FloodMetric
020 8156 6076
www.aegaea.com
Site Address:
Prepared On: 12 February 2025
Purpose: The FloodMetric report informs you about risk to your site from fluvial, pluvial, tidal and historic sources. This report also provides critical local policy information related to drainage and flood risk issues. Unless indicated, the FloodMetric report is unlikely to be suitable for submission to a Local Planning Authority. To the best of our knowledge the data is current and appropriate at the time of reporting.
| Canal Risk | Red | Amber | Green |
|---|---|---|---|
| Historical Risk | Red | Amber | Green |
| Regulatory Risk | Red | Amber | Green |
| Reservoir Risk | Red | Amber | Green |
| Rivers And Sea Risk | Red | Amber | Green |
| Watercourse Risk | Red | Amber | Green |
The site falls under the jurisdiction of a number of authorities that can be contacted for specific purposes.
- For flood issues the Lead Local Flood Authority is Hillingdon Council
- For development planning the Local Planning Authority is either Hillingdon Council or Slough Borough Council
- For water supply the Water Company is one of Thames WaterAffinity WaterAffinity Water
- For sewers and drainage the Water Company is Thames Water
- For flood models and details of flood defences the site is in the Thames and Hertfordshire and North London Environment Agency area
An initial search suggests the site is within Flood Zone 3 (11%), Flood Zone 2 (13%), and Flood Zone 1 (76%) as defined by the EA Flood Map for Planning and Strategic Flood Risk Assessment of the Local Planning Authority.
The nearest main river is on the site.
Pluvial flooding can occur during prolonged or intense storm events when the infiltration potential of soils, or the capacity of drainage infrastructure is overwhelmed leading to the accumulation of surface water and the generation of overland flow routes.
Our review of Recorded and Historical Flood Outlines within 250 metres of the site shows the site is within a historical flood extent.
Records of past flooding, mostly since 1946, are published in the Historic Flood Map.
Our review of Recorded and Historical Flood Outlines shows sewer, main river, and unknown fluvial flooding on-site in 2002/03, 2000, 1993, 2001, 2014, and 2021 caused by channel capacity exceeded (no raised defences).
There is no record of historical tidal or sea flooding within 250 metres of the site.
Our review of Recorded and Historical Flood Outlines shows other flooding near the site in 2014 and 2014 caused by groundwater/high water table and groundwater/high water table.
It is also possible for a property to have been affected in the past by heavy or prolonged rain or groundwater. Local Authorities often publish mapping of historical surface water flood incident records. Reviewing these records is out-of-scope for this statement. These would be reviewed in a more detailed report such as a Flood Risk Assessment.
We have compared flood zones taking into account flood defences with flood zones assuming that flood defences have failed. The site appears to be in an area that benefits from flood defences. However, this is only an indication and requires an expert to confirm it.
Local policy documentation does not identify the site as being in a Critical Drainage Area.
Critical Drainage Areas are places where drainage could cause flooding problems elsewhere. They are discrete geographic areas (usually a hydrological catchment) where multiple and interlinked sources of flood risk (surface water, groundwater, sewer, main river and/or tidal) cause flooding in one or more Local Flood Risk Zones (sometimes called surface water flood risk hotspots) during severe weather thereby affecting people, property, or local infrastructure. Land which has been identified by the Environment Agency as having critical drainage areas have a separate, yet similar, function and are referred to as Areas with Critical Drainage Problems (ACDPs).
Large waterbodies or reservoirs that have walls built above the surrounding ground level pose a risk of flooding. Walls could fail due to old age, accident, or because excess flood water has been added to the reservoir. Although a breach is unlikely the consequences would be significant, leading to rapid inundation of the downstream floodplain.
The site is not in an area where the EA provide specific flood alerts and warnings. The occupant of the dwelling should monitor Met Office Weather Warnings to be prepared for extreme weather events
To sign up for the EA flood warning service visit https://www.fws.environment-agency.gov.uk/app/olr/home or call Call Floodline for advice 24x7: Telephone: 0345 988 1188, Textphone: 0345 602 6340
The Local Planning Authority will make decisions on what is, and isn't, allowed to be built based on the policies stated in the National Planning Policy Framework and their Local Plan.
It is also possible that a new Local Plan is being created and that policies from that may be taken into account even though it hasn't been adopted yet.
The Local Authority may publish a number of Supplementary Planning Documents to amplify the policies of their local plan.
This Local Authority is part of Greater London Authority.
