Experimental investigation of wave attenuation and bulk drag coefficient in mangrove forest with complex root morphology

Abstract

Mangrove forests play a crucial role in coastal protection. Numerous experimental studies have been performed to examine the interaction between wave flows and mangrove forests, while the mangrove forests are commonly modeled as cylinder arrays, and the effect of the complex root system on wave attenuation is still unclear. A series of physical experiments were conducted to study the hydrodynamic characteristics induced by a 10-m-long mangrove forest model, which was built to reproduce a mature Rhizophora forest. Wave-attenuation analysis was performed to highlight the importance of considering the wave damping produced by mangrove roots under different water depths. In addition, the wave-attenuation coefficient β increases linearly with an increase in the relative wave height and wave steepness to varying degrees. In addition, the bulk drag coefficient (CD), a key parameter for quantifying the mean vegetation resistance, was obtained based on the classical model proposed by Dalrymple et al. (1984). The influences of several dimensionless hydrodynamic parameters on the CD are discussed in detail. The results show that the Ursell number and Keulegan–Carpenter number (KCd) have stronger correlations with CD for vegetation models without roots. The KCLe number calculated using effective characteristic length Le is suitable for describing the CD of the mangrove model for given water depth. Then, a new parameter, KCrv, was defined using the average pore velocity and vegetation-related hydraulic radius by considering the bulk characteristics of the wave and vegetation. Finally, a generic empirical formula was proposed for the CD to reveal the drag coefficient characteristics of vegetation with a relatively high correlation.