Borough of Strategic Flood Risk Mapping Assessment Level 2 Detailed Site Summary Tables

Site Code BR1

Site Name Brookfield

Site details Area 146.70ha

Current land use Mixed use including retail park, predominately greenfield.

Proposed site use Mixed Use • Wormleybury Brook • • Unnamed Ordinary Watercourse (north) tributary of Wormleybury Brook Unnamed Ordinary Watercourse (centre) tributary of Turnford Existing drainage • Brook features • Several Unnamed Ordinary watercourses (east) tributaries of Turnford Brook) • Unnamed watercourse (south) • • New River canal Proportion of site at risk FZ3b FZ3a FZ2 FZ1 6% 10% 12% 88% Multiple watercourses cross the site and therefore Flood zones 3a and 3b are associated with these. These include Wormleybury Brook in the north Sources of of the site flowing west to east, Turnford Brook across the centre of the site flowing west to east and an unnamed Ordinary watercourse that flows flood risk Fluvial west to east along the southern boundary of the site. Additional fluvial flood risk is posed by several tributaries of these watercourses within the site. The site’s eastern boundary is where the Environment Agency detailed hydraulic models commence, and therefore 2D modelling has been undertaken on the tributaries upstream of the A10 in order to provide an indication of flood risk. This should be confirmed in a site-specific Flood Risk Assessment. Proportion of site at risk (uFMFSW) 30-year 100-year 1,000-year 5% 8% 17% Surface Water The uFMfSW shows there are bands of surface water flooding adjacent to the watercourses that flow across the site in the north and centre or along the site boundary in the south. These bands are present in the 30-year event and increase in lower return periods. In the 1000-year there are widespread narrow bands of flooding across the site, but most prominently in the west. Borough of Broxbourne Strategic Flood Risk Mapping Assessment Level 2 Detailed Site Summary Tables

Site Code BR1

Site Name Brookfield

Site details Area 146.70ha

Current land use Mixed use including retail park, predominately greenfield.

Proposed site use Mixed Use The southern-most area of the site is within the reservoir inundation area of the North Flood Storage Area in the event of a breech. Mapping suggest depths in this area may be between 0.3 and 2.0 metres with velocities between 0.5 and 2.0 m/s. Reservoir The level and standard of inspection and maintenance required under the Act means that the risk of flooding from reservoirs is relatively low. The risk to development from reservoirs is residual but developers should consider reservoir flooding during the planning stage. Canal The New River Canal intersects the site. The Environment Agency’s historic flood map does not show the site as Flood history having flooded in the past. Defence Type Standard of Condition Protection Flood risk Defences management - - - infrastructure This site is not protected by any formal flood defences. Residual risk - The site is not covered by the Environment Agency’s flood warning Flood warning service. Emergency Dry access and egress for the site during fluvial flooding is available via planning Access and the A10 to the east in all scenarios. Dry access and egress during surface egress water scenarios is lost during the 30-year event due to surface water flooding along the unnamed road leading to the A10 Interchange. Climate change River Basin District Central Higher Upper allowances for Central End ‘2080s’ 25% 35% 70%

% increase in flood extent compared to Flood Zone 3a +1% +1% +2% Climate Climate change mapping indicates the following impacts for the future: Change • The increase in Flood Zone 3a outline with differing climate change allowances is minimal as flooding remains with a well-defined floodplain. Implications for • The depth, velocity or hazard of flooding may also increase in the the site area affected. • Climate change may also increase the extent, depth and frequency of surface water flooding. • Climate change modelling was also undertaken using 2D modelling. Borough of Broxbourne Strategic Flood Risk Mapping Assessment Level 2 Detailed Site Summary Tables

Site Code BR1

Site Name Brookfield

Site details Area 146.70ha

Current land use Mixed use including retail park, predominately greenfield.

