Laboratory Study of Wave Attenuation and Spectral Dissipation Induced by Mangroves with Stilt Root Morphology

Abstract

With rising global temperatures, the poleward migration of mangroves may enhance their contribution to coastal protection. Although the wave attenuation capacity of Rhizophora stilt roots is well established, their influence on wave spectral transformation remains less understood. This study investigates wave height attenuation and associated spectral dissipation using a near-prototype physical model of a Rhizophora mangrove forest. Wave dissipation was quantified using a bulk damping coefficient ( β ), which increased under shallower water depths, higher relative wave height ( H / h ), and greater wave steepness ( H / L ), with the latter exhibiting a clear linear relationship. To better represent the complex projected area of the root system, a depth-averaged equivalent diameter ( d _eq ) was introduced. Accounting for d _eq improved the collapse of β across different water depths for H / h and H / L . When d _eq was adopted as the characteristic length scale, the bulk drag coefficient ( C _D ) correlated more strongly with the Keulegan–Carpenter number (KC) than with the Reynolds number ( Re ). An existing KC– C _D relationship was expanded to include additional studies, with root diameter as the characteristic length scale demonstrating potential for a broadly applicable formulation for Rhizophora . A spectral analysis revealed preferential dissipation of high-frequency wave energy, particularly under emergent conditions, whereas this pattern diminished when the roots were fully submerged. This preferential dissipation was examined in the context of a frequency-dependent damping factor ( β _f ). The increase of β _f with frequency led to steepening of the spectral tail, particularly for steeper waves, which likely contributed to their greater attenuation.