Improving knowledge and decision-making on implementing infiltration systems

Abstract

The groundwater system in the Netherlands is increasingly pressured by climate change, pollution, and increased subsurface use, which disrupt groundwater recharge and -levels, impacting vegetation, groundwater users, and ecosystems. Rainfall primarily drives groundwater recharge, but urbanization and extended droughts hinder this process. Given the importance of stable groundwater levels, artificial recharge is gaining attention. Artificial recharge can be utilized with the implementation of infiltration systems, which infiltrate water into the subsurface through various techniques.

A specific project requiring an infiltration system is the reinforcement of the Hollandsche IJsseldijk between Krimpen aan den IJssel and Gouderak. An infiltration system is necessary because the flood defence is largely being reinforced by installing a sheet pile, which leads to declining groundwater levels and increased land subsidence. However, challenges arise due to infiltration in clay layers and limited knowledge regarding clogging risks, maintenance requirements, long-term sustainability, and legal regulations. This research aims to bridge these knowledge gaps and to develop an assessment framework for decision-making in infiltration system selection.

The research methodology includes literature research, expert interviews, decision-trees, and an Multi-Criteria Analysis (MCA). This ‘Assessment Framework’ was designed to compare and evaluate different infiltration systems, making it applicable to a wide range of similar projects. To evaluate applicability and functioning, the assessment framework was assessed for the intended infiltration system to be constructed for the Hollandsche IJsseldijk and refined through expert collaboration.

Findings from the literature research and expert interviews highlighted several key factors. Clogging risks are primarily influenced by water quality, oxygen intrusion, and temperature fluctuations. Maintenance needs are site-specific, with regular cleaning required to address clogging, while simpler system designs can reduce long-term upkeep. Experts also foresee infiltration systems becoming more widespread due to increasing droughts, but challenges such as system complexity, environmental impacts, and regulatory ambiguities remain significant.

Applying the developed framework to the Hollandsche IJsseldijk identified a horizontal infiltration system as the most suitable option. However, significant risks remain, including potential clogging, oxygen intrusion, soil heterogeneity, and clay swelling. Additionally, water safety assessments in the surrounding area are incomplete. These concerns, along with the framework's subjective nature, may affect the preferred option's determination.

Despite these challenges, the framework effectively aligns with the implementers’ conclusions for the infiltration system at the Hollandsche IJsseldijk, suggesting reliability in its application and usability for decision-makers. Further refinement of the assessment framework may be necessary to address site-specific challenges, particularly concerning water safety and maintenance.