Chelmsford Borough Council LDF: Level 1 SFRA

Transcription

1 Chelmsford Borough Council LDF: Level 1 SFRA Urban Capacity Study Sites PPS25 Flood Zones 2007 Figure B20-1 Preliminary Core Strategy Assessment Flood Zone Potential Housing Potential Employment 1, 2 & 3 Preliminary assessment shows that there is a varied distribution of the allocations between Flood Zone 1, 2 & 3. Development should utilise the low probability sites where possible. Some community and educational facilities are included as part of the proposal. The River Chelmer runs to the north of the allocated sites. These sites may benefit if the flood storage scheme was put in place Problem drainage areas Data not available The principles of the sequential test should guide the master planning of these sites. According to PPS25 residential development would not be permitted within the functional floodplain of the River Chelmer. Development of residential should, where possible, be steered into sites of lower Flood Risk, i.e. Flood Zone 1 or Flood Zone 2. Urban Capacity Study Sites Functional Floodplain Environment Agency Flood Zone 3 Environment Agency Flood Zone 2 DRAWING MAY BE USED ONLY FOR THE PURPOSE INTENDED AND ONLY WRITTEN DIMENSIONS SHALL BE USED This map is Based upon the Ordnance Survey mapping with the permission of the Controller of Her Majesty s Stationery Office. Crown. Unauthorised reproduction Infringes Crown and may lead to prosecution or civil proceedings. Chelmsford BC Licence Flood Risk Assessment In accordance with PPS25 if the site is over 10 dwellings or 1 hectare in size a FRA will be required to demonstrate that the development will not increase the risk of overland flow. Surface water attenuation onsite with respect to increased infiltration and storage will be required to ensure post development rates are equal to or lower than pre-development rates. Water recycling and rainwater harvesting could be considered as a means of reducing the amount of surface water from the site.

2 Chelmsford Borough Council LDF: Level 1 SFRA Urban Capacity Study Sites Figure B20-2 PPS25 Flood Zones 2107 Urban Capacity Study Sites Functional Floodplain plus 100 yrs of Climate Flood Zone 3 plus 100 yrs of Climate Flood Zone 2 plus 100 yrs of Climate USED ONLY FOR THE PURPOSE INTENDED AND ONLY WRITTEN This map is Based upon the Ordnance Survey mapping with the permission of the Controller of Her Majesty s Stationery Office. Crown. Unauthorised reproduction Infringes Crown and may lead to prosecution or civil proceedings. Chelmsford BC Licence The PPS25 current flood zones, (2007, series BX-1), were created by Black and Veatch as part of the River Chelmer Flood Risk Study (2006). This modelling has been refined for the PPS25 climate change scenarios (series BX-2, 2107), using more up to date topographical information and channel cross sections as part of the Halcrow modelling completed for the Environment Agency Flood Compensation Study (2007 ongoing). Details of the modelling approach and refinements are outlined in Annex A of this report.

3 Chelmsford Borough Council LDF: Level 1 SFRA North Chelmsford Area Action Plan PPS25 Flood Zones 2007 Figure B21-1 Preliminary Core Strategy Assessment Flood Zone 1 Potential Housing Preliminary assessment shows that any new allocations for housing development at Broomfield can reasonably be located in flood zone 1. Potential Employment N/A River Chelmer Problem drainage areas No data available. In accordance with PPS25 if the site is over 10 dwellings or 1 hectare in size a FRA will be required to demonstrate that the development will not increase the risk of overland flow. Functional Floodplain Environment Agency Flood Zone 3 Environment Agency Flood Zone 2 USED ONLY FOR THE PURPOSE INTENDED AND ONLY WRITTEN This map is Based upon the Ordnance Survey mapping with the permission of the Controller of Her Majesty s Stationery Office. Crown. Unauthorised reproduction Infringes Crown and may lead to prosecution or civil proceedings. Chelmsford BC Licence Flood Risk Assessment Due to the scale of the potential Greenfield development on these sites surface water considerations should be incorporated into the site master planning at the earliest opportunity. Surface water attenuation onsite with respect to increased infiltration and storage will be required to ensure post development rates are equal to or lower than pre-development rates, in this case greenfield. Water recycling and rainwater harvesting could be considered as a means of reducing the amount of surface water from the site.

