大亚湾不同波浪、水深与坡度条件下车叶型人工鱼礁的安全重量

Study on safe weight of artificial reef in different water depths and bottom slopes under Daya Bay wave

基于小振幅波和力学理论,以车叶型鱼礁为对象,对其在大亚湾不同波浪(台风、十年一遇、二十五年一遇、五十年一遇)、不同水深(6～20 m)、不同海床坡度(10-1、30-1、50-1)及附着生物等多种条件下的安全性进行了研究分析,确定了车叶型鱼礁的安全重量和大亚湾海域适于其投放的水深范围。结果表明,安全重量在深于12 m的水深范围内变化平稳,而在较浅的水深特别是在波浪发生破碎的前后过程大小差异明显,表现出随水深变浅先变大后变小的波动特征,二十五年一遇的波浪条件下3种坡度的安全重量最大值分别为42.4、22.4和20.3 t,对应水深为10、12和14 m。坡度对礁体安全重量的影响非常明显,坡度越小,安全重量越轻,各种波浪条件下30-1与50-1下的安全重量随水深的变化特征相似,而10-1与30-1的差异远大于30-1与50-1的差异。礁体的安全重量与波浪条件、水深和海床坡度有直接关系,水深过浅或海床坡度过大均可能导致礁体在较大波浪下失稳。从礁体稳定性与航道安全性两方面考虑,应选择大亚湾12 m以深、坡度平缓的水域,投放设计重量为22.4 t的车叶型人工鱼礁。

Artificial reefs（ARs）,constructed with reinforced concrete,have been deployed along many coasts to enhance fisheries aquaculture and diving tourism,protect habitats,and foster research all over the world for a few years.Though new techniques and developments were applied to ARs,unexpected problems have also occurred in deployment processes,stability,local scours,durability of materials,and design characteristics.Of these problems,the stability of ARs is a very important issue in preventing the failure of reef units due to wave and current action.It is indicated that the principal hydrographic factors to be considered in AR construction consist of installation depth,reef weight,wave height,currents,and tides.In this study,the authors analyzed stability of an cube artificial reef to determine safe weight and deployment depth at varying water depths（6-20 m）and bottom slopes（10-1,30-1,50-1）based on wave theory.FLUENT software and physics equations were used to estimate drag coefficients and waves forces.Drag coefficients for the reef is 1.5.Deep water offshore wave conditions in Daya Bay were transformed into shallow nearshore waters representative of the artificial reef site using linear wave theory.Four wave conditions were used in this study,including typhoon wave and different return period wave（10-year,25-year,50-year）.The results showed that the safe weights fluctuated with water depths,increased first and then decreased when wave broke.Under typhoon wave,the maximum values of safe weights were 21.4,16.8 and 14.2 t for 3 bottom slopes,which occurred at water depths of 6,7 and 7 m respectively.When wave height and period were increased,safe weight and corresponding water depth were increasing at the same time.The maximum values of safe weights were 42.4,22.4 and 20.3 t,which occurred at water depths of 10,12 and 14 m respectively under wave of 25-year return period.Bottom slope was closely related to safe weight.When bottom slopes were investigated for the same wave parameters and depths,safe weights decreased and vice versa when slopes increased.The distinction was much more evident between a bottom slope of 10-1 and the other bottom slopes（30-1,50-1）.The maximum value of safe weight for 10-1 could be 2.2 times the safe weight for 50-1 at the same water depth under wave of 25-year return period.The flat seabed is more proper for ARs deployment.It is concluded that safe weight design must take wave parameters,water depths and bottom slopes into consideration,and improper water depth or slopes might cause the failure of reef units.To prevent navigational hazards and loss of stability,the investigated reef was suggested to deploy at a water depth of 12 m or deeper with a design weight of 22.4 t in Daya Bay.The design weight and selected deployment depth can resist wave force of 25-year return period and satisfy the ARs engineering project requirement of safety.