Rainwater and flooding: how to manage water on your plot

Heavy rain quickly generates several hundred or thousand litres of water on a roof. How that water is collected, stored, infiltrated or drained plays an important role in managing runoff and preventing certain flooding problems.

A tank stores part of the water for reuse. Infiltration returns water to the ground where conditions allow. Drainage to an authorised outlet becomes necessary when infiltration is impossible. The right solution depends on the soil, the land, the building, the volumes and the applicable rules.

Since 1 January 2017, the Walloon Water Code has imposed a hierarchy for the projects concerned: infiltration first, then an artificial drainage route or a watercourse if infiltration is impossible, and finally the sewer network when other solutions are not possible. The aim: reduce network overload and preserve groundwater recharge.

A tank creates a buffer volume: part of the roof water is stored rather than discharged immediately, then reused for non-potable purposes. But a tank alone is not flood protection: its volume, the emptying capacity available before a rain event and the other devices on the plot matter just as much.

When the soil can no longer infiltrate enough, water concentrates towards the low points. Basements, garages, cellars and plant rooms are the most vulnerable. Depending on the layout, a sump or pumping station collects and drains the water to a suitable outlet.

A house can be affected by runoff without being beside a river: roads, sloping land, impermeable surfaces and natural flow paths concentrate water towards a plot. Wallonia provides flood hazard maps and data on certain runoff axes.

A coherent strategy combines several devices: tank, infiltration where possible, management of downpipes, suitable drainage, non-return valves as appropriate, sump and lifting pump for low areas. Each element is sized for the site.

During an intense event, flow rates become very high. A pump is chosen according to the incoming flow, the head, pressure losses and pipe diameter. For a critical installation, a high-level alarm, pump redundancy or a backup power supply can be studied.

Weber Water works on harvesting, pumping and rainwater management, studying the whole path of the water: collection, storage, possible infiltration, lifting and drainage.