Proposed site use Mixed Use • Source control techniques are likely to be suitable for this site. Mapping suggest groundwater flooding may be an issue at the site as such infiltration techniques may not be suitable. • Infiltration techniques may be suitable, providing the site is not at medium to high risk from groundwater flooding and they are located outside of GSP Zone 1. • Detention features may be feasible providing site slopes are <5% at Broadscale the location of the detention feature. If groundwater flooding is a assessment of medium to high risk to the site, then a liner may be required to possible SuDS mitigate against potential contamination issues. • Filtration systems are probably suitable providing site slopes are <5% and the depth to the water table is >1m. If the site has contamination Requirements issues, or is at medium to high risk from groundwater flooding, then for drainage a liner will be required. control and • All forms of conveyance features are likely to be suitable. Where slopes are >5%, features should follow contours or utilise check impact dams to slow flows. mitigation The site is partially located within GSP Zone 1 and 2. Infiltration techniques should only be used outside of GSP Zone 1 and following the Groundwater granting of any required environmental permits from the Environment Source Protection Agency for GSP Zone 2, although it is possible that infiltration may not be Zone permitted. Proposed SuDS should be discussed with relevant stakeholders (LPA, LLFA and EA) at an early stage to understand possible opportunities and constraints. This site has areas within its boundary designated by the Environment Agency as being a landfill site. A thorough ground investigation will be Historic Landfill required as part of a detailed site-specific FRA to determine the extent of Site the contamination and the impact this may have on SuDS. As such proposed SuDS should be discussed with the relevant stakeholders (LPA, LLFA and EA) at an early stage to understand possible constraints. The Sequential Test will need to be passed before the Exception Test is applied. The Exception Test will be required in the following scenarios If ‘More Vulnerable’ and ‘Essential Infrastructure’ is located in NPPF and • Exception Test FZ3a. planning requirements • If ‘Highly Vulnerable’ development is located in FZ2. implications • If ‘Essential Infrastructure’ is located in Flood Zone 3b Development will not be permitted in the following scenarios • ‘Highly Vulnerable’ infrastructure within FZ3a and FZ3b. ‘More Vulnerable’ and ‘Less Vulnerable’ infrastructure within FZ3b. Borough of Broxbourne Strategic Flood Risk Mapping Assessment Level 2 Detailed Site Summary Tables

Site Code BR1

Site Name Brookfield

Site details Area 146.70ha

Current land use Mixed use including retail park, predominately greenfield.

Proposed site use Mixed Use • At the planning application stage, a site-specific Flood Risk Assessment will be required if any development is located within Flood Zones 2 and 3. All sources of flooding should also be considered. • Flood risk at the site should be confirmed by undertaking more detailed hydraulic modelling at the site-specific level, which incorporates channel survey, as part of a site-specific Flood Risk Assessment. This 2D modelling provides an indication of flood risk for the purpose of the SFRA. • As above, climate change modelling should be undertaken using the relevant allowances for the type of development and level of risk. • Consultation with the Lead Local Flood Authority and the Environment Agency should be undertaken at an early stage. • Resilience measures will be required if buildings are situated in the flood risk area. • Onsite attenuation schemes would need to be tested against Requirements and the hydrographs of: guidance for site- o Wormleybury Brook o Turnford Brook specific Flood o Unnamed ordinary watercourse (south) Risk Assessment o Rags Brook to ensure flows are not exacerbated downstream within the catchment. • New or re-development should adopt exemplar source control SuDS techniques to reduce the risk of frequent low impact flooding due to post-development runoff. • Assessment for runoff should include allowance for climate change effects. • Risk from the watercourses identified in ‘Existing drainage features’ should be considered. • New development must seek opportunities to reduce overall level of flood risk at the site, for example by: o Reducing volume and rate of runoff o Relocating development to zones with lower flood risk o Creating space for water and flood storage. • Green infrastructure should be considered within the mitigation measures for surface water runoff from potential development and consider using Flood Zones 2 and 3 as public open space. Mapping Information Borough of Broxbourne Strategic Flood Risk Mapping Assessment Level 2 Detailed Site Summary Tables

Site Code BR1

Site Name Brookfield

Site details Area 146.70ha

Current land use Mixed use including retail park, predominately greenfield.

Proposed site use Mixed Use

Flood Zones Flood Zones 2 and 3a are based on the Environment Agency’s Flood Zone 2 and 3. The SFRA has identified Flood Zone 3b as land which would flood with an annual probability of 1 in 20 years. Flood Zone 3b has been derived from 2D hydraulic modelling, undertaken for this SFRA. Climate change The climate change allowances for the ‘2080’s epoch were modelled using 2D modelling techniques for the purposes of the SFRA. Due to the nature of the modelling, defences have not been accounted for and therefore mapping shows the ‘undefended’ scenario. The mapping provides a strategic assessment of climate change risk – developers should undertake detailed modelling of climate change allowances as part of a site-specific FRA. Surface Water The updated Flood Map for Surface Water has been used to define areas at risk from surface water flooding. Reservoir Reservoir flood risk depth and velocities are based on the ‘Flood risk from reservoirs’ mapping provided on the Government’s ‘Long term flood risk information’ website. Depth, velocity and hazard Depth, velocity and hazard mapping for the 1 in 100-year event (Flood mapping Zone 3a) have been taken from the modelling undertaken as part of this SFRA, as the site is not covered by detailed hydraulic models. As a result, there may be some small discrepancies between the extent of the depth, velocity and hazard grids with the extent of Flood Zone 3a on the Environment Agency’s website, because this study has used more up-to- date modelling data.