Flood mitigation solutions in buildings - Part One

Barely a year seems to pass nowadays without flooding being a feature of the national news. It is an emotive subject due to the degree of devastation it can cause – not only can severe flooding endanger life, but even at moderate levels can lead to substantial damage and disruption to normal patterns of life.

Water source contamination and inhibited access are significant problems, coupled with the ease with which internal fixtures and decorations can be damaged irreparably. A flooded house can be uninhabitable for months, and subsequent insurance costs can rise considerably as a result. And now the UK government’s plans to overhaul their flood-cover agreement with the insurance industry are coming under scrutiny, over concerns that a significant proportion of the population could find their property not only uninsurable, but also un-mortgageable and, in extreme cases, unsellable. Fortunately there are a number of measures that can be taken to minimise both the risks and impact of flood damage, both for new development and for existing buildings.

Flood avoidance strategies for new buildings

At the outset of a project, preliminary issues such as building siting and orientation, landform/topography and site location should all be considered, with respect to the proximity of the development to areas of potential flooding. Consultation with the Environment Agency (EA) and relevant Lead Local Flood Authority’s (LLFA) extensive flood risk information services should form part of the initial site analysis and data searches, to ascertain current and future risk of flooding from the various different sources. This information should be used to identify the development site at lowest risk of flooding, from those available.

Planning legislation

Under the previous legislation, Planning Policy Statement PPS25 in England and Wales sought to discourage building on floodplains without first undertaking a flood risk assessment, followed by the setting of minimum floor heights above predicted flood levels. The subsequent replacement by the National Planning Policy Framework (NPPF) broadens the requirement for Local Planning Authorities (LPAs) to “adopt proactive strategies to mitigate and adapt to climate change, taking full account of flood risk, coastal change and water supply and demand considerations”.

“…the essence of the policy is ‘safeguarding land from development that is required for current and future flood management&8221;

Planning policy on the issue of flood risk is quite explicit, and follows a specific sequence of measures. The procedure is set out in the NPPF and its supporting Technical Guidance document, and is summarised below. However, the essence of the policy is “safeguarding land from development that is required for current and future flood management”.

Initially, the LPA conducts a Strategic Flood Risk Assessment (SFRA), which evaluates flood risk across the extent of the area under their control. This SFRA supports and informs the Local Plan, in conjunction with the Flood Zone maps produced by the EA and LLFA, enabling them to target future development away from areas at highest risk of flooding.

The SFRA is also used to inform the sustainability appraisal (incorporating the Strategic Environmental Assessment Directive) of local development documents, and will provide the basis from which to apply the Sequential Test and Exception Test, in both the development allocation and development control processes. These tests are described below.

The EA (and also, now, the LLFA, who is typically the Local Authority [LA]) produce detailed flood maps which until recently have only identified sea and river flooding, ignoring the presence of existing defences. However, ‘flood risk’ is defined in the Technical Guidance document as including “all sources of flooding” and surface water-flood maps have now been added to the EA website. These maps classify flood risk into one of three categories, numbered Flood Zone 1, 2, 3 respectively. Zone 3 is further broken down into ‘a’ and ‘b’ subdivisions. Flood Zones 2 and 3 are illustrated on the maps, whereas Zone 1 (the lowest risk) is deemed to comprise all other land. The NPPF states that “the overall aim should be to steer new development to Flood Zone 1”, but that “where development is necessary”, it should be made “safe without increasing flood risk elsewhere”

Flood zone classifications

Flood Zone 1 is defined as ‘Low Probability’ with a less than 0.1% (1:1000) annual probability of river or sea flooding. Development proposals on sites over one hectare, must be supported by a Site-Specific Flood Risk Assessment (SSFRA). This must consider the vulnerability to flooding from all sources (including river and sea flooding), as well as the potential to increase the risk of flooding elsewhere due to surface water run-off from hard surfaces within those development proposals. It should also demonstrate how these flood risks will be managed so that the development remains safe throughout its lifetime, “taking climate change into account”. Furthermore, an evacuation plan for the development should be prepared, in consultation with the emergency services. There is no restriction on the type of development which may take place in this Zone.

“Minor developments are unlikely to raise significant flood risk issues unless they would have an adverse effect on a watercourse, floodplain or its flood defences; would impede access to flood defence and management facilities; or where the cumulative impact of such developments would have a significant effect on local flood storage capacity or flood flows.” – Technical Guidance, NPPF

Flood Zone 2, ‘Medium Probability’, is classified as having a risk of either:

• between 1% and 0.1% probability of river flooding; or
• between 0.5% and 0.1% probability of sea flooding.

