Implementing and upscaling the “Improved Drainage System”

Abstract

This master thesis explores the implementation of the Improved Drainage System (VDS) in the agricultural area of the Haarlemmermeer polder, aiming to reduce freshwater demand and mitigate waterlogging risks while maintaining suitable water quality and salinity for agricultural functions. The study investigates the effect of VDS compared to traditional water management in response to climate change and autonomous salinisation.

The traditional approach in water management focuses on meeting societal demands for agriculture, involving rapid drainage of excess water, and flushing of brackish surface water resulting from the permanent saline seepage flux. VDS, on the other hand, applies a flexible surface water level within specific margins and reduces the need for inlet water by eliminating the flushing of the polder.

The STOWA model, created by “Stichting Toegepast Onderzoek Waterbeheer” in collaboration with Witteveen & Bos and Waternet, is a calculation tool in Excel that utilizes a box model of the Water Balance. The STOWA model was utilized for this study, this tool proved suitable for studying the effects of the implementation of VDS on the water system regarding the water level, water demand, and water saliniy. However, the STOWA model is inadequate for the modeling of the water quality, because the STOWA model is a point-based model and is incapable of employing the spatial and temporal variability. The large parameter variation and uncertainties make STOWA unsuitable for making robust decisions regarding VDS implementation.

The research findings indicate that VDS reduces freshwater demand when comparing to the traditional management without flushing, resulting in lower inlet water requirements. Additionally, the results showed that without irrigation, no inlet water is needed to maintain the minimum set levels in the water system. Furthermore, the traditional management approach in the polder demonstrated its capability to handle anticipated extreme rainstorm events, suggesting that adaptive measures are not required to mitigate waterlogging risks caused by climate change. In contrast, internal autonomous salinisation does require measures to prevent salinisation of the surface water.

Implementing VDS consistently leads to higher surface water salinity compared to traditional management, contradicting the hypothesis that rainwater capture in VDS management would have a freshening effect. The findings from upscaling VDS to the entire Haarlemmermeer polder indicate that VDS can be applied in specific areas of the polder while maintaining agricultural functionality, depending on the flux and salinity of the phreatic seepage and the presence of seepage boils. Consequently, determining the freshwater-saltwater interface is crucial for assessing the suitability of an area for the implementation of VDS.