Submission Document SD23

E – Mersey Estuary

This statement includes the following:

1. Location map showing local-scale boundaries

2. Report on existing defences

3. Baseline understanding of shoreline dynamics and behaviour

4. Baseline scenario assessments (‘no active intervention’ and ‘with present management’)

5. Supporting information and references

C-191 North West England and North Wales SMP2 Appendix C – Baseline Processes

1 Location Map

C-192 E – Mersey Estuary

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2 Report on existing defences

Location Defence History Present Defences Residual Life – Natural Features Source and Assumptions (Do Nothing Scenario) Years

Fort Groyne, New Brighton Constructed 1983-85 Shore-connected 'reef block' >20 Sandy beach NFCDD. Defence type breakwater with rock stem. interpreted from photos in National Grid: Wirral Ground Shoreline (330963E 394520N) to Inspection 2007. (330996E 394498N)

Marine Promenade New Originally wall constructed Rock revetment with vertical >20 Mixed beach NFCDD. Defence type Brighton second half of 19th century. sandstone block seawall behind. interpreted from photos in Rock armour added 1990s. Wirral Ground Shoreline National Grid: Inspection 2007. (330996E 394498N) to (331205E 394166N)

Victoria Island, New Constructed 1983-85. Offshore reef block breakwater. >20 Sandy beach NFCDD. Defence type Brighton interpreted from photos in Wirral Ground Shoreline National Grid: Inspection 2007. (331216E 394390N) to (331296E 394237N)

New Brighton Pumping Constructed late 1980s Vertical precast concrete wave >20 Mixed beach NFCDD. Defence type Station dissipating seawall. interpreted from photos in Wirral Ground Shoreline National Grid: Inspection 2007. (331205E 394166N) to (331238E 394129N)

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Location Defence History Present Defences Residual Life – Natural Features Source and Assumptions (Do Nothing Scenario) Years

Mersey River Wall: Originally wall constructed Vertical sandstone block wall with >20 Mixed beach NFCDD. Defence type second half of 19th century. rock armour revetment in places. interpreted from photos in National Grid: Rock armour added late 1990s. Stepped concrete revetment Wirral Ground Shoreline (331238E 394129N) to upstream. Inspection 2007. (332549E 390845N)

Tower Groyne, New Constructed late 1990s Rock groyne >20 Mixed beach NFCDD. Defence type Brighton interpreted from photos in Wirral Ground Shoreline National Grid: Inspection 2007. (331359E 393812N) to (331522E 393849N)

Manor Groyne, New Constructed late 1990s Rock groyne >20 Mixed beach NFCDD. Defence type Brighton interpreted from photos in Wirral Ground Shoreline National Grid: Inspection 2007. (331642E 392750N) to (331798E 392803N)

Egremont Groyne, Egremont Constructed late 1990s Rock groyne >20 Mixed beach NFCDD. Defence type interpreted from photos in National Grid: Wirral Ground Shoreline (332023E 391877N) to Inspection 2007. (332152E 391955N)

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Location Defence History Present Defences Residual Life – Natural Features Source and Assumptions (Do Nothing Scenario) Years

Kingsway Tunnel to Ship Originally wall constructed Vertical sandstone block wall. >20 River channel Defence types interpreted from Ferry Terminal second half of 19th century. Google Earth (accessed 08/08/2008). National Grid: (332512E 390997N) to (332550E 390845N)

South of Ship Ferry Terminal Unknown Rock revetment Unknown River channel Defence types interpreted from Google Earth (accessed National Grid: 08/08/2008). (332550E 390845N) to (332577E 390731N)

Bus station to sewage works Unknown Concrete apron? Unknown River channel Defence types interpreted from Google Earth (accessed National Grid: 08/08/2008). (332577E 390731N) to (332580E 390651N)

Sewage works to Kings Unknown Rock revetment (possibly backed by Unknown River channel Defence types interpreted from Wharf wall). Google Earth (accessed 08/08/2008). National Grid: (332580E 390651N) to (332647E 390412N)

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Location Defence History Present Defences Residual Life – Natural Features Source and Assumptions (Do Nothing Scenario) Years

Kings Wharf to Bedford Unknown Vertical quay walls Unknown River channel Defence types interpreted from Road East Google Earth (accessed 08/08/2008). National Grid: (332647E 390412N) to (333440E 386832N)

Bedford Road East to The Unknown Rock revetment Unknown River channel Defence types interpreted from Dell Google Earth (accessed 08/08/2008). National Grid: (333440E 386832N) to (333851E 386147N)

The Dell to Seafield Road Unknown Vertical walls? Unknown River channel Defence types interpreted from Google Earth (accessed National Grid: 08/08/2008). (333851E 386147N) to (333996E 385958N)

Seafield Road to New Ferry N/A Natural beach N/A River channel Defence types interpreted from sewage works Google Earth (accessed 08/08/2008). National Grid: (333996E 385958N) to (334357E 385356N)

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Location Defence History Present Defences Residual Life – Natural Features Source and Assumptions (Do Nothing Scenario) Years

New Ferry to Dock Road Unknown Rock revetment Unknown River channel Defence types interpreted from North Google Earth (accessed 08/08/2008). National Grid: (334357E 385356N) to (334954E 384964N)

Dock Road North to Unknown Vertical walls? Unknown River channel Defence types interpreted from Magazine Lane Google Earth (accessed 08/08/2008). National Grid: (334954E 384964N) to (335669E 383820N)

Magazine Lane to Plantation N/A Natural beach N/A River channel Defence types interpreted from Road, Google Earth (accessed 08/08/2008). National Grid: (335669E 383820N) to (336133E 382672N)

Plantation Road to Unknown Vertical walls? Unknown River channel Defence types interpreted from Riverwood Road Bebington Google Earth (accessed 08/08/2008). National Grid: (336133E 382672N) to (336234E 382364N)

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Location Defence History Present Defences Residual Life – Natural Features Source and Assumptions (Do Nothing Scenario) Years

Riverwood Road to Eastham N/A Natural beach N/A River channel Defence types interpreted from Country Park Visitor Centre Google Earth (accessed 08/08/2008). National Grid: (336234E 382364N) to (336464E 381790N)

Eastham Country Park to Unknown Vertical quay walls Unknown River channel Defence types interpreted from Eastham Locks Google Earth (accessed 08/08/2008). National Grid: (336464E 381790N) to (337035E 381040N)

Manchester Ship Canal to Unknown Revetment at northern end, beach Unknown River channel Defence types interpreted from Poole Hall Sands with groynes. Embankment? Google Earth (accessed 08/08/2008). National Grid: (337035E 381040N) to (338801E 379079N)

Poole Hall Sands to Telford's Unknown Earth embankment Unknown Saltmarsh Defence types interpreted from Quay Google Earth (accessed 08/08/2008). National Grid: (338801E 379079N) to (340831E 377300N)

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Location Defence History Present Defences Residual Life – Natural Features Source and Assumptions (Do Nothing Scenario) Years

South Pier Road to Unknown Quay walls Unknown Saltmarsh Defence types interpreted from Canalside, Ellesmere Port Google Earth (accessed 08/08/2008). National Grid: (340831E 377300N) to (341000E 377200N)

Ellesmere Port to confluence Unknown Embankment with vertical walls in Unknown Saltmarsh Defence types interpreted from with River Weaver places on Manchester Ship canal Google Earth (accessed side. 08/08/2008). National Grid: (343262E 376970N) to (349965E 379749N)

Confluence with River Unknown Vertical walls separating the Mersey Unknown Saltmarsh Defence types interpreted from Weaver to landing stage from the Manchester Ship Canal. Google Earth (accessed 08/08/2008). National Grid: (349965E 379749N) to (349266E 381564N)

