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CIRIA Mitigation Indices

SUDs. The SUDs Manual a 800 page behemoth. In it newest form CRIA have introduced Pollution and Mitigation Indices. These are a simple concept when being applied at the correct time during the planning process. During the planning stage where designs can be changed to incorporate the below features, all is well and good. But applied as part of planning conditions, where layout is fixed then they can be problematic.

Land Uses

ciria_pollution_indicesAs one can see from the table to the right, any given land use has a set of Pollution Indices. Arranged as Suspended Solid, Metals and Hydrocarbons.

For the lower end of the scale these are easy to mitigate against. But for the more polluting land uses a very specific set of SUDs features will be required. 

This causes problems when a sites layout is fixing, yet above ground landscape style features are required to meet requirements.

For example Swales and Filter Strips are visible features that will need to shown on project drawings and as such it is not often possible to change these retrospectively, so what can you do about this?

Permeable pavement offer s the best choice for most, this can be laid in place of macadam and concrete surfaces where possible, and surface water is disposed of through the permeable pavement, which acts as a filter. This fits well in to most layouts.

If one is applying very retrospectively and the site is already finished, then what system is best?


ciria_mitigation_indicesIf you have missed the boat in terms of a visible solution then you will probably look to install a proprietary system. This is not an ideal choice as finding a proprietary system that has published Mitigation Indices is next to impossible.

In one instance we have agreed with the Drainage Officer that percentage removal rate can be substituted for the Mitigation Indice, where 10% removal would equate to a indices of 0.1 etc.

No this works out well for Oils (Hydrocarbons) and TSS as there are "units" available to deal with these that publish removal rates (or they can at least be derived from supplied data ) , but for metals a solution is tricky, and will likely rely on a qualitative assessment being made, and agreed by the assessing drainage officer.

So to conclude it is far better to implement these drainage features at the designs stage, rather then brush them under the carpet until conditioning.