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Simple Index Approach (SIA)

February 8, 2024

The Simple Index Approach (SIA) is a simple risk-based method for selecting the appropriate type and combination of sustainable drainage systems (SuDS) for the treatment of surface water runoff based off a variety of land uses and pollution risk hazards.

The SIA approach is outlined in CIRIA C753 The SuDS Manual and breaks down pollution into 4 categories (Very Low, Low, Medium, High). Each pollution categorisation and what is generally included in each, is shown below:

  • Very low – residential roofs
  • Low – commercial / industrial roofs
  • Medium – commercial yard and delivery areas and non-residential car parking
  • High – Haulage yards, Service yards, lorry parks, waste sites

Each pollution categorisation is then broken into 3 distinct pollution types with a value assigned as identified below:

  • Total Suspended Solids (TSS)
  • Metals
  • Hydro-carbons

Conveying runoff through different forms of SuDS features provides varied mitigation values for each of the 3 distinct pollution types. On the basis that the combined mitigation index value from SuDS is higher than the initial values associated with each pollution risk, the drainage system is deemed to have sufficiently mitigated the pollution hazard posed.

A few additional pointers on the SIA below:

  • Generally, it will be the external hardstanding areas which have the highest pollution risk which must subsequently be sufficiently mitigated by SuDS (roof runoff is generally Very Low to Low pollution risk).
  • Permeable paving is very good at mitigating pollution and generally can fully mitigate a medium pollution use by itself without any further SuDS needed.
  • Geo-cellular crates are not recognised within the SIA as providing any pollution mitigation value (another reason why open SuDS are preferable to underground SuDS such as geocell and tanks).
  • When in High-risk categorisation areas, it is suggested the use of SuDS is combined with proprietary pollution mitigation systems such as oil interceptors, forecourt and washdown separators to provide the overall mitigation needed.

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