Landing stage to downstream Unknown Vertical walls with vegetated Unknown River channel Defence types interpreted from Runcorn Bridge embankment. Google Earth (accessed 08/08/2008). National Grid: (349266E 381564N) to (350751E 383394N)

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Location Defence History Present Defences Residual Life – Natural Features Source and Assumptions (Do Nothing Scenario) Years

Runcorn Bridge to Old Quay Unknown Vertical walls separating the Mersey Unknown River channel Defence types interpreted from Bridge over ship canal from the Manchester Ship Canal. Google Earth (accessed 08/08/2008). National Grid: (350751E 383394N) to (352003E 383300N)

Old Quay Bridge to Norton N/A Natural river channel N/A River channel Defence types interpreted from Marsh Google Earth (accessed 08/08/2008). National Grid: (352003E 383300N) to (355943E 384810N)

Norton Marsh to drain Unknown Embankment Unknown Saltmarsh Defence types interpreted from opposite Penketh Bank Google Earth (accessed 08/08/2008). National Grid: (355943E 384810N) to (357020E 386302N)

Opposite Penketh Bank to N/A Natural river channel N/A Saltmarsh Defence types interpreted from Arpley Landfill Site Google Earth (accessed 08/08/2008). National Grid: (357020E 386302N) to (357536E 386494N)

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Location Defence History Present Defences Residual Life – Natural Features Source and Assumptions (Do Nothing Scenario) Years

Arpley Landfill Site to Unknown Sloped revetment - concrete? Unknown Saltmarsh Defence types interpreted from Richmond Bank Google Earth (accessed 08/08/2008). National Grid: (357536E 386494N) to (357597E 386693N)

Richmond Bank to works Unknown Embankment Unknown Saltmarsh Defence types interpreted from opposite Athertons Quay Google Earth (accessed 08/08/2008). National Grid: (357597E 386693N) to (359433E 387684N)

Opposite Athertons Quay to N/A Natural channel N/A Saltmarsh Defence types interpreted from Transporter Bridge Google Earth (accessed 08/08/2008). National Grid: (359433E 387684N) to (359662E 387646N)

Transporter Bridge to Unknown Vertical walls Unknown River channel Defence types interpreted from Monks Siding Google Earth (accessed 08/08/2008). National Grid: (359721E 387602N) to (359280E 387593N)

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Location Defence History Present Defences Residual Life – Natural Features Source and Assumptions (Do Nothing Scenario) Years

Monks Siding to building past Unknown Embankment Unknown Saltmarsh Defence types interpreted from Penketh Bank Google Earth (accessed 08/08/2008). National Grid: (359280E 387593N) to (356645E 386743N)

Building past Penketh Bank Unknown Vertical walls Unknown River channel Defence types interpreted from Google Earth (accessed National Grid: 08/08/2008). (356645E 386743N) to (356559E 386770N)

Past Penketh Bank to boat N/A Natural channel N/A River channel Defence types interpreted from park Google Earth (accessed 08/08/2008). National Grid: (356559E 386770N) to (356249E 386614N)

Boat park Unknown Vertical walls Unknown River channel Defence types interpreted from Google Earth (accessed National Grid: 08/08/2008). (356249E 386614N) to (356189E 386580N)

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Location Defence History Present Defences Residual Life – Natural Features Source and Assumptions (Do Nothing Scenario) Years

Boat Park to St Helens Canal N/A Natural channel N/A Saltmarsh Defence types interpreted from Lock Google Earth (accessed 08/08/2008). National Grid: (356189E 386580N) to (351567E 384278N)

St Helens Canal Lock to Unknown Vertical walls Unknown River channel Defence types interpreted from Runcorn Bridge Google Earth (accessed 08/08/2008). National Grid: (351567E 384278N) to (350988E 383754N)

Runcorn Bridge to Desoto Unknown Embankment topped by small Unknown River channel Defence types interpreted from Road vertical wall. Google Earth (accessed 08/08/2008). National Grid: (350988E 383754N) to (350350E 384026N)

Desoto Road to Ditton Unknown Embankment Unknown River channel Defence types interpreted from Brook Google Earth (accessed 08/08/2008). National Grid: (350350E 384026N) to (349573E 383915N)

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Location Defence History Present Defences Residual Life – Natural Features Source and Assumptions (Do Nothing Scenario) Years

Ditton Brook to central Unknown Embankment with rock facing on Unknown River channel Defence types interpreted from Pickering's Pasture channel side. Google Earth (accessed 08/08/2008). National Grid: (349573E 383915N) to (348814E 383304N)

Pickering's Pasture to Hale N/A Natural channel N/A River channel Defence types interpreted from Head Lighthouse Google Earth (accessed 08/08/2008). National Grid: (348814E 383304N) to (347261E 380915N)

Hale Head Lighthouse Unknown Walls Unknown River channel Defence types interpreted from Google Earth (accessed National Grid: 08/08/2008). (347261E 380915N) to (347177E 380944N)

Hale Head Lighthouse to N/A Natural channel N/A Saltmarsh Defence types interpreted from Liverpool Sailing Club Google Earth (accessed 08/08/2008). National Grid: (347177E 380944N) to (341302E 382548N)

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Location Defence History Present Defences Residual Life – Natural Features Source and Assumptions (Do Nothing Scenario) Years

Liverpool Sailing Club to N/A Natural beach N/A Unknown Defence types interpreted from Weaver Industrial Estate Google Earth (accessed 08/08/2008). National Grid: (341302E 382548N) to (340329E 383276N)

Weaver Industrial Estate to Unknown Vertical quay walls Unknown Unknown Defence types interpreted from Garston Docks Google Earth (accessed 08/08/2008). National Grid: (340329E 383276N) to (339228E 384314N)

Grassendale Esplanade Unknown Natural beach/embankment? N/A Unknown Defence types interpreted from Google Earth (accessed National Grid: 08/08/2008). (339228E 384314N) to (338709E 384885N)

Grassendale Esplanade to Unknown Vertical wall Unknown Unknown Defence types interpreted from centre Festival Park Google Earth (accessed 08/08/2008). National Grid: (338709E 384885N) to (336396E 386474N)

Festival Park Unknown Vertical wall with revetment or Unknown Unknown Defence types interpreted from natural beach in front. Google Earth (accessed National Grid: 08/08/2008). (336396E 386474N) to

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Location Defence History Present Defences Residual Life – Natural Features Source and Assumptions (Do Nothing Scenario) Years (335974E 386712N)

Festival Park to Huskisson Unknown Vertical wall Unknown Unknown Defence types interpreted from Dock Google Earth (accessed 08/08/2008). National Grid: (335974E 386712N) to (333200E 393184N)

Huskisson Dock to Unknown Short section vertical wall fronted Unknown Unknown Defence types interpreted from Huskisson Branch Dock by rock revetment. Google Earth (accessed 08/08/2008). National Grid: (333200E 393184N) to (333181E 393472N)

Huskisson Dock to Unknown Vertical wall Unknown Unknown Defence types interpreted from Brocklebank Dock Google Earth (accessed 08/08/2008). National Grid: (333181E 393472N) to (333018E 394190N)

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Location Defence History Present Defences Residual Life – Natural Features Source and Assumptions (Do Nothing Scenario) Years

Brocklebank Dock to Unknown Short section vertical wall fronted Unknown Unknown Defence types interpreted from Langton Dock by rock revetment. Google Earth (accessed 08/08/2008). National Grid: (333018E 394190N) to (332921E 394430N)

Langton Dock to Royal Unknown Vertical walls Unknown Unknown Defence types interpreted from Seaforth Dock Google Earth (accessed 08/08/2008). National Grid: (332921E 394430N) to (332116E 396031N)

Royal Seaforth Dock Constructed in 1970 Linear rock armour revetment. >20 Sand beach Sefton Annual Defence Inspection 2006. Residual life from Baseline National Grid: Report 2002. (331800E 395860N) to (331200E 397310N)

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3 Baseline understanding of shoreline dynamics and behaviour

LARGE SCALE: E - MERSEY ESTUARY

Interactions:

The Mersey Estuary lies within Liverpool Bay and extends from the mouth at Liverpool to the tidal limit at Warrington. The estuary has a bottle shaped plan-from; where the narrow deep entrance channel (the Narrows) opens into a shallow, wide middle basin (the inner estuary) that in turn, leads to a meandering channel (the upper estuary), which is more fluvial in form (Gifford and Partners, 2004). The estuary has a core area of 8,914 ha; 5,607 ha of which are intertidal sandflats and mudflats (ABPmer and HR Wallingford, 2007). Further data on the specific characteristics of the estuary are available from the Estuary Guide website.

