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Liverpool Waters Addendum to Flood Risk Assessment November 2011

Liverpool Waters Addendum to the Flood Risk Assessment – November 2011

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Liverpool Waters Addendum to the Flood Risk Assessment – November 2011

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Contents 1

PURPOSE OF THE REPORT ........................................................................................................... 3

2

REVIEW OF TOPOGRAPHICAL SURVEY .......................................................................................... 4 2.1 SCOPE OF TOPOGRAPHICAL ADDITIONAL SURVEY ............................................................. 4 2.2 REVIEW OF ADDITIONAL TOPOGRAPHIC SURVEY DATA ..................................................... 4

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ASSESSMENT OF SEA LEVEL RISE ................................................................................................. 6 3.1 BASIS OF ASSESSING SEA LEVEL RISE............................................................................... 6 3.2 SEA LEVEL RISE ASSESSMENT........................................................................................... 6 3.3 SUMMARY OF SEA LEVEL ASSESSMENT.............................................................................. 6

4

DEVELOPMENT PROPOSALS & PROPOSED FINISHED LEVELS .......................................................... 7 4.1 4.2 4.3 4.4

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EXISTING SITE LEVELS..................................................................................................... 7 DESCRIPTION OF VARIOUS DEVELOPMENTS PROPOSED WITHIN THE DEVELOPMENT PLAN. 8 PROPOSED FINISHED LEVELS AND FINISHED FLOOR LEVELS.............................................. 9 SAFE ACCESS AND EGRESS ............................................................................................. 10

DISCUSSION ON FLOODING & RELATED DEVELOPMENT ISSUES.................................................. 12 5.1 FLOODING MECHANISM RELATIVE TO THE SITE .............................................................. 12 5.2 OTHER CONSIDERATIONS .............................................................................................. 13

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CONCLUSIONS & RECOMMENDATIONS........................................................................................ 15

Appendices Appendix A

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Plan showing extent of Flood Zones 2 & 3 based on latest topographic survey.

Appendix B

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Assessment of Sea level Rise

Appendix C

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Correspondence with the Environment Agency

Appendix D

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Parameter Plan 010 - Liverpool Waters Indicative Masterplan

Appendix E

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Parameter Plan 001 - Liverpool Waters Site Location Plan 001

Appendix F

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Parameter Plan 008 - Liverpool Waters Parking Plan

Appendix G

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Liverpool Waters Flood Risk Assessment (October 2010)

Liverpool Waters Addendum to the Flood Risk Assessment – November 2011

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1

PURPOSE OF THE REPORT

1.1.1

This addendum report has been prepared to incorporate the review of the additional topographic survey data as identified as being required within the previous WYG Flood Risk Assessment dated October 2010.

1.1.2

Based on the findings of the new survey data the report will also provide some additional commentary and recommendations in respect of finished floor levels (FFL) and emergency access routes to support the current outline planning application for the development.

1.1.3

In preparing this addendum no consideration has been included for the additional affects of extreme wave height overtopping. This is in line with the original Flood Risk Assessment’s statement that inconclusive information is currently available from the Environment Agency to define these affects and that this issue will be addressed on a plot by plot basis for each detailed planning application as it comes forward.

1.1.4

It should be noted that in preparing this addendum a review has been undertaken of the recent Environment Agency’s report entitled “Coastal Flood Boundary Conditions for UK Mainland and Islands Ref SC060064” dated February 2011 which reviews existing sea levels and the rise due to climate change. However, this report confirms that the development site and the adjacent section of the River Mersey are deemed to be within the River Mersey estuary and as such the implications in respect of sea level rise are not directly relevant to the assessment of sea level rise contained within this addendum.

1.1.5

The addendum report has also addressed the issues raised within the Environment Agency’s letter dated 7th October 2010 and this along with other correspondence relevant to this addendum report is contained within Appendix C.

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2

REVIEW OF TOPOGRAPHICAL SURVEY

2.1

SCOPE OF TOPOGRAPHICAL ADDITIONAL SURVEY

2.1.1

As identified within the original WYG FRA dated October 2010, additional topographic survey data was required for the area from between Trafalgar Dock in the north down to West Waterloo Dock in the south. The additional survey was also to include the area of the existing depot building and the associated adjacent car parking areas.

2.1.2

The primary purpose of the additional survey which was undertaken in January 2011 was to confirm the previous assumption that the whole of the development site was located within Flood Zone 1.

2.2

REVIEW OF ADDITIONAL TOPOGRAPHIC SURVEY DATA

2.2.1

The new survey has been reviewed and the extents of the existing Flood Zone 2 (6.37m Above Ordnance datum (AOD)) and Flood Zone 3 (6.11m AOD) have been overlain onto the survey. A copy of Drawing No A062945-6/D100 showing the extent of the Flood Zones is contained within Appendix A.

2.2.2

From a review of the overlay drawing it can be seen that the extent of Flood Zone 3 (i.e. <6.11m AOD and coloured red) equates to the existing docks or the new Central Dock channel. There are some isolated areas within the site and to the west of the Central Dock Bridge within Flood Zone 3. However, these are all generally land locked and a result of low lying areas formed when the area was in filled. It is therefore considered that these localised areas will be infilled s part of the redevelopment and do not require any specific consideration in terms of policy.

2.2.3

The extent of Flood Zone 2 (i.e. <6.37m AOD and coloured pink) is however more extensive and apart from existing docks and the central channel, primarily includes the area around the Depot building and the adjacent car parking area to the west. As with the Flood Zone 3 areas there are also some minor areas to the west of the Central Dock bridge.

2.2.4

The remainder of the site is as previously assessed shown as being within Flood Zone 1 (i.e. >6.37m AOD).

2.2.5

Whilst the Flood Zone 2 areas are more extensive than those within Flood Zone 3 and not entirely ‘land locked’ it is still considered that given the overall context of both (i) the measures that have created this topography from the recent history on the site (i.e. demolitions, infilling operations etc) and (ii) the magnitude of the proposed redevelopment then the Flood Zone 2 footprint is of a rather arbitrary nature. It is therefore considered that it would be inappropriate to use the Flood Zone 2 footprint as a rationale for implementation of flood risk management policies distinct from the remainder of the site.

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2.2.6

Thus whilst the development proposals do reflect a general sequential approach to locating new buildings with public open space covering much of the Flood Zone 2 area, the real issue for the site as a whole (from a flood risk management perspective) is the predicted rise in sea levels and its implication for finished floor levels (FFLâ&#x20AC;&#x2122;s) and emergency access and egress routes. Dealing with this issue should be approached on the basis of the best estimates of the associated risks for the specific types of development and their exposure to risk rather than making distinctions based on an arbitrary Flood Zone 2 footprint.

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3

ASSESSMENT OF SEA LEVEL RISE

3.1

BASIS OF ASSESSING SEA LEVEL RISE

3.1.1

The proposed development will consist of both residential and commercial end use, each of which have a different design life and hence different values for assessing the relevant rise in sea level.

3.1.2

Using the criteria set out within Section 5.80 of the Liverpool City Council’s Strategic Flood Risk Assessment (SFRA) published in January 2008, the design life of the residential buildings has been assumed to be 100 years and 60 years for the commercial buildings.

3.1.3

As stated within Section 1, no account has been included for any revised sea level increases as set out within the recently published report by the Environment Agency titled “Coastal Flood Boundary Conditions for UK Mainland and Islands Ref SC060064 dated February 20112. This is due to the section of the River Mersey under consideration being outside of the scope of the report.

3.2

SEA LEVEL RISE ASSESSMENT

3.2.1

In undertaking the sea level rise assessment (which is contained within Appendix B), it has been assumed that the base level will be 6.11m AOD. This being the existing predicted 1 in 200 year tidal level without consideration of climate change or wave overtopping of the existing quay walls.

3.2.2

The assessment has been based on the net sea level rise criteria relating to climate change on coastal flooding as contained with Section 5.77 of the Liverpool SFRA and previously accepted by the Environment Agency. An additional 10% has been added to the figures to deal with the increases in peak river flows and wind speeds in relation to river estuaries as contained within Section 5.84 of the Liverpool SFRA and table B2 of PPS 25.

3.2.3

The assessment is based on a 30 year construction period for the works together with either a 100 year or 60 year design life.

3.3

SUMMARY OF SEA LEVEL ASSESSMENT

3.3.1

As assessed with the calculations contained within Appendix B the predicted sea level rise for the 1 in 200 year tidal level taking into account climate change for the two main types of development proposed within the development is as follows: Residential Development = 7.15m AOD Commercial Development = 6.95m AOD

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4

DEVELOPMENT PROPOSALS & PROPOSED FINISHED LEVELS

4.1

EXISTING SITE LEVELS

Northern Section 4.1.1

The existing site levels within the northern part of the site, (i.e. Bramley Moors/Nelson and the northern part of Salisbury and Collingwood Docks) is between 6.6m and 6.8m AOD.

4.1.2

Along the eastern frontage the ground levels rises to an average level of 7.1m AOD although the level rises locally upto 8.5m AOD at the Bascule Bridge between Stanley Dock and Collingwood Dock.

4.1.3

The western boundary (River Mersey frontage) is at an average level of 7.0m AOD behind a concrete river wall with a top level of 8.0m AOD.

4.1.4

The lock gates into Sandon Half Tide Dock to the north of Bramley Moors Dock being set at between 6.6m and 7.1m AOD.

Central Section 4.1.5

The central part of the site has an average level of between 6.6m and 6.9m AOD with some low lying levels as described within section 2.2 with the former lock gates at Salisbury Pier Head being set at 6.52 and 6.6m AOD.

4.1.6

The level of Regent and Waterloo Road which run along the eastern boundary falls from 8.5m AOD at the Bascule bridge down to a low point of 6.58 adjacent to the entrance road to the Depot building. The road then rises upto 7.0m AOD at the junction of Waterloo Road and Bath Street.

4.1.7

The western boundary (River Mersey frontage) is for the section north of the Central Dock bridge, at an average level of 6.9 - 6.9m AOD behind a concrete river wall with a top level of 8.0m AOD.

4.1.8

Along the section of the area approximately 140m south of Central Dock bridge, the average ground level is between 7.8m and 8.1 m AOD behind a river wall with a coping level of between 7.8m and 8.6m AOD. Along the remaining section down to the former entrance to Princess Half Tide dock, the average ground level is between 8.1m and 6.9m AOD behind the river front wall which has a coping level of 9.35m AOD. It should be noted that the river wall terminates at the former entrance to the Waterloo Dock which has now been infilled.

4.1.9

The former lock gates into Princess Half Tide dock being set at between 8.0m and 7.8m AOD.

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Princess Dock 4.1.10

Within the southern section (i.e. Princess Dock) the average ground level is between 7.3 and 7.1m AOD falling to between 6.9 and 6.4m AOD within the far southern part of the dock area.

4.1.11

The level along the River Mersey frontage being 7.8m at the northern end falling to 6.9m AOD at the southern boundary.

4.1.12

The Liverpool Waters Site Location Plan 001 identifies the above and a copy is contained within Appendix E.

4.2

DESCRIPTION OF VARIOUS DEVELOPMENTS PROPOSED WITHIN THE DEVELOPMENT PLAN.

4.2.1

Within the development as shown on the current masterplan contained within Appendix D there are to be several types of developments that each have differing requirements for establishing the finished levels in respect of tidal flooding order to meet the aspirations of the Liverpool SFRA and PPS 25. These being: i)

Residential Development

ii)

Commercial Development

iii)

Basement Car parking

iv)

Retained Historic Buildings

v)

Retained Historic Dock Walls

vi)

Public Open Spaces

vii)

External Spaces

4.2.2

The Liverpool SFRA also confirms that development classified as “more vulnerable” will be acceptable within Flood Zone 2 areas. This classification includes residential and commercial development which encompasses all of the proposed development proposals.

4.2.3

As a heritage led regeneration project the Liverpool Waters proposals include provision for retaining archaeological and historic structures and artefacts in situ. This means minimising the amount of land raising in the most sensitive parts of the World Heritage site.

4.2.4

The impact of this will be to remove the areas previously identified as being within Flood Zones 2 & 3. Any loss of flood storage being compensated for by increasing the general level of the site around the perimeter of the existing docks and thus providing additional flood storage freeboard.

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4.2.5

However, as even after raising ground levels it is anticipate that finished levels will still be within the zone of the predicted sea level rise then this will need to be taken into account in order to ensure that no buildings are at risk from tidal flooding and that adequate escape routes can be provided.

4.3

PROPOSED FINISHED LEVELS AND FINISHED FLOOR LEVELS

4.3.1

The Liverpool SFRA recommends within Appendix B1 that where parts of the site are within Flood Zone 2 and 3 that the minimum floor levels with regards to Flood Zone 3 shall also apply within Flood Zone 2.

4.3.2

In order to provide adequate protection against predicted rises in sea level and the affects of climate change the Liverpool SFRA recommends that the predicted sea level rise level is increased by 600mm to provide adequate freeboard. This increase being to take into account uncertainties in the sea level data and the affects of waves resulting from wind or surges due to low pressure.

Residential Development 4.3.3

Based on the predicted sea level rise up to the year 2115 as assessed under Section 3.3, the minimum FFL will be 7.15m + 0.6m = 7.75m AOD.

Commercial Development 4.3.4

Adopting a shorter design life for the proposed buildings and based on the predicted sea level rise up to the year 2101 as assessed under Section 3.3, the FFL will be 6.95m + 0.6m = 7.55m AOD.

4.3.5

The Liverpool SFRA does not provide any specific guidance as the minimum threshold levels of the entrance to the basement car parks. However, good practice is to set these at 300mm above the 1 in 200 plus climate change level. Based on the predicted 2101, 1 in 200 year level for commercial development of 6.95m AOD this would result in a minimum threshold level of 7.25m AOD. It should be noted that the design of the areas adjacent to any basement car park entrances should ensure that run off is directed away from the entrance.

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Appendix B – Guidance for Developers and Development Control – Liverpool SFRA January 2008.

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Retained Historic Buildings & Dock Walls 4.3.6

The current masterplan does not indicate that any of the retained historic buildings located within Flood Zone 2 & 3 will be reused to provide residential use. The key building to be considered is the existing building within the Clarence Graving Dock which may be adapted to provide a cafĂŠ or public information end use. In this scenario as it will not be possible to increase the internal floor level (due to planning issues) it proposed that the building is adapted to incorporate flood resilience measures in as much as any heritage constraints permit. Any other buildings that are to be reused should also adopt the same criteria.

4.3.7

The Clarence Graving Docks are both shown within the extent of Flood Zone 3, however as the existing dock basins are to remain behind the existing lock gates with no new development within them, then this will be an acceptable form of development.

4.3.8

As the existing dock walls and associated structures are viewed to be of historic importance it is anticipated that these will be retained at their existing level. Any increase in the general ground levels being provided outside of the extent of the historically important structures.

Public Open Spaces 4.3.9

Within the northern part of the Central Docks it is proposed to create two areas of public space. These being Central Park and Prospect Park. These areas are mostly created over the Flood Zone 2 area associated around the existing Depot building and the adjacent car park which has an existing ground level of 6.8m AOD. As this area is within the existing Flood Zone 2 area, it is proposed that parts of these new landscaped areas are kept at a low level to retain the existing flood storage capacity of the flood zone.

External Areas 4.3.10

The finished levels to the external areas around the proposed buildings will be set generally 150mm below the FFL of the proposed buildings with the areas falling away from the buildings. This will be in keeping with the proposed raising of the general site levels.

