Frequency-dependent wave damping by tidal wetlands under storm conditions

Abstract

Climate change is increasing the threat of coastal flooding worldwide, and the incorporation of tidal wetlands into coastal protection is being widely promoted. These circumstances have motivated extensive research on wetland-induced wave damping, while its distribution across the frequency scales remains poorly understood. Here, field observations during Typhoon Rumbia were adopted to examine the frequency-dependent wave damping by a Changjiang estuarine wetland, which consists of a salt marsh followed seaward by an unvegetated mudflat, under storm conditions. The results showed that the typhoon produced waves with approximately trimodal spectra comprising low-frequency swell (0.02–0.15 Hz), moderate-frequency wind-sea (0.15–0.48 Hz) and high-frequency components (0.48–0.70 Hz) at the mudflat edge. As the waves propagate landward across the mudflat and salt marsh, the integral wave heights are successively attenuated by 16 % and 32 % and the wind-sea waves continuously experienced the largest height attenuation. Mechanisms for the frequency-dependent wave damping were explored by discriminating energy dissipation and nonlinear transfer and reconstructing wave spectra induced by each of them. The frequency-dependent wave damping by unvegetated mudflats is controlled by nonlinear energy transfer, which shifts 10 % of the gross energy from the wind-sea to high-frequency components; the wind-sea waves’ attenuation is increased by 66 % but the high-frequency wave attenuation is reduced by 88 %, compared with the regime controlled solely by direct dissipation. Direct dissipation regulates the frequency-dependent wave damping across the salt marsh, and the contribution of nonlinear energy transfer to height attenuation of the wave components is below 15 %. The similar regime but differing mechanisms by which the studied wetland units damp waves of different frequencies provide new insights for wetland-wave interactions.