Extreme weather events such as heat waves and heavy rainfall are becoming more frequent due to climate change, threatening the liveability of urban environments. High building and infrastructure densities and impervious surfaces intensify heat and flood risks. This vulnerability has led to an increase in climate adaptation strategies, which often include the implementation of Nature based Solutions (NbS). In the Netherlands, one of the most widely used NbS is the swale, a green ditch that facilitates drainage, infiltration, and rainwater storage. Swales are seen as cost-effective and no-regret measures that, just as other NbS, can have multiple functions which provide multiple benefits. Despite these potential benefits, practitioners struggle with the design, development and maintenance of swales, partly due to limited local monitoring practices.

The research had two main objectives: (1) to explore current practice in research and in Dutch municipalities to identify performance criteria and monitoring techniques for swales to track the key functions of swales, and (2) to investigate motivators and challenges for municipal swale monitoring and develop a communication tool to address these.
A systematic literature review identified five key swale functions: water quantity, water quality, biodiversity, liveability and wellbeing. Water quantity and water quality functions are relatively well studied, with swales proven to reduce runoff and remove certain pollutants when properly designed and maintained. However, research is still lacking for the functions biodiversity, liveability, and wellbeing. The connections between the different functions were also explored, which showed the importance of recognising the connections and the influences of the different functions on each other. From this review, eight criteria were identified: infiltration rate, soil moisture, in- and outflow of pollutants, soil quality, vegetation development, temperature, resident experience and resident usage. For each criterion, quick-scan and in-depth monitoring techniques were proposed.

A survey among municipal workers who are involved with urban water management (mainly from South Holland) found that swales are seen as multifunctional objects, where water quantity and biodiversity are the most important functions. Monitoring is not part of the current practice, and swales are often only assessed on the functions during the design. However, there is interest in the implementation of monitoring, especially for water quantity and water quality. The main motivators for monitoring are to know the effect of swales on the long term and the ability to identify problems in time. The main challenges are keeping track of data and ensuring sufficient resources, like staff and budget.

To overcome these challenges, a workshop was designed to motivate practitioners to start monitoring the multiple swale functions. The workshop is aimed at municipal staff responsible for the organisation of maintenance for the different functions. By learning about the different functions and monitoring possibilities, the participants can create a monitoring plan that serves as a starting point for the actual implementation of monitoring. Evaluation by experts indicated that this workshop can motivate the target audience, because it increases the audience's awareness of swales, encourages them to consider the different functions and lets them practice with measuring, monitoring and retrieving data. It also provides them with a concrete and practical output that can be used in practice.

This study offers a framework to improve swale monitoring in the Netherlands. An overview of performance criteria with corresponding monitoring techniques is provided together with a communication tool that can bridge the gap between knowledge and practice.