In this paper the safety of a double-dike system (or twin dikes) is assessed. Such a system consist of two parallel lines of flood defences. During storms the combined strength of the parallel flood defences must prevent flooding of the hinterland. A culvert can be implemented for the tidal exchange of sea water to enable new land-uses in the area between the dikes such as aquaculture, saline agriculture, salt marsh restoration and clay extraction. We develop a general framework for assessing the safety of such double dike systems and apply a simplified version to the Double Dike between Eemshaven and Delfzijl (The Netherlands) to test this method. In doing so, we aim to quantify the flood protection benefits of parallel flood defenses and enable their use in multifunctional flood protection strategies.

Within the framework the transmission of hydraulic loads by the seaward dike to the landward dike in the case-study was described by overtopping, overflow and erosion of the outer slope, alongside discharge through the culvert in the event of a non-closure. For the subset of coastal double dike systems with a tall seaward dike (as in the case-study), the results show only a negligible improvement in flood protection compared to a single dike system. With the addition of a culvert in the first dike, flood risk will only be reduced by the second landward dike if its height is sufficient to retain water in the event of a non-closure during common storm events. These double dike systems are implemented for potential uses of the inter-dike zone, e.g. for nature restoration, rather than as a measure to primarily improve flood protection.

在本文中，评估了双堤系统（或双堤）的安全性。这样的系统由两条平行的防洪线组成。在风暴期间，平行防洪设施的综合力量必须防止腹地洪水泛滥。涵洞可用于海水的潮汐交换，以实现堤坝之间区域的新土地用途，如水产养殖、盐渍农业、盐沼恢复和粘土提取。我们开发了一个用于评估此类双堤系统安全性的通用框架，并将简化版本应用于 Eemshaven 和 Delfzijl（荷兰）之间的双堤以测试该方法。这样做的目的是量化并行防洪措施的防洪效益，并使其能够用于多功能防洪策略。

在该框架内，案例研究中通过外坡的漫溢、溢流和侵蚀，以及在未关闭的情况下通过涵洞排放的水力载荷在案例研究中通过向海堤坝向陆地堤坝的传递进行了描述。对于具有高大的向海堤坝的沿海双堤防系统的子集（如案例研究中），结果表明与单堤防系统相比，防洪方面的改进可以忽略不计。在第一条堤防中增加涵洞后，如果第二条向陆堤坝的高度足以在普通风暴事件期间未关闭的情况下保持水，那么洪水风险将仅通过第二条向陆堤坝降低。这些双堤系统的实施是为了堤间带的潜在用途，例如自然恢复，而不是作为主要改善防洪的措施。