WATERMAN CIVILS LIMITED
Minutes of Meeting for Hampton Court site Flood Risk Assessment
Thursday 12 January 2006 at 14.30
FrimleyBusinessPark, Frimley
Present:Owen Peat (OP)Environment Agency
Patricia Jones(PJ)Environment Agency
Stephanie Gladbach (SG) Allies and Morrison Architects
Denis Olette (DO) Allies and Morrison Architects
Richard A. Simmonds (RAS)Waterman Civils
Beatrice Libiszewska (BL)Waterman Civils
CcCaroline Dawson of Planning Potential
Evelyn Chan of Waterman Environmental
1. / Background / ActionWaterman Civils completed a PPG 25 Flood Risk Assessment (FRA) in support of the Planning Application for Jolly Boatman and Hampton Court Station site in East Molesey, London in 2004. Although the Environment Agency (EA) was consulted during its preparation, the FRA was never formally submitted for their approval as the scheme design has not been accepted by CABE and had to be changed. A new Masterplan is now being produced by Allies and Morrison.
The aim of the meeting was to present the new preliminary Masterplan Design and a potential solution to the Floodplain Compensation issues to the EA to achieve agreement on this and other flood issues.
2. / Introduction
RAS presented an introduction to the scheme and short history of the planning process. The site is located in a very prominent location in the vicinity of the second most popular visitor attraction in the Country, HamptonCourtPalace. In order for the Site to be developed and adjacent railway station to be refurbished in time for the boost of tourism during Olympics, a consensus should be reached between all stakeholders.
In terms of flooding, the development lies within the 1% annual probability (1 in 100 year return period) floodplain of the River Thames and River Mole and is not defended by flood walls or any alleviation schemes. Options for defending the residents at this site (secondary defences) together with compensation for loss of active floodplain will form the main subject of this meeting.
3. / Floodplain compensation
The EA has recently updated their model of the Lower Thames Flood Risk Mapping (LTFRM) and provided the most up-to-date 1 in 100 floodplain levels, at 8.41mAOD (Node 16.15D) in December 2005. The level has increased approximately 0.54m from the first quoted levels of 7.87mAOD and 0.3m from the second quotation which makes it more difficult to compensate on the very restrained site.
RAS has asked for an explanation of how the new levels were derived and why they increased by so much. OP replied that he is not a modeller himself but will provide contact to Polly Graham who is able to discuss details of the modelling. The EA policy is however, that they work with the most up-do dated and best information available at a time and we will need to provide compensation of the current floodplain levels of 8.41m AOD.
BL stated that she has done some preliminary calculations working with the latest available flood levels 8.41m AOD and believes that flood compensation at the site can potentially be achieved.
The idea is that the underground car park will be located at a basement level with the proposed ground levels levelled down to 7.4mAOD (ground levels in the Cigarette Island Park). Which is approximately 0.6m lower than the current car parking levels and will not only continue to be flooded in times of high probability floods (>1 in 50-years) but also increase the capacity of the floodplain for lower then 50 –years probability floods. The only areas taken out of the floodplain will therefore be the footprint of the buildings.
The area between the buildings will be flooded to a height of approximately 1m in the event of 1 in 100 –year event flood so the building exteriors will need to be made water resilient.
Placement of the building in the floodplain will result in displacement of approximately 450m3 of water, mostly between levels of 8.41m and 8.01m (current site level at the car park), which then can be compensated for on a level for level basis in areas not currently not in a floodplain mainly north and west of the hotel proposed at the Jolly Boatman site. Drawings were presented to illustrate and show that the flood compensation can be attained.
OP stressed out that having come to the solution is a positive outcome as the Floodplain Compensation is a major issue for the Environment Agency and should this solution be proved, the Agency will not have an issue with the development at this site and will comment positively on the planning application.
As the design of the scheme evolves, the flood risk team will be in touch with the Architect to undertake further designing earthworks to ensure that floodplain compensation is accomplished. The final drawings will be attached to the FRA.
