094 4,066 2,058 20 19,752 51,514 38% 20,008 14,278 8,500 50 34,500 51,514 67% 47,463 36,326 24,997

Figure 5.8: Existing Satchell Marsh saltmarsh and potential saltmarsh buffers Volumes required to achieve 50 m extension are greater than the typical yearly sediment volumes available from maintenance dredging in the Hamble, however volumes required for a 5 m extension are within the typical yearly volumes for all tidal levels calculated, and volumes required for a 20 m extension are within the typical yearly volumes for two of the tidal levels calculated: GREEN

Natural Sediment Supply – Levels sufficient to support aspirations – Natural sediment supply in Hamble system judged to be sufficient for all sites (Williams et al., 2010) and local saltmarshes are present: GREEN

Accessibility - When selecting areas for recharge, consideration of the accessibility and costs for appropriate vessels/plant/machinery to handle the material must be considered – Accessibility by vessel and plant/machinery from land and river relatively good: GREEN

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Contamination – Levels below Action Level 2 and licensed for disposal - All sediment samples from Hamble Marinas show some contaminants above Action Level 1 but none show contaminants above Action Level 2, and all sediments have been licensed for disposal at sea: AMBER

Local Economic/Recreational Activities – All current activities should be checked that they are not likely to be adversely affected by the creation of the recharge site – Access to site for recreational purposes, access through site for boat launching: AMBER

Socio-Economics – Value of protecting human infrastructure – Very high value, numerous houses, listed as flood risk: GREEN

Land Conservation Value - Sites selected for saltmarsh creation should not have a high conservation value (such as SSSIs, Ramsar sites etc.) – Majority of Satchell Marsh is designated SSSI, Ramsar, SPA and SAC (Figure 1.2 and Figure 2.10): RED

Risks – Risks to infrastructure or areas with natural value should be considered – Adjacent to Port Hamble and Hamble Yacht Services, significant liaison required: RED

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Little Marsh Little Marsh has lost 16% of its area since 1870 (Table 2.3, Figure 2.11 and Figure 5.9) but has otherwise remained relatively consistent in area.

Figure 5.9: Historic change in saltmarsh area at Little Marsh

Little Marsh - MCA Presence of existing natural saltmarshes – Indicates the existence of favourable conditions for saltmarsh creation – There are existing saltmarshes immediately adjacent to the site: GREEN

Elevation – Most successful marshes have been approximately 2.1 m OD – Majority of site between -2.3 m and 2.1 m relative to OD (Figure 2.12): GREEN

Drainage – An extensive creek system is required – Poor drainage, with only two linear creeks rapidly draining the area: RED

Surface gradient – Optimum is approximately 1-2% (<1:50). The calculated modal slope for Little Marsh saltmarsh is 2%: GREEN

Sediment particle size – Suitable for restorative works - Exact composition of dredged sediment to be used for recharge is unknown, but current data in this report (Section 4.2) indicate that fine grained sediments may be available as a result of marina dredging: AMBER

Sediment volume - Sediment volumes required for recharge should be similar to the volumes available from maintenance dredging - Volumes of sediment required to extend the existing Little Marsh by 5 m, 20 m, and 50 m buffers are shown in Table 5.7 and Figure 5.10.

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Table 5.7: Sediment volumes required to extend current Little Marsh saltmarsh extent by 5 m, 20 m and 50 m buffers

BUFFER TOTAL MUDFLAT VOLUME OF SEDIMENT REQUIRED (m3) SURFACE DISTANCE 2 MUDFLAT SURFACE AREA (m ) 2 MHWS MHW MHWN (m) AREA (m ) CONVERTED 5 1402 11,881 12% 860 515 204 20 3385 11,881 28% 3,681 2,606 1,545 50 7183 11,881 60% 11,002 8,532 6,041

Figure 5.10: Existing Little Marsh saltmarsh and potential saltmarsh buffers Volumes of sediment required to achieve 5 m, 20 m and 50 m extensions are within the typical yearly volumes for all of the tidal levels calculated: GREEN

Natural Sediment Supply – Levels sufficient to support aspirations – Natural sediment supply in Hamble system judged to be sufficient for all sites (Williams et al., 2010) and local saltmarshes are present: GREEN

Accessibility - When selecting areas for recharge, consideration of the accessibility and costs for appropriate vessels/plant/machinery to handle the material must be considered – Accessibility by vessel relatively good, however land access is difficult: AMBER

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Contamination – Levels below Action Level 2 and licensed for disposal - All sediment samples from Hamble Marinas show some contaminants above Action Level 1 but none show contaminants above Action Level 2, and all sediments have been licensed for disposal at sea: AMBER

Local Economic/Recreational Activities – All current activities should be checked that they are not likely to be adversely affected by the creation of the recharge site – Private marsh, no direct implication for immediate recreational activity on site, and limited near vessel access due to depth: GREEN

Socio-Economics – Value of protecting human infrastructure – Not protecting significant housing density, no recreational value: AMBER

Land Conservation Value - Sites selected for saltmarsh creation should not have a high conservation value (such as SSSIs, Ramsar sites etc.) – Little Marsh site designated SSSI, Ramsar, SPA and SAC (Figure 1.2 and Figure 2.12): RED

Risks – Risks to infrastructure or areas with natural value should be considered – Hamble Point Marina adjacent but not directly by marsh, control of sediment and liaison recommended: AMBER

Hamble Common Marsh Hamble Common Marsh has lost 70% of its area since 1870 (Table 2.3, Figure 2.13 and Figure 5.11) but shows accretion over the period 2000 – 2007, but has declined in area from 2007 – 2014).

Figure 5.11: Historic change in saltmarsh area at Hamble Common Marsh

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Hamble Common Marsh – MCA Presence of existing natural saltmarshes – Indicates the existence of favourable conditions for saltmarsh creation – Marsh is highly fragmented to patches of mid marsh and lower patches of pioneer species: AMBER

Elevation – Most successful marshes have been approximately 2.1 m OD – Majority of site between -2.4 m and 2.1 m relative to OD, but the most optimal areas are already have mid-saltmarsh, with the remaining mudflat at an elevation below 1 m relative to OD: AMBER

Drainage – An extensive creek system is required – Extensive existing creek system: GREEN

Surface gradient – Optimum is approximately 1-2% (<1:50). The calculated modal slope for Hamble Common saltmarsh is 2%: GREEN

Sediment particle size – Suitable for restorative works - Exact composition of dredged sediment to be used for recharge is unknown, but current data in this report (Section 4.2) indicate that fine grained sediments may be available as a result of marina dredging: AMBER

Sediment volume - Sediment volumes required for recharge should be similar to the volumes available from maintenance dredging - Volumes of sediment required to extend the existing Hamble Common Marsh by 5 m, 20 m, and 50 m buffers are shown in Table 5.8 and Figure 5.12.

Table 5.8: Sediment volumes required to extend current Hamble Common Marsh saltmarsh extent by 5 m, 20 m and 50 m buffers

BUFFER TOTAL MUDFLAT VOLUME OF SEDIMENT REQUIRED (m3) SURFACE DISTANCE 2 MUDFLAT SURFACE AREA (m ) 2 MHWS MHW MHWN (m) AREA (m ) CONVERTED 5 6,643 30,257 22% 5,009 3,251 1,480 20 19,999 30,257 66% 24,752 17,953 11,025 50 28,398 30,257 94% 43,642 33,705 23,555

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Figure 5.12: Existing Hamble Common saltmarsh and potential saltmarsh buffers Volumes required to achieve 50 m extension are greater than the typical yearly sediment volumes available from maintenance dredging in the Hamble, however volumes required for a 5 m extension are within the typical yearly volumes for all tidal levels calculated, and volumes required for a 20 m extension are within the typical yearly volumes for one of the tidal levels calculated: GREEN

Natural Sediment Supply – Levels sufficient to support aspirations – Natural sediment supply in Hamble system judged to be sufficient for all sites (Williams et al., 2010) and local saltmarshes are present: GREEN

Accessibility - When selecting areas for recharge, consideration of the accessibility and costs for appropriate vessels/plant/machinery to handle the material must be considered – Vehicle access unlikely (across protected Hamble Common), piping only likely option for vessel access: AMBER

Contamination – Levels below Action Level 2 and licensed for disposal - All sediment samples from Hamble Marinas show some contaminants above Action Level 1 but none show contaminants above Action Level 2, and all sediments have been licensed for disposal at sea: AMBER

Local Economic/Recreational Activities – All current activities should be checked that they are not likely to be adversely affected by the creation of the recharge site – Significant local recreational use for bird watchers, walkers, dog walkers etc.: RED

Socio-Economics – Value of protecting human infrastructure – Protecting public footpath/recreational area: AMBER

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Land Conservation Value - Sites selected for saltmarsh creation should not have a high conservation value (such as SSSIs, Ramsar sites etc.) – Hamble Common Marsh site designated SSSI, Ramsar, SPA and SAC (Figure 1.2 and Figure 2.14): RED

Risks – Risks to infrastructure or areas with natural value should be considered – Directly adjacent to Hamble Point Marina where known sediment deposition is an issue at this site: RED

Hook Saltmarsh at Hook (seawards-side of embankment) was recorded in 1946 (8,380 m2) but not before or since (Table 2.3 and Figure 2.15). No further information to support the loss is available.

Hook – MCA Presence of existing natural saltmarshes – Indicates the existence of favourable conditions for saltmarsh creation – No existing marsh, this is an intertidal creek of mud and gravel: RED

Elevation – Most successful marshes have been approximately 2.1 m OD – Majority of site between -2.4 and 1 m relative to OD: RED

Drainage – An extensive creek system is required – Single intertidal creek on seaward side of path: RED

Surface gradient – Optimum is approximately 1-2% (<1:50). The calculated modal slope for Hook saltmarsh is 1%: GREEN

Sediment particle size – Suitable for restorative works - Exact composition of dredged sediment to be used for recharge is unknown, but current data in this report (Section 4.2) indicate that fine grained sediments may be available as a result of marina dredging: AMBER

Sediment volume - Sediment volumes required for recharge should be similar to the volumes available from maintenance dredging - Volumes of sediment required to extend the existing Hook marsh by 5 m, 20 m, and 50 m buffers are shown in Table 5.9 and Figure 5.13.

Table 5.9: Sediment volumes required to extend current Hook saltmarsh extent by 5 m, 20 m and 50 m buffers

BUFFER TOTAL MUDFLAT VOLUME OF SEDIMENT REQUIRED (m3) SURFACE DISTANCE 2 MUDFLAT SURFACE AREA (m ) 2 MHWS MHW MHWN (m) AREA (m ) CONVERTED 5 8,498 206,852 4% 9,689 7,490 5,297 20 33,912 206,852 16% 58,083 46,862 35,486 50 77,894 206,852 38% 150,758 123,425 95,592

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Figure 5.13: Existing Hook feature and potential saltmarsh buffers Volumes required to achieve 20 m and 50 m extension are greater than the typical yearly sediment volumes available from maintenance dredging in the Hamble, however volumes required for a 5 m extension are within the typical yearly volumes for all three of the tidal levels calculated: AMBER

Natural Sediment Supply – Levels sufficient to support aspirations – Natural sediment supply in Hamble system judged to be sufficient for all sites (Williams et al., 2010) and local saltmarshes are present: GREEN

Accessibility - When selecting areas for recharge, consideration of the accessibility and costs for appropriate vessels/plants/machinery to handle the material must be considered –Vessel access may be limited due to distance and possible navigational issue as will require vessel presence close to or in main channel, vehicle access limited but may be possible: AMBER

Contamination – Levels below Action Level 2 and licensed for disposal - All sediment samples from Hamble Marinas show some contaminants above Action Level 1 but none show contaminants above Action Level 2, and all sediments have been licensed for disposal at sea: AMBER

Local Economic/Recreational Activities – All current activities should be checked that they are not likely to be adversely affected by the creation of the recharge site – Significant local recreational use for bird watchers, walkers, dog walkers etc.: RED

Socio-Economics – Value of protecting human infrastructure – Protecting public footpath/recreational area: AMBER

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Land Conservation Value - Sites selected for saltmarsh creation should not have a high conservation value (such as SSSIs, Ramsar sites etc.) – Hook site designated SSSI, Ramsar, SPA and SAC (Figure 1.2 and Figure 2.16): RED.

Risks – Risks to infrastructure or areas with natural value should be considered: AMBER

Bunny Meadows Marsh (South) Bunny Meadows Marsh (South) has gained 43% in area since 1870 (Table 2.3, Figure 2.17 and Figure 5.14). A decline is evident between 1870 and 1946, then the flooding of the area created significant growth from 1946 to 1971. Since then levels have fluctuated, but have remained relatively constant. Although the decrease between 2007 and 2014 may be regarded as significant.

Figure 5.14: Historic change in saltmarsh area at Bunny Meadows Marsh (South)

Bunny Meadows Marsh (South) – MCA Presence of existing natural saltmarshes – Indicates the existence of favourable conditions for saltmarsh creation – There are existing saltmarshes immediately adjacent to the site: GREEN

Elevation – Most successful marshes have been approximately 2.1 m OD – Majority of site between -2.7 and 2.1 m relative to OD: GREEN

Drainage – An extensive creek system is required – Remnant creek system in some areas, some conversion to reed bed in upper marsh to south east: AMBER

Surface gradient – Optimum is approximately 1-2% (<1:50). The calculated modal slope for Bunny Meadows (South) saltmarsh 2%: GREEN

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Sediment particle size – Suitable for restorative works - Exact composition of dredged sediment to be used for recharge is unknown, but current data in this report (Section 4.2) indicate that fine grained sediments may be available as a result of marina dredging: AMBER

Sediment volume - Sediment volumes required for recharge should be similar to the volumes available from maintenance dredging - - Volumes of sediment required to extend the existing Bunny Meadows Marsh (South) by 5 m, 20 m, and 50 m buffers are shown in Table 5.10 and Figure 5.15.

