PPG25 and FLOOD RISK ASSESSMENT

Policy

Susceptibility of land to flooding is a material planning consideration.
Development should not increase the risk of flooding elsewhere.

PPG25 requires a Sequential Test of flood risk. The Sequential Test identifies 3 'Flood zones'. Any proposed development can be categorised into a Flood zone, depending on the site's individual risk of flooding.
Local planners are requested to give priority to allocating/permitting sites within the lower flood risk zones.

Flood Zone 1
Risk Rating - Little or no risk
Annual probability of flooding:
River tidal & coastal < 0.1% (i.e. 1 in 1000 year)
Appropriate Planning Response
No constraints due to river, tidal or coastal flooding.

Flood Zone 2
Risk Rating - Low to medium risk
Annual probability of flooding :
River 0.1 - 1.0%, Tidal & coastal 0.1 - 0.5%
Appropriate Planning Response
Suitable for most development. Flood Risk Assessment appropriate to the scale & nature of the development is required. Warning & evacuation procedures should be considered.

Flood Zone 3
Risk Rating - High Risk
Annual probability of flooding, with flood defences where they exist:
River 1.0% or more, Tidal & Coastal 0.5% or more.
Appropriate Planning Response - depends on location:

Flood Zone 3 A - Developed areas
Suitable for residential, commercial & industrial development, provided that the appropriate minimum standard of flood defence can be maintained for the lifetime of the development. Suitable evacuation procedures are required.

Flood Zone 3B - Undeveloped & sparsely developed areas
Generally not suitable for residential, commercial & industrial development.
general purpose housing should not normally be considered - limited to job related accommodation (i.e. caretakers & operational staff)

Flood Zone 3C - Functional floodplains
Development should be wholly exceptional & limited to essential transport & utilities infrastructure.

Developments within 'high risk' areas
Where extensive areas of land fall within the higher risk zones, development may be required to avoid social & economic stagnation or blight. Careful consideration throughout the design and construction of such developments can lead to a 'managed' flood risk, acceptable to all parties involved.

Drainage
Urbanisation increases both the rate and volume of runoff, increasing risk of flooding elsewhere. This can be countered by introducing Sustainable Urban Drainage Systems, which try to mimic natural drainage using:

Infiltration
Evaporation
Storage

With the objective of limiting runoff from the developed site to the pre-development values, or less.

Development in the flood plain, or defence of land which normally floods, reduces storage of floodwaters and increases the risk of flooding elsewhere.

Climate Change
PPG25 requires consideration of Climate Change
This is expected to cause:

Rising sea level
Greater intensity of rainfall
Greater frequency of rainfall
Changing distribution of rainfall

More intense storms can cause greater storm surge in coastal regions. More intense rainfall means today's 100 yr storm event may become be a 10 yr event. Flood predictions are based on historical records, which may prove to be a poor guide to the future.

Flood Risk Assessments consider:

• Is the area liable to flooding?
• The probability of flooding occurring, now and in the future?
• Any flood defences and their effectiveness?
• The likely flood depths and rates of flow?
• The effects of climate change?
• Effect of runoff from the site?
• The effect of the site on floodwater storage or conveyance?

Fenland Hydrotech Services

We can:

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

The Developer has to assess:
Will the development be susceptible to flooding?
Will the development increase flood risk elsewhere?

Note: This is an overview of PPG25, and represents our interpretation of the guidelines. Interpretations by individual planners and Environment Agency offices may differ. If any of this affects you or your development, you should seek professional advice.

A summary of the guidelines can be downloaded as a pdf file (55k) and saved or printed. Click here

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