Walkthrough: a riverside site with a flood evacuation plan

A flood warning and evacuation plan does not make an unsafe site safe, and the fastest way to lose a riverside application is to offer one instead of safe access. This walkthrough follows an illustrative scheme of 34 flats on a brownfield riverside plot in Flood Zone 3a, where the site passed the sequential test on allocation grounds, the finished floor levels were set above the modelled design flood level with freeboard, and the dry pedestrian route off site could not be achieved. The evacuation plan was then part of a package rather than a substitute for one. All figures, policies and wording below are illustrative rather than taken from a real decision.

The order of work matters. Flood zone, then modelling, then levels, then access, then the plan.

Flood zone, sources and the modelling available

The site sat in Flood Zone 3a on the published flood map for planning, with a main river along the southern boundary and surface water risk shown along the access road. Three sources were in play: fluvial flooding from the river, surface water flooding from the catchment above the site, and a residual risk from a defence upstream.

The Environment Agency held detailed modelling for the reach, which it provided on request as a product. That saved a site specific hydraulic model, which is the single largest cost item on a flood constrained site. Ask for the modelling before commissioning anything, because the answer determines the budget.

Climate change allowances were applied for the river basin district and the development lifetime, taken from the Environment Agency's published allowance tables. Those tables are revised, so the Flood Risk Assessment stated the version used and the date it was checked. An assessment using a superseded allowance is the most common technical reason for an objection.

Sequential and exception tests

The site was allocated for residential development in the adopted local plan, and the allocation had been through the sequential test at plan making stage. That was recorded in the Sequential Test (Flood Risk) submitted with the application, with the plan reference and the examination evidence identified rather than a fresh search of alternative sites.

Residential development is more vulnerable development, so in Flood Zone 3a the Exception Test (Flood Risk) applied. Both limbs had to be addressed: wider sustainability benefits, and development safe for its lifetime without increasing flood risk elsewhere.

The second limb is where the case was argued. Safe for its lifetime is a technical question about levels, access and residual risk, and it is the limb the Environment Agency comments on.

Finished floor levels and freeboard

Levels were set from the modelled design flood level, including the climate change allowance, with freeboard above it. A 300mm freeboard is the figure in common use, and on this scheme it put residential floor levels around a metre above existing ground at the southern end.

That created three knock on effects, all normal and all worth anticipating:

• A level difference at the entrance, resolved by ramped external works and a lift serving the upper floors from an entrance lobby set at the raised level.
• A loss of usable ground floor area, since parking and plant occupied the undercroft while habitable rooms started at the raised level.
• Floodplain compensation, because filling within the floodplain displaces water. The scheme provided level for level compensatory storage on the eastern part of the site, modelled to show no increase in flood risk elsewhere.

No basement was proposed. On a Flood Zone 3a site, basement accommodation is a fight that is rarely worth having.

Safe access and egress, and the dry route question

The access road floods in the design event to a depth and velocity that made a dry pedestrian route off site impossible without works on third party land. This is the point at which many schemes are refused.

The package offered instead had four parts. First, safe refuge: every dwelling had access to floor levels well above the design flood level, with internal circulation allowing residents to move upwards without going outside. Second, a route to that refuge that did not cross external flood water. Third, flood warning: the site is within an Environment Agency flood warning area, with lead time measured in hours rather than minutes, and the scheme committed to registering the building for warnings. Fourth, a plan setting out who does what when a warning is received.

What the Environment Agency said, in substance, is what it usually says: evacuation and warning arrangements are matters for the local planning authority and its emergency planners, not for the Agency, and they do not substitute for safe access. The authority's emergency planning officer was then the decisive consultee. Engaging that officer before submission, rather than responding to their comments afterwards, changed the outcome here.

The evacuation plan and who it binds

A plan is only as good as the person obliged to operate it. The plan here identified the managing agent as the responsible party, with named roles rather than named individuals, and it was secured by condition with a requirement that it be reviewed at stated intervals.

The content followed a straightforward structure: the flood sources and warning lead times, the trigger for each action, the actions themselves, the location of refuge areas, the arrangements for residents needing assistance, the communication route to residents, and the arrangements for returning to the building afterwards.

Two provisions made the difference at committee. A register of residents who would need help to reach refuge, maintained by the managing agent with consent and data handling dealt with by the agent rather than described in the planning documents. And a commitment to an annual rehearsal, reported to the authority.

The honest limitation deserves stating. Plans depend on people acting, and compliance cannot be guaranteed by a planning condition. That is precisely why the levels and the refuge arrangements, which work without anyone doing anything, carried most of the weight in the case.

Flood resilient construction

Resistance keeps water out. Resilience accepts that water may enter and limits the damage. Both appeared in the scheme, with resilience emphasised because resistance measures fail when water stays up for days.

The specification included solid floors, water resistant finishes and insulation to the undercroft and lower structure, electrical distribution and sockets positioned above the design level, plant and lift motor rooms located at upper levels, and non return valves on drainage connections. The Drainage Strategy also had to show that surface water could not be discharged to a surcharged river during an event, which drove on site attenuation and a pumped overflow.

The documents submitted and the conditions attached

Eight documents dealt with flood risk, and the conditions mirrored them.

1. A flood risk assessment with the modelling, the allowance version, the levels, the compensation and the residual risk.
2. The sequential test reference to the allocation.
3. The exception test addressing both limbs.
4. A drainage strategy with attenuation, discharge rates and the pumped overflow.
5. A flood warning and evacuation plan.
6. A resilient construction specification with a drawing showing the levels of services and sockets.
7. Compensatory storage drawings and a volume calculation.
8. A statement of the emergency planning engagement with the authority.

The conditions required floor levels to be built as approved, compensatory storage to be formed before occupation, resilience measures to be installed as specified, and the evacuation plan to be implemented and maintained for the lifetime of the development. Where a scheme of this kind also sits in a sensitive area, screening can bring it into assessment territory, as our note on EIA screening when a sensitive area catches a small scheme describes.

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