4.3.11

The external finished levels will be set to ensure that any overland surface water flow is directed away from buildings and into the dock basins.

4.4

SAFE ACCESS AND EGRESS

4.4.1

In order to ensure that safe access and egress can be maintained to the proposed buildings within the development it will be necessary to ensure that any external areas over which emergency vehicles are required to travel, are within the guidance of FD2320/TR22.

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Framework & Guidance for Assessing and Managing Flood Risk for New Development

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4.4.2

Assuming that the coastal flood waters would be at a slow velocity due to the flows being attenuated from the initial storage within the dock basins and loss of energy from the waters being dissipated within the docks, it has been assumed that the mean velocity of the flood waters across the site will be generally 0.3 m/s or less .

4.4.3

Utilising Table 13.1 of FD2320/TR2 it can be seen that based on a mean velocity of 0.3 m/s and a flooded depth of 0.25m and a debris factor of 0.5 then the hazard factor would be 0.69 and the hazard rating assessed as being a very low hazard.

4.4.4

Based on the above and the critical 1 in 200 year flood level for 2115 of 7.15m AOD it can be shown that a minimum external ground level of 6.90m AOD (i.e. flooded depth of 250mm) will ensure adequate access for emergency vehicles and safe access and egress to the proposed buildings.

4.4.5

It should be noted that for the commercial buildings the predicted flood level for 2101 including an allowance for wave action and storm surge of 6.95m AOD and therefore based on the 250mm allowable flood depth then a minimum ground level of 6.70m AOD will ensure adequate access for emergency vehicles and safe access and egress to the proposed buildings.

4.4.6

It should be noted that as part of the detailed design of any emergency access routes, that where these are located adjacent to any existing bodies of water, then adequate signage should be provided so that in times of flood, emergency vehicles are aware of the presence of underlying deep bodies of water.

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5

DISCUSSION ON FLOODING & RELATED DEVELOPMENT ISSUES.

5.1

FLOODING MECHANISM RELATIVE TO THE SITE

5.1.1

The site is at risk from tidal flooding as a result of the 1 in 200 year event. The mechanism by which this will occur will be due to water entering the site as a result of high tides and the effects of storm surges and wave action (i.e. overtopping of perimeter structures).

5.1.2

At present the site is protected along a large proportion by a concrete river wall which has a varying coping level between 7.0m AOD within the northern part of the site to 9.3m within the southern part of the Central site. Within Princess Dock the coping level varies between 7.8m AOD within the northern part down to 6.9m AOD within the bottom southern part.

5.1.3

Between the sections of the river walls there are historic lock gate structures which have been sealed against the tide. The lowest top levels of these structures being 6.6m AOD for the Sandon Dock lock to the north, 6.52m AOD for the Salisbury Pier Head within the central area and 7.8m AOD for the former lack entrance into Princess Half Tide Dock.

Existing Tidal Flooding 5.1.4

The Extreme Sea level Study undertaken by the Environment Agency in 2008 has established that the current predicted 1 in 200 year event sea level is 6.11m AOD and for the 1 in 1000 year event 6.37m AOD. No site specific storm surge profile has been provided by the Environment Agency to take into account storm surge or wave action and as previously agreed the affects of wave height overtopping will be addressed within the detailed planning application process for each development.

5.1.5

However, the affects of climate change on sea level rises will result in this level rising by 2.5mm per year up to 2025, thereafter by 7mm per year until 2055 and 10mm per year until 2085. Calibrating the sea level rise from 2008 up to 2011 will result in a 1 in 200 year level being 6.118m AOD.

5.1.6

At present the site is protected from a tide level of 6.52m AOD, this being the minimum boundary level outside of the extent of the river walls at the lowest point at the Salisbury Pier Head. This provides a maximum freeboard against tidal flooding (based on still tidal levels) for the 1 in 200 event of 402mm.

Future Sea Level Rise 2011 - 2061 5.1.7

However, as each year passes then the affect of climate change will reduce the freeboard and using the predicted increases in sea level rise it can be seen that the Salisbury Pier Head Lock would be overtopped in the year 2061 in respect of the 1 in 200 year event.

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5.1.8

From the above it can be demonstrated that in the year 2061 the development will be at risk if tidal flooding and in the event of the maximum storm tide being reached, then flood waters will initially enter the site by overtopping the Salisbury Pier Head and cascade into the adjacent Trafalgar Dock basin. As the tide level increases the water level will rise and overtop the existing inner basin dock walls and flood the adjacent land areas.

5.1.9

This process will not result in an automatic flooding of the site, as initially the volume of water overtopping into the site will be dissipated throughout the numerous dock basins within the whole site. In reality due to the relatively short period when high tide will occur (max 2 hours) and the extensive areas of the dock basins and available freeboard, the volume of water should be retained within the dock basins.

5.1.10

It should be noted that the dock basins with the exception of the Bramley Moor Dock are all retained and not subject to tidal variations with the water level being retained within the range of 4.5 and 5.15m AOD. Therefore based on the upper level, the minimum freeboard available to provide storage of any flood waters will be 1.7m

Sea Level Rise 2115 5.1.11

In considering the maximum predicted future increases in tidal levels upto the year 2115, it has been identified that the still water level for the year 2115 taking into account climate change and predicted increases in sea level, that the 1 in 200 year tidal level will be 7.15m AOD

5.1.12

Under this scenario the tidal waters will primarily enter the site through the two former locks (i.e. Sandon Lock and Salisbury Pier Head) as well as breaching the existing river wall within the northern and far southern parts of the site.

5.1.13

The process will be as before in that the sea where it overtops the lock structures will flood direct into the dock basins, however, where the waters overtop the river frontage or river wall it will initially drain over the adjacent developments into either the adjacent dock basins or into the Central Dock channel.

5.1.14

Without detailed modelling it is not possible at this stage to identify whether there would be adequate storage available within the existing dock basin network to store all this water. However, it is generally the case that as both the period and rate of overtopping increases the tendency for the water level within the defended area is to equate to that outside the defences. Nonetheless following the strategy for setting the FFLâ&#x20AC;&#x2122;s properties will be above the still water level within the time frame and event severity considered.

5.2

OTHER CONSIDERATIONS

5.2.1

To assist with future proofing the development against climate change and the rising sea level consideration has been given within the car parking layout to providing under croft and podium car parking along the section fronting onto the River Mersey. This will ensure that any future flood waters overtopping the existing river wall will flow unimpeded overland into the Central Dock Channel and Waterloo West Dock. A copy of the Proposed Parking Plan 008 is contained within Appendix F

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5.2.2

It should be noted that as previously agreed with the Environment Agency, no specific consideration has been included for the affects of extreme wave action as these are to be considered as part of each detailed application for each site. However, the addition of the 600mm freeboard is normally considered as a suitable allowance to cover the affects of extreme wave action and therefore subject to the specific requirements of the Environment Agency in respect of extreme wave action, it may be acceptable to ignore these affects in setting minimum FFLâ&#x20AC;&#x2122;s.

5.2.3

Consideration should be given during the detailed design stage to ensure that any flood waters entering the non tidal docks can be drained out to restore the dock levels to the operational range of between 4.5 and 5.15m AOD

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CONCLUSIONS & RECOMMENDATIONS

This addendum report has identified the following: •

The additional topographic survey identified several areas that were previously assumed as being within Flood Zone 1 as being within Flood Zone 2 & 3. These areas primarily being around the western end of the Central dock bridge and around the existing Depot building and the adjacent car parking areas. The remainder of the site lies within Flood Zone 1

Generally a sequential approach has been utilised in establishing the masterplan with the areas identified as being within Flood Zone 2 being allocated for landscaped areas.

The existing 1 in 200 year still tidal level for the site of 6.111m AOD has been obtained from the Environment Agency’s Extreme Sea level Study dated 2008. Similarly, the 1 in 1000 year level has also been stated as being 6.37m AOD.

Allowing for the predicted rise in sea level to take into account climate change, the 2015 year sea level applicable to residential end use has been calculated as being 7.15m AOD and the 2101 year level applicable to commercial end use has been calculated as being 6.95m AOD.

The site is protected from flood waters along the River Mersey by either redundant lock structures or an existing concrete river wall.

The lock gate structures have a top level of 6.6m at the Sandon Lock to the north of the site, 6.52m AOD for the Salisbury Pier Head Lock and 7.8m for the Princess Half Tide Lock.

The coping of the river wall varies between 7m AOD in the north up to 9.3m AOD in the south part of the development adjacent to the Princes Half Tide Lock.

Within the Princess Dock basin there is no river wall and the existing quay wall levels vary between 7.8m AOD and 7.3m AOD over most of its length with the quay wall falling to a level of 6.9m AOD within the far southern section.

The Liverpool SFRA confirms that the “More Vulnerable” class of development is acceptable within Flood Zones 1 & 2.

Site levels within the site are generally anticipated to be increased by between 0.5 m and 1.0m in order to re use the material excavated from the construction of the basement car parks and in order to protect the existing historical dock structures.

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The site will be subject to flooding as a result of future increases in sea level due to the affects of climate change and these are required to be assessed and incorporated within the design of the development to ensure buildings are not at risk of flooding and that emergency access can be provided at all times.

Based on the increase in the rise of the still tidal levels, the site will not be at risk of flooding from the 1 in 200 year event until the year 2061. During this event flooding will only occur due to flood waters breaching the Salisbury Pier Head Lock structure. Any flood waters being retained within the existing dock basins and the additional freeboard provided due to the raising of the ground adjacent to the historic dock walls.

During later years the site will become at more risk of tidal flooding from the River Mersey. In the year 2115 (this being the maximum design life of the development and date for which tidal increase data is available) the flood level will rise to 7.15m AOD. This will result in inundation of low levels of the site where they are below the 7.15m level. The flood waters entering the site from not only the lock structures, but also by overtopping of the section of the existing river wall to the north of the development and over the existing quay wall top the south of Princess Dock.

The Liverpool SFRA recommends that FFL’s of any new buildings should be set 600mm above the 1 in 200 year flood level including the affects of climate change covering the design life of the buildings. This increase being to take into account uncertainties in the sea level data and the affects of waves resulting from wind or surges due to low pressure.

Based on the above and the predicted increase in sea levels, the minimum FFL for residential buildings will be 7.75m AOD and for commercial buildings 7.55m AOD and it is recommended that these levels are adopted as a minimum. It should be noted that as previously agreed with the Environment Agency, no specific consideration has been included for the affects of extreme wave action as these are to be considered as part of each detailed application for each site. However, the addition of the 600mm freeboard is normally considered as a suitable allowance to cover the affects of extreme wave action and therefore subject to the specific requirements of the Environment Agency in respect of extreme wave action, it may be acceptable to ignore these affects in setting minimum FFL’s.

Entrances to any proposed basement car parking levels shall be set at a minimum threshold level of 7.45m AOD with the ground levels leading to the entrances being graded away from the entrance.

Any historic buildings that are to be reused shall retain their existing FFL’s and be adapted to include flood resilience measures as appropriate.

The new landscape areas within Prospect Park and Central park shall be kept as low as possible to maximise the existing flood storage within the relevant Flood Zone 2 areas within which these are proposed.

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The final external ground levels will be set to ensure that any overland surface water flow is directed away from buildings and into the dock basins.

Based on the assessment criteria set out within FD2320/TR2, the hazard rating of the external areas during the 2115 1 in 200 year event has been shown based on a flooded depth of 250mm to be a very low hazard and sufficient to allow emergency vehicles to access all the buildings. Adopting these criteria minimum ground levels can be set at 6.90m AOD adjacent the residential buildings and 6.70m AOD adjacent commercial buildings.

Consideration should be given during the detailed design stage to ensure that any flood waters entering the non tidal docks can be drained out to restore the dock levels to the operational range of between 4.5 and 5.15m AOD.

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Appendix A Plan showing extent of Flood Zone 2 & 3 areas based on latest topographical survey

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Appendix B Assessment of Sea level rise due to Climate Change

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Addendum to Flood Risk Assessment

Appendix B â&#x20AC;&#x201C; Assessment of Sea Level Rise 1.0

ASSUMPTIONS

1.1

The rate of increase in sea level rise is based on the following as set out within Section 5.77 of the Liverpool SFRA and Table B1 of PPS 25 and is as follows: Net Sea Level Rise (mm/year)

N West England 1.2

2008-2025

2025-2055

2055-2085

2085-2115

2.5mm

7.0mm

10mm

13mm

The design life of residential buildings is assumed to be 100 years and for commercial buildings 60

years as set out within the criteria to be adopted within Section 5.80 of the Liverpool SFRA. 1.3

The construction period for the development is assumed to be 30 years.

1.4

The base 1 in 200 year tidal level is 6.11m AOD, this being based on the latest date for which the

predicted sea level rise has been based. 1.5

The maximum period to which the assessment is made is 2115 as this is the latest date for which

the increase in sea level is available.

WYG Engineering

part of the WYG Group

Liverpool Waters,â&#x20AC;&#x201C; Addendum to Flood Risk Assessment October 2010

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Addendum to Flood Risk Assessment

Appendix B – Assessment of Sea Level Rise

2.0

Residential Buildings

2.1

Assuming a 30 year build programme and a 100 year design life from completion of last building,

design life ends 2011 + 130 years = 2141. 2.1

The maximum period to be assessed will be 2008 to 2115 = 107 years.

Net Sea Level Rise up to the year 2115 (Using EA’s 2008 model as base) Period

Total Years

PPS25 – Sea Level Rise (mm/year)

Total (mm)

2008 - 2025

17

2.5

42.5

2025 - 2055

30

7

210

2055 – 2085

30

10

300

2085 - 2115

30

13

390 Total = 942.5mm

Therefore based upon the EA’s 1 in 200 year sea level model of 6.11m AOD (Extreme sea level study 2008, including surge). The 1:200 year sea level for years up to 2115 (inc. sea level rise) =7.053m AOD Extreme Wave Height From Section B6 of PPS25, a sensitivity allowance should be added for extreme wave height. From Table B.2, a 10% sensitivity factor should be added. Thus 94mm. Therefore based upon the EA’s modelled 1:200 year sea level of 6.11m AOD (Extreme sea level study 2008, including surge) and the guidance within PPS25 for precautionary sensitivity factors; The 1 in 200 year extreme sea level for years up to the year 2115 is approximately 7.15m AOD

WYG Engineering

part of the WYG Group

Liverpool Waters,– Addendum to Flood Risk Assessment October 2010

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Addendum to Flood Risk Assessment

Appendix B – Assessment of Sea Level Rise

3.0 Commercial Development Assuming a 30 year build programme and a 60 year design life from completion of last building, design life ends 2011 + 90 years = 2101. Net Sea Level Rise up to the year 2100 (Using EA’s 2008 model as base) Period

Total Years

PPS25 – Sea Level Rise (mm/year)

Total (mm)

2008 - 2025

17

2.5

42.5

2025 - 2055

30

7

210

2055 – 2085

30

10

300

2085 - 2101

16

13

208 Total = 760.5mm

Therefore based upon the EA’s 1 in 200 year sea level model of 6.11m AOD (Extreme sea level study 2008, including surge). The 1:200 year sea level for years up to 2101 (inc. sea level rise) =6.871m AOD Extreme Wave Height From Section B6 of PPS25, a sensitivity allowance should be added for extreme wave height. From table 13.2, a 10% sensitivity factor should be added. Thus 68.7mm. Therefore based upon the EA’s modelled 1:200 year sea level of 6.11m AOD (Extreme sea level study 2008, including surge) and the guidance within PPS25 for precautionary sensitivity factors; The 1 in 200 year extreme sea level for years up to the year 2101 is approximately 6.94m AOD NB: The above calculations only consider the contingency allowances for climate change within current guidance and not to the end of the estimated design life of 2141 for Residential Development.

