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Damage-cost methodology

Indicative scoping estimates — not a substitute for a full flood-damage appraisal under the EA's Project Appraisal Guidance.

Damage rates

Weighted Annual Average Damage (WAAD) per residential property, from the Multi-Coloured Manual via CIRIA BeST, uplifted to 2026 prices (×1.35 vs 2016):

• FZ3 (≥ 1 % AEP): £6,592 / yr per residential property
• FZ2-only (0.1–1 % AEP): £950 / yr per residential property
• +8 % indirect costs (emergency services, third-party)
• + £385 / yr intangibles for residential (health, disruption)

Building-type multipliers

Applied to base WAAD per JBA / MCM convention:

• Residential ×1.0 · Commercial / retail ×2.5 · Industrial / warehouse ×3.0 · Public buildings ×2.0 · Outbuildings (sheds, garages) ×0.1–0.5

NPV (100-year horizon)

Discounted with HM Treasury Green Book step-down rates:

• Years 1–30: 3.5 %
• Years 31–75: 3.0 %
• Years 76–100: 2.5 %

NPV factor ≈ 28.86 × annual damage.

Climate-change scenario

Linear +30 % uplift to peak river flow by 2080 (EA Central allowance for rivers, South West). Averaged over the 100-yr horizon this gives a ×1.21 NPV scalar.

Flood defences & damage avoided

The EA AIMS Spatial Flood Defences dataset (linestrings, with current Standard of Protection and asset attributes) is buffered to derive a benefitting zone:

• Fluvial defences: 75 m isotropic buffer
• Tidal / coastal defences: 150 m isotropic buffer
• Buffers halved for assets with current SOP < 25 years

Buildings inside the merged benefitting zone are reclassified as protected, with EAD reduced from the unprotected WAAD to the CIRIA BeST residual figure:

• Residual FZ3 residential: £53 / yr (1-in-200 SOP, no warning, 2026 £)
• Residual FZ2-only residential: £50 / yr
• Damage avoided = unprotected EAD − residual EAD (the financial benefit attributable to existing defences).
• Unprotected NPV = NPV of buildings that sit in FZ2/FZ3 but outside the benefitting zone — i.e. the protection gap.

This buffer-based proxy is conservative where defences front directly onto buildings (Plymouth, Tavistock) and may overstate protection where defences run along rural meanders. For the EA's authoritative Areas Benefitting from Defences polygon (FRAW), see the dataset link in the footer.

Railways

Railway lines are pulled from OpenStreetMap (railway=rail, railway=light_rail, railway=narrow_gauge) and grouped by line name. Sidings, yards, spurs and crossovers are excluded. Each line is intersected with FZ2 and FZ3 polygons in BNG (EPSG:27700) so lengths are metric. “Behind defences” uses the same buffered EA AIMS defence asset zone as the buildings analysis. Because some defences (notably the sea wall along the South Devon Main Line) protect track that also runs outside FZ2/FZ3, the behind-defences total can exceed the at-risk total — it represents all track that benefits from a defence asset.

Major caveats

• EAD assumes no advance flood-warning service of more than 8 h — i.e. a baseline used in BeST.
• Buildings inside FZ2/FZ3 are identified by centroid, so partial-flooding nuance is lost.
• OSM building tags are heterogeneous; default building=yes is treated as residential.
• OSM coverage in remote Dartmoor villages is patchy — counts and NPVs there are lower bounds.
• No allowance is made for vehicle damage, agricultural land, or critical-infrastructure losses.
• The benefitting-zone buffer is a pragmatic proxy. The EA's official ABD polygon is not publicly available via OGC API at the time of writing.