Managed Retreat Threshold Simulator

Coastal Defense Policy

Sea-level rise rate 8 mm/yr

Seawall height 1.6 m

Storm severity (surge scale) 0.5 m

Relocation cost / household $120k

Town Status

Year

Sea level

0.00 m

Freeboard

1.60 m

Annual flood risk

0.0%

Protect cost so far

$0k

Retreat (buyout) cost

$0k

Press Run to advance the clock and watch the two cost lines race.

How it works

Each simulated year the sea rises by the chosen rate, shrinking the seawall's freeboard (margin above mean sea level). Extreme storm surge on top of that mean level is modeled with a Gumbel (Type-I extreme-value) distribution, so the odds of the wall being overtopped that year are:

SL(t)      = SL0 + rate · t
freeboard  = wallHeight − SL(t)
P_flood(t) = 1 − exp( −exp( −freeboard / scale ) )

A flood year adds expected damage (P_flood × exposed value) to the running protect-cost line, on top of the wall's one-time build cost and yearly upkeep. The retreat-cost line is the one-time price of buying out every household and relocating the town inland — paid once, then flat forever.

Raising the wall lowers annual flood risk but raises the build/upkeep cost baseline.
A cheaper wall looks good in year 1 but the flood-risk term compounds as freeboard shrinks — watch for the crossover year where the protect line overtakes the flat retreat line.
This mirrors the real "Protect / Accommodate / Retreat" framework used in coastal-adaptation planning (IPCC AR6, WG2): each strategy wins for a different combination of sea-level-rise rate and defense cost.

Safe building Flooded this year Sea level

⚠ Couldn't load the 3D engineThree.js failed to load from the CDN. Check your connection and reload.