The geologically constrained mouth, known as ‘the Narrows’, stretches approximately 10km into the estuary. Moving inland from the Narrows, the estuary widens to around 5.5km in width within the shallow inner estuary. This section is predominantly characterised by shifting sand banks and three meandering channels: the Garston Channel (along the north bank); the Eastham Channel (along the south bank) and the Middle Deep Channel. The upper estuary is sinuous and becomes narrower as the estuary moves inland towards the tidal limit at Warrington. This section is characterised by two main channels that meander through highly mobile intertidal sandflats and mudflats which are exposed at low tide. Areas of saltmarsh fringe both the north and south banks along the majority of this section.

The Mersey Estuary is macro-tidal, with recorded tidal ranges which vary between 4m and 10m from neap to spring tides. The estuary is flood-dominant, although the ebb has a slightly longer phase than the flood (ABPmer, 2005). However, there is evidence that flood asymmetry is reducing over time (Gifford and Partners, 2004). The form of the estuary between the Narrows and Eastham creates a tidal amplification effect which has the affect of increasing tidal range along this stretch (Gifford and Partners, 2004).

Certain conditions such as very high tides (>10m at Liverpool) or lower tides combined with dry weather and reduced freshwater flows result in a tidal bore within the Mersey Estuary (POL, 2007). Under these favourable conditions the Mersey Bore may be seen in the inner estuary opposite Hale Point, about two and a quarter hours before high water in Liverpool.

The narrow mouth of the Mersey Estuary, combined with the shallow bathymetry and numerous sand banks within the inner estuary, limits propagation of waves into the estuary (Gifford and Partners, 2004). South westerly and north westerly winds act to funnel water into the estuary (EA, 2008). Locally-generated waves within the estuary, mainly from the west and south west, may influence sediment transport in intertidal areas.

The shallow nature of the north-eastern Irish Sea and Liverpool Bay results in an environment that is susceptible to some of the highest surge conditions in the UK. Resonance of water levels in the Irish Sea means that surges are often of short duration but intense, with highest current velocities concentrating in Liverpool Bay.

The Mersey Estuary has a relatively low freshwater input compared to its size. In addition to freshwater inputs from the River Mersey, other sources of freshwater enter the estuary at Howley Weir (18.8m3/s modal flow)

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and via the Manchester Ship Canal and other minor tributaries (15.2m3/s combined modal flow) (ABPmer, 2005). Freshwater flows vary seasonally from 25 to 200m3/s to flood flows which can exceed 1200m3/s (Halton Borough Council, 2008). The Mersey Estuary is a well-mixed estuary due to high tidal currents and low freshwater inputs.

The two main inputs of sediment to the Mersey Estuary are from offshore sources and fluvial sources, although fluvial sources are believed to be less significant. Erosion of the Ince Bank and soft cliffs at the Speke Garston Coastal Reserve, between Garston and Dungeon Lane, provides only a small contemporary source of sediment (HR Wallingford, 1999; Pye and Blott, 2004). The Manchester Ship Canal acts as a sediment trap, limiting the amount of fluvial sediment entering the estuary system. There are indications of an ebb delta seaward of the mouth of the Mersey Estuary, and a flood delta landwards of the Narrows. Similar to other estuaries along the north-west coast, the estuary is likely to be a strong sink for both mud and sand, with accumulation occurring in the inner estuary. Movement:

The Mersey Estuary is a glacially over-deepened valley, occupied by the Mersey and Irwell rivers, which formed along a geological fault (O’Connor, 1987). Following the last glaciation, rising sea levels flooded the valley forming the estuary; the approximate present outline of the estuary was established approximately 3,000 years ago (McDowell and O’Connor, 1977). Estuary capacity would have naturally reduced at a relatively slow rate over time; however, anthropogenic modification to the estuary has accelerated this process. Changes in tidal capacity within the Mersey Estuary have been attributed to:

• Natural changes in erosion and accretion patterns; and,

• Anthropogenic changes (principally reclamation, dock development, dredging, training wall construction and dredge spoil dumping).

Evidence suggests that prior to the late 19th century, the Mersey Estuary was in dynamic equilibrium, where net changes within the inner estuary were fairly limited and the capacity of the estuary held relatively constant due to periodic migrations of the channels (O’Connor, 1987). During the first half of the 20th century, the estuary underwent a significant period of morphological change. Changes to the ebb and flood tide hydrodynamics in Liverpool Bay, caused by the construction of training walls in the outer estuary, resulted in large-scale movement of sediment into the inner estuary, increasing intertidal area and reducing the estuary volume from 745Mm3 to 680Mm3 (Thomas, 2000). Since this time, a new equilibrium appears to have been reached and the rate of sediment movement into the estuary has slowed.

Accretion within the estuary has not been uniform. Most accretion has occurred in the inner estuary, with very little accretion in the Narrows and upper estuary (Halton Borough Council, 2008). Over the past 25 years a small net loss of sediment has been seen in the inner estuary (Blott et al, 2006), however, the overall trend for the past century has been that of siltation within the Mersey Estuary (Halton Borough Council, 2008). Analysis of navigation charts as part of the Mersey Gateway Project shows very little change along the estuary banks over the last hundred years; the maximum retreat was 12m within a 46 year period (Halton Borough Council, 2008).

In the inner estuary, changes in the position and size of the banks and channels have had a significant impact on the adjoining shorelines. In general, off-shore-diversion or infilling of near-shore channels leads to rising foreshore levels, mudflat and saltmarsh accretion, and cliff toe stabilisation. Conversely, periods of inshore

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channel migration and deepening lead to falling foreshore levels, mudflat and saltmarsh erosion, and cliff toe erosion (Pye and Blott, 2004).

Blott et al. (2006) suggest that the Mersey Estuary is likely to be relatively more sensitive to the effects of future changes in natural forcing factors, such as mean sea level, tides, waves and storm surges, than in the past when adjustments to training wall construction and dredging in the outer estuary were dominant. Modifications:

The Mersey Estuary has experienced a number of significant anthropogenic modifications over the last few hundred years, including:

• Dredging of channels for navigation purposes – Dredging began in 1833, providing access to the Ports of Liverpool and Birkenhead, although regular dredging commenced after 1890. At present, approximately 0.4 x 106 m3 of sediment is removed from the estuary each year and dumped in Liverpool Bay (Van der Wal and Pye, 2000).

• Construction of training walls – Training walls were first constructed in the outer estuary in 1909. Training walls were constructed along the face of Taylor’s Bank, in order to prevent the ongoing northward migration of the Crosby Channel and to prevent a smaller channel breaking through Taylor’s Bank. These walls were extended between 1910 and 1957 to include the Queens North, South, Askew Spit, Crosby West and Crosby East Training Banks (van der Wal and Pye, 2000).

• Construction of the Manchester Ship Canal – The Manchester Ship Canal was completed in 1894 to provide navigable access for seagoing ships from the Mersey Estuary to Manchester. Within the Mersey Estuary, the canal runs parallel to and along the south side of the estuary from Eastham, past Ellesmere Port, through the Runcorn Gap and to the south of Warrington.

