绿色海堤是传统结构工程与海岸生态系统共同组合而成的新型海堤, 用以应对未来海面上升、风暴加剧给低地海岸防护带来的挑战。需解决的问题主要有海岸生态系统消浪过程及生态系统在海堤体系中的配置方式。理论分析、现场观测、物模数模所获结果表明, 海岸生态系统确有显著的消浪功能: 1) 陆架泥区消浪, 其机制以再悬浮和浮泥运动为主, 底部摩擦为次; 2) 潮滩下部的粉砂细砂滩底部摩擦和推移质运动共同造成波能耗散, 而上部的泥滩则以再悬浮和悬沙输运为主; 3) 在盐沼、红树林、海草床等由植被构成的生态系统, 植物通过形态阻力、茎秆运动来阻滞水流、耗散波能, 其效能高于沉积物床面对波能的耗散; 4) 生物礁主要有珊瑚礁和牡蛎礁, 其消能作用主要通过床面摩擦和波浪破碎, 效能较高, 尤其是在风暴期间。生态系统如何成为海堤的有机组成部分, 尤其是侵蚀型海岸的生态位修复和绿色海堤整体设计, 还需进一步研究相关的科学问题: 与硬质工程结合的盐沼-牡蛎礁的适应性生物学; 未来环境变化条件下生态系统的稳定性; 绿色海堤生态系统空间配置及其与风暴事件的时间尺度匹配; 基于均衡剖面理论的海堤形态优化。

Green sea dyke represents a new concept of coastal defense, which combines traditional engineering structure with coastal ecosystem, to cope with the future trend of sea level rise and storm intensification. Before its application, however, the feasibility of such a system must be tested for low-lying coastal areas, where the risk of storm surge, storm-induced waves and shoreline erosion is greatest. The major issue is associated with the process of wave attenuation by the ecosystem and the way of using the ecosystem within the sea dyke system. For many years, wave energy dissipation has been an important research field for tidal flats, a typical environment of low-lying coasts, as well as beaches and rocky coast environments. Theoretical analysis, field observation and physical-mathematical modelling show that the coastal ecosystem indeed plays a significant role in wave energy dissipation: (1) resuspension and fluid mud movement dominate over bed friction in the mud area, in terms of wave height reduction; (2) wave attenuation occurs due to bed friction and bedload transport on the silt-sand flat at the lower part of the tidal flat, while on the upper mudflat it is caused mainly by re-suspension and suspended, fine-grained sediment transport; (3) in the vegetated ecosystems such as salt marsh, mangrove and seagrass beds, the efficiency of plants in reducing flow velocity and dissipating wave energy is higher than that of bare flats, due to plant morphological resistance and stem movement; and (4) wave attenuation is high when passing through biological reefs (e.g., coral and oyster reefs), especially during storms, with bed surface friction and wave breaking being the major mechanisms. Although the wave attenuation theory has been established, the optimization of the way the ecosystem is used within the sea dyke system requires further investigations, especially the techniques of ecological niche reconstruction on eroding coasts and the design of the sea dyke to improve structure safety. The relevant scientific problems include: adaptive biology for salt marsh plants and oysters in conjunction with hard engineering; stability of ecosystem in response to future environmental change; spatial configuration of ecosystem in the green sea dyke and the match between the ecosystem life cycle and the temporal scales of storm events; and the optimization of the sea dyke configuration, on the basis of the equilibrium coastal profile theory.

Most existing analytical and numerical models to quantify wave energy attenuation attributed to saltmarshes are based on the definition of a drag coefficient that varies with vegetation and wave characteristics and requires calibration, i.e., a case-specific variable. With the aim of determining a new variable to estimate wave energy attenuation without the use of calibration coefficients, wave attenuation caused by different saltmarsh species and the relationship with the ecosystem standing biomass are experimentally studied. Samples of four real saltmarshes with contrasting morphological and biomechanical properties, namely, Spartina sp., Salicornia sp., Halimione sp. and Juncus sp., are collected in the field and placed in a wave flume for testing under different regular and random wave conditions. Two meadow densities are considered, in addition to zero-density cases. Thus, wave damping coefficients are obtained in vegetated cases, β, and bare soil cases, β, and wave damping produced solely by the meadow standing biomass, β, is determined. The obtained wave damping coefficients are related to a new variable, the hydraulic standing biomass (HSB), which is defined as a function of the meadow mean height and standing biomass and incident flow characteristics. Linear fitting relationships between the wave damping coefficient and HSB are obtained, allowing β and β estimation without the need for calibration. Therefore, the use of these new relationships facilitates direct quantification of wave energy attenuation due to saltmarshes based on incident wave conditions, mean plant height and meadow standing biomass, variables that can be obtained from aerial images or remote sensing data, extending the applicability of the approach. Another key aspect is that this approach does not depend on any calibration coefficient and can be directly applied with knowledge of the abovementioned characteristics. This may represent a paradigm shift in the estimation of wave energy attenuation attributed to saltmarshes.© 2022. The Author(s).