- A. Current and expected flood risk from all sources (as defined in paragraph 9.2.12) across London should be managed in a sustainable and cost-effective way in collaboration with the Environment Agency, the Lead Local Flood Authorities, developers and infrastructure providers.
- B. Development Plans should use the Mayor's Regional Flood Risk Appraisal and their Strategic Flood Risk Assessment as well as Local Flood Risk Management Strategies, where necessary, to identify areas where particular and cumulative flood risk issues exist and develop actions and policy approaches aimed at reducing these risks. Boroughs should cooperate and jointly address cross-boundary flood risk issues including with authorities outside London.
- C. Development proposals should ensure that flood risk is minimised and mitigated, and that residual risk is addressed. This should include, where possible, making space for water and aiming for development to be set back from the banks of watercourses.
- D. Developments Plans and development proposals should contribute to the delivery of the measures set out in Thames Estuary 2100 Plan. The Mayor will work with the Environment Agency and relevant local planning authorities, including authorities outside London, to safeguard an appropriate location for a new Thames Barrier.
- E. Development proposals for utility services should be designed to remain operational under flood conditions and buildings should be designed for quick recovery following a flood.
- F. Development proposals adjacent to flood defences will be required to protect the integrity of flood defences and allow access for future maintenance and upgrading. Unless exceptional circumstances are demonstrated for not doing so, development proposals should be set back from flood defences to allow for any foreseeable future maintenance and upgrades in a sustainable and cost-effective way.
- G. Natural flood management methods should be employed in development proposals due to their multiple benefits including increasing flood storage and creating recreational areas and habitat.
- A. Lead Local Flood Authorities should identify - through their Local Flood Risk Management Strategies and Surface Water Management Plans - areas where there are particular surface water management issues and aim to reduce these risks. Increases in surface water run-off outside these areas also need to be identified and addressed.
- B. Development proposals should aim to achieve greenfield run-off rates and ensure that surface water run-off is managed as close to its source as possible. There should also be a preference for green over grey features, in line with the following drainage hierarchy:
- rainwater use as a resource (for example rainwater harvesting, blue roofs for irrigation)
- rainwater infiltration to ground at or close to source
- rainwater attenuation in green infrastructure features for gradual release (for example green roofs, rain gardens)
- rainwater discharge direct to a watercourse (unless not appropriate)
- controlled rainwater discharge to a surface water sewer or drain
- controlled rainwater discharge to a combined sewer.
- C. Development proposals for impermeable surfacing should normally be resisted unless they can be shown to be unavoidable, including on small surfaces such as front gardens and driveways.
- D. Drainage should be designed and implemented in ways that promote multiple benefits including increased water use efficiency, improved water quality, and enhanced biodiversity, urban greening, amenity and recreation.
The Local Planning Authority has 5 policies specifically about flooding:
- A. Current and expected flood risk from all sources (as defined in paragraph 9.2.12) across London should be managed in a sustainable and cost-effective way in collaboration with the Environment Agency, the Lead Local Flood Authorities, developers and infrastructure providers.
- B. Development Plans should use the Mayor's Regional Flood Risk Appraisal and their Strategic Flood Risk Assessment as well as Local Flood Risk Management Strategies, where necessary, to identify areas where particular and cumulative flood risk issues exist and develop actions and policy approaches aimed at reducing these risks. Boroughs should cooperate and jointly address cross-boundary flood risk issues including with authorities outside London.
- C. Development proposals should ensure that flood risk is minimised and mitigated, and that residual risk is addressed. This should include, where possible, making space for water and aiming for development to be set back from the banks of watercourses.
- D. Developments Plans and development proposals should contribute to the delivery of the measures set out in Thames Estuary 2100 Plan. The Mayor will work with the Environment Agency and relevant local planning authorities, including authorities outside London, to safeguard an appropriate location for a new Thames Barrier.
- E. Development proposals for utility services should be designed to remain operational under flood conditions and buildings should be designed for quick recovery following a flood.
- F. Development proposals adjacent to flood defences will be required to protect the integrity of flood defences and allow access for future maintenance and upgrading. Unless exceptional circumstances are demonstrated for not doing so, development proposals should be set back from flood defences to allow for any foreseeable future maintenance and upgrades in a sustainable and cost-effective way.
- G. Natural flood management methods should be employed in development proposals due to their multiple benefits including increasing flood storage and creating recreational areas and habitat.