• Port and industrial development – Development and expansion of ports and industry within the Mersey Estuary has resulted in land reclamation within the Narrows and the inner estuary. A total of 492ha had been reclaimed by the end of the 19th century. Construction of hard defences and embankments along much of the estuary shoreline and on-going dredging of approach channels has impacted on the estuary sediment system, influencing local patterns of erosion and accretion. Wider-scale interactions:

The influence of the estuary extends towards Formby Point along the Sefton coast to the north and towards Dove Point, Meols to the south (EA, 2008), and as such, the estuary has an important influence on these open coast frontages.

The net sediment transport direction along from the neighbouring North Wirral coast is generally eastwards and the Mersey is a major sink for sediments lost from the North Wirral frontage.

To the north-east of the estuary, there is a littoral drift divide at Formby, along the Sefton Coast, such that material is transported northwards into the River Ribble and southwards into the River Mersey; however, much of this sediment is retained within the beach-dune system of the Sefton Coast and therefore changes in the management of the Sefton coast are unlikely to have a significant impact on the Mersey Estuary as a whole. Similarly, in terms of sediment inputs, the Mersey Estuary is believed to contribute very little to the Sefton coast.

Changes in the estuary, and more specifically in its banks and channels, has historically had a significant influence on both the Wirral and Sefton open coastlines. Of significant importance to the regime of the Mersey Estuary,

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and to the adjacent open coastlines, is the impact of training walls, which have been constructed out into Liverpool Bay along the Crosby and Queens Channels for some 15km beyond the Narrows. These channels are maintained for navigation by dredging.

The evolution of the North Wirral coastline has been dominated by the influence of the Mersey and Dee estuaries. The low-lying land between Birkenhead and Wallasey embankment (Wallasey Pool) is thought to be an old channel of the River Mersey, with the north-eastern part of the Wirral coastline being an island. The original justification for the embankment was navigation; it was built to prevent the River Mersey from cutting a new outlet across the Wirral. The influence of the Mersey on the North Wirral shoreline was reduced by the training of the Crosby Channel, which started in the early 1900s. The training of the River Mersey concentrated ebb flows in the main channel with enhanced flood transport over sand banks on either side. It also resulted in reduced flow within other ebb channels, one of which fed sediment onto the North Wirral coast; this channel had become effectively closed by the early 1900s.

The construction of the training walls also impacted on the Sefton shoreline to the east. These works contributed to shoaling in the Formby Channel, deepening of the Queen’s Channel and growth of Taylor’s Bank (Pye and Neal, 1994). Dumping of dredge spoil also led to the development of Jordan’s Spit. The net effect of these works appears to have been an increase in wave focussing onto Formby Point (Pye and Neal, 1993), whilst to the south of Formby Point there has been continued accretion in the nearshore area and of the dunes. In the future, further growth in Taylor’s Bank and Formby Bank could increase the protection afforded to Altcar frontage, but may increase wave focusing onto Formby Point, increasing instability. Growth of these banks is, however, likely to slow as the Mersey Estuary adjusts to its modified hydrodynamic regime and the 2002 Admiralty chart shows that Taylor’s Bank had moved only slightly further to the east, indicating that the bank may have reached an equilibrium position with respect to its retaining training wall (Blott et al., 2006).

For the context of this report, the Mersey Estuary has been divided into three distinct regions:

• the Narrows (which covers the estuary between New Brighton and New Ferry on the southern bank and Seaforth and Dingle on the northern bank);

• the inner estuary (which covers the estuary between New Ferry on the southern bank and Dingle on the northern bank and Runcorn Bridge); and,

• the upper estuary (which covers the estuary between Runcorn Bridge and the tidal limit at Warrington).

LOCAL SCALE: THE NARROWS

Interactions:

The Narrows describes the geologically constrained section of the Mersey Estuary which extends between New Brighton and New Ferry on the southern bank and Seaforth and Dingle on the northern bank. Blott et al. (2006) describe the Narrows as an ice-deepened trough cut into the surrounding and underlying Permo- Triassic sandstones. Here the bedrock prevents any further erosion of the channel. Currently the channel which has a mean depth of 15m and an average width of 1km, which varies from 1.5km at its widest point, at New Brighton, to 0.75km at its narrowest point, at Pier Head (ABPmer, 2005).

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Tidal range within the Narrows (Gladstone Dock) varies between 10.5m on extreme spring tides and 3.5m on extreme neap tides (EA, 2008). Maximum tidal current velocities can reach 1ms-1 (on the flood tide) at the entrance to Queen’s Channel and around 2.2 ms-1 (on ebb and flood tides) in the Narrows on spring tides (Blott et al, 2006). These high tidal currents encourage scour and maintain channel depth (ABPmer, 2005). The tidal influx into the Narrows is on average 2,000m3s-1 during spring tides (Blott et al., 2006).

Offshore waves in Liverpool Bay are generally wind-generated. The Narrows is therefore vulnerable to wind- generated waves from the north-west (Gifford and Partners, 2004).

Littoral drift at the mouth of the Mersey is complicated due to the interaction of waves and strong tidal currents. Any sediment derived from the North Wirral and Sefton frontages is transported into the estuary via the Narrows. Strong tidal currents prevent sediment accumulation in this section of the estuary and lead to sediment being transported into the inner and upper estuary. Movement:

In the Narrows, morphological change has been severely limited by the surrounding solid rock outcrops, which rise to 60m above sea level. However, past land reclamation, in conjunction with dock development at Liverpool and Birkenhead, have constricted the Narrows further. Existing predictions of shoreline evolution:

Futurecoast (Halcrow, 2002) did not consider the Mersey Narrows and therefore no predictions are available from this study.

In the future, acceleration in the rate of sea level rise could result in increased water depths, tidal prism and current velocities within the estuary as a whole (Blott et al., 2006). However, within this geologically constrained section of the estuary, morphological change will be severely limited by the surrounding solid rock outcrops; therefore, any change within the Narrows are likely to be relatively small compared to the rest of the estuary.

LOCAL SCALE: INNER MERSEY ESTUARY

Interactions:

The ‘bottle-shaped’ inner estuary has a maximum width of 5.5km (ABPmer, 2005) and extends approximately 20km (EA, 2008), between New Ferry (on the southern bank) and Dingle (on the northern bank), and Runcorn Bridge. The margins of the inner estuary are formed mainly in Quaternary deposits up to 35m thick, with sandstone outcrops at intervals, notably at Hale and Runcorn (Blott et al., 2006). Within this section, three channels (Garston, Middle Deep, and Eastham) meander through extensive intertidal sandflats and mudflats. The inner estuary is bounded by large areas of saltmarsh on the southern margins between Ellesmere Port and Runcorn (EA, 2008), and a 5km stretch of undefended soft till (boulder clay) cliffs on the northern bank between Garston and Dungeon Lane, which are composed of slightly gravelly sandy muds (Pye and Blott, 2004). The Manchester Ship Canal runs parallel to and along the south side of the inner estuary from Eastham, past Ellesmere Port and through the Runcorn Gap.

Within the inner estuary, the mean spring tidal range is approximately 9m at Eastham (Halton Borough Council, 2008). The strong tidal currents in the Narrows reduce upstream as the estuary widens, leading to

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deposition of sand and mud, which form extensive intertidal banks at low tide (Blott et al, 2006). The large tidal range experienced within the estuary results in the estuary almost completely drying out at low tide.

Due to the constriction of the Narrows, there is very limited penetration of waves from the Irish Sea and wave energy is therefore relatively low in the inner estuary. Locally-generated waves within the inner estuary may influence sediment transport in intertidal areas; however, these waves are fetch-limited and are unlikely to exceed 2m in height (Gifford and Partners, 2004). The shallow bathymetry and extensive sand banks act to limit wave height and wave propagation further.

