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Development and flood risk

Development within an area at risk of flooding

If you are thinking about developing in an area shown to be at risk of flooding on the Development Advice Map, you will need to demonstrate that the flood risks and consequences can be acceptably managed in line with current national planning policy and in particular Welsh Government’s Technical Advice Note 15: Development and Flood Risk (TAN15).

It is often necessary to prepare a flood consequences assessment (FCA), to show how the flood risk to your development, or any increased flood risk elsewhere as a result of your development can be suitably managed or mitigated. TAN15 specifies that this assessment must be undertaken by a suitably qualified person.

Please note that producing an FCA does not ensure that the proposed development will fully satisfy the criteria of TAN15. It should also be noted that proposals for highly vulnerable development such as housing may not be an acceptable form of development in certain high risk areas. We would recommend you seek advice from the local planning authority prior to commissioning a FCA. 

Our role is to provide technical advice to the determining authority on the level and acceptability of flood risk from main rivers and the sea. We may also provide comment on any mitigation measures proposed. We do not provide advice on surface water flood risk or any localised flood risk issues. This advice should be sought as early as possible in the development planning process from the relevant Lead Local Flood Authority. The Lead Local Flood Authority, Highways Authority and Sewerage Undertaker should also be consulted on the suitability of site drainage proposals, including techniques available to calculate surface water rates of discharge (even where the receptor is main river) and soakaway infiltration. We do not provide advice in this regard.

Our advice on flood risk will be primarily based on existing information we may hold and the contents of any FCA submitted in support of a planning application. Details of the level of information required in a FCA can be found in TAN15 (Part 9, Section 7 and Appendix 1).

To assist in the preparation of your FCA, we have developed several Guidance Notes which contain technical advice and recommendations. There is a guidance note which outlines our recommendations on appropriate methodologies for flood frequency estimation and hydraulic modelling. There is also our Operational Guidance Note OGN100 for Modelling blockage and breach scenarios.

As part of our service for developers we can provide a preliminary opinion on your proposal, including whether a FCA is likely to be required, and an outline of what might need to be included in the assessment. Further information about this service, and how to request it, can be found in Our Service to Developers.

The evidence supporting the flood map has improved significantly over the years, but there are still places where it can be improved, either due to changes in the land, or more accurate survey and modelling. The process for submitting a Flood Map Challenge is laid out in our Guidance Note GN 029. It is important to remember that the extreme flood outline in the maps contains a combination of modelled and real past flood events.

If you require advice in addition to that provided by our preliminary opinion, for example a technical appraisal of your draft FCA, or a review of any hydraulic modelling undertaken, we would recommend you make use of our charged discretionary advice service. This is explained further in Our Service to Developers.

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