A study of the increase in subsurface freshwater on coral islands by slope seepage control: Experiment and modeling
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摘要:
地下淡水是支撑海岛居民生活用水保障和生态岛屿建设的重要战略资源。目前海岛淡水资源十分匮乏，鉴于我国灰沙岛的现实复杂性，亟需实施低成本、易操作且对岛礁稳定性及生态系统影响较小的增加地下淡水的措施。本研究提出了采用在海陆边坡处铺设防渗材料人为干预地下淡水体形成、增加淡水储量的措施，并通过砂箱试验和数值模拟相结合的方法在实验室尺度下分析了边坡防渗对灰沙岛淡水透镜体形成的影响，评估了不同因素在边坡防渗条件下增加地下淡水储量的效果，并以永兴岛为例定量评估了边坡防渗对地下淡水储量的影响。研究表明，边坡防渗通过改变地下水流场、增加淡水水头的方式增加了淡水储量。淡水体所需的稳定时间随之增加。在长、宽、高分别为50，5，35 cm的砂箱中，在35°的边坡处铺设14 cm长的隔水材料，淡水透镜体达到稳定后最大厚度由原有的13.7 cm增加至24.9 cm，淡水储量由561.8 cm3增加至1592.3 cm3，所需稳定时间由120 min增加至150 min。增加的淡水储量随降雨强度增加、砂体渗透系数减小、边坡防渗深度增加、防渗材料渗透系数减小而增加。若在永兴岛海陆边坡铺设2 m深的隔水材料，在未来30 a，淡水储量将由天然状态的3.4×106 m3增加至4.4×106 m3，增加原有储量的1/4。研究可为我国岛屿地下淡水科学管理、水资源安全保障提供理论支撑和实践指导。
Abstract:Fresh groundwater lenses are important resources for local inhabitants and ecosystems in coral islands. In view of the complexity of the coral islands, it is necessary to seek the measures of low-cost, effective and less impact on the stability and the ecosystem to increase freshwater reserves. Based on the sandbox experiments and numerical simulations, this study proposes an engineering measure to increase freshwater reserves by laying impermeable materials on the slope of an island. The effects of the measure on fresh groundwater lenses formation and different factors on increasing freshwater reserves in slope seepage prevention are evaluated. The practical feasibility of this measure is also discussed in case of the Yongxing Island. The results show that the slope seepage control increases the freshwater reserves by changing the groundwater flow field to increase the freshwater head. The stabilization time of the freshwater lens is also increased. After laying impermeable materials on slope of the sandbox (with the length, width and height of 50 cm, 5 cm and 35 cm, respectively), the maximum thickness of freshwater lens increases from 13.7 cm to 24.9 cm, the freshwater reserves increases from 561.8 cm3 to 1592.3 cm3, and stabilization time increases from 120 min to 150 min, respectively. The increased freshwater reserves develop with the increasing rainfall intensity and slope impermeable depth, and also extend with the decreasing permeability of coral sand and impermeable materials. The Yongxing Island is taken as an example, and laying impermeable materials with a depth of 2 m on the slope will increase the freshwater reserves from 3.4×106 m3 to 4.4×106 m3 in the next 30 years, increasing the original reserve by 1/4. This study can provide theoretical support and practical guidance for sustainable groundwater management and water resources security in island areas of China.
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Key words:
- slope seepage control /
- coral islands /
- freshwater lenses /
- numerical simulations /
- sandbox experiments
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表 2 参数赋值
Table 2. Parameters selection
参数选取 参数赋值 降雨强度/（cm·min−1） 0.2, 0.4, 0.6, 0.8, 1.0 砂体渗透系数/（cm·min−1） 12, 16, 20, 24, 28 边坡防渗长度/cm 2, 5, 8, 11, 14 防渗材料渗透系数/（cm·min−1） 0, 0.5, 1.0, 1.5, 2.0, 2.5 
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