上海市海塘防护能力现状的调查及分析

Study on the present defensive capability of the coastal levees in Shanghai

上海市海塘分布较广、岸线较长,建设历时跨度大,护面结构形式多样,为充分了解海塘的防护标准、现状结构情况和实际防御能力,分区域分段选取具有典型代表性的海塘岸段进行调查与分析。把上海市海塘分为崇明三岛、长江口南岸和杭州湾北侧3个大区域,其中崇明三岛包括崇明岛、长兴岛和横沙岛,长江口南岸包括宝山区、浦东新区和南汇区北侧,杭州湾北侧包括南汇区南侧、奉贤区和金山区。在收集各区段大量一线海塘资料的基础上,对各段海塘的现状结构进行较为系统地总结,对设防标准和实际防御能力进行了探讨,并提出了建议和应对措施。调查与分析表明,1997～2002年上海市海塘达标工程建设基本完成后,海塘的防御能力提高了,海塘的隐患消除了:农村及城乡结合部地区的海塘达到了防御100a一遇高潮位和11级风浪的组合标准;城市化地区及大型企业占用的海塘达到了防御100a一遇高潮位和12级风浪的组合标准,中长期将进一步加强至200a一遇高潮位的防护标准。达标后的海塘,近年历经多次台风的考验未出现缺口、决堤和大面积灾害性损坏的现象。随着上海市逐步向深水筑堤建设,由于缺乏了堤前高滩的消浪作用,加上堤前水深、浪强,在两个因素的综合作用下,超强台风对新建海堤将构成巨大威胁,潜在的风险也许在较长时间才能显现。新围海堤建成后,老海塘如果能够完全保留,或不得以只在局部开缺,或只削掉堤顶超高部分,在遭遇超标准风暴潮时,保留的二线老海塘替代挡潮,新老堤联合作用,必将极大地保障老堤内的防御水平,有效地控制和减少台风致灾范围,达到防灾减灾目的。因此,在有可能的条件下应尽量保留老海塘。

The coastal levees in Shanghai are distributed in broad areas of Chongming （including Chongming Island, Changing Island and Hengsha Island）, the southern bank of the Changjiang River mouth （ including Baoshan District, Pudong District and the north of Nanhui District） and the northern bank of the Hangzhouwan Bay （including the south of Nanhui District, Fengxian District and Jinshan District）. They were constructed over a large span of time with different surface structure. In order to learn the present defensive criteria and practical defensive capability, typical sections of the levee are selected for the study. A lot of practical and up-to-date data and information show that defensive criteria of the coastal levee were met by and large in the period from 1997 to 2002, when up-to-the-criterion engineering construction of the levee was finished in Shanghai. The coastal levee can withstand the combination of the highest high tide that happens once in 100 years and force 11 typhoon on the Beaufort scale in the country area and the junction between the country and the city proper, and the combination of the highest high tide that happen once in 100 years and force typhoon in the city proper and large factory area. The criteria in these areas will be further increased in the future, that can withstand the highest high tide which may happen once in 200 years. Large-scale catastrophic damage has not happen since 2003, although typhoon occurred frequently in the period. However, the wind power has not reached the designed defense criteria of the levee, so their practical defense capability remains to be tested. Because changeable typhoon is defined by unchanged wind speed and the defense criteria were defined in the absence of statistics means, they can not show the practical state and unreason in the criteria is inevitable. Now the levee in Shanghai is constructed in deep water. The absence of shoal in the front of the levee not only influences the zoology and river regime, but also causes safety problems against typhoon. In the absence of wave dissipation by shoal plus the influence of deep water and strong wave, ultra-strong typhoon greatly threatens the safety of the new coastal levee. Long time might be needed for the potential risk to manifest. If the old coastal levee can be reserved, or only part of it be demolished in case of absolute necessity, or the unwanted top be cut, the inside old levee can play a great role to minimize the damage when storm surge destroy the new levee. The combined action of the new and old coastal levees will surely reduce the harm from typhoon. Hence the old ones should be reserved as far as possible.