A future-proof rainwater management system

In the near future, the municipalities of Baarle-Nassau and Baarle-Hertog will face a variety of new redevelopment projects, building plans and road restructuring where rainwater will be separated as much as possible from the existing (combined) sewer system. Both municipalities need an overall vision for a structured and uniform approach to rainwater management, taking into account climate change and the resulting heavier downpours and more extended periods of drought. Water needs to be treated, buffered and discharged as efficiently as possible. The advantage of separating rainwater from the sewer system is that it will no longer be mixed with wastewater, thus reducing the load on the treatment system.

Many municipalities face the same problem: existing sewer systems are often overburdened by heavier rainfall, which can cause flooding and damage. The projects designed to address this problem often lack coherence. Actions are fragmented and often lack structure and a clear vision for the future, and striking a balance between dealing with extreme rainfall and preventing drought in the summer months is also challenging.

Iv was asked to draw up a rainwater management plan, which will serve as a blueprint for all future sewerage projects, redevelopments and building plans. Iv has gained a wealth of experience in establishing structural visions and ‘blauwe ader’ plans (plans involving spatial modifications to surface water and associated watercourses) and has developed a step-by-step plan that was also used in this project.

Starting point

The location of both municipalities is unique. The Belgian part consists of dozens of exclaves that, in turn, surround Dutch territory. The exclaves often cross roads and even buildings. Although both municipalities work closely together on sewerage and water management, the applicable laws and regulations differ.

The Belgian government has determined that owners of new-build homes need to buffer their rainwater. Depending on the roof’s surface area, up to 5,000 litres of water needs to be stored on the owner’s land. The water can be used to water the garden or to drain the rainwater slowly. This legislation, however, does not apply to Dutch territory. So when Iv drew up the rainwater management plan, buffering water on private property in the Belgian parts was taken into account.

In particular, the central area of both municipalities has a high concentration of paved surfaces and relatively few options for collecting or draining water. The current system cannot cope adequately with heavy rainfall.

Approach

To ensure the planning process runs smoothly, both the Dutch municipality of Baarle-Nassau and the Belgian municipality of Baarle-Hertog have been closely involved in the project from the outset. Several consultations were held with both municipalities, during which the input of historical and local knowledge was of great value. In addition, Iv has carried out various field inventories and identified specific area characteristics, opportunities and risks for integrating the future rainwater management plan.

One of the findings is that the groundwater level is relatively low, on average about 1.5 to 2 metres below the surface. Higher water levels occur only around Loveren, just below the surface. This low groundwater level can lead to drought stress during dry periods (summer) but also offers opportunities to buffer water during peak rainfall.

Following the inventory phase, Iv drew up a sketch design for optimising and redistributing rainwater using a ‘blauwe ader’ pipeline. This mainly involves draining water from flooded areas to the outskirts of the town via waterways. Because of the low groundwater level, these waterways are dry for much of the year. There is, therefore, an opportunity to store water on a large scale and allow it to seep slowly away. The ‘blauwe ader’ pipeline will be partly constructed as an infiltration pipe, meaning that most rainwater will seep into the ground locally during average rainfall. This will also prevent desiccation within existing built-up areas. The rainwater management plan takes into account future building plans and development projects, including the sizing of rainwater pipes to match the expected areas of paved surfaces.

The modelling used the test criterion of no water on the road during a ‘Storm 10’, increased by a factor of 10. Statistically, this heavy downpour occurs once every ten years and lasts 45 minutes, with a total rainfall of 35.7 millimetres and a peak rainfall of 75.6 millimetres per hour. All adjacent surfaces will be directly connected to this primary system. Fifty percent of roofs will be connected to the system, and public paved areas will be fully connected.

The plan consists of the primary system (blauwe ader) and marked areas where local measures, such as wadis and more minor actions on private land, will be taken. In addition to the primary system, several smaller areas have been identified with their own drainage location. Not all areas can be efficiently connected to the primary system.

Follow-up

In the meantime, Iv has been asked to develop other items into a more concrete (preliminary) design. In Baarle-Nassau and Baarle-Hertog, several major renovations of five centrally located roads are planned in the short term. The town centre will also be renovated in the coming years. With a supported rainwater management plan, both municipalities can confidently start work on these projects.