Flood Risk Assessments for Jersey Planning Applications

If your site is in a designated flood risk area, a flood risk assessment is not an optional extra. Without one the case officer cannot properly determine the application, and it will either be refused or sit unresolved while they ask for it.

Why Jersey has flood risk areas at all

Jersey’s flood risk comes from three directions, and they behave differently.

Coastal and tidal. Jersey has one of the largest tidal ranges in the world. Low-lying coastal land — parts of St Aubin’s Bay, the St Helier waterfront, Grouville — is exposed to tidal flooding, particularly when a spring tide coincides with a storm surge.

Fluvial. The island’s valleys carry streams that respond quickly to heavy rain. Valley-bottom sites can flood from watercourses that look insignificant in dry weather.

Surface water. Intense rainfall overwhelming drainage. This is the least predictable, and it is made worse by development that replaces permeable ground with hard surfacing.

Climate projections point to higher sea levels and more intense rainfall, which is why flood policy has tightened rather than relaxed.

When you will need an assessment

Expect a flood risk assessment to be required where:

• The site falls within a designated flood risk area
• You are building a new dwelling on a site with any flood exposure
• You are creating new habitable accommodation at ground or basement level
• You are significantly increasing the building footprint or the hard surfaced area
• You are changing the use of a building to something more vulnerable — for example converting a garage or outbuilding to living accommodation

That last one catches people. Converting a garage that has flooded before into a bedroom increases the vulnerability of the use, and that is exactly what flood policy is designed to scrutinise.

What the assessment contains

A flood risk assessment for a Jersey application should establish:

1. The sources of risk affecting the site — tidal, fluvial, surface water, or a combination
2. The likelihood and the depth of flooding, including an allowance for climate change over the lifetime of the development
3. The vulnerability of the proposed use
4. Mitigation — how the design responds
5. Residual risk — what remains after mitigation, and how it is managed
6. The effect on others — crucially, that your development does not increase flood risk elsewhere

That last point is often overlooked. Raising your site above the flood level may protect your building while displacing water onto a neighbour. An assessment that ignores this will not satisfy the department.

Mitigation that works

Design responses that are commonly accepted:

• Raised floor levels with adequate freeboard above the modelled flood level
• Flood-resilient construction — materials and finishes at low level that survive wetting and can be dried out
• Locating vulnerable uses upstairs, keeping ground floor for parking, storage or circulation
• Sustainable drainage — permeable surfacing, attenuation, soakaways where ground conditions allow, so runoff does not increase
• Safe access and egress in a flood event
• Flood-proof service routing — electrics dropped from above rather than run at floor level

How it affects what you can build

Flood risk rarely makes a site undevelopable on its own, but it shapes the design. It can dictate floor levels, which affects overall height, which affects the massing the planners will accept. It can rule out basements. It can determine where the habitable rooms go.

The practical lesson: find out about flood risk before you design, not after. A scheme designed and then adjusted for flood levels usually ends up compromised. A scheme designed around them from the start usually does not.

Do it in-house

We produce flood risk assessments alongside the drawings rather than subcontracting them, which matters because the assessment and the design have to agree with each other. A report that recommends a floor level the drawings do not show is worse than no report.

See flood risk assessments and the rest of our planning reports.

Check your site

Book a free consultation and we will establish whether flood risk affects your site, and what it means for what you want to build, before you commit to a design.