Water is the constraint that decides most things about a site before anyone has drawn a building on it. Whether it floods, what happens to what you discharge, whether the coast in front of it will still be there in fifty years — all of it is public information, and almost none of it is published in one place. The water, coast and flood map puts seventeen registers on one surface.

The river network, with flow attached

The base layer is the connected watercourse network of Great Britain, stitched into named chains rather than left as disconnected link fragments. Onto it sit the Environment Agency and SEPA gauging stations that actually measure flow and level, and the rain gauges that measure the input side.

The spacing of those gauges is the real limit on any question about flow. An ungauged reach is unmeasured, not dry — a distinction worth holding on to, because a map of measurements looks a great deal like a map of the thing being measured.

Storm overflows and what a spill is

Every monitored combined sewer overflow in England, Wales and Scotland, from Event Duration Monitoring returns published by ten water companies including Scottish Water. Each carries when it discharged and for how long.

A spill is a permitted event, not automatically a breach. Overflows exist so that sewers surcharge into a river rather than into houses; the question a map like this can answer is not "is this legal" but "how often, and into what" — which is why the overflows and the bathing waters sit on the same map.

Flood areas, and the difference between an area and a point

There are 4,208 flood warning and alert areas in England, held as their published boundaries rather than centroids. That distinction matters more than it sounds: a point cannot contain anything, so questions about which sites sit inside a warning area can only be answered from polygons. Wales publishes its own equivalent, included separately.

These are the places with a known, monitored flood mechanism — not everywhere that can flood. Absence from this layer is absence of a warning service, not absence of risk.

Which coast gets defended

The layer people tend not to expect is shoreline management policy: 7,501 points of Shoreline Management Plan policy across 21 named frontages, from National Coastal Erosion Risk Mapping. Each stretch of managed coast carries one of a small set of policies — Hold The Line defends it, Managed Realignment moves the defence back, No Active Intervention lets it go.

It is a statement about the future rather than a record of the present. A frontage marked No Active Intervention may have a working sea wall today and no commitment to maintain it, which makes this the single most consequential coastal constraint on siting anything with a long life.

Sources

OS Open Rivers, Environment Agency hydrology, rainfall and flood areas, SEPA, storm overflow feeds from ten water companies, NCERM 2024, Natural Resources Wales, bathing water quality, tide gauges, source protection zones and MMO port data — all under the Open Government Licence, and each named on the map itself.

You can also put any of it into a question on the CityMirror AI map, which reaches all 170 datasets at once and can relate any two of them by a spatial relationship.