4. / Voids / Action
Both RS and DO requested an explanation on the EA stand on voids and its usage for flood storage. OP explained that the EA stands on a position that no void can be accepted for flood compensation as from their experience they are usually enclosed at a later stage. The EA has no means to ensure, not even via Section 106 Agreement, that the will remain open during the whole life of the development (50 years) therefore they will not accept it in floodplain compensation.
5. / Access ramp to the underground car park / Action
The underground car park will be made water resilient and protected from being flooded via access by setting the top of the access ramp at 8.96mAOD (1% probability floodplain + climate change). Flood gates will be provided at times of forecasts of severe floods to prevent it from flooding.
6. / Finished Floor Levels
BL enquired which levels we should considered for the minimum finished flood levels for the residential dwellings:
- 600mm freeboard above the 1% probability flood levels 8.41m +600mm = 9.01m? or
- 300mm above the 1 % + 20% added for climate change flood levels 8.96m +300mm = 9.26m?
OP answered that he would agree to design the FFL at the 1 in 100-year flood level including 300mm for the climate change 8.96m providing that building will be made water resilient for further 300mm (to 9.26mAOD. For water proofing techniques OW directed DO to the EA website.
7. / Dry Access
BL progressed to the next issue of the meeting: provision of the dry access. Pedestrian access to land wholly outside the floodplain should be provided as a means of escape.
OW has stated that dry access is required to be provided only from residential developments. However as the scheme comprises of residential dwellings and a hotel a dry route will need to be provided to all buildings at a minimum level of 8.41m AOD. DO has commented that due to the FFL being at 8.96m AOD, the dry access will probably be provided at the same level.
8. River Thames Buffer zone
RAS explained that some works will be proposed to the access road to the CigaretteIslandPark. The road will need to be connected to the new Cross Roads proposed south of the Hampton Court Bridge and will provide main access to the development. As the road currently lies within 8m of the top of the River Thames Bank, drainage contsent will need to be applied for.
OP has confirmed that as the Flood Risk Consent (changed name of Drainage Consent) will definitely need to be applied for, as the road will be realigned away from the Landing Stage; he however did not envisage that this will present a major problem.
9. / River Mole Buffer Zone/BiodiversityBL confirmed that the edge of the River Mole is not well defined therefore the extent of the buffer zone and distance of the nearest building needs to be agreed. RAS presented photographs of the River Mole and River Thames to illustrate the current situation.
OP stated that the purpose of the buffer zone was to allow space for the maintenance of the river banks and maintaining natural character and of the banks biodiversity. OP confirmed that no buildings would be permitted within the buffer zone but the extend of it could be negotiable and shortened to possibly 5 metres. Building works and landscaping would be acceptable as long as they did not have an adverse impact on flood defence or biodiversity of the river and its banks. OP suggested that further contacts be made with conservation team of the EA to discuss the proposals in details.
10. / Surface Water Strategy
BL has noted that the site is currently approximately 98% impermeable and will probably remain so. OP noted that the EA will look to maintain the a ‘status quo’ i.e. not worsen the current conditions and not to increase the surface water run-off from the site.
Additionally, the EA promotes Sustainable Drainage Techniques (SUDS) and would recommend incorporating some of the techniques f.e. permeable paving into the design. Oil interceptors will have to be fitted before the water could be discharged into the surface water sewers. BL noted that due to the location of the site at the confluence of two major rivers, the groundwater levels (2.8m bgl and fluctuating) may prevent the use of infiltration techniques but will look at the issue.
11. / Groundwater / Action
RAS commented that the underground car park will be located below the groundwater levels. A drainage blanket or trenches will be provided to convey water below and around the structure.
12. / Summary
Agreed levels to be implemented in the scheme design:
- floodplain compensation: 8.41m AOD
- Minimum FFL levels for residential dwellings 8.96 m AOD
- Ramp to underground car park: 8.96m AOD
- Dry Access 8.96m (minimum 8.41mAOD)
13. / The Way Forward
BL to write up the notes of the meeting and distribute them to the EA for approval and to members of the design team. / BL
BL to continue liaising with DO and SB to achieve the EA’s requirements set out above and deliver Flood Risk assessment. / BL
OP to supply a copy of the Lower Thames Flood Study and contact to Polly Graham / OP