WYG Engineering

part of the WYG Group

Liverpool Waters,– Addendum to Flood Risk Assessment October 2010

creative minds safe hands


Appendix C Correspondence with the Environment Agency

Liverpool Waters Addendum to the Flood Risk Assessment â&#x20AC;&#x201C; November 2011

20


Appendix D Parameter Plan 010 Liverpool Waters Indicative Masterplan.

Liverpool Waters Addendum to the Flood Risk Assessment â&#x20AC;&#x201C; November 2011

21


Date: Scale:

October 2011 1:7500 @A3

Title:

Parameter Plan 010 - Liverpool Waters Indicative Masterplan

Chapman Taylor Chapman Taylor LLP Architects Masterplanners Designers

0

100m

500m


Appendix E Parameter Plan 001 – Liverpool Waters Site Location

Liverpool Waters Addendum to the Flood Risk Assessment – November 2011

22


BOUNDARY STREET

WELLINGTON DOCK

FULT ON STRE ET

REG ENT R OA D

BRAMLEY MOORE DOCK

BLACKSTONE STREET

FULTO N ST

BEACON STREET

NELSON DOCK DENBIGH STREET

BENTINCK STREET

BIRCH STREET

GREAT HOWARD STREET

REGENT ROAD

LIGHTBODY STREET

WALTER STREET

Leeds and Liverpool Canal Liverpool Locks

Lock

SALISBURY DOCK

STANLEY DOCK

COLLINGWOOD DOCK

G DOCKS

CLARENCE GRAVIN

TRAFALGAR DOCK

SHERWOOD STREET

SALTNEY STREET

CLARENCE GRAVING

DOCKS MADDRELL STREET

LOVE LANE

DUBLIN STREET

GLEGG STREET

REGENT ROAD

STONE STREET

DICKSON STREET

UPPER WILLIAM STREET COTTON STREET

STREET

LOVE LANE

GREAT HOWARD STREET

CARLTON

WHITLEY STREET

REGENT STREET

PORTER

WATERLOO

STREET

SPRAINGE

R STRE

ET

VULCAN

STREET

ROAD

LOVE LANE

RIVER MERSEY

VANDRIES

STREET

HOWARD STREET

OIL STRE

ET

CHADWICK STREET

KINGSWAY (Tunnel)

Ventilating Station KINGSWAY (Tunnel)

EAST STREET HOWARD GREAT

LL

DOCK

DOCK

PALL MA

RLOO

WATERLOO ROAD

WATE

LOO

R WATE WEST

LANYOR D

K ROA

GLASGOW STREET

GALTON STREET

PAISLEY STREET

GREENOCK STREET

PRINCES HALF TIDE DOCK

T EE

TS

RO

STR

R BE

LEEDS

STREET

LL

PA

T

E RE

ST

JESS

KING EDWARD

TH

STREET

L AL

M

BA

LIAM

WIL W OP AY

D

OL L HA T EE

TR

LS

PRINCES DOCK

RA

S PA

INCE

PR DE

TIT

ET

HE

RE

BA

ST

RN

S

ST

LD

LL HA

RE

IE

ET

RF

O

O

M

D OL

UA WQ

NE

T EE

Y

TR ES

L DA

RU

EET

RD FO

MS

STR

L APE

T

E RE

ST

CH

LE DA

ST

E RG

O GE Bank

'S K

L ST CA

C DO

T

NW FE

TE

K

IC

WA

LA

EE TR

S TE

ES

GA T EE TR RS

L.

SP

HO

ST

IC

.N ST

E

TH GO RE

ND RA

ST

T

EE

TR RS

TE

WA

E

S ICK SW UN BR

EE TR

T

LORD STREET

ST. NICH

KEY:

T

EE

TR

S OLA

S ICK SW

ET

UN

BR

PLAC

RE

ES

ST

E

M

JA

Liverpool Waters Outline Application Redline Boundary

Parameter Plan 001 - Liverpool Waters Site Plan

ET RE ST ET RE ST

Title:

ND

October 2011 1:7500 @A3

ND RA ST

Date: Scale:

RA ST

ND ISLA NN

MA

Other land in the ownership of the Applicant

Chapman Taylor Chapman Taylor LLP Architects Masterplanners Designers

0

100m

500m


Appendix F Parameter Plan 008 â&#x20AC;&#x201C; Liverpool Waters Parking Plan

Liverpool Waters Addendum to the Flood Risk Assessment â&#x20AC;&#x201C; November 2011

23


E: 8.0 1 level

E: 9.0 8 levels

E: 6.0

E: 7.0

2 levels

2 levels

E: 5.0 2 levels

E: 10.0 9 levels

E: 3.0 2 levels

E: 4.0 2 levels

E: 2.0

E: 1.0

2 levels

2 levels

D: 5.0 6 levels

D: 2.0

D: 4.0 D: 1.0

2 levels

2 levels

4 levels

C: 11.0 9 levels

C: 10.0 4 levels

C: 8.0 3 levels

C: 7.0 2 levels

C: 5.0 3 levels

C: 4.0 2 levels

C: 3.0 3 levels

C: 2.0 2 levels

C: 12.0 1 levels

C: 13.0 1 levels

B: 1.0 1 level

B: 2.0 2 levels

B: 3.0 6 levels

A: 3.0 2 levels

A: 2.0 2 levels

A: 4.0 2 levels

A: 1.0 3 levels

KEY: Liverpool Waters Outline Application Redline Boundary PRINCES DOCK

(neighbourhood A)

KING EDWARD TRIANGLE CENTRAL DOCKS

(neighbourhood B)

(neighbourhood C)

CLARENCE DOCKS

(neighbourhood D)

NORTHERN DOCKS

(neighbourhood E)

Underground Car Parking Podium Car Parking Multi Storey Car Parking Date: Scale:

October 2011 1:7500 @A3

Title:

Parameter Plan 008 - Liverpool Waters Parking Plan

Chapman Taylor Chapman Taylor LLP Architects Masterplanners Designers

0

100m

500m


Appendix G Liverpool Waters Flood Risk Assessment as submitted October 2010

Liverpool Waters Addendum to the Flood Risk Assessment â&#x20AC;&#x201C; November 2011

24


FLOOD RISK ASSESSMENT October 2010


REPORT CONTROL Document:

Flood Risk Assessment

Project:

Liverpool Waters, Liverpool Docks, Liverpool

Client:

Peel Land and Property (Ports) Ltd

Job Number:

A024649

File Origin:

A024649_Reports_FRA Final_Dec

Document Checking: Primary Author

R Browne

Initialled:

Contributor

WJ Wilkins

Initialled:

Review By

A P Fox

Initialled:

Issue

Date

Status

Checked for Issue

1

11/01/10

Final Issue

A P Fox

2

14/09/10

EIA Update Issue

A P Fox

3 4


Contents EXECUTIVE SUMMARY......................................................................................................................... 4 1

INTRODUCTION........................................................................................................................... 9 1.1 PROPOSED DEVELOPMENT ............................................................................................... 9 1.2 PURPOSE OF THE REPORT................................................................................................ 9 1.3 REPORT COMPLIANCE WITH PPPS25 ............................................................................... 11

2

SITE DESCRIPTION.................................................................................................................... 12 2.1 EXISTING SITE .............................................................................................................. 12 2.2 FLOOD RISK .................................................................................................................. 20

3

DEVELOPMENT PROPOSALS........................................................................................................ 29 3.1 3.2 3.3 3.4

4

PROPOSED DEVELOPMENT ............................................................................................. 29 DEVELOPMENT AND FLOOD RISK .................................................................................... 31 PROPOSED MITIGATION................................................................................................. 33 RESIDUAL FLOOD RISK .................................................................................................. 36

CONCLUSIONS AND RECOMMENDATIONS ................................................................................... 38

PPS25 PRACTICE GUIDE PRO-FORMA ........................................... ERROR! BOOKMARK NOT DEFINED.

Appendices

(under separate cover)

Appendix A

-

PPS25 Practice Guide Proforma

Appendix B

-

Site plans (existing and proposed)

Appendix C

-

Site photographs

Appendix D

-

Correspondence

Appendix E

-

Calculations


EXECUTIVE SUMMARY Peel Land and Property (Ports) Limited is looking to redevelop an approximate 2.2km stretch of Liverpool’s waterfront along the Eastern bank of the Mersey estuary. The derelict land is approximately 36Ha comprising mainly of redundant docklands within an approximate 64.5Ha overall red-line boundary which also envelops existing recent developments, such as offices at Princes Docks. The proposals consist of mixed-use development of residential, offices, hotel, community, retail and leisure uses. A schematic of the proposed development mix is included within appendix B.

WYG has been

commissioned to produce a Flood Risk Assessment (FRA) to accompany an outline planning application for the proposed development. The following conclusions and recommendations are drawn from this report: •

According to broad-scale flood mapping produced by the Environment Agency (EA), the majority of the site (typically the East of the larger parcel and also the smaller King Edward Street/Bath Street Parcel) fall within Flood Zone 1 (thus outside the extent of the 0.1% (1 in 1000) risk of flooding from either the sea or river flows), whilst a band of the Western boundary is shown as Flood Zone 3 and thus deemed as having greater than a 1% probability of river flooding in any one year or greater than 0.5% (1 in 200 year) probability of sea flooding in any one year. A small area of the site associated with Trafalgar Dock is shown as Flood Zone 2, thus deemed by the EA as having between 0.5% (1 in 200) risk of flooding and 0.1% (1 in 1000) risk of tidal flooding in any one year. Therefore, based upon broad scale mapping alone, the site is considered to have areas of ‘low’, ‘medium’ and ‘high risk’ potential of flooding from tidal or fluvial sources.

The Environment Agency undertook an ‘Extreme Sea Level Study’ in 2008 and determined that tidal influences from the River Mersey to be the overriding flood risk to the site. Estimations for the flood levels of the Mersey during 1 in 100 year (1% probability), 1 in 200 year (0.5%) and 1 in 1000 year (0.1%) flood events have been provided as 6.0m, 6.11m and 6.37m AOD respectively (including surge events) based upon the Liverpool Gauge at grid reference 332480,395240.

Existing on-site levels in comparison to the EA’s flood levels are typically over 6.14m AOD, behind the dock wall, which is not specifically a flood defence (coping level typically 6.53m and 9.31m AOD). Therefore none of the existing site should be classified as Flood Zone 3 (governed by the 6.11m, 1 in 200 year tidal event). A small area the recently infilled West Wellington Dock and an adjacent existing

4

Liverpool Water Flood Risk Assessment


Depot and car park do fall below the 1 in 1000 year tidal level of 6.37m AOD, thus potentially Flood Zone 2 (ignoring the presence of dock walls). •

Comparison between the above Mersey flood levels, the topographical survey and visual inspection of the areas constructed after the topographical survey was completed, have led us to deduce that the whole of the development area is in fact located within Flood Zone 1. It is recommended that this is confirmed by a further topographical survey locally to the ‘Central Dock Channel Bridge’ and the infilled area of West Waterloo Dock and existing depot and associated car park. This topographical survey update is currently being performed and will be made available in due course.

Liverpool City Council’s UDP shows that the development is designated within the ‘Strategic Investment Area’, whilst the northern half of the proposed development area down to ‘Central Dock Channel Bridge’ and the North of ‘Waterloo Quay’ is designated as ‘unimplemented UDP mixed use allocation’. Smaller pockets within the southern development area that appear to correlate with the development proposals are also denoted as ‘unimplemented UDP mixed use allocation’ However, the smaller development parcel enclosed within King Edward Street, Bath Street and Roberts Street is within the ‘strategic investment area’ though is unallocated.

Given the above and given the belief that the site to be located entirely within Flood Zone 1, it is considered that the PPS25 Sequential Test has been passed. Furthermore, from Table D3 of PPS25 (Flood Risk Vulnerability and Flood Zone Compatibility) the proposed development vulnerability classification is suitable for the site.

The development area is covered by Liverpool City Council’s Strategic Flood Risk Assessment (SFRA) of January 2008 which provides details of any known historical tidal, fluvial and pluvial flooding events.

The EA do not have any records of known historical flooding event to the site from river or tidal sources. The SFRA indicates the historical presence of 2no. ordinary watercourses within the vicinity of the site, ‘Beacon’s Gutter’ and River Pool. However, the SFRA also identifies both as having been historically abandoned and replaced by the public sewerage network.

Correspondence received from United Utilities (Appendix D) shows that they are aware of 2 no. public sewers flooding properties in the vicinity on their ‘at risk register’ as a result of overloaded sewers. They also can not rule out instances that have gone unrecorded or instances that have not qualified for inclusion on the register. The location of one of these sewers is beyond the influence of the site, while the other (St Nicholas Place on the southern boundary of the site) has an isolated low spot. With the proposed minimum finished floor levels proposed later, any flooding of the same sewer in the future is

5

Liverpool Water Flood Risk Assessment


only likely to route along (and be contained within) St Nicholas Place without compromising any plot buildings. Thus this at risk sewer is considered to be low risk of flooding the proposed development. •

We can confirm that UU reviewed the draft version of this FRA and confirmed it was acceptable in principal.

Liverpool City Councils Highways department records are a data source for the SFRA. No Highway within or within immediate vicinity of the development is highlighted within this data as having caused historical flooding. As per the records of public sewers flooding properties, it can not be ruled out that incidence have gone unrecorded within this data set.

Although ground water flooding is considered to be an increasing issue within Liverpool City centre, referencing the ‘Mersey Estuary Catchment Flood Management Plan’ ‘although the groundwater levels have been seen to increase, the risk to Liverpool from groundwater flooding is considered to be low’.

The flood risk from canal infrastructure failures is considered as part of the assessment. However, the open water relating to docks is to remain insitu following the redevelopment. The standing water level within these open docks is lower-lying than the anticipated thresholds of the proposed buildings, thus is expected to have the capacity to accommodate flood volumes from canal infrastructure failures without compromising the building thresholds. The risk of flooding due to canal infrastructure failure is therefore considered to be low.

LCC’s SFRA states that a 600mm freeboard should be provided above the 1 in 200 year Mersey Level, once contingency allowances for net sea level rises and increases in extreme wave height have been added for the design life of the development (though based upon their belief that areas of the site are within Flood Zones 2 or 3). Calculations have been made for the 1 in 200 year Mersey level using contingency allowances for net sea level rises and an uprise in wave height as 7.15m AOD in the year 2115, (based upon a 5 year construction period and 100 year design life).

This report recommends that all floor levels and emergency access routes are 7.25m AOD and above, giving a minimum recommendation of 100mm freeboard above the estimated 1 in 200 year Mersey level for the year 2115 (i.e. including climate change) and allowing a 300mm freeboard above the estimated 1 in 200 year Mersey level up to the year 2100 (6.95m AOD). This principle is less than the 600mm freeboard recommended within Liverpool City Council’s SFRA (based on the site being within Flood Zones 2 and 3), however because this report considers that the site is wholly within Flood Zone 1, it is considered that there is reasonable debate with the planners and the EA to adopt this strategy.

