Using the Simple Index Approach and DeepRoot Silva Cell for SuDS

DeepRoot has developed guidance on how the Silva Cell® product can be designed for bioretention in a sustainable drainage system (SuDS). Designers and engineers opt for SuDS as a greener and more sustainable method for mitigating the risk of flooding and pollution whilst also achieving additional amenity and biodiversity benefits. This blog discusses the Simple Index Approach (SIA), as outlined in The SuDS Manual, as a method for selecting appropriate SuDS features for a development site and how the Silva Cell Bioretention System can be designed using the methodology.

What Is The Simple Index Approach?

The Simple Index Approach is a simplified risk-based method for selecting appropriate sustainable drainages systems (SuDS) for the treatment of surface water runoff. The approach, as outlined in CIRIA C753 The SuDS Manual, assigns pollution hazard indices for a given land use based on the expected pollution hazard level. The contaminants considered are total suspended solids (TSS), metals and hydrocarbons. Suitable SuDS features are then selected to achieve a combined mitigation index score greater than the pollution hazard index and hence indicating the pollution has been suitably managed. The Simple Index Approach is suitable for land uses with low and medium pollution hazard levels only, as the water quality design method. For sites with a high pollution hazard level, additional water quality modelling should be considered.

How Is It Calculated?

At first, the considered site is given a pollution hazard level based on its land use (Table 26.2). From this, pollution hazard indices for TSS, metals and hydrocarbons are allocated.


The objective is to then select suitable SuDS features with an equal or greater total mitigation indices score (Table 26.3). The total mitigation indices score is calculated as shown below:

Pollution hazard level ≤Total mitigation indices

Total mitigation indices score= Mitigation indices1+0.5(Mitigation indices2n)

Mitigation Indices For The DeepRoot Silva Cell

The DeepRoot® Silva Cell® is a modular suspended pavement system that integrates water, soil and tree roots and delivers on-site stormwater management through bioretention.

The Silva Cell system enables the application of bioretention within urbanized areas without the loss of usable paved areas. Within the Silva Cell, the same features and layers of a typical bioretention system can be constructed. Based on this, the Silva Cell has the same mitigation indices as normal bioretention.

The Simple Index Approach is a fast and easy way to select appropriate SuDS features for mitigating the effects of surface water runoff. The Silva Cell Bioretention System can be used in this method to achieve high levels of solids, metals and hydrocarbons capture as surface water filters through the systems planting media. For additional support and guidance on surface water management and how the Silva Cell can be used in developments, download our Technical Simple Index Approach Design Guide on our website resources page, or reach out to Dr Chris Newton for expert advice and design assistance.

PDF to our Simple Index Approach Design Guide.

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