A number of the estuary’s freshwater sources flow into this section: Ditton Brook (1.38 m3/s mean flow) on the north bank, the River Gowy (1.23 m3/s mean flow) on the south bank, and the River Weaver (37.22 m3/s mean flow), which flows into the Manchester Ship Canal before connecting with the main estuary channels at Weaver Bend (Gifford and Partners, 2004).

Sediments within the inner estuary are mainly fine sand; however, medium-sized sand is more abundant towards the Narrows, where inherent strong tidal currents prevent the accumulation of fine sand (EA, 2008). Sediment transport is generally eastwards (i.e. upstream) within the estuary. Movement:

Historically the configuration of the main low water channel has changed significantly and erosion and accretion trends within this section are closely linked to such changes, although bridge and embankment construction and diversion of the River Weaver has now effectively stabilised the main channel in the east, fixing it onto the north of the estuary between Hale and Runcorn.

Movement of the low water channels has been attributed to changes in river flow (McDowell and O’Connor, 1977). In 1738, the inner estuary contained two main channels, one along the southern coastline, and one along the north (Pye and Blott, 2004; Blott et al., 2006). It was not until about 1842 that the three main channels, that are present today, developed. At this time the inner estuary was fringed by intertidal flats and saltmarshes, namely Stanlow, Ince and Score Banks in the south, and Dungeon Banks in the north Pye and Blott, 2004). Prior to construction of the Manchester Ship canal in 1894, the navigation channel oscillated over the whole width of the inner estuary, moving across both Frodsham and Ince Banks (ABP, 2005).

By 1936 all three channels had become more defined and in the east of the inner estuary the main channel had moved significantly northwards, resulting in erosion along Dungeon Bank to the north and accretion along Stanlow and Ince Banks to the south. By 1977 however, the main channel had moved south, resulting in accretion along Dungeon Bank and erosion along the edges of Stanlow and Ince Banks, although this was also accompanied by vertical marsh accretion (Pye and Blott, 2004). Between 1906 and 1977 sediment volumes in the inner estuary increased overall, particularly in the sub-tidal zone (Blott et al, 2006). However, recent studies suggest that the rate of infilling has slowed and consequently erosion is now evident (Pye et al, 2002; Blott et al., 2006).

A cliff erosion study commissioned by Speke Garston Coastal Reserve Steering Group (Pye and Blott, 2004) quantified historical erosion rates for the soft cliffs along the northern estuary shoreline, between Garston and Dungeon Lane. The study showed that over the past century, the soft cliffs have eroded no more than 10m in over the 100 year period. However, recession has occurred during two main periods, during the early part of the last century and during the past 10 to 15 years, which have been linked with movement of the deep water channel towards the northern shore. Analysis of recession rates over the past 25 years has shown that erosion rates are variable along the frontage. At the western end of John Lennon Airport, cliff toe recession has been in

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the order of 0.1 m/yr on average, while cliff erosion towards the eastern end of the Reserve has been slower, averaging less than 0.05m/yr (Pye and Blott, 2004). Erosion of the cliffs is associated with changes in intertidal and sub-tidal water levels in the Mersey estuary, but they are also affected by sub-aerial drainage. The toe of the cliffs lies just above the level of mean high water springs, but below the level of highest astronomical tide; therefore it is the extreme high tide levels that are likely to cause erosion and undermining of the cliff toe, and subsequent slumping and recession of the cliff itself. Existing predictions of shoreline evolution:

Futurecoast (Halcrow, 2002) did not appraise the Mersey Estuary and therefore no predictions are available from this study.

The overall response of the Mersey Estuary to sea level rise is uncertain. Possible responses include the vertical accretion of the estuary, the landward transition of the estuary (‘estuary rollover’) (where not constrained by topography or defences), or some combination of both. Work by Blott et al. (2006) found that the inner estuary is largely constrained by rock outcrops, till cliffs and artificial embankments and therefore changes are mainly due to the redistribution of sediments between and within the intertidal and subtidal zone, whilst the overall estuary planform remains more or less fixed. Blott et al. (2006) suggest that any acceleration in the rate of sea level rise could favour an increase in water depths, tidal prism and current velocities, which would increase the potential for sediment reworking and redistribution. This study also found that there has been a slight reduction in sediment volumes in the inner estuary since 1977 and the authors suggest that this could be due to a combination of the effects of continued dredging, reduced sediment supply from the outer estuary and rising sea levels: this trend may therefore continue in the future.

Work by Pye and Blott (2004) suggests that over the next 50 years, increases in mean sea level, surges and wave activity, combined with the continued migration of the main channel northwards, may have the potential to increase cliff recession rates at Speke Garston Coastal reserve by 50% in the future. The cliff erosion study (Pye and Blott, 2004) suggests the following rates of cliff erosion:

• West of the reserve cliff recession in the order of 2 to 3m over the next 20 years; 5 to 7.5m over the next 50 years (0.1 to 0.15m/yr); and,

• Speke Garston Coastal reserve cliff recession in the order of 1 to 1.5m over the next 20 years; 2.5 to 3.75m over the next 50 years (0.05 to 0.075m/yr).

LOCAL SCALE: UPPER MERSEY ESTUARY

Interactions:

The upper estuary, extending between Runcorn Bridge and the tidal limit at Warrington, is approximately 15km in length, is relatively narrow and restricted to a single main meandering channel between Hempstones Point and Warrington. This section is characterised by a highly mobile sandflat and mudflat area which becomes fully exposed at low tide. Between Runcorn and Hempstones Point the shallow intertidal area is actively reworked by two migrating low water channels, one to the south and one to the north, where substantial changes can occur on single tides. Saltmarshes border both banks of the estuary upstream of Runcorn, where a sandstone ridge constricts the estuary through the Runcorn Gap (EA, 2008).

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The tidal cycle is significantly affected by the geological restriction through the Runcorn Gap, resulting in the reduction of mean spring tidal range to 4.5m at Widnes and 2.9m at Fiddler’s Ferry in the upper estuary (Halton Borough Council, 2008).

Freshwater flows from both the River Mersey (typically in the range of 20-60m3/s (EA, 2008)) and Sankey Brook (2.61 m3/s mean flow (Gifford and Partners, 2004)) drain into the upper estuary. Despite the limited impact of freshwater discharge on the morphodynamics of the Mersey Estuary as a whole, low water channel movement in this section of the estuary may be influenced by the magnitude and duration of river flows, as well as tidal flows. Movement:

Within the upper estuary, the shoreline is geologically constrained through the Runcorn Gap and therefore morphological change has been limited in this location. Construction of the Sankey Canal and the Manchester Ship Canal, to the north and south of the estuary respectively, between Runcorn and Warrington, has constrained channel movement further.

Siltation within this part of the estuary since the last glaciation has followed the same pattern as that in the inner estuary, where the estuary has experienced gradual infilling. Here sub-tidal channels have decreased in depth and width, while intertidal areas have accreted vertically (Gifford and Partners, 2004).

The low water channel is highly mobile, moving substantially over single tides. Shifts in channel location appear to be oriented around the position of channel divergence at Hemstones Point. Mobility of this channel has been attributed to changes in river flow (McDowell and O’Connor, 1977). Existing predictions of shoreline evolution:

Futurecoast (Halcrow, 2002) did not appraise the Mersey Estuary and therefore no predictions are available from this study.

Assuming continued sedimentary infilling, it is predicted that this section will maintain overall stability under a scenario of future sea level rise. The general trend for siltation is likely to continue, however, the rate of siltation may decrease over time, although this will be dependent on the future balance of freshwater and marine sediment supply.

