Towards emission-free sea dike revetments

Abstract

Large parts of the Netherlands lay below mean sea level. An extensive network of flood protection measures is in place to prevent flooding. Periodic safety assessment and reinforcement are required to guarantee a resilient system. Currently, in the Netherlands, an extensive sea dike reinforcement program is underway to meet the safety standards mandated by the Dutch government. A dike reinforcement project encompasses several components, often interacting with each other. This study focuses on the outer dike revetment. Traditionally, the revetment design is optimised for financial costs. This study aims to optimise the dike revetment design for the lowest Environmental Cost Indicator (ECI). This quantifies the carbon dioxide equivalent emissions generated throughout the entire life cycle of a construction project or its constituent parts. This is pertinent in the current context as the Dutch Government harbors ambitions of achieving a fully circular economy by 2050, with an interim goal of a 50% circular economy by 2030. This ambition is to be implemented by companies within their organization. The revetment is optimised as a whole by assessing the entire outer revetment and applying different types of revetment based on their probability of failure. The primary objective of this study is to acquire knowledge on designing a dike revetment that fulfils technical requirements and has the lowest environmental cost indicator possible. The revetment types to study are loose rock, concrete elements (Basalton), interlocking elements (Verkalit), hydraulic asphalt concrete and grass. The following main research question is defined:

"What sea dike revetment design fulfils safety requirements and has the lowest environmental impact for the Lauwersmeerdijk-Vierhuizergat dike reinforcement project?"

To answer this question, a Python-based model was made. The model evaluates the probability of failure of each design option and calculates the corresponding ECI and financial costs. The LauwersmeerdijkVierhuizergat project, a Waddenzee dike reinforcement, is used as a case study. The required safety level for the Lauwersmeerdijk-Vierhuizergat outer revetment is 1/60.000 years. The model requires input for the design equations. The input consists of decision variables, control variables and hydraulic boundary conditions. The decision variables are the variables in the design equations that the designer can alter. The control
variables are the variables to which the designer has no influence. The hydraulic boundary conditions are determined for each water level discretised in steps of 0.2m. Next to the input for the probabilistic calculation, the input for the ECI and financial costs are required. The ECI data is obtained from the ’Milieudatabase’
(Schipper et al., 2022). The financial cost data is obtained from cost experts from Arcadis.

For loose rock, the probability of failure is calculated with the equations as defined by van der Meer (1988a). For the placed elements, the probability of failure is calculated with the equation defined by Klein Breteler and Mourik (2014). The asphalt revetment is designed with the uplift and wave impact equations as defined in TAW (2002). The grass revetment is designed with the use of the ’Gras erosie buitentalud’ (GEBU) tool. The equations for erosion according to Klein Breteler (2022a) are included in this tool. The design equations are evaluated using crude Monte Carlo analysis for each water level with corresponding hydraulic boundary conditions, decision parameters, and control parameters.

More than 2,000 designs, each meeting safety requirements, are made with varying transition heights. The optimal sea dike revetment design for environmental impact is the design with the lowest ECI score. When the
probabilistic approach is applied, this design has loose rock revetment at the lower section, Basalton for the middle section, and grass for the upper section. In general, it is concluded that loose rock contributes most to the ECI and should be limited as much as possible. The middle section is traditionally made of hydraulic asphalt concrete. However, when replacing this with placed elements, a lower ECI is achieved. The grass revetment often requires large volumes of high-quality clay from external locations, requiring long transport distances. This results in high ECI costs for thick clay layers. When comparing the revetment with the lowest ECI to the revetment design with the lowest financial costs, it is concluded that the design with the lowest financial costs has a large section with hydraulic asphalt concrete instead of placed elements. The study design costs €1890 per meter, with an ECI of €373 per meter. The financially most attractive design costs €1439 per meter, with an ECI of €458 per meter. The difference in financial costs is €451 per meter (24%) and €85 per meter ECI costs (23%). For the revetment design made by Arcadis, deterministic and semi-probabilistic
calculation methods are applied. The asphalt layer thickness is reduced by 50% when applying this method due to the large uncertainty in the probabilistic design. This results in a €12 per meter dike width lower ECI than the design with the lowest ECI made with the probabilistic approach.

Thus, when designing a sea dike revetment probabilistically, aiming to reduce the ECI as much as possible,
apply as little and the smallest possible loose rock grading, and replace the hydraulic asphalt concrete layer for placed elements.