Sutton Harbour Holdings PLC. March 2007

Transcription

1 Sutton Harbour Holdings PLC March 2007

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4 Executive Summary!! " WSP Development and Transportation has been engaged by Sutton Harbour Holdings PLC to assess the flood risks for the proposed commercial development of the existing Salt Quay House site at Sutton Harbour. The site is located on the North East Quay to Sutton Harbour, between the existing residential development on North Quay and the Boatyard site to the south that was recently granted planning permission for a mixed use development. WSP prepared a detailed Flood Risk Assessment (FRA) to support the Boatyard application and this document draws on the analyses and conclusions of that assessment. Sutton Harbour and its quays are protected from tidal flooding by the flood defence gates at the entrance to the inner harbour. This defence is owned, maintained and serviced by the Environment Agency (EA). The gates are operated by the Sutton Harbour lock keepers. In the Boatyard Flood Risk Assessment it was demonstrated that the site was not at risk of flooding in the 1 in 200 year tidal flood with a 100 year design life using the then draft PPS25 climate change predictions, assuming the existing flood defence gates close as intended. This FRA considers the residual risk of the gates failing in the open position and proposes mitigation measures to protect the ground floor of the proposed commercial development for the 60 years design life of the building. The site is in Flood Zone 3a as defined by Table D1 of PPS25, i.e. with a high probability of flooding when the presence of flood defences is ignored. The proposed development is purely commercial in its nature. Table D2 of PPS25 classifies such developments as being less vulnerable. The EA have confirmed that they will be looking for a 60 year design life for all new commercial developments. Taking the output from Tables D1 and D2, Table D3 concludes that such less vulnerable developments are appropriate for Zone 3a. During January 2007, an FRA was prepared for a commercial development on this site with a raised ground slab level some 500mm above the Q 200 flood level. Although this proposal was warmly received by the EA, it was considered unacceptable by Plymouth City Council (PCC) in respect of preventing the delivery of some key aspirations of the Local Plan and Area Action Plan. In planning and urban design terms, PCC are looking for an at grade solution with spill out from the active frontages onto the listed quayside in keeping with both the historic character of the harbour and the more recent developments that surround the inner harbour. The architect has accordingly redesigned the proposed development with a ground floor set at 3.65mAOD to provide level access to the quayside to meet with PPC s planning requirements. With a design life of 60 years for this proposed commercial development, the Q 200 tidal flood level with 60 years of climate change is 4.14mAOD. The architect has mitigated against this residual flood risk and designed the building with an external envelope constructed of flood resistant materials. Demountable flood defences are also proposed to be provided to the ground floor accesses in line with the proposals for the adjacent Boatyard site. These flood boards will be 1100mm high to provide flood defence up to a water level of 4.75mAOD offering just over 600mm freeboard above the design flood level. This FRA concludes that the proposed development can be undertaken without increasing the flood risk to itself or any other properties Salt Quay House - Flood Risk Assessment 1

5 1 Introduction #2# WSP Development and Transportation has been engaged by Sutton Harbour Holdings PLC to assess the likely flooding issues for the proposed redevelopment of the site known as Salt Quay House, Sutton Harbour, Plymouth The site is currently occupied by a warehouse which has been converted for office use. It is proposed to demolish the existing building and redevelop the site with a commercial development consisting of retail and office space. The site fronts onto the quayside and therefore external ground levels will have to remain broadly as at present The purpose of this report is to consider the existing flooding issues, impacts and any existing flood alleviation measures that are in place. The report will then consider the future commercial development and the impact of the flooding on it The report will cover all relevant aspects of the Department for Communities and Local Government publication Planning Policy Statement 25: Development and Flood Risk (PPS25) and is intended to address issues raised by the Environment Agency (EA) Salt Quay House - Flood Risk Assessment 2

