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Drainage Assessment - Major Applications

Information required and when requiredInformation Required/Guidance

Drainage Assessment - Majors

Required for all Major Application Types:

1. Outline

2. Full

3. Reserved Matters (if not sufficiently addressed at outline stage)

Devon County Council is the Lead Local Flood Authority and will comment on all major planning applications in relation to drainage and flooding matters.

Note: The Drainage Assessment could form part of the Flood Risk Assessment, as required for all Major Applications.

The level of information relating to surface water drainage management that must be submitted in support of planning applications should be proportional to both the scale of development and level of flood risk. Although not exhaustive, the lists contained within this section provide a guide as to the level of information required when assessing surface water drainage management.

Information Required for Outline Planning Applications

Location plan at an appropriate scale with a grid reference, showing geographical features, street names, watercourses, or other water bodies in the vicinity

Site survey showing the existing topography

Assessment of all existing flood risks to the site, including from sewer networks, groundwater, overland surface water flows, reservoirs, ponds, canals and other watercourses

Calculations of the current surface water runoff rates and volumes for the site

Calculations of the proposed surface water runoff rates and volumes for the site

Calculations of the surface water attenuation storage volume required for the 1 in 100 (+40% allowance for climate change) year rainfall event

Calculations of the long-term storage volume required to store the additional volume of surface water runoff caused by any increase in the site's impermeable area

Evidence that the site has an agreed point of discharge

Evidence that the hierarchy of drainage solutions has been followed, providing robust evidence as to the viability or otherwise of:

Discharge into the ground (infiltration)

Discharge to a surface water body (with written permission from the riparian owner)

Discharge to a surface water sewer, highway drain, or other drainage system (with written permission from South West Water Ltd., Devon County Council Highways, or the riparian owner, respectively)

Discharge to a combined sewer (with written permission from the riparian owner)

Evidence that the capacity of any receiving watercourse is sufficient to receive concentrated flows from the site

Explanations of the proposed flood risk mitigation measures 

Non-technical summary of the proposed surface water drainage management system

Outline operation and maintenance plan and timetable for the proposed surface water drainage management system over the entire

Plans of the proposed surface water drainage management system, demonstrating that it fits within the proposed site layout, and is practical and sustainable

Outline operation and maintenance plan and timetable for the proposed surface water drainage management system over the entire lifetime of the development

Information Required for Full, Reserved Matters or Discharge of Conditions Planning Applications

Location plan at an appropriate scale with a grid reference, showing geographical features, street names, watercourses, or other water bodies in the vicinity

Detailed site survey showing the existing topography

Assessment of all existing flood risks to the site, including from sewer networks, groundwater, overland surface water flows, surface water flows, reservoirs, ponds, canals, and other watercourses

Calculations of the current surface water runoff rates and volumes for the site

Calculations of the proposed surface water runoff rates and volumes for the site

Calculations of the surface water attenuation storage volume required for the 1 in 100 (+40% allowance for climate change) year rainfall event

Calculations of the long term storage volume required to store the additional volume of surface water runoff caused by any increase in the site's impermeable area

Evidence that the site has an agreed point of discharge

Evidence that the hierarchy of drainage solutions has been followed, providing robust explanations as to the viability or otherwise of:

  • Discharge into the ground (infiltration)
  • Discharge to a surface water body (with written permission from the riparian owner)
  • Discharge to a surface water sewer, highway drain, or other drainage system (with written permission from South West Water Ltd., Devon County Council Highways, or the riparian owner, respectively)
  • Discharge to a combined sewer (with written permission from the riparian owner)

Infiltration testing results at the location and depth of each proposed infiltration system

Groundwater monitoring over a 12-month period to demonstrate that the base of any infiltration component is at least 1 metre above the maximum anticipated groundwater level. This should take account of seasonal variations.

Evidence that the capacity of any receiving watercourse is sufficient to receive concentrated flows from the site

Detailed explanations and plans of flood risk mitigation measures

Technical summary of the proposed surface water drainage management system

Detailed plans of the proposed surface water drainage management system, demonstrating that it fits within the proposed site layout, and is practical and sustainable

Residual risk assessment to account for a failure of any part of the proposed surface water drainage management system

Detailed exceedance route plans to demonstrate that there is no residual risk of property flooding during events in excess of the return period for which the surface water drainage management system is designed

Detailed operation and maintenance plan and timetable for the proposed surface water drainage management system over the entire lifetime of the development

Details of the proposed community signage and engagement activities for each component of the proposed surface water drainage management system.

Where the development is close to or discharges to a SSSI, evidence will be required to demonstrate construction and operational phases will not impact upon water quality.


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