The Manchester Ship Canal Company Ltd & Anor, R (on the application of) v Environment Agency

When a public authority classifies infrastructure under its published flood-mapping policy, the policy must be interpreted objectively and applied according to its stated categories and purpose. Structures whose primary purpose is navigation or water-level control may be de facto flood defences, but they are not “formal defences” merely because they reduce flood risk. A policy-maker must follow its policy unless there is a legitimate reason to depart from it, while retaining sufficient flexibility to consider exceptional cases. The authority acted irrationally by treating canal sluices, whose primary purpose was to regulate water levels for navigation, as formal flood defences. An alternative challenge based on the assumed failure of the sluices failed because the relevant policy required a worst-case undefended scenario.

The claimants sought judicial review of the Environment Agency’s decision to classify sluices on the Manchester Ship Canal as “formal flood defences” for Flood Map and Flood Zone purposes. The Agency consequently assessed flood risk on the assumption that the sluices had failed in the closed position, causing land near the Canal to be placed in Flood Zone 3a rather than Flood Zone 2.

The claimants argued that the Agency had misinterpreted and misapplied its own policies and had acted irrationally by disregarding the very low probability of simultaneous sluice failure. They also contended that the Agency had applied its policies inflexibly and inconsistently. The central issues were the meaning of “formal defences”, the distinction between formal and de facto defences, and the lawfulness of the worst-case mapping assumption.

1. Ground 1 succeeded. The Agency’s decision was quashed because it failed properly to interpret, apply or have regard to its own policy, and was irrational in the Wednesbury sense.
2. Under PPS25, Flood Zones are an initial, precautionary and relatively crude mapping tool. They disregard formal flood defences and use an undefended worst-case scenario. More detailed assessments may later take account of actual flood risk, residual risk and Areas Benefiting from Flood Defences.
3. The expression “Flood Defences” in PPS25 refers to structures intended and designed to protect against flooding. The Agency’s policies distinguished formal defences, whose primary purpose is to limit the spread of flood water, from de facto defences, where flood protection is secondary or an unintended by-product.
4. The Canal sluices controlled normal water levels and were designed and maintained primarily to enable safe navigation. Their contribution to flood reduction made them de facto defences, but did not make them formal defences. The Agency therefore misclassified them by treating their secondary flood-reduction function as their primary purpose.
5. Published policies should be interpreted objectively. Where a policy has already determined that a particular type of structure is not a formal defence, the decision-maker cannot lawfully depart from that position without a legitimate reason. Policies should nevertheless not be applied with rigidity that excludes genuine exceptions or produces arbitrary results.
6. Ground 2 failed on the alternative assumption that the sluices were formal defences. The Agency had considered the evidence showing that simultaneous failure was highly remote, but the policy required formal defences to be disregarded for Flood Zone mapping. It was therefore lawful to assume that the sluices failed to open. The claimants had not challenged the lawfulness of the policy itself.
7. The court declined to determine whether the Canal should be assessed as a river or as infrastructure for Flood Zone purposes. That issue emerged after the hearing, the Agency’s position was not sufficiently clear, and the decision was being quashed on Ground 1.

The application for judicial review was allowed on Ground 1 only.