This research study has investigated the effects of sills as a nature-based countermeasure to reduce saltwater intrusion within Rijnmond Drechtsteden. The effects of this measure have been assessed regarding its impact on port logistics and freshwater supply in the region. The heavily dredged New Waterway provides the vital open connection to the sea for port operations. Meanwhile this same waterway allows salt to intrude landwards into the estuary, placing pressure on the freshwater network. With climate change leading to rising sea levels and extreme variations in river discharge, the impacts on both stakeholders are likely to increase. Previous research into bed shallowing indicated detrimental effects for port operations, therefore alternative nature-based solutions need to be investigated such as underwater sills.

The impact of a sill has been assessed using the Frame of Reference approach (Van Koningsveld, 2003). To determine the relationship between each stakeholder, a quantitative trade off has been defined using the comparison tool as per the method of Iglesias (2022). The tool facilitates the comparison of individual effects created by (nature-based) interventions using design parameters and performance indicators.

Via an extensive literature study, critical design parameters were defined for underwater sills. It was concluded that sill height and sill length would have the greatest impact on both the port and the freshwater supply in the region.
Then, the impact on the stakeholders was defined through choosing specific performance indicators and modelling techniques. The performance of port logistics was represented via the percentage of accessible tides and modelled using a simplified case in the nautical traffic model OpenTNSim. The performance of the freshwater supply was quantified using the percentage of freshwater availability, this was modelled using the concept of an idealised estuary in Delft Flexible 3D Mesh (DFM).
The research method takes the operational objective of the port as a base to select specific sill designs. This objective depends on tidally bound inbound vessels having access for 99% of all tides (de Jong, 2020). To comply with this requirement, sill heights of 0.1m, 0.2m, 0.3m and lengths of 2km, 5km, 10km were chosen. The model results show that sill height has a large impact on accessible tides, therefore severely limiting the range of heights that could be considered. Sill length has a lesser impact, allowing for a wider range to be assessed.

The effect of the sill designs on freshwater availability is negligible, with responses varying between 0.01% and 0.8%. This lack of response indicates that the chosen sill designs are too small to have a significant impact on salt intrusion in the channel. The sill designs led to access percentages between 100% and 70%, a wider range of percentages was investigated to see the effect of a more flexible port.
The resulting trade off curves all exhibit linear functions, indicating the lack of any trade off relation between the performance indicators and the sill designs. Since the sill designs fail to achieve both operational objectives, the overall strategic objective is not met. It can therefore be concluded that underwater sills are not feasible in the case study of the Rotterdam waterways.
The research results indicate that other nature-based solutions should be investigated to better suit the actively dredged channels in the Rotterdam waterways. Alternative methods which can synergise with this active dredging characteristic may pose to be significantly effective.