Seasonal flood risk evidence and the winter check

The national flood map shows river and sea flooding. It does not show the water that runs off the field behind your house in February, the ditch that backs up when the culvert blocks, or the groundwater that rises through the floor of the garage after a wet winter. Where the map and the ground disagree, the evidence that settles it is local, seasonal and gathered by someone who was there.

Winter is when that evidence exists. A site walked in August tells you almost nothing about how it behaves in January, and the gap between the two is where a flood risk assessment either becomes credible or does not.

Why winter evidence matters for a site

Three of the flood sources that affect small sites are seasonal in a way river flooding is not.

Surface water flooding follows intense rainfall and the routes it takes depend on ground conditions. The same storm produces a puddle on dry summer ground and a flowing sheet across saturated winter ground.

Groundwater flooding is the most seasonal of all. Levels rise through winter as rainfall recharges the aquifer, peaking in late winter or early spring, which is why a cellar can be dry for nine months and wet for three. Chalk and limestone catchments and springline villages are the classic settings.

Ordinary drainage capacity is also seasonal, because a system running at the top of its range after weeks of rain has nothing left for a storm.

None of that appears on a national map, and all of it is visible to anyone standing in the right place in February.

Photographs, dates and how to record them

Photographic evidence is accepted readily when it is recorded properly and dismissed when it is not.

Take the pictures from fixed viewpoints you can return to, and mark those viewpoints on a plan. Include something of known size in the frame, a gate post, a doorstep, a fence rail, so that depth can be judged. Keep the original files with their embedded date and location data rather than sending resized copies, because the metadata is what makes them evidence.

Write a short note with each set: the date, the time, the weather in the preceding days, and what you observed. A line saying water stood 150 millimetres deep across the access for six hours on the morning of 8 January is worth more than twenty photographs with no commentary.

Repeat the exercise. One wet winter proves an event. Three winters of records, including dry ones, prove a pattern.

Video is useful for flowing water, because depth alone does not convey whether a route is passable. A short clip from the same fixed viewpoint, with the date attached, answers a question that stills cannot.

Historic flood records and where they sit

Before gathering new evidence, collect what already exists.

The Environment Agency holds recorded flood outlines for historic events, which show where flooding has actually happened rather than where modelling says it could. The lead local flood authority, usually the county or unitary council, holds records of surface water and ordinary watercourse flooding and publishes investigation reports for the larger events. The authority's strategic flood risk assessment gathers much of this together and is published on its website.

Parish councils, highway authorities and water companies all hold fragments too. The water undertaker's records of sewer flooding incidents are worth requesting, and they feed directly into the Drainage Strategy for any scheme discharging to the public system.

Ask neighbours, and write down what they say with dates. Anecdote is weak evidence on its own and strong evidence when several accounts agree and align with a recorded rainfall event.

Groundwater and its season

Groundwater deserves separate treatment because it behaves differently from every other source.

It rises slowly, stays high for weeks, and does not respond to a single storm. It defeats conventional flood resistance measures, because water arrives through the floor rather than the door. And it is poorly mapped nationally, so site-specific evidence carries more weight here than anywhere else.

The evidence worth gathering is a monitoring record: a standpipe or borehole with readings taken through a winter, which gives a level rather than an impression. Where a basement or a lowered floor level is proposed, that record is close to essential, and it also determines the waterproofing approach the structural engineer will specify.

Start monitoring the winter before you need the answer. Retrofitting a year of groundwater data into a determination period is not possible.

Using local evidence in a sequential test

The sequential test asks whether the development could go somewhere at lower flood risk. Local evidence can cut both ways in that exercise, and honesty about it is what makes the assessment credible.

Where the mapped zone overstates the risk, site-specific survey and modelling can demonstrate that a parcel sits above the relevant level, which is a proper basis for refining the assessment rather than an argument for ignoring the map. Where local evidence shows flooding from a source the map does not cover, it has to be reported even though it makes the case harder. An assessment that omits known surface water flooding will be caught, and the credibility of everything else in it goes with it.

A Sequential Test is a short document that depends entirely on the quality of the evidence behind it, and it sits alongside the Flood Risk Assessment rather than inside it.

When the map and the ground disagree

National mapping is produced at national scale from modelled data and it is not intended to resolve a boundary at plot level.

Where the two disagree, the route is a site-specific assessment using surveyed ground levels, detailed flood data requested from the Environment Agency, and where necessary local modelling. Ground levels from remote sensing give a first view, though anything relied on for a threshold level needs a measured survey.

Say what you did and in what order. An assessment that explains why it departs from the published map, with survey levels and a data request behind it, reads very differently from one that simply asserts the map is wrong.

It is also worth checking whether the authority has designated a critical drainage area covering the site, because that changes the surface water requirements regardless of what the river map shows.

Reporting local evidence credibly

Four habits separate evidence that persuades from evidence that irritates a reviewer.

State dates and sources for everything, including anecdote. Distinguish what you observed from what you were told. Include the negative observations, meaning the winters when nothing happened, because a record that only contains flood days looks selected. And say what the evidence does not show, since a reviewer who finds an unacknowledged gap will assume there are others.

Seasonal evidence gathering follows the same discipline across disciplines, and the timing constraints are as real here as they are for the survey windows described in botanical survey season and habitat condition assessment.

This is general guidance, not legal advice. Check the position with your local planning authority or a qualified professional before relying on it.

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