When water becomes a challenge… and a solution

Rain falls on streets, squares and rooftops, picking up particles, heavy metals and microplastics. It then flows directly through the drainage system into waterways without undergoing any treatment. What was long regarded as a purely technical disposal activity is now a key environmental and planning issue. Rainwater must be managed to minimise environmental impact and enhance resilience to climate change in cities, but how exactly?

From water drainage to water treatment

Road drainage has undergone significant changes in recent decades. Whereas the emphasis used to be on rapid drainage, the focus today is on targeted treatment. Road runoff contains a variety of pollutants, particularly heavy metals, fine particles and tyre wear, which is increasingly recognised as a microplastics issue. Without adequate treatment, these pollutants end up directly in surface waters and groundwater, the quality of which can be significantly impacted.

Modern treatment facilities for road runoff rely on multi-stage processes: sedimentation for separating solids, and filtration for retaining fine particles and dissolved pollutants, the goal being to retain them before they enter the natural water cycle. Alongside purification capacity, floor space requirements and investment and operating costs also play a crucial role. Compact, efficient solutions are particularly in demand in urban areas, where space is already limited.

The sizing of treatment facilities and drainage systems also plays a crucial role. The design must take into account precipitation events, drainage areas and pollution loads in order to ensure reliable functionality during normal operation and periods of heavy rainfall. As demonstrated by practical examples, well-planned infrastructures can significantly improve water quality and offer economic benefits over the long term. Field trips to existing facilities demonstrate that different solutions exist depending on the location.

Decentralised thinking, integrated action

In addition to centralised road runoff treatment facilities, decentralised treatment systems are gaining in importance. While they do not achieve the same purification results as centralised systems and incur higher maintenance costs, they do allow for flexible adaptation to local conditions. The challenge lies in coordinating these systems across the entire network, an aspect that is becoming increasingly important.

Limitations of traditional sewage systems

Concurrently, existing drainage systems are under increasing pressure. In many places, rainwater is still discharged with wastewater. The higher frequency of extreme rainfall episodes driven by climate change is increasingly challenging the capacity of sewage networks. This results in more frequent discharges of combined sewage.

This is where the modelling of urban drainage systems can provide a valuable solution. Hydrological and hydrodynamic simulation models can be used to analyse complex interactions and project future scenarios. From this, a digital representation of the systems and their dynamics can be derived, facilitating understanding of how water flows and how reservoirs and pipelines interact.

Simulation modelling offers consistent answers to key questions such as: which areas are particularly vulnerable to intense rainfall? Which measures are most effective? And how can existing systems be efficiently expanded or adapted? At the same time, the models’ limitations also become apparent, often due to simplistic assumptions or uncertainties in the data. A purely normative approach, the relevance of which is increasingly being questioned in light of climate change, is supplemented by local and context-specific action.

Blue infrastructure is the solution

Rainwater management relying on blue infrastructure plays a key role in the development of new solutions. It integrates expanses of water, retention areas and infiltration elements into urban planning and complements technical systems with nature-based solutions. Concepts such as the ‘sponge city’ aim to absorb, store and use rainwater locally and release it gradually. They include in particular green basins, green roofs, open streams and permeable surfaces. These elements not only have a hydraulic function, but also improve the climate and quality of life in urban areas. Water becomes both visible and tangible, while also serving a practical purpose.

At the intersection of several specialities

This is where road runoff management, property drainage and modelling converge. The treatment of contaminated surface water is a key component of modern drainage systems, while modelling enables its optimal integration. Simulations help predict the performance of road runoff treatment facilities, optimise their design and understand their role within the overall system. At the same time, they demonstrate how decentralised treatment systems can be combined with elements of blue infrastructure to reduce peak runoff and improve water quality in a sustainable manner.

From technical details to an overall strategy

The shift in paradigm is evident. Road drainage is evolving from a technical necessity into a strategic tool for urban development. The integration of innovative treatment technology, data-driven modelling and blue-green concepts makes it possible to adapt water infrastructures specifically to meet future requirements

This ensures that water is safely drained away and can be used actively – as a resource for climate adaptation, a unifying element in urban spaces and the foundation for more resilient cities.