Only certain types (listed in the table below) of development are considered appropriate for this Zone, and all development proposals in this Zone (and above) must be accompanied by a SSFRA.
Flood Zone 3a is defined as ‘High Probability’, with a greater than 1% chance of annual river flooding, or greater than 0.5% sea flooding risk.

More vulnerable uses, and ‘essential infrastructure’, should only be allowed in this Zone if the Exception Test is passed (see below), and that infrastructure should be designed to be immune to flooding.

Flood Zone 3b is the floodplain land itself, and only ‘water-compatible’ uses should be permitted. Even then, no net loss of floodplain storage should result; water flows should not be impeded; and flood risk should not be increased elsewhere as a result.

The following table, adapted from the NPPF Technical Guidance document, illustrates the types of development that are considered suitable for each Flood Zone.

The development classifications themselves are as follows (adapted from the NPPF Technical Guidance document):

The NPPF Technical Guidance document notes three important points:

• Where buildings combine a mixture of uses, they should be placed into the higher of the relevant classes of flood risk sensitivity
• Developments that allow uses to be distributed over the site may fall within several classes of flood risk sensitivity
• The impact of a flood on particular uses will vary from one to another, so the flood risk management and mitigation measures may differ between uses within any particular vulnerability classification.

In all zones, developers and LAs should seek opportunities to reduce neighbourhood flood risk by virtue of the design, siting and use of sustainable drainage systems (SuDS). The Floods and Water Management Act 2010 establishes a SuDS Approving Body in unitary or county councils. This body must approve drainage systems in new developments and re-developments before construction begins. In Flood Zone 3a, there is a further requirement to attempt to provide (or preserve) flood storage space on site; while in Zones 3a and 3b, they should also seek opportunities to relocate existing development to land in lower-risk zones, where climate change is expected to increase flood risk.

Any planning applications submitted to the LA are then considered against both the Local Plan and the SFRA, for suitability from a flood risk point of view. In order to demonstrate compliance with the Local Plan from a flood risk point of view, applications in all Flood Zones except Zone 1 must undergo a Sequential Test. This is based on the SFRA (or on EA flood zones if this isn’t available), and follows the principle that “Development should not be allocated or permitted if there are reasonably available sites…with a lower probability of flooding”, and that “Where there are no reasonably available sites in Flood Zone 1… consider reasonably available sites in Flood Zone 2, applying the Exception Test if required.” And finally, “only where there are no reasonably available sites in Flood Zones 1 or 2 should the suitability of sites in Flood Zone 3 be considered”. Note that the Sequential Test isn’t required if the LA have already used it to prepare their development plan (and that the development proposals are in accordance with that plan), although a SSFRA may still be required.

Then, if this is passed, and if Table 1 of the Technical Guidance document states that it is necessary, the Exception Test should be applied. The details of the Exception Test are set out in the NPPF, but are described below.

The Exception Test requires that the development should –

• provide wider sustainability benefits to the community that outweigh flood risk; and the SSFRA must demonstrate that the development will be safe for its lifetime without increasing flood risk elsewhere; and
• where possible, reduce flood risk overall.

If both elements of the test are passed then the development may be approved. The following table, adapted from the NPPF Technical Guidance document, outlines which types of development require the Exception Test, for the different Flood Zones.

Having considered the results of the Sequential Test (and the Exception Test, if required), the Local Planning Authority (LPA) should only consider approval of “appropriate” development where –

• the site is organised to locate the most vulnerable development in areas of lowest flood risk if practicable;
• that development is “appropriately” flood resilient and resistant; and
• it incorporates SuDS.

A similar policy to the above applies to coastal areas, but the specific details are outside the scope of this article.

Notwithstanding the Sequential and Exception Test mechanisms that exist within planning policy, the key point to remember is that building on flood plains should be avoided, as their purpose is to hold excess water in a safe place away from development. Filling them with buildings achieves nothing more than moving the problem somewhere else. It was recently reported that some councils’ five-year housing supply allocations fall within Flood Zone 3 land, at least one of which was inundated in the February 2014 floods in the south east of England.

Parts in this series

This article is Part One of a series – ‘Flood mitigation solutions in buildings’. Links to other parts in the series are provided below:

Flood mitigation solutions in buildings – Part Two
Flood mitigation solutions in buildings – Part Three