6

Liverpool Water Flood Risk Assessment


Extreme wave height overtopping remains the only fundamental additional assessment / amendment which will be required to the FRA study. As the Environment Agency’s current overtopping data is inconclusive, the issues pertaining to overtopping (and their other concerns pertaining to design life beyond the current guidance in PPS25 and freeboard) will be addressed on a plot by plot basis for each detailed planning application as it comes forward. Thus, each detailed applications will require a FRA addendum, with flood resilient construction measures introduced on a plot by plot basis only if avoidance is not physically possible.

Infiltration techniques are not considered to be suitable for the development given the underlying firm to stiff clay strata.

No specific maximum permissible discharge figure has been provided by the Environment Agency. However, discharge to watercourse is considered to be the only suitable method of surface water discharge using either an existing connection directly to the Mersey, using a proposed connection, or discharging indirectly to the Mersey via the docks, subject to detailed drainage design.

It is anticipated that the proposed private drainage network will be formed using separate foul and surface water systems.

It is anticipated that the proposed private surface water sewers will be designed so no pipes surcharge during the 1 in 2 year event, the network doesn’t flood up to the 1 in 30 year event and drainage is modelled against a surcharged outfall, though subject to agreement with the Environment Agency and LCC as part of the drainage strategy.

It is anticipated that surface water run-off volumes up to the 1 in 30 year event will be attenuated onsite by, for example, allowing the existing open docks to fill above the usual standing water level, possibly in combination with underground tanks. Estimations for the 1 in 30 year attenuation have been calculated using the Windes software as 1800m3, based upon restricting the discharge from the site to the existing 1 in 2 year discharge figure (estimated to be 3645 l/s). This provides betterment on the existing drainage arrangement, which is believed to have no existing discharge controls. Should this volume be accommodated within the estimated 18Ha of dock waters, then this is estimated to cause the dock levels to rise by approximately 10mm.

Flows in exceedance of the 1 in 30 event up to the 1 in 100 year plus 30% climate change event will be retained on site by incorporating design guidance such as CIRIA Report C635. The precise requirements will be subject to detailed design but could, for example, accommodate the additional attenuation volumes required by again allowing the docks to rise above the usual standing water level. This could permit local and temporary flooding of the docks above normal operating level during exceedance

7

Liverpool Water Flood Risk Assessment


flooding until levels subside without compromising existing or proposed building floor thresholds. The volume of this exceedance has been estimated as a further 3350m3 of storage (5150m3 in total). Detailed design of such flooding however, would have to take into account the maximum water level that the docks would be allowed to raise without compromising any of the control features or canal traffic. This rise is estimated to be an additional 20mm above the 1 in 30 year attenuation level (30mm total). â&#x20AC;˘

As per the recommendations within B9 of Planning Policy Statement 25, precautionary sensitivity ranges for an increase in rainfall intensities should be considered to allow for the uncertainty brought about by climate change. The proposed drainage model and finished site levels should therefore demonstrate that no proposed or neighbouring thresholds are compromised during a 1 in 100 year plus 30% climate change event1 (based upon an estimated design life of the building of 100 years).

â&#x20AC;˘

A drainage strategy should be undertaken to assess the need for diversions to any existing drainage network to be re-utilised. It is also recommended that these networks are fully inspected to establish condition and identify any remedial works necessary.

â&#x20AC;˘

Residual risks may still remain to the development, such as collapse of a combined sewer overflow from the public sewer network or collapse of the dock walls or dock gates, which although are not designated sea defences, provide some defence to the development.

1

Table B2, PPS 25, Development and flood risk. Published December 2006

8

Liverpool Water Flood Risk Assessment


1

INTRODUCTION

1.1

PROPOSED DEVELOPMENT

Peel Land and Property (Ports) Ltd is looking to redevelop approximately 36Ha area that mainly comprises of former docklands that are historically related to Liverpool’s ship-building heritage. The redevelopment will compliment the existing city centre by providing a mixed-use development in an extension to Liverpool city centre core. The site lies to the North-west of the city centre, along a 2.2km stretch of the eastern bank of the Mersey Estuary, crossing the Central and Kirkdale Ward boundaries. The red line boundary to the main site is approximately 62Ha of which approximately 33.5Ha is proposed for redevelopment, whilst the remaining 28.5Ha is existing recent redevelopment. This boundary encompasses Bramley Moore and Nelson Docks to the North, down to West Waterloo and Princes Docks to the South. The development also includes a smaller parcel which is encompassed between King Edward Street, Bath Street and Roberts Street and approximate to 2.4Ha. As the overall planning boundary is approximate to 64.5Ha and given it’s location adjacent to the Mersey Estuary, WYG has been commissioned by Peel Land and Property (Ports) Ltd to produce a Flood Risk Assessment (FRA) to accompany an outline planning application for the proposed development.

1.2

PURPOSE OF THE REPORT

According to flood mapping produced by the Environment Agency (EA), the majority of the site (typically the East of the larger parcel and also the smaller King Edward Street/Bath Street Parcel) falls within Flood Zone 1 (thus outside the extent of the 0.1% (1 in 1000) risk of flooding from either the sea or river flows), whilst a band of the Western boundary is shown as Flood Zone 3 and thus deemed as having greater than a 1% probability of river flooding in any one year or greater than 0.5% (1 in 200 year) probability of sea flooding in any one year. A small area of the site associated with Trafalgar Dock is shown as Flood Zone 2, thus deemed by the EA as having between 0.5% (1 in 200 year) risk of flooding and 0.1% (1 in 1000) risk of flooding from either the sea or river flows. Therefore, based upon broad scale mapping alone, the site is considered to have areas of ‘low’, ‘medium’ and ‘high risk’ potential of flooding from tidal or fluvial sources.

9

Liverpool Water Flood Risk Assessment


Given the above and based on Planning Policy Statement 25 (PPS25)2 and current EA Standing Advice3, an FRA is required for the development. This is required in order to assess surface water runoff generated by the site, and to establish a management regime for the expected runoff. If not managed properly, surface water runoff from the site could potentially lead to increases in flood risk to other areas or the development itself. The minimum requirements for a Flood Risk Assessment are set out within Annex E of PPS25. These can be summarised as: •

Consider risk of flooding both from and to a development

Consider the effects of parts of the flood risk management infrastructure including raised defences, flow channels, flood storage areas and other artificial features together with the consequences of their failure

Consider the vulnerability of those that could occupy the development, taking account of the sequential and exception tests and the vulnerability classification including safe access

Consider and quantify the different types of flooding and identify flood risk reduction measures so that assessments are fit for the purpose of the decisions being made.

Consider the effects of a range of flooding events including extreme events

Include an assessment of the remaining risk (residual) after risk reduction measures have been taken into account

Consider how the ability of water to soak into the ground may change with development, along with how the proposed layout of development may affect drainage systems

Consider the effects of climate change4

From Table D1 of PPS25, the policy aims in Flood Zone 1 are;

For the developer and the local planning authority to seek opportunities to reduce the overall level of flood risk in the area and beyond through the layout and form of the development, and the appropriate application of sustainable drainage techniques.

2

3 4

Planning Policy Statement 25, Development and Flood Risk, published by the Department of Communities and Local Government, December 2006. http://www.environment-agency.gov.uk/research/planning/82584.aspx Annex E, Planning Policy Statement 25, Development and Flood Risk, published by the Department of Communities and Local Government, December 2006.

10

Liverpool Water Flood Risk Assessment


From Table D1 of PPS25, the policy aims in Flood Zone 2 are;

For the developers and local authorities should seek opportunities to reduce the overall level of flood risk in the area through the layout and form of the development, and the appropriate application of sustainable drainage techniques.

From Table D1 of PPS25, the policy aims in Flood Zone 3 are for the developer and the local planning authority to seek opportunities to:

1. Reduce the overall level of flood risk in the area through the layout and form of the development and the appropriate application of sustainable drainage techniques’; 2. Relocate existing development to land in zones with a lower probability of flooding; and

3. Create space for flooding to occur by restoring functional floodplain and flood flow pathways and by identifying, allocating and safeguarding open space for flood storage.5 In line with PPS25, this FRA will also consider other sources of flood risk, such as sewers, overland flow routes, groundwater flooding, infrastructure failures associated with ‘artificial flood sources’ and minor watercourses not shown on EA flood map. An FRA also considers how the development and adjacent properties will be affected by the uncertainty brought about by climate change during the development’s design life.

1.3

REPORT COMPLIANCE WITH PPPS25

To ensure that this report is in compliance with the requirements of PPS25, it was checked against the PPS25 Practice Guide pro-forma, which is included as Appendix A.

5

Planning Policy Statement 25, Development and Flood Risk, published by the Department of Communities and Local Government, December 2006.

11

Liverpool Water Flood Risk Assessment


2

SITE DESCRIPTION

2.1

EXISTING SITE

The site lies immediately to the North-West of inner Liverpool City Centre, along a 2.2km stretch of the Eastern bank of the Mersey Estuary, immediately North of the Royal Liver Building. The overall red line boundary is approximately 40 hectares with the Eastern boundary defined by the back of Highway associated with Regent Road (A5036), Waterloo Road and Bath Street, with the exception of East Waterloo Dock. The site is comprised of several docks relating to the city’s historical ship-building industry, being; Bramley Moore, Nelson, Salisbury, Collingwood, Trafalgar, West Waterloo, Princes-Half-Tide and Princes Dock, as shown below in Figures 1 and 26. The ferry terminal serving Douglas and Dublin provides the Western boundary of Princes Dock at the South, whilst the Kingsway Tunnel passes East/West beneath the site, under West Waterloo Dock. Approximately 28.5Ha within the main red-line planning boundary is existing recent redevelopment or existing retained dock waters. The approximate OS grid reference number at the development centre is SJ335915. A small parcel of the proposed development enclosed within Roberts Street, King Edwards Street and Bath Street is separated from the main development parcel by Bath Street. This parcel is irregular in shape and approximately 2.4Ha. All the docks, from Gladstone Dock in the North, down to Canning Docks in the South appear to be interlinked. Sluice gates are present to a number of these docks, allowing possible egress to the River Mersey. ‘Princes Half-tide dock’, ‘Salisbury Dock’, ‘Sandon Half-tide Dock’, ‘Brocklebank Dock’ and ‘Glandon Dock’ all have sluice gates out to the Mersey, with the Brocklebank Dock providing access for the main ferry terminal to Dublin and Douglas. Meanwhile, Collingwood Dock’s interconnectivity with Stanley Dock provides a link between the docks and the commencement of the Leeds/Liverpool Canal.

It was not

apparent during site visits as to which of the sluice gates (other than the main Brocklebank ferry terminal) are still operational. Recent redevelopment has taken place to the docks, completed early 2009 which is not reflected on the OS maps. A schematic of this redevelopment has been sourced from British Waterways and has been included within appendix B.

6

It is understood that this redevelopment in effect provided an extension to the

Crown Copyright 2009. All rights reserved. OS mapping generated using Promap (www.promap.co.uk), Licence Number 100020449. OS mapping reproduced under White Young Green Consulting Paper Map Reproduction Licence Number 100017603.

12

Liverpool Water Flood Risk Assessment


Leeds/Liverpool Canal, allowing safer passage for canal traffic through Salisbury and Trafalgar Docks. A large section of Trafalgar Dock and West Waterloo Dock have been in-filled with the exception of the creation of a new narrow 6.5m canal link from Trafalgar Dock to West Waterloo and Princes Docks. The extension of the canal links the aforementioned Docks to a modified Canning Dock to the South of the development area, passing between the River Mersey and ‘The Three Graces’. This new link sees the introduction of a new culverted section of canal beneath St Nicholas’ Place and beneath the access to the new cruise liner facility, with open basins on the Pier Head in front of the Liver Building and Port of Liverpool Building. The link enters a new basin at Mann Island before terminating at Canning Dock7. It was also apparent during the site visit that the ‘Seacombe’ Ferry Terminal has been upgraded and re-situated during recent redevelopment. As part of these regeneration works a new isolation structure has been constructed between Bramley Moore Dock and Nelson Dock and a new fixed bridge and lock installed at Princes Dock. A further lock has also been installed between the Mann Island basin and the modified Canning Docks. It is understood that these new controls have been introduced to assist navigation of the waters by canal boats, by preventing tidal influences from the northern and southern situated docks and thus control water levels within these docks between minimum and maximum operational water levels, thought to be 4.55 and 5.16m AOD respectively8. Existing levels to the top of dock structures vary between approximately 6.14m AOD and 9m AOD. Bramley Moore Docks in the North, down to Salisbury and Collingwood Docks typically have little variation in existing dock levels, at approximately 6.6m AOD, occasionally rising to meet levels of 7.0m AOD or 7.5m AOD at the Eastern boundary with Regent Road. Trafalgar Dock, West and East Waterloo Docks and Princes Half-Tide Docks have slightly greater variation in levels, ranging from approximately 6.3m AOD to a maximum of 8.5m AOD. Exceptions to this are a minor area of recently in-filled West Waterloo Docks at 6.14 AOD and the area associated with Clarence Graving docks, surveyed at 4.81m AOD. The existing levels to Princes Dock vary from 6.3m AOD, adjacent to St Nicholas’ Place to a maximum level approximate to 9.0m AOD adjacent to ‘New Quay’9.

7 8 9

British Waterways canal regeneration layout, Appendix B. General arrangement drawing L24298/01/2102D, of isolation structure, provided by British Waterways Topographical survey drawing provided by Peel Land & Property Ports.

13

Liverpool Water Flood Risk Assessment


The smaller development parcel enclosed between Roberts Street, King Edward Street and Bath Street slopes steeply from North-East to South-West from the King Edward Street boundary (approximately 17.1m AOD) to the Bath Street boundary (approximately 7.0m AOD).10

10

Topographical survey drawing provided by Peel Land & Property Ports.

14

Liverpool Water Flood Risk Assessment


Figure 1 - Site Location â&#x20AC;&#x201C; Liverpool Waters, Liverpool City Centre Crown copyright 2009. All rights reserved. Licence number 100020449.

15

Liverpool Water Flood Risk Assessment


Figure 2 - Site Location â&#x20AC;&#x201C; Liverpool Waters, Liverpool City Centre Crown copyright 2009. All rights reserved. Licence number 100020449.

16

Liverpool Water Flood Risk Assessment


Existing site plans and development plans are included within Appendix B. Photographs of the site have been included within Appendix C.