Table 5.10: Sediment volumes required to extend current Bunny Meadows Marsh (South) saltmarsh extent by 5 m, 20 m and 50 m buffers

Buffer Total Mudflat Volume of sediment required (m3) Surface Distance 2 mudflat surface Area (m ) 2 MHWS MHW MHWN (m) area (m ) converted 5 49,468 258,025 19% 5,566 3,077 965 20 120,558 258,025 47% 21,213 13,850 6,989 50 203,072 258,025 79% 52,559 39,250 26,287

Figure 5.15: Existing Bunny Meadows (South) saltmarsh and potential saltmarsh buffers Volumes required to achieve 50 m extension are greater than the typical yearly sediment volumes available from maintenance dredging in the Hamble, however volumes required for a 5 m extension are

AHTI_J2015_004 109 RIVER HAMBLE SOFT SE DIMENT HABITAT RETEN TION FEASIBILITY STUDY within the typical yearly volumes, and volumes required to achieve a 20 m extension are within typical yearly volumes for two of the tidal levels calculated: GREEN

Natural Sediment Supply – Levels sufficient to support aspirations - Natural sediment supply in Hamble system judged to be sufficient for all sites (Williams et al., 2010) and local saltmarshes are present: GREEN

Accessibility - When selecting areas for recharge, consideration of the accessibility and costs for appropriate vessels/plants/machinery to handle the material must be considered –Accessibility by vessel and plant / machinery from land and river are relatively good: GREEN

Contamination – Levels below Action Level 2 and licensed for disposal - All sediment samples from Hamble Marinas show some contaminants above Action Level 1 but none show contaminants above Action Level 2, and all sediments have been licensed for disposal at sea: AMBER

Local Economic/Recreational Activities – All current activities should be checked that they are not likely to be adversely affected by the creation of the recharge site – Impact during construction phase on walkers etc. due to probable path closure: AMBER

Socio-Economics – Value of protecting human infrastructure – Gardens of high value homes potentially impacted, site investigation needed to assess detail: AMBER

Land Conservation Value - Sites selected for saltmarsh creation should not have a high conservation value (such as SSSIs, Ramsar sites etc.) – Bunny Meadows South site designated SSSI, Ramsar, SPA and SAC (Figure 1.2 and Figure 2.18): RED

Risks – Risks to infrastructure or areas with natural value should be considered – Site behind seawall, no immediate risk to infrastructure: GREEN

Bunny Meadows Marsh (North) Bunny Meadows Marsh (North) shows considerable variation in area since 1870. There was a threefold increase between 1870 and 1946, due to the seawall breach. Levels remained relatively constant from 1971 onwards, however between 2007 and 2014 there was a significant decrease as noted with Bunny Meadows South (Table 2.3, Figure 2.19 and Figure 5.16).

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Figure 5.16: Historic change in saltmarsh area at Bunny Meadows Marsh (North)

Bunny Meadows Marsh (North) –MCA Presence of existing natural saltmarshes – Indicates the existence of favourable conditions for saltmarsh creation – Site access awkward thus ground truthing problematic to confirm marsh condition, channel which feeds marsh has grown and marsh appears fragmented with reed at upper limit: AMBER

Elevation – Most successful marshes have been approximately 2.1m OD – Majority of site between -0.34 and 1.8 m relative to OD thus sub optimal and would require discussion on options and impacts: AMBER

Drainage – An extensive creek system is required - channel which feeds marsh has grown and marsh appears fragmented, creeks possibly extant: AMBER

Surface gradient – Optimum is approximately 1-2% (<1:50. The calculated modal slope for Bunny Meadows (North) saltmarsh is 2%: GREEN

Sediment particle size – Suitable for restorative works - Exact composition of dredged sediment to be used for recharge is unknown, but current data in this report (Section 4.2) indicate that fine grained sediments may be available as a result of marina dredging: AMBER

Sediment volume - Sediment volumes required for recharge should be similar to the volumes available from maintenance dredging. Volumes of sediment required to extend the existing Bunny Meadows (North) by 5 m, 20 m, and 50 m buffers are shown in Table 5.11 and Figure 5.17.

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Table 5.11: Sediment volumes required to extend current Bunny Meadows Marsh (North) saltmarsh extent by 5 m, 20 m and 50 m buffers

BUFFER TOTAL MUDFLAT VOLUME OF SEDIMENT REQUIRED (m3) SURFACE DISTANCE 2 MUDFLAT SURFACE AREA (m ) 2 MHWS MHW MHWN (m) AREA (m ) CONVERTED 5 12,896 28,298 46% 6,784 3,501 670 20 25,136 28,298 89% 19,241 11,524 4,349 50 28,292 28,298 100% 22,125 13,415 5,408

Figure 5.17: Existing Bunny Meadows (North) saltmarsh and potential saltmarsh buffers

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Volumes required to achieve 20 m and 50m extensions to the MHWS tidal level is greater than the typical yearly sediment volumes available from maintenance dredging in the River Hamble. However volumes required for a 5m extension is within the typical yearly volumes, and volumes required to achieve a 20m extension are within typical yearly volumes for two of the tidal levels calculated: GREEN

Natural Sediment Supply – Levels sufficient to support aspirations - Natural sediment supply in Hamble system judged to be sufficient for all sites (Williams et al., 2010) and local saltmarshes are present: GREEN

Accessibility - When selecting areas for recharge, consideration of the accessibility and costs for appropriate vessels/plants/machinery to handle the material must be considered – Limited vessel access due to extensive shallow mudflats fronting much of the seawall, piping may be an option, vehicle access presumed difficult as site backed by mature woos: AMBER

Contamination – Levels below Action Level 2 and licensed for disposal - All sediment samples from Hamble Marinas show some contaminants above Action Level 1 but none show contaminants above Action Level 2, and all sediments have been licensed for disposal at sea: AMBER

Local Economic/Recreational Activities – All current activities should be checked that they are not likely to be adversely affected by the creation of the recharge site – Footpath and general area widely used for recreational purposes (walking, running, cycling) plus access to area down adjacent Crableck Lane so possible impact in construction phase: RED

Socio-Economics – Value of protecting human infrastructure – Gardens – Homes bordering the marsh area are at much higher level in woodland, thus marsh provides no distinct protection to properties: RED

Land Conservation Value - Sites selected for saltmarsh creation should not have a high conservation value (such as SSSIs, Ramsar sites etc.) – Bunny Meadows North site designated SSSI, Ramsar, SPA and SAC (Figure 1.2 and Figure 2.20): RED

Risks – Risks to infrastructure or areas with natural value should be considered - No immediate risk to adjacent sites as marsh behind wall, though possible risk during construction: AMBER

Crableck Marsh Crableck Marsh has lost 87% of its area since 1870 (Table 2.3, Figure 2.21 and Figure 5.18), although the area has accreted since 1984. However, the variability displayed in area may be an artefact of data quality for 1984 and 2000.

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Figure 5.18: Historic change in saltmarsh area at Crableck Marsh

Crableck Marsh – MCA Presence of existing natural saltmarshes – Indicates the existence of favourable conditions for saltmarsh creation – Limited existing marsh, patches of mid and lower marsh, highly fragmented and isolated, also patches of pioneer and Spartina spp.: RED

Elevation – Most successful marshes have been approximately 2.1 m OD – Majority of site between -2.4 and 1 m relative to OD: RED

Drainage – An extensive creek system is required – Remnant creek system evident on imagery: AMBER

Surface gradient – Optimum is approximately 1-2% (<1:50.). The calculated modal slope for Crableck marsh is 1%: GREEN

Sediment particle size – Suitable for restorative works - Exact composition of dredged sediment to be used for recharge is unknown, but current data in this report (Section 4.2) indicate that fine grained sediments may be available as a result of marina dredging: AMBER

Sediment volume - sediment volumes required for recharge should be similar to the volumes available from maintenance dredging - volumes of sediment required to extend the existing Crableck Marsh by 5 m, 20 m, and 50 m buffers are shown in Table 5.12 and Figure 5.19.

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Table 5.12: Sediment volumes required to extend current Crableck Marsh saltmarsh extent by 5 m, 20 m and 50 m buffers

BUFFER TOTAL MUDFLAT VOLUME OF SEDIMENT REQUIRED (m3) SURFACE DISTANCE 2 MUDFLAT SURFACE AREA (m ) 2 MHWS MHW MHWN (m) AREA (m ) CONVERTED 5 3,296 112,389 3% 2,380 1,485 632 20 13,466 112,389 12% 16,763 12,161 7,536 50 33,969 112,389 30% 52,290 40,191 27,996

Figure 5.19: Existing Crableck Marsh saltmarsh and potential saltmarsh buffers

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Volumes required to achieve 50 m extension are greater than the typical yearly sediment volumes available from maintenance dredging in the Hamble, however volumes required for a 5 m extension are all within the typical yearly volumes of sediment dredged, and volumes required for a 20 m extension are within the typical yearly volumes for two of the tidal levels calculated: GREEN

Natural Sediment Supply – Levels sufficient to support aspirations - Natural sediment supply in Hamble system judged to be sufficient for all sites (Williams et al., 2010) and local saltmarshes are present: GREEN

Accessibility - When selecting areas for recharge, consideration of the accessibility and costs for appropriate vessels/plants/machinery to handle the material must be considered – Vehicle and vessel access somewhat restricted due to marina infrastructure: AMBER

Contamination – Levels below Action Level 2 and licensed for disposal - All sediment samples from Hamble Marinas show some contaminants above Action Level 1 but none show contaminants above Action Level 2, and all sediments have been licensed for disposal at sea: AMBER

Local Economic/Recreational Activities – All current activities should be checked that they are not likely to be adversely affected by the creation of the recharge site – Impact on path users and potentially marina users during construction phase, also possible effect on vessel users as/if marsh develops, would need management: AMBER

Socio-Economics – Value of protecting human infrastructure – Site is protecting much used footpath: AMBER

Land Conservation Value - Sites selected for saltmarsh creation should not have a high conservation value (such as SSSIs, Ramsar sites etc.) – The southern mudflat area of Crableck Marsh is designated SSSI, Ramsar, SPA and SAC, as well as MPA. However most of the remnant saltmarsh falls outside of the designated area (Figure 1.2 and Figure 2.22): RED

Risks – Risks to infrastructure or areas with natural value should be considered - Risk to footpath infrastructure and significant risk to marina and berthing areas: RED

Universal Marsh Universal Marsh has lost 70% of its area since 1870 (Table 2.3, Figure 2.23 and Figure 5.20), however it appears to have accreted somewhat between 2000 and 2007, and remained stable between 2007 and 2014.

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Figure 5.20: Historic change in saltmarsh area at Universal Marsh

Universal Marsh – MCA Presence of existing natural saltmarshes – Indicates the existence of favourable conditions for saltmarsh creation – Existing saltmarshes at the site: GREEN

Elevation – Most successful marshes have been approximately 2.1m OD – Majority of site between -2.4 and 2.1 m relative to OD: GREEN.

Drainage – An extensive creek system is required – Linear creeks are present suggesting previous interference: RED

Surface gradient – Optimum is approximately 1-2% (<1:50). The calculated modal slope for Universal saltmarsh is 3%: AMBER

Sediment particle size – Suitable for restorative works - Exact composition of dredged sediment to be used for recharge is unknown, but current data in this report (Section 4.2) indicate that fine grained sediments may be available as a result of marina dredging: AMBER

Sediment volume - sediment volumes required for recharge should be similar to the volumes available from maintenance dredging - volumes of sediment required to extend the existing Universal Marsh by 5 m, 20 m, and 50 m buffers are shown in Table 5.13 and Figure 5.21.

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Table 5.13: Sediment volumes required to extend current Universal Marsh saltmarsh extent by 5 m, 20 m and 50 m buffers

BUFFER TOTAL MUDFLAT VOLUME OF SEDIMENT REQUIRED (m3) SURFACE DISTANCE 2 MUDFLAT SURFACE AREA (m ) 2 MHWS MHW MHWN (m) AREA (m ) CONVERTED 5 2329 26933 9% 1,921 1,301 678 20 8979 26933 33% 14,811 11,789 8,714 50 21169 26933 79% 47,773 40,304 32,663

Figure 5.21: Existing Universal Marsh saltmarsh and potential saltmarsh buffers Volumes required to achieve 50 m extension are greater than the typical yearly sediment volumes available from maintenance dredging in the Hamble, however volumes required for both a 5 m and 20 m extension are within the typical yearly volumes for the tidal levels calculated: GREEN

Natural Sediment Supply – Levels sufficient to support aspirations - Natural sediment supply in Hamble system judged to be sufficient for all sites (Williams et al., 2010) and local saltmarshes are present: GREEN

Accessibility - When selecting areas for recharge, consideration of the accessibility and costs for appropriate vessels/plants/machinery to handle the material must be considered – Accessibility by vessel and plant / machinery from sea and by land is relatively good: GREEN

Contamination – Levels below Action Level 2 and licensed for disposal - All sediment samples from Hamble Marinas show some contaminants above Action Level 1 but none show contaminants above Action Level 2, and all sediments have been licensed for disposal at sea: AMBER

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Local Economic/Recreational Activities – All current activities should be checked that they are not likely to be adversely affected by the creation of the recharge site – Possible effects on adjacent much used footpath and recreational boating moorings at Universal: RED

Socio-Economics – Value of protecting human infrastructure – Site is protecting much used footpath: AMBER

Land Conservation Value - Sites selected for saltmarsh creation should not have a high conservation value (such as SSSIs, Ramsar sites etc.) – Universal Marsh does not have local, European or international designations (Figure 1.2 and Figure 2.24): GREEN

Risks – Risks to infrastructure or areas with natural value should be considered - Risk to footpath infrastructure and significant risk to marina and berthing areas: RED

Swanwick Marsh Swanwick Marsh has lost 97% of its area since 1870 (Table 2.3, Figure 2.25 and Figure 5.22). There has been some accretion between 1984 and 2014. Data variability between 1984 and 2000 may be an artefact of data quality.

Figure 5.22: Historic change in saltmarsh area at Swanwick Marsh

Swanwick Marsh – MCA Presence of existing natural saltmarshes – Indicates the existence of favourable conditions for saltmarsh creation – Very patchy remnants on what was one of the Rivers largest marshes SIMON: RED

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Elevation – Most successful marshes have been approximately 2.1 m OD – Majority of site between -2.3 and 1.5 m relative to OD. Significant decline in marsh, remnant patches, squeezed by footpath infrastructure: RED

Drainage – An extensive creek system is required – Remnant creek system evident on images: AMBER

Surface gradient – Optimum is approximately 1-2% (<1:50). The calculated modal slope for Swanwick saltmarsh is 2%: GREEN

Sediment particle size – Suitable for restorative works - Exact composition of dredged sediment to be used for recharge is unknown, but current data in this report (Section 4.2) indicate that fine grained sediments may be available as a result of marina dredging: AMBER

Sediment volume - sediment volumes required for recharge are similar to the volumes available from maintenance dredging. Volumes of sediment required to extend the existing Swanwick Marsh by 5, 20, and 50m buffers are shown in Table 5.14 and Figure 5.23.

