Managing Rainwater in Your Garden: Geneva Solutions

The principle of managing rainwater in your garden is simple: it is better to let it soak away on site, or to store it, than to send it off as quickly as possible. In Geneva, where storms can be violent and soils are often clay, this means permeable surfaces, swales, infiltration trenches, targeted drainage and, if you wish, a tank for watering. Before changing how water flows across your property, check the cantonal and municipal rules.

Understanding the problem: where does the water go on your property?

Before choosing a solution, watch your land during a downpour. Good water management starts with an accurate picture of what happens, not with equipment.

• Where does the water come from? Roof, terrace, driveway, or the slope of a neighbouring plot or the street.
• Where does it end up? Lasting puddles on the lawn, a waterlogged wall base, a flooded garage entrance, water standing near the foundations.
• Where does it go next? Into a drain, into an infiltration area, or onto the neighbouring plot, which must be avoided.

Also note the areas that stay damp long after the rain: they point to poorly permeable soil or a hollow where water collects. A sketch of the plot showing the direction of flow will be useful when a professional visits.

Infiltrate rather than drain away

Sealed surfaces (concrete, asphalt, grouted paving) send all the water at once towards the drainage network and the lower part of the plot. Infiltrating on site spreads the water over time and recharges the soil.

Permeable surfaces

Stabilised gravel, grass-jointed paving slabs or draining paving blocks let water through to the subsoil. For a driveway or parking area, the choice of surface and its foundation directly determines how water is managed: our guide to driveway surfaces for your garage access details the options. If you still opt for a waterproof surface, plan slopes that direct water towards an infiltration area rather than towards the street or your neighbour; asphalt driveways should be designed with this in mind from the start.

Swales and rain gardens

A swale is a slight planted depression, usually long and shallow, that collects runoff and lets it soak in slowly. A rain garden follows the same principle in the form of a basin, planted with species that tolerate alternating wet and dry spells. These features blend into a landscape design and also serve as decorative elements.

Soakaways and infiltration trenches

Where space is limited, roof water can be directed to a soakaway or a trench filled with draining material, buried under the lawn. The water is held there briefly and then disperses into the ground. Their effectiveness depends on the nature of the soil: on clay, infiltration is slow, and the structure must be designed accordingly, or even supplemented with an overflow.

Draining damp areas and wall bases

Infiltration is not always enough. Some areas need to be dried out to protect structures and plantings.

1. Identify the source: surface water, a temporary water table or seepage behind a wall.
2. Divert surface water with a slope or a channel before it reaches the sensitive area.
3. Install drainage (a perforated pipe in a drainage trench) when water collects at depth.
4. Provide an outlet: an authorised discharge point, never the neighbouring plot.

Wall bases deserve particular attention: water building up behind a wall exerts pressure that can damage it. A gabion retaining wall has the advantage of being naturally draining, as our guide to the gabion wall in the garden also explains. Near cellars and foundations, never direct water towards the building: if dampness persists, have the structure examined by a building specialist as well.

Collecting rainwater for watering

A tank, buried or placed above ground, stores roof water for watering the garden. It helps with dry summers while holding back part of the storm water. A few pointers:

• water collected from a roof is suitable for watering, not for drinking;
• a filter upstream limits the entry of leaves and debris;
• an overflow must be provided and connected to an outlet or an infiltration area, because a full tank holds back nothing more;
• it pairs well with automatic irrigation, provided the pump and filtration are suited to the use.

The Geneva context

The lakeside climate alternates between summers that can be dry and sometimes intense storms: hardened soil absorbs the first drops poorly, and the water runs off. Clay soils, common in the canton, retain water and become saturated quickly; infiltration there is slow, so you need to multiply infiltration points and provide overflows. On sloping plots, water picks up speed: terraces, swales running across the slope and draining walls slow it down. The bise, meanwhile, dries out surfaces and increases the need for watering in summer, hence the value of a reserve.

As for the rules, the discharge of rainwater and connections to the networks follow cantonal and municipal provisions. They vary by plot and by municipality: check with your commune before changing a flow, creating a discharge point or connecting a drain. Constraints differ from one area to another, for example in the villas of Cologny, on the slopes of Veyrier or in the denser districts of Carouge. For an overview, see the page on a landscaper in Geneva.

Entrusting the project to a landscaper

The right solution depends on the soil, the slope, the areas to be treated and the rules that apply to your plot. A landscaper visits the site, observes how the water actually flows and proposes a coherent whole: surfaces, swales, drainage, storage. To request a quote, describe your project: a qualified landscaper will contact you within 24 hours for a visit and a detailed quote, free of charge and with no obligation. Find more advice in our landscaping guides.

Frequently asked questions

Can I send my roof’s rainwater to the neighbour’s plot or into the street?

No, you must not direct water onto the neighbouring plot, and discharge to the public road or the networks is regulated. Cantonal and municipal rules apply: check with your commune before making any change. It is better to infiltrate or store the water on your own land.

Does infiltration work on clay soil?

It works, but more slowly. Clay lets water through poorly, hence the value of multiplying infiltration features, providing overflows and, if needed, drainage. A professional assesses the land on site to choose the right combination.

What is the difference between a swale and a drain?

A swale is a shallow, planted ditch, visible, that collects and infiltrates water at the surface. A drain is a buried pipe that collects water at depth and carries it to an outlet. The two can complement each other on the same plot.

Is a rainwater tank enough to water the garden all summer?

It depends on the connected roof area, the size of the tank and the garden to be watered. It significantly reduces the use of mains water, but may not cover a very dry summer. Sensible watering, with suitable plants, makes the reserve last longer.

The base of my wall stays damp: what should I do?

First find out where the water comes from: roof, slope, sealed surface or water table. Divert the surface flow, then consider drainage with an authorised outlet. If the dampness reaches the cellar or foundations, have the building examined as well.