2.1.1

Existing Site Drainage

The docks provide the outfall route of seven combined sewer overflows into the River Mersey from the main city centre sewers. A schematic by Northwest Water Ltd of the flood alleviation scheme denoting the location of these outfalls and the associated control buildings has been obtained via Mersey Docks and is included within Appendix B. Outfalls ‘L1 Northern’, ‘L1A Bankhall Relief’, ‘L2 Bankhall’, ‘L3 Sandhills Lane’ and ‘L4 Beacons Gutter’ are all situated to the North of the proposed development area. Outfall ‘L8 Gower Street/Kings Dock meanwhile is situated some distance to the South of the development.11 Combined sewer overflow ‘L6 St Nicholas’ passes a short distance South of the southern site boundary, taking surcharged flows that back up from the main combined sewer within ‘New Quay’ and from the local combined sewers that connect upstream. Surcharge flows from the network pass to a 2400mm surface water outfall via a combined sewer overflow structure, situated South of St Nicholas Place. Northwest Water’s schematics suggest that this structure also acts as an outfall penstock chamber operated from the adjacent control building and controlling the overflow combined flows that enter the Mersey. The combined sewer overflow structure/overflow penstock chamber is represented on the sewer records sheet SJ3390SE (within Appendix B) as node 8410, whilst the control building is represented as node 8414, with both features seemingly adjoining the advised southern development boundary.12 Outfall ’L5 Battery Lane’ consists of two separate combined overflow sewers that pass between the Collingwood Dock and Clarence Graving Docks. Both these sewers provide an overflow from the ‘combined sewer overflow structure’ within Regent Road (denoted as node 6910 on United Utilities sewer records drawing SJ3391NE) once the main 2440mm sewer is surcharged. These 1200mm diameter and 1520x 920mm sewers pass East to West to a position just South of Salisbury Dock before converging and turning South-West beneath Trafalgar Dock. The discharge of combined flows into the Mersey from these overflows is controlled by an ‘Overflow penstock chamber’ located beneath the dock wall and denoted on the sewer records drawing SJ3391NW as node 3901.13

11 12 13

North West Water Limited’s Flood Alleviation drawings and location plans-See appendix B United Utilities sewer record drawings. Appendix B – (Printed 8th April 2009) North West Water Limited’s Flood Alleviation drawings and location plans-See appendix B

17

Liverpool Water Flood Risk Assessment


This penstock chamber also provides a surface water outfall location point from the Clarence graving yards’ dry-dock pumping station that although was full during the site visit appears to have been pumped dry during the date of the topographical survey.14 North West Water’s schematic drawings denote the presence of a control building for this penstock chamber, though no such feature is referenced on United Utilities public sewer records. However, the control building for the combined sewer overflow structure within Regent Road is shown on the public sewer records to be a sizeable building and associated access within the development boundary (node 6912)15. No on-site survey has been undertaken into the existing drainage arrangement and no detailed topographical survey was available for comment as part of this assessment. However, given the nature and the location of the dock structures adjacent to the Mersey, it is believed likely that surface water run-off from the majority of these surfaces pass over-ground directly into the docks and the Mersey or via gullies to the docks. Existing buildings are present around the perimeters of East Waterloo Dock and Princes Docks and to the East of the in-filled Trafalgar Dock, though again no public sewers are shown to be present on the drainage records and no on-site investigation has been carried out to determine whether the surface water from these buildings outfall to the public sewers or directly to the docks or to the Mersey itself. No visible signs were present during the site visit or within the information set as to any existing attenuation. It is therefore assumed that run-off from the existing development discharges freely, without restriction. The surface water from the smaller development parcel, between King Edward Street, Bath Street and Roberts Street appears to drain to the public sewers as United Utilities sewer records show small diameter combined public sewers within the parcel boundary. A 300mm diameter combined sewer is shown beneath the existing southern access connecting into the Bath Street sewer, whilst a 300mm diameter combined sewer is shown beneath the existing northern access connecting into the Robert’s Street sewer. A further 225mm diameter combined sewer at the North-East of this parcel into Roberts Street and a 1500x950mm combined sewer into Waterloo Road may also provide outfall to this parcel, though an on-site drainage investigation would be required to determine such outfalls. The development area is covered by Liverpool City Council’s Strategic Flood Risk Assessment dated January 2008. This document collates a good level of information as to any historical flooding events or known

14 15

United Utilities sewer record drawings. Appendix B – (Printed 8th April 2009) United Utilities sewer record drawings. Appendix B – (Printed 8th April 2009)

18

Liverpool Water Flood Risk Assessment


potential risks within the region, providing collation and alignment from the findings, guidance and policies within: 1. Mersey Estuary Draft Catchment Flood Management Plan (March 2007) 2. Liverpool Bay Shoreline Management Plan (1999): Sub Cell 11a – Great Ormes Head to Formby Point. 3. Adopted Regional Spatial Strategy (RSS) for the North West (2003) 4. Draft Regional Spatial Strategy (RSS) for the North West (2006) 5. ‘National Tidal and Sea Level Facility’/ Proudman Oceanographic Laboratory – Natural Environment Research Council which although a national resource is based within Liverpool (tidal levels for Liverpool taken at Gladstone Dock).16 No Level 2 report specific to Liverpool City Centre was available at the date of this report, as it is believed that the SFRA report of January 2008 is all encompassing. Existing surface water run-off from the overall development area has been estimated at 3640 l/s using Micro-Drainage ‘System 1’ software and based upon 36Ha of redevelopable area.

2.1.2

Wider area drainage

As described within the previous chapter, major public sewers are present within the vicinity of the development. The main Waste Water Treatment Works for Liverpool City centre is located 150m directly North of the development boundary, taking combined flows from the large diameter combined sewers that run Northwards up Regent Road as they pass the site. A combined sewer across Sandon Half Tide Dock provides an outfall/possible emergency overflow out of the treatment works to the mid-channel of the River Mersey, controlled by an outfall penstock chamber beneath the dock wall. Several other emergency combined overflows are present along the waterfront taking surcharge flows from the main sewers within New Quay, Waterloo Road and Regent Road to outfalls in the Mersey.17 Each combined sewer overflow is controlled by the use of outfall penstock chambers, whose locations are as denoted on North West Water’s

16 17

Pages 14 and 28 Liverpool City Council’s SFRA, January 2008 United Utilities sewer record drawings. Appendix B – (Printed 8th April 2009)

19

Liverpool Water Flood Risk Assessment


plans within appendix B, as are the associated control buildings. The historical frequency of use of these combined sewer overflows has not been advised by United Utilities. Map 2B within Liverpool City Councils Strategic Flood Risk Assessment denotes the routes of several ordinary watercourses through Liverpool City centre. An ordinary watercourse named ‘Pool River’ is denoted to route from the A59 Byron Street (some 1km to the East of the development) flowing South-West to a position close to Albert Dock and Salthouse Dock (approximately 750m South of the southern development boundary.18 A further watercourse named ‘Beacons Gutter’ is shown passing from East to West beneath Regents Road, approximately 150m North of the northern development boundary as it passes to the West of Regent Road towards the ‘Sandon Dock Waste Water Treatment Works’.

Liverpool City Councils

strategic flood risk assessment states that both ‘Beacons Gutter’ and ‘Pool River’ have been incorporated

into the sewerage system and can no longer be identified as arterial watercourses.19 It is therefore believed that the ‘Beacon’s Gutter’ combined sewer overflow is named after the former watercourse that previously passed via this route. Meanwhile, the public sewers denoted to connect via the ‘St Nicholas’ combined sewer outfall to the South of the development is thought to be along a similar route to the now nonexistent ‘River Pool’.

2.2

FLOOD RISK

2.2.1

Broad scale appraisal of flood risk

According to flood mapping produced by the Environment Agency (EA), the majority of the site (typically the East of the larger parcel and also the smaller King Edward Street/Bath Street parcel) falls within Flood Zone 1 (thus outside the extent of the 0.1% (1 in 1000) risk of flooding from either the sea or river flows), whilst a band of the Western boundary is shown as Flood Zone 3 and thus deemed as having greater than a 1% probability of river flooding in any given year or greater than 0.5% (1 in 200 year) probability of sea flooding in any given year. A small area of the site associated with Trafalgar Dock is shown as Flood Zone 2, thus deemed by the EA as having between 1% (1 in 100 year) risk of flooding and 0.1% (1 in 1000) risk of flooding from either the sea or river flows20. Therefore, based upon broad scale mapping alone, the site is considered to have areas of ‘low’, ‘medium’ and ‘high risk’ potential of flooding from tidal or fluvial sources. An extract of the Environment Agency’s flood risk map is shown within Figure 2, with more detailed maps within appendix B.

18 19 20

Map 2B, within appendices of Liverpool City Council’s SFRA of January 2008. Copy of same within appendix B Page 26, Liverpool City Council, Strategic Flood Risk Assessment, 2008 Environment Agency Flood risk map (downloaded 21.09.09) and EA’s ‘Basic FRA maps’ within Appendix B.

20

Liverpool Water Flood Risk Assessment


Regent Road, A5036

Waterloo Road

Site Location Trafalgar Dock

Kingsway Tunnel

Figure 2 â&#x20AC;&#x201C; Extract of EA flood risk map (Downloaded 21/09/2009)

KEY Approx Site Boundary.

Flood Zone 1, Low risk from river or sea flooding.

Flood Zone 2, Moderate risk from river or sea flooding.

Flood Zone 3a, High risk from river or sea flooding.

*More detailed versions of the broad scale flood maps are within appendix B

21

Liverpool Water Flood Risk Assessment


2.2.2

More detailed appraisal of flood risk

The Environment Agency’s broad scale mapping shows the site to be located within Flood Zones 1, 2 and 3, thus a preliminary assessment would suggest that the site is predominantly at low and medium risk of flooding from both tidal and fluvial sources, with a small area of Trafalgar Docks being at high risk of flooding.21 Liverpool’s Strategic Flood Risk Assessment, also denotes these areas as ‘High probability Area, 3a’.22 An Extract from the ‘Draft Mersey Estuary Management Plan (policy unit 11) states that ‘There are

approximately 373 properties and 28 commercial properties at risk of flooding with flood risk areas lying in the North of the unit and also near the waterfront. However, there is little history of flooding in this policy unit, and current flood risk management is thought to be appropriate in this heavily urbanised and industrialised unit.’ One tidal flood incident is reported within Liverpool City Council’s SFRA;

‘The only recording of flooding along the length of the Mersey within Liverpool is at Pier Head. This is reportedly due to exceptionally high tides backed up by strong winds from the West, causing the river to overtop the river wall at Pier Head (In the vicinity of the Royal Liver Building - immediately South of the development area). A small amount of water is reported to cover a restricted area of the Pier Head

promenade lasting for approximately 1 – 2 hours. Due to the frequency of this occurrence and the location, an emergency plan of sandbagging is in place’. ‘Additional information indicates that the docks are not designed to fulfill a flood defence role, with any such role being incidental. This information also indicates that in the event of extreme surges, flood waters can enter the port via lock gates at Gladstone (Approximately some 3km North of the development – downstream) so flood defence is ultimately provided by the east quays rather than the river wall’. The river walls to the Mersey however are currently considered to be in good condition.23 The EA’s mapping in this area is based on its broad scale national mapping programme, and is therefore not suitable alone for a detailed appraisal of flood risk for a specific development site. More detailed flood maps and Mersey levels have been acquired from the EA’s flood risk mapping and data management

21

22 23

Table D1, Planning Policy Statement 25, Development and Flood Risk, published by the Department of Communities and Local Government Map 5, Liverpool City Council, Strategic Flood Risk Assessment, 2008 Section 5.64, Liverpool City Council’s Strategic Flood Risk Assessment, January 2008

22

Liverpool Water Flood Risk Assessment


department in response to a pre-planning enquiry, a copy of which is in Appendix D. Maps 4A, 4B and 5 within Liverpool City Councils SFRA of January 2008 denotes an identical flood profile to that provided directly in the Environment Agency’s response to the pre-planning enquiry. As stated within the ‘list of maps’ on page 2 of the SFRA, the information source for these flood maps is the same Environment Agency flood risk zones. These maps and sea levels are sourced from their recent ‘Extreme Sea Level Study’ of 2008 with data provided from the gauge points at Liverpool gauge, grid reference 332480,395240 and South Crosby gauge at grid reference 331140,398269 and have the effects of surge already factored in.24 The closer of the two data-sets is the Liverpool Gauge data, giving river levels for the 1 in 100 year, 1 in 200 year and 1 in 1000 year events (including the effects of surge) as 6.0m, 6.11m and 6.37m AOD respectively. Directly comparing the 1 in 200 year Mersey levels of 6.11m AOD against the topographical survey, all dock structures can be seen to be above this level (typically 6.14m AOD and above) with the exception of the Clarence Graving Docks, (on occasion when the water has been pumped out from this basin area). Assuming that the topographical survey levels are correct, the dock structures adjacent to the River Mersey should therefore not be indicated as Flood Zone 3. The only exception to this is the non-developable areas of open water within the docks, localised reductions for the operation of bridge and gate control mechanisms and the ‘Clarence Graving docks’, located centrally within the main site parcel, that have been artificially lowered. This is subject to a more detailed topographical survey to determine that there are no small pockets of land below the 6.11m AOD level, such as post-construction levels within the recently infilled areas of Trafalgar Docks, which appears to have been completed following the original topographical survey found in Appendix B. Comparing the topographical survey with the 1 in 1000 year event of 6.37m AOD, the structures to Bramley Moore, Nelson, Salisbury, Collingwood Docks to the North are all above this level at 6.5m AOD upwards, whilst the sea/dock walls ‘protecting’ the development area are typically between a level of 6.53m AOD to 9.31m AOD. An existing car park area serving the depot adjacent to Trafalgar Dock, infilled areas of West Waterloo Dock and a minor area of Highway on the inside turn of St Nicholas Place situated to the South of the development boundary are nominally below the 1 in 1000 year event level. The minor area of Highway within St Nicholas Place (approx 6.28m AOD at its lowest point) is isolated from the Mersey by surrounding ground level of 6.40m AOD upwards and would therefore not flood should the Mersey reach the 1 in 1000 year level. It is apparent that without the presence of the dock walls, the aforementioned areas of West

24

Environment Agency River level plans- Appendix D

23

Liverpool Water Flood Risk Assessment


Waterloo dock and the depot car park could fall below the 1 in 1000 year (0.1%) Mersey Level, thus be at risk of flooding for events up to the 1 in 1000 year, thus Flood Zone 2. The extent that these areas are protected from tidal and surge influences of the Mersey are subject to the condition of the existing dock walls and availability of other migration routes of the Mersey to these areas, such as faulty sluice gates. This finding is also based upon the original topographical survey currently available, which is believed to have been carried out either prior to or during the redevelopment works to the docks. Visual inspection of this area suggested that the area of infilled West Waterloo Docks may have been raised above the 1 in 1000 year level during these works. It is therefore recommended that the topographical survey is revised locally to include the post-redevelopment levels, to concur with the visual interpretation that levels are in fact higher than the 1 in 1000 year threshold. This should confirm our assertion that the whole development area is located within Flood Zone 1. An up-to-date topographical survey has been commissioned local to this area and is currently awaited to confirm this assertion. It is assumed that the remaining profile of the body of water within Trafalgar Dock is shown as Flood Zone 2 on the EA’s mapping is the result of the modelling data having picked up the dock water level or bed level of this feature. All local structures surrounding this feature have levels that are significantly higher than this level at an approximate minimum of 6.80m AOD, whilst the flood maps exclusion of linked water features as Flood Zone 2 supports this conclusion. Therefore, it is believed again that this level should not be classified as Flood Zone 2 and should actually be designated as Flood Zone 1. Scoping opinion feedback based on the draft version of this FRA was received from the Environment Agency on 31st March 2010 and later on 15th April 2010 (effectively superseding the comments in the former letter). The main item in this correspondence called for an assessment of the affects of extreme wave / tidal overtopping height as well as the assessment of extreme sea level already performed. This data was finally made available for assessment on 29th July 201025. Unfortunately, this data didn’t really assist in clarifying the affects of wave overtopping as the model only indicated the word ‘overflow’ whenever the dock walls were breached and therefore doesn’t quantify maximum wave / water levels. This generally only occurred at isolated locations when assessing the most onerous storms such as 1/1000 year tidal levels or 1/200 year tidal events plus 100 year sea level rise. This issue was discussed in a meeting with the Environment Agency on 11th August 2010 and a strategy / way forward to deal with this gap in the assessment was agreed (see later).