Table 5.14: Sediment volumes required to extend current Swanwick Marsh saltmarsh extent by 5 m, 20 m and 50 m buffers

BUFFER TOTAL MUDFLAT VOLUME OF SEDIMENT REQUIRED (m3) SURFACE DISTANCE 2 MUDFLAT SURFACE AREA (m ) 2 MHWS MHW MHWN (m) AREA (m ) CONVERTED 5 1868 33798 6% 1,277 763 277 20 7340 33798 22% 3,871 6,416 3,871 50 18145 33798 54% 31,346 24,853 18,227

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Figure 5.23: Existing Swanwick Marsh saltmarsh and potential saltmarsh buffers Volumes required to achieve 50 m extension are greater than the typical yearly sediment volumes available from maintenance dredging in the Hamble, however volumes required for both a 5 m and 20 m extension are within the typical yearly volumes for the tidal levels calculated: GREEN

Natural Sediment Supply – Levels sufficient to support aspirations - Natural sediment supply in Hamble system judged to be sufficient for all sites (Williams et al., 2010) and local saltmarshes are present: GREEN

Accessibility - When selecting areas for recharge, consideration of the accessibility and costs for appropriate vessels/plants/machinery to handle the material must be considered – Accessibility by vessel and plant / machinery from land and river relatively good: GREEN

Contamination – Levels below Action Level 2 and licensed for disposal - All sediment samples from Hamble Marinas show some contaminants above Action Level 1 but none show contaminants above Action Level 2, and all sediments have been licensed for disposal at sea: AMBER

Local Economic/Recreational Activities – All current activities should be checked that they are not likely to be adversely affected by the creation of the recharge site – Potential implications during construction phase on footpath users and possible navigational issue for recreational vessels: AMBER

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Socio-Economics – Value of protecting human infrastructure – Site is protecting much used footpath: AMBER

Land Conservation Value - sites selected for saltmarsh creation should not have a high conservation value (such as SSSIs, Ramsar sites etc.) – Swanwick Marsh does not have local, European or international designations (Figure 1.2 and Figure 2.26): GREEN

Risks – Risks to infrastructure or areas with natural value should be considered - Risk to footpath infrastructure and significant risk to marina and berthing areas: RED

5.2.2 Remarks on MCA The MCA given above should not be taken as a recommendation of which marshes should or should not be potential beneficial use sites. It gives a colour-coded guidance to where marshes satisfy, or otherwise, particular factors taken from site knowledge and optimum values from the EA (2007) Saltmarsh Management Manual.

MCA can also be employed as a method of assessing confidence levels of the evidence base (e.g. available literature or data), through a confidence matrix. However, a confidence assessment in this study was not required as data were temporally and spatially comparable thus creating limited variance in quality and quantity and inherently supporting “confidence” in their suitability for subsequent analysis. Where data are derived from differing resources and acquired using differing techniques, confidence assessment would be of value, however for the present study, the data are sufficiently robust to negate the need (and value) for a confidence assessment.

5.3 Potential for other sediment management techniques A number of reviews and advice approaches for how to best approach saltmarsh and mudflat sediment retention and stabilisation methods have been undertaken, e.g. Colenutt (2001), Nottage and Robertson (2005), EA (2007) and, to a lesser extent, Williams et al. (2010). These studies have all considered saltmarsh management techniques other than (and/or in addition to) beneficial use of sediment. It is not the purpose of this document to recreate already available and useful text, however, a brief overview of sediment management methods, other than reuse of sediments, is provided here for context. Against this, opinion will be given as to the potential efficacy of sediment retention methods in relation to the River Hamble marshes.

5.3.1 Flow Modification The practice of slowing down water velocity, and hence increasing the potential for sediment deposition and retention, originated in Germany and Holland and first applied in the UK in the late 1980s/early 1990s. The practice is designed to slow the passage of water over the tidal cycle, in order to allow fine sediments to settle in a relative energy minima. Semi permeable structures are put in place to encourage slack water which slowly drains, allowing sediment to settle. The technique is somewhat analogous to the traditional methods of providing nutrient rich sediment to water meadows, and also to the role of pioneer species such as Spartina which promotes sediment retention through flow reduction between stems/leaves etc. (see Section 5.3.4 below).

Colenutt (2001) usefully summarises techniques available to reduce flow velocity as:

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 “provision of sedimentation polders or the use of baffle fences;  use of wave-energy reducing geotextile structures;  ‘construction’ of coarse-grained bunds seaward of the eroding frontage; and  vegetation planting / transplanting”.

EA (2007) indicates that the practice of using sedimentation fences was originally used in areas in southeast England, particularly in Essex, where continuing intertidal erosion was a problem (Holder and Burd, 1990). Examples include sites along the Dengie Peninsula (Essex). Monitoring of these sites suggested that their effectiveness was limited and, following several experimental studies, large scale flow modification using sedimentation fields is now only believed to be successful if the local sedimentary trend is towards accretion.

EA (2007) suggests that its use has declined in recent years, and is now used only in combination with a number of other techniques on a small scale. However this suggests that the use of such structures may still have potential on the smaller scale sites in the Hamble, particularly if some accretion is still occurring naturally (for examples see Plate 5.1 and Plate 5.2).

Plate 5.1: Brushwood and fence structure to enhance sediment retention, Lymington Marshes (Photo © and used with permission of Marine Space Ltd)

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Plate 5.2: Brushwood and fence structure showing enhanced sediment retention, Lymington Marshes (Photo © and used with permission of Marine Space Ltd)

When assessing the use of sedimentation fences, a number of potential negative effects must also be taken into consideration. These include disturbance to the marsh or mudflat during construction, washout of the fence material itself (i.e. fences are unsuitable where flow speeds are naturally high), and the potential for scour immediately adjacent to the structure. Fences may also constitute a hazard to boat traffic at high tide, which may be a significant issue within the Hamble, however given their typically location within the marsh itself this effect may be negligible.

5.3.2 Wave energy reduction Colenutt (2001) describes the use of geotextile structures to act as wave breaks or breakwaters to reduce the potential for erosion at the frontal area of a saltmarsh or mudflat. Nottage and Robertson (2005) also describe the use of larger-scale impermeable barriers that act as breakwaters, lying off the frontage of the marsh in exposed situations. Fetch within the Hamble is relatively short, the estuary is sheltered from prevailing winds thus wave energy is relatively limited, and hard breakwaters are unlikely to be required.

Colenutt (2001), Nottage and Robertson (2005) and EA (2007) also describe other wave break materials which may be more applicable within the Hamble, including brushwood, planking or traditional materials such as straw bales staked into place.

Breakwaters are usually positioned at or near low water, to encompass as much of the intertidal profile as possible and provide protection for most of the tidal cycle (EA, 2007). As with fences this may constitute a hazard to boat traffic at high tide and, given their potential positioning at or near the low

124 AHTI_J2015_004 RIVER HAMBLE SOFT SE DIMENT HABITAT RETEN TION FEASIBILITY STUDY water mark, this may be a more significant issue within the Hamble than placement of sedimentation fences. Any structure will require repeated assessment of its stability or reorientation and there may be a need for some effects modelling.

5.3.3 Bunds to hold material on the foreshore. Coarse material derived from dredging can be used in protective schemes and to hold finer grained material that is being used to raise saltmarsh levels landward of the coarse sediment. Cohesive clays can also be used in a similar manner to create bunds (see Plate 5.3 below). The behaviour of deposited coarse sediments is more predictable than fine sediment, and naturally transported fine sediments (or fine sediments deposited during beneficial reuse) may be more likely to remain at the disposal site if protected and held in place by a bund.

Plate 5.3: Clay bund at Wallasea Island (Photo © and used with permission of Roger Morris/Bright Angel Coastal Consultants)

The placement of clay bunds was employed on the River Orwell as a short-term way of raising mudflat levels on a highly eroded foreshore that had lost all of its fine grained sediment which had provided a bird feeding habitat. A similar system was also employed at Wallasea Island (now known as Allhallows Marsh) where compensatory habitat for loss of mudflat and saltmarsh at Lappel Banks and at Felixstowe was created by Defra (Dixon et al., 2008) (Plate 5.4 and Plate 5.5). This latter situation was designed to create saltmarsh rather than mudflat, and was constructed as part of a managed realignment project. A further high mudflat creation experiment was conducted at Poole Harbour in the 1990s.

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Plate 5.4: Development of salt marsh at Wallasea Island (Photo © and used with permission of Roger Morris/Bright Angel Coastal Consultants)

Plate 5.5: Gravel bund retaining finer grained sediment on the foreshore at Shotley (Photo © and used with permission of Roger Morris/Bright Angel Coastal Consultants)

In addition protective bunds have been used within the River Hamble, as partially noted to the south of Little Marsh (Plate 5.6).

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Plate 5.6: Little Marsh with semi permeable bund to left (south) of marsh – 2006 (Photo © and used with permission of Owl House Ltd)

Efforts to retain sediments within bunds have had mixed levels of success. For example the Shotley recharge project has certainly created habitat, but the gravel bund has rolled landwards and sediment levels behind the bund has had to be recharged. Sediment placement within clay retaining bunds on the Orwell has survived for several years, but is eventually expected to erode away. This is consistent with the concept's initial thinking: that the bunds would allow gradual release of sediment to recharge other mudflats and in the meantime the structures would help to reduce wave energy and erosion that was threatening the stability of flood defences.

Bunds have similar disadvantages to the placement of wave breaks. They can constitute a hazard to boat traffic at high tide and, given their typical construction with coarse sediments, may be a more robust obstacle to vessels than wave breaks consisting of natural materials. Again, their potential positioning at or near the low water mark may be a significant issue within the Hamble though micro-siting at possible locations may negate these concerns. The robust nature of the structure may mitigate the need for frequent assessment of its stability, however any requirement for reorientation will be much more complicated. Finally, the potential effect of such a structure on hydrodynamic processes means that the requirement for modelling prior to licensing may be increased. However, opportunities presented through personal communication with land owners and some regulators suggest pragmatic approaches in the Hamble may be considered whereby progress could be achieved in a localised scheme or trial.

5.3.4 Vegetation Planting EA (2007) indicates that vegetation planting can be used in a variety of situations as an exclusive technique or, more commonly, in combination with other restoration or habitat creation methods. Current velocity can be reduced, with an associated increase in depositional potential and decrease in erosive potential, by the energy dissipating effect of deliberately planted Spartina spp. EA (2007) notes, however, that:

“Saltmarsh vegetation can only be established successfully, if the physical as well as biological conditions are satisfactory. Natural colonisation should, therefore, be considered as the preferred option for saltmarsh vegetation establishment rather than artificial transplantation [note Garbutt, 2015,

AHTI_J2015_004 127 RIVER HAMBLE SOFT SE DIMENT HABITAT RETEN TION FEASIBILITY STUDY recommended against planting at Lincegrove and Hacketts]; although planting or seeding initiatives nonetheless may be useful in situations where there are no existing saltmarshes in the area and/or where natural colonisation is deemed to be undesirable from either a flood defence or a nature conservation perspective”.

EA (2007) also indicates a number of issues (many assessed in the MCA above) that should be considered when contemplating the suitability of a vegetation planting scheme, namely:

 Is there a history of saltmarsh growth? This will mean that it can grow in the area, but will also provide a seed supply for marsh succession and development;  Is the coastline experiencing rapid erosion? If so, a planted marsh may not survive long;  Is the site at a suitable elevation?;  Is the slope suitable for marsh succession?; and  Are the hydrographic conditions (including salinity) suitable for vegetation?

Potential negative effects of vegetation planting include the introduction of exotic species and problems with species competition, and species selected for planting should represent the natural species in the area, in order to maintain the biodiversity of the site. A planting campaign will require significant access to a marsh, and may cause disturbance to the marsh or mudflat during planting. EA (2007) suggests that transplanted vegetation should be taken from sources as close as possible to the intended planting site, since minor genetic differences may alter a plant’s ability to withstand particular environmental conditions. It may be difficult in the Hamble (where local saltmarshes are fragmented) to obtain a sufficient number of specimens.

In addition, under UK and European Legislation, the consent of the relevant statutory agency (e.g. Natural England) is required for the introduction of any species into designated sites. In relation to Hamble sites it as noted previously, Garbutt (2015) recommended against vegetation (Spartina) planting at Hacketts and Lincegrove, and felt it unviable at Bunny Meadows where modification of inflow pipes and sediment retention methods have been suggested by others (Dawson & Henville, 1992).

5.3.5 Scheme Risks Schemes to retain sediment whilst seemingly sensible, can go wrong in terms of a mismatch between the levels of sediment available, and the methods chosen as suitable to intercept and settle the available material (Williams et al., 2010). It is notable that in the Netherlands the option to promote sediment accretion, with the aim of increasing saltmarsh growth and thus reducing wave height, is now being considered as an integral part of coastal management soft engineering. It is being encompassed in cost benefit approaches (see Environment Bank, 2015) and ecosystem services models (van Loon- Steensma, 2015), though the authors note it is important to “involve experts” in terms of saltmarsh ecology and hydrodynamics and that “pilots are needed”; as noted by Natural England for the Solent region (Williams et al., 2010).

All the strategies described above will incur costs, although sedimentation fences and natural material wave breaks are likely to incur the lowest initial material outlay. There is an ongoing risk that any material that deposits within the marsh system (whether naturally or by recharge), may at some stage be eroded. Experience has shown that wave over-topping or increased flows due to storm events will lead to erosion of sediment back into the estuarine system. While some of this will certainly feed other

128 AHTI_J2015_004 RIVER HAMBLE SOFT SE DIMENT HABITAT RETEN TION FEASIBILITY STUDY inter-tidal mudflats and saltmarshes within the river, it is entirely possible that a proportion will end up back in the dredged pockets or basins and into the main channel itself, however this may be deemed an acceptable outcome after risks are identified to stakeholders.

