Water management in urban projects

In the city, every square metre of roof or car park sends rainwater straight to the sewer. Storms are becoming more violent, droughts longer, and regulations have followed suit. Here is how STARC and Optibim factor water management into your projects, from terraced houses to apartment blocks.

Why water needs to be considered from the sketch stage

A simple benchmark: 1 mm of rain on 1 m² produces 1 litre of water. A 100 m² roof therefore receives 2,000 litres during a 20 mm downpour. If all that water reaches the network within minutes, the sewers become overloaded and the neighbourhood's cellars suffer.

Impermeable ground creates a second problem. By preventing natural infiltration into the soil, it reduces the recharge of groundwater tables. In other words, the same city suffers both flooding and water shortages.

This is why we address water at the same time as siting, levels and surroundings. A swale or a tank added at the end costs more and works less well than a system planned from the outset.

What Walloon regulations require

In Wallonia, the principle is clear. Since 1 January 2017, the Walloon Water Code (Code de l'Eau) has required rainwater to be managed at plot level, following an order of priority:

• first, infiltration into the ground;
• then, if this is not possible, discharge into surface water or an artificial drainage channel (ditch, watercourse);
• only as a last resort, connection to the sewer.

These obligations must be complied with in urban planning and development projects. In practice, your planning permission application must demonstrate how water will be managed. SPW Territoire (the Walloon planning authority) has published a reference framework entitled Gestion durable des eaux pluviales (Sustainable rainwater management), which serves as a common basis for designers and municipalities.

Sanitation inter-municipal authorities also play a role. In Walloon Brabant, for example, the inter-municipal body IBW is consulted. In Liège province, AIDE also issues an opinion on rainwater management and checks the calculation assumptions used for sewers and storm basins.

Finally, at commissioning, CertIBEau comes into play. All buildings connected to the water supply for the first time have required this certification since 1 June 2021. Obtaining this certificate, which confirms the compliance of the installation, is the necessary condition for the connection to be permanently put into service. According to IDELUX, certification covers, among other things, any connections to alternative water sources (rainwater/well water), as well as the treatment and disposal of wastewater and rainwater. A badly connected tank can therefore hold up the opening of the water meter.

What Brussels regulations require

In Brussels, the 2006 Regional Town Planning Regulations (Règlement régional d'urbanisme, RRU) remain the reference. According to Bruxelles Environnement's Sustainable Building Guide, for new construction, installing a rainwater tank is compulsory, partly to avoid overloading the sewer network. This tank must have a minimum capacity of 33 litres per m² of roof area in horizontal projection. For a 100 m² roof, that amounts to at least 3,300 litres.

Local rules also deserve attention: certain municipal planning regulations (RCU) and specific land use plans (PPAS) may contain stricter requirements than the RRU. The RCU and PPAS of the relevant municipality should be checked when designing the project.

The Region goes further with integrated rainwater management (Gestion intégrée des eaux pluviales, GiEP), a pillar of the Water Management Plan. Bruxelles Environnement defines it as arrangements that store water as close as possible to where it falls, then discharge it through infiltration and evapotranspiration: the only outlets are therefore the ground and the atmosphere. As a rough guide, Bruxelles Environnement's training courses mention a buffer volume of at least 25 litres/m² of impermeable surface and a discharge rate of no more than 2 l/s per hectare, dropping to 0 l/s per hectare where the ground can infiltrate.

Two current developments deserve your attention. The draft new RRU provides that every construction must ensure integrated management of hundred-year rainfall events on the plot, though its final adoption remains to be confirmed. In addition, following an October 2025 court ruling, certain large undeveloped plots are now subject to stricter conditions: according to the Arius law firm, the file must then include the descriptive form, the CBS+ calculation and the climate note, and this temporary framework is expected to end with the new PRAS (Regional Land Use Plan, equivalent to GBP).

The solutions we implement

There is no one size fits all solution. We usually combine several systems:

• The recovery tank: it supplies the toilet, washing machine and outdoor tap. It reduces the water bill, but a full tank no longer buffers anything during a storm. Hence the value of a separate buffer volume with slow drainage.
• The planted swale: a wide, shallow, planted ditch that stores then infiltrates water. It also serves as a garden and cools the surroundings.
• The green or storage roof: it retains part of the rain and evaporates another part. This is useful in dense cities where land is scarce.
• Permeable surfacing: paving with widened joints, grass pavers, stabilised gravel for driveways and car parks.
• The infiltration well or trench: compact, but sensitive to clogging and dependent on the subsoil.

Bruxelles Environnement stresses the multifunctional aspect: management is described as integrated when these features occupy a multifunctional space, combining hydraulic, landscape and urban uses. This is also our approach as architects: a well designed swale enhances a project rather than cluttering it.

Our method, from survey to certificate

At our practice, water passes through several disciplines under one roof, in Gembloux.

1. The survey. BimBot carries out the survey of the plot and existing structures using 3D scanning and drone technology. Slopes, threshold levels, low points: this data shapes everything that follows.
2. Context analysis. We consult WalOnMap's maps. The karst atlas and the mapping of flood risks and drainage routes can completely change the strategy.
3. The permeability test. This checks whether the ground actually infiltrates water. On the loamy soils of Hesbaye, results are sometimes disappointing.
4. Sizing. Optibim calculates the volumes, discharge rates and infiltration surfaces, then drafts the hydrological note attached to the planning application.
5. Integrated design. STARC designs the surroundings, levels and drainage routes in keeping with the architecture and building services.
6. Completion. In Wallonia, as an approved certifier, Optibim can issue the CertIBEau at the end of the works.

Pitfalls to avoid

• Forgetting water in the budget. Earthworks, tanks, swales and planting represent a genuine cost item. It is best to identify this from the preliminary design stage.
• Infiltrating too close to foundations or a cellar, particularly when renovating older buildings.
• Assuming the ground infiltrates without testing it. A refusal to infiltrate discovered too late means the layout plan has to be revised.
• Sending water onto the neighbour's land. The Civil Code prohibits letting roof water run onto a neighbouring plot, and this remains a frequent source of disputes.
• Neglecting maintenance. Bruxelles Environnement recommends appointing a manager or person responsible for ensuring the upkeep of the systems. In shared ownership, this issue must be settled at completion.
• Wrongly connecting the tank to the internal network. Mixing rainwater with mains water is a classic cause of a non-compliant CertIBEau.

Are you planning a new build, a renovation or a housing project in Wallonia or Brussels, and finding water management a bit of a puzzle? The STARC and Optibim teams can analyse your plot, size the systems and prepare the note to be attached to your planning application. Contact us in Gembloux to discuss it.