- A. Lead Local Flood Authorities should identify - through their Local Flood Risk Management Strategies and Surface Water Management Plans - areas where there are particular surface water management issues and aim to reduce these risks. Increases in surface water run-off outside these areas also need to be identified and addressed.
- B. Development proposals should aim to achieve greenfield run-off rates and ensure that surface water run-off is managed as close to its source as possible. There should also be a preference for green over grey features, in line with the following drainage hierarchy:
- rainwater use as a resource (for example rainwater harvesting, blue roofs for irrigation)
- rainwater infiltration to ground at or close to source
- rainwater attenuation in green infrastructure features for gradual release (for example green roofs, rain gardens)
- rainwater discharge direct to a watercourse (unless not appropriate)
- controlled rainwater discharge to a surface water sewer or drain
- controlled rainwater discharge to a combined sewer.
- C. Development proposals for impermeable surfacing should normally be resisted unless they can be shown to be unavoidable, including on small surfaces such as front gardens and driveways.
- D. Drainage should be designed and implemented in ways that promote multiple benefits including increased water use efficiency, improved water quality, and enhanced biodiversity, urban greening, amenity and recreation.
- A) Development proposals in Flood Zones 2 and 3a will be required to demonstrate that there are no suitable sites available in areas of lower flood risk. Where no appropriate sites are available, development should be located on the areas of lowest flood risk within the site. Flood defences should provide protection for the lifetime of the development. Finished floor levels should reflect the Environment Agency's latest guidance on climate change.
- B) Development proposals in these areas will be required to submit an appropriate level Flood Risk Assessment (FRA) to demonstrate that the development is resilient to all sources of flooding.
- C) Development in Flood Zone 3b will be refused in principle unless identified as an appropriate development in Flood Risk Planning Policy Guidance. Development for appropriate uses in Flood Zone 3b will only be approved if accompanied by an appropriate FRA that demonstrates the development will be resistant and resilient to flooding and suitable warning and evacuation methods are in place.
- D) Developments may be required to make contributions (through legal agreements) to previously identified flood improvement works that will benefit the development site.
- E) Proposals that fail to make appropriate provision for flood risk mitigation, or which would increase the risk or consequences of flooding, will be refused.
- A) Applications for all new build developments (not conversions, change of use, or refurbishment) are required to include a drainage assessment demonstrating that appropriate sustainable drainage systems (SuDS) have been incorporated in accordance with the London Plan Hierarchy (Policy 5.13: Sustainable drainage).
- B) All major new build developments, as well as minor developments in Critical Drainage Areas or an area identified at risk from surface water flooding must be designed to reduce surface water run-off rates to no higher than the pre-development greenfield run-off rate in a 1:100 year storm scenario, plus an appropriate allowance for climate change for the worst storm duration. The assessment is required regardless of the changes in impermeable areas and the fact that a site has an existing high run-off rate will not constitute justification.
- C) Rain Gardens and non householder development should be designed to reduce surface water run-off rates to Greenfield run-off rates.
- D) Schemes for the use of SuDS must be accompanied by adequate arrangements for the management and maintenance of the measures used, with appropriate contributions made to the Council where necessary.
- E) Proposals that would fail to make adequate provision for the control and reduction of surface water run-off rates will be refused.
- F) Developments should be drained by a SuDs system and must include appropriate methods to avoid pollution of the water environment. Preference should be given to utilising the drainage options in the SuDS hierarchy which remove the key pollutants that hinder improving water quality in Hillingdon. Major development should adopt a 'treatment train' approach where water flows through different SuDS to ensure resilience in the system.
The Council will require new development to be directed away from Flood Zones 2 and 3 in accordance with the principles of the National Planning Policy Framework (NPPF).
The subsequent Hillingdon Local Plan: Part 2 -Site Specific Allocations LDD will be subjected to the Sequential Test in accordance with the NPPF. Sites will only be allocated within Flood Zones 2 or 3 where there are overriding issues that outweigh flood risk. In these instances, policy criteria will be set requiring future applicants of these sites to demonstrate that flood risk can be suitably mitigated.
The Council will require all development across the borough to use sustainable urban drainage systems (SUDS) unless demonstrated that it is not viable. The Council will encourage SUDS to be linked to water efficiency methods. The Council may require developer contributions to guarantee the long term maintenance and performance of SUDS is to an appropriate standard.
Base map and data from OpenStreetMap and OpenStreetMap Foundation (CC-BY-SA) © https://www.openstreetmap.org and contributors
Contains public sector information licensed under the Open Government Licence v3.0