With the need for expert opinion realised, and knowledge of the methods available, it is feasible, at this stage, to provide an overview of which marshes, and where, may benefit from an approach towards sediment retention for marsh vertical and forward growth (Table 5.15). This should be tempered with the knowledge that with the limited (or no) options for managed realignment or marsh retreat inland to overcome coastal squeeze, the temporal effects of retention methods may be limited. Table 5.15: Site assessment in terms of sediment retention aspirations

MARSH SEDIMENT RETENTION COMMENTS Hacketts Northern end (Lands End - below residents) Specifically mentioned in written (Garbutt Marsh may benefit from appropriate sediment et al., 2015) and oral communication, site retention such as fences of wave breaks to not to be interfered with (see section 3.1), ameliorate significant (Figure 2.3) though former was in relation to Spartina fragmentation and height loss (see section planting and comments may not pertain to 2.2.1). Imagery shows local resident placement Lands End. Site visit showed species rich of coarse material has caused localised diverse mid-level marsh in main area, with accretion. Decline slowed in recent years (3% patches of pioneer species. Lands End 2007-2014) and some gain in elevation highly fragmented, erosion starting from S- N from 1946 data. Large change noted from 1971-1984 Lincegrove Limited decline in last seven years, some Specifically mentioned in written (Garbutt Marsh localised accretion with elevation increase (see et al., 2015) and oral communication, site section 2.2.2). Low area loss 2007-2014, but not to be interfered with, though former height erosion. Retention methods such as was in relation to Spartina planting. Site fences may be used to address height loss, local visit showed species rich diverse mid-level trial possibly appropriate subject to NE marsh approval Mercury Circa 5% decline 2007-2014, edge erosion Marsh fronts relatively high density Marsh apparent as is height loss (topographical housing at Mercury Gardens, Hamble. The change to flatter profile) from upper (fen) value of this marsh in terms of coastal marsh (section 2.2.3). Retention methods not protection and recreational/social value recommended for upper marsh, though site should be considered (see Environment investigation may identify height loss aspects Bank, 2015) and appropriate management. Edge erosion noted, retention methods such as wave breaks or bunding to promote sediment accretion at toe of marsh for pioneer species may be beneficial. Satchell Limited information on the ecology of Satchell Local residents have significantly modified Marsh Marsh. Status described as unfavourable marsh for boat access, thus partially recovering (section 2.2.4) although this is in compromising marsh structure and context of wider SSSI. Localised accretion in potentially impeding coastal defence value. height, but loss in area. Will require detailed Local housing at risk of flooding (North discussion with regard to marsh status and Solent SMP) thus value of marsh for flood retention works, particularly regarding possible protection should be considered. Possible effects on local residents and river access. site for management options. Imagery suggests loss of natural drainage and partial conversion to fen, or upper marsh species with major boat access channels through and to rear of marsh.

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MARSH SEDIMENT RETENTION COMMENTS Little Marsh Marsh has accreted in height (see section Owner of Little Marsh interested to 2.2.5), but upper edge decreased (possibly due cooperate and investigate possible marsh to dewatering), fragmentation at river edge but enhancement through retention methods recent overall erosion limited (1%) 2007-2014. and repair of existing bund structure. Extension and repair, particularly at western end of existing bund and placement of bund or wavebreak retention structures towards toe of marsh along positon of previous pontoon may improve profile, enhance pioneer species and retain existing marsh. Hamble Highly fragmented structure (see section 2.2.6) Little Marsh is adjacent and perhaps Common significant height loss at upper marsh and identifies what may be achievable through Marsh isolated lower patches, area loss 15% 2007- sediment management. Economically 2014. Site may be significantly influenced by important site in terms of possible nearby sediment influx barriers (marina), thus sediment impacts on nearby commercial supply may be limited. Terrestrial runoff may marina, thus close consultation is advised. be a factor through creek. Possible fence retention structures useful, and desirable closer to Little Marsh where some areas of pioneer species are apparent. Hook Marsh Currently no saltmarsh on seaward side of Site possibly not viable in relation to footpath (section 2.2.7). Data suggest saltmarsh aspirations for intertidal offset inside was present in 1946 but no record before or footpath. after. Area is an intertidal creek with gravels and muds to outer edges. Sediment retention structures may be a viable option inside of spit, but liaison with partners developing offset intertidal area behind footpath would be required. Bunny Upper edge of marsh lost height and Dawson and Henville, (1992) commented Meadows topographic complexity from 2007-2014 (see that there are “high erosive velocities and South section 2.2.8). Reasons unclear. Large area considerable scouring of bed and banks” growth from initial wall breach but Significant where the culverts allow tidal flows in decline from 2007-2014. Sediment retention Bunny Meadows. Further, in this study methods suggested following water hydraulics comment has been made that the culverts survey (1992) plus redesign of water require modification and marsh features inlet/outlet structures inside wall. would benefit from some protection. Marshes cannot retreat landwards due to gardens and resident protection, though SMP suggests “adaptation”. Bunny Similar to Bunny Meadows South, loss of height Complex site with limited access. Former Meadows at rear of marsh and some central and toe grazing marsh, wall breached to allow tidal North areas in centre of marsh (section 2.2.9). Loss of ingress, but further landward retreat 16% in area from 2007-2014. May benefit from blocked by woodland. Options for modification to inlet channel and sustainable management may be sediment/flow retention methods inside wall to worthwhile. promote greater settlement. Crableck Significant height and area loss with another Commercially sensitive site, and whilst Marsh 6% of patchy remnants lost from 2007-2014 localised sediment retention may be (see section 2.2.10). Possible bund or wave beneficial, and may help with management break sediment retention options at last of infrastructure, would require close remnants associated with pontoon access liaison with marina operators to save structures at Universal Marina. marsh remnants. Universal Lost more than 50% of area from 1870-1971 The marsh is highly constrained in area by

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MARSH SEDIMENT RETENTION COMMENTS Marsh (see section 2.2.11). From 2007-2014 remnant marina structure to the south, private marsh loss is insignificant. Height of marsh gardens to the north and nearby pontoons marginally increased possibly related to regular in the river channel in front of the marsh. inundation. Possible limited options for fence Any proposal for sediment retention works or bund protection to encourage toe of marsh will need close liaison with the marina growth. However, the footpath now regularly operators and residents, though methods floods thus area for lower marsh may now be to promote accretion for marsh height may too inundated to achieve sufficient dewatering prove beneficial for coastal protection. for marsh growth. Swanwick From 1870-1971 88% of marsh lost (see section Significant height and area loss, impeded Marsh 2.2.12), the majority due to marina from landward migration and loss of construction which may also have affected sediment supply. Constrained by footpath sediment supply to remaining areas, though which is raised above the mudflat by circa would require study. Remnant on outside of 2-3m. This area was a large marsh which Brooklands meander declined to extent it is not has largely disappeared likely due to sea recorded, though patches remain. It is possible level rise, coastal squeeze and possibly that localised fencing may promote sediment sediment supply. This area may benefit accretion on mudflat, a trial may be valuable, from targeted studies to establish what but this area would benefit from sediment may be achieved. pathway study to inform validity of this approach.

It can be seen in some Hamble marshes that over heightening has resulted in a habitat change (Mercury and the upper of Little Marsh), to fen and unexpected Phragmites (Cope et al., 2008; Williams et al., 2010), equally in many sites on the Hamble, lower marsh has declined to mudflat and edge erosion is evident.

Saltmarshes are sensitive to change, but with enhanced sediment retention Spartina or other pioneer species may prosper and achieve some substrate stabilisation. Alternatively, marsh species may be less productive through inundation and reduced drainage leading to loss of growth, loss of binding roots leading to open areas and thus open water (Gedan et al., 2009).

Accordingly a balance of aspiration and what is practicable must be sought. Some locations on the Hamble appear potentially suitable for sediment retention approaches, and there is willingness from some private landowners to engage in the possibility. This seems an opportunity that the RHHA, regulators, stakeholders and partners could investigate. However, in light of the habitat and commercial/social sensitivity, close involvement of suitable expertise in retention methods, hydro and sediment dynamics is advised as noted by van Loon-Steensma (2015).

5.4 Potential environmental benefits of sediment management schemes As discussed, the Hamble estuary is clearly a site of significant human activity e.g. commercial harbour/marina use, recreation, flood defence etc., and the lower reaches are fringed with areas of residential or industrial development. These factors increase pressure on the estuary, including fringing saltmarshes and intertidal mudflats.

Saltmarsh and mudflat are highly productive, providing organic matter to the wider marine ecosystem and can provide habitat for feeding and over wintering birds and, when tidally inundated, pelagic and benthic marine species, and estuarine reduced salinity specialists. They are also important fish nursery grounds (including restored marsh (Gray, 2007)). They are also sinks and sources of sediments and

AHTI_J2015_004 131 RIVER HAMBLE SOFT SE DIMENT HABITAT RETEN TION FEASIBILITY STUDY nutrients and can have pivotal roles in local marine ecosystems and natural coastal protection and importantly they are recognised as a pollution sink and a sink for atmospheric carbon dioxide (Luisetti et al., 2014).

In common with many ecosystems and possibly analogous to mangrove (Friess et al., 2012) the value of coastal saltmarsh wetlands has been recognised relatively recently, with cost-benefit approaches highlighting the important of their ‘ecosystem services’ from social and economic aspects (e.g. Spurgeon, 1998; Fisher et al., 2009; Potts et al., 2014; Environment Bank, 2015). However, difficulty in valuing the benefits can arise when, within any system, interdependent ecosystem services are interlinked and complex. Potts et al. (2014) separate marine ecosystem services into four interlinked groups: basic pressures and structure; intermediate ecosystem services; final ecosystem services; and goods/benefits (Figure 5.24).

Figure 5.24: Marine ecosystem services as defined by Potts et al. (2014) The paper by Luisetti et al. (2014) focusses on the value of coastal zone ecosystem services associated with saltmarsh creation and estuary management. While this focusses on case studies within the Blackwater and Humber estuaries, there are a number of key environmental benefits identified which are of direct relevance to the Hamble.

One benefit specifically investigated by Luisetti et al. (2014) is the amenity and recreation value potential associated with the re-creation of intertidal habitats. This study indicated the valuation procedure is service and location specific, but that benefits could be estimated in terms of the increased amenity and recreational value of newly created intertidal habitats. Under the terms of the Luisetti et al. (2014) model, re-creation of saltmarshes in the Blackwater estuary resulted in significant increased value while there were negative outcomes associated with continued saltmarsh loss. The Hamble estuary has a very high recreational value, both from boating, but also in public benefit from the natural areas in terms of social wellbeing and perception, plus the marshes provide refuge and nursery areas for fish species associated with recreational and commercial fishing.

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Further environmental benefits indicated by Luisetti et al. (2014) which may have relevance to the Hamble estuary include flood defence, erosion control, contaminant and nutrient cycling, and food provision for fish species (including commercial species); the Hamble is a protected bass nursery area and saltmarsh has been shown as important for fish such as mullets, juvenile sea bass and young flat fishes (Laffaille et al., 2000). However, Luisetti et al. (2014) do indicate that care should be taking when extrapolating ecosystem service benefits between different estuaries, and especially between different coastal systems. A useful resource to enable informed consideration of the ecosystem services estuaries provide is given by the TIDE project which has taken study and definition of “services” further (see http://www.tide-toolbox.eu/reports/).

The flood defence benefits of salt marshes have been more widely recognized in recent literature. The predicted effects of sea-level rise may be mitigated by vertical accretion of salt marsh surface, while increasing marsh vegetation can stabilise coastlines (e.g. Gedan et al., 2009). Some studies (e.g. Shepard et al. (2011)) show that salt marshes decrease coastal damage by attenuating waves and decreasing storm surge, however Brisson et al. (2014) indicate that empirical data on the effects of salt marshes on wave attenuation and sedimentation are rare. There are, by contrast, a number of studies showing that loss of vegetation reduces marsh resiliency to sea-level rise, storm surge, and erosion (e.g. Baustian et al., 2012; Temmerman et al., 2012; Silliman et al., 2012).

The environmental benefits of saltmarshes as flood defences have been investigated recently by Brisson et al. (2014). This study looked at the ecosystem service of coastal protection linked to cordgrass/saltmarsh stabilised shores and coastlines and indicated that salt marsh recovery restored the provisioning of coastal protection through: (1) increased wave attenuation on healthy and recovered creek banks compared with creek banks experiencing die-off; and (2) increased shoreline stabilisation along healthy and recovered creek banks.

The paper by Foster et al. (2013) provides a useful and wide ranging examination of intertidal mudflat and saltmarsh conservation and sustainable use in the UK and considers the importance and value of saltmarshes; as well as reviewing their current status, characteristics, causes and consequences of loss, and the associated responses to loss. Foster et al. (2013) also highlight that where it exists in appropriate quantity and form, saltmarsh vegetation has a significant capacity to attenuate wave height and wave energy. This natural buffering function provides a first line of defence against coastal flooding, which considerably reduces the construction specifications of harder sea defences to protect the hinterland.

Intertidal mudflats and saltmarshes generate some of the highest and most valuable ecosystem services upon which humans and other species depend. The primary argument for their protection and reparation is, where practicable and reasonable, to secure and improve the continued delivery of these services, particularly for nature conservation and coastal defence purposes.

It is important to remember the nature of “ecosystem services”. Whilst some have questioned the term for fear of the exclusion of “non-valued” habitats (as saltmarsh used to be) in favour of those that humans view as important (see: Balmford et al., 2002; Wiens, 2007), financial valuation as a term of reference for human understanding may be the most effective way such habitats can be conserved.

For a saltmarsh restoration study undertaken in Huelva, Andalusia, Spain, Curado et al. (2014) noted that “only a few restoration projects incorporate public perception in their monitoring. However,

AHTI_J2015_004 133 RIVER HAMBLE SOFT SE DIMENT HABITAT RETEN TION FEASIBILITY STUDY participation of local people is a key process if social benefits are to be achieved”. Curado et al. (2014) went on to note that “most of [the region’s] citizens recognised the benefits of coastal marshes (75%), a perception which increased with level of education. The majority of the respondents showed a low level of knowledge about the ecological services and functions of salt marshes”. Thus it is strongly recommended that should beneficial use or passive structures (see below) be proposed, such education and information to both the private and, perhaps more significantly, commercial sectors in the heavily used Hamble be undertaken to demonstrate the value of these formerly misunderstood habitats.

5.5 Potential adverse environmental impacts There are inevitable environmental effects and potential impacts associated with the beneficial use of dredged material. This section is concerned only with the environmental effects which are likely to occur at the actual beneficial use site itself. The information presented does not consider the environmental effects associated with the actual dredging activity itself, whether from capital projects, or from maintenance activity. The Marine Management Organisation (MMO) has a duty to consider the direct and indirect effects associated with the dredging activity itself. Dredging will be subject to assessment as part of the Marine Works (Environmental Impact Assessment) (Amendment) Regulations (as amended 2011) application process (for capital works), through consideration of maintenance dredge protocols, and subject to Habitats Regulations Assessment (HRA) where necessary.

The MMO also has a statutory role, as the competent authority, considering what practical alternative disposal options are available, including the beneficial use of dredged materials, prior to granting a licence to dispose of dredged material at sea (Hiorns, 2011).

Hiorns (2011) classifies beneficial use into 4 categories of re-used of dredged material:

 Recycling and retention of dredged material within an estuarine or coastal sediment system;  Transport and re-use of dredged material to other geographical areas for habitat creation;  Re-use of dredged material as infill for reclamation or coast defence; and  Removal and re-use of contaminated dredged material for construction and infill.

The feasibility study for beneficial use within the River Hamble, falls under Point 1, and possibly part of Point 3.

