In a climate crisis where torrential rains and urban flooding are becoming more frequent and severe, rain gardens are emerging as an effective, accessible, and scalable solution for managing runoff water. These green infrastructures not only mitigate the effects of flooding but also beautify urban environments, promote biodiversity, and improve water quality. Here we explain how to implement this solution in your community.
What is a Rain Garden and How Does It Work?
A rain garden is a shallow depression filled with porous substrate and native vegetation, designed to capture, retain, and filter runoff from impermeable surfaces (roofs, streets, sidewalks).
How it works:
- Capture: Rainwater is directed into the garden through pipes or sloped surfaces.
- Temporary storage: Water accumulates in the depression during and immediately after rainfall.
- Infiltration: The porous substrate and plant roots allow water to slowly seep into the groundwater, recharging aquifers.
- Filtration: Plants and soil remove pollutants such as heavy metals, hydrocarbons, and excess nutrients.
- Evapotranspiration: Vegetation returns some of the water to the atmosphere, reducing the total volume of runoff.
Benefits of Rain Gardens
- Flood Reduction: They can absorb between 30% and 90% of runoff water, depending on their size and design.
- Improved Water Quality: They filter up to 80% of sediments and pollutants.
- Aquifer Recharge: They facilitate water infiltration into the groundwater, combating drought.
- Biodiversity: They attract pollinators and beneficial wildlife.
- Urban Beautification: They transform gray spaces into green areas.
- Reducing the Heat Island Effect: Vegetation mitigates high temperatures.
How to Design and Build a Rain Garden
1. Location and Size
- Ideal Location: Near runoff sources (downspouts, streets, parking lots), but at least 3 meters from foundations to avoid moisture problems.
- Size: Generally, between 5% and 10% of the impervious area drains into it. For a typical home, a garden of 5-10 m² is sufficient.
- Depth: Between 15 and 30 cm, with gentle slopes toward the center.
2. Site Preparation
- Excavation: Create a depression with a flat bottom and sloping sides.
- Substrate: A mixture of 50-60% sand, 20-30% compost, and 20-30% topsoil. This ensures drainage and nutrients.
- Gravel Layer: At the bottom, a thin layer of gravel improves drainage.
3. Plant Selection
Choose native species adapted to flood-drought cycles:
- For wet areas: Rushes (Juncus spp.), Calla lilies (Iris pseudacorus).
- For dry borders: Lavender (Lavandula spp.), Sage (Salvia spp.).
- Pollinator Attractants: Echinacea (Echinacea purpurea), Mint (Mentha spp.).
4. Inlet and Outlet System
- Inlet: From downspouts or drainpipes, with stones to disperse the water’s energy.
- Outlet: An overflow channel to direct excess water into the municipal drainage system in case of extreme rainfall.
5. Maintenance
- Initial Watering: Until plants are established.
- Weeding: Regular during the first year.
- Pruning and cleaning: Remove dry leaves and accumulated sediment.
Examples of Success in Cities
Copenhagen, Denmark: Following the 2011 floods, the city implemented 300 rain gardens as part of its climate adaptation plan.
- Portland, USA: Tax incentives for homeowners who install rain gardens, managing million liters of water annually.
- Mexico City: Projects in neighborhoods such as Xochimilco and Iztapalapa to reduce flooding and recharge aquifers.
Common Barriers and Solutions
- Lack of space: In densely populated areas, containerized or vertical rain gardens are preferred.
- Contaminated soils: Use phytoremediation plants such as sunflowers (Helianthus annuus) and lilies.
- Ignorance: Educational campaigns and community workshops.
Potential Impact
If 10% of urban households implemented rain gardens:
- Reduction of million liters of annual runoff.
- Aquifer recharge equivalent to thousands of Olympic-sized swimming pools.
- Creation of green corridors that connect fragmented habitats.
How to Involve Your Community
- Organize workshops with rain garden experts.
- Create collaborative maps of flood-prone areas.
- Request municipal support: some municipalities offer grants or technical assistance.
“Every drop of infiltrated rain is a victory against flooding and drought.” — Dr. Luis Zambrano, urban ecologist.
