Development and Flood Risk in the Nene Valley

The Environment Agency (EA) has reviewed the flooding events of Easter 1998, and produced policy and guidance for planners and developers in areas draining to the River Nene. The EA does not control development directly, but is a statutory consultee in the planning process.

The principal conclusions are that additional flood volume cannot be accepted in Northampton, and development in the Upper Catchment should contribute to the reduction of flood risk for storms up to the 1 in 200 year event. Similar policy principles are now being applied in the Nene catchment downstream of Northampton.

Background
The effects of conventional development on storm water drainage are twofold: ·

• The total volume of runoff is increased as infiltration and evapo-transpiration are reduced. ·
• The rate of runoff is increased

Where development takes place in the flood plain, the volume available for storage is also reduced and the flow of water may be impeded. The EA believe that past development, as well as changes in agricultural practice, have led to an increased risk of flooding in Northampton and the Nene Valley.

EA Policy

• The EA policy is that built development in the floodplain should be avoided.
• Development will be directed away from Greenfield sites in the floodplain.
• Severe restrictions will be placed on Brownfield development in the floodplain.
• Any permitted development outside the floodplain, but ultimately draining into Northampton, will need to include mitigating action against the effects of the development, leading to an overall reduction in flood risk.
• Proposed developments already in the planning system will be guided into line with these principles so far as possible.
• The EA will promote a strategic approach where possible rather than piecemeal flood protection installations.
• Promote Sustainable Urban Drainage.

Methodology
The river system cannot cope with additional unregulated runoff without increasing the risk of flooding to people and property. To regulate the impact of future developments, 'greenfield' runoff rates throughout the catchment have been assessed for storms of various return periods. Completed developments will be expected to discharge at lower flow rates, and lower total flow volumes, than the assessed greenfield runoff for rainfall events up to the 1 in 200 year storm.

Hydraulic and hydrological modelling of the catchment is fundamental to this policy. Developers will be expected to contribute to the cost of using and enhancing these models.

Implementation
Developers will be required to complete a pre-planning enquiry form. The EA will respond giving their assessment of the impact of the development.

Developers will be required to identify, implement and cover the costs of any necessary measures, including undertaking flood risk impact assessments, and provide additional information as required by the EA. This may include detailed surveys and modelling of reaches of rivers and other watercourses.

All flood risk measures should be operationally effective before the development commences. Responsibility for long-term maintenance should be established and a commuted sum to cover maintenance for 40 years will be required.

All new development proposals should incorporate the following as necessary:

• Sustainable Urban Drainage practice
• Source Control
• Minimisation of diffuse pollution
• On-sewer or open water space flow balancing

Fenland Hydrotech Services

We can:

• Prepare the pre-planning enquiry form
• Identify measures to reduce flood risk
• Prepare hydraulic models of sewers, rivers and whole catchments.
• Design Sustainable Urban Drainage Systems
• Negotiate agreements with Regulators.

EA Policy:

Developments should incorporate measures to ensure overall flood risk is reduced as a result of the change of use of the land.

SUDS River Modelling Urban Modelling

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