The benefits of a recharge or beneficial use scheme in the Hamble need to be considered against the potential impacts to existing habitat and species interests. Are there areas in the estuary where the intertidal and saltmarsh habitats will benefit, in the medium long-term, from the deposition of dredged material? The majority of the habitats within the Hamble region discussed here are part of the Southampton Water and Solent SPA and Ramsar site, or as features of the Solent Maritime SAC. Therefore, any beneficial use project will require statutory assessment of environmental effects as part of the processes required to satisfy the Marine and Coastal Access Act 2009, and also The Conservation of Habitats and Species (Amendment) Regulations 2012.

The Harbour Authority is a Section 28G body under the CRoW Act 2000 and a Section 40 body under the NERC Act 2006, and has to consider environmental effects of its projects on habitats and species listed under Section 41 of the NERC Act 2006.

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The majority of current beneficial use schemes are located in ecosystems where habitats and species are naturally tolerant of high sediment loads and periodic smothering, so environmental effects may not be significant, certainly where consideration is given to short-term impacts versus potential medium long-term benefits (UK Marine SACs Project, 2001). Further, it is possible that the beneficial use area is currently impacted, and thus in poor condition or non-functional, and therefore deemed a location suitable for restoration/enhancement. In these cases, to some degree the habitats and species at the recharge site may be deemed as ‘sacrificial’ in the larger scheme of site management i.e. it is acceptable that there will be initial direct impact at the site, with foresight that the location will benefit and improve in condition in the medium long term. In a conservation context, liaison with statutory consultees (Natural England etc.) will establish consensus on the habitat status (many listed as Unfavourable Recovering) and whether potentially suitable to undertake beneficial use project(s).

5.5.1 Environmental Effects Associated with Deposition of Dredged Material Environmental effects at the beneficial use location are associated with the deposition of dredged material, and can general be classified into: direct effects e.g. habitat removal or smothering; and indirect effects such as increased turbidity or alteration of sediment transport within the estuary. Bray (2008) interprets advice from The International Association of Dredging Companies (IADC) and the Central Dredging Association (CEDA). Bray (2008) presents a classification of the spatial footprint of effects as near-field (<1 km) and far-field (>1 km), and also considers the temporal envelope, with short- term effects (<1week) and those that may be more persistent i.e. long-term effects (>1 week) (Table 5.16). Whilst assessing environmental effects these thresholds should be considered indicative, and there may be requirements for estuary-specific modelling of physical and geomorphological envelopes to properly articulate the assessment envelope for a beneficial use project i.e. sediment plume footprints, sediment budget, sediment mobility prisms, etc. For The Hamble Estuary, the models and analyses presented within Section 5.2 of Williams-Hopley, (2014) (available on line) will be informative.

Table 5.16: Time–space matrix of potential environmental effects associated with dredged material placement (Adapted from: Bray, 2008) *Standard Operating Procedures NEAR-FIELD ENVIRONMENTAL FAR-FIELD ENVIRONMENTAL

EFFECTS (<1KM) EFFECTS (>1KM) Short-term Environmental Effects Smothering of organisms Offsite movements of sediments (<1 week) Turbidity by physical transport Reduced water quality Offsite movements of chemicals Disturbance – visual- and noise- by physical transport (mitigated related by S.O.P.*) Release of chemical contaminants/acute chemical toxicity (mitigated by S.O.P.*) Long-term Environmental Effects Altered substrate type Changes to hydrography and (>1 week) Altered community structure morphology Chronic chemical toxicity Changes to sediment budgets (mitigated by S.O.P.*) Bioaccumulation (mitigated by S.O.P.*)

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It is also important to understand the source of the dredged material as this may affect the types of environmental effects which may occur. Capital dredged material is generally more consolidated and coarse, compared to finer sediments of maintenance dredgings sourced from navigation channels (UK Marine SACs Project, 2001; Hiorns, 2011). Therefore, the behaviour of capital dredge arisings is considered to be more predictable than those for maintenance dredge material, as they are more likely to remain in situ at the location of deposition, rather than be re-mobilised through subsequent tidal phases and inundations, resulting in sediment re-suspension and run-off from the intertidal back into the subtidal channels and creeks (UK Marine SACs Project, 2001; Williams-Hopley, 2014). For the Hamble, capital dredge works have been limited (see ABPmer, 2011a) thus the material more regularly available for beneficial use projects will be maintenance dredge sediments.

Recharge of intertidal habitats with dredged materials that are coarser than the present intertidal sediments, can be used to protect saltmarshes from wave attack and erosion (Carpenter and Brampton, 1996; EA, 2007). However, although this technique has many benefits for flood defence purposes, the use of coarse sediments to recharge intertidal mudflats changes their nature considerably in terms of sediment processes and the habitats and species likely to evolve and colonise the site, post-discharge. Whereas the use of finer maintenance dredging material has value as it is most likely to resemble the intertidal sediment composition at the beneficial use area, and can be used to trickle feed sediment1 (Hiorns, 2011).

5.5.2 Direct Environmental Effects There are a small number of direct environmental effects associated with the deposition of dredged material onto the intertidal, or within the shallow subtidal, in estuarine habitats, and these are predominantly associated with short-term, near-field effects.

Removal of Habitat through Burial and Associated Smothering of Benthic Organisms Physical removal of a habitat through burial is the mostly likely short-term, near-field, direct effect associated with deposition of dredging material. This results in the smothering of benthic animals and plants at the recharge site, particularly if sediment is placed on the intertidal at too high a rate. Smothering can occur during the initial placement of material or due to more gradual accumulation as the discharge component of the project proceeds.

Settlement of suspended sediments from rainbowing or from re-suspension related to tidal processes can also smother or blanket subtidal and possibly adjacent intertidal, communities (EA, 2007).

The scale of impact is dependent upon the type of habitat and species present, with the majority of intertidal species tolerant of varying degrees of sediment deposition. High rates and high loads of deposition will most likely result in greater impacts to in situ communities, where steady trickle charging may more closely approximate natural sedimentation rates and allow greater adaptation to the deposition (UK Marine SACs Project, 2001).

1 Where periodic release of material in the water column increases the supply of fine sediment to intertidal areas via tidal currents.

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Turbidity and Water Quality Short-term increases in the level of suspended sediment, either directly due to trickle charging, or from re-suspension of deposited material, can give rise to changes in water quality affecting turbidity, reducing dissolved oxygen concentrations and light-penetration into the water column (Hiorns, 2011). For example, filter feeding organisms can be harmed by increased levels of turbidity clogging and damaging feeding and breathing structures. In addition, mortality and avoidance behaviour in fish have been observed in the face of artificially increased sediment levels (Appleby and Scarratt, 1989). However, these effects only occur when turbidity generated is significantly greater than the existing natural background levels and sedimentation rates in an area, but should beneficial use be considered as an option, the timing of dredge spoil placement in a saltmarsh area will need consideration into fish nursery/migration needs.

In most cases within the estuarine ecosystem benthic organisms are adapted to high elevations of suspended sediment particles, and it may be the case that recharge/beneficial discharge mimics storm- induced increased suspended sediment concentration, albeit at a higher frequency than naturally occurring events. However, these effects are considered short-term and generally near-field.

Visual- and Noise-related Disturbance The activity of depositing the dredged material itself can cause disturbance to marine life through noise, and also from the visual presence of the plant associated with the works. Certain wading bird and tern species are likely to demonstrate disturbance-related behaviours such as displacement from the mudflats and saltmarsh at the recharge site, increased ‘flightiness’, and abandonment of feeding and roosting sites, and possibly nests (UK Marine SACs Project, 2001; Hiorns, 2011). These effects can cause deterioration in the condition of individuals, and possibly mortality, however conservation objectives for designated sites are generally concerned with population-scale effects. Timings of the discharge operations can mitigate some effects i.e. the project avoids the nesting season of certain waders, or winter periods when migratory populations are present.

Activities such as rainbowing from dredgers will have a larger disturbance footprint than the use of trickle charging pipelines.

5.5.3 Indirect Environmental Effects

Changes to Hydromorphology and Geomorphology Deposition of dredged material alters the bathymetry and shore height at the site, which may in turn affect/alter the wave attenuation and tide characteristics (hydrography) within the system (Williams- Hopley, 2014). Alterations to the system can result in impact upon habitats through local erosion and accretion and also lead to changes in sediment transport and budget.

The magnitude of the effect will depend on the size of the recharge area, compared to the size of the cross-section of the estuary as a whole (Hiorns, 2011).

A change to the angle and/or slope of the shore (shallow subtidal and intertidal) alters the direction, size, and shape of the waves (Hiorns, 2011). This in turn can alter the amount of energy experienced on the foreshore and the erosion rate of intertidal habitats at other locations within The Hamble Estuary.

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Changes to Sediment Budgets This effect can generally be considered a positive one, and is in-part the reason for many beneficial use schemes in the UK. Capital and maintenance dredging can cause adverse effects on the sediment budget of an estuary and these impacts can often be mitigated by the beneficial use of dredgings, recycling and retaining material within the system.

Release of Chemical Contaminants If the dredged material contains elevated levels of contaminants, such as heavy metals or polycyclic aromatic hydrocarbons (PAHs), organic matter or nutrients, then contaminant-related effects may occur from the disposal of sediment. Results from previous dredges of Hamble sediments indicate that sediment contaminant levels were not above Action Level 2 (see 4.2) which requires managed disposal, thus in principle this should not be a barrier to their re-use in Hamble related projects. However, caution is advised as “pockets” of contamination can persist (see Steyl et al., 2013) which may lead to sediments being deemed unsuitable.

The disposal of dredged material is licensed by the MMO. Cefas advises the MMO on the potential impacts for contamination, and risks from disposal of the material. These procedures will be in place regardless of the destination of the dredged material (offshore disposal site or at a beneficial use site) and only suitable material will be authorised for use as part of any beneficial use project.

5.5.4 Long-term Benefits The following list is extracted from the UK Marine SACs Project (2011), which proposed several long- term benefits associated with beneficial use of dredged materials:

 The sediments can be retained within the estuary system and recycled into the intertidal habitats, replacing lost intertidal area;  Where available, clean, fine, dredged materials with appropriate organic content are able to support productive benthic communities, similar to natural intertidal flats, and can be re- colonised by fauna at the recharge site and from adjacent areas; and  With appropriate planning and time, the recharged intertidal habitat can closely resemble natural intertidal flats, both in appearance and function.

Hamble Estuary Beneficial Use - Stakeholder Engagement Investigations into saltmarsh regeneration projects and management of estuarine Marine Protected Areas (MPAs) has shown that early, and informed, engagement with stakeholders ensures more positive outcomes, with an increased perception of ownership of issues by users of the environment (Curado et al., 2014; McAuliffe et al., 2014 (who considered attitudes of recreational boaters); Williams-Hopley, 2014). As already discussed, it is important to note that a limited number of saltmarsh restoration projects incorporate public perception in their monitoring and in works on their inception (Curado et al., 2014). Efforts to engage the community and landholders should be encouraged in the Hamble which benefits from an interested and able local community.

Whilst investigating public perceptions and uses of natural and restored salt marshes (Iberian Peninsula, Spain), Curado et al. (2014) showed that local residents attached high importance to public consultation, and expected to be consulted about restoration works before they proceeded. Curado et al. (2014) posit

138 AHTI_J2015_004 RIVER HAMBLE SOFT SE DIMENT HABITAT RETEN TION FEASIBILITY STUDY that during the monitoring, participation of local people achieves, and allows optimisation of, social benefits which can bring added value to a beneficial use/restoration programme. In addition, this allows any potential problems of public use to be identified. Finally, clear engagement can result in improved management of restored ecosystems whilst delivering public amenity and education regarding the system (e.g. the Hamble Estuary).

Additional observations from the studies include:

 Monitoring is beneficial as it informs and improves restoration methodology for future applications and to solving unexpected problems;  The importance of mitigating perceptions of ‘top-down’ decisions during planning stages, so that stakeholders feel engaged;  Adopting co-management strategies; and  Stakeholder engagement events, public outreach and integration of local resource users in planning/implementation stages. McAuliffe et al. (2014) found that the recreational boating community indicated that the most important settings for boating were:

 Access to safe anchorages;  Boating in clean water; and  Viewing natural scenery.

There is potential for beneficial use to deny mooring access and to result in a perception of water quality issues. It is useful if any beneficial use/habitat restoration project can demonstrate, or result in, an increased area of more resilient natural surroundings, which secure the structure and functioning of the system in the long-term for the benefit of all stakeholders.

5.6 Potential for increased accretion from disposal of dredge spoil

5.6.1 Foreshore recharge Foreshore recharge aims to restore the functioning of saltmarshes and mudflats by introducing sediment to adjacent intertidal locations. Most recharge schemes use sediment dredged from berths, harbours or channels, thus providing a beneficial use for the material (EA, 2007). The concept of foreshore recharge has evolved over the past 20 years, starting with experimental 'trickle charging' in the Medway in the early 1990s.

The sediment used can be fine grained mud and sand, often obtained during maintenance dredging of berths or coarser sediment, typically during capital dredging and often from main channel areas, though this is not applicable in the Hamble. Coarser sediment can be used to create bunds and protection structures (see Section 5.3) (as seen by previous work at Little Marsh; time unknown), and the behaviour of deposited gravel and sand is more predictable than that of fine grained sediment. Should coarse grained sediments be used within the Hamble there is therefore a lower potential for this to be transported and redistributed to other locations.

Finer sediments can be used to raise saltmarsh and mudflat levels, but this is less likely to remain at the disposal site, unless protected or placed in very quiescent conditions (EA, 2007). However, the use of finer maintenance dredging material has ecological value as it is most likely to resemble the intertidal

AHTI_J2015_004 139 RIVER HAMBLE SOFT SE DIMENT HABITAT RETEN TION FEASIBILITY STUDY sediment composition at the beneficial use area, allowing more rapid colonisation of species from surrounding habitats, and evolution of comparable climax communities and habitats (Hiorns, 2011).

The methods for the placement of fine sediments have developed through a range of iterations, especially within the Stour and Orwell Estuaries where Harwich Haven Authority, in conjunction with the Port of Felixstowe, have undertaken numerous experimental and operational measures. The options include:

Foreshore placement. This can be achieved in a number of ways, including:

 Pumping maintenance sediment onto mudflats and sandflats. The highest profile examples of these techniques include sand placement onto the beach at Hayling Island (Chichester Harbour Conservancy) and onto Bournemouth beach (Poole Harbour Authority);  Experimental sediment placement onto saltmarsh at Lymington Harbour involves cohesive sediment;  Rainbowing cohesive sediment onto foreshores (mainly at Horsey Island, Essex);  Depositing sediment at the lowest point in the intertidal and allowing it to be carried up onto the shore by successive tides.

Subtidal placement using a split hopper barge, and placed as a mound of sediment on the sea bed that will be eroded and carried onto the foreshore on successive tides. Sediment in the hopper is substantially de-watered (as far as it can be).

