Active Attenuation Technology: A Practical Guide for Developers

Active attenuation represents a different approach to managing surface water on development sites. Unlike passive systems that rely solely on storage volume, active systems use mechanical controls to regulate discharge rates.

What Is Active Attenuation?

Active attenuation systems incorporate flow control devices, typically vortex valves or electronically controlled outlets, to manage the rate at which surface water leaves a site. These systems can respond to changing conditions and provide more precise control over discharge rates than passive attenuation alone.

The basic components include:

Storage tanks or chambers to hold surface water temporarily

Flow control devices that regulate the discharge rate

Monitoring equipment in some installations to track water levels

Control systems that can adjust discharge rates based on predetermined parameters

How Active Systems Differ from Passive Systems

Passive attenuation systems use fixed orifice plates or hydrobrakes to restrict flow. The discharge rate remains constant regardless of rainfall intensity or downstream conditions.

Active systems can adjust discharge rates. Some installations use simple mechanical devices that respond to water levels. More complex systems may incorporate sensors and electronic controls.

Applications in Development Projects

Active attenuation may be specified when:

Space is limited and maximum storage efficiency is required

Discharge rates must be precisely controlled to meet specific planning conditions

Downstream infrastructure has capacity constraints that require careful flow management

Multiple discharge points need coordinated control

Design Considerations

When considering active attenuation for a project, several factors require evaluation:

Site Assessment

The ground conditions, available space, and existing drainage infrastructure all influence whether active attenuation is appropriate. A topographical survey and ground investigation report provide essential information for design decisions.

Maintenance Requirements

Active systems require regular maintenance. Flow control devices need inspection and cleaning. Electronic components require periodic testing. Developers and building owners should understand these ongoing requirements before installation.

Planning Authority Requirements

Local planning authorities specify acceptable discharge rates and attenuation volumes. These requirements form the basis of any drainage design, whether passive or active. The design must demonstrate compliance with local authority standards and the National SuDS Standards.

Cost Implications

Active attenuation systems typically involve higher capital costs than passive systems due to additional equipment. Ongoing maintenance costs should be factored into whole-life cost assessments.

Integration with SuDS

Active attenuation can form part of a wider sustainable drainage strategy. It may be combined with:

Permeable paving to reduce runoff volumes

Swales and detention basins for additional storage and water quality treatment

Green roofs to reduce peak flows

Rainwater harvesting to reduce discharge volumes

The most effective drainage strategies often combine multiple techniques to manage both water quantity and quality.

Regulatory Framework

All drainage designs in England must comply with relevant regulations and standards:

Building Regulations Approved Document H covers foul and surface water drainage

National SuDS Standards set requirements for sustainable drainage

Local planning policies may impose additional requirements

Water company standards apply where connections to public sewers are proposed

Designs require approval from the local planning authority and, where relevant, the local water company.

When to Seek Professional Advice

Active attenuation systems require careful design to ensure they function correctly and meet regulatory requirements. Professional drainage designers can:

Assess site conditions to determine whether active attenuation is appropriate

Calculate required storage volumes and discharge rates

Select appropriate equipment for the specific application

Prepare drawings and calculations for planning authority approval

Coordinate with other consultants including architects, structural engineers, and civil engineers

The Design Process

A typical drainage design project follows these stages:

Initial consultation to understand project requirements and constraints

Site assessment using topographical surveys and ground investigation data

Preliminary design showing proposed drainage layout and attenuation approach

Detailed design including calculations, specifications, and construction drawings

Submission to authorities for approval

Support during construction to address any queries or required amendments

Making the Right Choice

The decision between passive and active attenuation depends on specific site conditions and project requirements. Factors to consider include:

Available space for attenuation storage

Required discharge rates and how precisely they must be controlled

Budget for both capital costs and ongoing maintenance

Complexity of the drainage strategy

Local authority preferences and requirements

A professional drainage designer can evaluate these factors and recommend the most appropriate solution for your project.