Have you double-checked that the drainage scheme complies with the mandatory category: Sur 1 ‘Management of Surface Water Runoff from developments’?
A Flood Risk Assessment is not mandatory under Sur 2 (Flood Risk). It is mandatory for
Sur 1 (Surface Water Run-off). This is required even for developments which are not shown to be in a Flood Zone and will require a Flood Risk Assessment (FRA) under the CSH. This is because the Environment Agency’s flood maps only show flooding from rivers or the sea. There are other sources of flooding which may affect a site and a Flood Risk Assessor will be required to investigate these, for example for Sur 2 rainwater and sewer (pluvial) and groundwater flooding.The report should be carried out in accordance with Planning Policy Statement PPS25 and the practice companion guide.
This section had a major update in April 08 to align the Code criteria with the government’s planning policy statement PPS25: ‘Development and Flood Risk’ and with the ‘Interim Code of Practice for Sustainable Drainage Systems (SUDS)’.
Just over a year on, some projects are struggling to meet the mandatory requirements, as they weren’t originally designed to these criteria.
To meet these standards it is necessary to consider design options at the master planning stage as the requirement will have an effect on site layout, development densities and built form as well as site infrastructure requests.
The criteria aim to encourage new developments to avoid runoff then if this is not possible, reduce runoff, and finally delay runoff, to ensure that any new development has minimal or no negative impact on the potential for local or ‘downstream’ flooding.
‘The peak rate of runoff should be no greater for the developed site than it was for the pre-developed site’, factoring in increases due to climate change of up to 30% over the life of the development. On some sites meeting this requirement will be relatively simple; on others it may be more challenging.
If the new development will increase the volume of runoff - where the overall impermeable area increases - the additional volume has to be reduced to zero, for example, by using infiltration methods, such as a soak away, or by reuse in the dwelling through a rainwater harvesting system.
If ground conditions won’t allow infiltration, you have to consider rainwater harvesting to reduce volume as far aspossible. If rainwater harvesting is not possible simply stating this, won’t comply, and would lead to a zero Code level.
Robust evidence must be provided based on technical performance or specific site constraints. The British Standard for Rainwater harvesting, BS 8515 2009, provides guidance to designers on sizing systems. Cost and maintenance burdens are not appropriate reasons for failing to meet the criteria required.
If there is any residual additional volume after using a soak away and/or rainwater harvesting, this must be discharged at a low rate as per the criteria hierarchy.
To go beyond the mandatory level, SUDS need to be specified to improve the quality of water discharged by either ensuring no discharge to the watercourse for the first 5mm of rain - roughly half of all rainfall events - or by having written agreements covering the SUDS ownership, maintenance and operation.
Key to success in meeting the criteria is to ensure the advisors are experienced in designing SUDS, up to speed with the Code criteria and are brought in at very early stages to ensure the most appropriate and cost-effective solutions can be applied.
Adoption of SUDS also needs to be negotiated early on, being aware that the adopter may need time to assess (possibly unfamiliar) maintenance liabilities in detail before entering into agreements.
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SUR1 - Surface Water Run-Off for the Code 2010
The Code now has the following changes:
• Clearer guidance in the Assessment Criteria
• Clearer definition of ‘Appropriately Qualified Professional’
• Clearer guidance in the Assessment Methodology
• Third mandatory requirement
• The allocation of credits has been split
SUR1 - Surface Water Run-Off
• Written into the Assessment Criteria is:
– If there is no increase in the impermeable area as a result of the new
development, then the peak rate and volume of run-off criterion does
not apply (Mandatory met).
– The additional predicted volume of rainwater discharge for the 100 year
6 hour event must be prevented from leaving the site by using infiltration
or other SUDS techniques e.g.
• Wet ponds
• Rainwater harvesting
• Soakaways
SUR1 - Surface Water Run-Off
• Appropriately Qualified Professional
A professional or team of professionals with the skills and experience to champion the use of SUDS within the overall design of the development at an early stage.
The professional or team of professionals must be capable of understanding the site’s particular surface water management needs and opportunities. In addition, they must have knowledge and experience in using SUDS-based solutions to influence the holistic design of a development’s drainage system and provide the robust hydraulic design calculations referred to in key guidance documents such as The SUDS manual (CIRIA C697, 2007) and Preliminary rainfall
runoff management for developments (EA/DEFRA, 2007). Suitable professionals may be found in a variety of disciplines, such as engineering, hydrology or a combination. Geotechnical advisers or specialists may be required for SUDS
techniques that allow infiltration.
SUR1 - Surface Water Run-Off
3rd Mandatory Requirement
• Designing for Local Drainage System Failure –
Demonstrate that the flooding of property would not occur in the event of local drainage system failure (caused either by extreme rainfall or a lack of maintenance).
• Further guidance - CIRIA publication C635 (2006) Designing for exceedence in urban drainage – good practice should be referred to for guidance.
• Evidence Requirements –
Drawings showing the proposed drainage solution, system failure flood flow routes, potential flood ponding levels and ground floor levels. Confirmation from the appropriately qualified professional that local drainage system failure would not cause an increase in the risk of flooding within dwellings either on or off site.
One credit can be awarded by ensuring there is no discharge from the developed site for rainfall depths up to 5 mm
One credit can be awarded if the run-off from all hard surfaces shall receive an appropriate level of treatment in accordance with The SUDS Manual to minimise the risk of pollution.
