For nearly 90 years, a 32-km dike on the Dutch coast has kept the North Sea from inundating a huge area of the Netherlands, around Amsterdam. Now, to counter the ravages of time and rising sea levels, work to upgrade the Afsluitdijk to handle a 10,000 year flood is well under way, although much delayed by erroneous design data.
政府项目正在将Xblocplus Armour（闭合堤防）放置约2 m的旧克兰驳船titaan的水平，同时用一层新开发的混凝土装甲块加强了面向海面的一侧。
正在加强三个现有组中，将两组新的水槽添加到正在加强的三个组中。西欧最大的六个泵将从IJSSELMEER（IJSSEL湖）排放水到Wadden Sea，这是一个在海岸和北海之间的潮间带沙子和泥平的系统。
An overview of the AfsluitdijkCredit: Rijkswaterstaat
该项目的其他元素包括风暴潮的屏障和湖边1公里长的蜿蜒曲折的河流，用于数百万的迁徙鱼，以在咸海和Ijsselmeer的淡水之间过渡。
The dike project is contractually scheduled for completion by Jan. 1 2023. Then, the special purpose company Levvel B.V. is due to start 25 years' maintenance under its public private partnership with Rijkswaterstaat, Netherlands’ water infrastructure authority.
But while most of the project is broadly on schedule, the sluices are being redesigned because water level and wave data provided to the contractor by Rijkswaterstaat were found to be underestimated.
In a letter to parliament this May, the then infrastructure minister, Cora van Nieuwenhuizen, confirmed that resulting redesign would push back the sluices' completion to 2025. Rijkswaterstaat had, she added, agreed to pay Levvel about $140 million toward the extra work and to trigger the disputes procedure to settle the full compensation.
A schematic of the rebuilt dike's profile.Credit: Rijkswaterstaat
A long wait
几个世纪以来，将北海远离Zuiderzee的愿望是内陆海洋5,000平方公里的愿望，对技术不足感到沮丧。根据官方历史，工程师Cornelis Lely在130年前制作了可行的设计。在成为水管理部长后，莱利（Lely）于1913年制定了政府项目政策。
1916年的一次重大洪水以及对农业用地的日益增长的需求帮助获得了1918年的议会批准。1920年的工作开始缓慢，并在十年中期加快了。堤防在1932年关闭，创建了1,100平方公里的开发和农业。
Of the 32 km closure, which carries a four lane highway, the clay core dike runs for about 24 km, says Lukas Meursing, Rijkwaterstaat's project environmental manager. Concrete structures, such as the sluices at the south end, account for the rest. The dike's sea face was originally lined with countless basalt blocks placed manually by an estimated 5,000 workers. The blocks are being covered with new concrete armor.
The two types of armoring blocks used on the dike.Credit: Rijkswaterstaat
Afsluitdijk "is our front door," says Meursing. It is the first line of defense in front of smaller dikes in the inner water. The system survived catastrophic floods in 1953, causing over 1,800 deaths mainly in the south west. While keeping the sea out, "there was some damage but its didn’t break," he adds.
The 1953 flood triggered the multibillion dollar, 25-year Delta Works program of storm barriers and other flood defences at the sea outlets of the Rhine-Meuse-Scheldt delta near Rotterdam. The catastrophe also led a increase in design flood risks by factors of up to 100 to an unprecedented one in 10,000 years.
根据Meursing的说法，10,000年的洪水将是海平面上升的结果，波浪大约是9m。
To upgrade the aging dike, Rijkswaterstaat in early 2018 selected the special purpose company Levvel B.V. for the design, build, finance and maintenance deal with a net present value then forecast at about $640 million. The construction cost, included in this sum, remains undisclosed.
1920年代原始堤防的建造. Credit: Rijkswaterstaat
Levvel is 46% owned by each Van Oord Aberdeen Infrastructure Partners B.V. and BAM PPP PGGM Infrastructure Coöperatie U.A. A financial investor holds the remaining 8%. A joint venture of BAM Infra and Van Oord Nederland is delivering the project under a design/build contract with Levvel.
Levvel项目总监Johan Vergeer说，这笔交易在2018年5月下旬在2018年5月下旬完成，其中大约7％的股权融资并为其余的贷款确保贷款。债务是由旨在利用基础设施私人投资的特殊欧盟设施保证的。
th Levvel收回其投资超过25年rough quarterly fees paid by Rijkwaterstaat. The payments will depend on performance in terms of preventing floods, ensuring adequate outflows of lake water to the sea, maintaining ship traffic through the locks and keeping the highway open, says Vergeer.
为了增加湖泊的排放能力，将添加大型泵和两组水槽。Credit: Rijkswaterstaat
Balancing risk and cost
Rijkswaterstaat would have had to roughly double the dike's height from around 7.5 m to withstand fully a 10,000 year flood during its 100-year design life, says Meursing. Instead, the dike is being raised only some 2m allowing for a potential overtopping rate of 10 liters per second per meter. That "can be held with a grass layer on the inner side of the dike," he says.
Another cost saving came from the selection of concrete armoring on the sea side. Using 75,000 XblocPlus units, newly developed by a BAM subsidiary, "enabled us to have a single layer to avoid (widening) the dike," says Cees van Oosterom, the contractor's design integration manager.
多面的2.3x2.8x1.1 m块，每个块重6.5吨，沿堤防下部的下部面部12 m宽的带中的浮台起重机放置在12 m宽的带中。Van Oosterom说，该阵列使堤防可以加剧，同时将其从脚趾到波峰的宽度约为46 m。
该制造商称，使用特殊块还将装甲部分中的混凝土量减少了56％以下，节省了约200,000立方米。
Chips cast into each block identify its coordinates in the dike for automatic positioning, says van Oosterom. They also record each block's history from the fully automated production facility at Harlingen, some 20 km away, he adds. Simpler shaped concrete blocks line the upper 17.5 m width of the sea face to slow high waves.
Van Oosterom补充说，承包商决定不完全依靠抽水从湖中排出多余的水。根据Meursing的说法，通过在三个现有组之间安装两个新的水槽，承包商将所需的泵送能力降低到每秒钟235立方米，大约将能源消耗减半。
Unfortunately, the sluices became the project's weak link when water flow data were found to be underestimates. The redesign is underway with major construction on the sluices expected to start in the second half of 2022, says van Oosterom.
The impact of the sluice delay has been "quite a considerable," adds Vergeer. But "it didn’t have a lot of impact on the dike itself". It is now impossible to complete the project on time in January 2023. But, while accepting some parts of the project will be incomplete, he adds, "we try to maintain that date".
