Mangrove Belts as Coastal Defence, Priced per Metre: Wave Attenuation, Growth Lag and Lifetime Cost Against a Seawall Baseline

Mangrove restoration is promoted as low-cost coastal defence, but its protection is rarely priced in the same units as a seawall. This paper computes, from stated assumptions, wave attenuation across a mangrove belt, the time-integrated cost of the seedling-to-canopy growth lag, lifetime cost per metre of shoreline including replanting, and a seawall baseline at the same target. A mature dense belt reduces a 1.0 m wave by 69.9% in 100 m; reaching 0.25 m transmitted requires 116 m dense, 213 m medium or 462 m sparse forest. Growth lag removes 7.08 of 50 full-equivalent years. A 150 m belt costs $716 per metre NPV over 50 years versus $3,855 per metre for a seawall, but a 150 m medium belt fails a 0.30 m wave target above 0.80 m incident (about 1-in-4-year), and surge is essentially unattenuated (0.015 m reduction for 150 m). SIMULATED, NOT MEASURED.