4 Chelmsford Borough Council LDF: Level 1 SFRA North Chelmsford Area Action Plan Figure B21-2 PPS25 Flood Zones 2107 Functional Floodplain plus 100 yrs of Climate Flood Zone 3 plus 100 yrs of Climate Flood Zone 2 plus 100 yrs of Climate USED ONLY FOR THE PURPOSE INTENDED AND ONLY WRITTEN This map is Based upon the Ordnance Survey mapping with the permission of the Controller of Her Majesty s Stationery Office. Crown. Unauthorised reproduction Infringes Crown and may lead to prosecution or civil proceedings. Chelmsford BC Licence The PPS25 current flood zones, (2007, series BX-1), were created by Black and Veatch as part of the River Chelmer Flood Risk Study (2006). This modelling has been refined for the PPS25 climate change scenarios (series BX-2, 2107), using more up to date topographical information and channel cross sections as part of the Halcrow modelling completed for the Environment Agency Flood Compensation Study (2007 ongoing). Details of the modelling approach and refinements are outlined in Annex A of this report.

5 Chelmsford Borough Council LDF: Level 1 SFRA North Chelmsford Area Action Plan PPS25 Flood Zones 2007 Figure B22-1 Preliminary Core Strategy Assessment Flood Zone 1 Potential Housing North Chelmsford Area Action Plan Preferred options (April 07). Preliminary assessment shows that any new allocations for housing development located in the greenfield areas to the north of the A12, to the east of broomfield and west of Boreham, can reasonably be located in flood zone 1. Potential Employment N/A River Chelmer to the West Problem drainage areas No data available. In accordance with PPS25 if the site is over 10 dwellings or 1 hectare in size a FRA will be required to demonstrate that the development will not increase the risk of overland flow. Functional Floodplain Environment Agency Flood Zone 3 Environment Agency Flood Zone 2 USED ONLY FOR THE PURPOSE INTENDED AND ONLY WRITTEN This map is Based upon the Ordnance Survey mapping with the permission of the Controller of Her Majesty s Stationery Office. Crown. Unauthorised reproduction Infringes Crown and may lead to prosecution or civil proceedings. Chelmsford BC Licence Flood Risk Assessment Due to the scale of the potential Greenfield development on these sites surface water considerations should be incorporated into the site master planning at the earliest opportunity. Surface water attenuation onsite with respect to increased infiltration and storage will be required to ensure post development rates are equal to or lower than pre-development rates, in this case greenfield. Water recycling and rainwater harvesting could be considered as a means of reducing the amount of surface water from the site.

6 Chelmsford Borough Council LDF: Level 1 SFRA North Chelmsford Area Action Plan Figure B22-2 PPS25 Flood Zones 2107 Functional Floodplain plus 100 yrs of Climate Flood Zone 3 plus 100 yrs of Climate Flood Zone 2 plus 100 yrs of Climate USED ONLY FOR THE PURPOSE INTENDED AND ONLY WRITTEN This map is Based upon the Ordnance Survey mapping with the permission of the Controller of Her Majesty s Stationery Office. Crown. Unauthorised reproduction Infringes Crown and may lead to prosecution or civil proceedings. Chelmsford BC Licence The PPS25 current flood zones, (2007, series BX-1), were created by Black and Veatch as part of the River Chelmer Flood Risk Study (2006). This modelling has been refined for the PPS25 climate change scenarios (series BX-2, 2107), using more up to date topographical information and channel cross sections as part of the Halcrow modelling completed for the Environment Agency Flood Compensation Study (2007 ongoing). Details of the modelling approach and refinements are outlined in Annex A of this report.

