Numerical Analysis of Wave Attenuation by Mangrove Forests

Abstract

Mangrove forests provide natural coastal protection by attenuating wave energy, although their protective effectiveness remains challenging to quantify. In particular, the contributions of vegetation density and of the bulk drag coefficient are seldom separated, and open-source wave-resolving models are rarely checked against prototype-scale physical-model data. This study aims to quantify wave attenuation across a mangrove band at its seaward edge (x = 35 m), its middle (x = 44 m) and its landward edge (x = 52 m); to isolate the independent effects of vegetation density and of the drag coefficient (C_d); and to assess the consistency of the model against a prototype-scale physical benchmark. The XBeach numerical model was applied in non-hydrostatic (wave-resolving) mode to a 104 m numerical flume, with vegetation schematised into root, trunk and canopy layers, each with its own stem diameter, density and drag coefficient. Simulations were performed for three scenarios — no mangroves, low-density mangroves (N = 25 stems m⁻²) and high-density mangroves (N = 50 stems m⁻²) — with additional sensitivity tests varying the drag coefficient (C_d = 2.0 and C_d = 1.5). The analysis focused on three key wave parameters: water level, zero-crossing wave height, and significant wave height. Results show that high-density mangroves reduce the band-averaged significant wave height by 24%, whereas low-density mangroves achieve a 15% reduction compared with unvegetated conditions, so halving the stem density approximately halves the reduction. Lowering the drag coefficient to C_d = 2.0 and C_d = 1.5 reduced the attenuation to 18% and 17%, a monotonic but sub-linear dependence, matching the trend observed in a prototype-scale physical model experiment. These findings highlight the significant influence of vegetation density and drag on wave attenuation and support the use of mangroves as effective nature-based solutions for coastal defence and sustainable coastal management