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Development Advice

Technical Advice Note 15 (TAN 15) - Development and Flood Risk

Introduction

As part of their regulation duties a planning authority will need to be satisfied that a development proposal is justified and that the consequences of flooding are acceptable.

TAN 15 is a technical advice note produced to provide technical guidance to supplement the policy set out in Planning Policy Wales (PPW), in relation to development and flooding. TAN 15 advises on development and flood risk, and provides a framework within risks arising from both river and coastal flooding, and from additional run-off from development in any location, can be assessed.

The general approach of PPW and TAN 15 is to advise caution with respect to new development in areas at high risk of flooding by setting out a precautionary framework to guide planning decisions. The overarching aim of the precautionary framework (in order of preference), is to:

1. Direct new development away from those areas which are at high risk of flooding.

2. Where development has to be considered in high risk areas (zone C) only those developments which can be justified on the basis of tests (outlined in sections 6 and 7 of TAN 15), are located within such areas.

Development Advice Zones

TAN 15 contains a development advice map (DAM). Three development advice zones (A, B, C) are described on the maps, which are attributed different planning actions. Zones A and B areas are areas of generally low risk (i.e. outside the main river flood plain). Zones C represents the extreme flood outline (equal to or greater than 0.1% flood risk), and is further subdivided into two zones, C1 and C2.

Zone C1: described as "areas of the floodplain which are developed and served by significant infrastructure, including flood defences".

Zone C2: described as "areas of the floodplain without significant flood defence infrastructure".

Nature of Development or Land Use

As particular flooding consequences may not be acceptable for particular types of development, e.g. allowing residential development in areas which are subject to high risk of flooding. The precautionary framework identifies the vulnerability of land uses to flooding by sub-dividing land usage in three categories:

1. Emergency Services including hospitals, ambulance / fire / police / coastguard stations, command centres, emergency depots, buildings used to provide emergency shelter in time of flood.

2. Highly vulnerable development including all residential premises (including hotels and caravan parks), public buildings (e.g. schools, libraries, leisure centres), especially vulnerable industrial development (e.g. power stations, chemical plants, incinerators), waste disposals sites.

3. Less vulnerable development including general industrial, employment, commercial and retail development, transport and utilities infrastructure, car parks, mineral extraction sites and associated processing facilities, excluding waste disposal sites.

Justifying the Location of Development (Section 6)

Section 6 of TAN 15 provides detail in relation to justifying new development within zones C1 and C2.

TAN 15 states that: 'new development should be directed away from zone C and towards suitable land in zone A, otherwise to zone B, where river and coastal flooding will be less of an issue'.
'...highly vulnerable development and Emergency Services in zone C2 should not be permitted. All other new development should only be permitted within zones C1 and C2 if determined by the planning authority to be justified in that location'.

Assessing Flood Consequences (Section 7)

A development proposal meets the tests outlined in section 6 of TAN 15, the justification will be in the knowledge that those developments will flood and will need to be planned accordingly.

Whether a development can proceed will depend upon whether the consequences of flooding of that development can be managed down to an acceptable level.

Further details on the assessment of flooding consequences is given in Section 7 and Appendix 1 of TAN 15.

Undertaking Flood Consequence Assessments

It is stated in TAN 15 that 'The first step in undertaking an (flood consequence) assessment must be for developers to consult the Environment Agency about the objectives of the assessment'.

Appendix 1 of TAN 15 provides further guidance on what is required when producing a Flood Consequence Assessment.

How this study relates to TAN 15

TAN 15 (see below) provides indicative guidance on threshold frequency and tolerable conditions for different types of developments. Table 1 shows the Threshold Frequency below which flooding of development should not be allowed. For example, residential development should be designed to be flood free during the 1% fluvial flood and/or the 0.5% tidal/coastal flood. If a development site passes the criteria in Table 1, then it also has to satisfy the following criteria in Table 2 for extreme events.

Based upon Table 2, the following parameters have been estimated using the results of the numerical modelling undertaken under this study:

- Maximum Flood depth;
- Maximum rate of water rise;
- Flood arrival time (speed of inundation);
- Maximum flood velocity.

Table 3 shows the relationship between the estimated flood probabilities shown on the produced maps and the TAN 15 criteria shown in Table 1 and linked to their associated approximate return period.

The study started in 2002 and has since gone through a number of iterations. The main outputs are the reports which can be downloaded from 'HR Wallingford Report and Drawings'. The TAN 15 maps produced provide the spatial distribution of the likelihood of flooding taking into account loading conditions (i.e. joint wave and water levels), the condition and geometry of the defences and the hinterland levels. They also show a more realistic flood extent than the IFM boundary that significantly overestimates the likely flood extent. These maps can assist planning department staff to make decisions on planning permission more quickly and accurately. They can also provide general guidance to the public on the likelihood of flooding. It is intended that the study will be checked / updated every five years.


  • TAN 15 Table 1: Guidance on Threshold Frequency for Different Types of Development

    This applies over the lifetime of the development.

    Type of Development Threshold Frequency
    Fluvial Tidal
    Residential 1% 0.5%
    Commercial / Retail 1% 0.5%
    Industrial 1% 0.5%
    Emergency Services 0.1% 0.1%
    General Infrastructure 1% 0.5%

  • TAN 15 Table 2: Guidance on Tolerable Conditions

    Type of Development Maximum Depth of Flooding (mm) Maximum Rate of Rise of Floodwaters (m/hr) Arrival Time of Inundation to Flood Risk Area (hrs)
    RESIDENTIAL (habitable rooms) Property 600 0.1 4
    Access 600
    COMMERCIAL & RETAIL Property 600 0.3 2
    Access 600
    INDUSTRIAL Property 1000 0.3 2
    Access 1000
    EMERGENCY SERVICES Property 450 0.1 4
    Access 600
    GENERAL INFRASTRUCTURE Property 600 0.3 2
    Access 600

  • Table 3: The Relationship Between Flood Probabilities and the TAN 15 Criteria

    Flood Probability TAN 15 Criteria Approximate Return Period (Years)
    0.01 1% 100
    0.005 0.5% 200
    0.001 0.1% 1000

 
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