Water Column placement in which the dredger does not de-water to its maximum ability and more fluid material enters the water column either by rainbowing or from a split hopper. This approach is intended to raise the suspended sediment concentration of the water column in the immediate vicinity of the desired location for sedimentation. A less targeted approach is agitation or water injection dredging to mobilise sediment and to use the tides to carry it away. Some of this sediment will re-enter the water column in locations where it may help to recharge mudflats, however, it may also arrive in unwelcome locations, and constraints in the Hamble suggest this may need careful consideration as a method. Similarly, overtopping to improve the density of material in the hopper may help to place sediment in the water column.

Noting the above, the techniques of rainbowing, sediment deposition at the lowest point of the intertidal, sub-tidal placement and water column placement are not as suitable for sediment deposition within the River Hamble as direct pumping/sediment placement.

5.6.2 Direct pumping or sediment placement This technique uses a floating pipeline running from the dredging vessel (Plate 5.7) and onto the dredge site (Plate 5.8). Colenutt (2001) indicates that this is a potentially cost-effective option, with no re- handling or storage outlay and limited additional transportation costs.

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Plate 5.7: Floating pipeline carrying fine grained dredge material to recharge site (Photo © MarineSpace)

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Plate 5.8: Fine grained material being pumped directly onto intertidal area (Photo © MarineSpace) There are operational and logistical considerations with this technique. The floating pipeline may restrict navigation and vessel movement, albeit temporarily and pipelines may need to be permanently positioned within the replenishment site, or repeatedly moved and removed.

Colenutt (2001) reports that sediment discharged through a pipeline typically has a high water content (Plate 5.8) and is geotechnically weaker than bottom-dumped material. Subsequently this sediment needs a greater length of time to dewater and consolidate, hence has the potential to flow and be remobilised on subsequent tides. There is, therefore, potential for such material to be transported, causing accretion in areas of deposition; potentially unwelcome locations. This is typically mitigated by the use of retaining structures such as bunds, polders (Plate 5.9, Plate 5.10) or sedimentation fences (Plate 5.11).

Plate 5.9: Construction of sediment retaining polder (Photo © MarineSpace)

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Plate 5.10: Sediment retaining polder following sediment placement (Photo © MarineSpace)

Plate 5.11: Sedimentation fence following sediment placement (Photo © MarineSpace)

Against this background of sediment placement options, it is important to recognise the potential for deposited sediment to be transported from the point of recharge. Whilst, as is the intention, some of this sediment will feed saltmarsh and mudflat, there is also the potential for some to be deposited in undesirable locations, e.g. back within the marina basins or main channel.

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The potential impacts of this deposition will depend on the magnitude and location of the settling i.e. how much sediment (thickness) settles, and where in the system this occurs. Subtidal, intertidal and saltmarsh habitats and species in the Hamble are naturally tolerant of high sediment loads and periodic smothering, so the environmental effects of increased deposition may not be significant (UK Marine SACs Project, 2001). However, deposition of re-suspended material at the River mouth, or within a marina, could result in additional maintenance dredging for a period of time. However it may be important to consider that any impacts are likely to be relatively short-term, versus the potential for substantial medium long-term bonuses associated with a beneficial use scheme.

While any detailed investigation of sediment transport pathways, or modelling, is beyond the scope of this study, some inferences on the potential fate of re-suspended sediments can be made by reference to data contained in other studies, in particular ABPmer (2011a) and Williams-Hopley (2014).

Both ABPmer (2011a) and Williams-Hopley (2014) note that the Solent and Southampton Water’s asymmetrical tidal system is present in the River Hamble as far as Mercury Yacht Harbour. The morphology of the estuary and the tidal wave form affects the sediment transport pathways in a complex manner. The approximately 50% longer flood tide (than ebb tide), and the long slack water period, present a potential upstream movement of suspended sediments with deposition occurring through the slack water period. A further confounding factor of the asymmetric tidal wave results in highest flows speeds at the bed during the ebb tide, leading to a bedload movement towards the mouth of the Hamble and Southampton Water Main Channel. It is understood that over the shorter term, erosion and accretion patterns are variable within the estuary, with the main channel being largely stable. The short-term variability is particularly the case for the reach between Bursledon Point and Crableck where erosion was identified from 1988-1994. From ABPmer (2011a) it is not clear if erosion was evident in the main channel or margins, though the latter is possibly implied.

Alternately, Williams-Hopley (2014) for the period 1976-1996 reported that “north of Bunny Meadows salt marshes [i.e. between Bursledon Point and Crableck] accreted indicating that sediment from the [Bunny Meadows] realignment site had been transported north or that sediment suspended through dredging of Mercury Yacht Harbour had contributed to mudflat accretion on the opposite shore”. Thus there is the potential for cross channel sediment transport. This was further considered by Williams- Hopley (2014) in that “the accretion of the estuary mouth indicates that the dredge channel acted as a sediment sink and attempted to re-establish its natural form. There was erosion of the channel and mudflat at the eastern edge of the channel indicating this to be the source of material for the western side”.

ABPmer (2011a) looked at the possible impact of marina developments on flow, and reported the effects were “largely localised to the immediate vicinity of each marina and, overall, the increase in tidal volume caused by marina construction manifested as a small increase in flow speeds on the ebb and flood tide near low water”. ABPmer (2011a) went on to consider the effects on sediment supply but felt this “was difficult to ascertain” but that bedform deepening may reduce upstream sediment supply with subsequent maintenance dredging allowing potential reduction in sediment supply to continue. ABPmer (2011a) does state that a cause and effect relationship cannot be implied “however, such a link can occur and is usually inferred”. Based on the above information, for areas such as the reach encompassing Swanwick Marsh to Crableck and Mercury, sediment tracer studies may assist in clarifying sediment pathways and thus possibly inform options for management.

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ABPmer (2011a) contains an evaluation of sediment disturbance and potential dispersal of sediment re- suspended as part of the maintenance dredging campaigns within the river. These modelled extents can also be used to estimate the potential dispersal of remobilised sediment following deposition on saltmarsh or mudflat as part of any potential future beneficial use project.

ABPmer (2011a) estimate that the fine grained sediment found in the estuary has a settling velocity in the region of 0.008-0.020 cm/s. ABPmer (2011a) states the extent of dispersal is dependent on the flow conditions at the dredge location, the settling velocity of the sediment, the height of disturbance in the water column and the net impact of the tidal propagation. Due to the asymmetrical tide (mentioned previously) the peak and average flow speeds are higher on the ebb tide when compared to the flood, with speeds during neap tides being about 50-60% of the speeds on spring tides.

The highest flows occur in the Port Hamble reach with peak flood and ebb flows of around 0.5m/s and 0.75m/s respectively (averages of 0.32m/s and 0.48m/s) ABPmer (2011a). Down estuary the flows are generally slower with flood and ebb peaks up to 0.3m/s and 0.5m/s respectively. Flows appear to be lowest near to the Mercury Yacht Harbour, but increase again through the Universal Marina reach.

ABPmer (2011a) modelling also estimates that, based on the overall average flood and ebb spring tide flow rates, the maximum extent of travel for re-suspended sediment will be approximately 8km during the flood, and approximately 5km on the ebb. The additional transport distances on the flood are driven by the longer duration available for upstream transport (as a result of the asymmetric tidal curve) and the settling lag as a result of the delay between local flow velocities falling below the threshold for maintenance of suspension, and the particles reaching the estuary floor.

Dredging on the ebb tide at locations in the lower reaches of the estuary, will result in a greater proportion of sediment being lost into Southampton Water Main Channel and out to the Solent, although much of this is considered likely to return on the flood tidal wave (ABPmer, 2011a). Flood tide dredging will result in the disturbed (and not retained) sediments being kept in the Hamble system. However, this has to be balanced with real world limitations of only dredging on flood tides, i.e. this may not be practical for marina operators.

Whether the sediment actually moves over the distances shown in the Hamble Dredge Plan (ABPmer, 2011a) (8km flood and just under 5km ebb) depends on the fall velocity and water depths. For the type of sediments associated with maintenance dredging, the fall velocity is very low meaning it will remain suspended for long periods with a tendency to travel the complete extent of the flow excursion. However, it is important to note that considerable concentration dilution would take place over time with settlement likely to occur during the high water slack period.

If these plume extents are taken as a potential scenario for sediment remobilised from a deposition site, then given that all of the potential saltmarsh disposal sites are within 5km of at least one marina, or the navigable channel, then there is the potential for sediment deposition in these areas.

The potential significance of sediment deposition is also related to the accumulation depth within the area of deposition. Using some simple assumptions and calculations, a potential sediment accumulation thickness can be estimated, however it should be noted that this is a very simplistic calculation, and included to provide some context to natural measures of deposition.

The following assumptions have been made:

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 ABPmer (2011a) suggests that the average volume of dredged sediment from a maintenance dredge campaign in Hamble Point, Port Hamble, Mercury, Universal and Swanwick is approximately 3,400 m3;  All of this sediment is assumed to be available for deposition on a recharge site;  In a worst-case scenario all of the sediment deposited on a recharge site is remobilised and dispersed along the plume extent in a single event;  A typical area of plume extent indicated in ABPmer (2011a) is approximately 40,100 m2 during a neap tide and 63,500 m2 during a spring tide (e.g. Figure 5.25); and  The remobilised sediment is dispersed and deposited evenly within the plume area. In reality this is unlikely - the majority of the sediment would likely re-settle in the vicinity of the area of remobilisation – however an investigation into the actual fate of the remobilised sediment is beyond the scope of this report. This assumption will, therefore, lead to an overestimation of thicknesses of deposition at the extremities of the plume, and an underestimation close to the site of resuspension, but does allow some context to the natural measures of deposition.

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Figure 5.25: Typical spring and neap plume dispersal envelope for the lower Hamble (from: ABPmer, 2011a)

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Assuming that the remobilised sediment is dispersed evenly within these plume areas, leads to an accumulation over the plume dispersal area of 0.08m (i.e. 3,400 m3/40,100 m2) on a neap tide and 0.05m (i.e. 3,400 m3/63,500 m3) on a spring tide; i.e. between 5cm and 8cm of sediment accumulation. While these thicknesses are small they are of a similar order of magnitude to the natural levels of siltation within the Hamble’s marinas, as reported in ABPmer (2011a) and reproduced in Table 5.17 below.

Table 5.17: Mean annual siltation rates in River Hamble marinas 1986-2010 (from ABPmer, 2011a)

MEAN ANNUAL SILTATION MARINA RATE (M/YR) Hamble Point Marina 0.17 Stone Pier Yard 0.03 River Hamble, HM Pier 0.11 Port Hamble Marina 0.10 Hamble Yacht Services 0.08 Mercury Yacht Harbour 0.10 Universal Marina 0.08 Swanwick Marina 0.05 As indicated, these calculations are highly conservative, and assume complete loss from a recharge site of the deposited sediment. A more realistic calculation may be made by comparison with the Lymington recharge site, where sediment retention structures were put in place prior to disposal. Here, an average change in sediment (potential loss, but possibly due to compaction and dewatering) of 15% was measured over an 11 month period (Lowe, 2013a). Applying a 15% loss to the calculation above would result in a loss of 510 m3 of the 3,400 m3 of deposited sediment. If this were to be deposited over the plume dispersal area shown in Figure 5.25, this would result in an accumulation of 0.012m (i.e. 510 m3/40,100 m2) on a neap tide and 0.008m (i.e. 510 m3/63,500 m3) on a spring tide i.e. between 8mm and 12mm of sediment accumulation.

Critically it is important to understand the range of sediment thicknesses deposited within any remobilised sediment plumes, and how these relate to likely plume extents along the tidal axes associated with the flood and ebb tidal wave and also across spring and neap cycles. The impact footprint will also be dependent upon the location of the beneficial use site, the methods used for deposition and any retention mechanisms/structures used.

There may be some short-term impacts associated with areas of sediment sink or accretion, especially if these overlap sensitive habitats or species (noting most in the Hamble are tolerant of smothering effects), or the locations of marinas and moorings (additional maintenance dredging may be required during deposition and sediment stabilisation phases). However, as discussed previously, the possible relatively short-term impacts of sediment deposition should be considered in the context of the potentially significant medium to long term benefits associated with beneficial use schemes.

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6. POTENTIAL SOURCES OF FUNDING

Question l: Potential sources of funding for any subsequent programme of research and/or practical projects.

6.1 Introduction In this current period of austerity sourcing funding for any ongoing research or practical projects is a challenging task, with much research funding being allocated to academic institutions, or requiring partnership with other organisations and academic bodies.

In research terms, any proposed projects would have to do something different from what has already been investigated. Much fundamental research associated with saltmarsh and mudflat restoration has been completed, so sources of funding for turning previously researched techniques into practice within the River Hamble seem more applicable for future work within the estuary.

The remainder of this section identifies a number of different organisations and/or funding streams, or strategies, which have previously funded marine environmental projects. It should be noted that many funding organisations stipulate that organisations applying for funding should have charitable status, or be not-for-profit organisations, and therefore Hamble Harbour Authority, or other project leads, may have to co-operate with other charitable/not-for-profit organisations in order to access these funding streams.

6.2 Esmée Fairbairn Foundation The Esmée Fairbairn Foundation is one of the largest independent grant-makers in the UK and its stated aim is “to improve the quality of life for people and communities throughout the UK both now and in the future. We do this by funding the charitable work of organisations with the ideas and ability to achieve positive change.”

The Foundation makes grants in the region of £35 million annually towards a wide range of work including in the environmental field. Amongst the Foundation’s environmental aims is “to address environmental degradation and biodiversity loss” and the Foundation funds organisations that take positive and practical action to address environmental challenges. Over the last five years, the Foundation has made £20 million worth of grants to work in the environment sector (Esmée Fairbairn Foundation, 2016).

Amongst the Foundation’s funding priorities is:

 Large-scale conservation of natural environments on land and at sea;  The Foundation has no deadlines, with applications possible at any time, and there is a two stage application process;  1st stage: online application;  2nd stage: a set of questions from a Grants or Social Investment Manager, with decisions made in 2-4 months.

Further details can be found here.