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4 Baseline Scenario Assessments

No Active Intervention Scenario Assessment Table: E – Mersey Estuary

Predicted Change for No Active Intervention Location Years 0 – 20 Years 20 – 50 Years 50 – 100 The Narrows There are various structures along the entire Many of the defences are assumed to fail during Residual remains of defences only. length of shoreline on both banks, consisting of this period, although actual residual lives are not seawalls and rock revetments. The residual life of known. these structures is unknown, but it is assumed that they will remain during this period. Assuming defences remain, there will be no Dock structures are not coastal defences and Sea level rise is expected to result in an increase in change in shoreline position during this period. therefore their future role and integrity will be water depths and tidal prism within the estuary The estuary is expected to remain relatively dependent upon future port operations, which and current velocities in the Narrows. However, stable during this period, with little intertidal would be uncertain under a no active morphological change within the Narrows will be change expected. intervention scenario. However, for the purposes severely limited by the surrounding solid rock of this no active intervention assessment, it has outcrops. been assumed that these structures will fail Further inundation of low-lying reclaimed land within this period. would re-expose and could eventually reactivate Past evolution of the Mersey Estuary has been cliffs along the original coastline. Additional dominated by changes resulting from human material will therefore be available as a source of intervention; it is therefore difficult to predict sediment to the estuary; however, this is likely to how the estuary will evolve in the future to only be limited due to cliff composition. natural processes. Sea level rise will result in an increase in water depths and tidal prism within the estuary as a whole, and current velocities in the Narrows. In the Narrows, morphological change is severely limited by the surrounding solid rock outcrops. Therefore, any morphological changes at the Narrows are likely to be relatively small

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Predicted Change for No Active Intervention Location Years 0 – 20 Years 20 – 50 Years 50 – 100 compared to the rest of the estuary. Failure of dock structures would result in localised flooding of reclaimed low-lying land, currently confined by the defences. Inner Mersey There are various structures along both banks, Remaining defences are assumed to deteriorate Residual remains of defences only Estuary consisting of earth embankments, seawalls and and fail during this period. revetments. The residual life of these structures is unknown, but it is assumed that the majority of defences will remain. Assuming defences remain, there will be no The key risk, as defences fail, will be the Any acceleration in the rate of sea level rise could change in shoreline position during this period. increased flood risk to low-lying areas. Periodic increase water depths, tidal prism and current Erosion of undefended cliffs would continue inundation will become more frequent and velocities, increasing the potential for sediment along the northern bank, providing some material flooding may potentially result. reworking both by waves and currents. to the estuary system. Undefended cliffs to the Acceleration in the rate of sea level rise would Inundation of low-lying land will increase as sea west of Speke Garston Coastal reserve could increase water depths, tidal prism and current levels rise, however in the inner estuary this is not erode by up to between 2-3m by year 20, while velocities, increasing the potential for sediment likely to have a significant impact on the tidal prism. along the reserve itself, cliffs could erode by up reworking both by waves and currents. The flood Trends of erosion and accretion along the banks of top between 1-1.5m by year 20. dominance which exists within the Mersey the inner estuary will continue to be dictated by Within the inner estuary the general trend for Estuary also means that eroded sediment is likely changes in the configuration of the channels, which reducing sediment volumes observed in the to be retained within the estuary the system, are generally free to migrate in this section of the 1970s may continue, resulting in a corresponding rather than exported. estuary. It is not, however, possible to predict how increase in water volumes. High ground will continue to fix the estuary at these channels may change in the future. Hale and Runcorn. Even where seawalls fail, it is High ground will continue to fix the estuary at likely that the infrastructure behind will continue Hale and Runcorn; therefore overall, there is little to fix the shoreline throughout this period. scope for broad-scale change in plan form of the Therefore, overall, there is little scope for broad- estuary. scale change in plan form of the estuary. Erosion of cliffs along the northern bank would Erosion of cliffs along the northern bank would continue to some provide material to the estuary continue to provide some material to the estuary system, but this is likely to be small compared to system, but this is likely to be small compared to volumes of sediment moving within the estuary the volumes of sediment entering and leaving the

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Predicted Change for No Active Intervention Location Years 0 – 20 Years 20 – 50 Years 50 – 100 estuary. Undefended cliffs to the west of Speke system as a whole. Undefended cliffs to the west of Garston Coastal reserve could erode by up to Speke Garston Coastal reserve could erode by up between 5 and 7.5m by year 50, while along the to between 10 and 15m by year 100, while along reserve itself, cliffs could erode by up to between the reserve itself, cliffs could erode by up top 2 and 4m by year 50. However these estimates between 5 and 8m by year 100. However these are for a worst case scenario and will be estimates are for a worst case scenario and will be dependent upon the channel configurations. dependent upon the channel configurations. Patterns of localised erosion and accretion along The frontage may also be affected by any changes the banks of the inner estuary are highly in the freshwater inputs, due to future climate dependent upon changes in channel position: the change, which may increase river flows which may inner estuary low water changes are generally in turn affect channel movement and erosion and free to migrate. It is not, however, possible to accretion patterns and shoreline exposure. predict how these may change in the future. There is the potential for the estuary to ‘rollover’, translating landward in response to sea level rise; however, topographic constraints at Hale, Helsby, Frodsham and Runcorn may hinder this process. Freshwater influx may increase if defences along the Manchester Ship Canal fail, which may in turn affect channel movement, erosion and accretion patterns and shoreline exposure. Upper Mersey There are various structures along both banks, Remaining defences are assumed to deteriorate Residual remains of defences only. Estuary consisting of earth embankments, seawalls and and fail during this period. revetments. The residual life of these structures is unknown, but it is assumed that the majority of defences will remain. Assuming defences remain, there will be no The key risk, as defences fail, will be the Any acceleration in the rate of sea level rise would change in shoreline position during this period. increased flood risk to low-lying areas. Periodic increase water depths, tidal prism and current The upper estuary is expected to remain inundation will become more frequent and velocities, increasing the potential for sediment relatively stable during this period. flooding may potentially result. reworking both by waves and currents.

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Predicted Change for No Active Intervention Location Years 0 – 20 Years 20 – 50 Years 50 – 100 Any acceleration in the rate of sea level rise Inundation of low-lying land will increase as sea would increase water depths, tidal prism and levels rise. current velocities, increasing the potential for The low water channel would continue to sediment reworking both by waves and currents. meander and configurations of the channel will However, it is predicted that the upper estuary influence future patterns of erosion, sediment will maintain overall stability under a scenario of transport and deposition within the intertidal future sea level rise. areas. It is not, however, possible to predict how High ground will continue to fix the estuary at channel configurations may change in the future. Runcorn; however, if defences along the The frontage will also be vulnerable to changes in Manchester Ship Canal and Sankey Canal fail the freshwater inputs, resulting from future climate channel would be able to meander more freely, change, which may change river flows and in turn resulting in localised marsh edge erosion. affect channel movement, erosion and accretion Configurations of the low water channel will patterns, and shoreline exposure. influence future patterns of erosion, sediment transport and deposition within the intertidal areas. It is not, however, possible to predict how channel configurations may change in the future. Freshwater influx from the River Mersey and surrounding areas has an influence on channel movement and therefore this stretch could be sensitive to changes in river flow.