7 Chelmsford Borough Council LDF: Level 1 SFRA North Chelmsford Area Action Plan PPS25 Flood Zones 2007 Figure B23-1 Preliminary Core Strategy Assessment Flood Zone 1 Potential Housing North Chelmsford Area Action Plan Preferred options (April 07) for housing. Preliminary assessment shows that any new allocations for housing development at Broomfield can reasonably be located in flood zone 1. Potential Employment N/A N/A Problem drainage areas No data available. In accordance with PPS25 if the site is over 10 dwellings or 1 hectare in size a FRA will be required to demonstrate that the development will not increase the risk of overland flow. Functional Floodplain Environment Agency Flood Zone 3 Environment Agency Flood Zone 2 USED ONLY FOR THE PURPOSE INTENDED AND ONLY WRITTEN This map is Based upon the Ordnance Survey mapping with the permission of the Controller of Her Majesty s Stationery Office. Crown. Unauthorised reproduction Infringes Crown and may lead to prosecution or civil proceedings. Chelmsford BC Licence Flood Risk Assessment Due to the scale of the potential Greenfield development on these sites surface water considerations should be incorporated into the site master planning at the earliest opportunity. Surface water attenuation onsite with respect to increased infiltration and storage will be required to ensure post development rates are equal to or lower than pre-development rates, in this case greenfield. Water recycling and rainwater harvesting could be considered as a means of reducing the amount of surface water from the site.

8 Chelmsford Borough Council LDF: Level 1 SFRA North Chelmsford Area Action Plan Figure B23-2 PPS25 Flood Zones 2107 Functional Floodplain plus 100 yrs of Climate Flood Zone 3 plus 100 yrs of Climate Flood Zone 2 plus 100 yrs of Climate USED ONLY FOR THE PURPOSE INTENDED AND ONLY WRITTEN This map is Based upon the Ordnance Survey mapping with the permission of the Controller of Her Majesty s Stationery Office. Crown. Unauthorised reproduction Infringes Crown and may lead to prosecution or civil proceedings. Chelmsford BC Licence The PPS25 current flood zones, (2007, series BX-1), were created by Black and Veatch as part of the River Chelmer Flood Risk Study (2006). This modelling has been refined for the PPS25 climate change scenarios (series BX-2, 2107), using more up to date topographical information and channel cross sections as part of the Halcrow modelling completed for the Environment Agency Flood Compensation Study (2007 ongoing). Details of the modelling approach and refinements are outlined in Annex A of this report.