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6.3 The Landfill Communities Fund The Landfill Communities Fund is a tax credit scheme enabling operators of landfill sites to contribute money to organisations enrolled with ENTRUST as Environmental Bodies. More than 53,000 projects have been approved by the Landfill Communities Fund since the inception of the scheme in 1996. Environmental Bodies carry out projects that comply with the objectives set out in The Landfill Tax Regulations 1996 (as amended by the Landfill Tax (Amendment) Regulations 2010 and the Environmental Permitting (England and Wales) Regulations 2010 ), which include:

6.3.1 The conservation of a natural habitat or of a species in its natural habitat This objective seeks the conservation or promotion of biodiversity through the provision, conservation, restoration or enhancement of a natural habitat, or the maintenance or recovery of a species in its natural habitat; and the proposed conservation site must be in the vicinity of a landfill site (interpreted as within 10 miles of a site). Any project needs to provide details of the conservation site, the conservation work proposed, the species or habitat that will be conserved by the project; and how far the project site is from a licensed landfill site. It is also considered best practice guidance that the species or habitat in question is listed in a Biodiversity Action Plan (BAP).

Further details can be found here.

6.3.2 Heritage Lottery Fund – Land and Natural Heritage – Landscape Partnerships The Heritage Lottery Fund (HLF) is one of the largest independent funding streams in the UK utilising money raised from the National Lottery. Its strapline is “From the archaeology under our feet to the historic parks we love, from precious memories to rare wildlife…we use money raised by National Lottery players to help people across the UK explore, enjoy and protect the heritage they care about.” The HLF has financed a community engagement scheme which encompasses a saltmarsh restoration project (http://www.touchingthetide.org.uk/our-projects/saltmarsh-restoration/).

The HLF fund awards grants towards a wide range of work, is a leading advocate for the value of heritage, and is the largest dedicated funder of heritage projects in the UK. It awards grants from the value of £3,000 up to over £5 million. It has £430 million to invest in 2016, and has invested £6.8 billion in grants for projects awarded to over 39,000 projects since 1994 (Heritage Lottery Fund, 2016).

The HLF has several funding streams for different projects / groups:

 Land and natural heritage;  Museums, libraries and archives;  Buildings and monuments;  Cultures and memories;  Industrial, maritime and transport;  Community heritage.

Of particular note is the Landscape Partnerships stream under ‘Land and natural heritage’: “By funding projects to work at a landscape-scale with local communities and landowners we aim to help more people appreciate the value of natural heritage whilst ensuring our landscapes can be more sustainably managed for the future.” Examples of projects that have received funding include ones looking at improving, expanding and connecting threatened habitats, and research and reparation of damaged

150 AHTI_J2015_004 RIVER HAMBLE SOFT SE DIMENT HABITAT RETEN TION FEASIBILITY STUDY heritage-related amenities. Beneficial use appears to match the checks/funding streams, especially if pitched along lines of future-proofing threatened habitats against climate change induced sea level rise, or reparation of damaged saltmarsh.

Of greatest relevance will be the Landscapes Partnerships programme (details here) which provides funding for landscape-scale schemes including habitat conservation, training in traditional rural skills, repairs to distinctive local buildings/features, and improving landscape access. Under this stream, projects associated with increased amenity, access to saltmarsh, watching wildlife etc. can be linked to beneficial use schemes.

Applications up to £100,000 can be made at any time with deadlines in place for all other applications (for guidance see here).These deadlines will vary, depending on how much is bid for, the grant programme, and where the project is based.

Private owners of heritage, including individuals and for-profit organisations, can apply under the Our Heritage grant programme (here) only. Public benefit from the project must be greater than private gain.

Further details can be found at here and here.

6.4 UK Government - Knowledge Transfer Partnerships with local Universities The Knowledge Transfer Partnership (KTP) scheme helps businesses to innovate and grow. It does this by linking them with a university and a graduate to work on a specific project (UK Government, 2016). Each KTP is a three-way partnership between a business, an academic institution and a graduate. The academic institution employs the recently-qualified graduate who works at the company. The graduate, known as the ‘associate’, brings new skills and knowledge to the business.

In practice a KTP can last between 12 and 36 months depending on the project and the needs of the business. It is part-funded by a grant. The amount businesses need to contribute is different for small and medium-sized enterprises (SMEs) and larger companies.

There is no indication of limits to the bids made for KTP funding (UK Government, 2016), however a KTP is part-funded by a grant. There are expectations towards contributing to the cost of the supervisor and the salary of the associate. The amount of contribution depends on the scale and length of the project, as well as the size of the company making the application. As an indication, SMEs contribute a third of the costs (the mean average annual contribution to a project for an SME is around £23,000). For larger business, contributions are expected to be half the cost of the project (mean average annual contribution to a project for a larger company is around £30,000).

There are specific deadlines for submission e.g. in 2016 closing dates for submission are: 11 May 2016; 06 July 2016; 07 September 2016; and 02 November 2016.

In the case of a beneficial use project in the River Hamble there are several universities that could be approached for a KTP funded project:

 Bournemouth University. Rachel Clarke - 01202 961 347 – [email protected]  University of Portsmouth,. Gill Prosser - 023 9284 2978 – [email protected]  University of Southampton,. Phil Jewell - 0238 059 8568 – [email protected]

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 Southampton Solent University. Bill Acharjee - 02380 319329 ext. 9011 – [email protected]

Further details can be found here.

6.5 INTERREG V Interreg Europe is a European initiative set up to assist funding regional and local projects to develop and deliver better policy. The initiative aims to create an environment and opportunities for sharing solutions, specifically aimed to ensure that government investment, innovation and implementation efforts all lead to integrated and sustainable impacts for people and places (INTERREG, 2016).

INTERREG V aims to get maximum return from the EUR 359 million financed by the European Regional Development Fund (ERDF) for 2014-2020. Interreg Europe exists to assist three types of beneficiaries:

 Public authorities – local, regional and national;  Managing authorities/intermediate bodies;  Agencies, research institutes, thematic and non-profit organisations.

Financial support from Interreg Europe must fall into one of the following four categories:

 Research and innovation;  SME competitiveness;  Low-carbon economy;  Environment and resource efficiency.

For beneficial use projects funding streams will sit within environment and resource efficiency. To successfully access INTERREG funds a consortium of partners is required with links to Member State governance bodies, regulatory bodies and/or statutory agencies. For a Municipal Port there would be limited funding streams from grant awarding bodies, but a joint bid with several other partner port companies located in Member States would be one way to try and access the fund.

It is worth noting that compiling an INTERREG bid is an onerous task and is recognised as an industry in itself, requiring a proposal project group to be set up before submitting any proposal.

There are discreet bidding periods associated with INTERREG V, running through the funds period 2014 – 2020. As an indication the first call for projects closed on 30 July 2015. Two hundred and sixty one (261) applications were received with 64 projects conditionally approved on 09 February 2016.

Further details can be found here.

6.6 Operator groups Precedent has been set in the recent past regarding UK Government departments and The Crown Estate instigating initiatives and co-operative funding streams partnered with industry. The primary aims of such groups are to reduce consenting risks to industry, to develop a better scientific evidence base, to deliver environmentally sound understanding, de-risking operations and to enable demonstrations/projects.

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Notable examples are the Aggregate Levy Sustainability Fund2, the Collaborative Offshore Wind Research into the Environment (COWRIE)3 , and Offshore Renewables Joint Industry Programme (ORJIP)4. Industry representation and funding input into these initiatives is a key component on the programme delivery, and also results in applied end products of value and use.

In the case of funding a sediment management/reuse project Hamble Harbour Authority may consider organising an ‘Operator Group’ which provides funds via an agreed (environmental) levy. This could be a flat rate or relative to the footprint of activity/effects in the estuary, such as number of berths/mooring owned/operated, or volume of annual dredging conducted/licensed.

The fund would be administered by a Steering Group with terms of reference to prioritise project-based expenditure which may include field trials of material deposition techniques, sediment entrainment investigations, costs of deposition plant, monitoring of beneficial use areas etc. The Steering Group would consist of core port and marina bodies, particularly Swanwick Marina, Deacons Boatyard, Elephant Boatyard, Mercury Yacht Harbour, Universal Marina, Port Hamble, Stone Pier Yard and Hamble Point Marina and possibly the Royal Southern Yacht Club (also being a marina and berth supplier), along with representation from Hamble Harbour Authority and Hampshire County Council, Natural England and Environment Agency. Another stakeholder body to consider would be the Green Blue organisation of the Royal Yachting Association who is based in Hamble and promote environmental works and research in the boating sector (see: here).

Details of the aforementioned operator groups can be found here, here and here.

6.7 Other Sources Other sources indicated are associated with regional and central Government groups. Defra (Department of the Environment, Food and Rural Affairs) and the EA operate the Grant in Aid (GiA) scheme by which flood risk management authorities may apply for a grant applicable to “any potential flood and coastal erosion risk management project costing over £5,000”. Details can be found here and here.

The Regional Habitat Creation Programme is supported by Defra through the EA and Natural England. In the Solent region the East Solent SMP states that it “seeks to replace saltmarsh habitats that are lost by the process of ‘coastal squeeze”. Specific details are limited, however, the approach to the scheme appears to be through the EA (see here) and there is a contact given as rebecca.reynolds@environment- agency.gov.uk.

2 The Aggregates Levy Sustainability Fund was established in 2002 and ran until March 2011, using revenue from the Aggregates Levy. The Levy itself was introduced as a means to better reflect the environmental costs of winning primary construction aggregates, and to encourage the use of alternative, secondary and recycled construction materials. A proportion of the revenue raised by the new Levy was allocated to a research fund, termed the Aggregate Levy Sustainability Fund. The stated objectives of the wider Levy Fund were to: minimise the demand for primary aggregates; promote environmentally friendly extraction and transport; and reduce the effect of local aggregate extraction. 3 Collaborative Offshore Wind Research into the Environment was set up by The Crown Estate as an independent body to carry out research into the impact of offshore wind farm development on the environment and wildlife, evolving into a charity which has gained global recognition for its scientific and educational work. 4 The Offshore Renewables Joint Industry Programme is a joint industry project involving the Carbon Trust, the Department of Energy and Climate Change, Marine Scotland, The Crown Estate and offshore energy project developers. The aim of ORJIP is to reduce consenting risks for offshore wind and marine energy projects.

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Regional Flood and Coastal Committees (RFCCs) have been developed to communicate flood risks, to promote investment in effective flood risk management and to develop “mutual understanding” of risks in a given area (see here). How this scheme is approached for funding is not immediately clear, however Hampshire County Council run and RFFC group (details here).

6.8 Indicative Costs There is limited information regarding the costs of saltmarsh/habitat restoration trials and projects. This is perhaps not surprising as the information will be commercially sensitive, and subject to competitive pricing between organisations. In addition, the pricing of works to instigate active saltmarsh restoration (placement of dredge spoil), or the more passive option of sediment retention methods, is problematic to delineate as saltmarshes are complex and not fully understood systems with which success is not guaranteed, but should restoration works prove viable and successful, the benefits can be notable.

Discussed in this document in relation to coastal protection, fish nurseries, biodiversity etc. (see section 5.4) the ecosystem services supplied by saltmarsh have now been realised as significant. But, as with all ecosystems, placing a financial value on this is difficult (though see Nunes et al. (2009) and Luisetti et al. (2014)) and has been questioned (Balmford et al., 2002; Wiens, 2007) particularly with regard to a lack of understanding as to what an ecosystem has to “offer” humans. If a major restoration or retention project is proposed, the engagement of expertise in assessing cost/benefit in marine/coastal ecosystem services may prove a useful investment for the RHHA when considering how to proceed. Alternatively, perhaps more pragmatic at the early stage is to potentially engage in relatively small scale trials with backing from landowners/managers after appropriate consultation with regulators, stakeholders etc.

6.9 Understanding and Value It is often discussed that saltmarsh systems are difficult to understand in terms of their ecology, physical habitat, what may be achieved in habitat restoration schemes, and what may have caused their initial decline. The possible reasons for saltmarsh decline in the River Hamble have been discussed, as have the possible methods for restoration. However, valuing restoration for direct costs (studies, works and materials) is problematic for a highly variable natural system, which needs to be worked with and is subject to difficult to predict biotic and abiotic variables. Thus, research to establish an analogy between a potentially less problematic cost model for a given civil engineering project, and that needed to recreate areas of saltmarsh through either passive (fences etc.) or active (beneficial use) methods, proved fruitless. However, basic pricing for materials and approaches are given below following short discussion on ecosystem service evaluation.

A useful report considering the ecosystem service value of saltmarsh was produced in 2015 (Environment Bank, 2015). As discussed above, work was funded through the Heritage Lottery Fund, via the Touching the Tide project (http://www.touchingthetide.org.uk/). Also see: https://www.cbd.int/financial/values/uk-naturalinvestments-2015.pdf p 31, for assessment of economic benefits versus risk.

The report found that restoration would “deliver ecosystem services” and commented that the value of ecosystem services provided would “significantly outweigh the cost of the restoration i.e. there is an economic rationale for such a project” (Environment Bank, 2015). The report provides a useful summary of the value (ecosystem service) of saltmarsh and soft coastal habitat (pp10), and discusses that coastal

154 AHTI_J2015_004 RIVER HAMBLE SOFT SE DIMENT HABITAT RETEN TION FEASIBILITY STUDY defence is the most important service provided with “up to 50% of wave energy attenuated in the first 10-20m of vegetated saltmarsh”.

The Environment Bank (2015) report also stated that soft sediment coastline gives “at least £3 billion worth of capital savings in sea-defence costs in England alone”. On that basis, as a cost benefit model calculated for coastal defence only, the Hamble saltmarsh/mudflat value could be assessed through a GIS analysis of total soft intertidal sediment habitat in England. This could then be divided into appropriate lengths (say 500m) and then weighted for infrastructure behind the marsh, and a basic coastal defence value can then be given; this does not account for the other “services” provided, which would require further valuation on their own merit. In circumstances such as Satchell Marsh, where the Environment Agency flood map shows the properties behind to be at risk (also see: http://www.northsolentsmp.co.uk/6572), the value of the degraded saltmarsh would need weighting and the benefits of restoration could be considered.

6.10 Costs AHTI and Marine Space have provided outline costs for some of the tasks associated with beneficial/retention of sediment projects based on knowledge of charging for such services. However, much of the pricing for these will be for specific expertise and as this will be of a competitive nature pricing cannot be obtained in advance of a specific quote. Pricing of materials and labour for beneficial use and retentions schemes have been based on the Lymington works and increased from their 2009 value (confidential quote) to allow for inflation at an average of 3.2% per annum (Bank of England Inflation Calculator).

Studies potentially needed to support this will be on a site by site basis. For example, a large scale scheme may require modelling of physical habitat, and the outcome of ecological change and habitat growth to be related to ecosystem services. However, this is a complex option and whilst funding routes have been suggested, with the current paucity of public funds financing large scale schemes may currently be difficult, though not unfeasible based on the potential “value” of the outcomes. It should also be noted that with a small scale trial and the agreement of the regulators and advisors, modelling and more complex supporting work may not be viewed as necessary for either a beneficial use scheme or a sediment retention approach. With appropriate expertise in sediment dynamics and ecology of the system(s), a desk based study for a specific location may be sufficient and there are locations in the Hamble where landowner participation in these aspirations may be a possibility.

