…what you need to know.

Here are 7 green solutions for stormwater management that can be used in cities. The first three and seventh items on the list are often the result of central (i.e. government or local government, i.e. political) decisions or investments, and in fact only the first item is something that cannot realistically be achieved through private investment. However, all the other elements can be implemented by blocks of flats and detached family houses.

It is important to note that while the impact of installing a single element may seem like a drop in the ocean, the combined impact of mass installation can be significant even at the urban level. In other words, the more people install such solutions, the better the city’s ability to manage rainwater will be.

Let’s look at the seven green solutions:

1. Maintain (or expand) existing green spaces

The most important task for cities is to maintain or, where possible, expand existing green spaces. Although we are approaching solutions from a specific rainwater management perspective, green spaces do not only play a role in rainwater absorption. Their role is significant

  • filtering out particulate matter, thus improving air quality, and

  • help to prevent urban heat islands, where heat from heated asphalt is trapped between buildings. In essence, they cool the city on hot days. To get a dramatic sense of this, you only have to walk into a leafy street in a heatwave from a square or street with no green space.

  • Green space also has a positive impact on general well-being. It supports healing, both mental and physical, as several studies have shown. It is no coincidence that more and more cities are trying to combine nature with the activities of health institutions.

  • Last but not least, it has a significant recreational role. Just think of Margaret Island in the summer. Picnics, leisure activities, social gatherings are all an integral part of how residents use the parks and green spaces.

2. Construction of drainage ditches

These are shallow ditches, usually installed alongside roads, that channel, store and slowly siphon off rainwater, relieving the burden on the sewer network. There are basically four types of trenches, which differ slightly in design and capabilities, but essentially perform the same function. Still, depending on the local environment and possibilities, one type of trench may be more desirable than another, provided your budget allows.

  • The most common is the classic grass swale. It is attractive because of its relatively cheap design. And even if modestly, it does filter rainwater from dirt.

  • An infiltration swale is actually a grassy ditch with a “dam” that can temporarily withhold and store water. Accordingly, it can do everything a grass ditch can do, but it’s better at silting and settling. But the price is higher because of the dam.

  • The bioswale bar is made using the bioretention principle. The siltation and filtration of rainwater is assisted by a special soil, a permeable layer, a perforated drainage pipe embedded in a gravel layer. This must be at least half a metre above the highest groundwater level. Its design is much more expensive, but it is superior to the previous solutions in both siltation and filtration.

  • Finally, a wet swale is a ditch planted with water-tolerant plants in waterlogged areas, mostly in places where the water table is already high, so that the level of the ditch is often visible.

3. Wider use of permeable pavements

  • Firstly, it helps to restore the natural hydrological balance under the pavement, while also retaining some of the rainwater and allowing it to slowly infiltrate into the soil.

  • Since urban flooding is often the result of run-off rather than just local rainfall, local stormwater management supported by such permeable pavements in places where large areas of pavement are required (e.g. parking lots) can be particularly beneficial, because it retains more water in place, so that the run-off is less of a burden on other areas. In other words, it can even make costly stormwater management investments elsewhere unnecessary.

  • It helps to prevent rainwater that is not captured from heating up as much as it would with impermeable asphalt, reducing the pressure on the flora and fauna of surrounding rivers or lakes (where urban rainwater flows into such catchments).
  • Finally, it can help filter stormwater to some extent from urban pollutants. This is especially true when it comes to permeable paving, such as the one pictured above, planted with plants.

4. Installation of water reservoirs / construction of reservoir cisterns

Although the construction of larger reservoirs/cisterns is likely to be a central or local government responsibility, smaller reservoirs can be put into use by anyone. Let’s see what are the main advantages of installing this tool:

  • It is probably the lowest-cost rainwater management method available to everyone and helps to retain rainwater. This not only relieves the sewerage burden (which in the long run means cheaper wastewater treatment), but also helps reduce urban flooding. The more people install such a device, the greater the impact. As the solution basically collects water run-off from the roofs, and the roof surface area, depending on the built-up area and the width of the roads, is approx. 20-35% of the total surface area of the city, it is easy to see that this is one of the most cost-effective ways of treating rainwater, as it can cover a relatively large area. Let us not forget that all this water would flow into the sewers.

  • Because of the previous point, it helps protect urban infrastructure and private property. It is not just about reducing the number or size of floods, but also about reducing the amount of water that falls in catchments in the event of an unexpectedly high rainfall event, which in turn reduces erosion damage.