25

Coastal Erosion Mapping – Liverpool Model Criteria Report Ref 9V6699/R/303018/PBor, 14 May 2010, Haskoning UK Ltd Coastal & Rivers

24

Liverpool Water Flood Risk Assessment


No levels have been provided on the topographical survey for the water levels or bed levels within the docks. The drawing of the new control structure between Bramley Moore Dock and Nelson Dock denotes operational minimum and maximum water levels of 4.55m and 5.16m AOD respectively.26 Assuming these levels to be indicative of the water level between Nelson Dock and Princes Dock, it is therefore considered that these areas would only flood during such storm events occurring during infrastructure failure of the dock walls. As mentioned within 2.1.2, the Strategic Flood Risk Assessment for the Liverpool area shows an ordinary watercourse, ‘Beacon’s Gutter’, terminating to the North of the site (in proximity to the Sandon Dock Wastewater Treatment Works) also River Pool, terminating to the South of the site in the vicinity of Albert Dock.27 No flood events have been advised by the Local Authority or the Environment Agency relating to these watercourses. Tables 6 and 7 within Liverpool City Council’s SFRA lists known historical fluvial flooding events, based upon the 1992 and 1989 watercourse flood alleviation studies. The watercourses in the vicinity of the development area were investigated as part of the 1989 study, as shown within Map 2B (See appendix B) with known flooding listed within Table 7. No known flooding is listed against either the River Pool or Beacon’s Gutter. However, these watercourses are described within the SFRA to no longer exist, having historically being replaced by the construction of the public sewerage system. On this basis, any recorded flood events in the vicinity of the site, relating to these former watercourse routes will appear as sewer flooding, appraised within the following section 2.2.3. An overview within the document regarding all fluvial flooding within the Liverpool area states that ‘The

1989 study indicates that at many locations, flooding is experienced on a number of occasions annually and in all the situations identified by this study, inundation occurs at a mean interval of 5 years of less’, but also goes on to state that ‘Since the 1967, 1988, 1989 and 1992 studies, measures have been put in place that

have solved or at least partially solved the source of flooding.’28

2.2.3

Other sources of flood risk

The main public sewers for the city centre pass along an interceptor sewer just outside the Eastern boundary, north-wards towards the Wastewater Treatment Works. Under normal conditions these would be treated as combined fouls within the Treatment Works. When the capacity of these sewers is surpassed, exceedence flows pass via the combined sewer overflow chambers to piped outfall sewers that discharge to

26 27 28

Isolation structure general arrangement drawing L24298/01/2101D provided by British Waterways Map 2B - Liverpool City Council, Strategic Flood Risk Assessment, Jan 2008 Page 34, Liverpool City Council, Strategic Flood Risk Assessment, Jan 2008

25

Liverpool Water Flood Risk Assessment


the River Mersey. CSO outfall L4 ‘Beacons Gutter’ is approximately 150m North (thus downstream) of the development area passing beneath Wellington Dock, whilst the combined sewer overflow ‘L6’ (St Nicholas) relating to the former River Pool is believed to route through the vicinity of Canning Place and Albert Dock, some 150m South-East of the site and upstream of the development. The potential risk of flooding from the capacity of these sewers being exceeded must be considered. Correspondence received from United Utilities (Appendix D) shows that they are aware of two public sewers flooding properties in the vicinity on their ‘at risk register’ as a result of overloaded sewers. The locations of these sewers are in St Nicholas Place (on the southern boundary of the site) and St John’s Lane (inland beyond the influence of the site). They also cannot rule out instances that have gone unrecorded or instances that have not qualified for inclusion on the register. It is anticipated that the flooding location on St Nicholas Street is the isolated low spot referred to earlier (6.28mAOD to 6.4mAOD). With the proposed minimum finished floor levels proposed later, any flooding of the same sewer in the future is only likely to route along (and be contained within) St Nicholas Place without compromising any plot buildings. Thus this at risk sewer is considered to be low risk of flooding the proposed development. We can also confirm that UU reviewed the draft version of this FRA and confirmed it was acceptable in principal. Copies of all correspondence with consultees are included in Appendix D. The SFRA for the Liverpool area collates the same data from United Utilities ‘DG5, at risk register’ and summarises the findings. The postcode associated with the development area is identified as having one property affected by flooding during the study period29. No further information is provided as to the location or size of this property in relation to the development boundary or the source or failure that led to the flooding. Liverpool City Councils’ Highway department are contributors to the SFRA. Table 8 in this SFRA provides a descriptive of the known Highway flooding events from a range of sources30. No Highway within or within immediate vicinity of the development is highlighted within this table. As per flooding to the public sewerage system, it cannot be ruled out that flooding incidents have gone unrecorded or not included within the register given that this data has been collated primarily for Highway maintenance purposes. The results of a culvert study within Liverpool City centre are included within section 5.48 of the SFRA. As described within 2.2.2 above, Beacon’s Gutter and River Pool are both believed to have ceased to exist as

29 30

Map 9 – Liverpool City Council, Strategic Flood Risk Assessment, Jan 2008 Table 8, Page 35, Section 5.4.4 Liverpool City Council’s Strategic Flood Risk Assessment, Jan 2008

26

Liverpool Water Flood Risk Assessment


watercourses. No other watercourses are known to pass within the vicinity of the site, neither are there any culverts listed within the table as poor condition.31 Ground water flooding can be summarised as flooding caused by the discharge of ground water held within aquifers or permeable underground strata. This is considered to be an increasing issue within Liverpool City centre, where past industrial requirements for the abstraction of groundwater have decreased, thus impacting on the water table. The main areas of water level increase are documented to be within the city centre and the tunnels beneath the city.32 Liverpool City Councils SFRA states that although historic groundwater abstraction has ceased, there is no known documented evidence of flooding from groundwater in the Mersey Estuary catchment, nor is there an increased risk of flooding.33 This statement follows the findings within the Mersey Estuary Catchment Flood Management Plan. Given and above and the likely elevation between the proposed buildings above the standing water levels within the docks, the risk of groundwater flooding is considered to be low risk. It is however recommended that ground water monitoring is undertaken within the site itself to establish any potential risk. The flood risk from infrastructure failures such as canals and sluice controls has been considered as part of the assessment. The Leeds/Liverpool canal adjoins the docks at Collingwood Dock (beneath Bascule Bridge). From this position the canal passes through several locks to higher topographical grounds to the East of the development. The open water relating to Collingwood Dock and Salisbury Dock are not redeveloped as part of the development scheme. It is therefore considered that there is a low risk of any canal failure surcharging the lower-lying water within the docks such as to threaten the thresholds of the proposed buildings. No other significant sources of flood risk to the site itself have been identified.

2.2.4

Summary of flood risk

The EA flood maps indicate that the site has areas within Flood Zones 1, 2 and 3a. This is based on the EAâ&#x20AC;&#x2122;s broad scale mapping and not always fully reflective of actual topographical levels, but the more detailed appraisal undertaken by WYG (as documented above) has concluded that this does not provided a true reflection of the site flood zoning. Closer inspection of the topographical survey data available,

31 32 33

Section 5.48â&#x20AC;&#x201C; Liverpool City Council, Strategic Flood Risk Assessment, Jan 2008 Section 5.60 Liverpool City Council, Strategic Flood Risk Assessment, Jan 2008 Sections 5.59- 5.63 Liverpool City Council, Strategic Flood Risk Assessment, Jan 2008

27

Liverpool Water Flood Risk Assessment


combined with the EA’s modelled Mersey flood levels, would suggest that the site should be classified as fully located within Flood Zone 1 and therefore at low risk of flooding from tidal/fluvial sources.

This

assumes that the small area of the recently infilled West Wellington Dock and existing adjacent car park is above the 1 in 1000 year tidal level. We have concluded this from visual inspection to date, as the topographical survey appears to have been performed prior to completion of the infilling of West Waterloo Dock and the creation of the 6.5m canal channel. It is therefore recommended that the topographical survey is updated to confirm the same and is currently being performed / awaited. An assessment of the affects of extreme wave / tidal overtopping height was performed as well as the assessment of extreme sea level data. Unfortunately, this data didn’t really assist in clarifying the affects of wave overtopping as the model only indicated the word ‘overflow’ whenever the dock walls were breached and therefore doesn’t quantify maximum wave / water levels. Agreement was achieved with the EA on the strategy for dealing with this issue in the future and will be described in detail later. No known flood events have been advised by the Local Authority or the Environment Agency relating to watercourses. Correspondence received from United Utilities (Appendix D) shows that they are aware of 2 no. public sewers flooding properties in the vicinity on their ‘at risk register’ as a result of overloaded sewers. They also can not rule out instances that have gone unrecorded or instances that have not qualified for inclusion on the register. The location of the one sewer with some influence over the site is along St Nicholas Place along the southern boundary of the site. However, this location is the lowest point of the development and is considered to be low risk of flooding the proposed development when the minimum proposed floor levels determined later are considered. Liverpool City Councils Highways department records are a data source for the SFRA. No Highway within or within immediate vicinity of the development is highlighted within this data as having caused historical flooding. As per the records of public sewers flooding properties, it can not be ruled out that incidence have gone unrecorded within this data set. Extracts from the Mersey Estuary Catchment Flood Management Plan confirm that there is no known documented evidence of flooding from groundwater in the Mersey Estuary CFMP catchment area. The flood risk to the site from canal infrastructure failures is also considered to be low.

28

Liverpool Water Flood Risk Assessment


3

DEVELOPMENT PROPOSALS

3.1

PROPOSED DEVELOPMENT

Peel Land and Property (Ports) Ltd is looking to redevelop approximately 36Ha area of derelict docklands that are historically related to Liverpool’s ship-building heritage. The redevelopment will compliment the existing city centre by retaining the open areas of water associated with the docks and maintain their function for canal traffic whilst consisting of residential, offices and a mix of ancillary uses. Layouts showing the existing and proposed site are included within Appendix B. One of the aims of PPS 25 is to steer development away from zones of high flood risk towards Flood Zone 1, the area of lowest flood risk. It has been demonstrated above that this application site falls within Flood Zone 1 and therefore, from Table D3 of PPS25 (Flood Risk Vulnerability and Flood Zone ‘Compatibility’), the ‘more vulnerable residential usage’ and ‘less vulnerable office usage’ proposed for this development is considered to be suitable for the site34. The development is therefore considered to pass the PPS25 Sequential Test by steering a suitably classified development to Flood Zone 1. The Draft Practice Guidance to PPS25 (2007) states that the developer is not required to apply the Sequential Test if a proposed development is located on a site which has been allocated for that type of development in a LDD that has been sequentially tested and supported by a SFRA. Liverpool City Council’s Strategic Flood Risk Assessment of January 2008 appends Map 12 which provides an overlay between Liverpool City Council’s Unitary Development Plan against flood risk35. The development is designated within the ‘Strategic Investment Area’, whilst the northern half of the proposed development area down to ‘Central Dock Channel Bridge’ and the North of ‘Waterloo Quay’ is designated as ‘unimplemented UDP mixed use allocation’. Smaller pockets within the southern development area that appear to correlate with the development proposals are also denoted as ‘unimplemented UDP mixed use allocation’ However, the smaller development parcel enclosed within King Edward Street, Bath Street and Roberts Street is within the ‘strategic investment area’ though is unallocated. The SFRA highlights potential flood risks within Liverpool City centre including listing the Eastern boundary of the site as ‘High Probability Area 3A’.

34 35

Refer to Table D2 and Table D3 of PPS25 (Flood Risk Vulnerability and Flood Zone ‘Compatibility’) Map 12 – Liverpool City Council, Strategic Flood Risk Assessment, Jan 2008

29

Liverpool Water Flood Risk Assessment


Specific guidance is provided within Liverpool City Councils SFRA regarding the both these development areas;

Sites

for

Various

Types

of

Development

Clarence

Dock/Trafalgar

Dock

(Site

M54)

and

Stanley/Salisbury/Collingwood Docks (Site M65) – the edge of this site is within Flood Risk Zones 2 and 3. As a major development site, consideration should be given to the design of the development and how this relates to the flood risk. Different types of development within the overall development may be more acceptable than others in line with Table D.2 of PPS25 – Flood Risk Vulnerability Classification. To comply with PPS25, the exception test would therefore only need to be met for some types of development. For example residential development (on the whole) would need to meet the exception test whereas buildings for shops may not need to and basement dwellings would not be acceptable at all. In terms of meeting the exception test, this area should be in general conformity due to it being a large area of brownfield land a key location in terms of regenerating the city. However the specific details of any proposal would need to be considered against the exception test, considering for example transport links in terms of creating a sustainable community. A flood risk assessment will therefore be required for this site, including in particular minimum floor levels. Two small sites – part of a large allocation – Sites for Various Types of Development – Princes Dock Site M3) remain unimplemented. These two sites are immediately on the waterfront and therefore floodrisk is an important consideration. As with sites M54 and M65 above, various types of development may be more acceptable than others with the exception test applying to certain types of development. With regards meeting the exception test, the location of these sites in closer proximity to the City Centre than M54/M65 ensures it is a sustainable location and again is on brownfield land. A flood risk assessment will be required for this site and it is essential that buildings comply with a minimum floor level (through advice from the Environment Agency). As stated within Section 2.2.2 of this report, none of the development area should be classified as Flood Zone 3 and should be reclassified as Flood Zone 1, subject to confirmation by topographical survey that the recently infilled area of West Waterloo Dock is above the 1 in 1000 year Mersey level. The proposed development mix will be residential, office and ‘mix of ancillary uses’. From Table D.2 of PPS 25, residential developments are classified as ‘more vulnerable’, whilst offices are classified as ‘less

30

Liverpool Water Flood Risk Assessment


vulnerable’.36 The vulnerability classification of the ancillary uses is likely to be classified as either ‘more vulnerable’ or ‘less vulnerable’. On this basis, an exception test and a sequential test would not be required as the development is considered to have passed the PPS25 Sequential Test as having steered a suitably classified development to Flood Zone 1. Liverpool City Council’s SFRA states that; The main flood risk issue in the city-centre is due to the River

Mersey. Special requirements have already been implemented in this area to take this risk into account. A minimum floor level is required for developments along the coastline (within flood risk zones 2 and 3) taking into account extreme tide levels and the impact of climate change over the lifetime of the development.

3.2

DEVELOPMENT AND FLOOD RISK

3.2.1

Flood risk to the site

As discussed in Section 2.2, the main flood risk to the site is from the River Mersey. The existing site is considered to be at low risk of fluvial flooding when direct comparison is made between the 1 in 200 year Mersey Levels (including surge) and the existing site levels. However, in developing the site for office and residential uses, the effects of climate change must be taken into consideration when setting development levels, providing an allowance based upon the design life of the development. None of the consultees are aware of any fluvial flood risk to the development from watercourses other than the Mersey. The Flood Risk Assessment (FRA) concludes that the proposed development is considered to be located entirely within Flood Zone 1 and is thus suitable for the proposed development use. By definition, there will therefore be no loss of functional floodplain to the River Mersey in either the construction or permanent phase. The site is therefore considered to be at low risk of fluvial flooding from the adjacent River Mersey. The FRA also concludes that the site is at low risk of pluvial, groundwater and infrastructure flooding. There is one sewer on United’ Utilities’ ‘At Risk’ register located on the southern boundary – the risk to the proposed adjacent plot from this will be minimised by adopting the mitigation measures recommended below. Detailed design of the site would therefore have to take into account the possibility that existing sewers may surcharge.

36

Table D2, Planning Policy Statement 25, Development and Flood Risk, published by the Department of Communities and Local Government

31

Liverpool Water Flood Risk Assessment


The following is a list of the limitations of this flood risk assessment: â&#x20AC;˘

The assessment of flood risk has been partially based on a topographical survey which we believe is out of date. The assessment makes some assumptions as to the likely levels and consequential flood zoning of the development. However, a new topographical survey has been commissioned to confirm this and is currently awaited.