6 2 Existing Land Uses and Existing Flooding Issues %2# 36( The proposed development site is currently known as Salt Quay House, Sutton Harbour in Plymouth and is at NGR SX A location plan can be seen as Figure 1 in Appendix A The site is bounded to the northeast by a builder s merchants whilst to the southwest by the quay and marina. Immediately to the south is The Boatyard site, for which planning permission was secured for a mixed use development late in To the northwest is the existing residential development of North Quay The existing Salt Quay House is a former warehouse building with a long frontage along North East Quay and a rear (northeast) elevation which follows a shallow S curve. It consists of two adjoining structures, the northwest section being of two storeys and the southeast a single storey with inserted mezzanines. It is proposed to demolish this building. %2% The existing site can be regarded as totally impermeable. The front elevation pitched roof currently discharges to outfall pipes through the listed quay structure into the harbour. The rear elevation pitched roof of the building drains to a combined sewer which runs through the builders merchant s yard. %2) 36( The site has been identified as at been at risk from flooding on the Environment Agency s indicative flood maps. A copy of the indicative flood map can be seen on Figure 2 in Appendix A An inspection of the Tidal Still Water Levels (2002) for South West England: Coast and Estuaries indicates that the 1 in 200 year or 0.5% annual probability tidal flood level is 3.68mAOD. A copy of the prediction spreadsheet is attached in Appendix B. %2+ 36( Sutton Harbour and the surrounding area are protected by the Sutton Harbour Flood Defence Scheme. The flood defence gates are owned, maintained and serviced by the EA. The gates are operated by the Sutton Harbour lock keepers The current arrangement for the operation of the flood defence gates will continue. The two pairs of gates have separate hydraulic systems. It would require both sets to fail in the open position for the harbour to be at risk of inundation. There is also a back up generator to cover for the eventuality of a power outage The gates are controlled by a computer controlled alarm system. As soon as the seaward tide level reaches 5.6m above chart datum (2.28mAOD) an alarm is activated and the gates automatically shut. If the gates fail to shut there is a manual override whereby the lock keepers can manually close the flood defence gates In advance of a predicted high tide, the EA also fax a warning to the Sutton Harbour lock keepers. However, in practice, the lock keepers with their tide tables, tide books and barometers know in advance if the automated system is likely to be activated Salt Quay House - Flood Risk Assessment 3

7 2.4.5 The EA currently have a proposed capital works scheme to replace the existing gates which are nearing the end of their operational life with new flood defence gates to keep the 1 in 100 year still water level out of the inner harbour. This will increase the level of protection to the properties behind the defence (NB: the Salt Quay site will have in excess of 1m freeboard in the 1 in 200 year tide event). The risk of the new gates failing in the open position should be less than the risk of the current gates failing, but with all mechanical devices a small but not ignorable risk exists It will be expected that as part of the ongoing operation of the existing and new gates that they are regularly tested and maintained as necessary by the EA. This will be particularly important for the gates to be in good working order in advance of predicted high tides The following analyses have been undertaken to assess the residual flood risk to the proposed development in the event of the defences failing completely and allowing the 1 in 200 year flood into Sutton Harbour Salt Quay House - Flood Risk Assessment 4

8 3 Future Land Uses )2# ( It is proposed to demolish the existing converted warehouse building and redevelop the site by the construction of a new commercial development consisting of office and retail space The proposed development will comprise approximately 30,000 square feet of office accommodation and ground floor retail units over 6 floors with a roof plant room. The footprint of the proposed building follows the line of the existing Salt Quay House generally with cantilevered office floors at upper levels projecting 3m towards the quayside. Construction will comprise of a steel frame and composite floor slabs. The primary entrances to the ground floor retail and office areas will be from the quayside elevation During January 2007, an FRA was prepared for a commercial development on this site with a raised ground slab level some 500mm above the Q 200 flood level. Although this proposal was warmly received by the EA, it was considered unacceptable by Plymouth City Council (PCC) in respect of preventing the delivery of some key aspirations of the Local Plan and Area Action Plan. In planning and urban design terms, PCC are looking for an at grade solution with spill out from the active frontages onto the listed quayside in keeping with both the historic character of the harbour and the more recent developments that surround the inner harbour Copies of the project architect s proposed ground floor plan, section and 3D sketch perspective of the proposed development are included in Appendix A The granite coping edge of the listed harbour wall in front of Salt Quay House has a level of typically 3.50mAOD. The ground floor entrance will be raised nominally to form a ground floor at 3.65mAOD into a reception area serving a ground floor office and access to upper floors. A separate at grade access will also be provided to a retail area facing out to the quayside. Again the proposed ground floor slab level is 3.65mAOD. )2% In the future the site will be covered by the building. Therefore the impermeable areas will remain as at present and accordingly there will be no increase in surface water runoff rates. However the whole roof will drain to the front elevation of the building and discharge into an existing piped outfall through the listed quay wall and into the harbour. This will reduce the runoff from the rear of the existing building which currently discharges to a combined sewer which will accordingly reduce the flood risk in this area Salt Quay House - Flood Risk Assessment 5

