Rain garden: the natural solution for sudden and heavy rains

How rain gardens capture rain and reduce run-off

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By Alex

A Rain Garden is a good landscape planning practice that can prove useful in avoiding damage from heavy and sudden rainfall. The climate changes we are witnessing in recent years, together with often inadequate water disposal infrastructure, contribute to the creation of phenomena such as flooding and inundation.

The first rain gardens date back to the 1990s. Today, they are finding increasingly wide application, as they are natural systems for managing meteorological runoff in a completely sustainable way. Among other things, they find use in both public (roads, driveways) and private gardens.

Let us then look more specifically at what these rain gardens consist of and how they work.

What are rain gardens

Rain gardens are nothing but slight depressions in the ground covered with plants and greenery having a very important technical function, which goes far beyond the purely aesthetic and hedonistic factor.

These are gardens designed specifically to manage and control large amounts of rainwater. Nowadays, with ongoing climate change, we are increasingly faced with violent heavy rainfall, which is then followed by long periods of drought.

The purpose of these gardens is to replace hydraulic engineering systems with devices that help regulate the natural hydrological cycle. In practice, the flow of rainwater is slowed down and the rainwater itself is purified through infiltration into the soil.

Structure of a rain garden

The structure of rain gardens is very simple. They are depressed basins (50 cm the max level from the ground level) that, taking advantage of the natural slope of the ground, accommodate excess rainwater that tends to accumulate on the impermeable surfaces adjacent to it, such as roads, roofs, parking lots and so on.

In terms of surface area, rain gardens must constitute at least 10 percent of the adjacent impermeable surface.

The basin, as we will explain further below, is made up of gravel, sand, organic compost, and site soil. This mix allows rainwater to run off gradually into the soil. In this way, in addition to not clogging the urban sewer system, the water itself is filtered by the soil, which releases it purified of about 30 percent of pollutants.

It is possible to supplement the rain garden system with a system for recovering runoff water so that it can be used, for example, for irrigation during drier periods.

Origin of rain gardens

The first rain garden was made in the 1990s in the USA. It immediately became very popular. Soon, the UK also began to look at this type of garden with interest, encouraging its use in urban areas in particular.

Currently, rain gardens are popular not only in the United States and In England, but also in China and Australia. In particular, China plans to implement a “Sponge city” program to mitigate floods that frequently affect various parts of the country.

What a rain garden is for

During very heavy rains, a rain garden collects rainwater and filters it, delivering it to the sewer system rather quickly and less polluted. In fact, the main purpose of these gardens is to contain possible flooding.

Basically, a rain garden:

  • collects rainwater
  • filters and purifies rainwater naturally
  • directs excess rainwater into existing drains
  • through the soil, separates water-soluble and toxic materials from the water
  • gives new vigor to groundwater
  • slowers the inflow of surface runoff so as to reduce the risk of downstream flooding
  • on a larger scale, helps greatly to counter road flooding
  • prevents water stagnation
  • by preventing water stagnation, it also prevents the proliferation of insects
  • if connected to a filtered stormwater storage and/or preservation system, gives the possibility of reusing the water itself, resulting in significant water savings
  • furnishes and decorates urban greenery and private gardens

Designing a rain garden

Considering its precipitous purpose and characteristics, designing a rain garden requires an assessment of the characteristics of the site where it is to be built.

Here are all the elements to be evaluated and established:

  • mapping the site including all structures and indicating areas where water flows as a result of rain
  • choosing impervious surfaces to manage with this garden
  • size of the garden
  • understand all slopes and identify the lowest points
  • climatic characteristics
  • atmospheric events that characterize the area, primarilythe frequency and intensity of precipitation
  • soil quality

The elements that make up a rain garden

In every rain garden, there are some basic elements that cannot be missing:

  • Draining soil composed mainly of sand (50 percent) and the rest of it of topsoil and compost. It performs various functions: it contributes to the absorption of heavy metals and various pollutants in general and provides structure and nutrients to plants
  • Water collection soil: a gravel bed with drainage pipes inside. It is used to convey water to the sewer system or collection tanks
  • Protective grass strip: herbaceous plants that cover the soil and serve to slow the incoming flow of water
  • Retention area: a depression of 10-20 cm of the soil is enough to restrain and collect water
  • Foil mulch: retains coarse organic matter and particles suspended in rainwater, and keeps the soil moist during the hottest and sultriest months
  • Plants, which must be carefully selected to withstand both excessive water and long dry spells

The zones of a rain garden

Typically, a rain garden is divided into 3 zones, depending on various moisture conditions.

  • Bottom: the central part, which can accommodate plants that like moist soil and, conversely, cannot stand dry soil.
  • Top: ideal for plants that like dry soils and do not suffer even for prolonged periods. It’s the driest and warmest of the three zones.
  • Terrace: suitable area for plants that tolerate both dry and wet soils equally.
rain garden
An illustration exemplifying the rain garden concept.

The ideal soil for rain gardens

The ideal soil for use in rain gardens consists, predominantly, of:

  • sand
  • mulch: creates shade, keeps the soil moist and shelters it from excessive heat, purifies the water from some pollutants in the rain, decreases the presence of weeds
  • rocks and gravel: very useful in areas of water inflow and outflow as they decrease the power of the water itself and limit the erosive process
  • organic compost: in addition to making the soil more fertile, it also filters out pollutants

The most suitable plants for rain gardens

The time of choosing plants to include in a rain garden is very delicate. Because of the special conditions in which they will have to live, they must in fact be plants that can withstand both very dry and very wet soils. For the depression zone of the garden, for example, botanical varieties will have to be chosen that, for short periods, are able to withstand submersion. The choice of plant essences should therefore be weighed rationally and not only on the basis of the aesthetic factor.

Therefore, the following factors should be taken into consideration when choosing rain garden plants:

  • the 3 rain garden zones: top, terrace and bottom
  • the area of origin of the species. The advice is toprefer native species, which are already accustomed to the climate of that specific place
  • the composition of the soil
  • the exposure: sun, shade, half-shade
  • the aesthetics (purely personal factor)
  • maintenance (more or less demanding)

In general, there must be a small number of plants in a rain garden, partly because each element must have enough space to grow develop well. On average, we recommend 6-10 plants per square meter. Among the botanical varieties chosen, avoid invasive species.

Also avoid plants whose roots need a lot of space, as well as plants whose roots tend to rot.

Maintenance and care of a rain garden

Generally speaking, a rain garden requires little maintenance. In the first 2 years, and in any case until the plants have grown enough, care should be taken to pull weeds regularly. After the first 2 years have passed, it will then be the plants themselves, which, now grown enough, will prevent weeds from developing.

From time to time, it is necessary to replenish the mulch condition, and, in addition, sediment and eroded material that has invaded the garden through water should be removed.

In the event that the garden remains flooded for too long, it may prove beneficial to divert the water so that the plants do not receive too much water.

In general, machinery or equipment that is too heavy should not be used for rain garden maintenance as it would cause soil compaction, thus creating an obstacle to drainage.

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