This note identifies key housebuilding experiences in achieving Code Certification for the Surface Water Credits. It has been prepared for technical representations to BRE and CLG.
In summary, it is considered that the current SUR 1 mandatory elements are overly prescriptive and often inappropriate to achieve their stated outcomes. They should be replaced with a performance based approach that could be supported by a decision tree reflecting individual site constraints. This would allow the maximum technical flexibility in bringing forward options to meet the individual planning, geological and hydraulic requirements of each site.
Context and planning issues
- Geological and contamination issues make many sites unsuitable for infiltration.
- Rainwater harvesting does not directly address the 6 hour volume attenuation requirements and is an inappropriate technical solution.
- The Environment Agency’s role in determining the drainage aspects of planning applications should be acknowledged as well as the full range of competent technical solutions that are recognised within the CIRIA SUDS Manual.
- Potentially complex and negative impacts on soil and groundwater conditions arising from wide‐scale infiltration need to be understood for individual sites. Such impacts can lead to soil instability both in the short and long term.
- In general, there seems little regulatory support for the technical requirements of SUR 1. The Environment Agency’s recent consultation dealing with Environmental Risk Assessments (Annex J for Groundwater) leaves it up to the developer to secure an Environmental Permit for several SUDS instances.
- There are a number of cases where the Local Authority as part of the planning process requires storm outflows from developments to achieve higher levels than those contained in the technical requirements. e.g. – a local ecology park.
Delivery and cost issues
- Many sites as defined by the Code are sub‐sets of larger developments for which surface water requirements are already defined.
- It is often the case that no space in practical terms is available within even low‐density sites for individual rainwater harvesting systems or further attenuation.
- The construction costs associated with rainwater harvesting are very high (typically £4,500 ‐ £6,200/dwelling). Concerns are evident about home buyer and market acceptability with a negative impact on sales values. Housing Associations have also expressed concern about operating procedures and maintenance costs of these systems.
- Rainwater harvesting systems have an associated significant gain in CO2 emissions that is in conflict with a primary objective of the Code.
- The 6 hour storm volume requirements are better addressed with properly engineered and site specific storm water attenuation systems.
- Rainwater harvesting systems do not work with green or brown roofs for which there is an increasing requirement in planning policies.
- Above ground rainwater harvesting tanks are liable to internal temperature rises which give rise to water contamination issues. The much larger rainwater harvesting tanks required to achieve attenuation suffer from a low churn rate with increased risk of water contamination from extended stagnation periods.
Posted on November 16th, 2009