  • The biggest advantage of rainwater harvesters is that the collected rainwater can be used later. Not for drinking, but for irrigation and other activities for which grey water is sufficient, it can be used at any time. This way we can reduce our water bills, as we don’t have to waste the expensive drinking water we buy on such things.

  • Finally, rainwater is much better for plants than drinking water. It is softer and contains no chlorine.

5. Installation of green roofs

By green roofs we are talking about roofs planted with vegetation. In effect, it is an attempt to replace the space taken by the building for nature. There are basically two types of green roofs: 1) low maintenance (extensive) green roofs, and 2) green roofs requiring full care and irrigation (intensive). Only intensive green roofs can offer full garden functionality and aesthetic value, while extensive variants are limited to drought-tolerant plants. Green roofs can be built on the roofs of most buildings. Although we may think that this is suitable only for flat-roofed buildings, and obviously the possibilities are wider there, there are more and more gable roofs nowadays, so it is not a reason to exclude them if you have one. What is true for green roofs is also true for watersheds: a green roof does not make an urban forest. But the combined effect of many green roofs is surprisingly significant. The benefits of a green roof in brief:

  • Of course, a green roof can also help to retain rainwater, as the garden and its plants can retain a significant amount of water (depending on the design and size of the garden, of course).

  • Green roofs also reduce the urban heat island effect. Now that the number of extreme hot days is already projected to roughly double in the medium term, the significance of this is not negligible. At the same time, it also reduces roof temperatures and has a positive impact on the average urban temperature. That is, if you count a drop of a few degrees in a heatwave as a positive.

  • Because green roofs are excellent insulators, they can reduce the surface temperature of the roof by up to 20 degrees in summer and significantly warm it in winter, making both cooling and heating the building more cost-effective. Of course, this is also a question of insulation, but the data available now suggests that green roofs will certainly reduce the running costs of summer air conditioners, which will often be even higher because of the heat that flows from the roof.

  • But insulation does not stop at heat. Green roofs are also significantly better at soundproofing. They help to absorb urban noise, especially low-frequency traffic sounds.
  • The improvement in air quality can certainly be included here. Rooftop gardens help to disperse dust and smog, reducing the city’s overall greenhouse gas emissions.
  • The overall well-being benefits of green spaces are unquestionable. A green roof attracts a wide variety of animals (insects, birds, etc.), making the urban environment a more pleasant experience, as it brings you closer to nature.
  • With the spread of green roof culture, a new trend towards sustainable urban agriculture can also be supported. You can even grow vegetables or fruit on green roofs.
  • Not to mention the added aesthetic value…

6. Installation of rain gardens

These are mostly low-lying areas, planted in the direction of the water flow, with a variety of drought and moisture-tolerant plants. Their design allows rainwater to be collected and drained away in 1-2 days. These can be installed in buildings or houses with gardens.

  • While this may not be the most important thing from a sustainability point of view, it is important to know that rain gardens reduce mosquito populations. The reason is that they collect and silt up water that would otherwise stand still and in which the dormice could breed. If not for the spit, that alone makes it worth installing one.
  • Of course, rain gardens are essentially about retaining rainwater and allowing it to drain away slowly. They also prevent or at least reduce the risk of flooding, either on your own property or (also) from water running off your property.

  • Because rain gardens catch, retain and slowly siphon rainwater, water stays in the soil longer, reducing the need for watering. So no need to waste drinking water on watering. If used in combination with a rainwater harvesting tank or tanks, the amount of water saved can be even greater.

  • Rain gardens are usually planted with deep-rooted, local flora that can take the strain. These often provide habitat or hunting grounds for many beneficial birds, butterflies and insects (including pollinators), displacing many pests that thrive in monoculture sites.

7. Planting organic gardens

Bioretention gardens are really rain gardens with a little extra engineering quirk. The first layer is a layer of soil specially optimised for infiltration, under which a bed of gravel is placed to retain water. In addition, the gravel bed has an extra drainage pipe underneath to control the water level, which drains excess water above a certain level outside the garden boundary, which is too much for the plants. The drain is actually a protruding pipe in the garden that acts as a drain when the water level rises too high. In fact, bioretention gardens should be thought of as a higher level rain garden, where targets such as the degree of filtration of pollutants, the saturated water conductivity of the soil, or the healthy environment of the plants planted can be more precisely set. Apart from this, it has the same positive effect as rain gardens.

Hopefully this summary has helped to give a more comprehensive picture of green solutions for urban water treatment.