â&#x20AC;˘

The assessment is based on model data provided by the Environment Agency rather than specific modelling for the scheme. While this is considered appropriate for the proposed development, WYG cannot be held responsible for any errors or omissions contained within this third party data.

â&#x20AC;˘

The extreme wave overtopping data provided by the Environment Agency was inconclusive when attempting to determine likely extreme water level over and above the extreme net sea level calculated in the assessment. As a result, it has been agreed with the EA that this part of the assessment will be deferred to detailed plot application stage until such data is more conclusive, or until modelling specific to each plot is necessary in lieu of no further conclusive data being available.

3.2.2

Flood risk from the site

For such developments that are more than 1ha and within Flood Zone 1, guidance within PPS25 states the policy aim that the development should not result in an increase in risk of flooding to either the site or adjacent developments and not result in the flooding of any properties for tidal events up to and including the 1 in 200 year plus climate change event.37 Having residential uses, the design-life of the development is believed to be 100 years plus, thus the uplift in peak rainfall intensity recommended within PPS25 for climate change is 30%.38 As per the guidance within PPS25 and CIRIA Document C635, consideration must be given to ensure surface water runoff from the site is managed without increasing flood risk elsewhere. Therefore, there is a need to consider mitigation against any flood risk from surface water runoff as well as accounting for future potential increases in rainfall intensity and frequency due to climate change. At detailed design stage, assessments should be undertaken to ensure that any exceedance flooding can be accommodated without

37 Table D1, Planning Policy Statement 25, Development and Flood Risk, published by the Department of Communities and Local Government, December 2006. 38 Table D2, Planning Policy Statement 25, Development and Flood Risk, published by the Department of Communities and Local Government, December 2006.

32

Liverpool Water Flood Risk Assessment


compromising building floor slabs or emergency access routes (subject to access and DDA requirements). The proposed drainage strategy to be adopted will ensure that any surface water run-off does not increase flood risk elsewhere.

3.3

PROPOSED MITIGATION

3.3.1

Flood risk to the proposed development

Liverpool City Council’s SFRA states that minimum floor levels should be set within Flood Zones 2 and 3 such that a 600mm freeboard is provided above the 1 in 200 year Mersey Level, once contingency allowances for net sea level rises and increases in extreme wave height have been added for the design life of the development. The current estimation (2008) by the Environment Agency for the 1 in 200 year Mersey level is 6.11m AOD, which includes for the effects of surge. Calculations have been made for the 1 in 200 year Mersey level using contingency allowances for net sea level rises and an uprise in wave height. The 1 in 200 year extreme sea level up to the year 2115 is estimated as 7.15m AOD, based upon a 5 year construction period and 100 year design life (calculations available within appendix E). The anticipated construction period is not yet known, though should it be more than the 5 years estimated above, then the design-life end will be beyond the current guidance available (year 2115). Given that we have determined the site to be located wholly within Flood Zone 1, it is recommended that no floor level or emergency access route is below 7.25m AOD as a minimum. This provides a 100mm freeboard above the 1 in 200 year Mersey level with climate change allowance, estimated to be 7.15m AOD up to the year 2115. This also allows for a 300mm freeboard above the estimated 1 in 200 year plus climate change Mersey level up to the year 2100 (6.95m AOD). Because the flood risk assessment concludes that the site is wholly within Flood Zone 1, it is considered that this strategy is proportionate to the low risk of flooding from astronomical tide / net sea level rise from the Mersey Estuary in the future. Extreme wave height overtopping remains the only fundamental additional assessment / amendment which will be required to this FRA study. The extreme wave height overtopping data contained in the EA’s ‘Coastal Erosion Mapping Model Criteria Report’ for Liverpool didn’t really help clarify the maximum overtopping water level as the data just indicates ‘overflow’ as soon as it breaches the dock walls and therefore doesn't quantify maximum levels.

33

Liverpool Water Flood Risk Assessment


A meeting was held with Stephen Sayce and Graham Todd of the EA on 11th August 2010 to discuss their formal comments on the FRA dated 15th April 2010 and to resolve the limitations in the EA’s model and agreeing the way forward. The primary approach agreed was to revise this FRA as an indicative outline assessment, highlighting the minimum likely requirements for the whole development. This includes recommending the above minimum level to avoid the affect of extreme astronomical tide with the effect of climate change allowed for. The issues pertaining to extreme wave height overtopping the dock walls (and their other concerns pertaining to design life beyond the current guidance in PPS25 and acceptable freeboard – see later) will be addressed on a plot by plot basis for each detailed planning application as it comes forward. If further overtopping data has not been developed by the EA at this time then individual hydraulic modelling may be necessary but this requirement can be argued on an individual case by case basis. Thus, each detailed application will require a FRA addendum, with flood resilient construction measures introduced on a plot by plot basis only if avoidance is not physically possible.. The issue of the amount of freeboard necessary (over and above the minimum 100mm recommended above) will need to be agreed with the EA taking into account the extreme wave height adjacent to that location (when determined) as well as proximity to the dock walls. It is anticipated that minimum floor level freeboard above extreme wave height will taper from 600mm (where dock walls breech) to the 100mm already advised at a distance from the dock wall where the combined effects of extreme sea level and wave overtopping are determined to be of no influence. This principle is less than the 600mm freeboard recommended within Liverpool City Council’s SFRA (for Flood Zones 2 and 3 only) and is therefore subject to approval by both the Environment Agency and Liverpool City Council. The other minor comments raised in the letter received from the EA on 15th April were also discussed at the above meeting – the EA was advised that UU had confirmed approval in principal to the FRA and had provided further information on the location of public sewer flooding.

34

Liverpool Water Flood Risk Assessment


3.3.2

Flood risk due to surface water runoff from the site

In order to ensure that surface water runoff from the site does not cause an increase in flood risk the management of runoff has been considered via a sequential approach, in line with Building Regulations. The following options for the disposal of surface water runoff were considered, in order of preference39: 1. A soakaway or some other infiltration system, 2. A watercourse 3. A sewer

Discharge to soakaway Analysis of the ground investigation report shows that the strata is typically a nominal layer of made ground underlain by firm to stiff clay40. Water strikes were generally between -0.76m and 5.13m AOD. Given the low permeability of the clay soils, it is considered that infiltration drainage techniques are unsuitable for use on this development site.

Discharge to watercourse No specific maximum permissible discharge figure has been provided by the Environment Agency at the writing of this report. However, discharge to watercourse is considered to be the only suitable method of surface water discharge from the development. Whether such a discharge is direct to the Mersey using an existing connection, directly to the Mersey using a proposed connection or indirectly to the Mersey via discharge to the docks is subject to a detailed drainage strategy. It is anticipated that the proposed private surface water sewers should be designed so no pipes will surcharge during the 1 in 2 year event, whilst the network should not flood for events up to the 1 in 30 year event as a minimum. Detailed design of the surface water network to the River may require consideration of a surcharged outfall. The level of surcharge and maximum permissible discharge would have to be agreed with the Environment Agency as part of the detailed drainage design. It is anticipated that surface water run-off volumes up to the 1 in 30 year event will be attenuated on-site by allowing for example, the existing open docks to fill above the usual standing water level, possibly in conjunction with underground tanks. Estimations for the 1 in 30 year attenuation have been calculated

39 40

Building Regulations H3(3), Rainwater drainage. WYG Geo-environmental Ground Investigation Interpretative Report, Draft August 2009

35

Liverpool Water Flood Risk Assessment


using the Windes software as 1800m3 (Main site parcel 1675m3, Bath Street parcel 135m3). These volumes have been based upon restricting the permissible discharge from the site to an estimate for the existing 1 in 2 year event discharge. This existing discharge has been estimated using ‘System 1’ of the ‘Win-Des’ programme as 3400 l/s for the main site area and 245 l/s for the smaller site area. Should this attenuation volume be accommodated within the estimated 18Ha of dock waters, then this is estimated to cause the dock levels to rise by approximately 10mm. Flows in exceedance of the 1 in 30 event up to the 1 in 100 year plus 30% climate change event will be retained on site by incorporating design guidance such as CIRIA Report C635. The precise requirements will be subject to detailed design but could, for example, accommodate the additional attenuation volumes required by again allowing the docks to rise above the usual standing water level, possibly in combination with underground tanks. This could permit local and temporary flooding of the docks above normal operating level during exceedance flooding until levels subside without compromising existing or proposed building floor thresholds. We estimate that a further 3350m3 of storage would be required to contain such exceedance flooding (5150m3 in total), without exceeding the aforementioned discharge. This is based upon an additional 3120m3 requirement for the larger parcel and an additional 230m3 for the smaller parcel, detailed design of such flooding however would have to take into account the maximum water level that the docks would be allowed to raise without compromising any of the control features or canal traffic. This rise in dock water levels is estimated to be an additional 20mm above the 1 in 30 year attenuation level (30mm total). As attenuation is not currently provided to control run-off from storm events in exceedence of the 1 year event, this proposal will constitute a net improvement on the existing situation.

Discharge to a sewer Discharge to a sewer is only applicable in lieu of inability to drain to soakaways or to a watercourse. It is assumed that the drainage strategy/ detailed drainage design will determine whether the smaller development parcel for example will drain to sewer rather than to the River Mersey, due to site limitations.

3.4

RESIDUAL FLOOD RISK

The calculations for the estimated maximum sea level including recommended contingency allowance for climate change are based upon the figures within Table B.1 which provides an uprise per year for climate change up to the year 2115. Although this meets the current guidance, should the construction period plus the design life exceed this period, then in theory there is a residual risk that sea levels continue to rise

36

Liverpool Water Flood Risk Assessment


beyond the available guidance, beyond the design life end of the development. A further assessment is therefore recommended prior to 2100 based upon the guidance at that time to establish whether further works are required to protect the site from future climate change increases. There is also the obvious residual risk associated with future extreme wave overtopping of the existing dock walls. The extents of this cannot be determined at this time due to limitations in the data available, so it is recommended that assessment pertaining to this is made on a plot by plot basis for each detailed application as it comes up. This should be performed and submitted as an addendum to this outline / minimum requirements FRA. The development proposals and the drainage strategy should consider the residual risks associated with the collapse or blockage of the public sewers that cross the site, such as the aforementioned combined sewer outfalls or sewer on the at risk register along St Nicholas Place. Given the existing road level here compared to the recommended minimum FFL determined earlier, the risk of flooding from such an infrastructure failure is considered to be low. Detailed design should consider the likelihood and impact of other infrastructure failures, such as failure of the canal locks, failure of the existing dock walls and/or the gates to the Mersey. Consideration should be given at detailed design stage to the risk of on-site flooding that may result from blockages to the new drainage network, as recommended within CIRIA Document C635. Regular, ongoing maintenance of the network will be required to ensure that the capacity of the system is maintained as it has been designed. In summary, if the aforementioned mitigation measures are incorporated into the design of this proposed development, then the type of development is considered appropriate for the site, and the residual flood risk to the development site will be minimal.

37

Liverpool Water Flood Risk Assessment


4

CONCLUSIONS AND RECOMMENDATIONS

The following conclusions and recommendations are drawn from this report: •

According to broad-scale flood mapping produced by the Environment Agency (EA), the majority of the site (typically the East of the larger parcel and also the smaller King Edward Street/Bath Street Parcel) fall within Flood Zone 1 (thus outside the extent of the 0.1% (1 in 1000) risk of flooding from either the sea or river flows), whilst a band of the Western boundary is shown as Flood Zone 3 and thus deemed as having greater than a 1% probability of river flooding in any one year or greater than 0.5% (1 in 200 year) probability of sea flooding in any one year. A small area of the site associated with Trafalgar Dock is shown as Flood Zone 2, thus deemed by the EA as having between 0.5% (1 in 200) risk of flooding and 0.1% (1 in 1000) risk of tidal flooding in any one year. Therefore, based upon broad scale mapping alone, the site is considered to have areas of ‘low’, ‘medium’ and ‘high risk’ potential of flooding from tidal or fluvial sources.

The Environment Agency undertook an ‘Extreme Sea Level Study’ in 2008 and determined that tidal influences from the River Mersey to be the overriding flood risk to the site. Estimations for the flood levels of the Mersey during 1 in 100 year (1% probability), 1 in 200 year (0.5%) and 1 in 1000 year (0.1%) flood events have been provided as 6.0m, 6.11m and 6.37m AOD respectively (including surge events) based upon the Liverpool Gauge at grid reference 332480,395240.

Existing on-site levels in comparison to the EA’s flood levels are typically over 6.14m AOD, behind the dock wall, which is not specifically a flood defence (coping level typically 6.53m and 9.31m AOD). Therefore none of the existing site should be classified as Flood Zone 3 (governed by the 6.11m, 1 in 200 year tidal event). A small area the recently infilled West Wellington Dock and an adjacent existing Depot and car park do fall below the 1 in 1000 year tidal level of 6.37m AOD, thus potentially Flood Zone 2 (ignoring the presence of dock walls).

Comparison between the above Mersey flood levels, the topographical survey and visual inspection of the areas constructed after the topographical survey was completed, have led us to deduce that the whole of the development area is in fact located within Flood Zone 1. It is recommended that this is confirmed by a further topographical survey locally to the ‘Central Dock Channel Bridge’ and the infilled area of West Waterloo Dock and existing depot and associated car park. This topographical survey update is currently being performed and will be made available in due course.

38

Liverpool Water Flood Risk Assessment


Liverpool City Council’s UDP shows that the development is designated within the ‘Strategic Investment Area’, whilst the northern half of the proposed development area down to ‘Central Dock Channel Bridge’ and the North of ‘Waterloo Quay’ is designated as ‘unimplemented UDP mixed use allocation’. Smaller pockets within the southern development area that appear to correlate with the development proposals are also denoted as ‘unimplemented UDP mixed use allocation’ However, the smaller development parcel enclosed within King Edward Street, Bath Street and Roberts Street is within the ‘strategic investment area’ though is unallocated.

Given the above and given the belief that the site to be located entirely within Flood Zone 1, it is considered that the PPS25 Sequential Test has been passed. Furthermore, from Table D3 of PPS25 (Flood Risk Vulnerability and Flood Zone Compatibility) the proposed development vulnerability classification is suitable for the site.

The development area is covered by Liverpool City Council’s Strategic Flood Risk Assessment (SFRA) of January 2008 which provides details of any known historical tidal, fluvial and pluvial flooding events.

The EA do not have any records of known historical flooding event to the site from river or tidal sources. The SFRA indicates the historical presence of 2no. ordinary watercourses within the vicinity of the site, ‘Beacon’s Gutter’ and River Pool. However, the SFRA also identifies both as having been historically abandoned and replaced by the public sewerage network.

Correspondence received from United Utilities (Appendix D) shows that they are aware of 2 no. public sewers flooding properties in the vicinity on their ‘at risk register’ as a result of overloaded sewers. They also can not rule out instances that have gone unrecorded or instances that have not qualified for inclusion on the register. The location of one of these sewers is beyond the influence of the site, while the other (St Nicholas Place on the southern boundary of the site) has an isolated low spot. With the proposed minimum finished floor levels proposed later, any flooding of the same sewer in the future is only likely to route along (and be contained within) St Nicholas Place without compromising any plot buildings. Thus this at risk sewer is considered to be low risk of flooding the proposed development.

We can confirm that UU reviewed the draft version of this FRA and confirmed it was acceptable in principal.