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With Present Management Scenario Assessment Table: E – Mersey Estuary

Predicted Change for With Present Management Location Years 0 – 20 Years 20 – 50 Years 50 – 100 The Narrows There are various structures along the entire Upgrade of defences may be required during this Upgrade of defences may be required during this length of shoreline on both banks, consisting of period to maintain current levels of protection. period to maintain current levels of protection. seawalls and rock revetments. The residual life of these structures is unknown, but works may be required during this period. Where defences exist, they will maintain the Where defences exist, they will maintain the Where defences exist, they will maintain the current shoreline position. The estuary state is current shoreline position. As sea levels rise, current shoreline position. As sea levels rise, there expected to remain as today during this period. there is likely to be some overtopping in certain is likely to be some overtopping in certain areas, areas, which may lead to breaching; therefore which may lead to breaching; therefore defence defence improvement works may be required improvement works may be required during this during this period. period. The Inner There are various structures along the entire Upgrade of defences may be required during this Upgrade of defences may be required during this Mersey Estuary length of shoreline on both banks, consisting of period to maintain current levels of protection. period to maintain current levels of protection. seawalls and rock revetments. The residual life of these structures is unknown, but works may be required during this period. Where defences exist, they will maintain the Defences will continue to maintain the shoreline Defences will continue to maintain the shoreline current shoreline position. position and reduce the risk from flooding. As position and reduce the risk from flooding. As sea Undefended cliffs to the west of Speke Garston sea levels rise, there is likely to be some levels rise, there is likely to be some overtopping Coastal reserve could erode by up to between 2- overtopping in certain areas, which may lead to in certain areas, which may lead to breaching; 3m by year 20, while along the reserve itself, cliffs breaching; therefore defence improvement therefore further defence improvement works may could erode by up top between 1-1.5m by year works may be required during this period. be required during this period. 20. In the inner estuary, the low water channels are Although the low water channels are generally free Little further change in the estuary form is generally free to migrate, but the margins of the to migrate, the margins of the estuary today are anticipated. estuary are largely fixed by high ground at Hale largely fixed by high ground at Hale and Runcorn, and Runcorn, by soft sediment cliffs (for example, by soft sediment cliffs (for example, between

between Garston and Widnes), and by sea walls Garston and Widnes), and by sea walls (Blott et al., (Blott et al., 2006). 2006); therefore there is little scope for any major

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Predicted Change for With Present Management Location Years 0 – 20 Years 20 – 50 Years 50 – 100 Overall, there is little scope for broad-scale change in plan form. However, the pattern of sand change in plan form of the estuary, but the banks, channels, tidal flats and saltmarshes will pattern of sand banks, channels, tidal flats and remain highly sensitive to changes in natural forcing saltmarshes is highly sensitive to changes in factors or human activities. natural forcing factors or human activities. Undefended cliffs to the west of Speke Garston Undefended cliffs to the west of Speke Garston Coastal reserve could erode by up to between 10 Coastal reserve could erode by up to between 5 and 15m by year 100, while along the reserve itself, and 7.5m by year 50, while along the reserve cliffs could erode by up to between 5 and 8m by itself, cliffs could erode by up to between 2 and year 100. However these estimates are for a worst 4m by year 50. However these estimates are for case scenario and will be dependent upon the a worst case scenario and will be dependent channel configurations. upon the channel configurations. There could be continued sediment loss, which There may be continued loss of sediment from could impact on the maintenance of defences. this stretch, as has recently been experienced since the 1970s. The Upper There are various structures along both banks, Upgrade of defences may be required during this Upgrade of defences may be required during this Mersey Estuary consisting of earth embankments, seawalls and period to maintain current levels of protection. period to maintain current levels of protection. revetments. The residual life of these structures is unknown, but works may be required during this period. Defences will continue to reduce the risk to the Defences will continue and reduce the risk to the Defences will continue to reduce the risk to the low lying areas from erosion and flooding. The low lying areas from erosion and flooding. Works low lying areas from erosion and flooding. Further upper estuary is expected to remain stable may be required to raise embankments in works may be required to raise embankments in throughout this period. response to sea level rise. response to sea level rise. Freshwater influx from the River Mersey and Freshwater influx from the River Mersey and surrounding areas has an influence on channel surrounding areas has an influence on channel mobility and consequently erosion and accretion mobility and consequently erosion and accretion patterns. Therefore this stretch could be patterns. sensitive to changes in river flow. The frontage will also be vulnerable to any change in rainfall patterns, which could affect river flows and therefore channel meandering and patterns of

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Predicted Change for With Present Management Location Years 0 – 20 Years 20 – 50 Years 50 – 100 accretion and erosion

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5 Supporting Information

No Active Intervention Data interpretation: E – Mersey Estuary

Assumptions made in predictions of coastal change for NAI Location Available data Uncertainty 0 to 20 years 20 to 50 years 50 to 100 years The Narrows Blott et al. (2006) contains Shoreline position held by Morphological change is Morphological change is The overall response of the estuary information on historical existing defences. severely limited by the severely limited by the form to sea level rise is uncertain. changes. Estuary state assumed to be surrounding solid rock surrounding solid rock Future integrity and position of stable. outcrops. outcrops. harbour structures is an uncertainty Assumed estuary remains Assumed estuary remains as they are not governed by coastal in similar state to today, in similar state to today, defence legislation. with continued supply of with continued supply of Limited data available on residual life sediment. sediment. of defences. Assumed that dock and Assumed that dock and other defences structures other defences structures will fail. will fail. Inner Mersey Recent studies suggest that Shoreline position held by Siltation rates slowly Siltation rates slowly The overall response of the estuary Estuary the estuary as a whole is in existing defences. reducing. reducing. form to sea level rise is uncertain. dynamic equilibrium, with Siltation rates slowly Morphological change is Morphological change is Patterns of erosion and accretion inherent localised erosion and reducing. limited by the limited by the depends on channel configurations, accretion resulting from Morphological change is surrounding topography. surrounding topography. which are not possible to predict – natural variations in this limited by the surrounding Erosion of undefended Erosion of undefended changes in Garston channel would balanced state. topography. soft cliffs (west of Speke soft cliffs (west of Speke affect rates of erosion along Speke Blott et al. (2006) suggests a Coastal reserve – rates present are a Erosion of undefended soft Garston Coastal Garston Coastal slight net loss of sediment in worst case scenario – the range is cliffs (west of Speke Garston Reserve) is assumed to Reserve) is assumed to the inner estuary since 1970s. considered sufficient to take account Coastal Reserve) is assumed be around 0.1- be around 0.1- of sea level rise. Pye and Blott (2004) suggests to be around 0.1- 0.15m/year, meaning up 0.15m/year, meaning up erosion rates for soft cliffs at 0.15m/year, meaning up to to between 5-7.5m to between 10-15m Future integrity and position of Speke Garston Coastal between 2-3m erosion may erosion may be expected erosion may be expected harbour structures is an uncertainty Reserve: by year 20: 1.0 to be expected by year 20. by year 50. by year 100. as they are not governed by coastal

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Assumptions made in predictions of coastal change for NAI Location Available data Uncertainty 0 to 20 years 20 to 50 years 50 to 100 years 1.5m along most of the Erosion of undefended soft Erosion of undefended Erosion of undefended defence legislation. Coastal Reserve frontage but cliffs (Speke Garston soft cliffs (Speke Garston soft cliffs (Speke Garston Limited data available on residual life 2.0 to 3.0m, where the Coastal Reserve) is assumed Coastal Reserve) is Coastal Reserve) is of defences. Mersey Way passes the to be around 0.05- assumed to be around assumed to be around In the inner estuary the greater western end of the airport 0.075m/year, meaning up to 0.05-0.075m/year, 0.05-0.075m/year, importance of freshwater flows runway. By year 50 year: 2.5 between 1-1.5m erosion meaning up to between meaning up to between means changes in precipitation could to 3.75m along most of the may be expected by year 20. 2.5-3.75m erosion may 5-7.5m erosion may be have an impact on meandering frontage and 5.0 to 7.5 m be expected by year 50. expected by year 100. behaviour. close to the runway edge (but Assumed defences will Assumed defences will Current trend of sediment loss may rate dependent upon position fail. fail in 20-50 year period. and size of the Garston be reversed. Channel). Rates include for sea level rise. Upper Mersey Limited information on Defences will continue to Key risk is from flooding: Key risk is from flooding: The overall response of the estuary Estuary historical and future changes provide protection from flood risk based on EA flood risk based on EA form to sea level rise is uncertain. available. flooding. Flood 2008 Maps. Flood 2008 Maps. In the upper estuary the greater Estuary assumed to remain Estuary assumed to Estuary assumed to importance of freshwater flows quite stable with continued remain quite stable with remain quite stable with means changes in precipitation could sediment availability. continued sediment continued sediment have an impact on meandering availability. availability. behaviour. Assumed defences will Assumed defences will fail. fail in 20-50 year period.

