基于数值波浪模型 SWASH 的竹排消浪性能优化

摘要

红树林可保护热带海岸免受淹没与侵蚀，从而降低全球沿海社区的洪涝风险。然而，城镇化与水产养殖等活动使红树林大面积消失，沿岸地区因此直接暴露于波浪作用之下。为在侵蚀岸段恢复红树林，人们采用竹桩加枝柴填料构成的透空结构为海岸挡浪，并营造有利于红树林定植的生境。但此类结构在风暴中往往丧失枝柴填料，维护费用高昂。本文提出一种仅由竖向竹桩、不填枝柴的新型设计以降低维护成本。以印度尼西亚德马克（Demak）为例，采用数值波浪模型 SWASH 预测结构的消浪效果，对各设计方案进行评价；通过分析现场实测资料与 WaveWatch III 数据确定德马克的设计工况。SWASH 先经室内试验验证，再用以研究最优排数及其最优间距（沿波浪传播方向）。计算表明：对由两排竹桩构成的结构，当沿波向排间距由 sx = 0.42 m 增至 sx = 5.8 m 时，透射率 Et/Ei 由 75% 降至 55%；间距再增大并不能进一步减小透射，且至少需要三排才能使透射率低于 50%——这是结构化生态修复中常用的消浪目标。本文由此识别出使竹结构消浪效果最大化的可行策略，可在无需枝柴填料的条件下减轻淤泥质海岸所受的波浪冲击，相比既有枝柴结构方案更经济、更便于施工与维护。此外，本文还提出了一种在 SWASH 中高效概化此类结构的新方法。

Abstract

Mangroves protect tropical coastlines from flooding and erosion, reducing flood risks along coastal communities worldwide. Nevertheless, mangrove forests have experienced considerable losses due to activities like urbanization and aquaculture, exposing coastal areas to wave attack. To restore mangroves at eroding coastlines, structures formed by bamboo poles and a brushwood filling are used to shelter the coast from waves, and create a favourable habitat for mangrove colonization. However, these structures often lose their brushwood filling during storms, resulting in high maintenance costs. This study proposes a new type of design to reduce maintenance expenses, consisting of only vertical bamboo poles without a filling of brushwood. Structure designs are evaluated for a case study in Demak, Indonesia, using the numerical wave model SWASH to predict wave attenuation through the structures. Field measurements and WaveWatch III data are analyzed to obtain the design conditions for the structures in Demak. SWASH is validated against laboratory experiments, and applied to investigate the optimum number of rows and their optimum spacing (in the direction of wave propagation). The model shows that for a structure consisting of two rows of bamboo poles, the transmission rate Et/Ei decreases from 75% to 55% when the row spacing in the wave direction is increased from sx = 0.42 m to sx = 5.8 m. Larger spacings do not result in less transmission, and at least three rows are needed to have a transmission rate lower than 50% - a common wave reduction target used in restoration efforts with structures. This study thus identifies potential strategies to maximize wave attenuation by bamboo structures, which can be used to reduce wave attack along muddy coasts without the need of a brushwood filling. Hereby, it provides an economically and user friendly alternative with respect to the previous brushwood structure designs, as it requires less material costs and maintenance. In addition, this study presents a new method to schematize these kind of structures in SWASH in an efficient way.