Integrated Stormwater Management
Sizing of the drainage plan and retention/infiltration structures
Project overview
Client: La Plaine Chassart
Location: Fleurus, Belgium
Industry: Quarries & Construction
Key results
Full regulatory compliance
The design meets all Walloon Region criteria for infiltration, retention, peak flow management, and controlled discharge to the outfall.
Operational and fire safety secured
The system integrates the specific fire prevention requirements prescribed by the City of Fleurus, including dedicated water volumes accessible within 50 metres of all buildings.
Rainwater valorisation designed
A recoverable potential of 1,886 m³ per year was identified and a collection system sized, covering toilet flushing, vehicle washing, and plant watering on site.
The Challenge
A logistics and construction site expanding its footprint, with new sealed surfaces generating stormwater that had to be managed, retained, and where possible, put to use.
La Plaine Chassart planned to relocate the activities of Mathias Bois to its site in Marbais, involving the construction of new buildings and the reconstruction and extension of existing structures. Every new building and hardstanding creates impermeable surface that prevents rainwater from soaking naturally into the ground. In Wallonia, that triggers a set of strict regulatory obligations: rainwater must be preferentially infiltrated on site, peak flows must be controlled before any water reaches a watercourse, and sufficient retention capacity must absorb intense rainfall events. In this case, part of the new impermeable surfaces also fell within a flood risk zone, requiring specific attention in the technical documentation.
The site added two further layers of complexity. Geologically, the site sits on a Carboniferous limestone karst zone, which required specific consultation with the Walloon authorities and careful attention in the design of infiltration structures. On the safety side, a fire prevention study commissioned by the City of Fleurus in November 2023 prescribed specific volumes of water to be kept permanently available for fire-fighting use, which had to be fully integrated into the water management design.
La Plaine Chassart mandated Revalio to produce a technical note on integrated stormwater management, advise on surface materials, and design the full drainage plan and retention and infiltration structures, all in support of the planning permit application.
What Revalio delivered
Revalio produced a technical note and designed a complete integrated stormwater system, covering drainage, infiltration, retention, and rainwater reuse in a single coherent solution.
Technical note for the planning permit
A technical note on integrated stormwater management was drafted, accounting for the specific prescriptions applicable to flood risk zones and supported by site photographs. The note addressed all relevant Walloon Region requirements and served as the technical foundation for the planning permit application.
Drainage plan and pre-treatment design
The total equivalent surface area generating runoff was calculated at 4,489 m², covering both building rooftops and hardstanding areas. The drainage network was sized according to the Manning-Strickler method, comprising 525 metres of PVC pipework ranging from 125 mm to 400 mm in diameter. The design separates water streams by origin. Clean roof runoff is collected via gutters and directed first to the rainwater storage tanks, with overflow routed to the infiltration basin. Hardstanding runoff, which may carry suspended solids and hydrocarbons from vehicle movements, passes through pre-treatment: a 13 m³ grit trap handles runoff from the manoeuvring apron around the soil storage building, and a combined 5 m³ grit trap and hydrocarbon separator treats water from the vehicle wash area.
Retention and infiltration basin
Using the Walloon Region's GTI methodology with a 30-year return period, exceeding the legal minimum of 25 years, the required retention volume was calculated at 217 m³ with a drainage time constraint of under 48 hours. Two in-situ infiltration tests had been carried out on site, returning significantly different permeability values. The more conservative value of 1.2×10⁻⁵ m/s was retained for sizing to reflect the heterogeneous ground conditions typical of an industrial site. The chosen geometry is a square dry filter basin of 18 m per side, 83 cm deep, with 25° bank slopes, giving a total footprint of 325 m².
Rainwater storage and fire reserve
A separate storage system, distinct from the retention infrastructure, was designed to keep harvested rainwater permanently available for non-potable uses. Based on a total roof area of 3,028 m², an annual rainfall of 814 mm, a runoff coefficient of 0.85 for metal roofing, and a swirl filter efficiency of 90%, the recoverable potential was calculated at 1,886 m³ per year. A 20 m³ concrete tank covers daily operational uses including toilet flushing, vehicle washing, and plant watering, equivalent to 170 m³ per year. A separate 120 m³ flexible tank provides the fire reserve required by the Hainaut-Est emergency zone, permanently available within 50 metres of all buildings.
How the Solution Works
The integrated system follows a clear logic: separate water streams by quality, treat each appropriately, and route each to its right destination.
Roof runoff, which is relatively clean, is collected and directed first to the rainwater storage tanks via a swirl filter that removes sediment before storage. Once the tanks are full, overflow continues to the infiltration basin. Hardstanding runoff, potentially contaminated by vehicle movements, passes through grit traps and where needed a hydrocarbon separator before reaching the basin, protecting the infiltration structure from clogging and the groundwater from contamination.
The basin itself acts as the hydraulic buffer for the site. During intense rainfall events, it captures peak volumes and releases them slowly through infiltration into the ground, recharging the local aquifer rather than sending a surge downstream. The 48-hour drainage design ensures the basin is ready to receive the next rainfall event within two days.
The storage tanks operate on a different logic entirely. Unlike the retention basin, which is designed to empty quickly, the tanks are designed to stay full, providing a permanent reserve of free, locally harvested water for daily non-potable uses on site. At current planned usage levels, only 9% of the total recoverable potential is being used, leaving significant capacity for future uses such as composting water supply, extended irrigation, or sharing with neighbouring sites.
Results
✓ Technical note on integrated stormwater management produced, addressing flood zone prescriptions and supporting the planning permit application.
✓ Full compliance achieved with Walloon Region stormwater management criteria, sized to a 30-year return period and a 48-hour drainage constraint.
✓ Infiltration basin sized at 18×18 m, 83 cm deep, with the conservative permeability value of 1.2×10⁻⁵ m/s retained to reflect site heterogeneity.
✓ Drainage network of 525 m designed and sized, with appropriate pre-treatment for each water quality stream.
✓ Fire prevention requirements fully integrated, with 140 m³ of permanently available water combining a 20 m³ concrete tank and a 120 m³ flexible reserve tank.
✓ Recoverable rainwater potential of 1,886 m³ per year quantified and a storage system designed, with 91% of potential available for future additional uses.
Need help with compliance or reduce your water consumption?
Managing stormwater on an expanding industrial or logistics site means satisfying geology, hydrology, planning regulations, and operational needs all at once. Whether you face karst constraints, flood zone obligations, Walloon infiltration requirements, or simply want to make the most of the rainfall that falls on your site, Revalio designs solutions that go beyond compliance.