9 Chelmsford Borough Council: Level 2 SFRA South Woodham Ferrers Figure B24 Hazard Zone Results Flooding velocities were generally low across the area, almost always well below 1.0 m/s for all scenarios. Therefore, water depth was the more significant contributing factor to the flood hazard classifications. 1 in 200 years + climate change scenario (Figure B27) RESULTS Location of Main Flood Depth THIS DRAWING MAY BE USED ONLY FOR THE PURPOSE INTENDED AND ONLY WRITTEN This map is produced from Ordinance Survey material with the permission of Ordinance Survey on the behalf of the controller of Her Majesty s Stationary Office. Crown copyright Unauthorised reproduction infringes Crown copyright and may lead to prosecution or civil proceedings. Scott Wilson GD The maximum flood depth observed during the 1 in 200 year climate change scenario is 4.98 metres, which was at , (element 6542), behind the flood defences approximately 600 metres southeast of the breach location. By contrast, the location of the maximum flood depths in both of the other, present day cases was at , (element 20717), 350 metres northeast of the breach location. The reason for this difference is that the climate change scenario is assumed to occur in 100 years time. It is assumed that any applicable local flood defences will have been raised above the modelled peak tidal level (plus a freeboard of 300 millimetres) during this time to account for the increased flood risk as a result of climate change. In other words, the climate change scenario only needs to consider flows through the breach itself, whereas the present day cases also take into account overtopping at various locations along the flood defences where the current level of protection is not adequate. For the present day South Woodham Ferrers models, much of the land upstream (to the east) of the breach is flooded via overtopping of the upstream defences. This leads to subsequent overtopping of the defences directly to the north and south of the breach as flows work their way towards the west. Therefore, should a breach occur during an extreme tidal event under the current flood defence regime, it would not be the only source of flooding for much of the area. Whereas, with upgraded flood defences in 100 years time, a breach in the flood defences would be the only source of flooding. This is the reason that the distributions of flood depth between the present and future situations are so different. The depth grid for the 1 in 200 year climate change scenario can be seen in Figure B27. Figure B25 shows the flood cell extent Flood Cell Description 1 in 200 Year event The location of deepest floodwater occurs 350 metres northeast of the breach location, in the swampland east of Compass Gardens and Saltcoats Park, with depths of around 4.3 metres. This occurs approximately 20 minutes after the breach. At this time there is also well above one metre of water in most of the areas to the east of Compass Gardens and Saltcoats Park and much of the nearby parkland is around 2-3 metres underwater at the highest peak of the tide. Much of the land to the southeast and southwest of the breach (Marsh Farm Country Park and land to the west) is also heavily inundated at depths of around 3 metres. Flood depths reduce towards the school and town centre as flooding makes its way to the west, but there is still a substantial amount of inundation. Houses to the north of Marsh Farm Country Park are largely unaffected. As a result, large amounts of land around Compass Gardens and Saltcoats Park, the school and almost all the land to the south of the breach is classified as High Risk. This eases into Low to Medium Risk areas towards Inchbonnie Road, but much of the land between Inchbonnie Road and Ferrers Road is inundated. There are also several large patches of land to the west of the school that is classified as Low to Medium Risk. 1 in 1000 Year event The location of deepest floodwater occurs 350 metres northeast of the breach location, in the swampland east of Compass Gardens and Saltcoats Park, with depths of around 4.9 metres. This occurs approximately 20 minutes after the breach. The flood depth and hazard situation for the 1 in 1000 year case is similar to that described above, but more High Risk zones are seen in the land between Inchbonnie Road and Ferrers Road. Almost all of the Compass Gardens and Saltcoats Park area is now classified as High Risk, as are the school & grounds and much of the Tutors Way and Overmead Drive area. The Low and Medium Risk areas are now more widespread to the west. As for the 1 in 200 year event, the entire area to the south (all land around Marsh Farm Country Park and land to the west) is classified as High Risk. 1 in 200 years + climate change scenario (Figure B26) The inundation during the 1 in 200 year event with climate change is solely from flows that pass through the breach. No overtopping occurs because the flood defences are assumed to have been raised to a suitable level. Therefore, although the peak tidal level for this scenario is higher than the 1 in 1000 year event, the area of inundation and flood hazard categorisations are very similar. In fact, there are marginally less areas of High Hazard witnessed during the climate change, most notably the western edge of Saltcoats Park towards Ferrers Road is now only Medium Risk. Otherwise, the extents and classifications are quite similar. Relevance to Development Considerations for particular Development areas: CH01 causes inundation to the east of South Woodham Ferrers and small areas to the periphery of the town to the south and southwest. Flood depths near to the school in South Woodham Ferrers are approximately 2 metres. General Considerations Specific Development Locations There are four allocated sites in South Woodham Ferrers (urban capacity sites) located within the 1 in 1000 year extent in a medium risk areas. All the land allocated for residential development or redevelopment is located within either the High or Medium Flood Hazard Zones. As this land is generally surrounded by existing residential development, there may be strong non-flood related pressures to redevelop these areas. If the increased flood risk from redevelopment is considered to be outweighed by the overriding pressure for new housing, developers should still aim to minimise the increase in flood risk as a result of development. This could be achieved by local land raising (providing all conditions are met), and using Sustainable Drainage Systems. In addition, ensuring finished floor levels are above flood water levels and using flood water compatible building materials will reduce the impact of a flood on the development. An appropriate flood warning and evacuation procedure would be required to reduce the risk to life from inundation.