9 4 Future Tidal Flooding Issues +2# 55#3%.."5(( ( An inspection of the Tidal Still Water Levels (2002) for South West England: Coast and Estuaries indicates that the 0.5% annual probability or 1 in 200 year return period tidal flood level is 3.68mAOD. A copy of the prediction spreadsheet is attached in Appendix B. +2% 55(! Annex B of PPS25 details the latest research on the issue of climate change. With regard to sea levels paragraph B5 states: Global sea levels will continue to rise, depending on greenhouse gas emissions and the sensitivity of the climate system. The relative sea level rise in England also depends upon vertical movement of the land, which is generally falling in the south-east and rising in the north and west. Allowances for the regional rates of relative sea level rise shown in Table B.1 should be used Table B.1 specifies recommended contingency allowances for net sea level rise in four periods from 1990 to The current EA guidance recommends a design life of 60 years for commercial developments, such as this. Therefore the following calculation has been undertaken to calculate what the 1 in 200 year return period tidal flood level will be in 60 years time, 2067, working from the base level of 3.68mAOD in = mm pa = 80mm = mm pa = 240mm = mm pa = 138mm Total predicted sea level rise from = 458mm +2) 555(( ( By adding together the two above predictions, the future design tidal level can be estimated Using the PPS25 guidance, the 1 in 200 year still water level in 2067 can be calculated as = 4.14mAOD. +2+ ( (4459 (31(" The site has been confirmed by the EA as being Zone 3a as defined by Table D1 of PPS25, i.e. with a high probability of flooding when the presence of flood defences is ignored The proposed development is purely commercial in its nature. Table D2 of PPS25 classifies such developments as being less vulnerable. The EA have confirmed that they will be looking for a 60 year design life for all new commercial developments Taking the output from Tables D1 and D2, Table D3 concludes that such less vulnerable developments are appropriate for Zone 3a. +2* 5(( Should a tidal flood occur, the surge would enter Plymouth Sound around the breakwater, and then into the Tamar and Plym estuaries where it would enter the Outer Salt Quay House - Flood Risk Assessment 6

10 Sutton Harbour before coming up against the lock gates that separates the outer and inner harbours These lock gates would be closed once a flood warning has been given. The outer gate is 12m wide and has a top level of 3.86mAOD whilst the inner gate is 14m wide and has a top level of 2.78mAOD. The tidal defence wall which is adjacent to the lock gates and separates the inner and outer harbours is at a level of 4.03mAOD. +2, 33 14(4457 ( ( To predict the rise in water level of the inner harbour adjacent to the proposed development site, a tidal overtopping analysis flood model was created as part of the Flood Risk Assessment undertaken for the adjacent Boatyard site The model used a three cycle tidal surge with the top of the cycle being equivalent to the predicted tide levels. As the tide rises above the outer lock gates sea water flows into the lock. Once the lock s water level has risen to the inner lock gates level, water spills over into the inner harbour. The analysis was based on the inner harbour impounded still water level being 2.28mAOD, as originally advised by the EA. In more recent correspondence with the EA it has been suggested that the operating regime has been revised and a higher impounded still water level of 2.48mAOD has been adopted, although no written evidence has been supplied to date to support this Using the PPS25 predictions, the defence is overtopped and the water level in the inner harbour is predicted to rise to 2.40mAOD. A summary of the methodology and results are attached in Appendix B The impact of the higher initial impounded water level (2.48mAOD) would be to reduce the amount of storage available for flood water overtopping of the gate. Therefore it is fair to assume that the maximum water level in the harbour if the overtopping analysis were to be rerun would be 200mm higher than shown in Appendix B; i.e mAOD, some 900mm below the existing quay wall level and over 1m below the ground floor slab level of the proposed Salt Quay House development. +2/ As part of the Boatyard FRA we also considered the effect of wave overtopping of the outer gate and wall. Because of the topography of the area, the wave fetches are comparatively short, but some localised wave action can lead to additional overtopping of the gate as witnessed in October However, because of the significant available storage volume in the inner harbour (600mm depth over 93,000m 2 inner harbour plan area = 55,000m 3 ), this is not considered to be a significant risk of additional flooding. (Note the 600mm is the minimum available depth above the impounded water level of 2.40mAOD and the lowest part of the quay wall at the Barbican at 3.0mAOD.) As initially agreed with the EA Development Control Officer for the Boatyard site, it was not considered necessary to calculate the likely volume of water that could overtop the defence given the comparatively huge available storage volume of the inner harbour. However in response to later communication with the EA we made a number of conservative estimates to calculate the likely scale of wave overtopping In Appendix B we have included a single sheet of calculations summarising the effect of potential wave overtopping of the gate and wall defences. We have calculated that a series of 300mm high waves hitting the defences for the duration of the tide being within 300mm the top of the wall results in the harbour filling by approximately 40mm Salt Quay House - Flood Risk Assessment 7

