Interception drainage

Interception drainage captures the first 5mm of all rainfall that falls on a site. This replicates greenfield conditions. The management of frequent events goes together with the control of extreme events to ensure that flood risk is not increased, and that water quality is not reduced because of a proposed development.

When SuDS design is only focussed on extreme events and infiltration into the ground is not viable, a site may be draining surface water to a watercourse or sewer at an agreed discharge rate when it would not drain there at all for frequent events, or during the initial stages of an extreme event. This can therefore increase flood risk if not addressed by interception drainage.

Designing for interception also helps to protect the morphology and ecology of the receiving surface water body.

All developments shall provide evidence that they have considered, assessed and designed for interception drainage. This means that it should be accounted for at the conceptual design stage. This is particularly important where it is not possible to drain rainwater into the ground due to a high water table or other geohazards, thus ruling out infiltration for interception. This is because the only other ways of achieving interception are through water reuse and evapotranspiration.

Water reuse is only deemed to be compliant when it is designed for regular daily demand for non-potable water. Water butts are not considered to meet these requirements but are encouraged as an additional beneficial feature in accordance with the SuDS hierarchy.

Features that deliver evapotranspiration require a (largely) flat vegetated surface. Including these features can impact the scale and layout of a development.

We welcome the use of hydraulic modelling to demonstrate compliance with interception standards. However, this is not always possible. When modelling software is used, this must be explained within the interception assessment of the drainage statement.

It is also possible for the interception assessment to use a simplified approach. Suggestions for this are offered by both The SuDS Manual and the NSfS. However, in places the guidance is unclear, particularly for lined features. We therefore offer the following information to assist in demonstrating compliance.

All features which are designed to deliver interception drainage must have longitudinal gradients of 1:100 or less. When calculating the contributing areas, they must include the full plan area of the interception features. This includes that which may not be included in the effective wetted area calculation.

When calculating the effective wetted area of a vegetated surface:

• no area within 5m of an outlet of a vegetated feature is included.
• features with side slopes may account for these where it is demonstrated what proportion of the surface would be wetted when conveying runoff for the 5mm rainfall depth event.
• the area should be below the outlet level for basins.
• it will be restricted to the area of specific provision for routing low flows directly to the outlet if it is at the surface (these are not advised in attenuation basins due to the restricted area for interception drainage).

Applicants are encouraged to review the guidance set out on section 24.8 of The SuDS Manual C75.

Where infiltration is viable and provides a total design solution (with an evidenced design infiltration rate higher than 1 x 10^-6m/s) interception design standards are met.   

The assessment must clearly state that interception is delivered via infiltration of runoff from all contributing areas within that (sub)catchment.

Water reuse must still be prioritised in accordance with the SuDS hierarchy. Where water reuse is not designed to deliver interception, water butts or other means of reuse that are not designed for regular daily demand are encouraged.

Water reuse is the highest priority runoff destination on the SuDS hierarchy and shall be incorporated wherever possible and appropriate.

Water reuse is only deemed to be compliant for interception drainage when it is designed for regular daily demand for non-potable water. Evidence of the approach and calculations that conform to BS EN 16941 must be submitted.

Where water reuse is not designed to deliver interception, water butts or other means of reuse that are not designed for regular daily demand are encouraged.

Evapotranspiration delivers interception where infiltration is not viable due to high groundwater or other geotechnical factors like unstable ground. This method relies on evaporation and transpiration from the surface and vegetated features.

The following features comply for the interception of their own surface area only:

• Green roofs.
• Permeable paving. 
• Swales with underdrains conveying water from other surfaces.

The following features can comply for a total contributing area of 4 times their effective wetted area:

• Bioretention areas and raingardens.
• Detention basins.
• Filter strips.

The following features can comply for a total contributing area of 5 times their effective wetted area:

• Swales with water conveyed at surface.

In addition to the general design guidance described above, where applicable, the depth of soil beneath the vegetated surface and above the liner must be 250mm with 20% voids.

The following features do not deliver interception drainage via evapotranspiration:

• Ponds with a permanent water pool.
• Any permanent wetted areas within a detention basin.

To demonstrate compliance where evapotranspiration is necessary to provide interception, we suggest that an effective wetted area plan is submitted. As part of the submitted interception assessment, this plan should be compared with the contributing area plan (required to support the modelling).

It must be clearly demonstrated what contributing areas drain to each interception feature and how compliance is achieved. This will be supplemented by an explanation and basic calculations in the interception assessment.

Where infiltration does not provide a total solution due to slow infiltration rates only (lower than 1 x10^-6m/s) and features are unlined.    

In addition to the evapotranspiration and water reuse methods described above, the following infiltration options may be considered:

• Permeable paving - for a total contributing area 2 times their effective wetted area.
• Filter strips and swales - for a total contributing area of 5 times their effective wetted area.
• Detention basins - for a total contributing area of 5 times their effective wetted area.

Interception drainage may only be delivered though water reuse or evapotranspiration. This can have a significant impact upon the scale and layout of the development.

In recognition that the National Planning Policy Framework states that SuDS should be proportionate to the nature and scale of the proposal we are willing to offer exceptions to the advice offered above for some minor and non-major applications.

In some cases, the planning officer may consider agreeing a departure from the NSfS, this should be discussed and agreed as part of pre-application advice. The SuDS consultee will not recommend or agree to a departure from the NSfS aside from the suggestions below.

CIL exempt and householder applications

Interception drainage maybe demonstrated with:

• infiltration features designed to meet extreme rainfall standards
• water butts or other means of reuse that are not designed for regular daily demand attached to all new downpipes
• raingardens and bioretention features attached to all new downpipes
• permeable surfacing