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With Present Management Data interpretation: E – Mersey Estuary

Assumptions made in predictions of coastal change for WPM Location Available data Uncertainty 0 to 20 years 20 to 50 years 50 to 100 years The Narrows Blott et al., (2006) include Shoreline position held by Shoreline position held Shoreline position held The overall response of the estuary information on historical existing defences. by existing defences. by existing defences. form to sea level rise is uncertain. changes. Estuary state assumed to be Estuary state assumed to Estuary state assumed to Future integrity and position of stable. be stable. be stable. harbour structures is an uncertainty Flood risk minimised by Flood risk minimised by Flood risk minimised by as they are not governed by coastal defences. defences. defences. defence legislation. Training walls remain in the Assumed that training Assumed that training outer estuary. walls remain in the outer walls remain in the outer estuary and that dock estuary and that dock and other defence and other defence structures will remain structures will remain. The Inner Recent studies suggest that Estuary state assumed to be Estuary assumed to Estuary assumed to The overall response of the estuary Mersey Estuary the estuary as a whole is in stable with no significant remain quite stable with remain quite stable with form to sea level rise is uncertain. dynamic equilibrium, with change, but localised areas continued sediment continued sediment Patterns of erosion and accretion inherent localised erosion and of intertidal accretion and availability. availability. depends on channel configurations, accretion resulting from erosion. Defences will constrain Defences will constrain which are not possible to predict – natural variations in this Flood risk minimised by channel movement. channel movement. changes in Garston channel would balanced state. defences. Flood risk minimised by Flood risk minimised by affect rates of erosion along Speke Blott et al., 2006 suggest a Shoreline position held by defences. defences. Coastal reserve – rates present are a slight net loss of sediment in worst case scenario-– the range is existing defences or fixed by Shoreline position held Shoreline position held the inner estuary since 1970s. considered sufficient to take account surrounding topography. by existing defences or by existing defences or of sea level rise. Pye and Blott (2004) suggests Erosion of undefended soft fixed by surrounding fixed by surrounding erosion rates for soft cliffs at cliffs (west of Speke Garston topography. topography. In the inner estuary the greater Speke Garston Coastal importance of freshwater flows Coastal Reserve) is assumed Erosion of undefended Erosion of undefended Reserve: by year 20: 1.0 to means changes in precipitation could to be around 0.1- soft cliffs (west of Speke soft cliffs (west of Speke 1.5m along most of the have an impact on meandering 0.15m/year, meaning up to Garston Coastal Garston Coastal Coastal Reserve frontage but behaviour. between 2-3m erosion may Reserve) is assumed to Reserve) is assumed to 2.0 to 3.0m, where the be expected by year 20. be around 0.1- be around 0.1- Current trend of sediment loss may Mersey Way passes the

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Assumptions made in predictions of coastal change for WPM Location Available data Uncertainty 0 to 20 years 20 to 50 years 50 to 100 years western end of the airport Erosion of undefended soft 0.15m/year, meaning up 0.15m/year, meaning up be reversed. runway. By year 50 year: 2.5 cliffs (Speke Garston to between 5-7.5m to between 10-15m Future integrity and position of to 3.75m along most of the Coastal Reserve) is assumed erosion may be expected erosion may be expected harbour structures is an uncertainty frontage and 5.0 to 7.5 m to be around 0.05- by year 50. by year 100. as they are not governed by coastal close to the runway edge (but 0.075m/year, meaning up to Erosion of undefended Erosion of undefended defence legislation. rate dependent upon position between 1-1.5m erosion soft cliffs (Speke Garston soft cliffs (Speke Garston and size of the Garston may be expected by year Coastal Reserve) is Coastal Reserve) is Channel). Rates include for 20.Siltation rates slowly assumed to be around assumed to be around sea level rise. reducing. 0.05-0.075m/year, 0.05-0.075m/year, meaning up to between meaning up to between 2.5-3.75m erosion may 5-7.5m erosion may be be expected by year 50. expected by year 100. Siltation rates slowly Siltation rates slowly reducing. reducing. The Upper Limited information on Estuary assumed to remain Estuary assumed to Estuary assumed to The overall response of the estuary Mersey Estuary historical and future changes quite stable with continued remain quite stable with remain quite stable with form to sea level rise is uncertain. available. sediment availability. continued sediment continued sediment Patterns of erosion and accretion Defences will constrain availability. availability. depend on channel configurations, channel movement. Defences will constrain Defences will constrain which are not possible to predict. Shoreline position held by channel movement. channel movement. In the upper estuary the greater existing defences or fixed by Shoreline position held Shoreline position held importance of freshwater flows surrounding topography. by existing defences or by existing defences or means changes in precipitation could Flood risk minimised by fixed by surrounding fixed by surrounding have an impact on meandering defences. topography. topography. behaviour. Flood risk minimised by Flood risk minimised by defences. defences.

E – Mersey Estuary C-226

Revision: 01/10/2009

References: E – Mersey Estuary

ABPmer (2005). Mersey Gateway – Phase II Modelling Study. November 2005.

ABPmer and HR Wallingford (2007). The Estuary-Guide: A website based overview of how to identify and predict morphological change within estuaries. Website prepared for the joint Defra/EA Flood and Coastal Erosion Risk Management R&D Programme. November 2007 http://www.estuary-guide.net/

Blott S J, Pye K, van der Wal D and Neal A. (2006). Long-term morphological change and its causes in the Mersey Estuary, NW England. Geomorphology, Vol. 81, p185-206.

Environment Agency (EA), 2008. Mersey Estuary Catchment Flood Management Plan.

Gifford and Partners (2004). New Mersey Crossing: Morphology Desk Study. Technical Report 3/03. Commissioned for Halton Borough Council.

Halton Borough Council (2008). The Mersey Gateway Project – Environmental Statement. Accessed from: http://www2.halton.gov.uk/merseygateway/content/esaddendum/

HR Wallingford (1999). Analysis of bathymetric surveys of the Mersey Estuary. Report No. IT 469.

McDowell and O’Connor (1977). Hydraulic Behaviour of Estuaries. Macmillan. London.

O’Connor B A (1987). Short and long term changes in estuary capacity. J. Geol. Soc. (Lond.) 144, p187- 195.

Proudman Oceanographic Library (POL) (2007). Dee and Mersey River Bores. Available from: http://www.pol.ac.uk/home/insight/merseybores.html. Website accessed October 2008.

Pye K and Blott S J (2004). Speke Garston Coastal Reserve: Soft Cliff Erosion Study. Kenneth Pye Associates Ltd. External Investigation Report ER505, 76 pp.

Pye K, Blott S J and van der Wal D (2002). Morphological Change as a Result of Training Banks in the Mersey Estuary, Northwest England. Internal Research Report CS14.

Pye K and Neal A (1993). Statigraphy and age structure of the Sefton dune complex: preliminary results of field drilling investigations. In: D Atkinson and J A Houston (eds) The Sand Dunes of the Sefton Coast. National Museums of Merseyside, Liverpool. p41-44.

Pye K and Neal A (1994). Coastal dune erosion at Formby Point, north Merseyside, England: causes and mechanisms. Marine Geology 119, 39-56.

Thomas C (2000). 1D modeling of the hydrodynamic response to historical morphological change in the Mersey Estuary. EMPHASYS Consortium Paper 9. Modeling estuary morphology and process. Final Report. Pp 55-61.

Van der Wal D and Pye K (2000). Long-term morphological change in the Mersey Estuary, North West England. Surface Processes and Modern Environments Research Group, Royal Holloway, University of London, Internal Research Report CS5, 21pp.

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Revision: 01/10/2009