Understanding the Simple Index Approach: How It Determines Your SuDS Design Requirements

When planning a new development, one of the most critical questions you'll face is: what type of sustainable drainage system (SuDS) features do you need? The answer lies in a straightforward yet powerful tool called the Simple Index Approach. This methodology has become the cornerstone of SuDS design in England, helping designers, developers, and local authorities determine exactly which drainage features are necessary for any given site.

What Is the Simple Index Approach?

The Simple Index Approach is a risk-based assessment method that evaluates how much treatment your surface water runoff requires before it can be safely discharged into the environment. Think of it as a scoring system that measures the pollution risk your development poses to receiving waters, whether that's a stream, river, or groundwater.

Introduced through the CIRIA SuDS Manual (C753), this approach provides a standardised way to determine the level of water quality treatment needed. Rather than leaving SuDS selection to guesswork or over-engineering, it gives you a clear, evidence-based answer.

How Does the Simple Index Approach Work?

The methodology assesses two key factors: the pollution hazard your site generates and the sensitivity of where that water will end up. By combining these elements, it produces an index score that directly translates into the number of treatment stages your drainage system must provide.

Pollution Hazard Assessment

Different land uses create different levels of pollution. A residential driveway produces far less contamination than a busy car park or industrial yard. The Simple Index Approach categorises sites into pollution hazard levels (typically low, medium, or high) based on factors such as:

• Traffic intensity and vehicle movements

• The type of activities taking place on site

• Potential for fuel, oil, or chemical spillages

• The impermeability and contamination risk of surfaces

For example, a residential development with standard driveways and roads would typically fall into the low to medium hazard category, whilst a lorry park or industrial estate would be classified as high hazard.

Receptor Sensitivity

Where your drainage discharges matters just as much as what's in it. The receiving environment's vulnerability determines how carefully you need to treat the water. Sensitive receptors include:

• Groundwater source protection zones

• Chalk aquifers and other permeable geology

• High-quality watercourses and habitats

• Areas with existing water quality issues

A discharge to a large, robust river system requires less intensive treatment than infiltration into a groundwater protection zone that supplies drinking water.

Calculating the Index

The Simple Index Approach combines your pollution hazard level with receptor sensitivity to produce a treatment requirement. This is typically expressed as the number of treatment stages needed, usually ranging from one to three stages, though some high-risk scenarios may require additional measures.

Each treatment stage represents a SuDS feature that removes pollutants through different mechanisms: sedimentation, filtration, biological uptake, or adsorption. The beauty of this system is its clarity. Once you know your index score, you know exactly how many treatment stages your design must incorporate.

Why the Simple Index Approach Determines Your SuDS Features

The Simple Index Approach doesn't just suggest SuDS features; it mandates them.

Here's why this methodology has become the determining factor in SuDS design:

Regulatory Compliance

Local planning authorities and water companies increasingly require developments to demonstrate compliance with the Simple Index Approach. Your planning application must show that you've assessed the pollution risk and incorporated sufficient treatment stages. Without this, you're unlikely to receive approval for your drainage strategy.

The approach aligns with National SuDS Standards and the requirements set out in planning policy, making it a non-negotiable element of the design process. It provides the evidence base that regulators need to approve your scheme.

Treatment Train Design

The number of treatment stages determined by the Simple Index Approach directly shapes your SuDS treatment train: the sequence of features that water passes through before discharge. If your assessment indicates you need two stages of treatment, you might incorporate:

• Permeable paving (Stage 1) followed by a swale (Stage 2)

• A filter drain (Stage 1) leading to a detention basin (Stage 2)

• Green roof (Stage 1) with bioretention areas (Stage 2)

Three stages might require an additional component, such as permeable paving, then a swale, followed by a wetland or pond. The approach ensures you're not under-treating high-risk runoff or over-engineering low-risk situations.

Feature Selection and Positioning

Understanding your treatment requirements influences which SuDS features are appropriate for your site. High pollution hazard sites might need features with greater pollutant removal capacity, such as bioretention systems or wetlands, rather than simple grass swales.

The approach also affects where you position features within your drainage network. Treatment should occur as close to the source as possible, with additional stages provided downstream. This "treatment train" philosophy ensures progressive improvement in water quality.

Cost and Space Planning

Knowing your treatment requirements early in the design process allows for realistic budgeting and space allocation. Three stages of treatment require more land and investment than a single stage. The Simple Index Approach provides this clarity upfront, preventing costly redesigns when you reach the planning stage.

Developers can make informed decisions about site layout, balancing treatment requirements with other land uses. Architects and engineers can collaborate effectively when everyone understands the non-negotiable drainage requirements.

Practical Application: A Worked Example

Consider a residential development of 50 homes with associated roads, driveways, and a small car park. The site will discharge to a local stream that feeds into a larger river system.

The pollution hazard assessment identifies the development as medium risk: residential roads and driveways with moderate traffic levels. The receptor assessment finds the stream has good water quality but is relatively small and sensitive to changes in flow and pollution.

Applying the Simple Index Approach, this scenario requires two stages of treatment. The design team might therefore incorporate:

• Stage 1: Permeable paving for driveways and some parking areas, providing initial filtration and reducing runoff volumes

• Stage 2: Swales along the road network, offering additional filtration, sedimentation, and biological treatment before discharge to the stream

This combination satisfies the treatment requirement whilst integrating naturally into the development's landscape. Without the Simple Index Approach, the design team might have under-provided treatment (risking planning refusal) or over-engineered the system (wasting money and space).

Common Misconceptions

Some developers and designers mistakenly believe that any SuDS feature will suffice, or that attenuation alone meets modern requirements. The Simple Index Approach makes clear that water quality treatment is just as important as flood risk management. An attenuation tank that simply stores and releases water provides minimal treatment; it won't satisfy a two-stage treatment requirement.

Another misconception is that the approach is overly complex or requires specialist knowledge. Whilst detailed assessments benefit from professional expertise, the fundamental principles are straightforward. Most residential developments fall into predictable categories, making the process relatively simple once you understand the framework.

The Future of SuDS Design

As climate change intensifies rainfall patterns and water quality regulations tighten, the Simple Index Approach will remain central to drainage design. It provides the flexibility to respond to site-specific conditions whilst maintaining consistent standards across all developments.

Future iterations may incorporate additional factors such as climate resilience, biodiversity enhancement, and amenity value, but the core principle (matching treatment to risk) will endure. Understanding this approach now positions you to navigate evolving regulations with confidence.

Getting Your SuDS Design Right

The Simple Index Approach removes ambiguity from SuDS design. It tells you precisely what treatment your development needs, which features can deliver that treatment, and how to arrange them for maximum effectiveness. This clarity benefits everyone: developers gain planning certainty, local authorities can enforce consistent standards, and the environment receives appropriate protection.

If you're planning a development, don't leave your SuDS design to chance. A proper assessment using the Simple Index Approach ensures your drainage strategy will satisfy regulators, protect water quality, and avoid costly revisions. 

Whether you're developing a single home or a major housing estate, understanding how the Simple Index Approach determines your SuDS requirements is the first step towards a successful, compliant drainage design.