11 Therefore, even with the combined effect of the 1 in 200 year tide, the worst case climate change allowance and wave action, the proposed development still has a freeboard of approximately 1m above the water level in the harbour, providing the gates close It is possible that larger waves will hit the defences, but in these cases only the crest is likely to overtop into the inner harbour. Therefore we have taken a conservative approach in assessing the impact of smaller waves occurring when the 1 in 200 year tide is coincident with the top of the defences and with all of the wave overtopping into the inner harbour If the existing Sutton Harbour Flood Defence Scheme is improved as proposed, then the risk of overtopping will be reduced accordingly. +2: 7 ( South West Water and Plymouth City Council Highways Department have been approached with regard to any flooding caused by their surface water systems. Following discussions both have confirmed that they have no record of surface water or highway flooding in this area, other than minor localised ponding due to leaves blocking road gullies By discharging the whole of the roof drainage into the harbour (at present the rear of the building discharges to a combined sewer), the flows in the sewer and resultant surface water flood risk will be lower Salt Quay House - Flood Risk Assessment 8

12 5 Miscellaneous Issues *2#! 63" As the flood levels are not predicted to raise above the harbour wall, access can be afforded at all states of the tidal cycle along Sutton Road. If the gates fail in the open position when a flood above 3.5mAOD is predicted, the early warning will activate the Flood Management Plan prepared by Sutton Harbour. In this circumstance the emergency access will be from the development to higher land on Sutton Road or Exeter Road. The Flood Management Plan prepared to discharge a planning condition on the Boatyard planning approval can be extended to include this immediately adjacent site. *2% To protect the environment from flooding during the construction period a localised runoff management system will be employed by the main contractor To prevent oils from polluting the environment, all oil stores will be bunded and maintained regularly during the construction period The contractor is to produce a method statement and agree it with the Environment Agency prior to the start of the construction. *2)! The private drainage will be designed to the requirements of the Building Regulations and handed over to the owners for maintenance The main drainage surface water system will be designed to Sewers for Adoption 6 th Ed and will be offered to South West Water for Adoption and future maintenance. *2+ 4( We have contacted South West Water to check whether there is sufficient capacity in the local sewer network. SWW have replied saying that they have no local capacity problems and providing that there are no unusually high discharges, then they would not envisage a problem in accepting the foul flows from the new development. The proposed reduction in discharge of roof drainage to the combined sewer will also increase the local capacity in the combined sewer. *2* 7 (" The rainwater will be collected and directed straight into the underground pipe system. Therefore, it is not envisaged that there will be any residual water quality issues. *2, ( 35";5! Until such time as the hazards relating to the site or location are known, we are unable to confirm that our recommendations or design will be acceptable in terms of safe buildability/maintainability Given the preliminary nature of our proposals and the limitations of our brief, we have not fully sought to obtain nor become aware of any significant information relating to hazards that might affect the construction or maintenance of this project Under the CDM Regulations, adequate information about the site must be provided by the client in order to allow the potential hazards to be reviewed by the designer and avoidance/mitigation measures taken where reasonably practicable Salt Quay House - Flood Risk Assessment 9