Table 6.1 gives an outline of costs (where available) for further study items suggested in this document and actual works to undertake restoration. For more detailed cost options and appropriate approaches, site specific discussions would be required.

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Table 6.1: Suggested costs for services/items for supporting studies and material/equipment * Prices are based on experience of the consultancy/academic sector and knowledge of time periods for short reports. Specific quotes are not available publicly, and cannot be sought for feasibility study, being commercially sensitive. ITEM/SERVICE DESCRIPTION RATE Supporting Services Basic Feasibility Study (site and To assess environmental Circa £10-25K depending on if project size dependent)* implications and assess what beneficial use or retention studies approach is required and size of project Ecosystem service study* To support cost/benefit model – Circa £15-20k larger scale projects Ecological modelling* To support projected habitat change Generally academic study, few – larger scale projects (see: consultants. Circa £15-20k (Fagherazzi et al., 2011)) Physical (sediment) modelling* To support trials or projects – larger £10-20k depending on scale and scale projects (with agreement) complexity of modelling requested Sediment tracer studies* To support trials or projects – larger £15-25k depending on amount of scale projects (with agreement) tracer required and number of grab samples collected Sediment analysis for PSA and To support trials or projects and to PSA: £60 per sample organics test sediment suitability for beneficial use project Terrestrial LiDAR and ground based To establish efficacy of schemes RHHA is considering LiDAR studies, study for post project monitoring thus will be aware of pricing and (option) these could encompass any scheme undertaken Capital Works Materials and Services example (based on lymington scheme) Materials and Labour Mobilisation and preparation Vessel and equipment preparation Circa £6.5k depending on project size Dredging and placing of material Method for placement site specific, £25 per m3 but potentially piping Timber piles To reinforce and support wave- Circa £15 each screen Filling and placement of geotubes Geotubes create across creek Circa £400 each barriers Construction and placement of Sediment retention structures £150 per m brushwood fences Engineering design fee Depending on project size/type Circa £12-18k Licence fee For disposal at sea, thus price may Up to 19,999m3 (encompasses total vary for beneficial use m3 pa) (ABPmer, 2011) £2,200 (see here).

The following sections will provide discussion and concluding remarks to the report.

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7. DISCUSSION

The RHHA has facilitated a research approach to investigating the spatial condition of saltmarsh and mudflat areas in the River, possible causal factors for change, and opportunities for future management where appropriate. This is in response to concerns developed within the Harbour Authority and also promoted by stakeholders, resident groups and individuals with regard to potential saltmarsh and mudflat erosion in the River; it is also in response to demand for opportunities for reuse of dredge spoil sought by both developers and regulators. The study is not exhaustive, as formal investigation into all factors implicated would necessitate long term research and analysis of a multifactorial approach. However it is, as far as possible, a comprehensive consideration of readily available literature and data on the status of Hamble River intertidal and saltmarshes habitats, sediment information, factors potentially involved in causing change, and the potential for restoration/management of sites. This is set against wider global research papers and grey literature which have clearly identified that the challenges facing Hamble saltmarshes are similarly noted in locations elsewhere. The information provided has been subject to analysis and discussion throughout the document. Reasons for decline, analysis of change, the potential effects of dredging, possibilities for restoration and related environmental effects have all been considered. This has led to a series of conclusions which will assist the RHHA and associated stakeholders to consider next steps. Whether this leads to saltmarsh restoration schemes at small or large scales can now be considered at an informed level, as can any supporting studies that may be required for suggested approaches given here The outcomes of this study have put the Hamble marshes into a wider context whilst focussing at the local level of difficulties. Accordingly the study indicates that:

 The reasons for decline in Hamble saltmarshes are multifaceted as seen in other locations. The most significant decline was land removal and reclamation for marina construction during the 1960s and 1970s. In addition to this, climate change, sea level rise/waterlogging and coastal squeeze have been implicated, particularly in areas protected by engineering restricting saltmarshes ability to achieve a natural retreat. These factors have been directly attributed to Hamble marsh decline, but also coupled with less well understood mechanisms, notably Spartina die back and the potentially enhanced waterlogging effect of the hybrid Spartina anglica. Dredging effects from marina construction combined with the Hamble sediment system accreting were suggested to achieve “balance”. Overall, as noted by other researchers, it appears likely that the encompassing effect on marsh decline in the Hamble is one of synergy. With gross environmental change the primary factor, exacerbated by human mediated influence at the catchment scale;  The implications of dredging on the Hamble soft sediment features were specifically investigated. Work has indicated that maintenance dredging can have very localised effects. However, research suggests that drawdown occurred after the marine development works of the 1960s-1970s, as the Hamble sediment system shifted toward equilibrium. Effects were suggested to include edge erosion and height loss as sediment supply decreased. In addition, localised effects have been suggested in more recent times at Mercury Marina, Hamble Point and Deacons; though the latter has no direct proximity to an existing marsh, Bursledon Marsh having disappeared by the 1970s due to marina and yard development;  Investigation into whether the maintenance dredging regime reduces sediment supply to soft sediment features revealed conflicting opinion. Certain research concluded that the complete

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removal of dredged sediment from the Hamble system, through deposition at offshore disposal sites, meant that intertidal mudflat and saltmarsh did have a reduced sediment supply to counter sea level rise. However, it has also been noted by other researchers that some intertidal areas and saltmarshes have accreted vertically in line with increasing inundation and associated sediment supply. Overall it was recognised that the Hamble is part of the wider Southampton Water system and that the River receives most of its fine material from marine sources (the River is flood dominated thus the major source of sediment is transported erosional material from the wider Southampton water / Solent system). Thus, by extension, sediment changes in the wider system also have implications for the Hamble, in conjunction with localised dredging in the Hamble (it was noted that ongoing maintenance dredging may reduce sediment supply to upstream areas) and the historic system change through marina creation;  With regard to the suitability of dredged arisings for possible use in beneficial projects, particle size data for River Hamble sediment are rare. However, data were available for some upstream locations allowing assessment of sediment consistency. This showed sediment to be slightly gravelly muds and sandy muds. As the sediments dredged from the River, allowing for coarser material, will generally comprise that which could have deposited on intertidal soft sediment features, it is assumed the arisings will be suitable on physical aspects alone, though this will need clarification through testing. Chemical and organic content will require analysis and PSA data will be collected on a case by case basis to support any projects taken forward;  From the above information, it is reasonable to consider that the dredge arisings are at least potentially suitable for restorative works on River Hamble soft sediment features. This is set in the context of thresholds for pollutant levels and organic material requisite for successful marsh colonisation. Studies would be required to confirm these aspects and, whilst contaminants are recorded (most recently) as below Action Level 2 and sediments are licensed for at sea disposal, it should be remembered that pollutants can exist in pockets and can affect sensitive ecological systems therefore appropriate replicated sampling may be useful prior to any beneficial use;  The suitability of Hamble River marshes for beneficial use restoration required consideration of numerous factors including those that led potentially to decline in the first place, those that may lead to material being suitable for restorative works, and the location of saltmarshes which may be suitable. Overall, whilst recognising that a cautionary approach has led to little work of this nature in the region, equally, restorative projects for use of dredge arisings on already stressed habitats requires careful thought and may have regulatory issues;  The MCA approach used here provides an overview of marsh status in relation to factors assessed, and provides a framework for decision makers to differentiate between potentially suitable sites. The qualitative assessments are highly simplified, and the judgements of the research team should be tested by a stakeholder group before decisions are made. It should also be noted that at this stage the criteria are unweighted, i.e. they are all assessed to be of equal value. When moving forward with site-selection a stakeholder group may decide that certain criteria are more important than others and should be weighted accordingly. This may alter the outcomes of the MCA;  If a beneficial use or retention scheme is considered further as a result of this, or further supporting work, it is recommended that suitable liaison, consultation and appropriate

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targeted studies proceed, e.g. sediment dynamics, localised impact assessment, and suitable deposition and retention methods. But note, this report has also discussed that smaller projects may not require greater detailed study, allowing for a pragmatic view by regulators and stakeholders;  As with many soft sediment intertidal and coastal habitats in the UK, the marshes in the Hamble are subject to significant stressors, many of which have been discussed here. The perceived barriers to beneficial use of sediment may be overcome by alternative approaches, e.g. passive sediment retention methods. Consideration of structures used in UK schemes was undertaken, with local (River Hamble) evidence that appropriate attempts to enhance sediment settlement can work; as seen (albeit some time ago on a small creek section) at Lands End and, at Little Marsh (time unknown, but believed to be in the 1920/30s). Considering inherent risks, suggestions have been made for potentially appropriate methods to use at Hamble intertidal mudflats and saltmarshes, though it is an imperative that these are considered at the appropriate scale with suitable consultation and supporting data. Following these processes and procedures, there may be an opportunity for a realistic trial / small scale project in the River;  The environmental and societal benefits of saltmarsh and mudflat restoration are much clearer as a result of recent research developing our understanding of the inherent value of these habitats and systems. The ecosystem service concept helps to clarify the value that soft sediment habitat features have at the individual and community scale, and to society as a whole, and their role in sustaining healthy marine ecological communities and systems is increasingly apparent, as is their relative fragility;  Enhancement of ecosystem service, notably through the provision of naturally resilient coastal protection services and, by extension, ecological value of highly conserved sites can only be a positive outcome. However, to deliver these benefits the involvement of an active community and commercial and private river users is highly desirable, most likely critical. The benefits must be discussed in conjunction with the risks to ensure the aims of a pragmatic approach are achieved;  Environmental impacts from beneficial use are difficult to discuss other than at the general site level. Therefore, should a site-specific project be conceived, it should be supported with an appropriate level of impact assessment. Effects on conservation features from sediment suspension may include smothering resulting in physical habitat loss, and short term effects on water quality and related ecosystems/communities and species (e.g. fish and shellfish). Equally, physical placement can cause disturbance effects thus timing will be a significant issue with avoidance of bird nesting and overwintering periods being a likely requisite, though this may prove awkward as most dredging is undertaken in the winter period. Any beneficial scheme should engage at an early stage with the statutory and technical advisory bodies and regulators, local authorities, along with notable non-governmental organisations, and local representatives groups, in addition to the stakeholder groups highlighted in the preceding point;  Sediments deposited at recharge locations can, potentially, be redistributed within the system and may be deposited in less welcome locations, such as the navigation channel or re- deposited within the marinas. Conservative assessment undertaken here indicates that there is a risk, but relatively recent works at Lymington showed the benefit of retention structures coupled with sediment placement. Whilst there are risks of sediment redistribution, these are

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likely to be reliably short term, and when considered against the potential (conservation and ecosystem service) benefits, such projects should be evaluated and where feasible taken forward to actual outcomes;  Financing for beneficial use trials/projects or sediment retention approaches is potentially difficult in a current UK atmosphere of limited public funds and future political uncertainty. There are possible funding roots through philanthropic sources, or through engagement with user groups and the local community, plus landowners may be able to supply “in kind” value.  The potential costs associated with additional works have been presented, however, to obtain detailed costs would require a specific project description, and would be commercially confidential. Accordingly, reasonable estimates based on experience and a previous Solent based beneficial use scheme, have been supplied as guidelines.

In summary, aspirations for beneficial use or sediment retention studies or projects in the River Hamble should be taken forward to practical outcome if the support and agreement can be achieved. The possible benefits may well outweigh the risks, and whilst success is by no means guaranteed, the possibility to take such a scheme past the talking stage is a useful opportunity for the RHHA and any project partners. It would also be used as an example for other similarly pressured estuaries in the UK.

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8. CONCLUSIONS

 This study should be viewed as an end point to this research approach, at least for the Hamble. It is considered that the next step is either to decide to do nothing, or to take forward a practical trial/project for beneficial use of dredge spoil or retention options. Subsequent need for detailed research/modelling will be defined by the scale of project considered plus regulator opinion, with smaller schemes benefiting from a pragmatic approach;  The work identifies that the major saltmarsh loss period was in the 1960s-1970s when physical removal for the creation of marinas was undertaken on the River Hamble. Subsequent loss is mainly considered to be related to coastal squeeze and sea level rise, though other complex biotic and abiotic factors have had a role in saltmarsh decline;  In considering dredging, there is a clear distinction between historic construction impacts and ongoing maintenance dredging, the former having had significant deleterious effects, the latter being less clear. Research results on causality in the River Hamble are ambiguous. Currently there is no clear proven relationship that dredging is increasing saltmarsh decline at the estuary level, however it is apparent that it has caused localised impacts;  On the basis of currently available data, saltmarsh at the Hamble Estuary level have not declined significantly in recent years, however local variance to this does exist with some fragmentation and erosion apparent. However, these more recent variations are somewhat clouded by the accuracy of the available data which can over or underestimate change;  From this point forward, monitoring of further change or loss can be undertaken with a good level of accuracy using terrestrial LiDAR. The high resolution, small error margin (compared to satellite/aerial) and repeated application will provide a highly useful dataset against which future change can be measured and protection of the saltmarsh optimised. This can also be applied to assess the efficacy of any restoration or retention scheme. It has no immediate financial benefit, but is strongly recommended for the longer term to enable accurate monitoring of soft sediment habitat change in the River Hamble;  No locations have been clearly identified for major beneficial reuse projects. This may be discussed in more detail by stakeholders and regulators, but the evidence given here does not support a major beneficial use scheme. However, small viable independent projects could be undertaken by interested parties with support from stakeholders. On localised scale the following projects can be suggested:  Lands End (Hacketts Marsh) – sediment retention structures to offset fragmentation and to demonstrate viability of such schemes;  Satchell Marsh - management measures through either do nothing other than control access, or set up sediment retention structures to decrease damage to marsh through artificial creeks created by residents and significant loss through marina creation;  Little Marsh – options from localised recharge at edge, retention structures and improved drainage to promote growth of lower marsh at “cliffing” edge;  Hamble Common Marsh – potential for small scale beneficial re-use in conjunction with sediment retention structures plus options to manage localised dredge draw down factor.  Whilst such projects might seem small scale in terms of the Hamble Estuary overall, it is important to understand that the cumulative effects of such projects could be of net benefit to the soft sediment habitats in the Hamble;

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 More schemes could be considered which comprise:  Bunny Meadows North and South – consider sediment retention and possible recharge, but, only feasible with significant modification of inlet/outlet culverts to control/reduce very high tidal flows;  Swanwick Marsh – possible use of sediment retention structures to promote localised low marsh colonisation. Recent Spartina growth suggests this may be feasible, but this will be a larger scale, longer term scheme.

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REFERENCES

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