Liverpool City Councils Highways department records are a data source for the SFRA. No Highway within or within immediate vicinity of the development is highlighted within this data as having caused historical flooding. As per the records of public sewers flooding properties, it can not be ruled out that incidence have gone unrecorded within this data set.

39

Liverpool Water Flood Risk Assessment


Although ground water flooding is considered to be an increasing issue within Liverpool City centre, referencing the ‘Mersey Estuary Catchment Flood Management Plan’ ‘although the groundwater levels have been seen to increase, the risk to Liverpool from groundwater flooding is considered to be low’.

The flood risk from canal infrastructure failures is considered as part of the assessment. However, the open water relating to docks is to remain insitu following the redevelopment. The standing water level within these open docks is lower-lying than the anticipated thresholds of the proposed buildings, thus is expected to have the capacity to accommodate flood volumes from canal infrastructure failures without compromising the building thresholds. The risk of flooding due to canal infrastructure failure is therefore considered to be low.

LCC’s SFRA states that a 600mm freeboard should be provided above the 1 in 200 year Mersey Level, once contingency allowances for net sea level rises and increases in extreme wave height have been added for the design life of the development (though based upon their belief that areas of the site are within Flood Zones 2 or 3). Calculations have been made for the 1 in 200 year Mersey level using contingency allowances for net sea level rises and an uprise in wave height as 7.15m AOD in the year 2115, (based upon a 5 year construction period and 100 year design life).

This report recommends that all floor levels and emergency access routes are 7.25m AOD and above, giving a minimum recommendation of 100mm freeboard above the estimated 1 in 200 year Mersey level for the year 2115 (i.e. including climate change) and allowing a 300mm freeboard above the estimated 1 in 200 year Mersey level up to the year 2100 (6.95m AOD). This principle is less than the 600mm freeboard recommended within Liverpool City Council’s SFRA (based on the site being within Flood Zones 2 and 3), however because this report considers that the site is wholly within Flood Zone 1, it is considered that there is reasonable debate with the planners and the EA to adopt this strategy.

Extreme wave height overtopping remains the only fundamental additional assessment / amendment which will be required to the FRA study. As the Environment Agency’s current overtopping data is inconclusive, the issues pertaining to overtopping (and their other concerns pertaining to design life beyond the current guidance in PPS25 and freeboard) will be addressed on a plot by plot basis for each detailed planning application as it comes forward. Thus, each detailed applications will require a FRA addendum, with flood resilient construction measures introduced on a plot by plot basis only if avoidance is not physically possible.

Infiltration techniques are not considered to be suitable for the development given the underlying firm to stiff clay strata.

40

Liverpool Water Flood Risk Assessment


No specific maximum permissible discharge figure has been provided by the Environment Agency. However, discharge to watercourse is considered to be the only suitable method of surface water discharge using either an existing connection directly to the Mersey, using a proposed connection, or discharging indirectly to the Mersey via the docks, subject to detailed drainage design.

It is anticipated that the proposed private drainage network will be formed using separate foul and surface water systems.

It is anticipated that the proposed private surface water sewers will be designed so no pipes surcharge during the 1 in 2 year event, the network doesn’t flood up to the 1 in 30 year event and drainage is modelled against a surcharged outfall, though subject to agreement with the Environment Agency and LCC as part of the drainage strategy.

It is anticipated that surface water run-off volumes up to the 1 in 30 year event will be attenuated onsite by, for example, allowing the existing open docks to fill above the usual standing water level, possibly in combination with underground tanks. Estimations for the 1 in 30 year attenuation have been calculated using the Windes software as 1800m3, based upon restricting the discharge from the site to the existing 1 in 2 year discharge figure (estimated to be 3645 l/s). This provides betterment on the existing drainage arrangement, which is believed to have no existing discharge controls. Should this volume be accommodated within the estimated 18Ha of dock waters, then this is estimated to cause the dock levels to rise by approximately 10mm.

Flows in exceedance of the 1 in 30 event up to the 1 in 100 year plus 30% climate change event will be retained on site by incorporating design guidance such as CIRIA Report C635. The precise requirements will be subject to detailed design but could, for example, accommodate the additional attenuation volumes required by again allowing the docks to rise above the usual standing water level. This could permit local and temporary flooding of the docks above normal operating level during exceedance flooding until levels subside without compromising existing or proposed building floor thresholds. The volume of this exceedance has been estimated as a further 3350m3 of storage (5150m3 in total). Detailed design of such flooding however, would have to take into account the maximum water level that the docks would be allowed to raise without compromising any of the control features or canal traffic. This rise is estimated to be an additional 20mm above the 1 in 30 year attenuation level (30mm total).

As per the recommendations within B9 of Planning Policy Statement 25, precautionary sensitivity ranges for an increase in rainfall intensities should be considered to allow for the uncertainty brought about by climate change. The proposed drainage model and finished site levels should therefore

41

Liverpool Water Flood Risk Assessment


demonstrate that no proposed or neighbouring thresholds are compromised during a 1 in 100 year plus 30% climate change event41 (based upon an estimated design life of the building of 100 years). â&#x20AC;˘

A drainage strategy should be undertaken to assess the need for diversions to any existing drainage network to be re-utilised. It is also recommended that these networks are fully inspected to establish condition and identify any remedial works necessary.

â&#x20AC;˘

Residual risks may still remain to the development, such as collapse of a combined sewer overflow from the public sewer network or collapse of the dock walls or dock gates, which although are not designated sea defences, provide some defence to the development.

41

Table B2, PPS 25, Development and flood risk. Published December 2006

42

Liverpool Water Flood Risk Assessment


FLOOD RISK ASSESSMENT – Appendices October 2010


Liverpool Water Flood Risk Assessment


Appendix A PPS25 Practice Guide Proforma

Liverpool Water Flood Risk Assessment


PPS25 PRACTICE GUIDE PRO-FORMA The table below provides an easy cross-reference between the PPS25 Practice Guide Pro-form (ref. Appendix C of PPS25 Practice Guide) and this WYG Flood Risk Assessment. PPS25 Practice Guide section 1

Description and location

WYG Report ref. Para.

Page

1a

Development description and location?

1.1

9

1b

EA vulnerability classification?

3.1

29

1c

Consistency with Local Development Documents?

3.1

29

1d

Evidence that the sequential test or exception test (PPS25) has been applied in the selection of this site for this development type.

3.1

29

2

Definition of the flood hazard

2a

What are the sources of flooding that could affect the site?

1.2

9

2b

Description of flood mechanics for all sources and any historical flooding.

2.2.2

22

2c

Existing surface water drainage arrangements for the site?

2.1.1

17

3

Probability

3a

EA Flood Zone classification?

2.2.1

20

3b

Reference to any Strategic Flood Risk Assessment covering this site?

2.2.1

20

3c

Probability of the site flooding taking account of the contents of the SFRA and of any further site-specific assessment?

2.2.2

22

3d

Existing rates and volumes of run-off generated by the site?

2.2.1

20

3.2.2

32

4 4a 5

Climate change Influence of climatic change on the site? Detailed development proposals

5a

Details of the development layout, referring to the relevant drawings.

3.1

29

5b

Where appropriate, demonstrate how land-uses most sensitive to flood damage have been placed in areas within the site that are at least risk of flooding.

N/A

N/A

3.3.1

33

6 6a 7

Flood risk management measures How will the site be protected from flooding, including the potential impacts of climate change, over the developmentâ&#x20AC;&#x2122;s lifetime? Off site impacts

7a

How will you ensure that the measures to protect your site from flooding will not increase flood risk elsewhere?

3.3.2

35

7b

How will you prevent run-off from the completed development causing an impact elsewhere?

3.3.2

35

8

Residual risks

8a

What flood-related risks will remain after you have implemented the measures to protect the site from flooding?

3.4

36

8b

How, and by whom, will these risks be managed over the lifetime of the development

3.4

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Liverpool Water Flood Risk Assessment


Appendix B â&#x20AC;&#x201C; SITE PLANS

Liverpool Water Flood Risk Assessment


Appendix C â&#x20AC;&#x201C; SITE PHOTOGRAPHS

Liverpool Water Flood Risk Assessment


View East along southern boundary (St Nicholas Place)

View looking North from St Nicholas Place towards Princes Dock

Liverpool Water Flood Risk Assessment


View North from South East corner of development area showing existing buildings adjacent either side of Princes Dock

View North along sea wall and Princes Parade from South East corner of development area

Liverpool Water Flood Risk Assessment


Sea wall construction, looking South-West from South-West corner of development with Cruise-liner facility in the background

Sea wall and floating landing stage, looking South along Western boundary

Liverpool Water Flood Risk Assessment


View North along sea wall and Princes Parade towards Princes Jetty

View North-East from Princes Parade across towards William Jessop Way

Liverpool Water Flood Risk Assessment


Western elevation of Princes Half Tide Dock, from the South

Eastern elevation of Princes Half Tide Dock, from the South

Liverpool Water Flood Risk Assessment


View from Princes Dock Causeway Bridge, along Princes Dock towards the Liver building to the South/South-East

View South/South-East from Princes Dock Causeway Bridge, down William Jessop Way

Liverpool Water Flood Risk Assessment


View North-East across Princes Half-tide dock to new-build

View North from Princes Dock Causeway Bridge across Princes Half-tide dock towards West Waterloo Dock and â&#x20AC;&#x2DC;Waterloo Quayâ&#x20AC;&#x2122; development either side of East Waterloo Dock

Liverpool Water Flood Risk Assessment


View North to East Waterloo Dock from East of Princes Half-tide dock

View West along northern building line of Waterloo Quay development and associated car parking over approximate route of â&#x20AC;&#x2DC;Kingsway Tunnelâ&#x20AC;&#x2122;

Liverpool Water Flood Risk Assessment


View North to infilled area of West Waterloo Dock and new Central Dock Channel Bridge over new 6.5m ‘Central Docks Channel’

View West along northern building line of Waterloo Quay development and associated car parking over approximate route of ‘Kingsway Tunnel’

Liverpool Water Flood Risk Assessment


View South over new 6.5m ‘Central Docks Channel’ and infilled area of West Waterloo Dock, taken from new ‘Central Dock Channel Bridge’

View West over infilled area of West Waterloo Dock out towards the Mersey, taken from new ‘Central Dock Channel Bridge’

Liverpool Water Flood Risk Assessment


View South-East from new ‘Central Dock Channel Bridge’ towards Waterloo Quay development

View North over new 6.5m ‘Central Docks Channel’ and infilled area of Trafalgar Dock, taken from new ‘Central Dock Channel Bridge’

Liverpool Water Flood Risk Assessment


View North-East over depot car park, with depot in the background, taken from new â&#x20AC;&#x2DC;Central Dock Channel Bridgeâ&#x20AC;&#x2122;

View North between depot (RHS) and depot car park (LHS)

Liverpool Water Flood Risk Assessment


View East from Depot access road towards Porter Street

View North-West from Depot access road over open land (East of Trafalgar Dock)

Liverpool Water Flood Risk Assessment


View North from Depot access road over open land adjacent to Trafalgar Dock

View North-East from Depot access road over open land (East of Trafalgar Dock)

Liverpool Water Flood Risk Assessment


View west over Clarence Graving Docks

View west from Bascule Bridge, across Collingwood Dock with Salisbury Dock and Victoria Tower in the background

Liverpool Water Flood Risk Assessment


View north-west from Bascule Bridge over Collingwood Dock

View north along Regent Road of Bascule Bridge (between Collingwood Dock and Stanley Dock, with Stanley Warehouse (RHS)

Liverpool Water Flood Risk Assessment


View south along Regent Road of Bascule Bridge over Collingwood Dock with site access (leading to north of Collingwood Dock) to the right-hand-side

View South-West across Collingwood Dock

Liverpool Water Flood Risk Assessment


View West across Collingwood Dock

View south-west over dockland to north of Nelson Dock, with warehouses to RHS, Nelson Dock to LHS

Liverpool Water Flood Risk Assessment


View North-West of Bramley Moore Dock

View North from North-East corner of site, showing Bramley Moore Dock (Wellington Dock in the background LHS and waste water treatment works background RHS)

Liverpool Water Flood Risk Assessment


Appendix D â&#x20AC;&#x201C; CORRESPONDENCE

Liverpool Water Flood Risk Assessment


Appendix E â&#x20AC;&#x201C; CALCULATIONS

From Table B1 of PPS25, Recommended contingency allowances for net sea level rises;1

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Planning Policy Statement 25, Development and Flood Risk, published by the department of Communities and Local Government, December 2006.

Liverpool Water Flood Risk Assessment


Net Sea Level Rise (mm/year)

N West England

2008-2025

2025-2055

2055-2085

2085-2115

2.5mm

7.0mm

10mm

13mm

Assuming a 30 year build programme and a 100 year design life from completion of last building, design life ends 2139. However, guidance on climate change allowances only currently available up to year 2115. Therefore, using this limit of current guidance (106 years from the date of this report); Net Sea Level Rise up to the year 2115 (Using EA’s 2008 model as base) Period

Total Years

PPS25 – Sea Level Rise (mm/year)

Total (mm)

2008 - 2025

17

2.5

42.5

2025 - 2055

30

7

210

2055 – 2085

30

10

300

2085 - 2115

30

13

390 Total = 942.5mm

Therefore based upon the EA’s 1 in 200 year sea level model of 6.11m AOD (Extreme sea level study 2008, including surge). The 1:200 year sea level for years up to 2115 (inc. sea level rise) =7.053m AOD Extreme Wave Height From Section B6 of PPS25, a sensitivity allowance should be added for extreme wave height. From table 13.2, a 10% sensitivity factor should be added. Thus 94mm. Therefore based upon the EA’s modelled 1:200 year sea level of 6.11m AOD (Extreme sea level study 2008, including surge) and the guidance within PPS25 for precautionary sensitivity factors; The 1 in 200 year extreme sea level for years up to the year 2115 is approximately 7.15m AOD Assuming a 15 year build programme and a 75 year design life from completion of last building, design life ends 2099-2100. Therefore, using 91 years from the date of this report);

Liverpool Water Flood Risk Assessment


Net Sea Level Rise up to the year 2100 (Using EA’s 2008 model as base) Period

Total Years

PPS25 – Sea Level Rise (mm/year)

Total (mm)

2008 - 2025

17

2.5

42.5

2025 - 2055

30

7

210

2055 – 2085

30

10

300

2085 - 2100

15

13

195 Total = 747.5mm

Therefore based upon the EA’s 1 in 200 year sea level model of 6.11m AOD (Extreme sea level study 2008, including surge). The 1:200 year sea level for years up to 2115 (inc. sea level rise) =6.86m AOD Extreme Wave Height From Section B6 of PPS25, a sensitivity allowance should be added for extreme wave height. From table 13.2, a 10% sensitivity factor should be added. Thus 75mm. Therefore based upon the EA’s modelled 1:200 year sea level of 6.11m AOD (Extreme sea level study 2008, including surge) and the guidance within PPS25 for precautionary sensitivity factors; The 1 in 200 year extreme sea level for years up to the year 2100 is approximately 6.93m AOD Say 6.95m AOD

NB: The above calculations only consider the contingency allowances for climate change within current guidance and not to the end of the estimated design life of 2140 (estimated to be 30 year build programme + 100 year design life) See recommendations within body text of report regarding future flood study.

Liverpool Water Flood Risk Assessment


Liverpool Waters Addendum to the Flood Risk Assessment â&#x20AC;&#x201C; November 2011

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Liverpool Waters - Flood risk addendum