13 6 Mitigation Measures and Residual Risks,2#! 643! If the flood defence gates operate as designed, the inner harbour will have in excess of 1m freeboard. The model produced for the adjacent Boatyard site of the tidal inundation for the inner harbour predicted that the future water level will rise by 0.121m to 2.401mAOD The granite coping edge of the harbour wall in front of Salt Quay House has a level of typically 3.50mAOD; therefore, the proposed development will not be flooded during the design event. The finished ground level at the development area is to be 3.65mAOD which is over 1m above the predicted defended 1 in 200 year flood level However, this Flood Risk Assessment considers the residual risk of flooding in the event of both gates failing in the open position. The two pairs of gates have separate hydraulic systems. It would require both sets to fail in the open position for the harbour to be at risk of inundation. There is also a back up generator to cover for the eventuality of a power outage. However if all of these situations arose and the gates couldn t be closed manually (which is the failsafe position), then there is a residual risk of the whole area around Sutton Harbour flooding, and the following are put forward as mitigation measures to address this risk With a design life of 60 years for this proposed commercial development, the Q 200 tidal flood level with 60 years of climate change is = 4.14mAOD The architect has mitigated against the residual flood risk and designed the building with an external envelope constructed of flood resistant materials. Demountable flood defences are also proposed to be provided to the ground floor accesses in line with the proposals for the adjacent Boatyard site. These flood boards will be 1100mm high to provide flood defence up to a water level of 4.75mAOD offering just over 600mm freeboard above the design flood level The Flood Management Plan for the installation of the demountable flood defences for the Boatyard site could be extended to include the ground floor entrances to Salt Quay House. Figure 3 in Appendix A shows the indicative flood boards required to protect the ground floor against the residual flood risk. The exact size and form of these flood boards will be finalised at the detailed design stage In this situation, after receiving the appropriate early warning of a predicted high tide above the ground floor slab level of 3.65mAOD, the flood boards can be installed to protect Salt Quay House It is also proposed that the building is placed on the EA early warning flood risk register in the same way that all properties around Sutton Harbour and the Barbican area should be. However, due to the predictable nature of tidal events, the EA will be able to offer much earlier flood warnings than can be expected for fluvial flooding It is also recommended to raise all electrics for ground floor properties by a minimum of 600mm above floor level The impermeable areas will not increase due to the development and therefore, the surface water runoff will not increase. South West Water has confirmed that they do not have any suitable connections to their surface water network. Therefore, the surface water from the whole roof will discharge into Sutton Harbour using the same points of discharge through the listed quay wall as at present Salt Quay House - Flood Risk Assessment 10

14 ,2% 5(( It is understood that funding has been received by the Environment Agency to carry out works on the flood defence gate and the wall between the inner and outer harbours. This will enhance the protection afforded to the proposed development and the existing properties around the harbour and the historic Barbican area. The latest correspondence from the Environment Agency has confirmed that the current proposal is to raise the level of the existing defence by 170mm. According to the EA, this is intended to offer protection from extreme tidal flooding until The Defra / EA Flood Risk Assessment Guidance for New Development (FD2320/TR2) includes a section on safe access and exit from new developments in flood risk areas. Table 13.1 summarises the danger to people for different combinations of depth and velocity. For the proposed Salt Quay House, the existing listed quayside level is typically 3.5mAOD. The Q 200 flood level with 60 years of predicted sea level rise due to climate change is 4.14mAOD. Therefore we are considering a worst case depth of water of 640mm over the quayside if the flood gates fail in the open position as previously described Although no modelling or analysis of flood flow velocities has been undertaken as part of this FRA, it is reasonable to accept that the velocities will be very low. As a result Table 13.1 concludes that there is a danger for most. However because of the predictable nature of tidal flooding and the placing of the proposed development on the EA s early flood warning system, a safe means of egress can be provided from the development to higher land on Sutton Road and Exeter Road. It must always be borne in mind that this residual risk of flooding can only occur if both of the EA flood defence harbour gates fail in their open position Salt Quay House - Flood Risk Assessment 11

15 7 Conclusions And Recommendations /2# 43( The site is in Zone 3a as defined by Table D1 of PPS25, i.e. with a high probability of flooding when the presence of flood defences is ignored The proposed development is purely commercial in its nature. Table D2 of PPS25 classifies such developments as being less vulnerable. The EA have confirmed that they will be looking for a 60 year design life for all new commercial developments Taking the output from Tables D1 and D2, Table D3 concludes that such less vulnerable developments are appropriate for Zone 3a With a design life of 60 years for this proposed commercial development, the Q 200 tidal flood level with 60 years of climate change is 4.14mAOD. The architect has designed the building to provide level access to the quayside to comply with Plymouth City Council s planning vision for this location In the event of the EA flood defence gates failing, a Flood Management Plan will be activated and the demountable flood defences installed. These 1100mm high flood boards will provide a 600mm freeboard above the design flood level In conclusion it can be seen that the proposed development can be undertaken without increasing the flood risk to itself or adjacent properties. /2% 4!! It is recommended that approval for the scheme is given and that the Environment Agency release a Letter of Conformity to the Local Planning Authority Once planning permission has been obtained, the above outline proposals are reviewed and a detailed design is undertaken. This design must then be reviewed for technical, environmental, health and safety and for CDM compliance Salt Quay House - Flood Risk Assessment 12

16 Appendices - Figures & Calculations

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