Guidance for architects, and developers on flood risk, drainage and phase 1 contaminated land reports is summarised below with links to the key summary documents.
Barnet is primarily residential with some light industrial history and several small former landfill sites. The investigation for potentially contaminated sites concentrates on current or previously industrial areas, though information on past and present land use will be gathered for the whole borough. The underlying geology of the borough consists of London clay, which reduces the likelihood of offsite migration of contaminants. The borough also has few ground water protection zones.
For potentially contaminated land, the developer will be required to carry out a Preliminary Risk Assessment which will help determine the potential for contamination at a development site.
Where necessary a full site investigation should be undertaken, considering both the possible risk to future users of the site and hazards to ground and surface water quality.
Before development can start, planning conditions may require that appropriate remedial measures are agreed with the planning authority and carried out in line with current guidelines, having regard to relevant legislation (Part 2A of the Environmental Protection Act and Contaminated Land Regulations). The developer will be required to provide a report verifying that the works have been carried out as agreed. This will normally be achieved by setting conditions on planning permissions.
Barnet’s range of guidance does not state whether a Site Walkover is required at the Phase 1 stage. If not specified in the developer’s planning condition, the developer should correspond with the Council’s Contaminated Land officer and an environmental consultant such as GeoSmart to review whether a walkover may be required following details being provided of the Site, its proposed development and past uses.
The London Borough of Barnet has experienced flooding incidents every year since 2010; even though this flooding was in general local these events highlight that flooding can occur at any time with potentially devastating consequences. The surface water flooding experienced in 2012 further reinforces the risk faced by the London Borough of Barnet from local sources. With the expectancy for flooding frequency and severity to increase due to a changing climate, flood risk management has never been more important.
All development proposals within Barnet must comply with the flood risk assessment and management requirements set out in the NPPF and the associated technical guidance on flood risk over the lifetime of the development and have regard to measures proposed in the Thames River Basin Catchment Flood Management Plans (CFMPs).
Dollis Brook, Pymme’s Brook and Silk Stream are the primary watercourses which drain the majority of the Barnet area. Mutton Brook, Dean’s Brook, Folly Brook, Decoy Brook and Edgwarebury Brook are significant tributaries, which flow into Dollis Brook and Silk Stream.
Sewer flood risk
The most significant areas which have encountered DG5 flooding is in the northern area of Edgware, to the west of the borough boundary; the Postal District code HA8 has experienced 234 counts of sewer flooding within Barnet. The Postal sector HA8 8 has recorded the highest number of sewer flooding incidents with 143. Other Postal sector areas with significant historical sewer flooding include NW9 7, with 85 recorded incidents, and HA8 9, with 68 sewer flooding incidents.
Surface water flood risk
Thirty three Critical Drainage Areas (CDAs) were identified within the Borough. These were the areas within Barnet which were considered to be at the highest risk of surface water flooding.
Flooding from artificial sources – reservoirs and canals
The south west boundary of Barnet is marked by the River Brent and the Silk Stream confluence at the Brent Reservoir. The 598,000 m² Brent Reservoir is owned and maintained by the Canal and River Trust Charity and feeds the Paddington Arm of the Grand Union Canal. There are three reservoirs located within the London Borough of Barnet: Arkley reservoir, which runs alongside Rowley Green Lane; Bury Farm reservoir; and Stoney Wood. Failure of the defences of these reservoirs could have significant consequences to properties in the vicinity.
Epping Forest District Barnet Council
Reducing or slowing the amount of rainfall (run off) entering the drainage network is important to help reduce flood risk both in Barnet and further downstream. The borough has 14kms of streams and brooks. The North London Strategic Flood Risk Assessment identified fluvial flooding from Dollis Brook, Silk Stream, Pymmes Brook and their associated tributaries as the primary source of flood risk in the borough. Surface water flooding in Barnet presents a low to moderate risk, and sewer flooding as low risk.
Sustainable Urban Drainage techniques such as porous paving should be used where possible to reduce flood risk and the Mayor’s drainage hierarchy applied.
A Drainage Strategy will be required when submitting a planning application for the following types of development:
The drainage strategy requires information on the proposed SuDS and surface water runoff discharge destination in line with Policy 5.13 of the London Plan. It also requires supporting calculations on the greenfield and proposed development’s peak discharge rates and water storage volumes for different rainfall events. These calculations need to ensure that proposed developments are designed to the Non-statutory technical standards for sustainable drainage systems.
Where the SuDS Suitability Mapping indicates that infiltration-based SuDS are potentially suitable or uncertain, the drainage strategy must investigate the use of infiltration techniques through site-specific infiltration testing or bore hole records. This level of evidence must be provided to justify use of any non-infiltration-based surface water management techniques.
SuDS need to be designed with the landscape features of the development site in mind, maximising additional benefits including, but not limited to, environmental, water quality and amenity enhancement.
Permission to connect to the local sewer network and pipes should be sought from the relevant Water and Sewerage Company. Evidence demonstrating that an agreement in principle for any proposed new sewer connections has been reached must be provided as part of the drainage strategy. Failure to do so could impact the detailed design and overall drainage strategy for the site.
All applications with surface water drainage are required to complete the Barnet Sustainable Drainage Assessment Form for Full or Reserved Matters applications.
Groundwater flooding in Barnet has been found to be a relatively low risk due to the impermeable geology (primarily London Clay) and depth of the groundwater table.
If, for any minor or major development, the development site intersects with an area with >= 25% susceptibility to groundwater flooding, the applicant should address this issue by providing a Screening Assessment (as a minimum) that either confirms low risk of impacts (and therefore no further work is needed) or advises the level of impact and the associated mitigation actions proposed.
GeoSmart can assist you with such an assessment.